674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 1
Diploma in
Marine Industry Surveying 2008/09 Marine Corrosion & Coatings Module K
Basics of Corrosion Environmental Effects Forms and Mechanisms of Corrosion Survey Equipment and Methods Choosing a Paint System
AUTHOR Mr. Peter Morgan
K
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:12
Page 2
Marine Corrision & Coatings
MPI Group, as a body, are not responsible for any opinions expressed in this module by contributors. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior permission of MPI Group. © Marine Publications International Ltd and Lithgow Associates 2007
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Marine Corrision & Coatings
11:12
Page 1
Module K
Author Mr. Peter Morgan
Page 1
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 2
Module K
Marine Corrision & Coatings
Marine Corrosion & Coatings
Module K
CONTENTS
1
2
3
4
1.1
Corrosion Costs and Economics.
5
1.2
Definition of Corrosion
5
ELECTRO-CHEMISTRY
6
1.3
Atomic Structure and Ion Formation
6
1.4
Electrolytes, Electron Flow and Batteries
7
1.5
Electron Flow
8
1.6
Half Cells
9
1.7
Redox Reactions
9
1.8
Cell Potentials
10
1.9
Corrosion Potentials / Electro-chemical Series
10
1.10
Cell Voltage
11
1.11
pH and Acidity. Hydrogen Ion Concentration
12
CORROSION PROCESSES
13
2.1
Corrosion Sites, Anodes and Cathodes
13
2.2
Passivation
13
2.3
Process Factors affecting Corrosion Mechanisms
14
2.4
Temperature
14
2.5
Concentration Effects
14
2.6
Conductivity Effects
14
2.7
Velocity Effects
14
2.8
Pressure
14
2.9
Galvanic Effects
15
2.9a
Anode size effect in Galvanic Corrosion
15
ENVIRONMENTAL EFFECTS
16
3.1
Marine Atmosphere and Seawater Corrosion
16
3.2
Dirt Deposits and Cargo Deposits
16
3.3
Condensation Corrosion
18
FORMS AND MECHANISMS OF CORROSION
19
4.1
Oxygen Concentration Cell Formation
19
4.2
Mechanism of Oxygen Corrosion
19
4.3
Crevice Corrosion
20
4.4
Pitting corrosion
21
4.5
Galvanic Corrosion
22
4.6
Carbon Dioxide Corrosion
24
4.7
Hydrogen Sulphide Corrosion
24
Page 2
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 3
Module K
Marine Corrision & Coatings
5
6
7.
4.8
Environmental Stress Cracking
24
4.9
Microbiologically Induced Corrosion (MIC)
25
4.10
Other Mechanisms and Types of Corrosion
26
METHODS OF CORROSION CONTROL
28
5.1
Materials selection for corrosion protection
29
5.2
Steels and Irons
29
5.3 Non-Ferrous Materials
30
5.4
31
Thermosets and Thermoplastics
COATINGS
32
6.1
32
Types of Coatings
6.2 Surface Preparation and Application of Coating
32
6.3
Coating Types and Application
33
6.4
Effect of Coatings on Cathodic Protection Design
36
6.5
Coating Evaluation and Inspection Measurements
36
CATHODIC PROTECTION
38
7.1
Theory
38
7.2
Impressed Current Cathodic Protection (ICCP)
39
7.3
Protective Potentials and Potential Measurements
41
7.4
Cathode Current Density
42
7.5
Importance of Coatings for CP
43
7.6
Over Protection / Under Protection
43
7.7
Types and Properties of Anodes. Anode Weight, Shape and Life
45
7.8 Calculations for Design
46
7.9
Impressed current cathodic protection
48
7.10
Power Sources
48
7.11
Types and Selection of Anodes
48
7.12
Calculations for Design
49
7.13
Interference Current
49
7.14
Transformer / rectifiers
51
7.15
Potential Surveys
51
7.16
C.P. Maintenance Factors
52
7.17
CP SAFETY
53
8.0
CORROSION PREVENTION MAINTENANCE
55
9.0
IDENTIFICATION OF DEFECTS
56
10.0
SURVEY METHODS AND EQUIPMENT
57
10.1
57
11.0
Non-Destructive Inspection Methods.
CASE HISTORIES
59
Service History
59
Page 3
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 4
Module K
Marine Corrision & Coatings
12
13
METHODS OF CORROSION CONTROL - COATINGS
64
12.0
Minimising Corrosion Effects
64
12.1
Corrosion Prevention using Protective Coatings
64
12.2
What are Coatings?
64
CHOOSING A PAINT SYSTEM
71
Exposure Testing of Paint Films
71
Performance Expectation of Coatings
71
Coating Systems and their Selection
71
14
COATING SPECIFICATIONS
78
15
PRACTICAL PAINTING CONSIDERATIONS
80
16
SURFACE PREPARATION - ALTERNATIVE METHODS
82
Blast Pots/Hoses/Nozzles
85
17
PAINT APPLICATION: QUALITY CONTROL
93
18
PAINT FAULTS
95
19
FURTHER INFORMATION
101
Corrosion Societies
101
APPENDIX A
102
APPENDIX B
112
Marine Terminology and Construction
112
Naval Vessels
112
Page 4
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 5
Module K
Marine Corrision & Coatings
1 BASICS OF CORROSION 1.1
Corrosion Costs and Economics.
UK Industry
Cost in £ Million
Building and Construction Food Industry General Engineering Marine Industry Government Metal refining / fabrication Oil, Gas and Chemical Industry Power Transport Water Total
250 40 110 280 55 15 180 60 350 25 £1365 million
Table showing cost of Corrosion in the USA 1995 compared with 1975
All Industries
1975 (US $ Billion)
1995 (US $ Billion)
Ratio 1995 /1975
Total
82.5
296.0
3.59
Avoidable
33.0
104.0
3.15
1683.7
7033.6
4.18
4.90
4.21
GNP Total % of GNP
Since 1995 it is estimated that real term corrosion costs for the oil and gas industry have been reduced by 50%. However the above Table for corrosion costs averaged through all industries in the United States shows a saving between 1975 and 1995 of only 14%
1.2
Definition of Corrosion
Corrosion is the deterioration of a substance, usually metal, or the deterioration of its mechanical and metallurgical properties due to its reaction with the environment. In some cases there is no visible evidence of this deterioration that can lead to the sudden failure (environmental cracking) of the material for no immediately apparent cause. Iron, the main component of steel is thermodynamically unstable and tends to return to its oxide state. Most other metals including aluminium and zinc show this tendency to return to an oxide ore state or other thermodynamically more stable compound.
