Corrosion Tutorial
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CORROSION TUTORIAL Dated: June 14, 2004
Hota GangaRao Eung H Cho Sucharitha Bachanna Srinivas Aluri Robert Creese
Constructed Facilities Center West Virginia University, Morgantown, WV 26506
Corrosion Tutorial
FOREWORD Corrosion is the result of chemical reaction between a metal and its environment. It is the tendency of the refined metal to return to its mineral state. Corrosion engineers study the corrosion mechanisms to determine the causes of corrosion and the methods to minimize the resulting damage. Also, corrosion engineers design and apply various methods to prevent corrosion by practical and economical means. The importance of corrosion is not only related with economy, but also with safety issues. The loss of material due to corrosion results in the failure of the machines, structures, bridges, etc. resulting in damages worth billions of dollars. The annual direct cost of corrosion and of protection against corrosion in the United States for Department of Transportation alone is estimated to be around 276 billion in the year 2001. Direct costs mean the costs of replacing corroded structures and machinery. Indirect costs resulting from actual or possible corrosion are more difficult to evaluate and maybe more than $276 billion. Some of the major industries affected by corrosion are 1) Defense, 2) Nuclear power plants, 3) Aircraft, 4) Pipeline, 5) Storage Tanks, 6) Highways and bridges, 7) Water systems, 8) Gas distribution, 9) Transportation, 10) Petroleum, 11) Oil and natural gas, 12) Chemical plants, etc. In this tutorial, some of the basics issues dealing with corrosion are explained. The essential elements of electrochemistry that are needed to understand the basics of corrosion reactions are presented in Chapters 1 and 2. Corrosion has been classified in many different ways. One way is to classify by the forms in which corrosion occurs. The forms of corrosion are discussed in Chapter 3. Corrosion of composites is typically called Aging.
Composites have different mechanical properties compared to most
metals. Hence aging of composites is discussed in Chapter 4. Use of composites is rapidly becoming prevalent in many applications. Some of the important applications are discussed in Chapter 5. Although corrosion cannot be prevented, its rate can be retarded using many different methods. Some of the important methods are discussed in Chapter 6. Chapter 7 consists of brief information on typical metals and composites that are used extensively in the industry. chemical kinetics of the corrosion reactions.
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Chapter 8 deals with advanced electro-
Corrosion Tutorial
TABLE OF CONTENTS 1
FUNDAMENTALS OF CORROSION
1
1.1
ELECTROCHEMICAL CELL
1
1.2
STANDARD ELECTROCHEMICAL POTENTIAL
3
1.3
NERNST EQUATION
5
1.4
FREE ENERGY AND ELECTRODE POTENTIAL
6
1.5
POTENTIAL MEASUREMENT OF HALF-CELL REACTION
7
2
PASSIVITY AND ELECTROCHEMICAL CORROSION MEASUREMENTS
9
2.1
PASSIVITY
9
2.2
TAFEL EXTRAPOLATION
12
2.3
LINEAR POLARIZATION RESISTANCE METHOD
13
2.3.1 DERIVATION OF THE POLARIZATION RESISTANCE
13
2.3.2 PRINCIPLE OF MEASUREMENT
14
2.3.3 ADVANTAGES OF POLARIZATION RESISTANCE MEASUREMENTS
15
2.3.4 ERRORS AND LIMITATIONS IN THE USE OF POLARIZATION RESISTANCE MEASUREMENTS
15
2.4
16
OTHER METHODS TO DETERMINE POLARIZATION RESISTANCE
2.4.1 ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY
16
2.4.2 ELECTROCHEMICAL NOISE
16
3
18
FORMS OF CORROSION
3.1
UNIFORM CORROSION
3.1.1
18
ATMOSPHERIC CORROSION
20
3.1.1.1
Mechanism
20
3.1.1.2
Prevention of Atmospheric Corrosion
21
3.1.2
GALVANIC CORROSION
21
3.1.2.1
CORRODED END (ANODIC OR LEAST NOBLE)
26
3.1.2.2
PROTECTED END (CATHODIC OR MOST NOBLE)
28
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3.1.2.3
Factors Affecting Galvanic Corrosion
30
3.1.2.4
Galvanic Series
31
3.1.2.5
Prevention of Galvanic Corrosion
33
3.1.3
STRAY CURRENT CORROSION
33
3.1.3.1
Direct stray current corrosion
33
3.1.3.2
Alternating Stray Current Corrosion
34
3.1.3.3
Telluric Effects
34
3.1.4
GENERAL BIOLOGICAL CORROSION
36
3.1.4.1
Causes and Prevention of Biological Corrosion
36
3.1.4.2
Prevention of biological corrosion
37
3.1.5
MOLTEN SALT CORROSION
38
3.1.6
CORROSION IN LIQUID METALS
39
3.2
LOCALIZED CORROSION
3.2.1
39
PITTING CORROSION
39
3.2.1.1
Initiation of Pitting Corrosion
40
3.2.1.2
Propagation of Pitting Corrosion
40
3.2.1.3
Prevention of Pitting Corrosion
42
3.2.2
CREVICE CORROSION
43
3.2.2.1
Initiation and Propagation of Crevice Corrosion
43
3.2.2.2
Prevention of Crevice Corrosion
45
3.2.3
PACK RUST
45
3.2.4
FILIFORM CORROSION
46
3.2.4.1 3.2.5
3.3
Prevention of Filiform Corrosion
48
LOCALIZED BIOLOGICAL CORROSION
49
ENVIRONMENTALLY INDUCED CRACKING
3.3.1
STRESS CORROSION CRACKING
49 49
3.3.1.1
Metallurgical Effects
50
3.3.1.2
Electrochemical Effects
50
3.3.1.3
Prevention of Stress Corrosion Cracking
51
3.3.2
SULFIDE STRESS CRACKING
52
3.3.3
LIQUID METAL EMBRITTLEMENT
52
3.3.4
SOLID METAL INDUCED EMBRITTLEMENT
53
3.3.5
CORROSION FATIGUE CRACKING
54
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3.3.5.1
Comparison with Stress Corrosion Cracking
55
3.3.5.2
Prevention of Corrosion Fatigue Cracking
55
3.3.6
HYDROGEN INDUCED CRACKING
56
3.3.6.1
Comparison with Stress Corrosion Cracking
56
3.3.6.2
Prevention of Hydrogen Induced Cracking
57
3.4
MECHANICALLY ASSISTED DEGRADATION
3.4.1
EROSION CORROSION
3.4.1.1 3.4.2
57
Prevention of Erosion Corrosion
58
IMPINGEMENT CORROSION
3.4.2.1
57
58
Prevention of Impingement Corrosion
59
3.4.3
CAVITATION CORROSION
59
3.4.4
FRETTING CORROSION
60
3.5
METALLURGICALLY INFLUENCED CORROSION
3.5.1
INTERGRANULAR CORROSION
61 61
3.5.1.1
Exfoliation / Lamellar Corrosion
61
3.5.1.2
Weld Decay
62
3.5.1.2.1
3.5.1.3
Prevention of Weld Decay
63
Sensitization (Intergranular Corrosion of Austenitic Stainless Steels)
63
3.5.2
DEALLOYING
65
3.5.3
DEZINCIFICATION
65
3.5.3.1 3.6
Prevention of Dezincification
65
HIGH TEMPERATURE CORROSION
65
3.6.1
OXIDATION
66
3.6.2
SULFIDATION
66
3.6.3
CARBURIZATION
67
4
AGING OF COMPOSITES
69
4.1
INTRODUCTION
69
4.2
COMPOSITION
69
4.2.1
FIBER REINFORCEMENTS
70
4.2.2
RESIN SYSTEMS
71
4.2.3
FILLERS
71
4.2.4
ADDITIVES
71
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4.3
FIBER REINFORCED POLYMERS
72
4.3.1
CHARACTERISTICS OF FIBER REINFORCED COMPOSITES
75
4.3.2
SHORT-TERM MECHANICAL AND HYGRO-THERMAL BEHAVIOR
78
4.3.2.1
Thermal Coefficient and Conductivity
78
4.3.2.2
Moisture Diffusion/Plasticization
81
4.3.2.2.1
Diffusion Through Unreinforced Epoxy, Vinyl Ester, Polyestor and Phenolics
82
4.3.2.2.2
Effect of Moisture on Fiber-Matrix System
87
4.3.2.2.3
Effect of Temperature and Polymer Structural Variables on Sorption of Water
94
4.3.3
LONG TERM MECHANICAL AND HYGROTHERMAL BEHAVIOR (AGING)
4.3.3.1
Creep Theory
104
104
4.3.3.1.1
Effect of moisture and temperature on Creep
115
4.3.3.1.2
Effect of Physical Aging on Creep
119
4.3.3.1.3
Effect of Ultraviolet (UV) Radiation on Creep
120
4.3.3.2
Fatigue and Fracture
121
4.3.3.2.1
Fatigue Process
124
4.3.3.2.2
Fatigue in Unidirectional Composites
127
4.3.3.2.3
Fatigue in Multidirectional Composites
130
4.3.3.3
Aging Due to Environmental Factors
4.3.3.3.1
Environmental Factors Influencing the Durability of Composites
131 132
4.3.3.4
Knockdown Factors
144
4.3.3.5
Durability Models
147
4.3.3.5.1
4.4 5
Analytical Methods to Predict the Effects of Environment on Composite Materials
SUMMARY AND CONCLUDING REMARKS APPLICATIONS OF COMPOSITES
5.1
147
155 156
APPLICATIONS OF COMPOSITES FOR DEFENSE PURPOSES
156
5.1.1
AIRCRAFT SYSTEMS
156
5.1.2
GROUND SYSTEMS
158
5.1.3
INDIVIDUAL AND CREW SERVED SYSTEMS
158
5.1.4
ROCKET AND MISSILE SYSTEMS
159
5.1.5
SHIPBOARD SYSTEMS
160
5.2 5.2.1
APPLICATIONS OF COMPOSITES FOR CIVILIAN PURPOSES AUTOMOTIVE
161 161
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5.2.2
INFRASTRUCTURE
162
5.2.3
CONSTRUCTION
162
5.2.4
TRANSPORTATION
163
5.2.5
BIOMEDICAL
164
5.2.6
COMPUTER PRODUCTS
164
5.2.7
CORROSION RESISTANT PRODUCTS
165
5.2.8
ELECTRICAL
165
5.2.9
RECREATIONAL
166
5.2.10
MARINE
166
6
RETARDATION METHODS FOR CORROSION
6.1
CATHODIC PROTECTION
6.1.1
168 168
SACRIFICIAL ANODE SYSTEM
168
6.1.1.1
Advantages of Sacrificial Anode Systems
170
6.1.1.2
Disadvantages of Sacrificial Anode Systems
170
6.1.2
IMPRESSED CURRENT CATHODIC PROTECTION
170
6.1.2.1
Advantages of Impressed Current Cathodic Protection
172
6.1.2.2
Disadvantages of Impressed Current Cathodic Protection
172
6.2
COATINGS
172
METALLIC
172
6.2.1
6.2.1.1
Hot Dipping
172
6.2.1.2
Chemical Vapor Deposition (CVD)
173
6.2.1.3
Ion Vapor Deposition (IVD)
173
6.2.1.4
Spraying
173
6.2.1.5
Electroplating
174
6.2.2
INORGANIC
175
6.2.2.1
Portland Cement Coatings
175
6.2.2.2
Ceramics
175
6.2.2.3
Chromate Filming
175
6.2.2.4
Phosphate Coatings
176
6.2.2.5
Nitriding
176
6.2.3
6.2.3.1
ORGANIC
176
Binders
176
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6.2.3.2
Pigments
176
6.2.3.3
Solvents
177
6.2.4
6.3
NONSTICK COATINGS
177
INHIBITORS
177
6.3.1
ANODIC INHIBITORS
178
6.3.2
CATHODIC INHIBITORS
179
6.3.3
MIXED INHIBITORS
179
6.3.4
APPLICATIONS
179
6.4 7
ANODIC PROTECTION
179
METALS AND COMPOSITES DICTIONARY
7.1
METAL ALLOYS
7.1.1
182 182
CAST IRON
182
7.1.1.1
White Cast Iron
182
7.1.1.2
Malleable Iron
183
7.1.1.3
Ductile Iron
183
7.1.1.4
Gray Cast Iron
183
7.1.1.5
High Silicon Cast Iron
183
7.1.2
STEELS
184
7.1.2.1
Plain Carbon Steels
184
7.1.2.2
Low Alloy Steels
184
7.1.2.3
Stainless Steels
185
7.1.3
ALUMINUM AND ITS ALLOYS
185
7.1.4
MAGNESIUM AND ITS ALLOYS
186
7.1.4.1
Lead and its Alloys
186
7.1.5
COPPER AND ITS ALLOYS
186
7.1.6
NICKEL AND ITS ALLOYS
187
7.1.7
ZINC AND ITS ALLOYS
187
7.1.8
CADMIUM
187
7.1.9
TITANIUM AND ITS ALLOYS
188
7.1.10
COATED ALLOYS
188
7.1.11
MOLYBDENUM
188
7.1.12
TANTALUM
188
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7.1.13
TUNGSTEN
189
7.1.14
ZIRCONIUM
189
7.1.15
GOLD
189
7.1.16
PLATINUM
189
7.1.17
SILVER
190
PLASTICS
190
7.2 7.2.1
THERMOSETS
190
7.2.2
THERMOPLASTICS
191
7.3
CERAMICS
191
7.4
COMPOSITE MATERIALS
191
7.4.1
ORGANIC MATRIX COMPOSITES (OMCS)
192
7.4.2
METAL MATRIX COMPOSITES (MMCS)
193
7.4.3
CERAMIC MATRIX COMPOSITES (CMCS)
193
7.4.4
PARTICULATE REINFORCEMENTS
193
7.4.5
WHISKER REINFORCEMENTS
194
7.4.6
CONTINUOUS FIBER REINFORCED COMPOSITES
194
7.4.7
BRAIDED FABRICS
194
7.4.8
HYBRID FABRICS
195
7.4.9
KNITTED OR STITCHED FABRICS
196
7.4.10
WOVEN COMPOSITES
196
7.4.11
CARBON FIBER REINFORCED PLASTICS
197
7.4.12
GLASS FIBER REINFORCED PLASTICS
197
7.4.13
ARAMID FIBER REINFORCED PLASTICS
197
7.5
MANUFACTURING PROCESSES OF COMPOSITES
7.5.1
OPEN-MOLD PROCESSES
197 197
7.5.1.1
Hand Lay Up
198
7.5.1.2
Tube Rolling
198
7.5.2
CLOSED-MOLD PROCESSES
199
7.5.2.1
Resin Transfer Molding (RTM)
199
7.5.2.2
Vacuum Assisted Resin Transfer Molding (VARTM)
200
7.5.2.3
Resin Injection Molding Process
200
7.5.2.4
Compression Molding
201
7.5.2.5
Pultrusion
202
7.5.2.6
Extrusion
203
7.5.2.7
Filament Winding Process
203
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7.6
COMPOSITES TERMINOLOGY
204
7.7
REAGENTS
212
7.7.1
SULFURIC ACID
212
7.7.2
HYDROCHLORIC ACID
212
7.7.3
NITRIC ACID
213
7.7.4
HYDROFLUORIC ACID
213
7.7.5
PHOSPHORIC ACID
214
8
CORROSION KINETICS
8.1
215
POLARIZATION
215
8.1.1
ACTIVATION POLARIZATION
215
8.1.2
CONCENTRATION POLARIZATION
220
8.2
MIXED POTENTIAL THEORY
222
8.3
EXPERIMENTAL POLARIZATION CURVES
224
LIST OF FIGURES
Figure 1-1: Electrochemical Cell _________________________________________________ 2 Figure 2-1: Schematic Active-Passive Behavior of the Anodic Polarization of a Metal _______ 9 Figure 2-2: Schematic Representation of Corrosion of Stainless Steel with Two Oxidation Reagents ___________________________________________________________________ 10 Figure 2-3: Comparison of Galvanostatic and Potentiostatic Anodic Polarization Curves_ __ 11 Figure 2-4: Cathodic and Anodic Polarization Curves________________________________ 12 Figure 3-1: Uniform Corrosion
17
Figure 3-2: Example of uniform corrosion damage on a rocket assisted artillery projectile
18
Figure 3-3: Atmospheric corrosion of galvanized anti crash railing due to marine aerosol condensation on wooden post
19
Figure 3-4: Galvanic Corrosion of Brass Coupled With Black Iron
20
Figure 3-5: Mechanism of Galvanic Corrosion of a Two Metal Couple
21
Figure 3-6: Galvanic cell showing corrosion process in its simplest form
22
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Figure 3-7: Corrosion rate determination for a two electrode process system
23
Figure 3-8: Corrosion rate determination for a three electrode system
24
Figure 3-9. The introduction of a less noble metal will decrease the corrosion rate of the more noble metal.
25
Figure 3-10: Effect of cathode to anode ratio in galvanic corrosion
28
Figure 3-11: Occurrence of Stray current corrosion in pipelines.
33
Figure 3-12: Ionic current flow onto the pipeline
34
Figure 3-13: Current flow onto pipeline at coating discontinuities
35
Figure 3-14: External stray current sources.
36
Figure 3-15: Corroded surface of carbon steel in its natural condition
37
Figure 3-16: Pitting in Aluminum
39
Figure 3-17: Propagation of Pitting Corrosion
41
Figure 3-18: Crevice Corrosion
43
Figure 3-19: Mechanism of Crevice Corrosion
44
Figure 3-20: crevice corrosion in rivets
45
Figure 3-21: A crevice formed into an open atmosphere
46
Figure 3-22: Example of Pack Rust
46
Figure 3-23: Mechanism of Filiform Corrosion
47
Figure 3-24: “worm like” filiform corrosion tunnels.
48
Figure 3-25: Filiform Corrosion Causing Bleed Through a Welded Tank
49
Figure 3-26: Stress Corrosion Cracking Showing Branched Cracks in Aluminum Plates
50
Figure 3-27: Schematic of Active-Passive Behavior of the Anodic Polarization of a Metal
51
Figure3-28: Liquid Metal Embrittlement
53
Figure 3-29: Solid Metal Induced Embrittlement of a cadmium plated B7 bolt
54
Figure 3-30: Brittle crack in a cadmium plated B7 bolt from solid metal induced embrittlement ____________________________________________________________________________55 Figure 3-31 Impingement corrosion in a bent tube
59
Figure 3-32: Exfoliation of Aluminium
62
Figure 3-33:Exfoliation of aircraft component
63
Figure 3-34:Intergranular corrosion in stainless steel
64
Figure 3-35: Intergranular Corrosion of 7075-T6 aluminum adjacent to steel fastener
65
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Figure 4-1: Fibers as reinforcement a) Random Fibers________________________________70 Figure 4-1: Fibers as reinforcement b) Continuous Fibers (Long)______________________ _70 Figure 4-2: The combined effect on modulus of the addition of fibers to the resin matrix_____ 73 Figure 4-3: Typical Sorption Curve (Vijay et al., 2001)______________________________ 83 Figure 4-4: The Sorption Curves for Epoxy, Vinyl ester, and Isopolyester Resin When Exposed to the 3 Different Solutions (Chin et al., 1999)_________________________________________84 Figure 4-5: Fickian Diffusion Curves for Epoxy in (a) Water, (b) Salt Solution, and (c) Concrete Pore Solution at 22 °C (Chin et al., 1999)__________________________________________ 86 Figure 4-6: Thermal Expansion Measured by Stokes (Stokes, 1990)_____________________ 88 Figure 4-7: Moisture-Induced Thermal Expansion vs. Temperature (Stokes, 1990)_________ 88 Figure 4-8: (A) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen as a function on the humidity of conditioning environment. (Stokes, 1990) ___________________ 90 Figure 4-8: (B) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in Across Ply Direction (Stokes, 1990)_____________________________________________________91 Figure 4-8: (C) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in Ply Direction (Stokes, 1990)________________________________________________________92 Figure 4-8: (D) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in Wrap Direction (Stokes, 1990)________________________________________________________93 Figure 4-9: Difference Between (a) Diamine and (b) Aniline Hardener __________________ 99 Figure 4-10: Bonds Between Glass Fiber and Coupling Agent. _______________________ 102 Figure 4-11: Typical Creep Behavior of Plastics (GangaRao, 2001) ___________________ 105 Figure 4-12: Maxwell's Model__________________________________________________ 108 Figure 4-13: Kelvin Model_____________________________________________________110 Figure 4-14: Four Element Model _______________________________________________111 Figure 4-15: Behavior of creep and stress relaxation in four element model ______________112 Figure 4-16: Behavior of creep when subjected to a series of stresses ___________________113 Figure 4-17: Schematic Representation of the Effects of Time, Temperature, and Moisture on Creep Compliance (Liao,1998) ________________________________________________ 116 Figure 4-18: Moisture Absorption Behavior_______________________________________ 118 Figure 4-19: Effect of Physical Aging on Creep_____________________________________120
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Figure 4-20: Fatigue Damage Mechanism in Unidirectional Composites Under Loading Parallel to Fibers: (a) Fiber Breakage, Interfacial Debonding; (b) Matrix Cracking; (c) Interfacial Shear Failure [Talreja, 1987]________________________________________________________125 Figure 4-21: Two-Stage Strength Degradation Model for Fatigue Reliability of Composites [Talreja, 1987] _____________________________________________________________ 127 Figure 4-22: Comparison of S-N Curve for Three Different Unidirectional Composite Materials [Curtis and Dorey, 1986] _____________________________________________________ 128 Figure 4-23: Comparison of S-N curve for Four Different Materials with Different Carbon Fibers in Same Epoxy Resin [Curtis and Dorey, 1986] ______________________________ 129 Figure 4-24: Fatigue Life Diagram of Unidirectional Composites Under (a) Loading Parallel to Fibers, (b) Off-Axis Loading (Dotted line correspond to on-axis loading) [ Talreja, 1987] __ 130 Figure 4-25: Normalized S-N Curves for (0/±45) CFRP Laminates with Varying Percentage of 0o Fibers [Curtis and Dorey, 1986] _____________________________________________ 131 Figure 5-1: F-22 Raptor Aircraft -Tactical Fighter Aircraft __________________________ 157 Figure 5-2: RAH-66 Comanche ________________________________________________ 157 Figure 5-3: a.GAU –19A b. F18C/D ____________________________________________ 158 Figure 5-4: Reactive Armor and XM-301 Gun ____________________________________ 158 Figure 5-5: Objective Crew Served Weapon ______________________________________ 159 Figure 5-6: Delta II _________________________________________________________ 160 Figure 5-7: Missiles from the Hydra 70 Family____________________________________ 160 Figure 5-8: Destroyers _______________________________________________________ 161 Figure 5-9 Goalkeeper: In-Ship Defense System ___________________________________ 161 Figure 5-10: Composite Fire Truck Panels _______________________________________ 162 Figure 5-11: All Composite Bridge, Laurel Lick, CFC-WVU _________________________ 162 Figure 5-12: Energy Plant Towers ______________________________________________ 163 Figure 5-13: Third Rail Protection in Monorail System _____________________________ 163 Figure 5-14: MRI Units ______________________________________________________ 164 Figure 5-15: Composite Computer Chip _________________________________________ 164 Figure 5-16: Waste Water Plant________________________________________________ 165 Figure 5-17: Telecommunication Towers_________________________________________ 166 Figure 5-18: Recreational Products_____________________________________________ 166
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Figure 5-19: Sheet Piling and Fender Applications_________________________________ 167 Figure 6-1: Steel Tank Protected by Sacrificial Anode System ________________________ 169 Figure 6-2: Mechanism of Anodic Protection System _______________________________ 170 Figure 6-3: Steel Tank Protected by Impressed Current System _______________________ 171 Figure 6-4: Mechanism of Impressed Current Systems Explained Using Anodic Polarization Curves ____________________________________________________________________ 171 Figure 6-5: Mechanism of Electroplating ________________________________________ 174 Figure 6-6: Effects of Applied Anodic Current on the Behavior of Active-Passive Alloy ____ 181 Figure 7-1: Particulate Reinforcement___________________________________________ 193 Figure 7-2: Whisker Reinforcement _____________________________________________ 194 Figure 7-3: Continuous Fiber Reinforcement _____________________________________ 194 Figure 7-4: Braided Fabrics___________________________________________________ 195 Figure 7-5: Hybrid Fabrics ___________________________________________________ 195 Figure 7-6: Knitted or Stitched Fabrics __________________________________________ 196 Figure 7-7: Woven Fabric ____________________________________________________ 196 Figure 7-8: Hand Lay Up _____________________________________________________ 198 Figure 7-9: Tube Rolling _____________________________________________________ 199 Figure 7-10: Resin Transfer Molding Machine (CFC-WVU) _________________________ 200 Figure 7-11: VARTM –Tabletop Model of VARTM and Schematic Process of Manufacture _ 200 Figure 7-12: Injection Molding Machine (CFC-WVU) ______________________________ 201 Figure 7-13: Compression Molding Machine (CFC-WVU)___________________________ 202 Figure 7-14: Schematic Representation of Pultrusion Process (Bedford Plastics) ____________ 202 Figure 7-15: Basic Extruder___________________________________________________ 203 Figure 7-16 a: Winding Machine Showing Carriage and Mandrel b: Filament Winding____ 204 Figure 8-1: Activation Polarization _____________________________________________ 216 Figure 8-2: Butler-Volmer Equation and Tafel Equation ____________________________ 220 Figure 8-3: Combined Polarization _____________________________________________ 222 Figure 8-4: Behavior of Metal M in Acid Solution__________________________________ 223 Figure 8-5: Behavior of Coupled Metals in Acid Solutions ___________________________ 224 Figure 8-6: Showing Cathodic and Anodic Polarization Curves_______________________ 226
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LIST OF TABLES Table 1-1: E0 Values for Metals __________________________________________________ 4 Table 1-2: E0 Values for Common Oxidation Reagents ________________________________ 5 Table 3-1: Galvanic Series of Metals/Alloys in Seawater _____________________________ 32 Table 4-1: Diffusion Coefficients of Epoxy, Vinyl Ester, and Isopolyester Resins___________ 85 Table 4-2: Variation of Equilibrium Moisture Content with Degree of Cure ______________ 98 Table 4-3: Effect of Hardener on Equilibrium Moisture Content ______________________ 100 Table 4-4:Calculated Values of Gth _____________________________________________ 103 Table 4-5: Knockdown Factors (Vijay,1998) ______________________________________ 146
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1 FUNDAMENTALS OF CORROSION In the design and fabrication of machines and structures, the choice of the material depends on many factors, including its corrosion behavior. Although corrosion resistance is an important factor, the final choice frequently depends on many other factors like economics, availability, etc. In the case of applications that require aesthetic appeal, appearance is the most important consideration. Corrosion resistance or chemical resistance of a material depends primarily on thermodynamics, electro-chemistry, and metallurgy among others.
Thermodynamics
can determine whether or not the material is prone to corrosion. Electro-chemistry along with environmental factors, design, etc., can predict the approximate rate of corrosion. Metallurgical factors influence the corrosion resistance of the material.
Physical
chemistry and its various disciplines are very useful for studying the mechanisms of corrosion reactions, the surface conditions of metals, and other basic properties. This chapter introduces the fundamentals of the electro-chemistry needed to understand the basic corrosion mechanism.
The electrochemical cell, standard electrochemical
potential, Nernst equation, and free energy and electrode potential are briefly discussed.
1.1 Electrochemical Cell Corrosion is an electrochemical reaction between the metal and its environment. In order to determine how corrosion occurs, we must understand the formation of a corrosion cell. The corrosion cell consists of an anode, a cathode, an electrolyte, and an anionic membrane. Figure 1.1 shows an electrochemical cell, which consists of a steel anode, a platinum cathode, sulfuric acid catholyte and an anionic membrane (allows only negatively charged ions to pass through the membrane).
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Corrosion Tutorial
Figure 1-1: Electrochemical Cell
At the anode, iron is oxidized and dissolved into the electrolyte. Fe = Fe2+ + 2e-
…(1.1)
Equation (1.1) is an anodic reaction and it is also called half-cell reaction. This reaction cannot occur alone; it needs a partner cathodic reaction. The cathodic reaction takes place on the surface of the platinum electrode according to: 2H+ + 2e- = H2 (g = ‘gas’)
…(1.2)
This cathodic reaction is also called half-cell reaction. We can see that cathodic reaction is a reduction of H+ to H2 (g). The anionic membrane allows only the anions (negatively charged ions) of the sulfate ions to pass through.
This transfer of anions is needed to maintain the electrical
neutrality of the solutions at both anodic and cathodic compartment. For example, the dissolution of Fe introduces Fe++ ions into the anodic compartment. So, one mole of Fe++ introduced needs one mole of SO42- transferred from the cathodic compartment to maintain the electrical neutrality.
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1.2 Standard Electrochemical Potential Standard electrochemical potential is defined as the potential under standard conditions, i.e., 25 °C and 1 atmosphere pressure and when the reactants of the reaction have unit activity. The list of E0 values for various metals is provided in Table 1.1. Table 1.2 shows the E0 values for common oxidation reagents. The E0 values for both tables are given for cathodic reaction, as per the international convention.
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Corrosion Tutorial
Table 1-1: E0 Values for Metals
Reaction
Standard Potential E0 (volts vs. SHE)
Au3+ + 3e- = Au
+1.498
Ag+ + e- = Ag
+0.799
Hg22+ + 2e- = 2Hg
+0.799
Cu2+ + 2e- = Cu
+0.342
Pb2+ + 2e- = Pb
-0.126
Sn2+ + 2e- = Sn
-0.138
Ni2+ + 2e- = Ni
-0.250
Co2+ + 2e- = Co
-0.277
Cd2+ + 2e- = Cd
-0.403
Fe2+ + 2e- = Fe
-0.447
Cr3+ + 3e- = Cr
-0.744
Zn2+ + 2e- = Zn
-0.762
Al3+ + 3e- = Al
-1.662
Mg2+ + 2e- = Mg
-2.372
Na+ + e- =Na
-2.71
K+ + e- = K
-2.931
Note: SHE = Standard Hydrogen Electrode
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Noble
Active
Corrosion Tutorial
Table 1-2: E0 Values for Common Oxidation Reagents
Reactions
Standard Potential E0 (volts vs. SHE)
Cl2 + 2e- = 2Cl-
+1.358
O2 + 4H+ + 4e- = 2H2O (pH 0)
+1.229
NO3- +4H+ + 3e- = NO + 2H2O
+0.957
O2 + 2H20 + 4e- = 4OH-
+0.82
Fe3+ + 3e- = Fe
+0.771
O2 + 2H2O + 4e- = 4OH- (pH 14)
+0.401
Sn4+ + 2e- = Sn2+
+0.15
2H+ + 2e- = H2
0.000
2H2O + 2e- = H2 + 2OH- (pH 7)
-0.413
2H2O + 2e- = H2 + 2OH- (pH 14)
-0.828
The E0 values can be calculated from ΔG0 (GFE = Gibbs Free Energy) value of the reaction. For example, the reverse reaction of reaction (1.1) is: Fe2+ + 2e- = Fe
…(1.3)
The GFE change for reaction (1.3) is 20.30 kcal/mole. But, ΔG0 = - nFE0
…(1.4)
where F is the Faraday constant and is 23.06 kcal/equivalent-volt and n is the number of electrons or equivalents/mole. Then, the E0 of reaction (1.3) will be -0.44 eV.
1.3 Nernst Equation Nernst equation is used to calculate E values from the E0 values. The E values are under the conditions that deviate from the standard condition as defined in section 1.2. The Nernst equation can be derived from:
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Corrosion Tutorial
ΔG=ΔG0+RTln(K)
…(1.5)
where R is the gas constant (=1.987cal/mole-K), T is the absolute temperature, K is the equilibrium constant, and F is the Faraday constant (=23060 cal/equivalent-volt). Substitution of equation (1.4) and the similar term for ∆G into Equation (1.5) yields: E=E0–(RT/nF)*ln(K)
…(1.6)
Substitution of R, F, and T (=2980K) into Equation (1.6) and converting ln (log to the base e) to log (log to the base 10) yields:
E = E0 −
1.987 * 298 * 2.303 log( K ) n * 23060
E = E0 −
0.059 log( K ) n
…(1.7)
Equation (1.7) is called the Nernst Equation.
1.4 Free Energy and Electrode Potential The overall electrochemical reaction is a combination reaction of cathodic and anodic reactions. If the number of electrons is not identical in both half-cell reactions, mathematical arrangement should be made to equate the number of electrons, so that the overall electrochemical reaction results in no electrons. Since potential is an intensive (having same potential value for every subdivision of a system) property, the potentials of the half-cell reactions cannot be added in order to determine the free energy of the overall reaction. The free energy of the overall reaction can be determined as follows: First E value of the half-cell reactions should be calculated using the Nernst equation. Then, these values should be converted to free energy, ΔG. In this procedure, n for each half-cell reaction should be determined to give no electrons in the overall reaction. Then the free energies of the half-cell reaction can be added to obtain the free energy of the overall reaction.
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For example, the overall reaction for Figure 1.1 can be written as: Fe + 2H+ = Fe++ + H2
…(1.8)
The cathodic reaction is Equation (1.2) and the anodic reaction is Equation (1.1). To calculate the free energy of reaction (1.8) when [Fe++] = 10-2 molar, H+ = 10-1 molar and PH 2 = 1 atmosphere The E value for cathodic reaction is: E = 0−
PH 0.059 log +2 2 = −0.059 2 [H ]
…(1.9)
Then the ΔG = -2F(-0.059) = 2.72 kcal/mole. Similarly the E value for reaction (1.3) is:
E = −0.44 −
0.059 1 = −0.499 log 2 [ Fe ++ ]
This E value is the potential for the cathodic reaction.
…(1.10) Since we need the
potential of the anodic reaction, its potential should be 0.499 eV. Then the free energy is:
ΔG = -2F(0.499) = -23.01 kcal/mole. Now the free energy of the overall reaction (1.8) is the sum of those for the halfcell reactions, which is -20.29 kcal. The sign of this free energy indicates that the overall reaction is spontaneous. If
ΔG is negative, the reaction is spontaneous because the energy level at the final state is lower than that of initial state. Also the reverse is true. When ΔG is zero, the reaction is at equilibrium.
1.5 Potential Measurement of Half-Cell Reaction The reference electrode is needed to measure the potential of the half-cell reaction. Since the potential of reference electrode is known and we measure the
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Corrosion Tutorial
potential difference between the half-cell reaction and the reference electrode, we can measure the potential of the half-cell reaction. The measurement should be conducted in a circuit with a high impedance value. Otherwise, a significant current flows through the circuit and then the potential changes to give a wrong value of the half-cell reaction. There are several types of reference electrodes.
The standard electrode is
prepared with platinum wire immersed in a chamber, which contains 1 molar H+ ion, and pure hydrogen gas is bubbled through the solution. The potential of this reference electrode is given as: 2H+ + 2e- = H2 E0 = 0 eV
…(1.11)
Thus, the standard hydrogen electrode is not practical because it involves flammable hydrogen gas. Thus, other reference electrodes are used. One of them is calomel electrode and it has the following half-cell reaction: Hg2Cl2 + 2e- = 2Hg + 2Cl- E0 = 0.241 eV
…(1.12)
The potential at the saturation of chloride ion is 0.241 eV. The next reference electrode is copper-copper sulfate electrode and it has the following half-cell reaction: CuSO4 + 2e- = Cu + SO4- E0 = 0.318 eV
…(1.13)
The potential at the saturation of copper ion is 0.318 eV. The next reference electrode is silver-silver chloride electrode and it has the following half-cell reaction: AgCl + e- = Ag + Cl- E0 = 0.222 eV
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…(1.14)
Corrosion Tutorial
2 PASSIVITY AND ELECTROCHEMICAL CORROSION MEASUREMENTS
2.1 Passivity When an oxidizer such as ferric ion is added to a solution in which a metal (e.g., iron, nickel, chromium, titanium) is immersed, a typical polarization curve as shown in Figure 2.1 is produced.
ipass (passive current)
Oxygen evolution Etp transpassive passive
E
icc (critical current)
Epp (passivation potential)
active Ecorr (corrosion potential)
Log i Figure 2-1: Schematic Active-Passive Behavior of the Anodic Polarization of a Metal
In the active zone, the curve follows Tafel behavior. However, when the voltage increases (by adding more oxidizer), the current starts decreasing until it reaches ipass (passive current).
This passive current is retained until the potential reaches Etp
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Corrosion Tutorial
(transpassive potential). Above Etp, the current starts increasing again like in the active zone. The passive zone has the lowest current due to the formation of metal hydroxide/oxide film. This film is usually impermeable for the corrosion reagents such as oxygen and water thus reducing the metal corrosion drastically.
At Etp the film
becomes unstable making the film break down, and the current starts increasing above its potential. The passivity is one of the important aspects in corrosion. Passive films are intentionally produced to control the corrosion rates. For example, in anodic protection, applying a potential to form a passive film protects acid storage tanks. Titanium alloys are frequently used in an aggressive environment to induce passive film. Since titanium is an active metal, it induces passive film easily. Stainless steels are generally immune to atmospheric corrosion where oxygen is involved but they are not immune to acid environment. The mechanism is illustrated in Figure 2.2. It can be seen from Figure 2.2 that the higher potentials with oxygen can induce passivity while the lower potential with acid induces higher corrosion rate.
2H2O+O2+4eE
2H++2e-=H2
Log i Figure 2-2: Schematic Representation of Corrosion of Stainless Steel with Two Oxidation Reagents
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Corrosion Tutorial
There are two methods to generate anodic polarization curves.
Figure 2.3
compares both methods. The dotted line is produced by galvanostat where potential is measured at each increment of current. Thus, the passivity cannot be revealed by this method.
However, since the potentiostat measures current at each increment of
potential, the passive zone will be revealed.
This potentiostat should be used to
measure the passivity.
Galvanostat
E Potentiostat
Log i Figure 2-3: Comparison of Galvanostatic and Potentiostatic Anodic Polarization Curves
The methods available for measurement of corrosion by electrochemical polarization are as follows: 1. Tafel extrapolation 2. Polarization resistance Polarization measurements are found to be useful in engineering and research applications due to its inherent advantages such as accuracy, speed, continuous monitoring and non-destructive measurements.
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Corrosion Tutorial
2.2 Tafel Extrapolation The Tafel extrapolation technique uses the data obtained from the cathodic polarization measurements. Consider a metal M in de-aerated acid solution. In Figure 2.4, the solid lines represent the experimentally measured cathodic polarization and anodic polarization data. As the current density increases, the region is found to be linear compared to less current density values. This linear region is referred to as Tafel region. To determine the corrosion rate from this measurement, the Tafel region is extrapolated to obtain the corrosion potential. The accuracy of this technique is better than the conventional techniques. The disadvantage of this system is that it cannot be applied for systems having more than one reduction reactions.
2H++2e-=H2(g)
Experimental
Ecorr
icorr
Tafel equation
E Fe=Fe2++2e-
Log i
Figure 2-4: Cathodic and Anodic Polarization Curves
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Corrosion Tutorial
2.3 Linear Polarization Resistance Method Linear Polarization Resistance Method (LPR) is the only corrosion monitoring method that allows corrosion rates to be measured directly in real time. The response time and data quality makes this technique superior to other methods. This method is limited to electrolytically conducting liquids.
2.3.1 Derivation of the Polarization Resistance It is experimentally observed that iapp is almost linearly related to applied potential within a few millivolts of polarization from Ecorr. Stern and Geary simplified the kinetic expression to provide an approximation to the charge transfer controlled reaction kinetics given by Equation (2.1) for the case of small overvoltage with respect to Ecorr.
iapp = I corr [exp(
2.3( E − Ecorr )
βa
) − exp(
2.3( E − Ecorr )
βc
)]
…(2.1)
where βa = anodic Tafel slope
βc = cathodic Tafel slope Icorr / icorr = corrosion rate/corrosion current density Ecorr = corrosion potential Equation (2.1) can be approximated mathematically by taking its series expansion (e.g., ex = 1 + x + x2/2! + x3/3! +….) and by neglecting higher terms when
ΔE/β < 0.1. This simplified relationship has the following form Rp = [
E iapp
](E - Ecorr )→0 =
βa * βc 2.3icorr ( β a + β c )
...(2.2)
Rearranging icorr =
βa * βc B = 2.3R p ( β a + β c ) R p
…(2.3)
Rp is the polarization resistance
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Corrosion Tutorial
βa = anodic Tafel slope βc = cathodic Tafel slope B is the proportionality constant The units of Rp are ohms as obtained from E-Iapp data when the applied current is not normalized by electrode area (such data must be multiplied by electrode area to yield Rp (Ω-cm2)). If the electrode area is doubled, the measured Rp value in ohms is halved, and this intrinsic polarization resistance value remains the same. This gives the result that corrosion rate per unit area is independent of electrode area. However, the working electrode area must be known to calculate the corrosion rate. The B factor is dominated by the smaller of the two anodic and cathodic Tafel slopes (βa, βc), if unequal. Therefore, cathodic mass transport control results in B =
βa/2.3 and similarly anodic mass transport control results in B = βc/2.3. Knowledge of Rp, βa, and βc enables direct determination of the corrosion rate at any instant in time using equation (2.3). iapp is approximately linear with potential within ± 5 mV to 10 mV of Ecorr . Consequently the method is called linear polarization method (LPR). The extent of approximately linear E – iapp region can vary considerably among corroding systems. It can be infered that corrosion rate is inversely proportional to the Rp.
2.3.2 Principle of Measurement Before making any measurements, residual potential difference between the reference electrode (R) and the test electrode (T) should be nullified.
After which,
current will flow from the auxiliary electrode (A) onto the test electrode. The flow of current between the auxiliary electrode and test electrode will increase until the test electrode potential is shifted 10 mV with respect to the reference electrode. The current (ΔI) required to sustain the 10 mV potential shifts is proportional to the corrosion rate of the test electrode.
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Corrosion Tutorial
2.3.3 Advantages of Polarization Resistance Measurements •
Rapid Measurement.
•
Accuracy of the measurement of even smaller corrosion values.
•
Can be used to monitor corrosion continuously which cannot be inspected visually or by other methods.
2.3.4 Errors and Limitations in the Use of Polarization Resistance Measurements •
The Stern-Geary relationship is only mathematically valid only when ΔE is equal to zero
•
Curvature of polarization curves about the corrosion potential, Ecorr can cause errors in the measurement of polarization resistance when linearization techniques are involved.
•
This equation is valid only for activation controlled process
•
This method is not applicable under the special non-Tafel conditions corresponding to passivity or cathodic diffusion limiting current densities.
•
The corrosion rate icorr must be much larger than any other exchange currents of half-cell reactions, or the latter rates will dominate the polarization resistance measurement.
•
Resistances from the presence of films on the electrodes and the electrolyte resistance between the working and reference electrode in high resistivity media can produce an underestimation of corrosion rates due to IR (current times resistance) losses on ΔE and must be compensated to obtain accurate measurements.
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2.4 Other Methods to Determine Polarization Resistance 2.4.1 Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy is a well-established laboratory technique used to determine the electrical impedance of the metal-electrolyte interface at various AC frequencies.
Impedance measurements combine the effects of DC
resistance with capacitance and inductance.
In order to make impedance
measurements, it is necessary to have a corrosion cell of known geometry, a reference electrode, and instrumentation capable of measuring and recording electrical response of the test corrosion cell over a wide rate of AC excitation frequencies. Again, with the evolution of more rugged computers, some investigation of this method is now being made in the field. AC impedance is capable of characterizing the corrosion interface more comprehensively and with good quality equipment specifications of achieving measurements in lower conductivity solution or high resistivity coatings. AC impedance measurements can be used to predict corrosion rates and characterize systems under study and are commonly used for performance studies of chemical inhibitors and protective coatings to evaluate the resistance of alloys to specific environments etc.
2.4.2 Electrochemical Noise Electrochemical noise is a monitoring technique, which directly measures naturally occurring electrochemical potential and current disturbances due to ongoing corrosion activity. It has the same media conductivity limitations and requirement for a known electrode area as other electrochemical techniques.
It is generally less
quantitative than linear polarization resistance for corrosion rate calculations; it is useful in detecting transient effects in marginally conductive situations. Laboratory and field interpretations are still under development. One advantage is that this technique can provide a large quantity of information, which can be useful in determining what is actually happening in real time with corrosion activity in piping or equipment being monitored.
One disadvantage is that there is too much information to analyze,
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especially, when there is a contact but acceptable level of corrosion activity in the system or another source of potential electrical disturbance in the system.
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3 FORMS OF CORROSION The various forms of corrosion that are prevalent in metals are discussed in terms of their characteristics, mechanisms, and preventive measures in this chapter.
3.1 Uniform Corrosion Uniform (or general) corrosion is a form of corrosion where there is uniform reduction of thickness over the surface of a corroding material.
This is the most
important form of corrosion on the basis of tonnage wasted. Uniform corrosion can be easily measured and predicted. This leads disastrous failures relatively rare. The breakdown of protective coating systems on structures often leads to this form of corrosion.
Figure 3-1: Uniform Corrosion. (Reference:http://corrosion.ksc.nasa.gov/html/unifcor.htm )
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Figure 3-2: Example of uniform corrosion damage on a rocket assisted artillery projectile (Reference: http://www.corrosion-doctors.org/Forms/projectile.htm)
By selecting suitable materials and protective coatings, uniform corrosion can be controlled. Cathodic protection and corrosion inhibitors are other preventive methods. “It is relatively easy to monitor uniform corrosion; generally the simplest methods suffice (coupons, ER, NDT techniques for thickness measurements). Much data on uniform corrosion has been published that can be used for design purposes and estimating a corrosion allowance". In most practical cases, corrosive environments tend to differ from "textbook" cases (even small differences can be very significant). Furthermore, actual uniform corrosion rates tend to vary with time; this variability is not accounted for by single "textbook values". Corrosion monitoring is therefore advisable. Unexpected rapid uniform corrosion failures can occur if the material's surface changes from the passive (low corrosion rate) to the active (high corrosion rate) state. The resultant increase in uniform corrosion rate is typically several orders of magnitude. This undesirable transition can occur if the passive surface film is disrupted by mechanical effects, flow rate changes, a chemical change in the environment etc. Realtime corrosion monitoring systems can detect such transitions.” (This excerpt is taken from www.corrosion-club.com)
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3.1.1 Atmospheric corrosion Atmospheric corrosion is defined as the corrosion or degradation of the material exposed to the air and its pollutants. This has been identified as one of the oldest forms of corrosion and has been reported to account for more failures in terms of cost and tonnage than any other single environment. The rate at which the corrosion takes place and the severity is primarily dependant upon the properties of the surface formed electrolyte, which in turn depends upon the factors such as relative humidity, climatic conditions, temperature, atmospheric pollutants etc.
3.1.1.1 Mechanism of Atmospheric Corrosion Atmospheric corrosion is an electrochemical process, which takes place in the presence of an electrolyte. Depending upon the climatic conditions, relative humidity and temperature, a certain humidity level is reached which tends to form a thin electrolyte on the metallic surfaces. The following reactions take place. Anodic reaction 2M Æ 2M2+ + 4eCathodic reaction O2 + 2H2O + 4e- Æ 4OH-
Figure 3-3: Atmospheric corrosion of galvanized anti crash railing due to marine aerosol condensation on wooden post (Reference: http://www.corrosion-doctors.org/AtmCorros/AtmCorr.htm)
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It is an established principle that if a change occurs under which a system is in equilibrium, the system will tend to adjust itself so as to nullify the effect of that change. Therefore, in the presence of an electrolyte, atmospheric corrosion proceeds by balancing cathodic and anodic reaction. The anodic reaction involves the dissolution of the metal in the electrolyte, while the cathodic reaction involves reduction. Oxygen from the atmosphere is readily supplied to the electrolyte in thin film conditions.
3.1.1.2 Prevention of Atmospheric Corrosion There are two approaches to prevent Atmospheric corrosion. The first is a temporary one which is used during storage and which consists of lowering of atmospheric humidity by using a desiccant, heating devices, or by treating the surface with vapor phase or surface inhibitors. Permanent solution is by applying organic, inorganic and metallic coatings effectively.
3.1.2 Galvanic Corrosion When two dissimilar metals are coupled and immersed in an electrolyte solution, a galvanic cell is formed. In a galvanic cell, the more active metal is corroded while the noble metal is protected. Corrosion of active metal is increased and of noble metal is decreased upon galvanic coupling. Figure 3.4 shows galvanic corrosion between the pipe made of black iron and a brass valve.
Figure 3-4: Galvanic Corrosion of Brass Coupled With Black Iron (Reference: http://www.eci-ndt.com/gallery_a.htm)
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When two dissimilar metals M and N, with N being more active, are connected and immersed in an acid solution, the corrosion mechanism may be depicted in a graphical representation as shown in Figure 3.5.
2H+ H2
M
N
N2+
2eFigure 3-5: Mechanism of Galvanic Corrosion of a Two Metal Couple
The more noble metal M undergoes the cathodic reaction while the active metal N undergoes the anodic reaction. The electrons produced by the dissolution of N are consumed by the cathodic reaction that takes place on the noble metal surface. Thus, corrosion of the noble metal retards while that of the active metal accelerates because of the galvanic effect generated on the metal surface. The magnitude of the galvanic current flow is controlled by the potential difference and the total resistance to current flow as shown in the Figure 3-6, which can be broken down further into the following resistance components. Resistance to current flow of the electrolyte. Resistance to current flow in the conducting materials and in the connection between them. Resistance associated with polarization behavior of the anode and cathode. The Figure 3-6 illustrates a cell showing the corrosion process in its simplest form. This cell includes the following essential components: a metal cathode, metal anode, a metallic conductor between anode and the cathode and an electrolyte in
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contact with the anode and the cathode. They are so arranged to form a closed electric path.
Figure 3-6: Galvanic cell showing corrosion process in its simplest form (Reference: http://www.edstrom.com/Resources.cfm?doc_id=131)
The cathode is positively charged and anode is negatively charged. The difference in charge provides potential voltage which is the driving force for the current to flow in the cell. Hydrogen gas is produced at the cathode and no destruction will occur while the anode gives off its ions in the form of rust, this is where the corrosion occurs. The rate of which depends upon the relative sizes of the anode and cathode and also the potential difference between cathode and anode. If, for instance, the anode is very small compared to the cathode, the rate of corrosion would be very rapid. The opposite would be true if there was a very large anode compared to the cathode. When contact with a dissimilar metal is made, however, the self-corrosion rates will change: corrosion of the anode will accelerate; corrosion of the cathode will decelerate or even stop.
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This is shown in Figures 3-7 to 3-9. Figure 3-7 shows the corrosion rate for a single metal in solution. E (V) Cathodic Reaction 1
Ecorr
Anodic reaction 2
log Current Density 2 μA/cm
icorr
Figure 3-7: Corrosion rate determination for a two electrode process system (Reference: http://www.egr.uri.edu/che/course/CHE534w/chapter3EnivronmentalCorrosion.htm)
Figures 3-8 and 3-9 shows the rate determination when a third electrode process is added at a potential between the first two electrode reactions. The rule that must be applied is that the ‘total oxidation rate must be equal to the total reduction rate.’
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E (V)
Cathodic Reaction 1 Anode Reaction 3
Ecorr 1+2 Total Cathode 1+3
Cathode Reaction 3
Anodic Reaction 2
log Current Density 2 μA/cm
icorr 1+2
Figure 3-8: Corrosion rate determination for a three electrode system. (Reference: http://www.egr.uri.edu/che/course/CHE534w/chapter3EnivronmentalCorrosion.htm)
From Figure 3-8, it is seen that the corrosion rate for electrode 2 has increased from icorr to icorr 1+2 as it is the only anodic reaction. Two cases are shown in Figures 3-8 and 3-9; when the corrosion potential for three electrodes is above the two electrode potential and when the three electrode corrosion potential is below the two electrode potentials respectively. In Figure 3-8 the resulting corrosion potential is more negative than the third electrode reverse potential, thus contributing to the cathodic reaction and protecting the third electode from corrosion. The second electrode dissolution rate increased significantly by the introduction of the third electrode processes. In Figure 3-9, the resulting corrosion potential from the three electrodes is more negative than the double electrode potential. In this case both the second and third electrodes are corroding, but the third electrode corrodes at a lower rate than the second electrode.
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E (V)
Cathodic Reaction 1 Anode Reaction 3
Ecorr 1+2 Cathode Reaction 3
Total Anode 1+3
Anodic Reaction 2
log Current Density 2 μA/cm
icorr 1+2
Figure 3-9. The introduction of a less noble metal will decrease the corrosion rate of the more noble metal. (Reference: http://www.egr.uri.edu/che/course/CHE534w/chapter3EnivronmentalCorrosion.htm)
Both these Figures 3-8 and 3-9 show that introducing a more anodic metal will decrease the corrosion rate in a more noble metal. This is the process behind galvanic corrosion. It can also be used for protection by galvanizing. The seawater Galvanic Series, shown in table 3-1 below can be used to predict which metal will become the anode and how rapidly it will corrode. The metals below are arranged according to their tendency to corrode galvanically. Metals with negative voltage charges (anodic–least noble) are listed first, followed by metals with positive charges (cathodic–more noble).
3.1.2.1 CORRODED END (ANODIC OR LEAST NOBLE) •
MAGNESIUM
•
MAGNESIUM ALLOYS
•
ZINC
•
ALUMINUM 5052, 3004, 3003, 1100, 6053
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•
CADMIUM
•
ALUMINUM 2117, 2017, 2024
•
MILD STEEL (1018), WROUGHT IRON
•
CAST IRON, LOW ALLOY HIGH STRENGTH STEEL
•
CHROME IRON (ACTIVE)
•
STAINLESS STEEL, 430 SERIES (ACTIVE)
•
302, 303, 304, 321, 347, 410,416, STAINLESS STEEL (ACTIVE)
•
NI - RESIST
•
316, 317, STAINLESS STEEL (ACTIVE)
•
CARPENTER 20 CB-3 STAINLESS (ACTIVE)
•
ALUMINUM BRONZE (CA 687)
•
HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE)
•
LEAD - TIN SOLDERS
•
LEAD
•
TIN
•
INCONEL 600 (ACTIVE)
•
NICKEL (ACTIVE)
•
60 NI-15 CR (ACTIVE)
•
80 NI-20 CR (ACTIVE)
•
HASTELLOY B (ACTIVE)
•
BRASSES
•
COPPER (CA102)
•
MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905)
•
SILICON BRONZE
•
NICKEL SILVER
•
COPPER - NICKEL ALLOY 90-10
•
COPPER - NICKEL ALLOY 80-20
•
430 STAINLESS STEEL
•
NICKEL, ALUMINUM, BRONZE (CA 630, 632)
•
MONEL 400, K500
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•
SILVER SOLDER
•
NICKEL (PASSIVE)
•
60 NI- 15 CR (PASSIVE)
•
INCONEL 600 (PASSIVE)
•
80 NI- 20 CR (PASSIVE)
•
CHROME IRON (PASSIVE)
•
302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE)
•
316, 317, STAINLESS STEEL (PASSIVE)
•
CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825
•
NICKEL - MOLYBDEUM - CHROMIUM - IRON ALLOY (PASSIVE)
•
SILVER
•
TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.)
•
GRAPHITE
•
ZIRCONIUM
•
GOLD
•
PLATINUM
3.1.2.2 PROTECTED END (CATHODIC OR MOST NOBLE) From the list 3.1.2.1, it is true that each metal has a different electrical potential when immersed in the same electrolyte (an electrically conductive fluid such as sea water). As a result, if two dissimilar metals are placed in the same electrolyte, their different electrical potentials will produce a voltage that can be measured on the two pieces of metal. According to the potential difference of these two metals, the current flows from higher voltage metal to the lower one. This action raises the voltage of the lower-voltage metal above its natural potential. To establish the equilibrium, the lowervoltage metal discharges a current in to the electrolyte. The current passes through the electrolyte back to the higher-voltage metal and completes the electrical circuit between the two pieces. The current flowing through the electrolyte is generated by an electrochemical reaction that steadily consumes the lower-voltage metal a process known as galvanic corrosion.
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In couple A, the aluminum rivet is comparatively small, and the C/A ratio is large. In couple B, the situation is reversed: the stainless steel rivet is small, and the C/A ratio is also small. Corrosion of the aluminum rivet in couple A will be severe. However, corrosion of the large aluminum plate in couple B will be much less, even though the potential difference is the same in each case. The two major factors affecting the severity of galvanic corrosion are (1) the voltage difference between the two metals on the Galvanic Series, and (2) the size of the exposed area of cathodic metal relative to that of the anodic metal. Corrosion of the anodic metal is both more rapid and more damaging as the voltage difference increases and as the cathode area increases relative to the anode area.
Figure 3-10: Effect of cathode to anode ratio in galvanic corrosion (Reference: http://www.ocean.udel.edu/seagrant/publications/corrosion.html)
The effect of the second factor, the cathode-to anode area ratio, C/A, is illustrated in Figure 3-10 for a rivet in a plate. In both couples A and B, aluminum is the anode, and stainless steel is the cathode. In couple A, the aluminum rivet is comparatively small, and the C/A ratio is large. In couple B, the situation is reversed: the stainless steel rivet is small, and the C/A ratio is also small. Corrosion of the aluminum rivet in couple A will be severe. However, corrosion of the large aluminum plate in couple B will be much less, even though the potential difference is the same in each case.
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3.1.2.3 Factors Affecting Galvanic Corrosion Area Effect The current flowing between anode and cathode will be the same independent of the surface area of each electrode. For the anodic and cathodic reactions, it is the current rather than current density which is equal. Since the corrosion occurs at anode, the rate of corrosion depends on the current density in the anode. To minimize the galvanic corrosion, the anode area should be very large compared to the cathode area. Inorder to protect the system from galvanic corrosion, the cathode of the system should be painted. If the paint is damaged, then small cathode to large anode area ratio is formed which results in minimizing corrosion rates. Conversely, if the anode is painted, then the damage to the paint causes large cathode to small anode ratio resulting in large corrosion rates in the anode and thus penetrating into the metal. The anode to cathode area effect is an important characteristic. It is important in several other forms of corrosion including pitting corrosion, crevice corrosion, stress corrosion cracking and corrosion fatigue.
Distance Effect Distance effect is another important factor for galvanic corrosion. Galvanic corrosion rates are the largest at the interface between the anode and cathode and decrease with distance away from the contact region. The transportation of the ions becomes more difficult when the distance between anodic and cathodic reaction site increases thus decreasing the corrosion rate. Essentially the resistance of the electrolyte increases with distance. This is an important factor in determination of the form of corrosion. If a galvanic corrosion is suspected, then according to the rule as explained, the rate of corrosion adjacent to the galvanic contact region should be higher. If the corrosion rate is far away from this area then different type of corrosion may be involved. For example if corrosion appears at a constriction some distance from a galvanic contact, then erosion corrosion was the cause and not the galvanic corrosion.
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Distance Apart in the Galvanic Series Galvanic series is an empirical listing of the corrosion resistance of metals. Its advantage over the Redox series is that it refers to alloys in a real environment. Metal at the top of the system are highly cathodic while metals at the bottom highly anodic. For selection purposes metals close together on the list are desirable as there is little driving force for corrosion to be accelerated. Clearly, it is undesirable to connect metals widely spaced on the galvanic series.
3.1.2.4 Galvanic Series Galvanic series is a list of metals/alloys according to their corrosion potentials. The list is formed by polarization of two or more half-cell reactions to a common mixed potential, Ecorr on the corroding surface. Corrosion potentials in the galvanic series are measured in real or simulated service conditions.
The galvanic series gives only
tendencies for galvanic corrosion, not the rate. In the galvanic series, the potential changes due to changes in electrolyte composition and temperature. For every change in environmental conditions, a new series needs to be established. Table 3.1 lists some of the metals/alloys for the galvanic series in seawater. In Table 3.1, titanium is a noteworthy element. Titanium is a very active metal with E0 value being -1.63 V for the reduction of:
Ti2+ + 2e- = Ti
…(3.1)
However, it has a very noble corrosion potential. This may be due to the fact that titanium surface is easily covered with passive film with normal potential oxidizing reagents such as oxygen.
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Table 3-1: Galvanic Series of Metals/Alloys in Seawater
Noble
Platinum
⇑
Gold Titanium Silver Hastelloy C (62 Ni, 17 Cr, 15 Mo) 18-8 Stainless steel (passive) Inconel (passive) (80 Ni, 13 Cr, 7 Fe) Cast Iron Wrought Iron Copper Red Brass Cadmium Manganese Bronze Aluminum 52SH Tin Lead Zinc Magnesium Alloys
⇓
Magnesium
Active
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3.1.2.5 Prevention of Galvanic Corrosion •
Coupling of two dissimilar metals should be avoided when they are much apart in the galvanic series.
•
In the case where such coupling is necessary, break the electrical contact between the two metals by using insulation such as gaskets and rubber.
•
Do not paint anodic area because if the paint is ruptured in a small area, the corrosion rate in the area will increase drastically. Instead, paint the cathodic area.
3.1.3 Stray Current Corrosion Stray current corrosion is the current flowing through unintended paths due to some kind of leakage from extraneous sources. The damage caused to the metal components due to this unwanted current refers to stray current corrosion. Metals like aluminum under soil or water are affected due to this kind of corrosion. This current mostly originates due to bad earthen systems of electrical equipments and eventually leaks through the metal structures or other conductive systems. The common sources of these stray current include electric railway systems, cathode protection systems of nearby equipment or pipelines, DC-driven elevators, etc. The mechanism involved in stray current corrosion is that an electrolysis cell is formed that forces the metal structure through which it passes to act as an anodic site. Thus local oxidation occurs and the metal is consumed rapidly.
3.1.3.1 Direct Stray Current Corrosion This type of corrosion occurs due to direct current from sources like rail transit system, DC welding equipment. Again this type of corrosion can be classified as dynamic current corrosion when the current is not steady or flows irregularly and as static current corrosion when the flow is steady. The effects of direct stray current are very severe compared with alternative currents.
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3.1.3.2 Alternating Stray Current Corrosion This type of corrosion is caused due to alternate currents from sources like overhead AC power lines.
3.1.3.3 Telluric Effects The disturbances in the earth’s magnetic field called geomagnetic activity may lead to the production of dynamic stray currents. These currents induced naturally due to the geomagnetic activity are called telluric effects. These may flow onto a buried pipeline varying the magnitude of current flow and the position of current pick-up. The discharge areas will also fluctuate with time.
Figure 3-11: Occurrence of Stray current corrosion in pipelines. (Reference:http://www.corrosion-club.com/stpickup.htm)
Basic Theory A protective coating is used as the primary form of protection for buried pipelines. Additionally, cathodic protection is designed to provide protection at coating discontinuities. Thus a combination of a protective coating system and cathodic
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protection system is used to reduce the corrosion risk factor. Figure 3-12 schematically illustrate current flow in an impressed current cathodic protection system (the principle is similar for a sacrificial anode system). Under these idealized conditions, current flows through the electrolyte (soil) onto the pipeline, in the form of ionic current.
Figure 3-12: Ionic current flow onto the pipeline. (Reference:http://www.corrosion-club.com/sttheory.htm)
The schematic diagram in Figure 3-13 details the current flow onto the pipeline at coating discontinuities, under the protective influence of the cathodic protection system.
Figure 3-13: Current flow onto pipeline at coating discontinuities. (Reference:http://www.corrosion-club.com/sttheory.htm)
Current flow in the electrolyte that does not originate from the cathodic protection system designed to protect the pipeline is referred to as stray current. Such external stray current sources interfere with the normal operation of the cathodic protection system leading to corrosion problems.
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Figure 3-14: External stray current sources. (Reference:http://www.corrosion-club.com/sttheory.htm)
3.1.4 General Biological Corrosion Microorganisms such as Iron Related Bacteria (IRB), Sulfate Reducing Bacteria (SRB) and Heterotrophic Aerobic Bacteria (HAB) grow on iron and steel pipes that lead to the corrosion of these metals. This type of corrosion can occur through aerobic and anaerobic corrosion. The other name for this corrosion is Biofouling.
3.1.4.1 Causes of Biological Corrosion Bacteria Bacteria may be either aerobic or anaerobic, aerobic bacteria sustains only when oxygen is present. Sulfur is oxidized to produce sulfuric acid and thus corrosion is accelerated. Metals are subject to corrosion when the bacteria deplete the oxygen supply or releases metabolic products. On the other hand, anaerobic bacteria, can survive only when free oxygen is not present. The metabolism of these bacteria requires them to obtain part of their sustenance by oxidizing inorganic compounds, such as iron, sulfur, hydrogen, and carbon monoxide. The resultant chemical reactions cause corrosion.
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Fungi Microorganism feed on organic materials and this leads to the growth of fungi. Low humidity does not kill microbes; it slows their growth and may prevent damage due to corrosion. Temperatures between 68 F and 108 F and relative humidity between 85 and 100 percent are ideal conditions for the growth of most of the microorganisms.
3.1.4.2 Prevention of Biological Corrosion Microbial growth must be removed completely to avoid corrosion. Microbial growth should be removed by hand with a firm non-metallic bristle brush and water. Removal of microbial growth is easier if the growth is kept wet with water. Microbial growth may also be removed with steam at 100 psi. Protective coating is used when using the steam for removing microbial growth. A simple, yet effective method for monitoring the population size and/or activity of specific groups of bacteria is Biological Activity Reaction Test. Below is an image of a carbon steel plate section that is bio-corroded. In the analysis chamber of the ESEM XL30 TMP Philips Electron Microscope, the image was obtained in high vacuum. The corrosion product is brown. Bacteria produce tubercles and, under the tubercles a small hole. Bacteria collect inside the holes and produce FeS. Biological corrosion also affects copper and aluminum alloys.
Figure 3-15: Corroded surface of carbon steel in its natural condition. (Reference: http://www.esemir.it/images/bcorr2_i.htm)
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3.1.5 Molten Salt Corrosion The corrosion of metal containers by molten or fused salts is known as molten salt corrosion. This phenomenon mostly occurs at high temperatures.
3.1.5.1 Mechanism of Molten Salt Corrosion Metal dissolution and Metal oxidation are the two common mechanisms involved in this corrosion. Molten salt corrosion can be identified as the intermediate form of corrosion between molten metal and aqueous corrosion. Many principles that apply to aqueous corrosion also apply to molten salt corrosion, such as anodic reactions leading to metal dissolution and cathodic reduction of an oxidant. The corrosion process is mainly electrochemical in nature, due to the ionic conductivity of most molten salts. Molten salts are partially electronic conductors as well as ionic conductors. This fact allows for reduction reactions to take place in the melt. Cathodic reactions increase causing a substantial increase in corrosion growth. Molten salt systems operate at higher temperatures than aqueous systems.
3.1.5.2 Types of Molten salts
Molten Fluorides
Chloride Salts
Molten Nitrates
Molten Sulfates
Hydroxide Melts
Carbonate Melts
In nuclear reactor cooling systems, fluoride melts are used. Corrosion in fluoride molten-salt melts is enhanced because protective surface films are not formed. Nickel-
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base alloys show better corrosion resistance than iron-base alloys. Chloride salts attack steels rapidly with preferential attack of carbides. In order to prevent molten salt corrosion, a material should be selected that will form a passive non-soluble film in the melt. Minimizing the entry of oxidizing elements such as oxygen and water into the melt is important.
3.1.6 Corrosion in Liquid Metals Liquid metals are widely used in nuclear reactors and nuclear power systems. Liquid lithium, mercury, potassium and cesium are some of the examples of liquid metals, used in the above said applications. Liquid metal corrosion may be formed due to the dissolution of the solid metal. The metal surface may react with the liquid metal due to the presence of some impurities, to form an alloy or due to compound reduction.
3.2 Localized corrosion Localized corrosion is defined as the selective removal of material by corrosion at small areas and zones in contact with corrosive environment. It usually takes place when there is intense attack at the localized sites than at the rest of the original surface. It is mostly accompanied with other destructive processes such as stress, fatigue, erosion and other forms of chemical attack. The following are different types of localized corrosion.
3.2.1 Pitting Corrosion When passive metal surface is exposed to a chloride solution, corrosion is initiated by forming pits on the metal surface. Pitting is a form of extremely localized attack that results in holes in the metal. Pits are difficult to determine because they are
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smaller in size and they will be covered with the corrosion products. Figure 3-16 shows pitting corrosion on aluminum surface.
Figure 3-16: Pitting in Aluminum (Reference: http://www.clihouston.com/pitting.htm)
3.2.1.1 Initiation of Pitting Corrosion Pitting corrosion is initiated when the passive film is attacked by chloride ions in solution. Most of the metal oxide/hydroxide films are soluble by chloride ions. Further, hydrogen ions are produced by the hydration of metal chloride. These hydrogen ions further attack the substrate of the metal matrix, because there would not be passive films formed in the presence of chloride ions. Stainless steel is well known to be susceptible to pitting corrosion. The reason is that upon hydration of CrCl3, very low pH is produced. D. A. Jones and B. E. Wilde observed that upon hydration of 1 N CrCl3 (N=normality) produced a pH of 1.1 while 1 N of FeCl2 and NiCl2 produced pH of 2.1 and 3.0, respectively. These results show that stainless steel, which contains chromium, is most susceptible and nickel alloys are least susceptible to pitting corrosion.
3.2.1.2 Propagation of Pitting Corrosion Once the pits are initiated in the presence of chloride ions, the metal ions produced by corrosion attract more chloride ions by Coulomb forces into the pits,
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making the pit solution more concentrated. Since the solubility of oxygen is virtually zero in concentrated solutions, no oxygen reduction occurs within a pit. The metal ions are hydrated and the aqueous metal hydroxide is transported to outside of the pit where it is further oxidized in the case of steel.