Page 5
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 6
Module K
Marine Corrision & Coatings
ELECTRO-CHEMISTRY 1.3
Atomic Structure and Ion Formation
Atoms are the minute building blocks of all matter and consist of a nucleus and circulating electrons. The diameter of the path for circulating electrons can be a factor of 10,000 x larger than the diameter of the nucleus. The nucleus consists of uncharged particles called Neutrons and positively charged (+ve) Protons. The orbiting electrons have an equal negative (-ve) charge to the Protons and the atom is electrically neutral. Corrosion reactions and processes are ordinary chemical reactions in which the atoms gain or lose electrons. If the atom gains or loses one (or more) of its electrons it becomes an ION with an electrical charge. Loss of n electrons equals a gain of n positive charges on the atom nucleus that becomes a CATION. Gain of n electrons equals a loss of positive charge and the atom becomes an ANION. (n can be 1, 2 or 3) Chemical and corrosion processes are actually the exchange of the -ve charged electrons between the various atoms involved. Ion Examples:
Na+ Sodium cation,
Ca++ Calcium cation,
Fe++ Ferrous cation, Fe+++ Ferric cation, Cl Chloride anion, SO4- - Sulphate anion Molecules Molecules are formed when two or more atoms combine together with a strong bond between them. Nearly all gases occur normally in the form of molecules. Hydrogen (H), Oxygen (O), Nitrogen (N) and Chlorine (Cl) occur as pairs of atoms (Molecules) and their formulae are written as H2, O2, N2 and Cl2. Other gases such as Carbon Dioxide (CO2) Hydrogen Sulphide (H2S) and Water Vapour (H2 O) also occur as molecules. For example the Carbon Dioxide Molecule Formula is: O=C=O The water molecule H2O is formed from a hydrogen cation H+ and an anion OHWater Molecule, Hydrogen & Hydroxyl Ions Ionic Compounds A large group of substances known as inorganic salts exist as solids in an ionic form. Ordinary salt. Sodium Chloride (NaCl) consists of sodium cations regularly arranged next to chlorine anions. The +ve and -ve charges on the ions attract them together and the charges also neutralise each other so that the solid sodium chloride is electrically neutral as shown in the following diagram.. Na+ Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+ ClNa+ Cl- Na+ Cl- Na+ Cl- Na+
Page 6
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 7
Module K
Marine Corrision & Coatings
Arrays of these unit cells are built up to form crystals. The way in which the atoms of a metal are bonded together is different to that in the molecule or in the ionic crystal. Apart from the regular arrangement of unit cells the atoms share their electrons between every other atom in the crystal. Electrical neutrality is still maintained because the electrons stay within the crystal, and their charges are still neutralised by the charges on the protons in the atomic nuclei. It is this special atomic and electron arrangement in metals which gives them their special properties of electrical conductivity, hardness, brightness and formability.
1.4
Electrolytes, Electron Flow and Batteries
To create corrosive conditions at normal environmental temperatures corrosion cell is required. The corrosion cell works in the same way as the well-known dry battery cell. A typical corrosion cell is shown in Figure 1 o
For low temperature corrosion (< 200 C) to take place, an electrically conductive solution that will allow cations and anions to move freely through its bulk must be present. A solution of this type is called an electrolyte. Water containing dissolved ionic salts is the most common and the best electrolyte, and is always present in electrolytic corrosion. Seawater is on of the best electrolytes and is the main cause of marine corrosion. Water corrosion is greatly increased if it contains dissolved oxygen. The formation of ions in solution and the movement of the ions towards one or other of the electrodes (anode or cathode) are called electrolysis. The movement in the electrolyte occur as a result of the difference in electrical potential between the anode and the cathode. This is the driving force. (Electro-motive Force. EMV or Volts) If there is no difference in potential there is no driving force between the anode and the cathode and corrosion cannot occur.
Page 7
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 8
Module K
Marine Corrision & Coatings
METALIC CONNECTION PATH ELECTROLYTE LEVEL
Fe++
H+
CORROSION
H+ H+ Fe++ e-
ANODE
H+
CATHODE
ELECTROLYTE: DILUTE In any corrosion reaction the loss of electrons at the anode must equal the gain of electrons at the cathode FIGURE 1 ELECTROLYTIC CELL-BATTERY
In the battery cell, shown above, the electrolyte is a paste of zinc chloride in water. On closing the electric circuit there is a chemical action and a flow of electrons in the circuit caused by the potential difference of 1.5 volts between the cathode and anode.
1.5
Electron Flow
Two different metals or conductors must be connected in the battery or corrosion cell to enable current to flow. Negative charged electrons flow from the part of the cell which starts corroding, called the Anode, through the metallic part of the circuit to the part of the cell which does not corrode, called the Cathode. This flow of electrons produces a measurable electrical potential difference across the circuit. Due to the flow of electrons the electrical neutrality of the electrodes is lost. The anode becomes positively charged because it loses electrons through the metal portion of the circuit. This +ve charge will attract the flow of -ve charged anions in the electrolyte. In the same way the cathode gains electrons through the metal path and becomes negatively charged. The cathode now attracts +ve charged cations in the electrolyte.
Page 8
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 9
Marine Corrision & Coatings
Module K
Corrosion occurs at the anode because atoms of the metal at the surface lose electrons and become positively charged. These positively charged ions go into solution, and are transported towards the cathode under the influence of its negative charge.
1.6
Half Cells
In Figure 1 the anode reaction is a half-cell. The cathode reaction is also a half-cell and the two together (cathode + anode) make a complete cell. The total reaction occurring is the sum of the reactions of the half cells. For example. The anode reaction is: The cathode reaction is:
Zn = Zn++ + 2e2H+ + 2e- = H2
The sum of the reaction is: Zn + 2H+ = Zn++ + H2 The concept of half-cells is important in the measurement of corrosion potentials and in cathodic protection where special copper/copper sulphate half-cells are used. These uses will be discussed in greater detail later.
1.7
Redox Reactions
Where any two half-cells are coupled together they form a complete cell. The loss of electrons or gain in +ve charge by a metal is known as Oxidation and this type of reaction always occurs at the anode. The process does not necessarily form oxides. Chlorides and other salts may also be formed, but oxidation is one of the fundamental corrosion processes. Only one type of reaction occurs at the anode. M = M++ + 2e For an iron anode this reaction is: Fe = Fe++ + 2eAt the anode, metal is always dissolved as cations with the loss of electrons and oxidation. At the cathode however, any one of three reactions may take place: 1. 2H+ + 2e- = H2 (Hydrogen gas bubbles) 2. O2 + 4H+ + 4e- = 2H2 O (Water) 3. O2 + 2H2O + 4e- = 4OH- (Hydroxyl ions) Reaction 1. is the reduction of hydrogen ions and is very common in acid solutions. (The H+ comes from the acid) Reaction 2. is called the reduction of oxygen and occurs in acid solutions containing air or oxidising agents. Reaction 3. is also reduction of oxygen but occurs in neutral or alkaline solution. 1. 2. & 3 are called Reduction reactions. The Hydrogen ions are reduced to hydrogen gas. Oxygen molecules are reduced to water or hydroxyl ions.
Page 9
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 10
Marine Corrision & Coatings
The first reaction takes place quite rapidly in acid solutions, but very slowly in alkaline or neutral media. It can be speeded up by dissolved oxygen (O2) as shown in Reaction 2. In a complete cell oxidation occurs at one half cell and reduction at the other cell. The overall reaction is known as a Redox Reaction (Reduction / Oxidation)
1.8
Cell Potentials
The driving force of a corrosion cell is determined by the difference in electrical potential (volts), measured between the anode and the cathode in the metallic path of the closed circuit, with a high resistance voltmeter. The potential difference is measured across the cell as shown in Figure 2
Electrictity Conventional Flow
V Electron Flow Electrolyte
CATHODE
ANODE
FIGURE 2: MEASURING CELL POTENTIAL Measured potential differences (E) can range from zero up to 3 volts. A zero voltage indicates that no chemical (corrosion) action will occur, or that an equilibrium condition will exist. A few millivolts show a low corrosion driving force and the possibility of corrosion occurring. A potential of several hundred millivolts or more indicates a very high corrosion driving force and the possibility of very high, unacceptable corrosion rates.
1.9
Corrosion Potentials / Electro-chemical Series
In laboratory work the corrosion potential of a metal is usually measured against a standard half-cell called a platinum-hydrogen electrode. This technique allows the corrosion potentials of various metals to be compared with a standard reference. The cell is used to define a galvanic series of metals arranged in order of their potentials as measured against the reference standard. This series is also known as the Electrochemical Series of Metals. A table of potentials for different metals in seawater is shown in the electrochemical series. (Table 1) In practise cells such as the saturated calomel cell, copper sulphate cell and the silver/silver chloride cell are used in the field.
Page 10
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 11
Module K
Marine Corrision & Coatings
Electrochemical Series (sea water) reference a saturated calomel electrode (SCE) TABLE 1 METAL PURE MAGNESIUM MAGNESIUM ALLOY ZINC ALUMINIUM ALLOYS 7072 and 6065 PURE ALUMINIUM MILD STEEL GREY CAST IRON HIGH SILICON CAST IRON 18:8 STAINLESS STEEL, ACTIVE LEAD ADMIRALTY BRASS, COPPER ALUMINIUM BRONZE SATURATED CALOMEL (REF. CELL) 18:8 STAINLESS STEEL, PASSIVE DUPLEX STAINLESS STEEL PASSIVE INCONEL 625 HASTALLOY C TITANIUM GRAPHITE SILVER GOLD
POTENTIAL (v) -1.65 -1.53 -1.03 -0.95 to -0.85 -0.9 -0.6 to -0.55 -0.55 -0.4 -0.4 -0.3 to -0.2 -0.35 -0.35 -0.30 0.00 -0.1 to +0.1 +0.15 . +0.2 +0.24 0 to +0.15 +0.3 +0.8 +1 TABLE 1
The more negative the potential of the metal the more reactive (easily corroded) the metal is likely to be. e.g. Magnesium is much more easily corroded than copper. Potentials between Dissimilar Metals Using the above Table or similar Tables that use the Hydrogen reference Electrode, the Copper / Copper Sulphate Electrode (CSE) or other standards, the potential of a cell is easily calculated by simple subtraction between the half-cell potentials. Example: The Electrochemical series potential difference between the SCE reference electrode and Aluminium is: 0.00 - (-0.9) = +0.9 volts Example: Between Grey Cast Iron and Aluminium Bronze the potential difference is: -0.30 - (-0.55). = +0.25 volts
1.10
Cell Voltage
The potential (volts) of a corrosion cell indicates the corrosion driving force and the amount of current flowing (amps) gives an indication of the volume of metal corroding in unit time. Current flow is measured with a low resistance ammeter. The amount of metal lost in a corrosion reaction can often be found by applying the law that the mass of a substance liberated at an electrode is proportional to current flow × time. (Faraday’s Law). One Amp for one second = one Coulomb and 96,500 Coulombs will liberate or dissolve the gram equivalent weight of a metal.
Page 11
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 12
Marine Corrision & Coatings
The most reactive metals such as Potassium (K) Magnesium (Mg) Aluminium (Al) and Iron (Fe) have negative electrode potentials and are anodic in character when related to the Pt/H2 Electrode. In the presence of hydrogen ions these anodic metals will more readily corrode than the less reactive and more noble metals such as platinum and gold that have positive electrode potentials. If two metals in the galvanic series are coupled in a corrosion cell, the driving force or tendency for corrosion to occur in the anodic metal is proportional to the potential difference between them. For example, if zinc and steel are coupled the potential difference between the metals is: -0.5 - (-1.03) = +0.53V. The zinc corrodes at a moderate rate and the steel does not corrode. If, in the same cell, the steel is now replaced by silver the potential difference is: + 0.8 -(-1.03) = +1.83V. The zinc is at a more -ve electrode potential and its potential has greatly increased. In this case the actual corrosion rate of the zinc also increases considerably although the silver does not corrode. This principle is important in design to prevent dissimilar metals contacting one another and causing corrosion and in the application of sacrificial corrosion protection to be discussed later.
1.11
pH and Acidity. Hydrogen Ion Concentration
The symbol pH is a chemical shorthand for a method of measuring the acidity or alkalinity of a solution. The acidity of an aqueous solution is a measurement of the negative logarithm of the hydrogen ion concentration. This is better known as the pH value. The symbol p is derived from the German word potenz, and means the logarithmic exponent, or power of concentration. The letter H is the chemical symbol for hydrogen. A pH scale of 0 to 14 has been adopted. 0 is a very strong acid. (High H+ ion concentration) and 14 is a strong alkali with a very low H+ ion concentration. Pure water is considered to be neutral, neither acid nor alkaline and it has a pH of 7.0. The pH of a liquid is extremely important as most acid solutions with low pH values (acids) are very corrosive to metals, and if very acid they can also damage the skin and eyes of operators unless precautions are taken. Alkalis are often beneficial in improving the corrosion resistance of steel, but can be very damaging to metals such as zinc and aluminium. Strong alkalis also damage the skin and eyes.
Page 12
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 13
Module K
Marine Corrision & Coatings
2 CORROSION PROCESSES Corrosion occurs due to the anode and cathode reactions in electrolytic cells as already discussed. On any given piece of metal such as pipes or tanks the corrosion cells form at numerous microscopic sites on the surface.
2.1
Corrosion Sites, Anodes and Cathodes
The anodic sites corrode as shown in Figure 3 with the loss of metal ions. The usual reaction at cathodes is the production of hydrogen gas in acid solutions, reduction of oxygen to form water in oxygenated slightly acid solutions or the production of hydroxyl ions (OH-) in neutral oxygenated solutions.
CORROSION SITES ON METAL SURFACE
CATHODE
H+ Fe++ Cl-
H+
ANODE
CATHODE
H+ Cl-
ELECTROLYTE
ANODE
CATHODE CORROSION CELL ON STEEL PLATE POSITIVE CURRENT FROM ANODE TO CATHODE THROUGH ELECTROLYTE FIGURE 3 CORROSION SITES ON A METAL SURFACE Corrosion (oxidation) of metals usually produces a soluble or easily detached corrosion product that is lost from the surface to expose new metal, which continues to corrode. In some cases the corrosion product forms a thin adherent film that protects the metal from further attack, as in stainless steel. The metal is then said to be passivated. Corrosion rates are measured in mm per year or, in USA practise, in mils (thousandths of an inch) per year and the rates are affected by process variables.
2.2
Passivation
Metals that can be passivated corrode normally in moderate concentrations of oxidising agent but may suddenly show a 10-3 o 10-6 decrease in corrosion rate as the oxidising agent concentration is increased still further into the Passive region. Eventually at very concentrations of oxidant the corrosion rate will increase again in the transpassive region.
Page 13
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 14
Marine Corrision & Coatings
Metals showing this effect include stainless steels in mild oxidising agents and carbon steels in concentrated sulphuric or concentrated nitric acid. The passivation effect of Stainless Steel is due to a thin layer of inert chromium oxide on the steel surface. Other passivation effects are due to thin oxide or other inert chemical layers. Sometimes the film is easily destroyed by moderate mechanical damage or by changes in oxidising conditions. Rapid corrosion may then occur.
2.3
Process Factors affecting Corrosion Mechanisms
2.4
Temperature o
Corrosion rates generally increase with temperature and may double for every 20 C increase in the reaction system temperature.
2.5
Concentration Effects
Increased chemical concentration usually increases corrosion rates up to a limiting value. Further increases in the concentration may cause no further increases in the corrosion rate or may even cause it to decrease.
2.6
Conductivity Effects
Increased electrical conductivity in a solution (or a soil) indicates increased ionic activity, and therefore a probable increase in corrosion rate. Seawater with a high conductivity or low resistivity is much more corrosive than fresh water. Conductivity = 1 / resistivity and resisistivity is the property usually measured in corrosion with a value in ohm cm. Seawater has a resistivity of 30 ohm cm. The corrosion pattern occurring on a ship is rather different due to the ships movement. The ship is continually passing through new supplies of oxygenated water so the supply at the ships hull surface is never used up in the corrosion reactions. Instead of occurring in the splash zone the maximum corrosion occurs at or just below the water line where the oxygen content and is high and the movement of water is greater than occurs in the splash zone of a static structure. The band of maximum corrosion occurs over a depth ranging from the unloaded to fully loaded water lines.
2.7
Velocity Effects
It can be seen from the above statement that Increased velocity can cause increased corrosion due to increases in the amount of corrosive substance passing over the surface in a given time. At high velocities and in turbulent conditions fluids may also cause mechanical damage to materials, accelerating corrosion effects and producing severe attack known as erosion corrosion. This occurs particularly on ships propellers and on various pump impellers that may be used for pumping water, controlling ballast tanks or loading cargoes.