The growth of the pit is by two distinct
mechanisms. One is by acid produced upon the hydration of metal ions. The other is formation of an electrical circuit between an anodic reaction of metal dissolution which takes place at the pit and the cathodic reaction of oxygen reduction reaction which takes place outside the pit. These mechanisms can be seen graphically in Figure 3.17 in the case of steel. The hydration of ferrous ion is given by Fe2+ + 2H2O = Fe(OH)2(aq) + 2H+
…(3.2)
We can see that the hydration produces an acid, which further corrodes the pit. The reaction is:
Figure 3-17: Propagation of Pitting Corrosion
Fe + 2H+ = Fe2+ + H2
…(3.3)
The other mechanism where iron is dissolved with oxygen can be written as: 2Fe + 2H2O + O2 = 2Fe2+ + 4OH-
…(3.4)
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Finally the oxidation of Fe(OH)2 to Fe(OH)3 can be written also: 4Fe(OH)2(aq) + O2 + 2H2O = 4Fe(OH)3(s)
…(3.5)
Pitting corrosion occurs when a passive film or another protective surface layer breaks down locally. After this initiation (local breakdown of the film) an anode forms where the film has broken, while the unbroken film (or protective layer) acts as a cathode. This will accelerate localized attack and pits will develop at the anodic spots. The electrolyte inside the growing pit may become very aggressive (acidification) which will further accelerate corrosion. Pitting occurs mainly in the presence of neutral or acidic solutions containing chlorides or other halides. Chloride ions facilitate a local breakdown of the passive layer, especially if there are imperfections in the metal surface. Initiation sites may be non-metallic inclusions, e.g. sulfides, micro crevices caused by coarse grinding, or deposits formed by slag, suspended solids, etc. When the metal corrodes in the pit, dissolved metal ions generate an environment with low pH and chloride ions migrate into the pit to balance the positive charge of the metal ions. Thus the environment inside a growing pit gradually becomes more aggressive and repassivation becomes less likely. As a result, pitting attacks often penetrate at a high rate, thereby causing corrosion failure in a short time. The pits often appear to be rather small at the surface, but may have larger cross-section areas deeper inside the metal. Since the attack is small at the surface and may be covered by corrosion products, a pitting attack often remains undiscovered until it causes perforation and leakage.
3.2.1.3 Prevention of Pitting Corrosion Selection of an alloy, which is resistant to pitting corrosion in the chloride medium (e.g. Nickel Alloy). Elimination of stagnant areas because this area can be a cathodic site. Equipment should be designed for complete drainage avoiding areas that retain standing solution.
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3.2.2 Crevice Corrosion Crevice corrosion is similar to pitting corrosion in the initiation and propagation mechanisms.
But these two are differentiated.
Pit corrosion occurs on the open
surface of metal while crevice corrosion occurs on hidden areas of the metal surface. The crevice corrosion is usually associated with small volumes of stagnant solution retained on gasket surfaces, lap joints, and surface deposits under bolt and rivet heads (Figure 3-18). This form of corrosion is called crevice corrosion or deposit or gasket corrosion.
Figure 3-18: Crevice Corrosion (Reference: http://corrosion.ksc.nasa.gov/html/corr_forms.htm)
Environmental factors affecting crevice corrosion are sand, dirt, corrosion products and other solids.
These deposits act as a shield and create a stagnant
condition. The deposit could also be a permeable corrosion product. Contact between metallic and non-metallic surfaces can cause crevice corrosion as in the case of a gasket.
Wood, plastics, rubber, glass, concrete, asbestos, wax and fabrics are
examples of materials that can cause this type of corrosion.
3.2.2.1 Initiation and Propagation of Crevice Corrosion
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The basic mechanism of initiation and propagation of crevice corrosion are basically the same as those of pitting corrosion. The difference is that the cathodic site of the crevice corrosion is a small amount of water retained by gasket or similar materials. The mechanism of crevice corrosion is illustrated in Figure 3-19.
O2 OH-
e
-
Fe2++2H2O=Fe(OH)2+2H+
Crevice
Fe+2H+=Fe2++H2 Fe=Fe2++2eFigure 3-19: Mechanism of Crevice Corrosion
Figure 3-19 shows a connection of two plates by a bolt. The upper left hand side corner retains a small amount of water, which can serve as a cathode site. Oxygen is dissolved into water and reduced to consume electrons supplied by the anodic reaction of steel corrosion at the hidden crevice site. After initiation of crevice corrosion, more chloride ions are attracted and help propagate the crevice corrosion. Otherwise, the anodic half-cell reactions cannot take place because of the formation of passive film. This anodic site has high concentration of iron and is unsuitable for cathodic site because oxygen cannot be dissolved in the high concentrated solution.
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Figure 3-20: crevice corrosion in rivets (Reference:http://www.corrosion-doctors.org/Localized/Crevice.htm)
3.2.2.2 Prevention of Crevice Corrosion Crevice corrosion should be prevented at the design stage itself.
•
Design vessels for complete drainage to avoid sharp corners and stagnant areas.
•
Inspection of equipment and removing deposits frequently.
3.2.3 Pack Rust Pack rust is a form of corrosion typical in steel components that develop crevice in to an open atmospheric environment. This particular form of corrosion is often used in relation to bridge inspection to describe built-up members, of steel bridges which are already showing signs of rust between steel plates.
Figure 3-21: A crevice formed into an open atmosphere
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Figure 3-22: Example of Pack Rust (Reference:http://www.corrosion-doctors.org/Bridges/Pack-rust.htm)
3.2.4 Filiform Corrosion Filiform Corrosion is a special type of crevice corrosion. Filiform corrosion has been observed on steel, magnesium and aluminum surfaces covered by tin, silver, gold, phosphate, enamel and lacquer coatings.
Filiform corrosion is an unusual type of
attack, since it does not weaken or destroy metallic components but only affects the surface appearance.
It appears under thin coatings.
It initiates on the scratched
defects of the coatings and propagates in the form of thread like filaments. The filament has two ends: one is head and the other is tail. The head is anodic site where metal is dissolved while the tail is cathodic where oxygen is reduced. The mechanism of filiform corrosion is depicted in Figure 3-23.
O2+H2O
Fe2++2H2OÆ Fe(OH)2+2H
+
OHFe(OH)3
e-
O2+H2O
e-
Fe2+ H+ Fe2+ Fe(OH)2
Steel
OH-
O2+H2O
e-
Figure 3-23: Mechanism of Filiform Corrosion
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A special form of oxygen concentration cell is formed on metal surfaces having an organic coating leading to filiform corrosion. This corrosion starts under the painted surface when moisture permeates the coating on the metal. It has a distinguished pattern and is known by its characteristic worm-like trace. Filiform corrosion occurs when the relative humidity of the air is between 78 and 90 percent and the surface is slightly acidic. The most important environmental variable in filiform corrosion is the relative humidity of the atmosphere. If the relative humidity is lower than 65 percent, the metal is unaffected; and for relative humidity higher than 90 percent, corrosion primarily appears as blistering. Filiform corrosion normally begins as small, microscopic defects in the coating and then grows out into a network. The paint or plating should have certain properties that could sustain or avoid any causes that lead to this type of corrosion. Lacquers and "quick-dry" paints are most susceptible to this problem. Their use should be avoided unless absence of an adverse effect has been proven by field experience. Low water vapor transmission and excellent adhesion are some of the characteristics that are to be exhibited by the coatings that are to be used. Polyurethane finishes are especially susceptible to filiform corrosion. This corrosion usually attacks steel and aluminum surfaces. The traces never cross on steel, but they will cross under one another on aluminum which makes the damage deeper and more severe for aluminum. . Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality. The effected area should be treated properly and a good protective finish should be applied. If the corrosion is not removed and allowed to grow, the corrosion can lead to intergranular corrosion. This could be seen especially around fasteners and at seams. Filiform corrosion can be removed using glass bead blasting material with portable abrasive blasting equipment or sanding. Zinc-rich coatings should be considered for coating carbon steel because of their cathodic protection quality. Filiform corrosion can be prevented by storing aircraft in an environment with a relative humidity below 70 percent, using coating systems having a low rate of diffusion
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for oxygen and water vapors, and by washing the aircraft to remove acidic contaminants from the surface.
Figure 3-24: “worm like” filiform corrosion tunnels. (Reference:http://www.faa.gov/avr/afs/300/pdf/2g-CH6_2.pdf)
Figure 3-25: Filiform Corrosion Causing Bleed Through a Welded Tank (Reference: http://corrosion.ksc.nasa.gov/html/filicor.htm)
3.2.4.1 Prevention of Filiform Corrosion •
Controlling the humidity to lower values (e.g. less than 65%).
•
Corrosion resistant substrates of stainless steel, titanium or copper do not exhibit filiform corrosion.
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•
Coatings and inhibitors eventually retard corrosion to some extent, but not completely.
3.2.5 Localized Biological Corrosion In this case, biological organisms are the sole cause or accelerating factor in the localized corrosion. The reason behind this is that organisms do not form continuous film on the metal surface. The large fouling organisms in marine environments settle as individuals, and it is often a period of months or even years before a complete cover is built up. Microscopic organisms also tend to settle on the metal surfaces in the form of discrete colonies or spots rather than continuous films.
3.3 Environmentally Induced Cracking Environmentally Induced Cracking (EIC) is due to brittle mechanical failures that result in presence of tensile stress and a corrosive environment. EIC includes stress corrosion cracking (SCC), corrosion fatigue cracking (CFC) and hydrogen embrittlement or hydrogen induced cracking (HIC).
3.3.1 Stress Corrosion Cracking Stress corrosion cracking (SCC) is due to the low static tensile stress of an alloy exposed to a corrosive environment. Three conditions must be present together to produce SCC: a critical environment, a susceptible alloy and some component of tensile stress. Figure 3.9 shows SCC of aluminum plate with branched cracks.
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Figure 3-26: Stress Corrosion Cracking Showing Branched Cracks in Aluminum Plates (Reference: http://corrosion.ksc.nasa.gov/html/corr_forms.htm)
3.3.1.1 Metallurgical Effects Pure metals are more resistant to SCC than alloys of the same base metal, but they are not completely immune. Higher strength alloys are more susceptible to SCC. The cracks are usually produced by SCC by normal to the tensile component of stress. Transgranular failures are less common than intergranular ones, but both may exist in the same system, or even in the same failed part, depending on the conditions. Intergranular failure suggests that some inhomogeneity exists at the grain boundaries. Sometimes, specific chemicals react with alloy components to make the alloy more brittle which develops cracks. For example, ammonia reacts with copper in brass to cause SCC because ammonia can form various complex ions with cupric ion. Hydrofluoric acid may react with nickel and copper in the nickel-copper alloy (Monel alloy 400) because fluoride forms compounds of NiF2 and CuF2. Chloride ions cause SCC to austenite stainless steels, probably because chromium in the alloy may react with chloride ions to form CrCl3.
3.3.1.2 Electrochemical Effects Electrochemical potential has a critical effect on stress corrosion cracking (SCC). SCC usually occurs where the passive films are less stable. The regions near the two ends of the passive zone as shown in Figure 3.27
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ipass (passive current)
Oxygen evolution Etp transpassive passive
E
icc (critical current)
Epp (passivation potential)
active Ecorr (corrosion potential)
Log i Figure 3-27: Schematic of Active-Passive Behavior of the Anodic Polarization of a Metal
In the upper zone, SCC and pitting are associated in adjacent or overlapping potential ranges. In lower zone, SCC occurs where passive film is relatively weak.
3.3.1.3 Prevention of Stress Corrosion Cracking Prevention of SCC requires elimination of one of the three factors: tensile stress, environment or susceptible alloy.
•
When a tensile stress is applied, the magnitude will be reduced by the shotpeening. Shotpeening is a process in which small, hard particles of 0.1 to 1.0 mm are projected at high velocities onto the surface to increase the compressive stress.
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•
Removal of residual stress may be accomplished by stress-relief annealing. Annealing may be impractical for some stainless steels, which sensitize and become susceptible to intergranular attack.
•
Reducing the oxidizing agents.
•
Inhibitors are also effective in limited situations.
•
Changing the composition of the alloying elements for lower strength.
•
Choosing an alloy, which is resistant to the particular environment.
•
Cathodic protection will usually stop SCC, but will accelerate HIC. Caution must be exercised with higher strength alloys, which may fail by HIC when cathodic protection is applied.
3.3.2 Sulfide Stress Cracking (SSC) When metals and metal alloys come into contact with moist hydrogen sulfide, sulfidic environments undergoes corrosion damage, commonly known as sulfide stress cracking. This causes brittle failures in steels and other high strength alloys. The acidic effect of the sulfides combined with stress in the environment leads to this type of corrosion. SSC is also called hydrogen sulfide cracking, sulfide cracking and sulfide corrosion cracking.
3.3.3 Liquid Metal Embrittlement (LME) When liquid metal comes into contact with a metal, there will be decrease in its ductility. This phenomenon is called liquid metal embrittlement. The decrease in ductility can result in brittle failure of a normally ductile material. Tensile strength can also be reduced along with the ductility of the metal. Small amounts of liquid metal are sufficient to result in embrittlement. LME shows many characteristics of both stress corrosion cracking and hydrogen embrittlement corrosion. Like local de-passivation prior to stress corrosion cracking, LME requires an incubation period for the liquid metal to penetrate the oxide or passive layers.
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Liquid metal embrittlement can occur at loads below yield stress and thus failure can occur without significant deformation or obvious deterioration of the component.
Figure3-28: Liquid Metal Embrittlement (Reference:http://www.hgtech.com/Corrosion/Hg%20LME.htm)
Intergranular or transgranular cleavage fracture is the common fracture modes associated with liquid metal embrittlement. However reduction in mechanical properties due to de-cohesion can occur. Hence a ductile fracture mode results occurring at reduced tensile strength. Some events that may permit liquid metal embrittlement under the appropriate circumstances are brazing, soldering, welding, heat treatment and hot working. In addition to an event, it is also required to have the component in contact with a liquid metal that will embrittle the component.
3.3.4 Solid Metal Induced Embrittlement The embrittlement caused above the melting point of the embrittler is Liquid metal embrittlement and the same phenomena below the melting point are pertained as Solid metal embrittlement. LME can be considered as a prerequisite for the occurrence of SMIE. Notched tensile specimens of various steels are embrittled by solid cadmium. Intimate contact between the solid and the embrittler, the presence of tensile stress, crack initiation at the interface is the necessary conditions for SMIE to occur. The susceptibility to SMIE is stress and temperature sensitive and does not occur below a
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specific threshold value. In LME, a single crack usually propagates to failure. But SMIE requires multiple cracks to propagate.
Figure 3-29: Solid Metal Induced Embrittlement of a cadmium plated B7 bolt. (Reference:http://www.hghouston.com/x/56.html)
Figure 3-30: Brittle crack in a cadmium plated B7 bolt from solid metal induced embrittlement (Reference:http://www.hghouston.com/x/57.html)
3.3.5 Corrosion Fatigue Cracking Corrosion fatigue cracking (CFC) is a brittle failure of an alloy caused by fluctuating stress in corrosive environment. This type of corrosion occurs because the cyclic stresses do not give enough time for the metal to recover its structural integrity. The crack propagation rate depends on the frequency and cyclic amplitude of the
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stress. The frequency of cyclic stress is important in CFC. Lower frequency leads to greater crack propagation per cycle. Very high frequencies eliminate the effects of the corrosive environment. CFC cracks propagate perpendicular to the principal tensile stress. Increasing the ratio R, of the minimum to the maximum stress in the cycle generally decreases the resistance to corrosion fatigue cracking.
At ambient
temperature the stress ratio R has no effects, but at high temperatures when creep is possible, R has its effects. The susceptibility to corrosion fatigue cracking increases due to stress raisers such as notches.
3.3.5.1 Comparison with Stress Corrosion Cracking •
CFC is similar to SCC in that a corrosive solution induces brittle fracture in an alloy that is normally ductile in non-corrosive environment.
•
CFC occurs due to cyclic stress whereas SCC occurs due to static stress.
•
CFC cracks propagate perpendicular to the principal tensile stress whereas SCC cracks propagate normal to the tensile stress.
•
CFC cracks are usually transgranular and blunt whereas SCC cracks are usually transgranular or intergranular and sharp.
•
Corrosion products are usually absent in case of SCC whereas it is present in CFC.
3.3.5.2 Prevention of Corrosion Fatigue Cracking Corrosion fatigue cracking (CFC) can be minimized using inhibitors, cathodic protection, and reduction of oxidizers or increase in pH. Removal of cyclic stress by designing will prevent CFC. Shotpeening increases the resistance to CFC.
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3.3.6 Hydrogen Induced Cracking HIC usually occurs in hardened or high stressed steels. The presence of atomic hydrogen results in decrease in the toughness or ductility. HIC has been recognized classically as being of two types. The first is known as blistering. When hydrogen builds up at metallurgical inhomogeneities (traps), nascent hydrogen (H atom) which will recombine to form molecular hydrogen gas. Accumulations of this gaseous molecular hydrogen develop high pressures sufficient to rupture inter-atomic bonds, forming microscopic voids (blisters). The second type of HIC results from hydrogen being absorbed by solid metals. This type is especially prevalent in iron alloys because of the solubility difference of hydrogen in ferrite (BCC) and austenite (FCC). The solubility of hydrogen in austenite is higher than that in ferrite. Upon forming ferrite from austenite below their phase transformation temperature of 723 °C, hydrogen in the ferrite is supersaturated and thus leads to embrittlement of the metal, which causes HIC. However, the face centered cubic (FCC) stainless steels and FCC alloys of copper, aluminum and nickel are more resistant to HIC because of their inherent high ductility and low diffusivity for hydrogen.
But reactive alloys of titanium, zirconium,
vanadium, niobium and tantalum, which are embrittled by insoluble hydrides, are susceptible to HIC.
3.3.6.1 Comparison with Stress Corrosion Cracking HIC is similar to stress corrosion cracking in that brittle fracture occurs in a corrosive environment under constant tensile stress. Cathodic protection aggravates HIC but suppresses or stops SCC. SCC cracks are usually branched, whereas CFC cracks are unbranched. SCC cracks are mostly intergranular whereas the HIC cracks are usually transgranular. If the alloys are cold worked, the cracks will be intergranular for HIC. Failures by HIC are usually maximized at/or near room temperature.
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3.3.6.2 Prevention of Hydrogen Induced Cracking •
Selection of alloys, which are more resistant to HIC such as FCC stainless steel and copper.
•
Stop cathodic protection.
3.4 Mechanically Assisted Degradation Mechanically assisted degradation of metals is defined as the degradation which involves both corrosion mechanism and a wear or fatigue mechanism. In this velocity, abrasion, hydrodynamics, etc. play a major role and has a significant effect in the corrosion behavior.
3.4.1 Erosion Corrosion Erosion corrosion is due to the relative movement between a corrosive fluid and the metal surface.
Erosion corrosion is characterized in appearance by grooves,
gullies, waves, rounded holes, and valleys, and usually exhibits a directional pattern. Sometimes passive films are developed on the metal surface, which has high corrosion resistance such as aluminum, lead and stainless steels. Erosion corrosion results when these protective surfaces are damaged. Metals that are soft such as copper and lead are quite susceptible to erosion corrosion. The corrosive media, which cause erosion corrosion, are gases, aqueous additions, organic systems, and liquid metals. All types of equipment exposed to moving fluids are subject to erosion corrosion. Some of these are piping systems, such as bends, elbows and tees, valves, pumps, blowers, centrifugals, propellers, impellers, agitators etc and equipments subjected to spray.
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3.4.1.1 Prevention of Erosion Corrosion •
Choose materials with better resistance to erosion corrosion such as aluminum and stainless steel.
•
Avoid sharp elbows and angles in the design of equipment.
3.4.2 Impingement Corrosion Impingement is a process resulting in a continuing succession of impacts between particles and a solid surface. Impingement corrosion is a form of erosion corrosion associated with impingement action of liquids. It may be accelerated by entrained gas bubbles. More specifically it is caused by the impingement action of water carrying entrained gas bubbles and striking the metal surface at an angle. It is not the result of mechanical erosion of the metal itself but is the result of removal of the film of corrosion products by erosion which is ordinarily protective at lower velocities.
Figure 3-31 Impingement corrosion in a bent tube (Reference:http://www.alu-info.dk/Html/alulib/modul/A00115.htm)
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Impingement corrosion is usually seen at or near the entrance of the tubes and in bends. It takes the form of pitting or grooving and eventual perforation of the wall at that location while the remainder of the tube shows no sign of corrosion. Parts like pump casings, pump shafts and impellers, nozzles and valve seats, tubes are some of the examples where impingement corrosion can be seen.
3.4.2.1 Prevention of Impingement Corrosion Suitable resistant material (harder materials) is to be chosen. Design, shape and geometry are some of the aspects that are to be considered. Inhibitors and coatings are to be applied. It is desirable to separate out solids, water or gas early in a flow system in order to avoid two phase flow.
3.4.3 Cavitation Corrosion Cavitation corrosion occurs under conditions of severe turbulent flow and rapid pressure changes. The cavitation process is responsible for the breakdown of the protective surface film on the metal. This depassivation results in an accelerated corrosion and causes gas pockets and bubbles to form and collapse. High flow velocities and the entrainment of solid particles are the main causes of erosion corrosion. Bubble implosions are responsible for the breakthrough of the passive film in cavitation corrosion. These bubbles may result from boiling phenomena or may arise because of the release of dissolved gases from the fluid as a result of pressure drops. This type of corrosion can be seen at the suction of a pump, at the discharge of a valve or regulator, at pipe elbows and expansions. This form of corrosion will eat out the volutes and impellers of centrifugal pumps. Cavitation should be designed out by reducing hydrodynamic pressure gradients and designing to avoid pressure drops
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below the vapor pressure of the liquid and air ingress. Resilient coatings and cathodic protection can be considered as control methods. Large pressure changes should be avoided and surface layers should be hard to prevent from cavitation corrosion.
3.4.4 Fretting corrosion The rapid corrosion that occurs at the interface between contacting highly loaded metal surfaces which are enhanced through slight vibratory motions. It is caused by the combination of corrosion and the abrasive effects of corrosion product debris often seen in component with moving or vibrating parts. Pits or grooves and oxide debris characterize this damage. Bolted assemblies and ball bearings are most vulnerable cases of fretting corrosion. Contact surfaces exposed to vibration during transportation undergo the effects of fretting corrosion. The metal surface gets exposed to the atmosphere, while the oxide film is removed due to this effect. This rubbing occurs at small amplitudes. While the rubbing motion continues, fatigue cracks are initiated by high shear stresses. As a result of surface initiated fatigue, wear particles break out of the material, become trapped between the surfaces and oxidize. Apart from causing dismounting problems, these oxidized particles prevent free axial displacement and introduce increased load in the bearing, which in severe cases may lead to immediate bearing failure. Parameters that need to be controlled in fretting corrosion evaluations include corrosive environment, contact load, amplitude and frequency of load fluctuations, temperature and availability of moisture. Increased surface hardness and use of lubricants are some of the preventive methods to be used for fretting corrosion. Bearing loads has to be reduced on mating surfaces.
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3.5 Metallurgically Influenced Corrosion This is classified as a result of the significant role that metallurgical factors affect corrosion. The metallurgical influences considered are the relative stability of the component alloys, metallic phases, metalloid phases and local variations in composition in a single phase.
3.5.1
Intergranular Corrosion Intergranular corrosion (IGC) can be caused by depletion of one of the alloying
elements in the grain boundary areas. An example is stainless steel upon extended exposure to temperatures between 400 and 510 °C, typically during welding. In this case, chromium precipitates as chrome carbide in the grain boundaries, which depletes the element and thus causes intergranular corrosion along the grain boundaries. This is because chromium is the element that can reduce the corrosion by forming the passive film.
3.5.1.1 Exfoliation / Lamellar Corrosion Exfoliation corrosion is a special form of intergranular corrosion. It is usually seen in high strength aluminum alloys and carbon steels. By the force of expanding corrosion products at the grain boundaries, the metallic surface grains get lifted up and results in this form of corrosion. Corrosion products building up along these grain boundaries exert pressure between the grains. Thus a leafing effect can be observed eventually giving a layered appearance to the metal. Alloys that have been extruded or worked heavily undergo this type of damage. The damage often begins at end grains seen in machined edges, holes and grooves. It may progress through an entire section.
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Figure 3-32: Exfoliation of Aluminium (Reference:http://corrosion.ksc.nasa.gov/html/intercor.htm)
Figure 3-33:Exfoliation of aircraft component (Reference:http://www.corrosion-doctors.org/Forms/exfol-examp.htm)
3.5.1.2 Weld Decay Sensitization of austenitic stainless steels during welding is known as weld decay. During welding of austenitic stainless steel, at the interface of base metal and weld the temperature was too high to sensitize the alloy. However, areas next to the
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base metal/weld are sensitized since it reaches the critical temperature range of 425 °C to 815 °C. Thus IGC occurs along this weld decay zone.
3.5.1.2.1 •
Prevention of Weld Decay
Solution annealing is heat treatment of an alloy to high temperature just below its melting point to dissolve the alloying elements, followed by quenching to prevent the precipitation of chromium carbides.
•
Low carbon alloy modifications allow a minimum reduction of chromium by precipitation along the grain boundaries.
•
Stabilized alloys containing niobium and titanium in austenitic stainless steels will not reduce chromium content because these elements will react with carbon in the sensitization temperature range.
3.5.1.3 Sensitization (Intergranular Corrosion of Austenitic Stainless Steels) Numerous failures of 18-8 stainless steels have occurred because of intergranular corrosion. This happens in environments where the alloy should exhibit excellent corrosion resistance.
When these steels are heated in approximately the
temperature range of 425 °C to 815 °C (800 to 1500 °F), they become sensitized or susceptible to intergranular corrosion.
Here sensitizing means that chromium
precipitates as chromium carbide leaving the grain boundary areas with less chromium. Above 815 °C, the chromium carbides are soluble and below 425 °C, the diffusion rate of carbon is too low to permit the formation of carbides. However, in the temperature range between these two temperatures, conditions are optimum to precipitate chromium with carbon.
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Figure 3-34: intergranular corrosion in stainless steel
Figure 3-34 shows a stainless steel which corroded in the heat affected zone a short distance from the weld. This is typical of intergranular corrosion in austenitic stainless steels. This corrosion can be eliminated by using stabilized stainless steels (321 or 347) or by using low-carbon stainless grades (304L or 3I6L). Heat-treatable aluminum alloys (2000, 6000, and 7000 series alloys) can also have this problem.
Figure 3-35: Intergranular Corrosion of 7075-T6 aluminum adjacent to steel fastener (Reference:http://www.faa.gov/avr/afs/300/pdf/2g-CH6_2.pdf)
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3.5.2 Dealloying Dealloying or selective leaching is the removal of one or more active element(s) from the alloy. Dealloying is a form of corrosion found in copper alloys, gray cast iron, and some other alloys. Dealloying can be controlled by the use of more resistant alloys such as inhibited brasses and malleable or nodular cast iron.
3.5.3 Dezincification Dezincification occurs when the more active element of zinc in an alloy is preferentially leached in a corrosive environment leaving the copper behind. Common yellow brass consists of 30% zinc and 70% copper. Dezincification is readily observed with the naked eye because the alloy assumes a red or copper color contrasting with the original yellow.
There are two types of dezincification and both are readily
recognizable. One is uniform or layer type and the other is localized or plug-type.
3.5.3.1 Prevention of Dezincification Dezincification can be minimized by reducing the aggressiveness of the environment or by cathodic protection, but in most cases these methods are not economical. A preferable method is to select a resistant brass. Usually when the zinc content is less than 15 weight percent, the brass is immune due to dezincification. For example, red brass (15% Zn) is almost immune. Another method is the development of better brasses with the addition of 1% tin to a 70-30% brass. Further improvement can be obtained by adding small amounts of arsenic, antimony or phosphorus as inhibitors.
3.6 High Temperature Corrosion Corrosion can occur when metal is exposed to an oxidizing gas at high temperatures. This form of corrosion that does not require the presence of a liquid
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electrolyte is known as high temperature corrosion. It is also called as dry corrosion, tarnishing, high temperature oxidation and scaling. As temperature increases, rate of attack also increases. The surface film thickens as a result of reaction at the scale-gas or metal-scale interface due to the ion transfer through the scale. This behaves as a solid electrolyte. High temperature scales are usually thought of as oxides, but also be sulfides, possibly carbides, or a combination of these. High temperature corrosion is a widespread problem in various industries such as heat treating, aerospace and gas turbine, power generation (nuclear and fossil fuel) etc.
3.6.1 Oxidation Oxidation is the most important high temperature corrosion reaction. Alloys intended for high-temperature applications are designed to have the capability of forming protective oxide layers. Regardless of the predominant mode of corrosion, oxidation participates in the high temperature corrosion reactions. Depending on the base alloy composition and the intended service temperature, alloying requirements for the production of specific oxide scales have been translated into minimum levels of scale-forming elements. These protective oxide scales are formed at all the surface discontinuities, wherever the alloy surface is exposed to the ambient environment. Hence there is a possibility that notches of oxides are formed at occluded angles in the surface, which may eventually serve to initiate or propagate cracks under thermal cycling conditions.
3.6.2 Sulfidation Sulfidation is defined as the reaction of a metal or alloy with a sulfur-containing species to produce a sulfur compound that forms on or beneath the surface on the metal or alloy. When sulfur activity of the gaseous environment is sufficiently high,
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sulfide phases, instead of oxide phases can be formed. Sulfidation attack mainly occurs at sites where the protective oxide has broken down. If sufficient sulfur enters the alloy so that the available chromium and aluminum is converted to sulfides, then the less stable sulfides of the base may form due to different morphological and kinetic reasons. These base metal sulfides are responsible for the accelerated attack as they grow much faster than the chromium or aluminum oxides or sulfides and have low melting points. As long as the sulfur is present in small amounts as sulfides, there is little danger of accelerated attack. Once sulfides have formed alloys they have a tendency to get oxidized to form new sulfides in grain boundaries or at the sites of chromium or aluminum, which act to localized stress or reduce the load-bearing section.
3.6.3 Carburization Carburization is defined as the increase of the carbon content of steel due to interactions at elevated temperatures with the environment. It occurs kinetically in many carbon-containing environments like carbon, carbon monoxide and hydrocarbons. Carbon has large influence on the mechanical properties of the steel like hardness, strength etc. Carburization therefore results in the formation of a hard top layer that is more brittle than the core material. Carbon forms carbides (like Cr23C6, Cr3C 2 or Cr7C3), depleting the metal matrix locally of chromium and making it more sensitive to corrosion. Minor alloying elements can exert an influence on the susceptibility to carburization of various alloys. In particular silicon (1.5 to 2.0%), niobium, tungsten, titanium, nickel and the rare earths have been noted as promoting resistance to carburization, whereas lead, molybdenum, cobalt, zirconium and borium are considered detrimental. Carburization is based on carbon transport across the metal/gas interface which is very slow, and alloy pretreatments likely to promote such scales forms smooth
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surfaces or pre-oxidation. This oxide film will decrease the carburization attack. Because of the high solubility of carbon in austenite, austenitic steels carburize more readily than ferritic steels.
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4 AGING OF COMPOSITES
4.1 Introduction A composite material is a complex solid material, composing of atleast two materials, that, when combined, remain discrete but function interactively to form a useful material. The composite is designed to exhibit the best properties that cannot be predicted by simply summing the properties of its constituents. This combination of materials is done by physical means unlike the chemical bonding that takes place in the alloys of monolithic materials etc. Composites are made of fiber reinforcements, resin, fillers, and additives. The fibers provide stiffness and strength. The resin offers high compressive strength and binds the fibers together into a matrix. The fillers serve to reduce cost and shrinkage. The additives help to improve not only the mechanical and physical properties of the composites but also workability. A true composite might be considered to have a matrix material completely surrounding its reinforcing material. The matrix holds the reinforcement to form the desired shape while the reinforcement improves the overall mechanical properties of the matrix. After designed properly, the combined material produces characteristics not attainable by either constituent acting alone.
4.2 Composition Composites are composed of resins, reinforcements, fillers, and additives. Each of the above mentioned constituents play a vital role in the processing of the final product. Resin holds the composite together thus influencing the physical properties of the final product, while the reinforcement provides the mechanical strength. The fillers and additives are used to impart special properties to the final product.
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4.2.1 Fiber Reinforcements The fiber is an important constituent in composites. The fiber generally occupies 30% - 70% of the matrix volume in the composites. The primary function of fibers is to carry load along the length of the fiber to provide strength and stiffness in one direction. The fibers can be chopped, woven, stitched, and/or braided. They are usually treated with sizing such as starch, gelatin, oil or wax to improve the bond as well as binders to improve the handling. The most common types of fibers used in advanced composites for structural applications are the fiberglass, aramid, and carbon. The fiberglass is the least expensive and carbon being the most expensive. The cost of aramid fibers is about the same as the lower grades of the carbon fiber.
a. Random fiber (short fiber) reinforced composites
b. Continuous fiber (long fiber) reinforced composites
Figure 4-1: Fibers as the reinforcement (Fibrous Composites) (Ref: http://www.efunda.com/formulae/solid_mechanics/composites/comp_intro.cfm)
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4.2.2 Resin Systems The resin is another important constituent in composites. The primary function of resin is to transfer stress between the reinforced fibers and protect them from environmental damage. A thermoplastic remains solid at room temperature and becomes soft when heated. It may be shaped or molded while in a heated semi-fluid state and become rigid when cool. On the other hand, thermoset resins are liquids or low melting point solids in their initial form. Once cured, solid thermoset resins cannot be converted back to their original liquid form. Thermosetting resin will cure permanently at elevated temperatures. This characteristic makes the thermoset resin composites very desirable for structural applications. The most common resins used in composites are the unsaturated polyesters, epoxies, vinyl esters polyurethanes and phenolics.
4.2.3 Fillers It is cost effective to fill the voids in a composite matrix purely with resin as the resins are expensive. Hence fillers are added to the reisn matrix to control the material thus improving its mechanical and chemical properties. The three major types of fillers used in the composite industry are the calcium carbonate, kaolin, and alumina trihydrate. Some other common fillers are mica, feldspar, wollastonite, silica, talc, and glasses.
4.2.4 Additives Different varieties of additives are used to improve the material properties, aesthetics, manufacturing process and performance. The additives are divided into different groups – catalysts, promoters, and inhibitors; coloring dies; and releasing agents. The most common man-made composites can be divided into three main groups:
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Polymer Matrix Composites (PMC’s) – These are also known as Fiber Reinforced Polymers (or Plastics). They use a polymer based resin as the matrix, and variety of fibers such as glass, carbon, and aramid as the reinforcement. These ate the most common used composites. Metal Matrix Composites (MMC’s) – These materials use a metal such as aluminum as the matrix and fibers such as silicon carbide is used as the reinforcement. This is mainly used for automotive industry. Ceramic Matrix Composites (CMC’s) - Used in very high temperature environments, these materials use a ceramic as the matrix and reinforce it with short fibers, or whiskers such as those made from silicon carbide and boron nitride.