2.8
Pressure
An increase in pressure increases the amount of gas such as oxygen or carbon dioxide that can be dissolved in the electrolyte (usually water). This gives an increase in the concentration of corrosive gases and causes increased corrosion rates. This is not normally a problem in marine corrosion.
Page 14
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 15
Marine Corrision & Coatings
2.9
Module K
Galvanic Effects
Dissimilar metal contacts in an electrolyte can accelerate the corrosion of the more active metal. This effect occurs frequently in marine design and maintenance such as where brass fittings are screwed into a steel structure, steel components are fixed to an aluminium hull; bronze propellers are connected to steel shafts. Materials such as admiralty brass or bronze fittings are commonly attached to steel structure and can accelerate the corrosion of the steel. Pipefittings and heat exchanger tubes are other items commonly affected by galvanic corrosion. The less noble material will be at risk of corrosion and requires additional protection. (Refer to Table 1 )
2.9a
Anode size effect in Galvanic Corrosion
If the anodic material (material at a risk of corrosion) has a large surface area in relation to the area of the cathodic (non-corroding metal) then the amount of corrosion should not be severe because the electrical current causing the corrosion is distributed over the large area of the anode and produces a low current density. Since current density is directly related to corrosion rate the corrosion rate should be low. However if the exposed anode area is small in relation to the cathode area the corrosion rate can be very high. The classic example of this effect occurred over 200 years ago when the Royal Navy tried to protect its wooden ships from marine fouling by cladding the hulls with copper. The copper sheet was attached to the hull by iron rivets. This produced very large cathodes (Cu) and small anodes. Within a few months the rivets corroded away causing the copper to disbond from the hulls.
Page 15
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 16
Marine Corrision & Coatings
3 ENVIRONMENTAL EFFECTS 3.1
Marine Atmosphere and Seawater Corrosion
The commonest form of atmospheric corrosion is the uniform rusting of steel due to the combination of iron with water and oxygen from the air. Rusting of iron to form various oxides. Fe + 2H2O + O2 = 2Fe(OH)2 (Unstable, oxidises) 2Fe(OH)2 + H2O + ½O2 = Fe(OH)3
(Ferric hydroxide)
Fe(OH)3 = H2O + FeO(OH)
(usually yellow)
2FeO(OH) = H2O + Fe2O3
(orange dehydrated oxide)
Reaction rates are modified by the following: The amount of water in the air (humidity) and rainfall, contaminants in the air, salt spay and blown sand in marine environments or sulphuric acid in industrially polluted areas. Many harbours in industrial ports can suffer from the double affect of marine salt and industrial air pollution. Corrosion rates in coastal areas have been measured at 400 times the corrosion rate in desert atmospheres and corrosion on specimens 25m from the seashore has been measured at 12 times the corrosion rate at locations 250m from the shoreline. High air and metal surface temperature can also increase corrosion. Chemical reaction rates, including corrosion usually double for every 10 to 20oC increase in temperature. High surface temperatures can cause rapid evaporation of water and then of course the corrosion stops. Remember there cannot be any electrolytic corrosion without an electrolyte (water). However the evaporation can cause salts to concentrate on the surface. At some later period when the surface is again wetted the concentrated salts will be dissolved and may form exceptionally high concentrations of salt or sulphate that can then have an increased corrosion effect.
3.2
Dirt Deposits and Cargo Deposits
Accumulation of dirt or mud deposits on metal surfaces can attract moisture and set up corrosion cells that can cause severe corrosion often in crevices or corners where the corrosion can be very damaging. Many ships carry corrosive cargoes that are held in specially designed holds. If these cargoes spill in the wrong areas such as on deck they can cause severe deposit corrosion problems. The corrosion in these cases occurs as pitting and the mechanism by which it occurs is usually an oxygen concentration cell as described in the next chapter. Cargo holds are subject to severe environments including corrosive and abrasive wear due to the loading of bulk materials through inlet pipes, wear due to tractor movement within the holds and salt water that may be deliberately added as ballast or accidentally leaked into the hold. The worst conditions apply when a corrosive cargo is saturated with seawater through hatch leakages. Corrosive cargoes include salt, gypsum, organic and inorganic fertilizers, iron ore and numerous other materials.
Page 16
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 17
Module K
Marine Corrision & Coatings
Sea Water Immersion Corrosion Sea water can only dissolve some 15 parts per million of oxygen, however oxygenated sea water is extremely corrosive to iron and carbon or low alloy steels which need to be well protected by coatings. The pattern of seawater corrosion and the effect of oxygen are shown in Figure 4. Where oxygen content is less than 0.1 ppm as in deep quiet water or sea mud the corrosion can be negligible. The sections of harbour sheet piling that are buried in the mud are often uncoated and do not corrode significantly.
SEA WATER CORROSION OF STEEL AT VARIOUS DEPTHS Static Structure Marine atmosphere
Splash zone
Corrosion of steelpilng
High tide
Low tide
Quiet seawater Mud line
Typical corrosion rate of steel, mpy
FIGURE 4 SEA WATER CORROSION OF STEEL AT VARIOUS DEPTHS Static Structure Ships hulls are subject to heavy general corrosion at and above the waterline due to continuous expose to oxygenated water. Well below the waterline the corrosion is more likely to be local pitting associated with marine growths, deposits or variations in the metal plate composition, as at welds. Seawater corrosion of ballast tanks is a major problem on ships. The movement of the water in the tanks and by transfer pumps can cause the water to be saturated with oxygen and be extremely corrosive. Lack of drainage in some designs allows local stagnant areas to develop where corrosion cells under deposits can form and where bacterial activity can also increase corrosion rates. (Bacteria / microbiological effects are discussed later)) High standards of painting are required to prevent corrosion with special formulations to reduce marine growth. Typical protective systems are heavy duty (200 to 300 microns) epoxy based coatings for hulls and tanks. Galvanised steel is often used for submerged and splash zone steel structures. Seawater usually causes general corrosion in carbon and low alloy but can cause pitting under deposits and marine growth or bio fouling. Pitting corrosion is the standard form of failure in type s 304 and 316 stainless steels and in aluminium alloys. The extent of pitting can be severe and is usually greatest in the higher strength copper containing aluminium alloys.
Page 17
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 18
Marine Corrision & Coatings
3.3
Condensation Corrosion
Condensate corrosion is a particularly aggressive form of atmospheric corrosion in empty or partly filled storage tanks. The moisture-laden air is sucked into the tank and the tank cools at night. Condensation forms on the interior walls and roof. Frequent cycles of condensation and drying concentrates dissolved salts and can cause severe corrosion. High quality coatings, sealed tanks, and inert atmosphere blanketing are techniques used to control this type of problem. Tanks on land may also be designed with a floating roof that allows no air space above the tank contents.
Page 18
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 19
Module K
Marine Corrision & Coatings
4 FORMS AND MECHANISMS OF CORROSION Atmospheric rusting and external corrosion of all metal structures are subject to accelerated corrosion due to oxygen in contact with an electrolyte (water) Air and water are the two items that cause the greatest amount of corrosion damage due to their abundance in the natural environments. Iron oxides FeO(OH), Fe2O3 and Fe3O4 are formed by steel corrosion and heavy localised corrosion pitting occurs in soil and under deposits where oxygen concentration cells can occur.
4.1
Oxygen Concentration Cell Formation
This is one of the commonest causes of pitting corrosion and is the cause of pitting corrosion in the oil and gas industry. It is also a major cause of internal corrosion in systems wherever oxygen can be introduced.
4.2
Mechanism of Oxygen Corrosion
Figure 5 shows a very common corrosion problem at a point where some form of local deposit blocks of the possibility of air contacting the surface while the surrounding area is still in contact with the air. The deposit may be dirt, bacteria slimes, residual cargo remains. (Iron ore causes severe problems) or items left after maintenance work. e.g. Gloves left in a hold or tank has been a source of this type of problem. The differential concentration of oxygen in the water or soil causes an electrolytic cell to form. The cell reaction causes oxygen to be reduced at the cathode. 2H2O + O2 + 4e- = 4OH For every anode reaction there must be a corresponding anode reaction. Therefore the area deprived of oxygen becomes the anode and corrodes. Fe = Fe++ + 2e-
Page 19
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 20
Module K
Marine Corrision & Coatings
OXYGEN CONCENTRATION CELL FORMED BY DIRT DEPOSIT ELECTROLYTE
High oxygen at deposit surface Cathode
Salt, mud,water low oxygen at centre Anodic area
High oxygen at deposit surface Cathode Steel Plate
Steel Plate Corrosion
This type of corrosion cell occurs under deposits and in crevice corrosion and in pitting FIGURE 5 The problem is usually overcome by ensuring the area is kept free of debris or by painting. Problems frequently occur under residual deposits left in cargo holds or even on ships decks. Cleaning, hosing and regular maintenance are helpful.
4.3
Crevice Corrosion
Crevice corrosion can occur on pipe flange joints and under gaskets or under the heads of nuts and bolts as shown in Figure 6. The crevice corrosion reaction mechanism is generally considered to be another form of the oxygen concentration cell reaction. There is also evidence that in many cases the corrosion mechanism is actually the same as that in pitting corrosion.
Metals, which have protective oxide layers on the surface and are prone to pitting corrosion, such as 304 / 316 type austenitic stainless steels, are also prone to crevice corrosion, particularly in seawater. Crevice corrosion is controlled by sealing the crevices or by opening them to sufficient width for flow to occur and reduce the differential concentration effects.
Page 20
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 21
Module K
Marine Corrision & Coatings
CREVICE CORROSION
NUT
ANODE AREA (ACTIVE)
OXYGEN STARVATION
WASHER
CATHOD AREA (PASSIVE) STAINLES STEEL
FIGURE 6 CREVICE CORROSION
4.4
Pitting corrosion
A wide range of environments and mechanisms, each of which may form different types of pits, can cause pitting. Oxygen concentration cells and CO2 corrosion cause wide interconnecting pits. Chlorides in oxidising solutions cause the formation of deep small diameter pits in metals that are covered with partially protective surface films. This type of corrosion occurs frequently and can be very damaging to plant and pipelines. Pitting can be caused by several sets of circumstances as follows: localised differences in metal composition, which create a galvanic action between grains and grain boundaries in the metal surface. A local break down of the protective coating on a surface. This is particularly common in austenitic stainless steels that normally have a thin protective oxide layer over the exposed surfaces. This layer provides the steel with its corrosion protective properties and is known as the passivating surface. If the passivation film is removed rapid corrosion can follow. In practise local small areas of film damage occur frequently. In oxidising conditions the damage on stainless steels may be self-healing but if shielded under deposits, cells are set up in which the unprotected area is strongly anodic to the surrounding cathodic area and pitting commences. As the pit deepens metal is lost as Fe++ and oxygen reduction takes place on surrounding surfaces. As reaction proceeds an excess +ve charge builds up in the pit and attracts Cl ions from the electrolyte. A high concentration of ferric chloride, FeCl3 develops and the product hydrolyses. FeCl3 + 3H2O = 3HCl + Fe(OH)3 The acid HCl concentration increases in the pit and the reaction rate accelerates. The mechanism is known as an autocatalytic reaction and explains the rapid penetration rates that occur. Rates of +500 mils/year (12.5mm/year) are common and thick pipeline sections can be penetrated in less than one year. See Figure 7.
Page 21
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 22
Marine Corrision & Coatings
PITTING CORROSION MECHANISMS O2
Na+ O2 CLOH-
Na+ CLOH-
O2 Fe++ OHALKALI OH-
CL-
ALKALI
CATHODE PRECIPITATE
CATHODE
CLFe++ Fe++ CLACID
Fe++
PRECIPITATE
CL-
ANODE ACID
FIGURE 7 PITTING CORROSION MECHANISMS
4.5
Galvanic Corrosion
The Table of the Electrochemical Series lists metals in an order according to the ease with which they corrode. If any two dissimilar metals are electrically connected in a corrosive solution, the one that is most active, and has the most -ve potential, will corrode. The driving force of corrosion on the least noble metal is directly proportional to the potential difference between the metals. The actual rate of corrosion is proportional to the amount of current passed. Accelerated corrosion, or in some cases decreased corrosion, of a metal caused by bringing it into electrical contact with a dissimilar metal, is called Galvanic Corrosion. In Figure 8 there is an open circuit between the metals in the first cell and no flow of electricity. The zinc corrodes fairly rapidly and the iron corrodes at a slower rate. If the pieces of metal are connected through an external circuit the corrosion pattern changes. Zinc corrosion increases and iron corrosion is stopped. The galvanic effect with the zinc protecting the iron is the principle of cathodic protection. The lower part of the Figure shows typical practical problems that occur when dissimilar metals are in contact in a corrosive environment In engineering structures, many different metals come into contact with each other. This is particularly true of process vessels and heat exchangers. Conditions for galvanic corrosion to occur can be summarised as follows: • • •
Presence of dissimilar metals in electrical contact Presence of a corrosive electrolyte Small anode and large cathode
Small cathodes and large anodes, as in the copper rivet example, create cells with lower anode current densities and, consequently, lower corrosion rates than occur with large cathodes and small anodes as in the aluminium example.
Page 22
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 23
Module K
Marine Corrision & Coatings
The smaller the difference between dissimilar metals on the electrochemical and galvanic series table, the lower is the corrosion driving force and the lower is the potential for corrosion to occur. Prevention of Galvanic Corrosion Dissimilar metals should be separated with suitable electrical insulating material. When insulation is not possible, make the more anodic metal the easier to replace when corrosion becomes evident. Otherwise ensure that the anodic metal is thicker and of larger surface area than the cathodic metal to reduce corrosion rates.
V OPEN CIRCUIT
CLOSED CIRCUIT
Zinc corrodes more rapidly than iron
Zinc corrodes rapidly, iron is protected
GALVANIC EFFECT OF ZINC COUPLED TO IRON IN A CORROSIVE ELECTROLYTE (Sacrificial Protection) COPPER RIVET STEEL GALVANIC CORROSION OF STEEL UNDER COPPER RIVET
ALUMINIUM RIVET STEEL GALVANIC CORROSION OF ALUMINIUM RIVET IN CONTACT WITH STEEL PLATE
FIGURE 8 GALVANIC CORROSION
Page 23
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 24
Marine Corrision & Coatings
4.6
Carbon Dioxide Corrosion
Carbon dioxide (CO2) gas forms only 1% of the atmosphere. Carbon dioxide (CO2) readily dissolves in water (H2O) to form carbonic acid (H2CO3), a weak acid that in o saturated solution at 15 C and 1 bar (14.5 psi or 000kPa) pressure, has a pH of 3.6. due to dissociation to produce hydrogen ions. H2CO3 = H+ + HCO3The solubility of CO2 is increased by pressure and decreased by temperature. Solubility of dissolved minerals, particularly calcium carbonate, is increased by the presence of carbon dioxide in water. This increases the conductivity of the electrolyte and therefore increases the corrosion tendency. CO2 corrosion is not a significant gas in marine environments. The gas may be present in natural gas and liquefied gas carried by special transporters but does not normally present a corrosion risk.