4.3 Fiber Reinforced Polymers The mechanical properties of resins such as epoxies, polyesters are not very high when compared to metals and hence they have limited use for the manufacture of their own structure. But their ability of ease formation into complex shapes makes them more desirable. Glass, aramid and boron have high tensile and compressive strengths but in solid form, these properties are not apparent because when stressed, surface flaws will cause the material to crack and fail before its breaking point. In order to overcome this problem, the material is produced in the fiber form, thus restricting the flaws to a small number of fibers while the remaining exhibiting the material’s strength. Therefore a bundle of fibers reflect the optimum performance more accurately. However, fibers alone can only exhibit tensile properties along the fiber’s length. Inorder to obtain exceptional properties, resin systems are combined with reinforced fibers such as glass, aramid, carbon etc. The resin matrix protects the fibers from damage caused by abrasion and impact and also distributes the load applied to the composite between each of the individual fibers. Since the Fiber Reinforced Composites combine a resin system and reinforcing fibers, the properties of the resulting composite will combine some properties of the resin on its own with that of the fibers, which are summarized as shown in the Figure 4-2.
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Fiber
Tensile Stress
FRP Composite Resin
Strain
Figure 4-2: The combined effect on Modulus of the addition of fibers to a resin matrix.
The mechanical properties and composition of FRP composites can be tailored for their intended use. In addition to the manufacturing process to fabricate the product, the type and quantity of materials will also affect the mechanical properties and performance. Important considerations for the design of composite products are as follows:
•
Type of fiber reinforcement
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Fiber volume fraction
•
Orientation of fiber (0o, 90o, +/- 45 o or a combination of these)
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Type of resin
•
Cost of product
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Manufacturing process
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Volume Production
•
Service conditions
Composites are manufactured using two major processes. They are: open mold process and closed mold process. Types of open mold processes are hand lay up and
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tube rolling. Closed mold processes include, resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), resin injection molding, compression molding, pultrusion, extrusion and filament winding process.
Composites when compared to
conventional materials are expensive, but are still widely used in various applications. They are used in defense equipments, infrastructure, medical equipment etc. This is because of the advantages offered by composites in comparison to conventional materials.
•
They have high stiffness, strength, or dimensional stability.
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They increase toughness (impact strength).
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They increase heat-deflection temperature.
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They increase mechanical damping.
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They reduce permeability to gases and liquids.
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They increase some electrical properties (e.g., increase electrical resistivity).
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They decrease water absorption.
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They decrease thermal expansion.
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They increase chemical wear and corrosion resistance.
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They maintain strength/stiffness at high temperatures while under strain conditions in a corrosive environment.
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They increase secondary uses and recyclability, and to reduce any negative impact on the environment.
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They improve design flexibility.
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They have low density.
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They are easy to handle.
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They have good fatigue response and damage tolerance.
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They are tailored to loading conditions to optimize structural performance.
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Radar Transparency.
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They are non-magnetic and have high dielectric strength (insulator).
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Tailored surface finish.
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Potential for real-time monitoring.
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4.3.1 Characteristics of Fiber Reinforced Composites Fiber-reinforced polymer (FRP) composites offer many advantages, such as enhanced corrosion resistance and high specific mechanical properties, over traditional structural materials. However, composite laminates also have inherent weaknesses, such as susceptibility to impact damage and long-term aging. Aging of composites may depend on environmental factors on the composite material when exposed for a period of time and type of liquids. This may occur as a result of the matrix degradation, fiber degradation, or fiber-matrix interface degradation.
Some of the causes of
corrosion/aging of composites are absorption of solvents, oxidation, UV radiation, and thermal degradation, etc. An FRP component may sustain constant or repeated loading under environmental exposure for a prolonged period of time. These service conditions render environmental effects, creep, fatigue, and weathering effects on FRPs, which are to be considered in design. The physical or chemical weathering conditions occur when a composite material is subjected to mechanical loadings such as static load, fatigue and creep or when it is exposed to chemical solutions like alkaline, acid or aqueous solutions. As a result of the above mentioned environmental factors, changes with exposure time take place in the mechanical properties such as strength, stiffness, creep, and fatigue life and chemical properties such as glass transition temperature. Short term behavior of FRP composites are determined to establish parameters such as mechanical resistance (axial, bending, shear and combinations), thermal behavior (thermal coefficient and conductivity), and moisture diffusion/plasticization. Long-term responses of composite materials are determined to establish the properties of aging in composites due to sustained loads, cyclic loads and environmental conditions. The response of a composite has been classified in terms of its short-term behavior and long-term behavior.
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Short term Mechanical and Hygro-Thermal Behavior Thermal Coefficient and Conductivity: A lower coefficient of thermal expansion of glass fibers in relation to resin produces residual stresses within the material microstructure during temperature drop. In cold regions, the difference in curing and operating temperatures of the composite material may be as high as 2000F; thus resulting in residual stresses that are high enough to cause microcracking within the matrix and the matrix-fiber interfaces. Matrix tensile strength reductions up to 50% may be possible because of residual stress build-up under low temperature effects. Temperature affects the rate of moisture absorption as well as the mechanical properties of a composite. Mechanical properties of fiber reinforced composites change when the material is exposed to elevated temperatures (370C to 1900C). Increase in temperature may accelerate time-dependent effects such as creep and stress relaxation. Similarly, evaluation of composite systems at low temperatures is essential since high strength and stiffness degradation rate under thermal cycling is observed in cold region structures. The increase in stiffness at low temperature is attributed to crystallization and instantaneous thermal stiffening, which is dependent upon polymer type and temperature. The decrease in temperature can lead to possible increase in: 1) modulus; 2) tensile and flexural strength; 3) fatigue strength and creep resistance; and 4) adhesive strength. Also, decrease in temperature can lead to possible reduction in: 1) elongation and deflection; 2) fracture toughness and impact strength; 3) compressive strength; and 4) thermal coefficient. These and other mechanical properties that are functions of temperature will be evaluated.
Moisture Diffusion / Plasticization: Water penetrates FRPs through two processes: diffusion through the resin, and flow through cracks or other material flaws. During diffusion absorbed water is not in the liquid form, but consists of molecules or groups of molecules that are linked together by hydrogen bonds to the polymer. The molecules dissolved in the surface layer of the polymer migrate into the bulk of the material under a concentration gradient. Water penetration into cracks or other flaws occurs by capillary flow. Water also penetrates at the fiber-matrix interface. It is reported that the primary mechanism of moisture through the cracks is an after effect. Moisture
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pickup leads to loss of chemical energy, which is attributed to hydrolytic scission of ester groups. However, increased hydrostatic pressure reduces water uptake due to closing of micro cracks. Diffusion of water into the resin causes swelling stresses. The equilibrium content of water determines the magnitude of swelling stresses. The chemical composition of resin influences the solubility of water in the resin and its susceptibility to hydrolysis. Exposure of composites to moisture for longer duration results in resin (matrix) plasticization and interface bond strength reduction. In this report, diffusion coefficients, rate of hydrolysis and property degradation are reviewed as a function of different parameters such as moisture concentration, humidity level, composite thickness, fabric configuration and temperature. The effects of moisture on long-term strength and stiffness of the FRP composites and hybrids are discussed.
Long Term Mechanical and Hygro-Thermal Behavior (Aging) Creep: Creep of composites is significant as compared to fibers particularly when sustained stress levels exceed particular threshold levels. For example, maximum sustained stress for GFRP reinforcement embedded in concrete is limited to 20% of the failure stress. The relationship between survival probability (creep-rupture probability) and logarithm of creep-rupture time will be included in addition to the effects of initial sustained strains with respect to moisture absorption and fracture. Different creep models are also discussed.
Fatigue and Fracture: This section mainly focuses on the models that predict the fatigue life of composites and hybrids, and fracture initiation. The review consists of empirical fatigue strength theories, strength and stiffness based degradations under fatigue.
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Aging due to Environmental Effects: In-service FRP composites and hybrids exposed to harsh environments may lose strength, stiffness, and bond, thus compromising structural integrity and safety. Change in properties of polymers in the absence of load is referred to as ‘aging’. Since different factors influence mechanical properties of polymer composites, aging is a complex phenomenon. Aging phenomenon can be very significant during the service life of strengthened concrete-frame or masonry wall, i.e., over its 75-year service life. Review on durability and aging in this report includes effects of different pH conditions, temperature and sustained stress level on structural composites.
4.3.2 Short-Term Mechanical and Hygro-Thermal Behavior 4.3.2.1 Thermal Coefficient and Conductivity A lower coefficient of thermal expansion of glass fibers in relation to resin produces residual stresses within the material microstructure during temperature drop. In cold regions, the difference in curing and operating temperatures of the composite material may be as high as 200°F; thus resulting in residual stresses that are high enough to cause microcracking within the matrix and matrix-fiber interfaces. Matrix tensile strength reductions up to 50% may be possible because of residual stress buildup under low temperature effects. Temperature affects the rate of moisture absorption as well as the mechanical properties of a composite.
Mechanical properties of fiber reinforced composites,
change when the material is exposed to elevated temperatures (37°C to 190°C). Increase in temperature may accelerate time-dependent effects such as creep and stress relaxation. Similarly, evaluation of composite systems at low temperatures is essential since high strength and stiffness degradation rate, under thermal cycling is observed in cold region structures. For example, high stiffening (50 to 100 times over ambient temperature of 78°F) of resins at low temperatures reduces the desirable movement of elastomeric bearing pads under bridge seats, leading to serious malfunction of the pads and consequent failure of the structure.
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stiffness at low temperature is attributed to crystallization and instantaneous thermal stiffening, which is dependent upon polymer type and temperature. The decrease in temperature can lead to possible increase in: 1) modulus; 2) tensile and flexural strength; 3) fatigue strength and creep resistance; and 4) adhesive strength. Also, decrease in temperature can lead to possible reduction in: 1) elongation and deflection; 2) fracture toughness and impact strength; 3) compressive strength; and 4) thermal coefficient.
Thermal Coefficient Temperature effects can induce the dimensional change of a composite, in addition to its mechanical deformations. Composites have two coefficients of thermal expansion.
α1 refers to expansion in the direction of the fibers, while α2 is the
expansion in the direction perpendicular to the fibers. The resins used have thermal expansion coefficients that are positive (~10 to 20x10-6/C), and the fibers can have either a low value of α or even a negative value, as is the case with carbon fibers. The thermal expansion in the direction of the fibers, α1 is computed according to Schapery, 1968.
α1 =
1 [α f E f (T )Vf + αmEm (T )Vm ] E1 (T )
…..(4.1)
and in the direction perpendicular to the fibers α 2 = (1 + Vf )α f Vf + (1 + Vm )α m Vm − α1V12
……(4.2)
where vf, vm, and v12 are the values of the respective Poisson’s ratio. It is noteworthy to point out that the moduli of elasticity of the constituent parts are shown as function of time, T, to account for creep. Thermal mismatch between fiber and matrix may cause matrix cracks in composites under severe temperature fluctuations. Such stresses can be predicted by complex FEA of the microstructure or by complex micro-mechanical models of the same
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(Bowles, 1991). Propagation of such mode of damage has been observed only for very high or very low temperature applications.
Thermal and Electrical Conductivity Transport Properties
The properties of materials (such as heat conduction, permeation, electrical conduction, etc.,) are grouped together and termed the transport properties, because they deal with diffusion through the composite. Equations are suggested by [Hashin, 1968; Halpin, 1984; and Springer, 1981] for both the longitudinal and transverse directions in an FRP composite. The longitudinal properties are computed through the rule of mixtures as follows: k 1 = Vf k f + Vmk m
…(4.3)
where k1, kf, and km are the transport properties of the composite in the longitudinal direction, in the fiber, and in the matrix, respectively. For the transverse coefficient, the property k2 is computed by the Halpin-Tsai equation: k 2 1 + ξηVf = km 1 − ηVf
….(4.4)
where ⎛ Kf ⎞ ⎜⎜ − 1⎟⎟ K ⎠ η= ⎝ m ⎛ Kf ⎞ ⎜⎜ + ξ ⎟⎟ ⎝ Km ⎠
……(4.5)
⎛a⎞ log(ξ) = 3 log⎜ ⎟ ⎝b⎠
……(4.6)
and a and b are the dimensions of the fiber along and perpendicular to the direction of measurement of the transport coefficient.
These equations accurately
predict the thermal and electrical conductivities of carbon-epoxy composites for Vf up to 60%.
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4.3.2.2 Moisture Diffusion/Plasticization Water penetrates FRPs through two processes: diffusion through the resin, and flow through cracks or other material flaws. During diffusion, absorbed water is not in the liquid form, but consists of molecules or groups of molecules that are linked together by hydrogen bonds to the polymer. The molecules dissolved in the surface layer of the polymer migrate into the bulk of the material under a concentration gradient. Water penetration into cracks or other flaws occurs by capillary flow. Water also penetrates at the fiber-matrix interface. It is reported that the primary mechanism of moisture pickup is diffusion through resin, and transfer of moisture through the cracks is an after effect. Moisture pickup leads to loss of chemical energy, which is attributed to hydrolytic scission of ester groups.
However, increased hydrostatic pressure reduces water
uptake due to closing of micro cracks. The rate of degradation of the polymer composite exposed to fluid environment is related to the rate of sorption of the fluid. The sorption behavior depends on the type of fluid, fluid concentration, temperature, externally applied stress, hydrostatic pressure, the state of the material, and the chemical structure of the polymer and fiber/matrix interface. It is governed in the most part by the chemical structure of the resin, the degree and type of cross-linking and the presence of voids. The sorption rate depends on the properties of the constituents, volume fraction and orientation of the fibers, and process variables. Temperature increases the rate of absorption. Diffusion coefficients have been found to increase with temperature. The damage done in FRP due to the diffusion of fluids depending on the exposure time may be plasticization and swelling of the matrix, which can be restored by drying.
It could also result in irreversible damage such as matrix cracking,
debonding of the fiber/matrix interface region, damage to fibers, and delamination caused by swelling or internal stresses. Improved bonding between fiber and matrix, particularly the chemical bonding tends to delay the corrosion process.
Moisture decreases the glass transition
temperature, Tg, of the polymer matrix, due to Plasticization of the matrix as a result of the disturbance of the Van der Walls bonds between the polymer chains.
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The tensile strength and modulus, compressive strength and modulus, flexural strength and modulus, transverse tensile strength and modulus, short beam shear strength and modulus, impact strength and interlaminar strength are reduced due to exposure to moisture. Fracture energy or fracture toughness seems to increase in some cases and reduce in some cases.
4.3.2.2.1 Diffusion Through Unreinforced Epoxy, Vinyl Ester, Polyestor and Phenolics Chin et al. [1999] studied the sorption and diffusion of water, salt water and concrete pore solution in epoxy (EPON 828 RS), vinyl ester (Derakane 411-350PA) and polyester (Aropol 7240-T15) matrices. Typical dimensions of the polymer film used in the experiments were 25mm X 25mm and the thickness of the vinyl ester and polyester film ranged from 230 to 260 microns whereas the thickness of epoxy film was 300 microns. They observed Fickian diffusion in all the resins and solutions. When a concentration gradient exists in a material, such as a solid polymer, there is a natural tendency for the concentration difference to be reduced and eliminated by the process of mass transfer. The mass flux is generally proportional to the local concentration gradient; this is often known as Fick’s first law, and the constant of proportionality is called the diffusivity or the diffusion coefficient.
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Figure 4-3: Typical Sorption Curve (Vijay et al., 2001)
The Figure 4-3 shows sorption curves for epoxy, vinyl ester and isopolyester resins. The curves for moisture content ratio show an initial linear region up to about Mt/M∞ = 0.6 followed by a region concave to the abscissa. Uptake was rapid for the first
10 hours, and then slowed between 10 and 100 hours as equilibrium was attained. The epoxy resin has a higher concentration of hydrophilic hydroxyl groups located along the backbone thus exhibiting higher moisture uptake than other resins.
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Figure 4-4: The Sorption Curves for Epoxy, Vinyl ester, and Isopolyester Resin When Exposed to the 3 Different Solutions (Chin et al., 1999)
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Table 4-1: Diffusion Coefficients of Epoxy, Vinyl Ester, and Isopolyester Resins
Diffusion Coefficient, D Matrix
Epoxy
Vinyl Ester
Isopolyester
(x 10-9 cm2 s-1)
Sorbent 22 °C
60 °C
Distilled water
0.53
13.6
Salt solution
1.04
8.54
Pore solution
0.67
9.82
Distilled water
6.88
19.0
Salt solution
8.75
24.5
Pore solution
8.72
24.3
Distilled water
41.9
Salt solution
---
Pore solution
8.89
---
The sorption curves for epoxy, vinyl ester, and isopolyester resin when exposed to the 3 different solutions are shown in the Figure 4-4.
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Figure 4-5: Fickian Diffusion Curves for Epoxy in (a) Water, (b) Salt Solution, and (c) Concrete Pore Solution at 22 °C (Chin et al., 1999)
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Although vinyl ester resin exhibited the lowest equilibrium moisture uptake, it had higher diffusion coefficients than epoxy. Since diffusion coefficient is a function of both permeability and solubility, it is concluded that the permeability of vinyl ester is greater than epoxy. Moisture sorption into polymeric material lowers the glass transition temperature (Tg); Hence reduces the maximum working temperature of the material. Hiltz and Keough [1992] studied the influence of absorbed water on the Tg of a poly amideimide using dynamic mechanical analysis (DMA) and DSC. The results indicate a decrease in Tg from 568 K to 477 K as the weight of absorbed water increased from 0% to 4.25%.
This established a clear correlation between Tg and amount of water absorbed. The reduction in Tg is due to plasticization of the resin, where the chemical structure was not affected. Decrease in Tg is also attributed to increase in the chain mobility and voids. A reduction in Tg is also accompanied by swelling of the sample. Both swelling and plasticization are reversible and the thickness and properties of the dried sample are restored. Wong and Broutman [1985] found the epoxy EPON 828 samples to regain their initial thickness and Tg upon drying them after saturation. Especially in vinyl ester resins, hydrolysis of the matrix is observed for long periods (>4000 h) [Buck et al., 1998]. Water causes the ester linkages to break into acid and alcohol groups. Ghorbel and Valentin [1993] studied the changes in structure with time of aging and found that the above reactions were contributing to hydrolysis of the resin.
4.3.2.2.2
Effect of Moisture on Fiber-Matrix System
Stokes [1990], in his work made cylindrical specimens of FM5055 carbon phenolic, which was heated uniformly at a constant rate of 5.50C/sec. The specimens were 0.635 cm in diameter and 2.54 cm in length and were fabricated such that the direction transverse to the fabric plane was aligned with the axial direction of the specimen. As the specimens were heated, strain in the axial direction was measured as
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a function of temperature. The oven chamber in which the specimens were heated was maintained at zero percent relative humidity. Stokes measured the transverse thermal expansion strain for specimens containing three different initial moisture contents: 0%, 4%, 8% initial moisture. The measured results from Stokes are shown in Figure 4-6.
Figure 4-6: Thermal Expansion Measured by Stokes (Stokes, 1990)
Figure 4-7: Moisture-Induced Thermal Expansion vs. Temperature (Stokes, 1990)
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Strain response due to water is obtained by subtracting the strain response for the 0% moisture specimen from the response for the 4% and 8% moisture specimens. Separated strain profiles for 4% and 8% moisture levels obtained through subtraction technique are shown in Figure 4-7 referred to as the moisture-induced thermal expansion. Stokes [1990] measured the swelling response of a typical carbon phenolic composite in the three primary material directions. The data obtained suggest that at low and high relative humidities, the incremental increase in moisture absorption can be attributed primarily to the resin whereas at intermediate relative humidities, water is moving largely into the carbonized fibers. Figure 4-8 (A) shows the mean equilibrium moisture content of each set of specimens from each conditioning chamber as a function of the relative humidity of the conditioning environment. A sigmoidal (shape of the alphabet S) relationship was found between the two variables. Figures 4-8 (B), (C) and (D) show the equilibrium linear swelling of each specimen of FM 5055 in the three primary orthogonal directions (warp, fill, and across ply) as a function of equilibrium moisture content of the material. Using regression analysis, a fourth-degree polynomial was fit to each of the equilibrated data sets as shown in Figures 4-8 (B), (C) and (D). The larger swelling in the across-ply direction, as opposed to the two fiber directions, is evident.
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Figure 4-8: (A) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen as a function on the humidity of conditioning environment (Stokes, 1990)
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Figure 4-8: (B) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in the Across Ply Direction (Stokes, 1990)
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Figure 4-8: (C) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in the Fill Direction (Stokes, 1990)
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Figure 4-8: D) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in the Wrap Direction (Stokes, 1990)
From the work as shown in figures 4-8 (A) to 4-8 (D), Stokes concluded that large increases in moisture content at intermediate moisture levels resulted in negligible increases in volume of the composite. Moreover, less than 20% of the water absorbed by the composite can be accounted for by the swelling of the material, which indicates a large free volume within the composite. The unaccounted volume of water absorbed by the fully saturated composite indicates that dry FM 5055 has a free volume of approximately 11.5% and an apparent density of approximately 1.61 g/cm3. It was proposed that the phenolic network polymer is responsible for the majority of the absorption of water at low and high moisture levels, whereas the fibers absorb most of the intermediate moisture content. These findings appear to indicate that there are multiple sites for the absorption of water in the cured resin and that these sites vary in their affinity for water.
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4.3.2.2.3 Effect of Temperature and Polymer Structural Variables on Sorption of Water Temperature
Temperature increases the rate of sorption. The diffusion coefficient has been found to increase with temperature according to the Arrhenius relationship [1981] D = D0 exp(− E / RT )
….(4.7)
where E is the activation energy for diffusion. Bonniau & Bunsell (1981) calculated E to be of the order of 11000 cal/mol for (Diglycidyl ether of bisphenol A) DGEBA epoxy cured with different hardeners. Marsh et al. (1984) in their experiments calculated E for neat resins and composites to be 9500 cal/mol and 9940 cal/mol, respectively. The two values seem to be consistent. In addition, the value of activation energy does not seem to vary too much between neat resins and epoxies glass fiber composites. Verghese et al. (1999) too found that the diffusion coefficient follows an Arrhenius relation with E/R= 4650 K. At higher temperatures (close to Tg), the sorption approaches that of the Fickian response.
However, under extreme conditions (i.e. temperatures close to boiling)
irreversible damage to the resin and composites was observed. Ishai [1975] found significant damage to the resin (shown by Scanning Electron Microscopic (SEM) studies of cut samples) and glass fibers at temperatures close to boiling. Significant damages included extensive cracking, debonding and degradation of the glass fibers. Bunsell and Dewimille [1983] studied sorption in DGEBA composites and found similar results. Significant damage was found to reduce fiber regions, which was attributed to imbalance of stress distribution. However, damage at lower temperatures was negligible. Samples immersed in distilled water at room temperature for over 3 years showed no damage to their structure. An important conclusion of their study was that accelerated aging could not be simulated by subjecting the resin to higher temperatures because under these conditions other phenomena (such as permanent damage to
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resin) take place. This study also led to the conclusion that at higher temperatures, other factors controlled the sorption process, i.e., cracking/crazing of the resin and degradation of the interfacial layer.
Presence of --OH group in Polymer
Interactions of water with polar groups within the polymer chain of vinyl ester resins have been confirmed through FTIR (Fourier Transform Infrared Spectroscopy) studies on saturated samples. FTIR studies made on vinyl ester resins before and after aging showed changes in the shape of the peak at 1450 cm-1, which was attributed to a stretching vibration of the carbon monoxide and the bonding of the primary alcohol OH bond. Verghese et al. demonstrated the influence of polar OH groups [1999]. Specific interactions through hydrogen bonding were observed in Derakane 441-400 vinyl ester resin composites (which had an OH group in its structure) as against their model resin in which the OH groups was substituted for a CH3 group. The model resin did not exhibit the thermal spiking phenomenon, while the saturation level of the resin was lower as compared to the Derakane 441-400 vinyl ester resin.
Nature of Water
Water may exist as either “bound” or “unbound” within the system.
The
existence was confirmed by 2H – NMR studies by Klotz et al. [1996] on diglycidyl ether bisphenol A resin cured with dicyandiamide, and/or diamino-diphenyl sulfone. Woo and Piggott [1987] observed that the effective dielectric constant of water is only 55-77% that of free water. Hence, the mobility of water within the polymer is between that of ice and free water. It was believed that water might cluster within a polymer, and water molecules from hydrogen bonds may become bound with polymer groups. However, contrary theories were postulated [Netravalli et al., 1984].
Changes in the Resin Structure
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Changes taking place within the polymer due to the sorption of water may be reversible or irreversible. Weitsmen [1988] and Wong et al. [1985] showed that slow rearrangement of the polymer chains takes place due to the ingress of the water molecule. This rearrangement of polymer chains causes changes in the free volume and hence the second and subsequent sorption is comparatively faster. Wong and Broutman [Part I and II, 1985] studied the sorption of water into EPON 828 resin (DGEBA cured with m-phenyldiamine/aniline) and concluded that Fickian sorption was taking place although the diffusion coefficient was concentration dependent.
They proposed that the water molecules caused polymer chains to
rearrange and did not cause any damage to the epoxy. However, this rearrangement was not permanent and the polymer collapsed to its original form when annealed or heated to temperatures greater than its Tg. When this sample was subjected to the sorption experiment after annealing, the behavior was found to be the same as the original sample. The researchers also observed that the post-cured sample did absorb more water. Due to additional cure, the resin had more fractional free volume and more–OH Groups, which caused increased water absorption. McMaster and Soane [1989] also found the second and third sorption to be faster than the first. The diffusion coefficient for one such result was first sorption, 3.5 x 10-9 cm2/sec and second sorption 4.2 x 10-9 cm2/sec. Hence the history of the polymer has a definite bearing on subsequent sorption. Marsh et al. [1984] reported that the resins retained some moisture on drying. The irreversible changes that may take place are cracking/crazing, and degradation of the matrix/fiber interface are shown by Ishai [1975], and Dewimille and Bunsell [1983]. They took SEM photographs, which clearly show extensive cracking attributed mainly due to imbalance in stress and degradation of the interface. In some cases, cracks developed in the fiber rich zones. This is because the water plasticized the resin and rendered the resin rich zones more resistant to damage. Netravalli et al. [1984] used (Differential Scanning Calorimetry) DSC technique to obtain the glass transition temperatures and curing energies of wet and dry samples. They concluded that the absorbed water can act as a plasticizer, but the effect was
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reversible. They also found that the curing energies progressively decreased as the amount of water taken in increased. The water was found to cause the unreacted epoxide groups to react, thereby causing some permanent changes in the resin structure.
Cure
The effect of curing temperature and time of cure has been reported by a number of researchers. In all cases, the amount of water sorbed has been found to increase with the curing. Wong and Boutman [1985] found this in their study of DGEBA/mphenyldiamine/aniline epoxy resin. Sahlin and Peppas [1991] found the post cured TGDDM/DDS resin to consistently sorb more water than the one without the post cure. The extent of cure of a resin can be found using Torsional Braid Analysis (TBA), Torsion Pendulum and FTIR (Fourier Transform Infrared Spectroscopy). The Tg of a sample increased as the time of an isothermal cure was increased and hence the extent of cure increased. Enns and Gillham [1983] cured DGEBA with stoichiometric amount of DDS at 175°C for four different times: 50, 100, 180 and 600 minutes. Six hundred minutes was not enough to fully cure the sample, as the Tg was below the Tg∞ (approx. 215 °C) of the fully cured sample.
The samples were then subjected to a humid
environment at 25 °C. Four different humidities were used, 31%, 51%, 79.3% and 93%. The result of the sorption experiments was that the sample with the highest percent of cure absorbed more water at all the conditions.
More cured resins exhibit higher
sorption because the highly cross-linked specimen has a lower density and consequently a greater free volume. Similarly, a greater cure leads to the formation of more –OH groups. A typical result is shown in Table 4-2.
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Table 4-2: Variation of Equilibrium Moisture Content with Degree of Cure Cure time
Tg °C
(minutes)
Density (g/ml)
D x 109 cm2/s
M∞
50
135
1.237
1.074
1.69
100
160
1.2357
1.136
1.7707
180
185
1.2339
1.65
1.58885
600
195
1.2334
1.737
1.7170
From the viewpoint of mechanical properties, it is desirable to have a fully cured polymer. This is because a partially cured resin has a lower modulus compared to a fully cured resin, and it displays more creep. The most common method of determining the extent of cure is with the help of a differential scanning calorimeter (DSC). During the course of a curing reaction, heat is liberated, and the instantaneous rate of energy evolution can be measured using a DSC. The extent of reaction, α , then is simply the ratio of the total heat evolved to the heat of reaction. When dα dt is plotted as a function of time, t, it is found (Gupta, 2000) that data can be described by the following equation.
(
)
dα n = k1 + k 2 αm (1 − α ) dt
…..(4.8)
in which the ks are temperature-dependent rate constants while m and n are temperature independent constants. White and Mather [1991] used an ultrasonic cure monitor technique to assess the simultaneous extent of cure and mechanical property development during the cure on an epoxy resin EPON 815/V 140 and compared the results with DSC monitoring. The modulus extent was derived and presented as a characterization parameter similar to the degree of cure in thermal cure characterization. The results have shown that the degree of cure does not accurately reflect the mechanical property development during cure. In comparison to the thermal degree of cure, the modulus extent shows that
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significant mechanical property development is still occurring in the later stages of cure when the degree of cure is fully developed. In other words, if the goal is to determine how close a sample is to being fully cured, the ultrasonic method is likely to provide the needed information while the thermal method may not provide such information. The former method is a non-destructive method.
Hardener
A hardener is used in the cure of an epoxy resin to cross-link the epoxy chains, thus giving structural rigidity. The commonly used hardeners are Diamino Diphenyl Sulphone (DDS), diphenyl diamine, dicyandiamine (DICY), anhydride and Lewis acid hardeners. Due to their polarity, hardeners influence the sorption of water. Sahlin and Peppas [1991] cured TGDDM resin with different amounts (5, 15, 25, 35, 45 wt %) of DDS. The sorption increased as the amount of the hardener was increased. Within a reasonable deviation, the increase was linear with the amount of water. The polarity of the hardener also had an effect. Diamant et al. [1981] attempted to keep the polarity of the resin a constant by replacing the diamine hardener with aniline in DGEBA (Figure 4-9). By keeping the polarity a constant, morphology of the polymer has changed as the length of the matrix between cross-linking increased, thus reducing the density of cross-links in the resin without affecting the polarity and increased chain mobility.
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Figure 4-9: Difference Between (a) Diamine and (b) Aniline Hardener
Contrary to expectations, the resin cured with only diamine hardener was observed to have the maximum sorption, which is attributed to (and confirmed by etching experiments) regions that had a greater cross-linking density, which surrounded areas of lower density. Higher cross-linking density caused hindrance to the movement of the water molecules and effectively reduced the sorption. However, the inherent polarity of diamine and aniline was not taken into account. Diamine is more polar than aniline and hence could have accounted for the increased uptake. In addition to polarity of individual hardeners, the presence of excess hardener increases the affinity of the resin towards water. A residual hardener, for example, DICY means greater affinity for water. DGEBA was cured with 5, 15 and 25 phr (parts per hundred parts of resin) of triethylene tetramine (TETA) at 100 °C. The results are tabulated in Table 4-3 [1982].
Table 4-3: Effect of Hardener on Equilibrium Moisture Content %TETA
Tg (dry) °C
Tg (Wet) °C
M∞ % (70 °C)
5
109
105
1.92
1.5
15
142
109
3.3
2.7
in DGEBA
100
M∞ % (20 °C)
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25
95
52
8.6
10.8
It is clear that the amount of water absorbed increased with the amount of the hardener and temperature. However there was a discrepancy from expected Tg. Thus, important parameters that influence sorption of water are:
•
Extent of cure, like curing temperature and time of cure.
•
Type of hardener used to cure the resin as well as the amount used.
•
Amount of free volume present in the resin.
•
Sizing agent (whether it forms a good bond or not between the matrix and fiber).
•
Environment like pH, temperature, etc.
•
Effect of Glass Reinforcement
Influence of glass fibers in polymers on the sorption of water can be determined by sorption experiments on neat samples as well as composites. The polymer resin used should be cured with the same hardener/catalyst as well as under the same conditions. Contradictory results have been obtained on the above subject. Marsh et al. [1984] studied the sorption of water in bisphenol A and cresol novolac epoxy cured with dicyandiamide. Neat resins and composites with 40% E-type glass were studied at 75°C /100% RH. The sorption of water in the glass composite was the same as that of the neat resin.
Both the neat resin as well as the glass composite showed an
intermediate saturation before the onset of residual moisture. These similarities led the authors (Marsh et al., 1984) to the conclusion that water did not enter the interface between the matrix and the fiber; hence, there was no difference in the sorption of neat resin and composites. On the other hand, Ishai [1975] showed that the behavior of neat resins was quite different from composites. In the case of diffusion of moisture into Epon 828 resin with E-glass fibers, significant damage was found not only to the
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interface but also to the glass fiber (confirmed by SEM pictures). The degradation was signaled by a drastic change in the sorption curve. Plotting the Relative Weight Change (RWC) Vs. Relative Length Changes (RLC) also showed the onset of degradation. The extent of degradation was more for samples exposed to extreme environments, i.e., temperatures of 80°C. Also the degradation of the interface was considerably less at lower temperatures i.e. 20°C. Similar results were obtained by Dewimille and Bunsell [1983], who found that composites (DGEB/Anhydride with E-Glass) degrade when exposed to water at high temperatures (80°C and above). Similar sorption studies were also made on vinyl ester glass fiber reinforced composites. Pai et al. studied the effect of glass fiber lay-up sequencing in various acidic environments [Parts I and II, 1997], using six different types of resins including vinyl esters. They found that the composite with the Chopped Strand Matrix exhibited least resistance to all liquids (water, 15%, 25% and 35% Sulfuric acid). Although the diffusion process became sluggish as the concentration of sulfuric acid increased, the saturation levels were much higher. They also studied the extent of degradation on the composite.
They assessed the fiber/matrix interface by performing an interlaminar
shear strength Test. The loss of the shear strength increased as the concentration of sulfuric acid increased, showing an increased rate of degradation with increased acidic pH. In order for a composite to function properly, there must be a chemical bond between the matrix and the reinforcing fibers so that the applied load (applied to the matrix) can be transferred to the fibers. In 'fiber glass' the fiber is inorganic while the matrix is organic, and these two do not bond readily unless the fibers are treated to modify their surface. Silica (SiO2) is hygroscopic, i.e., however slow it absorbs water onto its surface where the water breaks down into hydroxyl (-OH) groups. The coupling agent takes the form of a silane (R-SiX3), where R is an organic radical that is compatible with the polymer matrix and X is a hydrolisable organic group such as an alcohol. The most common silane couplant is tri-ethoxy-silane. Heat will force the elimination of water between the -OH pairs at the hydrated silica surface and the silane
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as well as between the adjacent silane molecules forming a strong bond between the matrix and the fibers as shown in Figure 4-10.
Figure 4-10: Bonds Between Glass Fiber and Coupling Agent.
If the bonds are chemical, then the presence of the glass will have a negligible effect. If, however, the bonds are weak and can be displaced by the hydrogen bonding due to water, debonding and degradation of the glass fiber can be pronounced. This was studied by Ritter et al. [1998].