4.7
Hydrogen Sulphide Corrosion
Hydrogen Sulphide (H2S) is a colourless, inflammable and highly toxic gas. It is heavier than air and has a very strong smell of rotten eggs. Quite low concentrations are sufficient to cause rapid death. H2S can occur in fresh or salt waters containing large amounts of rotting vegetation. The gas is often present in sewers, sewer outfalls and in rotting organic matter in shallow waters. However the concentration is rarely enough to cause safety problems or the accelerated corrosion of steels.
4.8
Environmental Stress Cracking
Stress Corrosion Cracking (SCC) This type of corrosion is one of the most important and dangerous causes of failure in metal structures and vessels. It is a combination of corrosion and mechanical stress and can only occur when the structure, in the area of the corrosive substance, is under a tensile stress. The stresses may be due to many sources: • mechanical loads • internal pressure • weld restraint • thermal stresses • residual surface stresses Cracking may proceed slowly at first, but when the load bearing capability of the metal has been reduced sudden complete failure occurs due to mechanical overload. This rapid failure, and the fact that SCC is often difficult to detect, makes this type of corrosion very dangerous. Brasses undergo stress corrosion in ammonia solution. Carbon steel is susceptible to stress corrosion in contact with carbonates, in strong, hot caustic soda and in nitrates (NO3-) solutions. Ships cargoes that may contain nitrates or carbonates can sometimes cause a risk of stress corrosion cracking if they become wet and are in contact with steel surfaces. Inspectors and surveyors should be aware of the risk.
Page 24
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 25
Marine Corrision & Coatings
Module K
Austenitic Stainless Steel Stress Corrosion Cracking Stainless steels of the 18% chromium and 8% nickel type are readily stress corroded in brines or sea o water at temperatures above 60 C. The Chloride Ion (Cl-) in association with oxygen or oxidising agents usually causes this type of attack on stainless steel. Chloride Stress Corrosion Cracking. (CSCC) CSCC in stainless steel often has a characteristic branched appearance, like an aerial view of a river estuary system, when studied under a microscope. The cracks can be intergranular or transgranular in o form. A stress of 60% of the ultimate tensile strength and a temperature of 60 C is normally required to induce this type of cracking. One of the principle areas of stainless steel CSCC has been on heated and insulated pipes and vessels where chloride from the environment or from the insulation, concentrates on the steel surface and causes cracks to form. On board ship there are may be a number of stainless steel systems that could be affected by this type of failure. Heat exchangers and hot water pipework may be affected. Some specialist ships have extensive cargo areas and pipes in stainless steel for carrying foodstuffs or special corrosive chemicals. 0 A pipe that is not heated can still be at risk. For example piping on a deck may heat up to well over 60 C in hot sunlight. Small amounts of salt water left in the pipe could also heat up and then cause cracking at weld joints or bends. Hydrogen Embrittlement The presence of atomic hydrogen in some metals, particularly in high strength steels and in titanium, reduces the metal ductility rendering it brittle. This phenomenon is known as hydrogen embrittlement. Until a steel containing hydrogen actually cracks, there is no permanent damage and in many cases the original properties of the steel can be restored by suitable heat treatment to bake out the hydrogen. Stress raisers increase the effect of hydrogen embrittlement. Hydrogen from corrosion reactions and electro-plating processes is a cause of hydrogen embrittlement. Cathodic over protection is also thought to pose a risk to high strength quenched and tempered steels. Chromium plated and cadmium plated high strength steels can fail due to this effect.
4.9
Microbiologically Induced Corrosion (MIC)
Many types of bacteria can live in tanks, vessels, cargo holds, and pipelines in the slimes that often coat submerged pier supports and sheet piling. Bio-fouling, the build up of small shell fish, crustaceans and barnacles on submerged structure and ships hulls can also act as centres for bacteria growth. In optimum environments the organisms reproduce rapidly and large colonies can grow in a few days. During growth the bacteria can convert nutrients into highly corrosive chemicals including hydrogen sulphide and sulphuric acid. Colonies of bacteria can also act as deposits and create concentration cells. All of these activities can lead to increased corrosion in a system. Bacteria slimes can also grow to an extent where they can completely block pumps and process pipes. Two principle forms of bacteria are encountered. Anaerobic Bacteria. These bacteria live and grow where there is no free oxygen. The commonest types are known as Sulphate Reducing Bacteria (SRB’s). These bacteria absorb sulphate from water or other nutrients such as sewage, various oils and even bitumen coatings. They then reduce the sulphate to sulphite and finally to hydrogen sulphide that is then free to cause corrosion.
Page 25
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 26
Marine Corrision & Coatings
o
o
Conditions for the reproduction of anaerobic bacteria are generally a Temperature range of 20 C to 40 C, a pH of 5.5 to 8.5, adequate nutrients and absence of oxygen. Many corrosion problems encountered inside fuel tanks, ballast tanks, sewage systems and pipework are due to a combination of agents, including bacteria, which all combine and accelerate the corrosion rate. Corrosion effects can include sulphide stress cracking and the formation of large diameter localised pits that often have an appearance similar to miniature lakes. Problems of pipe blockage can occur due to colony growth and the deposits can also cause hot spots and insulating conditions in heat transfer. Aerobic Bacteria These are bacteria that can only grow in conditions where oxygen is present. Control of Bacteria Methods of control include design to avoid stagnant areas, manual washing where accessible, additions of biocide dosing and sterilising of water in tanks and vessels. Gluteraldehyde is a major biocide applied in batch doses. Chlorine is commonly used to sterilise all types of water.
4.10
Other Mechanisms and Types of Corrosion
Erosion Corrosion At high velocities corrosive fluids and gases impinging on a metal surface will increases the rate of attack due to mechanical wear which is superimposed on the corrosion process. Ships propellers, rotating at high speed in a corrosive environment are particularly prone to this type of problem. The effect is called erosion corrosion and it can be very rapid. It is shown schematically for a pipe in Figure 9. Clean gases or pure water alone can cause erosion corrosion if the velocity is very high, and particularly if the flow is turbulent or perpendicular to the metal surface. The presence of sand or sediments greatly increases the rate of attack and lowers the An example of this is use of 90:10 Cu:Ni alloy tubes in seawater heat exchangers. The tubes perform well in clean seawater at velocities up to 3 metres/second. At higher velocities they may suffer from erosion corrosion. However if the water contains mud or sand, as in an estuary water, the velocity at which attack occurs is reduced dramatically, and may be ¼ meter/second or less. Erosion corrosion occurs on sharp pipe bends, tee pieces, pump impellers and on ships propellers. Erosion corrosion is controlled by increasing the bend radii, reducing angles of tee’s to form shallow angle Y inlets, increasing pipe diameter to reduce velocity, smoothing out irregular surfaces, removing sand or deposits and also by changing material.
Page 26
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 27
Module K
Marine Corrision & Coatings
EROSION CORROSION OF PIPE ELBOW Laminar Flow
Internal Erosion Corrosion
Turbulence
Velocity + 25m/second, Gas Velocity +5m/second, Liquid with sand, debris FIGURE 9 EROSION CORROSION OF PIPE ELBOW Cavitation Cavitation is a special form of erosion corrosion. This is caused when vacuum bubbles are created in turbulent flow of high velocity; these bubbles collapse creating small areas of high stress and severe pitting can occur as pieces of metal are torn out of the surface by the mechanical forces involved in the implosion of the bubbles. Cavitation occurs at places where there are large and rapid changes of pressure. Typical examples are on the trailing edges of impellers and ships propellers or on downstream areas of high pressure reducing or proportioning valves. Corrosion Fatigue Fatigue is the failure of a metal at a stress considerably below its normal yield strength when it is subjected to continued cyclic stress. If this cyclic stress is imposed in a corrosive environment the fatigue life of the metal will be substantially reduced. Failure often occurs by intergranular cracking. The time to failure by fatigue is related to the amplitude of the cyclic stress and the number of stress cycles that occur. This is shown in Figure 10.