They studied the propagation of a crack in
monolithic glass (soda-lime and fused silica) untreated glass-epoxy interface and glassepoxy interface sized with 2-amino propyl triethoxy silane (3-AMPS) using a double cleavage drilled compression test. The increase in resistance of the silane bonded epoxy interface is attributed to chemical bonding of epoxy to the glass via the coupling agent.
The rupture or
debonding will take place only by breaking of the Si-O-Si bonds formed between the glass and the silane-coupling agent. However, the highest threshold energy release rate, Gth for the soda-lime glass is lowered as the alkali molecules can bond with water and hence prevent the silane molecules from properly bonding to the glass surface. The Gth calculated is given in the Table 4-4.
Table 4-4: Calculated Values of Gth
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Gth Jm-2
Specimen Soda-Lime glass (SLG)
1.69
Fused Silica (FS)
2.76
Untreated SLG-Epoxy
0.25
Untreated FS-Epoxy
0.25
Silane treated SLG-Epoxy
1.32
Silane treated FS-Epoxy
3.31
4.3.3 Long term Mechanical and Hygrothermal Behavior (Aging) Polymers and composites used for the renewal of civil infrastructure will be exposed to complex infrastructure service environment conditions like range of combinations of stress, time, temperature, moisture, radiation, chemical, and gaseous environments and are expected to perform more than forty years. These materials should be required to go through a series of specifications based on inherent, and residual mechanical, physical, and thermal properties after accelerated service environment exposure conditions. The lack of understanding of the fundamental parameters controlling long-term materials performance necessarily leads to overdesign and in-service prototype evaluations and, furthermore, inhibits greater utilization. The determination of long term mechanical and hygro-thermal behavior of these materials and the understanding of the phenomena in relation to civil infrastructure renewal are critical to the further use of FRP composites in civil infrastructure.
4.3.3.1 Creep Theory Creep is defined as the continuous deformation (strain increase) over a prolonged period under constant load. i.e., when a polymeric material is subjected to a constant, sustained load, it deforms instantly, but then the deformation continues over a
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long period. This phenomenon of increasing strain is known as creep. It is a very slow plastic deformation process that occurs at stress levels below the yield point. Conversely, if a constant strain is imposed on a polymeric material, it exhibits instantaneous stress, but then the stress increases with increase in time; this is known as stress relaxation. It is a universal phenomenon and is exhibited by most materials, including metals. Polymers behave viscoelastically, and exhibit creep and stress relaxation to a high extent. Viscoelasticity arises because polymers are long-chain molecules, and under stress, parts of a molecule or even entire molecules can rearrange and slide past each other. This is especially easy above the polymer glass transition temperature Tg. Furthermore, creep and stress relaxation are more pronounced in thermoplastics than in thermosets; cross linking in thermosets restrict polymer chain mobility. The presence of fillers and reinforcements can further restrict creep. If the deformations are large enough, chain rupture may occur, particularly in thermosets where the chains are crosslinked into a network. Environmental factors such as temperature, moisture, and irradiation can all exercise their effects on molecular activity in the polymer, thus altering the macroscopic (creep) behavior. At high stress levels, this leads to relative slip between fiber and matrix. Rupture of fibers may also occur, resulting in higher fiber stresses in surrounding intact fibers, thus increasing elongation and rate of creep over time.
Constant Constant Strain dε/dt Primar
Secondar
Teritiray
y
y II
III
Time, t
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tf
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Figure 4-11: Typical creep behavior of plastics
The properties of viscoelasatic materials are dependent on time, temperature, and rate of loading; instantaneous test results cannot be obtained to define material response under sustained stress or deformation. Consequently, a sustained load is applied to a specimen in one of several standard configurations (such as tension, compression, or flexure) at constant temperature and the corresponding deformation is measured as a function of time. Since the deformations are small, strain gages are generally employed. In principle, the creep deformation should be linked to an applied stress. Thus, as the specimen elongates the cross sectional area decreases and the load need to be decreased to maintain a constant stress. In practice, it simpler to maintain a constant load. When reporting creep test results the initial applied stress is used. The effect of constant load and constant stress is shown in Figure 4-11. For FRP, a four-stage response is generally observed: 1. rapid initial elongation (ε0) of the specimen; 2. Rapid reduction in response rate (primary creep, stage I); 3. Steady state (secondary creep, stage II); and 4. A rapid increase in response and fracture (tertiary creep, stage III). In general this effect (dashed line for constant stress) only really manifests itself in the
tertiary region, which is beyond the
region of interest in the secondary region.
Creep and Relaxation
FRP with glass fibers is expected to have very limited creep in the longitudinal direction, as compared to the substantial creep in the transverse direction and under shear stress. The load in FRP composite materials is carried primarily by the fibers, which behave elastically. Thus creep and stress relaxation are not as significant in composites as in the bulk polymer matrix itself, particularly along the direction of fibers, which is typically designed to be the primary load path.
For
a
purely
elastic
material, the stress-strain behavior is not dependent on time, even if it exhibits a nonlinear stress-strain relationship. This is because stress is a unique function of strain.
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σ = Е0ε
……. (4.9)
ε = J0σ
…… (4.10)
This resembles the behavior of a spring. E0 is the elastic modulus and J0 is the elastic compliance. When a sinusoidal strain is applied, the strain and stress are in phase, meaning that they follow each other and related only by proportionality constant. For a purely viscous material, such as a Newtonian fluid, the stress is proportional to the rate of strain. σ(t) = η
dε dt
……(4.11)
When a sinusoidal strain is applied, the stress and strain are out-of-phase, i.e., when the strain is maximum, the stress is zero, and when the strain goes through zero, the stress is maximum. This resembles the behavior of dashpot. Polymers, as well as most real materials exhibit a combination of elastic and viscous responses. They can therefore be represented as combination of springs and dashpots (Maxwell’s model). From equations 4.10 and 4.11
εt =
σ (t ) E0
+
1
t
η ∫0
σ (r )dr
…..(4.12)
For a creep test, where a constant stress is applied at t=0, the compliance is found by integrating the above equation as
J (t ) =
1 t + E0 η
………………………....(4.13)
and creep increases passively with time. If the elastic and viscous parts are subjected to the same strain, the applied stress is the sum of the first part of Equations 4.10 and 4.11. This is a parallel combination of a spring and dashpot attributed to Kelvin
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model. For a creep test under constant stress, integrating for the strain, we get the compliance as: E 0t
− 1 J (t ) = [1 − e η ] E0
….. (4.14)
and creep is predicted to attain a constant value with increasing time. Clearly the creep behavior shown in the Figure 4-11 is more complex than what can be predicted based on the above equations.
Overview of Creep Models
Maxwell and Kelvin Models are used for representing the creep of thermoplastic and thermoset resins. If the deformations and stresses are small and the time dependence is weak, creep and stress-relaxation tests are essentially the inverse of one another. Therefore, to a first approximation stress relaxation data can be converted into creep by the following equation.
⎛ ε (t ) ⎞ ⎛ σ ⎞ ⎜⎜ ⎟⎟creep = ⎜⎜ 0 ⎟⎟relax ⎝ σ (t ) ⎠ ⎝ ε0 ⎠
… (4.15)
where, ε0 = initial strain in a creep test ε(t) = creep strain after time t σ0 = intial stress measured at the beginning of a stress-relaxation test σ = stress after time t
Maxwell’s Model
Maxwell’s model consists of a spring (Hookean) and a dashpot (Newtonian) in series as shown in Figure 4-12.
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Figure 4-12: Maxwell’s Model (Ref: http://www.matter.org.uk/matscicdrom/manual/pm.html)
The modulus of the spring is E, and the viscosity of the dash pot is η. In a stressrelaxation experiment, the model is given a fixed strain ε while the stress σ is measured as a function of time as shown in equation 4.16. dε 1 dσ σ = + =0 dt E dt η
….(4.16)
Since, ε = σ/E for the spring and σ/η=dε/dt for the dashpot, the solution of the equation is ⎛E⎞
⎛ −t ⎞ ⎟ ⎠
−⎜⎜ ⎟⎟ ⎜ σ = e ⎝ η ⎠ = e⎝ τ σ0
…(4.17)
where = حη/E is known as the relaxation time. Since a dashpot deforms instantly, all the initial deformation takes place in the spring; later the dashpot starts to relax and allows the spring to contract. Most of the relaxation takes place when t is close to .ح Mathematically, the stress relaxation can be written as ⎛ −t ⎞ ⎟ ⎠
σ (t ) σ ⎜⎝ τ Er = = e ε σ0
…..(4.18)
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Kelvin Model
Kelvin Model consists of a spring and dashpot connected in parallel as shown. In thiis the elastic and viscous parts are subjected to same strain, and hence the applied stress is the sum of the first part of (4.15) and (4.16). For a creep test under constant stress
Figure 4-13: Kelvin’s Model
E 0t
− 1 E(t ) = [1 − e η ] E0
…… (4.19) Neither the Maxwell nor the Kelvin model can describe the creep of thermosets. A more realistic model is the four-element model. Maxwell’s model does not describe the creep behavior completely. According to Maxwell model, as t goes to infinity the stress ratio becomes zero, which indicates that all creep and corresponding creep stress are recoverable. Whereas, in the four element described later, fraction if creep is not recoverable for the flow that occurred in the dashpot with viscosity η3, which models the actual behavior of polymers. Therefore, the Four Element Model accounts for the actual creep behavior.
Four Element Model
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Figure 4-14: Four Element Model
A four-element is shown in Figure 4-14. When a constant load is applied, the spring with modulus E1 deforms first. Later, the dashpot with the viscosity η1 deforms. Finally, it is the spring E2 and dashpot η2 that deform. The total elongation is the sum of the individual elongation of the three parts.
ε=
σ0 E2
[1 − e
−
E 2t
η2
]+
σ0 E1
+
σ0 t η1
….. (4.20) If at time t1, the load is removed, the spring with modulus E1 retracts instantly. The equation for subsequent creep recovery is:
ε = ε 2 e ( − ( t −t ) / τ + 1
σ 0 t1 η3
………
… (4.21)
where
ε=
σ0 E2
(1 − e (−t1 / τ ) ) ……. (4.22)
As tÆ ∞, it is found that the creep of dashpot with viscosity η1 cannot be recovered. Equation 4.22 is shown in the Figure 4-15:
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Figure 4-15: Behavior of Creep and Stress Relaxation in Four Element Model
Master Curves
In order to compute the spectrum of relaxation or retardation times, experimental data covering about 10-15 decades of time are needed. This is a very fine consuming proposition. In practice, the data can be obtained in short intervals but over a series of constant temperatures, and then they can be superimposed to extend the time scale of measurement. In this task, Boltzman superposition principle and the principle of timetemperature superposition are employed. According to the Boltzman superposition principle, the response of a material to a given load is linear and additive. For the case of creep, if there are several stresses σ0, σ1, σ2, σ3, σ4,……… σi applied at times 0,t1, t2, t3, t4,…. ti, the total creep is
ε (τ ) = J (t )σ 0 + J (t − t1 )(σ 1 − σ 2 ) + .... + J (t − t i )(σ i − σ i −1 )
…..(4.23)
where the creep ε(t) at time t depends on the compliance function J(t), which depends on temperature. Figure 4-16 illustrates the behavior for a polymer that obeys the following equation 4.21.
ε (τ ) = Kσt n
….(4.24)
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Figure 4-16: Behavior of Creep when subjected to series of stresses
where K and n are temperature dependent constants. It describes a system when subjected to a series of stresses σi and ti, creep is given by the sum of identical responses. It has also been found that the creep curves made at different temperatures can be superposed by horizontal shifts along a logarithmic time scale to give a single creep curve. This procedure was originally suggested by William, Landel and Ferry (WLF), and the result is called a master curve. The extent of shifting is given numerically by an equation called the WLF equation, which holds between Tg or Tg+10K and about 100K above Tg. Above the upper limit of applicability of the WLF equation, one may use an Arrhenius equation with a low activation energy. Note that master curves can be made from stress relaxation data, dynamic mechanical data, or creep data. Sometimes a vertical shift may be needed in addition to the horizontal shift and the vertical shift may be needed in addition to the horizontal shift and the vertical shift may depend on temperature. Aging and heat treatments may also affect the shift factors. For these reasons the vertical shift factors are largely empirical.
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Creep and Relaxation in Composite Laminates
Correspondence Principle: To find the relaxation corresponding to the compliance of the Maxwell model, the correspondence principle is used. The correspondence principle is valid for a linearly viscoelastic material, i.e., non-linearity is in time, not on stress. Stress only magnifies the deformations in a linear, proportional way. According to the correspondence principle, any viscoelastic problem in the time domain can be solved as an elastic problem in the Laplace domain. By transforming all the equations to the Laplace domain, it reads
…(4.25)
J(s) s =1/ ( E(s) s)
where s is the Laplace variable. Taking the Laplaces of the Maxwell model equation, multiplying by s2 and inverting we get the Laplace transform of the relaxation
E(s) = E/ ((E/η) + s)
…(4.26)
Taking the inverse Laplace transform, the relaxation time domain is
E (t) = E0e-(E0/η) t
…..(4.27)
If the material follows a Maxwell model, this equation should model well the data of relaxation test. The Kelvin model does not have a simple relaxation equation because the stiffness at t=0 is infinity, and because the stress is applied suddenly to the viscous component. Increasingly complex equations can be used in order to fit the experimental data better, but the number of parameters and the complexity increases accordingly. The concept and methods of analysis remain the same.
Unidirectional Composites
To predict the creep behavior of unidirectional lamina micromechanics is used. Since all the micromechanics models are formulated for elastic fiber and matrix, they
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must be used in Laplace domain. Models that include empirical adjustable parameters cannot be used because the time dependence of the adjustable parameters is not known, so it is not possible to take the Laplace transform of them. Luciano and Barbero [1995] used micromechanics of periodic microstructure to find analytical, complicated expressions for the creep and relaxation tensors of unidirectional composites. From these, they obtained all the creep compliances and relaxation functions in all directions for the composite, such as E1, E2, G12, G23 and all the Poisson components. All that is needed are the elastic properties of the fibers, the fiber volume fraction, and representation of the creep behavior of the matrix. Simple micro-mechanics formulas such Halpin -Tsai cannot be used because they contain adjustable parameters. Since the properties of the matrix often differ from the bulk properties, Harris and Barbero [1998] suggested that creep tests be performed on the composite to back-calculate the creep behavior of the matrix. This has not been done to present. On the contrary, Harris and Barbero, as well as many others, struggled to perform creep tests on bulk matrix.
Laminated Composites
The approach for prediction of laminate creep is to use classical lamination theory in the Laplace domain. Harris and Barbero [1998] used such a method to predict composite behavior of various laminates. Although the concept is simple, the equations are too complicated for hand calculations. Instead, a simple program or spreadsheet can be used.
4.3.3.1.1
Effect of moisture and temperature on Creep
Moisture and time, like temperature and time, often have an interchangeable effect on the creep behavior of polymers. Moisture absorption in polymer-matrix composites will result in the development of residual stresses and will plasticize the resin. Both of these effects can accelerate the time-dependent behavior of material. The effects of time, temperature and moisture on the creep compliance are illustrated
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schematically in the Figure 4-17. Usually, creep compliance is increased with an increase in moisture content and temperature over time. Although there are some studies on creep-rupture for pultruded FRP in fluids, data on viscoelastic behavior of the materials under the influence of fluid absorption are scarce. Moreover, the moisture absorption level is history-dependent, and therefore sorption behavior under temperature and/or humidity cycling is not the same as under a constant humidity and temperature level.
Figure 4-17: Schematic representation of effects of time, temperature, and moisture on creep compliance. (Liao,1998)
When moisture is absorbed in a polymer-matrix composite the resin is plasticized. If the resin swells, stress will be generated. The first of these effects will soften the polymer and increase creep. Parasyuk et al. [1987] investigated the effect of water immersion on compressive creep of uni-directional glass-reinforced composites. In the dry-condition, creep ceased after sometime when the applied stress was low. However, when the applied stress exceeded 85% of the ultimate strength, a steady state creep rate (dε/dt=constant) was reached. In liquid media, similar behavior was observed but considerably lower stress levels compared to tests conducted in air. Furthermore, at a given stress level, the rate of creep increased with immersion in water. This was attributed to the redistribution of
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stresses between glass fibers and the polymer matrix, and between damaged and undamaged reinforcing elements. These general conclusions remained unchanged when data were obtained at higher temperatures. Wang and Wang [1980] have investigated the influence of moisture, temperature and stress on the tensile creep behavior of Scotchply 1002 glass/epoxy composites. Unidirectional laminates were loaded at angles 00, 450, and 900 with respect to the fiber direction. At from temperature with low stress 6.2 MPa and moisture content (mass fractions from 0.5% to 0.94%), 00 laminates exhibit minimal creep compared to the 900 and 450 laminates, but exhibited a significant increase in creep deformation when the temperature was increased to 1020C. Creep of the 450 and 900 laminates was also strongly influenced by moisture and temperature, even at low stress levels.
Figure 4-18: Moisture Absorption Behavior (adapted from Weitsman [116])
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Moisture absorption behavior of composite materials can be categorized into several types, as shown in Figure 4-18. Curve LF represents linear Fickian behavior, where after a rapid initial take-off the moisture weight gain gradually attains equilibrium. Glass fiber reinforced plastics (GFRP) exhibit such behavior under specified conditions. For instance, E-glass/vinyl ester with acryl-silane or epoxy silane surface treatment follows linear Fickian behavior for water absorption up to 80°C. Curve A represents pseudo-Fickian behavior where the moisture weight gain never reaches equilibrium after the initial take-off. Curve B is two-stage diffusion with an abrupt jump in moisture weight gain after initial take-off. The cause for the jump is attributed to a change of environment such as temperature, applied load, or relative humidity. This is due to the fact that there are some periodic changes in the aforementioned environments. Curves C and D represents the most adverse situation pertained to the material performance. The rapid moisture weight gain depicted by curve C results from large deformation or damage of the material, for example, fiber/matrix debonding and matrix cracking, which are often irreversible. For curve D, moisture weight gain follows a decreasing trend after the initial take-off, also an irreversible process as a result of leaching out of the material from the bulk following chemical or physical break-down. Sorption process involving severe circumstances such as elevated temperatures, external load, and high solvent concentration will often result in behavior described by curves C or D. For instance, glass/polyester at 40°C and under an applied load of 50% ultimate tensile stress displayed type D behavior; S-glass/epoxy followed curve C in water at above 80°C. In general, the moisture absorption behavior depends on temperature, applied load, type of media, time, and material system, and is inseparable from other performance aspects concerning durability. The rate of fluid sorption and the quantity absorbed are governed for the most part by the chemical structure of the resin, the degree of cross-linking, the type of crosslinking, and presence of voids. As a consequence, the diffusion process can be actively controlled by using a matrix with lower uptake or lower permeability, a hybrid matrix composite.
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4.3.3.1.2
Effect of Physical Aging on Creep
Physical aging is a process, occurring below the glass transition temperature Tg, where the macromolecules gradually change their packing in order to approach the equilibrium free volume state. The gradual approach towards equilibrium affects the mechanical properties of the polymer, often resulting in a material that is stiffer and more brittle, so that the compliance is decreased (or the modulus increased) than expected in a viscoelastic material without aging. In terms of free volume theory, it can be visualized that as the free volume decreases towards its equilibrium values; the mobility of chain segments is hindered, giving rise to a stiffer response. Aging is a characteristic of the glassy state and is found in all polymer glasses. The effects of physical aging continue until the material reaches volume equilibrium. The time required to reach volume equilibrium depends on the aging temperature. During this time the mechanical properties may change significantly. Effects of physical aging on long-term performance of FRP could be substantial, especially when the material is subjected to an aging time as long as 50 or more years, typical for infrastructure. In general, as polymeric materials physically age, they become stiffer and more brittle, show less stress relaxation, or delayed increases in creep compliance. In some cases this may be “desirable” if the changes are understood and predictable.
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Figure 4-19: Effect of physical again on creep behavior.
Physical aging can lead to changes in the short-term as well as long-term mechanical properties. Effects of physical aging on long-term performance of FRP can be substantial, especially on aging the material for 50 years or more. This is because the materials become glassier. In general, as polymeric materials physically age, they become stiffer and more brittle, show less stress relaxation, or delayed increases in creep compliance. In some cases this may be “desirable” if the changes are understood and predictable. It is found that the short-term creep compliance curves shift to longer times with increasing initial aging time, and then the long-term compliance decreases relative to simple extrapolation of the short-term compliance. In addition, the process of physical aging is found to be both temperature and load dependent. This phenomenon has been investigated extensively in the recent past, and it is well understood.
4.3.3.1.3
Effect of Ultraviolet (UV) Radiation on Creep
Ultraviolet radiation that reaches the earth’s surface comprises about 6% of the total solar radiant flux and has wavelengths between 290 nm and 400 nm. Radiation below approximately 290 nm is effectively eliminated by stratospheric ozone. The remainder of the solar radiation is composed of visible (52%) and infrared (42%) radiation. Since most polymers have bond dissociation energies on the order of the 290
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nm to 400 nm wavelengths in the ultraviolet region, they are greatly affected by exposure to this portion of the solar spectrum. Chemical changes induced by UV exposure are the result of a complex set of processes involving the combined effect of UV and oxygen. Bond dissociation is initiated by the absorption of UV radiation, resulting in chain scission and/or cross linking; subsequent reactions with oxygen result in the formation of functional groups such as carbonyl (C=O), carboxyl (COOH), or peroxide (O-O). The effects of UV exposure, or photo degradation, are usually confined to the top few microns of the surface. However, in some cases, degradation at the surface of a polymeric component has been shown to affect mechanical properties disproportionately, as flaws that result from surface photo degradation can serve as stress concentrators and initiate fracture at stress levels much lower than those for unexposed specimens. The effect of ultraviolet radiation is also compounded by the action of temperature, moisture, windborne abrasives, freeze-thaw and other environmental components. The effect of UV radiation on the creep behavior of different polymeric materials was investigated by Regel et al. [1967].
By irradiating a loaded specimen in the
wavelength interval from 248.3 to 300.0 nm, they found that during the time that the UV radiation was turned on, creep strain increased sharply. On turning off UV radiation, the creep rate returned to the original value, showing that the creep-rate change was reversible. This increase occurs at each and all steps of creep and is fairly general, as it is exhibited by a large number of polymers. Regel et al. (1967) suggested that the increase in creep of exposure to UV radiation was the result of increased stress relaxation or easier relative movement of polymer chains.
4.3.3.2 Fatigue and Fracture Fatigue is defined as the failure or rupture of a plastic article under repeated cyclic stresses, at a point below the normal static breaking strength. Fatigue failure occurs when a specimen has completely fractured into two parts, has softened or has
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otherwise significantly reduced in stiffness by thermal heating or cracking. It can also be arbitrarily defined as having occurred when the specimen can no longer support the applied load within the deflection limits of the apparatus. Fiber Reinforced composites have a rather good rating regards to life time in fatigue. The same does not apply to the number of cycles to initial damage nor to the evolution of damage. Composite materials are inhomogeneous and anisotropic, and their behavior is more complicated than that of homogeneous and isotropic materials such as metals. The main reasons for this are the different types of damage that can occur (eg. Fiber fracture, matrix cracking, matrix crazing, fiber buckling, fiber matrix interface failure, delaminations,…), their interactions and their different growth rates. The following are the parameters that influence the fatigue performance of composites
• Fiber type • Matrix type • Type of reinforcement structure (unidirectional, mat, fabric, braiding..) • Laminate Stacking Sequence • Environmental conditions (mainly temperature and moisture absorption) • Loading conditions (stress ratio R, cycling frequency …) and boundary conditions
There are a number of differences between the fatigue behavior of metals and fiber reinforced composites. In metals, the stage of gradual and invisible deterioration spans nearly the complete lifetime. No significant reduction of stiffness is observed during the fatigue process. The final stage of the process starts with the formation of small cracks, which are the only form of macroscopically observable damage. Gradual growth and coalescence of these cracks quickly produce a large crack and final failure of the structural component. As the stiffness of a metal remains quasi unaffected, the linear relation between stress and strain remains valid, and the fatigue process can be simulate d in most of the cases by a linear analysis and linear fracture mechanics.
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In fiber-reinforced composite damage starts very early and the extent of the damage zones grows steadily, while the damage type in these zones can change (e.g., small matrix cracks leading to large size delaminations). The gradual deterioration of a fiber reinforced composite-with a loss of stiffness in the damaged zones – leads to a continuous redistribution of stress and reduction of stress concentrations inside a structural component. As a consequence an appraisal of the actual state or a prediction of the final state (when and where final failure is to be expected) requires the simulation of the complete path of successive damage stress. Studies comparing glass and carbon fibers suggest carbon fiber composites are superior in fatigue performance in terms of fatigue life and rate of damage development. The matrix also influences fatigue performance at a low number of cycles. It was found that the influence of the matrix on the quasi-static properties is on the position of the knee point in the stress-strain curve. It is a quantity defined by the intersection of two linear parts on the stress strain curves. Fatigue loading above the knee point results in degradation of stiffness followed by sudden drop in stiffness leading to the death of the composite material and loading below knee point is the no-failure situation.
Low
deflection fatigue is matrix and interface dominated while high deflection levels include matrix cracking, fiber matrix debonding, glass fiber fracture at the tensile surface, and plane fiber buckling and delamination on the compressive side. Fatigue behavior of composite laminates is both frequency and temperature dependent. Higher frequencies and higher temperatures tend to reduce the fatigue life of the composite material. The effects of elevated temperatures, humidity, and other corrosive fluids (such as acids) on FRP are to shorten their fatigue life, compared to those fatigued without imposed environment. Jones et al. (1983) noted that the fatigue degradation rate of GFRP preconditioned in boiling water is slower compared to dry specimens.
The
authors attribute this phenomenon to several factors; first, Plasticization of the resin may result in an increase in the long life fatigue strain in the resin.
Second,
preconditioning in boiling water permitted relaxation of the thermal strains introduced during processing. Third, stress transfer capacity of the resin-fiber interface will be reduced by boiling, which in turn will reduce the stress concentration in the vicinity of the broken fibers or resin micro-cracks.
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Romans et al. (1971) have shown that the susceptibility of different epoxies to water is different, which suggests that the effect is matrix and/or interface controlled and there is a close relationship between chemical structure and environmental susceptibility. Specimens exposed to water have reduced fatigue life and is attributed to the combined effects of mechanical fatigue and localized pressure exerted by water trapped in the micro cracks. The fiber matrix interface region has a controlling effect on the environmental fatigue of composites. The rate of reduction on the off-axis specimens in water is higher than the unidirectional specimens, implying direct impact of water on the interphase region. It has been indicated that fluid absorption during fatigue is faster than under static condition, and there is a dynamic interaction of fatigue behavior and the environment.
4.3.3.2.1
Fatigue Process
Unlike homogeneous materials, FRP composites accumulate damage in general rather than developing localized damage, and fracture does not always occur by propagation of a single macroscopic crack.
The damage accumulation in these
materials is micro structural, which includes fiber/matrix debonding, matrix cracking, delamination and fiber fracture [Mathews, 2000]. unidirectional
composites
primarily
depends
on
Fatigue damage mechanism in loading
mode
(e.g.,
tensile,
compressive, bending, torsion or combinations) and on the loading direction i.e., parallel or inclined to the fiber direction. In unidirectional fiber reinforced composites, fractures in fibers occur but the accumulation is slower and the life of the composite is not dependent on fatigue fractures in fibers. But matrix micro cracks transverse to the loading axis develops and propagates, thus breaking fibers or causing interfacial failure, leading to the failure of the composite. In uni-directional composites, fatigue damage is initiated by debonding between fiber and matrix. Typically, the damage mechanism in tensile fatigue is of three stages as shown in Figure 4-20 [Talreja, 1987] namely: Fiber breakage Matrix cracking
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Interfacial shear failure
Figure 4-20: Fatigue Damage Mechanism in Unidirectional Composites Under Loading Parallel to Fibers: (a) Fiber Breakage, Interfacial Debonding; (b) Matrix Cracking; (c) Interfacial Shear Failure [Talreja, 1987]
Mechanical fatigue is the most common type of failure of structures in service. The fatigue behavior of composite materials is conventionally characterized by a Wöhler or S-N curve. For every new material with a new lay-up, altered constituents or different processing procedure, a whole new set of fatigue life tests has to be repeated for such a characterization. If the active fatigue damage micromechanisms and the influence of the constituent properties and interface were known, it would be possible, at least qualitatively, to predict the macroscopic fatigue behavior. A study of the fatigue damage mechanisms would also give indications of the weakest microstructural element, which is useful information in materials selection for improvement in service properties. In tensile fatigue of a multidirectional laminate, the critical elements are the longitudinal plies, which are the last to fail. Fiber breakage is due to the failure of the weakest fiber in the laminate due to excess stress, which causes shear-stress concentration at the interface i.e., close to the tip of the broken fiber, leading to debonding of the fiber from surrounding matrix. The debonded area leads to matrix cracking when the stresses exceed the fatigue limit. Under low strains, approximately 50% of ultimate tensile strain of matrix, a matrix crack stops at the interface. However, at high strains, the stresses at crack tips exceed the fracture stress leading to fiber pullout or breakage of adjoining fibers due to higher stresses. Strength degradation is assumed also to take place in these two stages reflecting the development of the underlying damage process.
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In the first stage, a general weakening of the material is assumed and is considered dependent on damage parameters representing the stage of damage. A power law is assumed for the rate of increase of the damage parameter [Talreja, 1987], taking it as a function of an effective stress.
A relationship between the residual
strength (R) and the initial strength (R0) is then given as: ⎛ N R = R c + (R 0 − R c )⎜⎜ 1 − ⎝ Nc m' =
Nc =
⎞ ⎟⎟ ⎠
m'
…(4.28)
1 (1 + m)
…(4.29)
1 k (1 + m )S m
…(4.30)
where Rc, Nc - residual stresses and number of cycles at the CDS, respectively; m’, m, k - material constants, and S- maximum applied stress, respectively. In the second stage, strength degradation is assumed to result from the localized zones of damage which are conceptually replaced by a single crack capable of releasing the same amount of elastic energy as that released collectively by the various crack growth mechanisms.
Residual strength (R) is related to a characteristic
dimension of the "equivalent” crack C through a fracture mechanics type relationship R = αC −1 / 2
…(4.31)
where α is the material constant characterizing material toughness [Talreja, 1987]. The evolution of damage is expressed by the rate of growth of the crack dimension, which is assumed to depend on the current state of damage given by the current crack dimension. Assuming a power law for the crack growth, a relationship between residual strength and the applied maximum stress is derived. This relationship forms the basis for determining the probability distribution of the residual strength and the probability distribution of the number of cycles to attain the Characteristic Damage State.
The complete two-stage strength degradation model for fatigue reliability
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analysis of composites is illustrated in Figure 4-21 where R is the initial strength, Rc and Nc are the residual strength and the number of cycles corresponding to CDS, S is the maximum applied stress and Nf is the number of cycles to failure.
Figure 4-21: Two-Stage Strength Degradation Model for Fatigue Reliability of Composites [Talreja, 1987]
4.3.3.2.2
Fatigue in Unidirectional Composites
The S-N curve for carbon fiber, glass fiber and aramid fiber in the same standard epoxy matrix is shown in Figure 4-22. The use of stiff fibers such as carbon fibers results in low strains (1.0 - 1.8%) to failure and less stiff fibers like glass lead to relatively higher strains (2.5 – 3.5 %) to failure. Hence, the curve is steep for glass fibers while it is shallow for carbon fibers in Figure 4-22. The slope of the curve (Figure 4-21) is a function of the strain in the matrix [Curtis and Dorey 1986]. The S-N curve for carbon fibers with different stiffness in the same standard epoxy resin is shown in Figure 4-21.
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Figure 4-22: Comparison of S-N Curve for Three Different Unidirectional Composite Materials [Curtis and Dorey, 1986]
It can be seen that there is little improvement in the fatigue behavior with change in fiber stiffness. This is because the fatigue behavior of composites is dependent on the strain in the matrix and interfacial characteristics rather than fiber strength. Due to this reason, plots of mean strain rather than stress versus log cycles to failure are commonly used for composite materials (Figures 4-22 and 4-23).
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Figure 4-23: Comparison of S-N curve for Four Different Materials with Different Carbon Fibers in Same Epoxy Resin [Curtis and Dorey, 1986]
A typical fatigue life diagram (Figure 4-24a) for a unidirectional composite under loading parallel to fibers is shown. In Figure 4-24a, fatigue limit of the matrix is defined as the maximum strain below which no cracks or only non-propagating cracks maybe initiated in the matrix material. This matrix material property is taken as the lower limit of the progressive matrix damage. It can be seen that as the fiber stiffness reduces, distinct progressive damage band (matrix cracking) is observed before fiber breakage.
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Figure 4-24: Fatigue Life Diagram of Unidirectional Composites Under (a) Loading Parallel to Fibers, (b) Off-Axis Loading (Dotted line correspond to on-axis loading) [ Talreja, 1987]
For off-axis loading angles between 0o and 90o, the tip of crack initiated in the matrix will be subjected to two displacement components, i.e., an opening normal to the fibers and a sliding parallel to the fibers. This leads to a mixed mode crack growth parallel to the fibers. The limiting values of crack tip displacement will depend on the off-axis angle; with crack tip displacement increasing with an increase in off -axis angle. The fatigue life diagram for off-axis loading is shown in Figure 4-24 (b). It was found that for off-axis angles more than a few degrees, the fiber breakage bond would be lost, as matrix and/or interfacial cracking will become the predominant damage mechanism for strain up to fracture strain. [Talreja, 1987]
4.3.3.2.3
Fatigue in Multidirectional Composites
The damage mechanism in multidirectional composites is similar to off-axis loading in unidirectional composites except that delamintion is found to occur in these laminates [Talreja, 1987]. In multidirectional composites, the first event of failure is debonding of transverse fibers.
The debonded crack then grows towards the ply
interface causing stress concentration in the interfacial layers.
This leads to
delamination and hence overstressing of the 0o ply; thus, the increase in the number of non-axial plies in a laminate reduces the strength and stiffness since only fewer fibers
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are available to support the mean applied stress in the axial direction [Mathews, 2000]. For a multidirectional laminate, Figure 4-25 gives the comparison of S-N curve for varying percentage of 0o plies Angled ply layers with fibers typically at ±450 can also develop intraply damage, which causes a small reduction in strength and stiffness [Curtis and Dorey, 1986]. Also stress concentration is developed at the ends of intraply cracks, which causes delamination between the layers. Multidirectional laminates also develop edge-induced stresses because of different elastic properties of the layers, often giving rise to delamination between layers.
Figure 4-25: Normalized S-N Curves for (0/±45) CFRP Laminates with Varying Percentage of 0o Fibers [Curtis and Dorey, 1986]
4.3.3.3 Aging Due to Environmental Factors Several researchers have investigated the changes in properties and performance of composites under various environments, when they are manufactured with different constituent materials. In the following sections, an extensive review of the
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literature dealing with durability/aging of composites is carried out. Special attention is paid to understanding the existing experimental and analytical methods to qualify and predict the effects of constituent materials and environmental conditions on composites.