CORROSION FATIGUE
% stress
100 Steel test piece cycles to failure in a non-corrosive environment
80 60
Steel test piece cycles to failure in a corrosive environment
40
10
2
10
3
10
4
10
5
10
6
10
Cycles to Failure FIGURE 10 FATIGUE AND CORROSION FATIGUE
Page 27
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 28
Module K
Marine Corrision & Coatings
CYCLIC STRESS Constant amplitude reverse cyclic stress pattern
+ Stress (S) MPa
_
n = No. of cycles
Exagerated diagram of wave actions on hull stress and fatigue
Bending Moment
Wave
Wave
Bending Moment in opposite direction
Wave
FIGURE 11 CYCLIC STRESS
A distinguishing feature of corrosion fatigue is the presence of numerous cracks in addition to the one that caused failure. Fatigue failures can occur in all types of structures and rotating equipment. Catastrophic failure of several bulk tankers in recent years has been attributed to corrosion fatigue by a mechanism similar to that shown in Figure 11. If considerable structural corrosion has also occurred in the critical stress areas fatigue cracks can develop and spread until eventually there are so many cracks that the load bearing capability of the structure is exceeded and a rapid complete failure ensues that can cause the ship to break in half. The corrosion may be a result of a cargo getting wet or leaking in poorly protected holds.
Page 28
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 29
Marine Corrision & Coatings
Module K
5 METHODS OF CORROSION CONTROL 5.1
Materials selection for corrosion protection
A number of internationally recognised specifications exist for selecting materials. The American Society produces these for Testing Materials, ATM. The American Iron and Steel Institute AISI. British Standards Institute BSI and European Standards Institute Euronorm.
5.2
Steels and Irons
Over 80% of ships, marine structures, pipelines and vessels in marine associated industries are made of carbon or low alloy steel, usually with application of some form of protective coating. British Standard, Euro Standard and ASTM Specifications are the primary documents for the purchase of carbon and low alloy steels. Carbon increases strength but decreases ductility in steels. Alloys such as manganese confer strength; chromium and molybdenum confer corrosion resistance. Other alloy elements assist in grain refining and improving machinability. Some high strength load bearing steels e.g. AISI4340, high strength steel plates and duplex stainless steel forgings require heat treatment to achieve optimum properties. ANNEALING; Slow cool in air from a high temperature. NORMALISING: Natural cool in air. QUENCHING AND TEMPERING: Produces tough, high strength structures. Safety critical and very high pressure pump casings and valve bodies are generally made from high integrity low alloy steel forgings. For less critical items cast irons can be used. A range of high (15%) nickel cast irons known as Ni-Resists are excellent for moderately corrosive conditions. Steels containing more than 12% chromium are classed as stainless steels. 12% to 14% chromium steel (AISI 405) without any nickel is a typical ferritic stainless steel. Steels with higher alloy content and many non-ferrous materials are considerably more expensive and their use has to be fully justified.
Page 29
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 30
Module K
Marine Corrision & Coatings
Typical Stainless Steel Compositions:
Type of Steel
A ISI or UNS No Grade
Composition % Cr
C
Ni
Mn
Mo
Si
N
Austenitic 304L
18-20
0.03
8-12
2
-
1
Austenitic 316L
16-18
0.03
10-14
2
2-3
-
Duplex
S310803
21-23
0.03
4.5-6.5
2
2.5-3.5
0.08-2
Duplex (Feralium 255)
S32550
24-27
0.04
4.5-6.5
1.5
2.9-3.9
0.1-0.25
Super Duplex (Zeron 100)
S32760
24-26
0.03
6-8
1
3-4
0.2-0.3
Super Austenitic
N08028
26-28
0.03
39.5-42.5 2.5
3-4 (Sanicro 28)
Austenitic stainless steels are prone to pitting and crevice corrosion in chloride waters, especially in the absence of oxygen which maintains the passive film surface.
5.3 Non-Ferrous Materials Nickel based Alloys Chromium free nickel alloy such as Alloy 400 63% min. Ni, 28-34% Cu. (Monel 400) and the higher strength K-Monel 63% min Ni, 27-33% Cu and 2.3 -3.3% Al are suitable for service in neutral and reducing conditions. They are excellent for use in seawater, fire water systems and many heat exchangers as shafts, impellers and tubes. They are susceptible to failure in moderate to strong oxidising conditions. Nickel copper alloys are also susceptible to corrosion by sulphur compounds. Chromium containing nickel alloys can be used in oxidising conditions and very severe environments, pump shafts, valve trim and other critical areas. Suitable materials for these conditions include the following:Inconel 600 (UNS N006600) 75%Ni 16%Cr 8%Fe Inconel 625 (UNS N006625) 61%Ni 22%Cr 9%Mo 5%F Incoloy 825 42%Ni 21.5%Cr 30%Fe 3%Mo 2%Cu Hastalloy C276 (UNS 102761) +50%Ni 16%Cr 5%Fe 16%Mo 4%W High strength Alloy X750 is used extensively for springs in corrosive service. Copper Alloys Cartridge Brass 70% Cu, 30% Zn , This is very ductile and used for low strength, low corrosion resistant fittings and tubing. Finished items o must be stress relieved at 280 C otherwise they are susceptible to stress corrosion. Admiralty Brass 70%Cu. 29%Zn, 1%Sn has improved corrosion resistance and is used extensively in heat exchanger tubes. Aluminium Bronze Copper aluminium alloys Cu + 2-12% Al have good resistance to corrosion and erosion corrosion and are very useful in seawater as castings and forgings.
Page 30
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 31
Marine Corrision & Coatings
Module K
Copper Nickel Copper Nickel 90/10 CuNi and 70/30 CuNi alloys are widely used for seawater piping, condenser tubing and firewater pipework. Aluminium Alloys Lightweight alloys with a wide range of strengths are available. They have moderate corrosion resistance to seawater. All are susceptible to acid and alkali corrosion with susceptibility increasing with increasing strength. lightweight makes aluminium attractive for ships deck housings and structures and some fittings. Aluminium is also extensively used for the complete hulls of small boats. Its comparatively low strength and low modulus of rigidity makes it unsuitable for the hulls or highly stressed sections of larger ships of (say) plus 100 tons dead weight. Zinc, Cadmium and Magnesium. Zinc is used extensively in the form of hot dipped galvanizing for the protection of steel in seawater and in marine atmosphere environments. It corrodes at a linear rate directly related to coating thickness and also provides sacrificial protection to the steel. Galvanizing is more commonly applied on static structures than on ships. Bolts and fasteners that are galvanized can seize up in seawater due to zinc corrosion products filling the threads. Magnesium alloy is sometimes used for fittings on high cost racing boats because of its lightweight and relatively high strength. High cost and high susceptibility to corrosion make it impractical for most ship applications. Zinc and magnesium bars and other shapes are also used as sacrificial anodes on ships hulls, propellers heat exchangers, subsea piping and marine piling. Cadmiun Cadmium is an excellent marine atmosphere protective electro-plating for steel. High toxicity during the plating process and production of toxic fumes if vaporised by welding has caused it to be replaced by zinc tin alloys and other materials, less toxic but with generally poorer performance.
5.4
Thermosets and Thermoplastics
Thermoset plastics such as glass, aramid or carbon fibre reinforced epoxies are extensively used in the manufacture of hulls and deck structures on a wide variety of small to medium sized boats and yachts. Royal Navy mine sweepers have been made of these materials to avoid magnetic fluxes triggering mines. The Worlds biggest Composite ship, the yacht Mirabella V with a 75m long fibreglass hall and 90m tall fibreglass mast was completed in 2003. The materials have very high resistance to sea water corrosion but cost, strength and fabrication problems make them unsuitable for very large commercial ships, except as parts of deck structures, cable trays, walking grids, storage tanks, rigid piping and instrument housings. Thermoplastics Thermoplastics such as PVC and polyethylene are used for flexible piping and cable ducting..