4.3.3.3.1 Environmental Factors Influencing the Durability of Composites Environmental factors such as moisture (water, humid air or liquid), temperature (free-thaw cycling, elevated temperature, fire) and other weathering conditions (physical, chemical, and UV rays exposure) affect the performance of both neat resin and composite materials. The physical weathering condition occurs when either the neat resin or a composite material is subjected to mechanical loadings such as static load, fatigue and creep. The chemical weathering condition occurs when either the neat resin or composite materials is exposed to chemical solutions such as alkaline, acid or aqueous.
When composite materials are subjected to these environmental factors,
mechanical and chemical properties such as strength, stiffness, creep, fatigue life, glass transition temperature, and interfacial bond strength of the composite material change with exposure time.
Degradation of mechanical properties depends mainly on: 1)
chemical and physical structure of the polymer, 2) physical state of the material, 3) additives, 4) time and temperature, 5) moisture and pressure, and 6) nature of stress [Kilen, 1983]. Composite material property changes over a long service life (» 50 years) can be predicted by experimental methods simulating the environmental conditions in the laboratory.
The moisture and temperature distribution inside the composite
materials, commonly known as “moisture problem,” can be predicted by analytical methods such as Fick’s diffusion law [Springer, 1981].
4.3.3.3.1.1 Effect of Moisture The effect of moisture on composite materials occurs when the composite materials are exposed to humid air, water or any liquid.
Depending on the
environmental conditions and the condition of the material, the material either absorbs
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or loses moisture as manifested by weight gain or weight loss [Springer, 1981]. Moisture absorption and desorption in a composite material eventually leads to the degradation in its properties. Moisture affects the composite materials at the matrix or the fiber level, and even at the interface of fiber/matrix level.
Effect of moisture in resins
Moisture content in resins depends on various factors such as type of resins, fillers, additives, nanomers, type of liquid to which the resin is exposed to, temperature, etc. First, moisture penetrates through the resin and later moisture is transferred through the cracks [Springer, 1981]. Neat resins show higher moisture absorption than the composites show, because in the resins the matrix swells when exposed to moisture. The swelling of the matrix causes stress within the material, which eventually tends to decrease the strength and stiffness. However, many resin systems tend to recover their properties upon drying. Further, the percentage of moisture absorption varies from resin to resin based on their chemical structures. Chin [1999] observed the moisture uptake for vinyl ester and isopolyester resin exposed to distilled water, salt water and concrete pore solution at 22oC and 60oC.
For vinyl ester (at both ambient
temperature and 60oC), salt-water uptake was higher than pure aqueous or alkali solution uptake. Similar results were observed in the case of isopolyester at ambient temperature, but at 60oC, mass loss occurred after a certain period in alkaline solution and salt water. This is attributed to the fact that in a polyester resin, ester groups are distributed along the main chain, making them more available to hydrolysis reactions at higher temperatures. The structure and morphology of a resin affect the moisture uptake.
In general, a high concentration of polar functional groups can promote
increased sorption of polar penetrants [Apicella et al., 1982]. Among epoxy, vinlyester, and isopolyester resins, the sorption was greater for epoxy compared to the other two resins because more hydroxyl groups are present in epoxy matrix. Most of the resins or composites follow the Fickian process, and a non-Fickian behavior occurs when the resins or composites undergo damages such as cracks. Diffusion in all three liquids (water, salt, alkaline) and in three resins (epoxy, vinyl ester and isopolyester) followed the Fickian process [Chin, 1996].
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Effect of moisture in glass fibers
Studies conducted by Ehrestein and Spaude [1984] showed that both glass fiber and glass fiber-resin bond are susceptible to degradation through moisture content. Glass fibers can lose up to 10% of their bending strength when exposed to moisture. While resins recover their lost properties while drying, glass fibers do not recover their properties but tend to corrode, eventually leading to loss of effective cross-section. Results from a companion study of Karbhari et al. [1998] show that water accelerates the rate of crack growth in glass with the degradation being more severe and following different mechanisms with higher temperature exposures. Acceleration is thought to be the result of two factors: 1) reduction in surface energy of glass fibers after exposure to moisture that reduces the energy required for interfacial crack formation, or debonding and 2) reduction in energy required to break the Si-O bonds. The strength of E-Glass is time dependent in the presence of moisture and is susceptible to stress corrosion. Stress corrosion in turn is dependent on the type of attacking fluid, wherein more concentration of corrosive fluid leads to greater detriment to glass fibers.
Effect of moisture in composites
Moisture content in composites depends on the type of composites and environmental conditions that the composite is exposed to over a certain period and range of temperature.
When composites with polymer matrix are placed in a wet
environment, the matrix begins to absorb moisture. The moisture absorption of most fibers used in practice is negligible; however aramid fibers alone absorb significant amount of moisture when exposed to high humidity [MIL-HDBK-17, 1997]. The effect of temperature and moisture on Kevlar/epoxy laminates was studied by Allred [1984], who showed that in the saturated state, the room temperature flexural strength of the laminates decreased by 40% over the dry state. Similarly, at an elevated temperature, the strength drop was found to be in the range of 60 – 70%. When composites are exposed to humid air, the moisture content depends on the relative humidity in the air. When composites are exposed to acidic or alkaline liquids, the moisture content depends on the type of the attacking liquid. The maximum
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moisture content in the Graphite/epoxy composite was found to be lower in salt solution (1.25%) than in distilled water [Springer, 1981]. A similar trend was noted in the Eglass/vinyl ester composite with maximum moisture content of 0.18 ~ 0.29% in the salt solution compared to the moisture content of 0.22 ~ 0.33% in the tap water [Vijay, 1998].
The maximum moisture content was nearly twice in the alkaline solution
compared to tap or salt water. This hypothesizes that the degradation of the material has started at the interface of the fiber/matrix level. In the case of E-glass /polyester composites, the interface tends to become more hydrophilic when exposed to moisture and the following is noted: 1) The fibers weaken due to crack growth that is accelerated by water in the resin; 2) The resin swelling produces radial stresses at interface that is reinforced by water pressure, and leads to fiber debonding and consequent weakening of composite; 3) plasticization of resin by water results in increase in viscoelasticity. When a composite absorbs moisture, the swelling coefficient of fiber is lower than the matrix. Free swelling of layers does not take place and, consequently, internal stresses are developed. These internal stresses can be calculated as in MIL-HDBK-17 [1997].
4.3.3.3.1.2 Effect of Temperature Temperature plays a vital role in the durability of composites.
Temperature
affects the rate of moisture absorption and chemical and mechanical properties of the composites. The magnitude of these temperature effects depends on the type of liquid to which the composite is exposed.
The mechanical and visoelastic behavior of
composites could degrade dramatically at elevated temperatures and under freeze-thaw cycles, which is discussed below.
Elevated temperature
When the fiber reinforced composite materials are exposed to elevated temperature, mechanical properties such as modulus, tensile and flexural strength, fatigue strength and creep resistance and adhesive strength may decrease [Kelen, 1983].
Composite materials generally degrade when exposed to moisture, but the
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degradation is further enhanced at elevated temperatures mainly due to matrix degradation. An increase in temperatures may also cause time-dependent effects such as creep and stress relaxation [Janas and McCullough, 1987], whereas low temperatures may result in brittle failures.
Vinyl ester composites aged at high
temperatures in an aqueous environment showed an overall decrease in strength with an increase in the duration of conditioning at a given condition, particularly at higher temperatures [Buck, 1998]. At an elevated temperature, alkaline solution will be the most damaging solution to glass composites and also to the neat resins in terms of tensile strength. The alkali affects the fiber/matrix debonding as well as the fibers. As the temperature increases, the pH value of an alkaline system also increases, thereby increasing the corrosiveness inhibited by the alkaline solution.
At 99oC the alkaline
solution was found to be more corrosive and weight losses found to be about two to five times greater at temperatures higher than at 66oC.
Freeze-thaw cycles
The excess moisture in composites generally expands upon freezing and causes internal stresses, which initiate cracks or delaminations. Due to the formation of cracks, the mechanical properties of a composite degrade largely under freeze-thaw cycling. For example, the ultimate tensile strength of Kevlar laminates was found to decrease by 23% after 360 cycles and by 63% after 1170 cycles when subjected to two hour temperature cycles from –20oF to 125oF [Allred, 1995]. Similarly, there was significant strength and stiffness loss of GFRP composites in alkaline solution under freeze-thaw cycling (0 – 70 oF) [Vijay and GangaRao, 2000]. An average strength reduction of about 10% was observed for glass reinforced isopolyeser and vinyl ester structural plates when treated in 4% salt solution and exposed at both room temperature and cyclic temperature of 0 – 70oF [Ajjarapu, 1994]. The durability of pultruded vinyl ester composites was investigated by means of freeze-thaw tests. Samples were subjected to three levels of exposure, 100, 300 and 500 cycles. The strength and strain to failure were found to be approximately 50% lower for E-Glass/vinyl ester and E-Glass/epoxy samples when subjected to 4.4oC to –17.8oC freeze-thaw cycling temperature [Lesko, 2000]. In the work done by Vergheese, et al. [1999], differential scanning calorimetry
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(DSC) was used to identify the nature and presence of freezable water for each constituent material within an E-glass/vinyl ester composite, i.e., matrix and interface. Heat flow measurements during thawing taken for a single cycle (-150oC to +50oC) on saturated, unreinforced vinyl ester resin samples indicated no melt endotherm and thus the absence of freezable water. This was attributed to the fact that water resides in the free volume of the resin. Since this free volume size is in the order of about 4.20 A, these voids are thermodynamically too small for water to freeze. It is highly unlikely to freeze water in highly cross-linked amorphous polymers.
This is due in part to
geometric space constraints in addition to hydrogen bonding, which further impede the freezing process.
However, the crack dimensions in composite systems are large
enough to facilitate the freezability of water.
The freezability of water leads to
accumulation of damage due to the growth of cracks. In one other study, Dutta, [1994], concluded that low temperatures stiffen the polymer composites. The flexural behavior of composites is matrix-dominated and the increase of bending and shear modulus at lower temperature controls the composite’s flexural properties. Thus, polymer matrix and its type play a crucial role in the composite’s behavior at subfreezing temperatures. Low temperature thermal cycling has shown that both bending and shear moduli degrade in case of plain weave glass composites.
4.3.3.3.1.3 Effect of Solutions with Different pH Levels Effect of salt solution
The effect of salt solution on a composite is very low compared to the effect of other environmental aging conditions. This is because salt molecules are larger than other liquid molecules; hence the diffusion rate is slower.
The slow diffusion rate
prevents early damage of fiber/matrix interface and the glass fibers. In fact, several researchers have observed a strength gain in composites when exposed to salt solution. Strength gain is mainly due to post curing which improves the properties of matrices in the composites. Strength gain (less than 10%) and stiffness loss (less than10%) were observed for a glass/vinyl ester composite when conditioned in salt
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water at room temperature [Vijay, 1998]. The reduction in strength and stiffness is greater when composites in salt solution are exposed at elevated temperature because the rate of diffusion increases at elevated temperatures. The tensile strength loss of about 19% was observed in a glass/epoxy composite conditioned in salt solution at 60oC. From the SEM micrographs, it was concluded that the failure of composites was due to matrix degradation; i.e., cohesive failure [Kajorncheappunngam, 1999].
The
degradation in tensile strength (when exposed to salt solution) can be reduced to some extent by adding chopped strand mats in the composites. The ester linkages in the outer layers of the continuous strand mat bind the free water through hydrogen bonds in the surface layers, hence reducing its effect through the thickness. In general, the degradation in strength and stiffness of composites exposed to salt solution is insignificant compared to other aging liquids.
Effect of aqueous solution\acid solution
Acid affects on the composite in terms of reduction of strength and stiffness at room temperature. When composites are exposed to acids, the acid diffuses through the matrix and subsequently reaches the surface of glass fibers.
Once the acid
contacts the glass fibers, the ion exchange takes place between glass fibers and acid, eventually leading to surface shrinkage. The surface shrinkage causes internal stress within the fibers and initiates cracks in the fibers. Sometimes, the ions in the matrix cause fracturing of matrix, thereby increasing the rate of diffusion and leading to debonding of fibers.
The reduction in tensile strength of a glass/epoxy composite
exposed to acid solution over a 5-month period was 73% at room temperature, but it was only 48% at elevated temperature [Kajorncheappunngam, 1999].
At elevated
temperatures, the effect of acid on strength reduction is low because the ion exchange reaction reduces at elevated temperature, and hence the damage to glass fibers is low compared to room temperature damage. Durability of composites in an aqueous solution greatly depends on the type of fiber that reinforces the composite. For example, boron free glass fiber shows improved performance over traditional E-glass fiber because the former has improved corrosion
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resistance. ECRGLAS (boron-free glass) laminates were found to have 30% higher flexural strength than E-glass laminates.
The aqueous solutions cause dramatic
increase in hydrolysis of Kevlar 49 yarn, especially in the combination of temperature and stress. The degradation in such glass fibers can be protected to some extent by choosing the appropriate resin system, application of gel coats and providing appropriate protective coatings. The efficacy of application of gel coats and protective coating has been shown by the marine industry to prevent blistering, jackstraw, matrix degradation and fiber attack [Altizer et al., 1996].
Effect of alkaline solution
Significant loss in strength and stiffness occurs when the composites or neat resins are exposed to alkaline conditions. This loss is attributed to the fact that the rate of moisture absorption in composites is more when exposed to alkaline solution than other liquids.
The rate of strength and stiffness loss was about twice in alkaline
environment for E-glass/vinyl ester composite over that for glass composite exposed to salt environment [Vijay, 1998]. The alkaline solution mainly attacks at the interface of fiber/matrix debonding.
The hydroxide ions in an alkaline environment attack the
primary component of glass (silica) and cause the breaking of Si-O-Si bonds in glass fiber.
This results in fiber corrosion and reduction in strength. At an elevated
temperature alkali has a greater effect on the strength and stiffness of the composites. This is attributed to better matrix cross-linking reaction and becomes brittle.
The
brittleness leads to matrix cracking, thus reducing the strength and stiffness of the composite. A reduction of about 70% in tensile strength and ultimate strain to failure was observed in a glass/polyester composite (Vijay and GangaRao, 1999). The effect of alkali can be anywhere in the composites, i.e., fibers, matrix or at the interface of fiber/matrix. Hence, durability of composites in alkaline solution can be improved by selecting proper fiber and/or resin. For example, corrosive resistant glass fibers and alkali resistant resins can be used to make composites. Boron free glass fibers (ECR) show improved performance over traditional E-glass fibers because of their improved corrosion resistance.
Composite with Advantex glass fibers (boron free glass) had
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about 95% tensile strength retention, while those with E-glass had only about 85% retention in the tensile strength Devalpura [1998]. Glass fibers are more sensitive to alkali environment when compared to aramid or carbon fibers. The penetration of alkali into such fibers is mostly time dependent. For GFRP bars, the penetration of alkali increases with time. The glass fibers deteriorate in the area where alkali has penetrated, which reduces strength of the bars. The strength of GFRP bars decreased with time when immersed in alkali solution, whereas those of AFRP and CFRP bars were not decreased [Katsuki, 1995]. Higher alkali concentration increases the degradation on composites. GFRP bars immersed in 5gm/L of sodium hydroxide (over a 4 month period) had about 20% reduction in strength while those in 20 gm/L of sodium hydroxide had about 30% reduction in strength [Alsayed, 1998]. With respect to performance of resin in the alkaline solution, vinyl ester has better alkaline resistance compared to other resins and exhibits excellent strength and stiffness properties.
The increased distance between cross-linkages in vinyl ester
implies that it does not completely polymerize. Performance characteristics of vinyl ester changes with cure time. The ultimate tensile strength of vinyl ester samples after 1300 hours of immersion in alkaline solution was about 70 MPa while that of isopolyester was only about 50 MPa. The pH level of an alkaline solution is another important factor, which acts as a catalyst in degrading the glass fibers. Cementitious extract with pH 10 buffer and water had the greatest degradation in composite strength. This is hypothesized due to greater concentration of Ca ions available for formation of calcium hydroxide crystals at the surface of glass fibers. The effect of alkali solution on composite materials can be potentially reduced by fiber sizings (to promote fiber/resin debonding) and by selecting alkali resistant resins and corrosive resistant fibers [Altizer et al., 1996].
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4.3.3.3.1.4 Effect of Environmental Aging on Glass Transition Temperature (Tg) The glass transition temperature (Tg) generally indicates the thermal stability of a composite material. The Tg varies depending on the type of aging environment and the exposure temperature of composites.
The aging solutions (depending on moisture
absorption) lower the glass transition temperature (Tg) and enhance the apparent phase separation in composites due to the effect of polymer plasticization. The effect of plasticization was greater in glass/epoxy composites than it was in epoxy neat resin samples. This was probably due to exposed edges that allowed solution to diffuse easily into the composite than the neat resin [Kajorncheappunngam, 1999]. In contrast to neat resins, the reinforcing fibers in a composite and the resulting interface regions actually enhance resin plasticization and hydrolysis, slowing down Tg kinetics. When the composites are exposed to acidic environment, Tg increases rapidly due to the hydrogen bonding, and then remains constant, once the hydrogen bond formation reaches its maximum state due to the finite number of accessible active sites available for H+ ions.
The performance of composite material in terms of Tg varies with
temperature. At ambient temperature, no significant changes were observed in Tg for vinyl ester and isopolyester resins following 1300 hour immersion in water, salt solution, and concrete pore solution [Chin, 1985]. As the cure temperature increases, the glass transition temperature also increases.
This is because, at elevated temperature,
additional cross-linking reactions take place and there is continued resin curing process. At
temperatures
above
glass
transition,
polymer
molecules
have
sufficient
instantaneous mobility to get back towards equilibrium during temperature changes. When a polymer is quenched from above-to-below glass transition temperature, the lack of instantaneous mobility results in free volume in the system. This change in free volume during the movement towards equilibrium results in altering mechanical properties of the bulk polymer [GangaRao et al., 1995].
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4.3.3.3.1.5 Effect of Ultraviolet (UV) Rays The UV radiation affects polymeric composites, especially at and near the surface [Ashbee, 1989]. However, no significant damage in terms of composite material properties has been found [Ashbee, 1989]. In addition, accelerated UV exposure tests under wet/dry cycles on marine fabrics with polyesters or nylon fibers were found to have significant reductions in tensile strength and elongation [Moore and Epps, 1992]. Based on the above data, it is clear that the resins and fibers made of polymers have significant property changes. On the other hand, FRP composites primarily driven by glass or carbon fibers do not vary significantly.
4.3.3.3.1.6 Structural and Manufacturing Factors Influencing Durability of Composites Durability of composites not only depends on environmental factors such as moisture, temperature, physical and chemical exposure, but also to some extent on structural factors. The factors that influence structural properties are: 1) type of fibers (glass, aramid, carbon etc.), 2) type of resins (epoxy, vinyl ester, polyester etc.), 3) type of composites (epoxy/glass, vinyl ester/glass etc), 4) fiber orientations, 5) fiber volume fractions, 6) thickness of the composites, 7) interfacial bond, 8) manufacturing techniques 9) others. Performance of a composite as function of some of the structural factors is addressed, herein.
Fibers
The performance of composites under aging conditions depends on the type of fiber reinforcement.
Since the fibers are the main load resisting constituents of
composites, the early degradation of the fiber should be avoided. The fiber should be selected in such a way that they are alkali resistance, because alkali solution breaks the glass bonds in the fiber leading to fiber breakage. Generally, boron free glass fibers are corrosion resistant and perform well when compared to the traditional E-glass fibers. As mentioned earlier, the penetration of alkali occurs more in glass fibers than in aramid or carbon fibers. Although carbon fibers do not absorb moisture and are resistant to many
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chemical solutions, carbon composites do absorb moisture, when formulated with polymer matrices. Although the degradation of carbon fibers may not occur by itself, the oxidation reaction at the carbon cathode degrades the matrix material in that location causing degradation of the composites.
Similarly, ECR-glass has an enhanced
chemical resistance, especially in acid environment [EUROCODE, 1996]. A chopped strand mat is used for a smooth finish of a composite. The chopped strand mats are actually resin rich surfaces and increase the moisture absorption and diffusion coefficient. Diffusion coefficient for laminates with chopped strand mats at the surface was found to be considerably higher than with continuous fabrics. The fiber volume content is one of the factors responsible for the durability of composites. Interlaminar shear strength and impact strength of the laminates were measured and found to have the properties of the composites with higher fiber content degraded faster than those with lower fiber content [Singh, 1991]. The fiber orientation plays a vital role in the moisture absorption. In one study, Karbhari et al. [1998] found higher moisture absorption but lower strength degradation in triaxial fabrics compared to the uniaxial fabrics. The extra moisture absorption effects in specimens with triaxial fabrics were hypothesized due to absorption along the fiber-resin interfaces with increased directionality resulting in increased crossover or contact points.
Resins
The durability of composites varies with the type of resin used in the composites. Failure in the composites primarily initiates at the resin level. Since resins have more void contents, they are easily attacked by the aging solutions.
Most of the aging
solutions penetrate the resin and reach the core, thus degrading the fibers or the interface of fiber/matrix. Hence, one should be careful in selecting the type of resins. In one study, bisphenol polyster was found to perform much better as compared to the isophthalic polyester, because the former was more corrosion resistant. Also several studies have proven that vinyl ester has good stability against harsh environments compared to the polyester. This is mainly attributed to the chemical structure of vinyl ester resins. Polyesters have double bonds at about 250g/mol level while vinyl esters
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have reactive double bonds at about every 500-1000 g/mol. In addition, performance characteristics of composites with vinyl ester change with cure time.
Manufacturing Techniques
Manufacturing techniques play a partial role in the durability of composites. Possibility of high void content exists in a composite during manufacturing.
The
presence of voids in the components increases the moisture absorption and diffusion coefficient, which eventually leads to degradation in strength and stiffness on a longterm basis. Controlling the line speeds can reduce the void fractions during pultrusion. Pultruded samples made at different line speeds (4, 8, and 12 ipm) were aged, then different conditions and mechanical properties were evaluated [Garland, 2000]. Based on the microscopy results, no difference was observed in the void fraction due to the effect of line speeds; hence, no difference was observed in the rate of moisture absorption. Pull speeds at higher line speeds may provide some differences in void fractions, fiber wet-out and degree of cure, which may then show the rate of degradation in the strength and stiffness when exposed to environmental conditions. Hence, the void contents in the composites should be kept as low as possible during manufacturing process, i.e. less than 0.5% if possible and certainly no more than 1%.
4.3.3.4 Knockdown Factors The mechanical properties of a composite material, such as strength in tension, bending, shear etc. can be obtained by conducting experiments at coupon, component or system level on non-aged specimens or by using the analytical formulae given in section 1.4.1. In order to calculate the nominal strength and stiffness values for design, the base values obtained should be multiplied with modification factors for actual field and environmental conditions. These modification factors are called knockdown factors. Equations 4.32 and 4.33 are equations used to calculate nominal strength and stiffness. (AASHTO LFRD Bridge Design Specifications) F=F0CfCmCcCaCst
…(4.32)
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E=EoCm
…..(4.33)
where F= Nominal resistance in bending (b), or torsion (t), or compression (c), or shear (s) F0 =Base resistance of b,t,c, or v E = Nominal modulus for b,t,c,or v Eo = Base modulus for b,t,c,or v Cf = Size effect factor for dimensions of width, depth, span etc. Cm = Moisture content factor and/or humidity factor with pH variation Cc = Environmental factor, which varies with the FRP material exposure to different temperature levels Ca = Physical aging factor that varies with number of years of service Cst = Sustained load factor These knockdown factors or reduction factors are established through different tests and field evaluations. These factors can be used when no test data are available. For example, if the tensile strength of a bigger diameter bar is required and the data are not available then the size effect factor can be used to establish these values. Vijay, (1998) has given a table to account for various knock down factors by conducting tests on 3 GFRP bars each, weathered under natural atmospheric exposure for 3, 7 and 10 years and tested in tension for this research.
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Table 4-5: Knockdown Factors (Vijay,1998) Factor
Notation
Parameter
Eqn.1.1 & 1.2
Knockdown Factor (Reduction Coeff.) 1.00-#4 0.85-#5
Size effect Factor
Cf
Diameter
0.70-#6 0.65-#7 0.60-#8
Moisture Content Factor
Cm
Sustained Load Factor (20%-40% on GFRP bar)
Cst
Temperature Factor to be used with (Cm and Cst)
Cc
Physical Aging Factor
Ca
Salt (Ph ≈ 7)
0.9-0.75
Alkaline (Ph ≈ 13)
0.8-0.65
Salt/Water
0.85-0.70
Alkaline
0.70-0.40
Mean Annual Temperature (T0F)
1(T ≤ 52.5 0 F ) (T − 52.5) 1− for 100 (52.5 ≤ T ≤ 92.5 0 F )
(In combination with alkalinity and stress)
0.90
Notes:
size effect factor can be used only for interpolating bigger bar diameter strengths when smaller diameter bar (in this table #4 is chosen as reference) is tested. values in this investigation were correlated for a mean annual temperature of 52.50F and hence knock-down factor of 1 is chosen at that temperature. If mean annual temperature is more than 52.50F then a minimum reduction of 0.1 is applied for every 100F increase in the mean annual temperature. This is an empirical approach purely based on strength reductions in GFRP bars under accelerated aging at 69.80F, 93.680F and 1500F in addition to natural weathering
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results of Litherland et al., 1981. Mean annual temperatures above 900F are not expected in any part of the globe. A limit of 0.4 is provided as the reduction factor for combined effects (Vimala, 2002). This factor is given considering that all the environmental factors that cause aging do not act at the same time and place.
4.3.3.5 Durability Models Various analytical models used to predict the effect of environmental factors on the durability of the composite materials is collected and cited in the paragraphs, which follow.
4.3.3.5.1 Analytical Methods to Predict the Effects of Environment on Composite Materials
4.3.3.5.1.1 Effect of Moisture Following parameters are necessary to describe the behavior of composite material exposed to an environment with temperature ‘Ta’ and moisture content ‘ca’ as a function of time ‘t’ [Springer, 1981]. The temperature distribution inside the material as a function of position and time T (x, t)
The moisture concentration inside the material as a function of position and time c (x,t)
The total amount (mass) of moisture inside the material as function of time m(t) Changes in “performance (e.g., physical, chemical or mechanical property)” of the material as a function of time P(t)
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When changes in temperature and moisture inside the material are to be determined and not the performance, analytical methods can be employed if the diffusion process is “Ficikian”
Fickian Diffusion
The diffusion process is said to be “Ficikian” if the following conditions are met [Springer, 1981]. Heat transfer through the material is by conduction only and can be described by Fourier’s law The moisture diffusion can be expressed by a concentration-dependent form of Fick’s law Energy (Fourier) and mass transfer (Fick) equations are decoupled Thermal conductivity and mass diffusivity depend only on temperature and are independent of moisture concentration or the stress levels in the core.
Analytical predictions based on Fickian diffusion are a function of geometry, boundary conditions, initial conditions and material properties such as density (ρ), specific heat C, thermal conductivity K, maximum moisture content Mm and a relationship between the maximum moisture content and the ambient conditions [Shen and Springer, 1981].
Fourier’s Equation of Heat Transfer
Fourier’s equation of heat transfer is given by:
ρC
∂T ∂ ∂T = Kx ∂t ∂x ∂x
…(4.34)
where ρ = density, C = specific heat, T= temperature, t= time, x = distance, and K x = thermal conductivity.
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Fick’s Equation of Mass Transfer
Fick’s equation of mass transfer is given by: ∂c ∂ ∂c = Dx ∂t ∂x ∂x
…(4.35)
where c = moisture concentration, Dx= diffusion coefficient, x = distance, t = time
Diffusion is said to be non-Fickian [Shen and Springer, 1981], if: Cracks develop in the material or delamination occurs leading to an altered structural representation Moisture propagation is dominated by fiber-matrix interface Many composites under ambient conditions follow Fickian diffusion and this process is extensively modeled, whereas the non-Fickian diffusion models are sparsely used because excessively cracked specimens are removed from service well before the non-Fickian phenomenon sets in, leading to lack of experimental data on specimens under service.
Moisture Content (M)
In practice, the percent of moisture content M of a composite is defined on weight gain
M=
Weight of the Moist FRP ( w ) − Weight of the dry FRP ( w d ) x100 …(4.36) Weight of the dry FRP ( w d )
Mm is the maximum moisture content that can be attained under given
environment and is found to be insensitive to the temperature but dependent upon the moisture content in the environment. For a composite material exposed to humid air, Mm depends upon the relative humidity (φ) , and is given by:
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Mm=aφb (for humid air exposure)
…(4.37)
where a and b are constants. For a composite material immersed in a liquid, Mm is a constant with time after the material reaches saturation level. Mm=constant (for liquid immersion)
…(4.38)
Diffusion Coefficient (Dc)
The diffusion coefficient characterizes the speed at which moisture is transported through the material. The temperature dependence of diffusion coefficient for a rectangular composite exposed to moisture is characterized by Arrhenius relationship [Rao et al., 1981]. Dc = D0 exp −Ed / RT
…(4.39)
where Do = diffusion coefficient with respect to reference temperature, Ed = activation energy for diffusion, R = universal gas constant, T = absolute temperature.
Diffusion coefficients (Dc) can be calculated by using [Rao et.al, 1981] 2 2 ⎡⎛ ⎛ M2 − M1 ⎞ ⎤ ⎞ h ⎟ ⎥ ⎟ ⎜ D c = π ⎢⎜⎜ ⎢⎝ 4Mm ⎟⎠ ⎜⎝ t 2 − t 1 ⎟⎠ ⎥ ⎣ ⎦
…(4.40)
where h = thickness of the composite, Mm= maximum moisture content, t1 and t2 = time
taken to reach the moisture contents M1 and M2.
Diffusion Coefficient for Alkaline or Salt Solution (k)
For FRP circular sections such as bars immersed in an alkaline or salt solution, the diffusion coefficient is a function of the depth of liquid penetration, concentration of
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the liquid, exposure duration and the bar diameter [Katsuki, 1995] as given in the following eqn. x = s ⋅k ⋅C⋅ t
…(4.41)
where x = depth from surface of rod C = alkali or salt concentration (mol/l) t = curing time (hrs) k = diffusion coefficient of alkali or salt in rod.
Effect of Temperature
Empirical relations for predicting hygrothermal degradation effects have been studied at Lewis Research Center of NASA and given by [Chamis, 1984]. PmD ⎡ TGW − T ⎤ = Pmo ⎢⎣ TGD − T0 ⎥⎦
0.5
…(4.42)
where PmD = degraded mechanical property, Pmo = mechanical property at reference
condition, TGW = glass transition temperature of wet resin, TGD = glass transition temperature of dry resin, To = reference temperature at which PmD is determined The manufacturer supplies TGD, whereas TGW is given by empirical relation: TGW = (0.005M2 – 0.1M + 1) TGD (M≤ 10%)
…..(4.43)
where M = weight % moisture in the resin It is clear that the matrix with the higher TGD will yield a composite with better resistance to hygrothermal degradation.
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4.3.3.5.1.2 Prediction of Residual Stresses Due to Moisture and Temperature Moisture absorption at low temperatures results in matrix swelling and reduction in strength and stiffness. If uniform strain in longitudinal direction and uniform stress in transverse direction are assumed, residual stresses in the matrix and fiber along the longitudinal direction due to matrix swelling and thermal shrinkage is given by [Hahn, 1976]: where
σmL =
Vf E f Em [(α f − αm )(T − T0 ) + (β f c f − βm − c m )] VmEm + Vf E f
…(4.44)
σ fL =
Vm σmL Vf
…(4.45)
where E= elastic modulus, V= Volume ratio, α = coefficient of thermal expansion, β =
coefficient of moisture swelling, c = specific moisture content, T = temperature σ L = longitudinal stress, To = stress-free temperature (usually cure temperature), Subscript m and f correspond to matrix and fiber respectively.
4.3.3.5.1.3 Prediction of Rate of Degradation in Composite Strength under Harsh Environment Ajjarapu (1994) suggested the rate of degradation of the composite materials under harsh environmental conditions using the following relationship:
σ t = σ 0 e − λt where
…(4.46)
t < 450 days, λ = 0.0015, σ o = initial tensile strength, σ t = tensile
strength at time t,
σ
t > 450 days
=σ
o
…..(4.47)
/2
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From the experiments, it was concluded that the maximum reduction was 50% in 450 days. However, beyond 450 days, the strength did not change considerably. However, for the design purposes, the strength of the material due to long-term aging should to be taken a minimum of 50% of the initial ultimate strength.
4.3.3.5.1.4 Fatigue Damage Models This section deals with damage models and fracture energy absorption in composites.
Fatigue Damage Model
The fatigue damage can be measured by the variable D, which is a function of the number of cycles applied on a composite. Cyclic loading causes the damage to increase from Di to Df after N cycles at which point catastrophic failure of the composite laminate occurs. (Di is zero for undamaged material.)
Assuming that the damage
accumulation rate depends on the cyclic rate amplitude, Δσ, the load ratio, R, and on the current level of D, then: dD = f (Δσ,R, D) dN
…(4.48)
Above eqn. is valid when temperature, frequency, etc., are constant or have negligible effects (Beaumont). The lifetime, Nf, (the number of cycles to increase D from Di to Df) is therefore:
Nf =
Df
dD
∫ f (Δσ , R, D )
…(4.49)
Di
Relation between the axial Young’s modulus, E, of the composite laminate and the accumulated damage, D is given by: E = E0 g(D )
…(4.50)
where Eo is the initial or undamaged modulus. Therefore:
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1 dE = g' (D ) E0 dN
…(4.51)
where g’ stands for the derivative of g with respect to D. Differentiating and substituting into Eq (4.49), we get: ⎡ ⎛ E ⎞⎤ ⎡ ⎛ E ⎞⎤ 1 dE = g' ⎢g−1 ⎜⎜ ⎟⎟⎥ f ⎢ Δσ,R, g−1 ⎜⎜ ⎟⎟⎥ E0 dN ⎝ E0 ⎠ ⎦ ⎣ ⎝ E0 ⎠ ⎦ ⎣
…(4.52)
where g-1 is the inverse of g: ⎛E⎞ D = g' ⎜⎜ ⎟⎟ ⎝ E0 ⎠
…(4.53)
The function g(D) has to be established either experimentally or theoretically before determining function f. Function g(D) depends on the properties and the lay-up of the composite laminate, and not on how the damage, D, was introduced. Data of (E/Eo) is obtained as a function of N and, knowing g(D), the function of f (Δσ, R, D) is determined experimentally using : f (Δσ,R, D) =
1 ⎛ dE ⎞ ⎜ ⎟ ⎡ −1 E ⎤ E0 ⎝ dN ⎠ g' ⎢g ⎥ ⎣ E0 ⎦ 1
…(4.54)
The right hand side of the equation is evaluated for different values of Δσ at a constant (E/Eo) and R, for different values of R at constant Δσ and R. The function f is determined from the plot of these results. The mechanical and hygrothermal properties of a composite material have been elaborated upon in the earlier subheadings. The aging of composites with relation to various environmental factors and analytical models used to predict the effects of environment on composite materials have also been discussed. Following this subdivision we will discuss the applications of composite materials in various fields. The reader can thus appreciate the variability of the use of composites in various fields and application of composites especially in the field of defense.
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4.4 Summary and Concluding Remarks Technical issues concerning the long-term and short-term response of composite materials are dealt with in this chapter. Issues include mechanical properties, fluid sorption and damage, thermal coefficient, creep, fatigue, and aging due to environmental effects. The mechanical and hygrothermal properties of the composite are established first in terms of short-term response.
The aging of the composite
material due to physical and chemical factors are elaborated upon. This chapter brings to light to the reader that composite material also age under corrosive environments. But the rate of aging is slower than in the case of metals. An important point worth mentioning is the effect of size on performance. Most of these results obtained from the tests conducted, are from coupon level specimens. The size of FRP structural elements for an actual application is much larger. A proper correlation between these two parameters in size should be done to implement these results in the field. At the present systematic studies on the size effect is not available, and there is no clear conclusion on whether or not a size effect exists.
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5
APPLICATIONS OF COMPOSITES
This chapter deals with the applications of metal and composites.
Special
emphasis is given to the application of these materials in the manufacture of defense equipment.
Composites are vastly used in defense applications and also in other
industries such as construction, automotive, recreational, etc.
The cost factor of
composite material makes it suitable for manufacture of weapons in the field of defense. Thus the applications of composites can be broadly classified as those used in defense industry and in various other civilian purposes.
5.1 Applications of Composites for Defense Purposes The use of composites in the field of defense is multipurpose. The application of composites for defense equipment is elaborated in the following paragraphs.
5.1.1 Aircraft Systems Composites are widely used in the manufacture of aircrafts.
The need for
improved materials to withstand varied environments is leading to the demand of composites in various fields of defense.
The F-22 Raptor aircraft (Figure 5-1) is the
next-generation air superiority fighter for the Air Force to counter emerging worldwide threats. It is designed to penetrate enemy airspace and achieve a first-look, first-kill capability against multiple targets. The F-22 is characterized by a low-observable, highly maneuverable airframe; advanced integrated avionics; and aerodynamic performance allowing supersonic cruise without afterburner. The Raptor was built with a requirement for a fighter to replace the earlier fighters, with emphasis on agility, stealth and range. The F/A-22 raptor construction is 39% titanium, 24% composite, 16% aluminum and 1% thermoplastic by weight. Titanium is used for its high strength-to-weight ratio in critical stress areas, including some of the bulkheads, and also for its heat-resistant qualities in the hot sections of an
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aircraft. Carbon fiber composites have been used for the fuselage frame, the doors, intermediate spars on the wings, and for the honeycomb sandwich construction skin panels.
Figure 5-1: F-22 Raptor Aircraft -Tactical Fighter Aircraft Reference: http://www.airforce-technology.com/projects/f22/
The RAH-66 Comanche is a helicopter designed for armed reconnaissance, attack and air combat missions.
Figure 5-2: RAH-66 Comanche Reference: http://www.gdatp.com/Products/2002/arm_systems/rh66_comanche/RAH66.html
Externally mountable aircraft guns are also manufactured using composite materials. The GAU-19A (Figure 5-3a) is an externally mounting 12mm gatling gun and the F18C/D is a 20mm gatling gun system.
The F18C/D system (Figure 5-3b) is
lightweight and the compact design is mounted on a common pallet structure. Lightweight components include a fiber reinforced plastic inner drum and rotating scoop disc assemblies. The GAU-19/A gun is especially effective in high clutter environments and for engagements that are inside missile envelopes.
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Figure 5-3: a.GAU –19A b. F18C/D Reference: http://www.gdatp.com/Products/2002/aircraft.htm
5.1.2 Ground Systems The use of composites in armor systems enables the manufactures to decrease the weight of the armor thus enabling capability of further increasing the protection without increasing self-weight. Composites are resistant to the harsh environments and the impact loads. The ammunition the armor utilizes is a very insensitive high energy explosive with reduced weight.
This promotes for carrying a larger quantity of
ammunition for attack purposes.
Figure 5-4: Reactive Armor and XM-301 Gun Reference: http://www.gdatp.com/Products/2002/ground.htm
5.1.3 Individual and Crew Served Systems The Objective Crew Served Weapon (OCSW) is the next generation replacement for current heavy and grenade machine guns. It is truly a lightweight, two-man portable
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system that incorporates the most modern technology advancements in fire control, materials and munitions. The use of lightweight composite material made it feasible for the manufacturer to develop a crew system to be handled by two men.
Figure 5-5: Objective Crew Served Weapon Reference: http://www.generaldynamics.com/
5.1.4 Rocket and Missile Systems One of the rockets made of composite component parts is the DELTA II rocket (Figure 6-4). It makes use of the lightness and durability of composites to bring about the added features required for the increased serviceability and service life of the rocket. Delta II can launch single, dual, or multiple payloads on the same mission. To accommodate these varying requirements, one of the companies designed a variety of payload attach fittings. The company also builds several fairing types to enclose and protect payloads on the launch pad and during ascent. Fairing is the front end of the rocket that serves to protect the satellite being launched from the external environmental factors. Some factors affecting the fairing are aerodynamic loads, vibration, noise, temperature extremes, dirt, dust, rain, snow, and micrometeorites that may be encountered as the satellite is launched and accelerates through the atmosphere into space. A composite fairing is offered in a 3-m diameter (10-ft) size. An aluminum 2.9-m (9.5-ft) diameter fairing is available, as is a 3-m diameter stretch composite fairing for certain payloads. The fairing being made of composite reduces the weight of the entire system.
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Figure 5-6: Delta II Reference: http://www.boeing.com/defense-space/space/delta/delta2/delta2.htm
Figure 5-7: Missiles from the Hydra 70 Family Reference: http://www.fas.org/man/dod-101/sys/missile/hydra-70.htm
5.1.5 Shipboard Systems Ships such as the carriers, battleships that are used in warfare need to carry heavy loads and need to resist high wear and tear. Ships are also prone to high rate of corrosion due to the environment, in which they operate, which is highly corrosive. The need to design and manufacture better battleships and cruisers for combat is the high priority in the defense.
This brings about the use of composites that have better
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corrosion resistance and higher fatigue lives. The weapon systems mounted on ships for battle are also built using composites making the weapons lighter and increasing their service life.
Figure 5-8: Destroyers Reference: http://www.chinfo.navy.mil/navpalib/factfile/ships/ship-dd.html
Figure 5-9 Goalkeeper: In-Ship Defense System Reference: http://www.gdatp.com/Products/2002/arm_systems/goalkeeper/goalkeeper.html
5.2 Applications of Composites for Civilian Purposes 5.2.1 Automotive Automotive body parts, body panels, structural components, and under hood parts, especially made of steel, are subjected to severe corrosion under high
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temperature. Steel is prone to high corrosion under harsh environments.
The design
and manufacturing versatility of composites along with high endurance to corrosion allows replacement of steel with composites in automotive parts.
Figure 5-10: Composite Fire Truck Panels Reference: http://www.pultrude.com/automotive.html
5.2.2 Infrastructure Aging bridges, highways, buildings, weapons, machinery etc. in infrastructure are subjected to corrosion and high internal stresses. Repair, rehabilitation and retrofit of these infrastructures have set a stage for composites. In order to bring about this rehabilitation, FRP laminates, wraps and jacketing systems are used. The retrofitting of columns using FRP wraps is a fast developing technique to increase the service life of a bridge.
Figure 5-11: All Composite Bridge, Laurel Lick, CFC-WVU
5.2.3 Construction Pultruded profiles play a dominant role in composite construction applications. They are used in a number of construction purposes such as bridge decks, support systems for bridge decks, corrosion resistant walkways, and railings. They are also
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used as reinforcing material in concrete as steel bars corrode at a rapid rate and FRP bars have higher corrosion resistance. A number of panels made of FRP material are used together in infrastructure development i.e., energy plant towers, cooling towers, pedestrian bridges and many other structures.
Figure 5-12: Energy Plant Towers Reference: http://www.pultrude.com/construct.html
5.2.4 Transportation Composites are used in various transportation vehicles such as aircrafts, rockets and heavy-duty vehicles. They have replaced aluminum, steel and other traditional materials due to their high resistance to corrosion and their ability to withstand high temperatures. In transportation systems such as pedestrian walkways they are used as handrails etc. They are used, also in railways in order to wrap the existing wooden crossties.
Figure 5-13: Third Rail Protection in Monorail System Reference: http://www.pultrude.com/construct.html
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5.2.5 Biomedical Composites are being applied today to medical devices ranging from artificial limbs to light weight tubing used in invasive surgery. Magnetic resonance imaging units and electromagnetic shielding application units are built using composites.
Figure 5-14: MRI Units Reference: http://www.gemedicalsystemseurope.com/euen/rad/mri/products/vhi/vhi.html
5.2.6 Computer products Composites are used to manufacture computer-housing products, computer boards, outer frame and even the microprocessors and chips.
The high factor of
corrosion inhibition of composites enhances its use in the building of component parts of the computer
Figure 5-15: Composite Computer Chip Reference: http://news.bbc.co.uk/1/hi/sci/tech/2053539.stm
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5.2.7 Corrosion Resistant products Composites are used to manufacture various corrosion resistant products. They are used to manufacture pipes in sewer systems, which lasts longer then the traditional pipes.
Corrosion resistant composite tanks, scrubbers and pressure vessels have
expanded further into the industrial sector. Equipment used in wastewater treatment plants must withstand sustained exposure to highly corrosive chemicals and composites are proving the best materials to be used in such environments.
Figure 5-16: Waste Water Plant Reference: http://www.pultrude.com/ww.html
5.2.8 Electrical Electrical cables, optic fibers, cable supports, telecommunication towers, power transformers, power transmission lines etc. are manufactured using composites. They have various advantages over conventional material. They are corrosion resistant, fire, smoke and toxic safe and have high ranges of electrical and thermal insulation. They also boast of minimum maintenance, minimum initial cost and ease of installation.
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Figure 5-17: Telecommunication Towers Reference: http://www.pultrude.com/elec.html
5.2.9 Recreational Composites have successfully replaced wood and conventional materials such as steel in various consumer recreational products such as fishing rods, tennis racquets, golf club, Paddles, windsurfing masts, kites, bicycle handlebars and various other fastenings. Composites are found in most of the outdoor sports and recreational activity equipments.
Figure 5-18: Recreational Products Reference: http://www.pultrude.com/consumer.html
5.2.10 Marine Corrosion in marine environments has led to opportunities for composites in waterfront applications such as marine fenders and pilings. Some of the composite
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pilings used are pilings made with vacuum-assisted technology, filament wound composite piling jackets filled with cement, and extruded thermoplastic pilings reinforced with extruded thermoset composite rebar.
Figure 5-19: Sheet Piling and Fender Applications Reference: http://www.pultrude.com/mar.html
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6 RETARDATION METHODS FOR CORROSION
There are four main methods used for retardation of corrosion process. They are:
Cathodic Protection
Coatings
Inhibitors
Anodic Protection
These methods can be employed individually or in combination with each other.
6.1 Cathodic Protection A metal in contact with corrosive environment starts to corrode due to electrochemical reactions. This effect results in metal dissolution and oxygen reduction reactions when the oxidizing reagent is oxygen.
Cathodic Protection reduces the
corrosion rate by supplying electrons to the metal structure.
There are two main
techniques employed in cathodic protection. They are: •
Sacrificial Anode System
•
Impressed Current Cathodic Protection
6.1.1 Sacrificial Anode System The sacrificial anode is a more active metal than the metal structure to be protected (Figure 6-1). The metal to be protected and the sacrificial anode is a coupling for galvanic corrosion. The dissolution of sacrificial anode “takes over” for the dissolution reaction of the metal to be protected. This mechanism is shown in Figure 62. The sacrificial anode makes the metal to be protected as cathode. This sacrificial anode method cannot be used on acid because the consumption of sacrificial anode is
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very high. Thus, this method is rather suitable to protect the metal structure exposed to oxygen environment in neutral solution. Sacrificial anodes are installed at every 10 feet to protect the structure (e.g., pipeline). So, this system cannot be implemented for very long structures (pipelines). This system is economical and suitable to a pipeline that runs through short distance, such as 0.5 miles.
Figure 6-1: Steel Tank Protected by Sacrificial Anode System
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OH-
H2O O2 e-
e-
Fe
Zn
Zn2+
Figure 6-2: Mechanism of Anodic Protection System
6.1.1.1 Advantages of Sacrificial Anode Systems •
Simple and easy to design.
•
Low maintenance requirements.
6.1.1.2 Disadvantages of Sacrificial Anode Systems •
Used only where small current requirements are needed.
•
Soil resistance should be low.
6.1.2 Impressed Current Cathodic Protection In this method current is provided to the structure by some means such as AC supply, which is converted to DC by means of a rectifier and supplied, to the ground beds of usually graphite, silicon cast iron or precious material surrounded with carbonaceous backfill. The system is shown in Figure 6-3. The current drains from there and reaches the structure to be protected. To complete the circuit the negative terminal of the DC power supply is connected to the structure to be protected and the positive terminal is connected to the ground bed system. The mechanism is seen in Figure 6-4. In this method, electrons are supplied to the metal structure, reducing the corrosion rate. The applied voltage is Eapp as shown in Figure 6-4. Thus, the corrosion rate is reduced from icorr to i’corr. This method is not suitable to acid environment because current requirement will be too high. This method is only suitable to oxygen corrosion in neutral environments like the sacrificial anode method. Unlike the sacrificial anode method, this method can be applied to pipelines, which run long distances.
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Figure 6-3: Steel Tank Protected by Impressed Current System
MÆ M++eE Eap O2+2H2O+2e-
i’corr
icorr Log i
Figure 6-4: Mechanism of Impressed Current Systems Explained Using Anodic Polarization Curves
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6.1.2.1 Advantages of Impressed Current Cathodic Protection •
High output capacity.
•
Single installation can protect a large structure.
6.1.2.2 Disadvantages of Impressed Current Cathodic Protection •
Greater maintenance requirements.
•
Depends on availability of power supply.
6.2 Coatings Coatings are used to cover or spread the material to be protected against corrosion with a finishing, protecting or enclosing surface. There are mainly three types of coatings. They are •
Metallic
•
Organic
•
Inorganic
6.2.1 Metallic Metallic coatings are with more active metals than the substrate metal. Metallic coatings are further classified as follows:
6.2.1.1 Hot Dipping The process of hot dipping is carried out by immersing a pre-treated substrate in a bath of molten metal or alloy. Hot dipping is used to increase corrosion and wear resistance. Coatings of all low melting-point metals and alloys such as zinc and aluminum are deposited. The steels coated with such metals are called galvanized steels. The coating of 55% zinc and 45% aluminum is used because the addition of
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aluminum suppresses the transport of iron into the coating; thus reinforcing the physical property of the coating.
6.2.1.2 Chemical Vapor Deposition (CVD) Chemical vapor deposition is a process in which gaseous molecules of a metal, alloy or polymer is deposited on the surface of the substrate in the form of thin film or powder.
Gaseous compounds of materials to be deposited are transported to a
substrate surface where a thermal reaction/deposition occurs at temperatures around 1000 °C.
CVD offers many advantages over other processes.
It can deposit any
element or compound and it can also produce high purity and high density coatings economically. The disadvantages are high process temperature, loss of hardness and significant dimensional changes and geometric distortions. The disadvantage of high temperature CVD process can be overcome by using plasma enhanced CVD process, which usually requires a temperature range of 200-300 °C.
6.2.1.3 Ion Vapor Deposition (IVD) Ion Vapor Deposition is a process by which aluminum is vaporized at high temperatures and adheres to metal substrate. The process requires a vacuum chamber and a vaporizing source. IVD coatings can be applied to a wide variety of metallic substrates, aluminum alloys, carbon steels and stainless steels. The advantages of IVD coatings are uniformity of thickness, applicability to complex shapes and prevention of hydrogen embrittlement. The disadvantages are unable to coat deep holes and simpler methods are not available to repair the coatings.
6.2.1.4 Spraying Spraying is the deposition of the metallic coating by dispersing it on the metallic substrate to be protected. All spraying processes feed the material into the heating device where it is heated, melted and sprayed on the surface. The molten particles strike the surface and form a thin coating on the substrate. As they cool, they form a lamellar structure to form the desired coating. Some common spraying process include flame spraying, plasma arc spraying, electric arc spraying, high velocity oxy-fuel etc.
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The thermal sprayings are capable of competing with paintings and plating for atmospheric corrosion.
6.2.1.5 Electroplating Electroplating is a deposition of metal ions onto the metallic substrate by passing current. A schematic is shown in Figure 6-5 for electroplating zinc ions on steel. Zinc chloride is added to the cathode compartment. Zinc ions are attracted to the cathode where they are reduced to elemental zinc and are deposited on the cathode surface. The cathodic reaction is: Zn2+ + 2e- = Zn
…(6.1)
e-
eH+
cl-
O2
cl-
Zn2+
Anode (Inert alloy)
Cathode (Steel)
Anionic membrane
Figure 6-5: Mechanism of Electroplating
Chloride ions are transported through the anionic membrane to the anode compartment in order to maintain the electrolytic neutrality. At the anode, water is dissociated by the reaction: 2H2O = 4H+ + O2 + 4e-
…(6.2)
At the anode, the electrolytic neutrality is maintained by producing hydrogen ions against the influx of chloride ions from the cathode compartment. Some of the
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advantages are plating samples having complex shapes, cheap, rapid technique and no sophisticated instrument is needed for the process.
Another advantage is that the
coating properties of strength, uniformity and grain size can be improved by adding various additives. However the disadvantage is that the electrode position requires higher energy.
6.2.2 Inorganic Inorganic coatings perform conversion coatings, Portland cement, ceramics, and glass on surface of the materials. They are further described below.
6.2.2.1 Portland Cement Coatings Portland cement coatings are used to protect the pipes made of cast iron or steel on both water and soil environments.
The coatings can be applied by centrifugal
casting for interior surfaces of pipes and spraying for exterior surfaces. The advantages are low cost and easy to repair.
The disadvantage is that coatings can be easily
damaged by mechanical or thermal shock and also easily attacked by sulfate-rich water.
6.2.2.2 Ceramics Ceramic coatings are used to improve the wear and corrosion resistance to withstand high temperature applications such as internal combustion engines, rocket nozzles and exhaust passages of hot gases. These coatings can also be applied to steel to provide acid and heat resistance.
6.2.2.3 Chromate Filming Chromate conversion coatings or chromate filming is a process by which the material is protected by applying chromate using immersion. The most suitable method for the formation of film will be the treatment with chromic acid. These films are about 5mm thick and form a color depending on the base alloy.
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6.2.2.4 Phosphate Coatings Phosphate coating is a type of chemical conversion process that transforms the surface of a base metal with a non-metallic crystalline coating. These coatings are produced by holding the metal in a hot bath of phosphoric acid and zinc. Manganese and zinc phosphate coatings are most commonly selected for their wear-resistant properties. It also can retain oil such as petroleum products, which can avoid rusting.
6.2.2.5 Nitriding Nitriding procedures are thermo-chemical processes, where the surface of the material is enriched with nitrogen. In this process, nitrogen is diffused into the bulk material from outside. media.
Nitriding can be done using solid, liquid or gaseous nitrogen
It is typically carried out in the temperature ranges of 500-575ºC.
The
advantages of Nitriding are increased fatigue strength, excellent dimensional stability without distortion, completely non-toxic, environmentally clean, and low energy consumption.
6.2.3 Organic Coatings consisting of organic materials like esters, etc. are called organic coatings. They are composed of hydrocarbon and are further classified into:
6.2.3.1 Binders Binders are usually resins or oils, but can be inorganic compounds such as soluble silicates. The binder is the film-forming component in the paint. The nature and amount of binder determines the performance properties such as toughness, adhesion, and color retention.
6.2.3.2 Pigments Pigments are finely ground organic or inorganic powders that provide color, opacity, film cohesion, and corrosion inhibition. The distinction between powders that
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are pigments or dyes is based on their solubility. Pigments are usually insoluble and dispersed in the material whereas dyes are readily soluble in a solution.
6.2.3.3 Solvents Solvents are used to dissolve the binder and to facilitate application of the paint. Solvents are usually organic liquids or water. A true solvent is a single liquid that can dissolve the coating. Solvent is often used to describe terpenes, hydrocarbons, and chlorinated solvents.
6.2.4 Nonstick Coatings These coatings provide a polished and smooth surface. The non-stick quality of the surface is due to the silicone, which is the non-stick, hydrophobic and lubricious component.
It offers superior surface protection in both fresh and salt-water
environments, and extends the life of steel, iron, aluminum, concrete, and a wide range of other substrates.
6.3 Inhibitors Corrosion inhibitors are used to reduce the corrosion rate of metallic surfaces in a corrosive environment by adding chemical compounds. These inhibitors will reduce the rate of anodic, cathodic or both reactions. Inhibitors are adsorbed on the metal surface by physisorption or chemisorption. Physisorption occurs due to inter-molecular forces between the metallic surface and inhibitor molecules. Chemisorption occurs due to the formation of equivalent chemical bonds between the metal surface and inhibitor molecules. After the inhibitor molecules are adsorbed, they form a protective film on the metal surface. Inhibitors can be transported to the metal surface by various methods. Some of them are: 1. Dispersing the inhibitor in the corrosive liquid 2. Adding the inhibitor to paints 3. Emitting vapors of volatile inhibitors onto the metal surface
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Inhibitors are classified into three types. They are: 1. Anodic inhibitors 2. Cathodic inhibitors 3. Mixed type inhibitors
6.3.1 Anodic Inhibitors These inhibitors protect the metal by forming a passive film on the metal surface. This film retards the rate the anodic reaction. However, if the concentration of the oxidizer is too low, the inhibitors are not effective. This is because cathodic polarization curve will intersect the active region of the anodic polarization curve, giving high corrosion rate. In this case, the anodic inhibitors are not suitable for usage. Some examples of anodic inhibitors are sodium chromate, phosphate and molybdate. Sodium chromate (Na2CrO4) is used for inhibiting metal corrosion, pigment formulation, and oil well drilling. Sodium chromate is used in certain types of corrosion control applications as well as in the formulation of pigments, where alkalinity is required. Since sodium chromate is hygroscopic (having the characteristic of drawing moisture from the atmosphere), the anhydrous form is better suited for areas where high humidity could cause caking and handling problems. Inorganic phosphates are compounds of phosphorus and oxygen that exists in many forms and is usually combined with other elements (metals such as sodium, potassium, calcium, and aluminum). Salt or ester of a phosphoric acid is used as nontoxic inhibitors. Molybdate is a salt of molybdenum containing the group of MoO4 or Mo2O7. It is an environmentally benign inhibitor. Molybdate inhibitors have been proven effective as demonstrated in numerous outdoor exposure studies and used for commercial applications for more than 30 years.
Molybdate inhibitors are effective in both
conventional salt-spray tests and the newer cyclic salt-spray/UV exposure tests (e.g., ASTM D 5894).
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6.3.2 Cathodic Inhibitors These inhibitors form a protective coating on the metal surface like anodic inhibitors but retard the rate of cathodic reactions on the metal surface. Some common examples of cathodic inhibitors are zinc salts (e.g., ZnSO4), sodium sulfonates (NaRSO3) and phosphonates (H2PO3). Sodium sulfonates are used as additives to oils and lubricants used in functional objects to extend the maintenance period of the object in oily, waxy or greasy coatings. Phosphonates are extensively used in scale/corrosion inhibition, metal finishing, ore recovery, oil drilling, industrial cleansing, pulp, paper, textile dyeing, and crop production. Phosphonates are derivatives of phosphates.
6.3.3 Mixed Inhibitors These inhibitors influence both the anodic and cathodic reactions. Many organic compounds are of this type. Some examples are aliphatic, cycloaliphatic, aromatic and heterocyclic amines, organic esters, etc.
6.3.4 Applications They are used in open water-cooling systems, closed cooling systems of motor vehicles, in primer of the anti-rust paint, and in closed containers.
6.4 Anodic Protection This technique is used for tanks in which strong acids are stored. The tank metal should exhibit active-passive behavior in order to be applied by this technique. The idea is to increase the potential, which is in the middle of the passive zone. In this situation corrosion rate will be much reduced. Consider Figure 6-6 in which the anodic polarization curve intersects with the cathodic polarization curve at the active region having the corrosion rate icorr under normal conditions. A device called potentiostat is needed to increase the potential. As the potential increases from Ecorr to E’corr the applied current density decreases from icorr
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to i’corr. The optimum range for this applied current density would be in the middle of the passive region for the system to be more effective. Anodic protection can be implemented using a potentiostat.
It has three
terminals. One is connected to the structure to be protected which is anode; another to the auxiliary electrode which is a cathode and third to the reference electrode. The potentiostat maintains constant potential between the structure and the reference electrode. The optimum potential can be determined by the anodic polarization curve. Many reference electrodes are used in the anodic protection systems along with the conventional reference electrodes such as glass and calomel electrodes. The primary advantage of anodic protection is its applicability in extremely corrosive environment whereas cathodic protection can be utilized only in weak to moderate corrosive environments. The current requirements are very low in anodic protection whereas they are very high in cathodic protection and increases as the corrosivity of the environment increases. Thus, the operating costs in anodic protection systems are lower than those of cathodic protection systems. The limitation of anodic protection systems is that it is only applicable to activepassive alloys. The installation costs are high due to complex instruments including potentiostats and electrodes.
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Transpassive
iapp E’corr
Passive
E Active
Ecorr
i’corr
icorr
Log i Figure 6-6: Effects of Applied Anodic Current on the Behavior of Active-Passive Alloy
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7 METALS AND COMPOSITES DICTIONARY
7.1 Metal Alloys 7.1.1 Cast Iron Cast iron is a common term that applies to high carbon iron alloys containing silicon. The common alloys include gray cast iron, white cast iron, malleable cast iron and ductile/nodular cast iron. Most cast irons contain carbon content between 3.0 to 4.5 The melting points of cast iron ranges from 1150 to 1300 °C, which is
wt%.
considerably lower than that of steels. Thus, they are easily melted and amenable to casting.
Furthermore most cast irons are very brittle because of the high carbon
content. The cementite (Fe3C) is a metastable phase and is dissociated into ferrite (almost pure iron) and graphite under some conditions. For most cast irons, the carbon exists as graphite and both microstructure and mechanical properties depend on composition and heat treatment. The important structural constituents of cast iron are graphite, ferrite, and cementite.
These are the least expensive of the engineering
metals. They can be alloyed for improvement of corrosion resistance and strength. These materials are brittle and exhibit practically no ductility.
7.1.1.1 White Cast Iron When silicon content is less than 1% and the cooling rate is rapid, most carbon exists as cementite instead of graphite. This is called white cast iron. These irons are extremely hard and brittle because of the cementite inclusion.
Thus it is used for
applications that necessitate a very hard and wear resistant surface such as rollers, crushers, and grinding mills.
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7.1.1.2 Malleable Iron Heating white cast iron between 800 to 900 °C decomposes the cementite forming graphite into clusters.
It has high strength and appreciable ductility or
malleability. It is used in numerous applications in automobiles (such as connecting rods and transmission gears), locomotives, road machinery, pipefittings, valves, etc.
7.1.1.3 Ductile Iron Ductile iron is also called nodular cast iron. Adding magnesium to the gray cast iron before casting produces a distinctly different microstructure and mechanical properties.
The mechanical properties of ductile irons can be improved by heat
treatments. It is used in applications such as automobile crankshafts, main shafts and rotors for machinery drive etc.
7.1.1.4 Gray Cast Iron The carbon and silicon contents of gray cast irons lie between 2.5 to 4 wt% and 1.0 to 3.0 wt%, respectively.
Because of the high silicon content, cementite
decomposes into ferrite and graphite. The graphite exists in the form of flakes. Gray cast iron is weak and brittle in tension. However its strength and ductility are much higher under compressive loads.
They are very effective in damping
vibrational energy. It is used as base structures for machines and heavy equipment that are exposed to vibrations.
7.1.1.5 High Silicon Cast Iron High silicon cast iron is produced by increasing the silicon content in gray cast iron to about 14%. It is usually corrosion resistant in many environments. The notable exception is hydrofluoric acid. Their inherent hardness makes them corrosion resistant to erosion corrosion. A straight high silicon iron such as Duriron contains about 14.5% silicon and 0.95% carbon. This composition is suited to provide the best combination of
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corrosion resistance and mechanical strength. The excellent corrosion resistance of high silicon irons is due to the formation of passive SiO2 surface layer, which forms during exposure to the environment. The high silicon cast iron has a major application in pipelines and fittings.
7.1.2 Steels Steels are basically iron-carbon alloys in which the carbon content is usually less than 1.0 wt%, along with the addition of other alloying elements.
The mechanical
properties of steels depend on its carbon content. Steels are classified into different types depending on their carbon content. They are plain carbon steels, low alloy steels and high alloy steels.
7.1.2.1 Plain Carbon Steels Plain carbon steel contains carbon as the only alloying element. As the carbon content increases, brittleness increases. The mechanical properties of this steel are controlled by the carbon content. The plain carbon steels are further classified as lowcarbon (0.3%), medium-carbon (0.3 to 0.6%) and high-carbon (0.6 to 1.0%).
Low
carbon steels are used in automobile body panels, tin plate, and wire products. Medium carbon steels are used in shafts, axles, gears, crankshafts, couplings, and forgings. High carbon steels are used in spring materials and high strength wires.
7.1.2.2 Low Alloy Steels Low alloy steels contain alloy content less than 8%. Carbon steel is usually alloyed individually or in combination with small quantities of chromium, nickel, copper, molybdenum, phosphorus, and vanadium to produce low alloy steels.
Better
mechanical properties can be obtained by increasing the alloy content, but the most important advantage is better corrosion resistance to atmospheric corrosion. Strengths are higher than ordinary carbon steel. These materials can be used for stampings, forgings and boiler plates.
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7.1.2.3 Stainless Steels The most important advantage of stainless steels is their resistance to corrosion and good mechanical properties at atmospheric and elevated temperatures. Chromium is the main alloying element and the steel should contain at least 11%. Chromium tends to form passive film and exhibit excellent resistance to many environments. Stainless steels form pits when exposed to chloride environments. The stainless steels are classified into three groups based on their structure, composition, and characteristics.
They are austenitic stainless steels, ferritic stainless steels, and
martensitic stainless steels.
Austenitic stainless steels are also called as work
hardening steels. The strength and hardness of this steel can be improved by cold working. The most predominant alloying elements are chromium and nickel, which is usually in the range of 16 to 26% and 6 to 22%, respectively. Austenitic stainless steels are the most corrosion resistant because of the high chromium contents and the nickel addition. They are produced in the largest quantities. The applications include chemical processing equipment. Ferritic stainless steels contain 11.5 to 27% chromium and the carbon content is usually low. These stainless steels are also called non-hardening steels because they cannot be hardened by heat treatment. These steels find applications in automotive industry. Martensitic stainless steels are also known as hardened alloy steels. Chromium content will be usually in the range of 11.5 to 18%. The mechanical properties can be improved by quenching. These steels find major applications in oil and gas industry.
7.1.3 Aluminum and its Alloys Aluminum alloys are a mixture of base metal aluminum with one or more alloying elements, which can be metallic or non-metallic (such as silicon). The aim of alloying is to enhance the properties of the base metal, (e.g. its strength and corrosion resistance, etc). Aluminum has a specific gravity of 2.7. Aluminum alloys have a strong resistance to corrosion because of an oxide skin that forms as a result of reactions with the atmosphere. This corrosive skin protects aluminum from most chemicals, weathering conditions, and even many acids. However alkaline substances are known to penetrate
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the protective skin and corrode the metal. Aluminum has high conductivity for heat and electricity. It finds applications in aerospace and automotive industries.
7.1.4 Magnesium and its Alloys Magnesium is the lightest metal having specific gravity of 1.74. Magnesium is a silvery-white metal that is used as an alloy element for aluminum, lead, zinc, and other nonferrous alloys. Magnesium is ductile and suitable for all metal working processes. Magnesium forms a film to protect against corrosion. However, it is easily corroded by chlorides, sulfates, and other chemicals. Magnesium is easily susceptible to corrosion in marine environments. Hence magnesium is usually anodized to improve its corrosion resistance.
Magnesium is non-toxic and non-magnetic.
Magnesium finds various
applications in the aerospace parts such as fuselage, engine parts and other accessories. Magnesium is extensively used in printing and textile industries.
7.1.4.1 Lead and its Alloys Lead is one of the oldest metals used. It is used for water piping. Lead has excellent atmospheric and sea-water corrosion resistance.
Lead also possesses
excellent corrosion resistance against sulfuric acid. Lead and its alloys are used in piping, sheet linings, solders, storage batteries, radiation shields, cable sheath, bearings, roofing, and ammunition. Lead is soft, easily formed and has a low melting point. In the process industry, lead is alloyed with 0.06% copper to improve corrosion resistance.
It is rapidly attacked by acetic acid and generally not used in nitric,
hydrochloric, and organic acids. The strength of the lead can be improved by addition of antimony or calcium.
7.1.5 Copper and its Alloys Copper is a base material alloyed with tin, zinc, nickel, etc. to enhance its strength. The mechanical properties can be improved by cold working. When copper is alloyed with tin, it forms bronze. Similarly, copper alloyed with zinc forms brass. The
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most popular brasses or bronzes are cold heading brass, naval bronze, and leaded brass. Copper possesses higher electrical conductivity, thermal conductivity, corrosion resistance, and ductility. The major applications of copper alloys are power utilities, lighting and wiring devices, electronics, appliances, electric cords, etc.
7.1.6 Nickel and its Alloys Nickel has low corrosion rates in acid solutions in the active state.
Alloying
additions of copper, molybdenum, and tungsten lowers the corrosion rate further. Nickel is one of the major alloying elements in the stainless steels, corrosion resistant steels, and high temperature steels. Nickel alloys possess high strength, toughness, and ductility.
Some of the important nickel alloys are Monel, Constantan, German
silver, Hastelloy D, Iconel, etc.
The nickel alloys are used extensively for vessels,
piping, pumps, and in the production of most minerals and organic acids.
7.1.7 Zinc and its Alloys Zinc is not a corrosion resistant metal but it is utilized as a sacrificial anode for cathodic protection of steel. Its use is in galvanized steel for piping, fencing, nails, etc. It is also utilized in the form of bars or slabs as anodes to protect ship hulls, pipelines, and other structures. Zinc alloy parts are made by die casting because of their low melting points. Many automobile components such as grilles and door handles are die cast but are usually plated with corrosion resistant metals.
7.1.8 Cadmium Cadmium is used exclusively as an electroplated coating. Cadmium is soluble in hydrochloric acid, sulfuric acid and nitric acid. Cadmium can be used to protect steel, but it is not as effective as zinc. Cadmium is more expensive than zinc and its salts are toxic.
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Cadmium plating is utilized on high strength steels in aircraft because of improved resistance to corrosion fatigue.
It can be soldered and hence it finds
applications in radio and television industries.
7.1.9 Titanium and its Alloys Titanium is highly reactive forming a continuous, stable, protective, and adherent oxide film on the surface in presence of oxygen and moisture. Titanium alloys are often unsuitable for hot, concentrated, reducing acids in which the oxide film cannot form. Titanium finds wide spread application in the aircraft industry.
7.1.10 Coated Alloys Alloy materials, which have a coating material on it, are called coated alloys. They have more resistance to corrosion. The coatings used should be compatible with the underlying alloy materials.
7.1.11 Molybdenum Molybdenum shows good resistance to hydrofluoric, hydrochloric, and sulfuric acids, but oxidizing agents such as nitric acid cause rapid attack. It is used as an alloying element for steel to increase its hardness, toughness, strength, and abrasion resistance. Molybdenum is lighter in weight, easier for fabrication, and possesses high ductility. Molybdenum finds applications in electronic tubes and in pressure applications where galling is a problem.
7.1.12 Tantalum Tantalum is widely used in handling chemically pure solutions such as hydrochloric acid.
Tantalum possesses high thermal conductivity and corrosion
resistance. But tantalum is easily attacked by alkalies. Tantalum is used for surgical implants.
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7.1.13 Tungsten Tungsten has the highest melting point temperature.
Its chief use involves
strength at high temperatures. A common example is filament in bulbs. Tungsten shows good resistance to acids and alkalies except nitric acid.
7.1.14 Zirconium Zirconium finds extensive application in the ceramic industry. zirconium is an important structural metal in atomic energy plants.
Moreover,
Zirconium has
excellent corrosion resistance due to its protective oxide film. This metal exhibits good corrosion resistance to alkalies and acids except for hydrofluoric acid, hot concentrated hydrochloric, and sulfuric acids. Zirconium has found some applications in hydrochloric acid service. Its corrosion resistance is affected by impurities in the metal such as nitrogen, aluminum, iron, and carbon.
7.1.15 Gold Gold is one of the oldest metals used. Gold and its alloys are used in jewelry, dental inlays, electrical contacts, plating, tableware and decorative purposes. Gold is not attacked by dilute nitric acid and sulfuric acid. But gold is attacked by concentrated nitric acid, chlorine, and bromine, mercury and alkaline cyanides. The color, stability and luster made this metal useful in jewelry and coinage.
7.1.16 Platinum Platinum exhibits good mechanical properties such as high temperatures and wear resistance. The inertness of platinum is attested by its extensive service as a catalyst. It is very expensive and hence it is used for limited applications. Platinum has excellent corrosion resistance and oxidation resistance in hot oxidizing gases. Platinum is attacked by aqua regia, hydroiodic, hydrobromic acids, ferric chloride, chlorine and bromine. It is used in spark plugs, electrical resistors, burner nozzles, and as anodes or cathodes in electrolytic cells.
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7.1.17 Silver Silver is best known for its use as coinage, jewelry and tableware.
Silver
possesses high electrical and thermal conductivity, stability, and luster. Silver finds extensive applications in the photography industry. Silver is also widely used in the chemical industry as solid silver and also as loose, clad, or brazed linings.
Silver
tarnishes in the presence of hydrogen sulfide and sulfur bearing environments. It is highly resistant to organic acids. Silver is attacked by nitric acid, hot hydrochloric acid, mercury, and alkaline cyanides and may be corroded by reducing acids if oxidizing agents are present.
7.2 Plastics A plastic is a material that contains organic substance of larger molecular weight, is solid in its finished state. Plastics are classified into two types thermoplastics and thermosets.
Thermoplastic materials soften when heated and return to the original
hardness when cooled. Some common examples of thermoplastic materials are nylon, acrylic and cellulose. On the other hand thermosets harden when heated and retains the hardness after cooling. phenolics and polyesters.
Some common examples of thermosets are epoxies, Plastics are generally much weaker, softer, and more
resistant to chloride ions and hydrochloric acid, less resistant to concentrated sulfuric and oxidizing acids such as nitric, less resistant to solvents.
7.2.1 Thermosets A thermoset is a resin in liquid form before curing (eg. epoxy or polyester), which undergoes a chemical reaction by the action of heat, catalyst, ultraviolet light, etc., to become a relatively insoluble substance. Upon the application of heat or catalysis the liquid resin becomes rigid due to curing process. The resultant product is less sensitive to temperature and it is nonrecyclable. The chemical bonds are formed through crosslinking of molecules and this provides thermal stability without any flow of resin upon
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heating. A thermoset composite is a material containing a thermosetting polymeric matrix.
7.2.2 Thermoplastics Thermoplastics are those materials whose change upon heating is substantially physical rather than chemical. They are largely one-dimensional or two-dimensional molecular structures such as nylons, polycarbonates, acetals, and polysulfones. These are linear or branched structures and they are recyclable. The polymer melts and flows upon heating and has a number of heat sensitive properties.
Individual polymer
molecules are held together by weak forces, such as Van der Waal forces, hydrogen bonds and dipole- dipole interactions. Thermoplastic matrices reinforced with carbon, glass or aramide fibers are called thermoplastic composites.
7.3 Ceramics A ceramic material consists of compounds of metallic and non-metallic elements. Ceramic materials find extensive applications in jet engines and atomic reactors. Ceramics include brick, stone, fused silica, stoneware, glass, clay tile, porcelain, concrete, abrasives etc.
Ceramic materials have low tensile strength and high
compressive strength. The strength of the material depends on cross-sectional area, composition and surface conditions. Most ceramic materials exhibit good resistance to chemicals with exception to hydrofluoric acid and alkalies. Ceramic parts are usually formed by pressing, extrusion or slip casting.
7.4 Composite Materials A composite material is made by combining two or more materials to give a unique combination of properties.
Composites maintain an interface between
components and act in concert to provide improved specific or synergistic characteristics not obtainable by any of the components acting alone. Composites
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include: (1) fibrous (composed of fibers usually in a matrix), (2) laminar (layers of materials), (3) particulate (composed of particles or flakes, usually in a matrix), and (4) hybrid (combinations of any of the above).
7.4.1 Organic Matrix Composites (OMCs) The matrix in the composite is a chemical compound containing carbon molecules with the exception of CO2, which is considered inorganic. These composites in which fibers are embedded in an organic matrix are called OMCs. They are primarily used in aerospace structures as they have high resistance to corrosion and fatigue damage. OMCs are further subdivided as:
Polymer Matrix Composites (PMCs)
A polymer is an organic compound, natural or synthetic, whose structure can be represented by a repeated small unit (mer). These long molecular chains consist of repeating chemical units held together by covalent bonds formed by a polymerization reaction. PMCs consist of polymer resin as the matrix. When the matrix material is such a polymeric chain, then it is called a PMC.
PMCs are typically used in low-
temperature structural applications such as in civil-structures, biomedical implants, automobiles, and airframe structures.
The fibers typically provide the stiffness and
strength to the composite and can be made from a wide variety of materials including glass, graphite, kevlar, and boron. The fibers can be arranged in almost any fashion, ranging from totally random to highly structured and organized. Carbon Matrix Composites (CMCs)
Extra steps of carbonizing and densifying the original polymer matrix typically form carbon matrix composites.
They are commonly refered to as carbon-carbon
composites.
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7.4.2 Metal Matrix Composites (MMCs) MMCs include metallic matrix materials reinforced with continuous fibers such as boron, silicon carbide, graphite or alumina, wires including tungsten, beryllium, titanium and molybdenum and discontinuous materials such as fibers, whiskers and particulates. The reinforcements are chosen to increase stiffness, strength, as well as heat, and wear resistance.
The principal motive of using metal matrix composites was to dramatically
extend the structural efficiency of the metallic materials while retaining their advantages including high chemical inertness, high shear strength and good property retention at high temperatures.
7.4.3 Ceramic Matrix Composites (CMCs) Materials consisting of a ceramic or carbon fiber surrounded by a ceramic matrix, primarily silicon carbide. Ceramic materials are inherently resilient to oxidation and deterioration at elevated temperatures. Ceramic matrix composite focuses on achieving useful structural and environmental properties at high operating temperatures.
7.4.4 Particulate Reinforcements Particulate reinforced composites include those reinforced by spheres, rods, flakes and many other shapes of roughly equal axes.
These can be classified as
discontinuous reinforcements.
Figure 7-1: Particulate Reinforcement Reference: http://www.asm-intl.org/pdf/spotlights/IntroToComposites.pdf
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7.4.5 Whisker Reinforcements Whisker reinforcements are short, discontinuous fibers of polygonal crosssections, made of a large number of materials such as graphite, aluminum oxide, silicon carbide, silicon oxide, boron carbide and beryllium oxide.
Figure 7-2: Whisker Reinforcement Reference: http://www.asm-intl.org/pdf/spotlights/IntroToComposites.pdf
7.4.6 Continuous Fiber Reinforced Composites Continuous fiber reinforced composites contain reinforcements whose lengths are much greater than their cross sectional dimensions. If any increase in length of the fiber does not provide further increase in elastic modulus or strength of the composite, then the composite is considered to be continuous fiber reinforced.
Figure 7-3: Continuous Fiber Reinforcement
Reference: http://www.asm-intl.org/pdf/spotlights/IntroToComposites.pdf
7.4.7 Braided Fabrics Braided fabrics are fabrics in which two sets of continuous fibers are interwoven symmetrically about an axis. Braided fabrics are engineered with a system of two or more yarns intertwined in such a way that all of the yarns are interlocked for optimum load distribution. Biaxial braids provide reinforcement in the bias direction only with fiber angles ranging from ± 15o to ± 90o. Triaxial braids provide reinforcement in the bias direction with fiber angles ranging from ± 10o to ± 80o and axial (0o) direction.
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Figure 7-4: Braided Fabrics Reference: http://www.asm-intl.org/pdf/spotlights/IntroToComposites.pdf
7.4.8 Hybrid Fabrics The term hybrid refers to a fabric that has more than one type of structural fiber in its construction. If low weight or extremely thin laminates are required, a hybrid fabric will allow the two fibers to be presented in just one layer of fabric. Typically, the available fiber orientations include the 0o direction (warp), 90o direction (weft or fill). It would be possible in a woven hybrid to have one fiber running in the weft direction and the second fiber running in the warp direction, but it is more common to find alternating threads of each fiber in each warp direction.
There are several types of hybrid
composites characterized as: (1) interply or tow-by-tow, in which tows of the two or more constituent types of fiber are mixed in a regular or random manner; (2) sandwich hybrids, also known as core-shell, in which one material is sandwiched between two layers of another; (3) interply or laminated, where alternate layers of the two (or more) materials are stacked in a regular manner; (4) intimately mixed hybrids, where the constituent fibers are made to mix as randomly as possible so that no overconcentration of any one type is present in the material; (5) other kinds, such as those reinforced with ribs, pultruded wires, thin veils of fiber or combinations of the above.
Figure 7-5: Hybrid Fabrics Reference: http://www.netcomposites.com/education.asp
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7.4.9 Knitted or Stitched Fabrics Stitched fabrics are produced by assembling successive layers of aligned fibers. Typically, the available fiber orientations include the 0o direction (warp), 90o direction (weft or fill), and +45o direction (bias).
The assembly of each layer is then sewn
together. This type of construction allows for load sharing between fibers so that a higher modulus, both tensile and flexural, is typically observed.
This allows for
maximum resin flow when composites are manufactured.
Figure 7-6: Knitted or Stitched Fabrics Reference: http://www.enae.umd.edu/ASC/abstracts/abs102.pdf
7.4.10 Woven Composites A planar woven fabric composite is a fabric produced by interlacing two or more sets of yarns, fibers, rovings, or filaments where the elements pass each other essentially at right angles and one set of elements is parallel to the fabric axis.
Figure 7-7: Woven Fabric Reference: http://www-mech.eng.cam.ac.uk/ccm/projects/mpfs/departabs2.pdf
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7.4.11 Carbon Fiber Reinforced Plastics A composite material composed of carbon fiber as filler reinforcement and plastics as matrix. The most important feature of it is that it has excellent mechanical properties though it has low density. They have high stiffness to weight ratios and provide better fatigue characteristics to the composite by reducing the amount of strain in the polymer matrix.
7.4.12 Glass Fiber Reinforced Plastics In this type of FRP the reinforcement is provided by glass fibers. Glass fibers exhibit the typical glass properties of hardness, corrosion resistance, and inertness. Tensile strength of glass fibers reduces with increases in temperature and with chemical corrosion.
7.4.13 Aramid Fiber Reinforced Plastics The trade name of this type of fiber is Kevlar. Aramid fibers have high-energy absorption during failure, which make them suitable for impact and ballistic protection. They have low compressive strength, they creep absorb moisture and are sensitive to UV radiation.
7.5 Manufacturing Processes of Composites 7.5.1 Open-Mold Processes Open mold processes make use of a single cavity mould and requires little or no external pressure. The mold is waxed and sprayed with gel coat. After the gel coat cures, catalyzed resin is sprayed along with fibers. The fibers are chopped directly into the resin stream. The laminate is then compacted by hand with rollers. The composite is then cured, cooled and removed from the reusable mold.
The open mold technique
produces emission of volatile organic compounds. Types of open mold spray up are hand lay up and tube rolling.
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7.5.1.1 Hand Lay Up An open mold process in which the reinforcements are applied to the mold and the composite is built by hand. Resins are impregnated by hand into fibers, which are in the form of woven, knitted, stitched or bonded fabrics. This is usually accomplished by using rollers or brushes, with an increasing use of nip-roller type impregnators for forcing resin into the fabrics by means of rotating rollers and a bath of resin. Laminates are left to cure under standard atmospheric conditions. Curing is normal at ambient temperature but if heating is done the curing process is accelerated.
Figure 7-8: Hand Lay Up Reference. http://www.netcomposites.com/education.asp?sequence=55
7.5.1.2 Tube Rolling Tube rolling is the manufacturing process used to produce finite length tube and rod. The material is precut in particular patterns to achieve the required ply schedule and the fiber architecture. The pieces are laid and the mandrel is rolled over under pressure.
Fibers must be continuously realigned to impart bending strength.
technique is more environmentally friendly.
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Figure 7-9: Tube Rolling Reference: http://www.harrisonrods.co.uk/production.htm
7.5.2 Closed-Mold Processes A technique used in composite fabrication that utilizes a two-piece mold (male and female). The processes are usually largely automated. Various methods are used to transfer liquid resin from an external source into the dry preform that will be placed in a two-sided matched closed mold. Closed mold processes include RTM, VARTM, resin Injection molding, compression molding, pultrusion, extrusion, and filament winding process.
7.5.2.1 Resin Transfer Molding (RTM) A closed-mold pressure injection system that allows for faster cure times as compared to contact molded parts. The process uses polyester matrix material systems in association with cold-molding and reinforcement material types such as continuous strand, cloth, woven roving, long fiber and chopped strand. In RTM, resins and catalyst are metered and mixed in the dispenser equipment before they are injected/infused into the mold.
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Figure 7-10: Resin Transfer Molding Machine (CFC-WVU)
7.5.2.2 Vacuum Assisted Resin Transfer Molding (VARTM)
Figure 7-11: VARTM –Tabletop Model of VARTM and Schematic Process of Manufacture Reference: http://www.ncat.edu/~sasmith/C2.pdf
7.5.2.3 Resin Injection Molding Process Resin injection molding is similar to the resin transfer molding process except that it injects a resin/catalyst into the mold in two streams, so that mixing and the resultant chemical reaction occur in the mold instead in the dispensing head. The material is forced from an external heated chamber through a runner or gate into a cavity of a closed mold by means of a pressure gradient, independent of the mold's clamping force.
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Figure 7-12: Injection Molding Machine (CFC-WVU)
7.5.2.4 Compression Molding Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, and pressure is applied to force the material into contact with all mold areas. material has cured.
Heat and pressure are maintained until the molding
Compression molding is a high-volume, high-pressure method
suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand. The advantage of compression molding is its ability to mold intricate parts of large dimensions.
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Figure 7-13: Compression Molding Machine (CFC-WVU)
7.5.2.5 Pultrusion Pultrusion is a continuous automated closed molding process that is cost effective for high volume production of constant cross section parts. This process relies on relies on reciprocating or puller/clamping systems to pull the fiber and resin continuously from a resin impregnating bath through a heated steel die. Excess resin is squeezed out by the shaped bushings. The compacted part then enters the die where it cures.
Figure 7-14: Schematic Representation of Pultrusion Process (Bedford Plastics)
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7.5.2.6 Extrusion Extrusion is a process used for forming composite preformed materials from mixtures of a matrix powder and short fibers suitable for MMCs. It is a thermoplastic process whereby pellets, granules, or powder are melted and forced through a die under pressure to form a given, continuous shape.
Figure 7-15: Basic Extruder Reference: http://www.reliance.com/prodserv/standriv/appnotes/d7741.pdf
7.5.2.7 Filament Winding Process "Filament Winding" is a highly automated process in which fiber yarn (Carbon Fiber, Fiberglass, Kevlar, etc.) is pulled from a large spool, pulled through a bath of resinous polymeric material (epoxy, etc.), and "wound" upon a tool, or mandrel. A release agent is applied to the mandrel before winding which enables the composite part to be removed later. The mandrel is then placed under tension in the winding machine, which rotates the mandrel while moving a carriage that applies the composite material. A computer controls these motions, and ensures that the composite material will be applied accurately. Once the composite material is applied, a special non-stick plastic film is wrapped under tension around the part. This film is applied to provide compaction to the composite, and is removed later. The mandrel is then placed in a computer-controlled oven, and special heating profiles harden the polymeric resin, solidifying the composite material. The mandrel is then removed by placing the mandrel
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/ part into an extraction machine which pulls the mandrel out of the part. The part can then be machined, finished, and painted into a final form.
Figure 7-16 a: Winding Machine Showing Carriage and Mandrel b: Filament Winding Reference a: http://www.advancedcomposites.com/winding.htm Reference b: http://www.netcomposites.com/education.asp?sequence=57
7.6 Terms Related to Composites Abraded – Make the surface rough by mechanical means such as wirebrushing. Accelerated Aging - Any set of conditions designed to produce in a short time
the results obtained under normal conditions of aging. Additives – This material modifies and enhances the final FRP product. Adherent – A member of a bonded joint. Adhesive – A substance capable of holding two materials together by surface
treatment. Adhesive Failure – Failure caused due to the rupture of the adhesive bond
leading to the separation of the adhesive - adherend interface. Aging - The effect of environmental factors, like moisture, temperature, etc. on
the composite material when exposed for a period of time.
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Aging Factor – Aging Effects due to physical and chemical exposure are
accounted for using aging factors in design methodology. Anisotropic Material – Elastic properties are different in all the directions. Aramid Fibers - A manufactured fiber in which the fiber forming substance
consists of a long chain synthetic aromatic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings. Axial Strength – The strength of the material in tension or compression when
the load is applied length wise along the axis. It may be tensile or compressive strength. Bending Strength - The strength of a material in bending, expressed as the
stress on the outermost fibers of a bent test specimen, at the instant of failure. Bending Modulus - The ratio, within the elastic limit, of the applied stress on a
test specimen in bending, to the corresponding strain in the outermost fibers of the specimen. Bending Rigidity - Stiffness due to bending forces. Bi-directional Fibers/Fabrics (2-D) – Continuous rovings placed in a plane in
any two directions. Block Shear Failure - Combination of tension failure along one plane and shear
failure along another plane of the same component. Bolt Shear Failure – Is caused by high shear stress in the fastener. Bond – The adhesion of one surface to another, with the use of an adhesive or
bonding agent.
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Carbon Fibers – Fibers produced by the pyrolysis of organic precursor fibers
such as rayon. PAN or pitch in an inert atmosphere. Chemical degradation – The degradation or the deterioration of a material due
to chemical attack. Cleavage Failure – Characterized by a single-plane cleavage failure where the
apparent laminate transverse tensile strength is less than the corresponding in plane shear strength. Coefficient of Thermal Expansion - The coefficient of thermal expansion
(linear) is the change in length per unit length of material, for a one degree Centigrade change in temperature. Cohesive Failure – Is caused due to the failure of the adhesive/adherend when
subjected to loads exceeding the adhesive/adherend strength. Composite - Composites are a combination of a reinforcement fiber in a polymer
resin matrix, where the reinforcement has an aspect ratio that enables the transfer of loads between fibers, and the fibers are chemically bonded to the resin matrix. Composite Moment of Inertia - Moment of inertia due to joint action between
the composite deck and girder. Crazing – Fine cracks at or near the surface of a plastic material. Creep – Time dependent deformation of a material (or structure) under constant
load. Debonding – Separation of fiber from the matrix. Delamination – The separation of the layers of material in a laminate. Delams – Delamination in the FRP deck.
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Ductility – Ability of material to undergo large deformations before fracturing. Ductility Factor – The ratio between the difference of total impact energy and
energy to peak impact force to energy to peak impact force. Durability – The ability of a structure to maintain strength and stiffness
throughout the service life of the structure. ECR Glass – ECR glass fibers are boron free, very resistant to chemical attack
and have similar properties to E glass. Effective Flange width - In an FRP composite deck system, the shear
deformations tend to reduce the longitudinal stresses as they progress transversely from the web of the girder (shear lag effect). In engineering practice, this problem is accounted for assuming only a portion of the flange to the effective. This reduced flange width is referred to as the effective flange width. Elastic Deformation - Nonpermanent deformation, after which body returns to
original shape or volume when deforming force is removed. Epoxy Resin – Resins, which may be of widely different structures but which are
characterized by the reaction of the epoxy group to form a cross-linked hard resin. Extensional Bending Coupling Stiffness – Stiffness due to coupling of in-plane
and bending forces. Factored Resistance – Product of nominal resistance and the resistance factor,
which is provided to account for variability in material properties of FRP. Failure strain – The ultimate strain at which the FRP material fails. Fatigue - The failure or decay of mechanical properties after repeated
applications of stress.
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Fiber – A single homogeneous strand of material having a length of at least 5
mm, which can be spun into a yarn or roving, or made into a fabric by interlacing in a variety of methods. Fiber Direction - The orientation of the major axes of the fiber weave as related
to the designated zero direction. Fiber Reinforced Polymer – Fiber reinforced polymer materials consist of fibers
of high strength and modulus embedded in or bonded to a matrix with distinct interfaces (boundary) between them. Fire Retardant Resins – Resins that are inflammable. Flexural Strength - The strength of a material in bending, expressed as the
stress on the outermost fibers of a bent test specimen, at the instant of failure. Flexural Modulus - The ratio, within the elastic limit, of the applied stress on a
test specimen in flexure, to the corresponding strain in the outermost fibers of the specimen. Gel Coat – A quick setting resin used in molding process to provide an improved
surface for the composite. Glass Transition – The reversible change in an amorphous polymer or in
amorphous regions in partially crystalline polymer from a viscous or rubbery condition to a hard and relatively brittle one. Glass Transition Temperature – Temperature range over, which a plastic
changes from a rigid state to softened (rubber-like) state, resulting in significantly degraded mechanical and electrical properties. This occurs in a narrow temperature range, rather than a sharp point such as a freezing point or boiling point.
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Global Deflection – The maximum global deflection of the girder in a typical
composite structure. Impact Resistance – Resistance offered by the structure against impact loads. In-Plane Shear Loading – Adherend shear loads that produce shear stresses in
the bond line in lap and strap joints. In-Plane Stiffness - Stiffness in the in-plane direction due to in-plane forces. Isotropic – Elastic properties are same in all the directions; hence, the material
contains an infinite number of planes of material property symmetry passing through the same point. Lamina – A single ply or layer in a laminate of layers. Limit State - A condition beyond which the structure ceases to satisfy the
provisions for which it was designed. Limit Stresses – A value of stress beyond which the structure ceases to satisfy
for provision for which it was designed. Local Deflection – Local deflection in the top flange of FRP deck. Matrix - Polymeric material used as binder for reinforcing fibers in FRP. Net-tension Failure – Occurs when the specimen is narrow compared to the bolt
diameter and the crack propagates transverse to the loading. Nominal Resistance – The resistance of a component or connection to force
effects, as indicated by the dimensions specified in the contract documents and by maximum permissible stresses, deformations, or specified strength of materials. Nonductile Material – A material, which is brittle, or a material that can undergo
very little plastic deformation.
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Orthotropic Material – FRP materials containing three orthogonal planes of
material property symmetry.
Overlay – The laying of FRP material or asphalt over a composite deck in order
to prevent further corrosion or for decorative purposes. Overloads – Loads that are heavier than the design loads of the structure. Protective Coatings – A protective and finishing enclosure surface for the FRP
material. Resin - Polymeric material used as binder for reinforcing fibers in FRP. Resistance Factor – Factor, which depends on, event of rupture of the section
due to tension, flexure, shear, torsion or combination. Rovings – A number of ends, tows, or strands collected into a parallel bundle
with little or no twist. Rule of Mixtures - Formed sub laminates undergo the same axial strain but not
necessarily the same transverse strain, based on the linear volume fraction relationship between the composite and its corresponding constituent properties. Service Limit State – Limit states relating to stress, deformation and cracking
under service loads. Shear - An action or stress resulting from applied forces that tend to cause two
contiguous parts of a body to slide relative to each other in a direction parallel to their plane of contact. Shear Rigidity – Stiffness due to shear forces.
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Shear Modulus - The ratio of shearing stress τ to shearing strain γ within the
proportional limit of a material. Shearlag – The non-uniform shear distribution within any given material. Shear-Out Failure - Occurs as the section of the material parallel to loading
pushes past the remaining specimen. It is caused by shear stress and occurs along the shear- out planes. Single Lap Joint – A joint formed by overlapping one laminate over the other. Stitched Fabrics (3-D) – Multidirectional rovings in a plane with various layers in
the direction perpendicular to the plane and stitched together (yarn, glass fiber, etc) either in-plane or out of plane. Temperature Gradient - Temperature gradient is the rate of change of
temperature with distance in any given direction at any point i.e, the temperature gradient from the top to bottom of composite deck. Tensile Shear loading – Adherend shear loads that produce shear stresses to
the bond line in lap and strap joints. Tensile Strength – Strength of a material in tension. Thermally Induced Loads – Loads induced due to thermal gradient in a FRP
composite Deck. Thermoplastic – A plastic that can be repeatedly softened by heating and
hardened by cooling through a temperature range characteristic of the plastic, and when in the softened stage can be shaped by flow into shapes by molding or extrusion. Thermoset – A plastic that is substantially infusible and insoluble after being
cured by heat or other means.
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Unidirectional Fibers (1-D) – Continuous long fibers in one direction for which
strength and stiffness is maximum along the direction of fibers. Void Content - Amount of voids existing in a given material Wearing Surface - Overlay material applied on the surface of the FRP Deck. Young’s Modulus – The ratio stress/strain within elastic limit.
7.7 Reagents 7.7.1 Sulfuric Acid Sulfuric acid is a very strong acid. It is a dense, colorless, and corrosive liquid. It is used in manufacturing fertilizers, pigments, dyes, drugs, and explosives. It is also used in petroleum refining and metallurgical processes.
The commercial industrial
concentrations for sulfuric acid are 78%, 93% and oleum. Steel is the most common material used for storage and transportation of sulfuric acid. Dilute acids attack steel more rapidly than the strong acids. Moreover, the corrosion rate of steel increases as the temperature of the acid increases when the concentration of the acid is up to 70%. Above 70% concentration, there is no reliable data available to predict the corrosion rate. Aeration of sulfuric acid in the storage tanks will lead to drastic efforts. Cast iron shows similar effects like steel. High silicon cast iron containing 14.5% silicon has better performance against corrosion. Usage of high silicon cast iron should be avoided in case of fuming acids.
Copper alloys are not generally used for sulfuric acid
applications.
7.7.2 Hydrochloric Acid Hydrochloric acid is used in the manufacture of fertilizers, dyes, artificial silk, and pigment for paints.
It is used in removing scales in boilers, clean membranes in
desalination plants, and to clean other metals for coatings. Hydrochloric acid is one of the most corrosive of the non-oxidizing acids in contact with copper alloys, and is
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handled in dilute solutions. It is soluble in benzene, alcohol, and ether. It is insoluble in hydrocarbons. It is incompatible/reactive with metals, hydroxides, amines, and alkalis. Hydrochloric acid’s fumes have an acid, penetrating odor.
Aqueous solutions of
hydrochloric acid attack and corrode nearly all metals except mercury, silver, gold, platinum, tantalum, and certain alloys.
It may be colored yellow by traces of iron,
chlorine, and organic matter.
7.7.3 Nitric Acid Nitric acid is a corrosive, non-volatile and inorganic acid.
It is used in the
manufacture of fertilizers, dyes, explosives, and other organic chemicals. It is a strong, monobasic acid and an oxidizing agent. In the presence of traces of oxides, it attacks all base metals except aluminum and special chromium steels. It is soluble in water (cold and hot), and ether. It increases the flammability of combustible organic and oxidizable materials and can also cause ignition of some of these materials.
7.7.4 Hydrofluoric Acid Hydrofluoric acid is a weak acid.
It finds extensive applications in various
industries such as glass industry, leather industry, textile industry, fertilizers, etc. Magnesium shows excellent corrosion resistance to hydrofluoric acid. Steel is used in handling the hydrofluoric acid when the concentration is between 60 and 100%. Monel shows excellent corrosion resistance at all concentrations and temperatures. aeration and presence of oxidizing salts leads to increase in corrosion rate.
But Lead
shows good resistance to hydrofluoric acid in concentrations below 60% at room temperature.
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7.7.5 Phosphoric Acid Phosphoric acid usually exists as a crystal or clear liquid. It is either an oily, thick, colorless, and odorless liquid, or a thick, colorless, unstable crystalline solid. It is used in the manufacturing of phosphates, fertilizers, electric lights, and soft drinks. It is used as an acid catalyst, soil stabilizer, antioxidant in food, bonding agent for refractory bricks, and gasoline additive. It is also used in the rust proofing and polishing of metals, cotton dyeing, tile cleaning, extracting penicillin, hot stripping for aluminum and zinc substrates, ceramic binding, water treatment, process engraving, electro-polishing, coagulating of rubber latex, operating lithography, photoengraving operations, and pickling. Phosphoric acid is incompatible with strong caustics and most metals. It readily reacts with metals to form flammable hydrogen gas. The liquid can solidify at temperatures below 21oC. It is corrosive to ferrous metals and alloys. It is soluble in alcohol and hot water. It can form three series of salts: primary phosphates, dibasic phosphates, and tribasic phosphates. It is deliquescent and hygroscopic. It is a chelating agent. It has a low vapor pressure at room temperature.
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8 CORROSION KINETICS
The corrosion reaction rates can be predicted using the thermodynamic principles only when the reactions are in equilibrium. When the corrosion reactions are not in equilibrium, the corrosion kinetics needs to be considered. Polarization, mixed potential theory, and experimental polarization curves are discussed in this chapter.
8.1 Polarization The polarization is a state where current flows in a circuit, where the cathodic and anodic reactions take place. Polarization can be divided into two types. They are activation polarization and concentration polarization. Activation polarization is related to chemical reaction control while concentration polarization is related to diffusion control.
8.1.1 Activation Polarization The activation polarization can be derived using Figure 8.1. The solid line and the dotted line represent the status of non-polarization and polarization, respectively. The directions of these reactions are conveniently chosen as Anodic
H2
⎯⎯ ⎯⎯→ Cathodic
←⎯ ⎯ ⎯ ⎯
2 H + + 2e −
…(8.1)
The activation energies of the forward and reverse reactions are ΔG f * and ΔGr * . When the forward reaction is polarized by a potential E, the activation energy for the forward reaction becomes: ΔG f * − nFE + (1 − α )nFE = ΔG f * − α nFE and the activation energy for the reverse
reaction also becomes: ΔGr * + (1 − α )nFE
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∆G*F –ά nFE ∆G*r
*
∆G F
∆G*r + (1-ά)nFE
nFE
nFE (1-ά)nFE
ά
(1-ά)
Figure 8-1: Activation Polarization
Then the rates of the anodic and cathodic reactions become: Ra = PH 2 K a = PH 2υ e
−(
ΔEa ) RT
Rc = [ H ]K c = [ H ]υ e +
+
−(
…(8.2) ΔEc ) RT
…(8.3)
where ΔEa and ΔEc are the activation energies of the forward and reverse reactions, respectively. But generally i = nFR where i is the current density (amp/cm2) or (Coulomb/cm2-sec), F is the Faraday constant 96500 (Coulomb/equivalent) and R is the reaction rate (moles/cm2-sec).
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Substitution of Equation (8.2) into (8.4) yields: r −( ΔG f * − α )nFE ) ) i = nFRa = PH 2 nFυ exp( RT
= PH 2 nFυ exp( −
…(8.4) ΔG f *
RT
r α nFE ) i = PH 2 ka exp( RT where ka = nf υ exp(
) exp(
α nFE RT
)
…(8.5)
−ΔG f *
RT
)
r s r But when E = Erev i = i0 . Erev is equilibrium potential where i and i are same as i0 . Here i0 is called exchange current density. Then, the equilibrium rate can be written as:
i0 = PH 2 ka exp(
α nFErev RT
…(8.6)
)
Dividing Equation (8.5) by (8.6) yields: r i =
i0 PH 2 ka exp(
α nFErev RT
)
PH 2 ka exp(
α nFE RT
)
r α nF ( E − Erev ) i = i0 exp RT
…(8.7)
But E - Erev = η . η is called overvoltage and it is a potential for driving force. Then Equation (8.7) becomes: r α nFη i = i0 exp RT
…(8.8)
The rate of reverse reaction can be derived similarly as: s −( ΔGr * + (1 − α )nFE ) i = nF R e = [ H + ]nFυ exp( ) RT
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= [ H + ]nFυ exp( = [ H + ]kc exp( −
−ΔGr * −(1 − α )nFE ) exp( ) RT RT
(1 − α )nFE ) RT
where kc = nFυ exp(
…(8.9)
−ΔGr * ) RT
s But i = i0 when E = Erev i0 = [ H + ]kc exp(
−(1 − α )nFErev ) RT
…(8.10)
Dividing Equation (8.9) by (8.10) yields:
s i =
i0 −(1 − α )nFE [ H + ]kc exp( ) −(1 − α )nFErev RT [ H + ]kc exp( ) RT
s − (1 − α )nF ( E − E rev ) ) i = i0 exp( RT
But η = E - Erev
s −(1 − α )nFη i = i0 exp( ) RT
…(8.11)
The net anodic current is given by: r s ia = i − i
ia = i0 exp(
α nFηa RT
) − i0 exp(
−(1 − α )nFηa ) RT
…(8.12)
Equation (8.12) is called Butler-Volmer equation. We can see that as the r s s overvoltage increases i increases but i decreases. Thus when ηa >> 0, i becomes negligible. By convention ηa takes positive values. Then Equation (8.12) becomes:
ia = i0 exp(
α nFηa RT
…(8.13)
)
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ηa =
2.303RT i log α nF i0
…(8.14)
Equation (8.13) or (8.14) is called Tafel equation.
We can see that Tafel
equation does not consider the reverse reaction rate, where β a =
2.303RT is known as α nF
the anodic Tafel slope. In the case of cathodic reactions, the net cathodic current is given by: s r ic = i − i
ic = i0 exp(
−(1 − α )nFηc α nFηc ) − i0 exp( ) RT RT
…(8.15)
Equation (8.15) is also called Butler-Volmer equation. But when ηc
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