Page 31
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 32
Marine Corrision & Coatings
6 COATINGS 6.1
Types of Coatings
Corrosion management by coatings has been used extensively since the mid 18th century when natural bitumen’s were the common protective material. Major advances in the 20th century were the development of sophisticated epoxies, improved pigments, faster drying and curing to meet demands for improved performance and faster application and re-coat times. Since the 1970’s there has been a big demand for improved high performance marine coatings. However these demands have coincided with new requirements to reduce the toxicity of pigments and solvents and also to reduce the amount of volatile organic compounds (VOC’s) given off to pollute the atmosphere during coaing application and drying. These needs conflict with one another, and consequently a large amount of coating and process has had to be undertaken to develop the wide range of modern coatings. The main recent development milestone areas are: • • • • •
Improved resins for chemical and water resistance Moisture tolerant coatings for applying to wet surfaces High solids coatings for applying >250μm DFT in one coat 100% solids low viscosity resin coatings for high build and low volatile organic compound (VOC) emission Chromate and Lead free coatings of low toxicity
Tin free low toxicity environmentally friendly coatings for the protection of ships hulls. These modern coatings are also designed to keep the hulls free from bio-fouling and achieve low friction in the water, thereby saving fuel and allowing increased speeds. Epoxy and polyester powder coatings for high build and zero VOC emission Glass flake filled epoxies and polyesters for exceptional chemical and abrasion resistance. Water based high performance coatings for low VOC emissions Improved quality control and quality assurance on materials and application procedures There is now a wide choice of both general purpose and highly specialised coatings available to the Specifier.
6.2 Surface Preparation and Application of Coating Ship coatings, offshore structures and all steel systems requiring a high standard of corrosion protection requires abrasive blasting using iron shot, copper slag or grit before being painted. Abrasive blasting cleans and roughens the metal surface to provide a key for the adhesion of the paint. If the surface is too smooth the paint will not adhere. If the surface is too rough, then high points may stick up through the surface of the coating and reduce its efficiency.
Page 32
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 33
Module K
Marine Corrision & Coatings
Surface Profile Selecting the correct grade of blasting grit produces the surface profile or peak to valley height. The profile is required to obtain good paint adhesion even when the surface is clean. The surface profile is measured as peak to valley height or as a centre line average between the peaks and valleys as shown in Figure 11. The profile should be varied to suit the type of coating. Heavy-duty coatings of + 250microns dry film thickness need a profile of 50 to 75 microns. See Section 8 for methods of measuring the profile.
SURFACE PROFILE Rogue Peak Peak Trough Amplitude
FIGURE 11a Surface Cleanliness Surfaces must be properly cleaned, by using blasting, grinding, wire brushing, mechanical sanding and chipping, or solvent washing techniques. Cleanliness of the surface profile is necessary to get a good bonding of the primer coat. Bare metal surfaces easily corrode in any humid or moist atmospheric conditions. This type of corrosion results in the formation of an oxide film, which is not bonded to the metal and interferes with the paint bonding. Blasting Standards • Steels Structures Painting Council of America S.S.P.C. • NACE Standards for surface preparation by Abrasive Blast Cleaning • ISO STD.8501 Standard for Painting Steel Surfaces • British Standard Specification for Surface Finish of Blast Cleaned Steel for Painting. B.S. 7079 Dry Film Thickness (DFT) Once a coating has been applied and has dried, it is necessary to monitor the thickness to ensure that the specified amount has been applied to the surface.
6.3
Coating Types and Application
Heavy duty two pack coating and lining systems, based on organic resins, are one of the most frequently used forms of controlling corrosion. Virtually all external steel work is painted. Underwater and splash zone areas are coated with high build paint systems. Interiors of many tanks and vessels are lined or coated with specialised resin systems.
Page 33
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 34
Marine Corrision & Coatings
Additional coating systems used are the metallic coatings, such as, hot dip galvanising, metal spraying and electro or electroless plating. Systems based on organic (carbon containing) resins can be paint coatings or linings.
Paint Coating Systems Paints are made up from a mixture of solvent resin and pigments. Sometimes special additives called catalysts and hardeners also have to be added to the paint just before it is used. This depends on the systems chosen. All paint must be thoroughly mixed to make the paint flow correctly. Many paint systems consist of three different layers as follows: a. Primer Coat, b. Undercoat, c. Finishing Coat. Thickness is usually specified in microns (μm) or mils (thousandths’ of one inch) for the dry film thickness (DFT). 1 mil = 0.001inch = 25 μm.
40mils = 1mm
Typical heavy duty coating thickness may vary from 200 to 500 μm (5 to 20 mils) in thickness. Special bituminous coatings, glass flake coatings and 3 layer FBE / PE coatings may be up too 3mm thick. Specifying Paints A European Standard ISO 12944 provides a classification of environments and the paint types and thickness needed to give various life times in the given environment. The Standard is not clear for requirements on immersion service and internal pipelines. Some typical coatings that might be used are as follows: Ships hull Coating 2 coats of Epoxy to a DFT of + 500microns + 1 coat of antifouling paint. Note: Modern antifouling paints are highly specialised. They contain compounds that are toxic to bio fouling organisms but have much lower toxicity than the Tri Butyl Tin compounds that were used before 2000. Some coatings rely on self-polishing to retain freedom from bio-fouling. This is the gradual deliberately engineered loss of paint during service. The paint loss also carries away the fouling organisms and maintains a clean surface. It has been estimated that severe fouling of a ships bottom can add 30% to the fuel bill. Sea water Piling and structures Immersion and Splash Zone 1. 2. 3. 4.
3 coat epoxy to a Total Dry film thickness (DFT) of 400 microns 1 coat epoxy primer + 1 coat high build epoxy to a DFT of + 400 microns Coal tar epoxy to 400 microns 2 coats epoxy glass flake to 500 microns (+15 year life)
Marine Atmosphere Coatings 1. 2 coats epoxy mastic + 1 coat polyurethane to 350 microns DFT 2. 2 coats epoxy + 1 coat polyurethane to 350 microns DFT
Page 34
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 35
Marine Corrision & Coatings
Module K
Marine Immersion 1. 2 coats of epoxy to a DFT of +500 microns Buried or Immersed Pipeline External Coating 1. Fusion Bonded Epoxy 650 microns 2. Coal Tar Enamel 3mm 3. 3 layer FBE + adhesive + polyethylene (2.5 to 3mm) Tank Interiors 1 coat Epoxy High Build 350 microns DFT Note. Potable water tanks must have coating certified for use by relevant authority Pipe Interiors 1. Epoxy 2. FBE 3. Cement 4. Polyethylene Liner The above are only general examples and should not be used for specification purposes. Some paints and their uses are: Zinc Silicate Primer: Used in damp corrosive conditions where a lot of mechanical damage may occur. Also heat resisting. Not for permanent immersion. Epoxy Coatings:
Most commonly used high performance systems for external coatings and many tank linings. Epoxies are sometimes loaded with granite or silica fillers and applied as non slip abrasion resistant surfaces for decks and floors.
Polyurethane:
Used as topcoat on hulls, tank exteriors and platforms to give durable good appearance. Phenolic / Epoxy Phenolic: Used on the interior of tanks containing hot solutions.
Alkyds:
General purpose paints.
Glass Flake Polyester:
Modern Chemical (acid) resistant coatings for vessels and tanks. (Expensive but long life)
Chlor -rubber:
Fairly cheap but soft chemical resistant coating. (now being phased out due to environmental unacceptability)
Fusion Bonded Coatings These coatings consist of dry powdered resins. Epoxy is the most commonly used type of resin in a system called fusion-bonded epoxy (FBE). In order to apply the coating, the components have to be pre-heated by an electrical induction heater and the powdered resin is sprayed onto the surface. The resin melts and spreads over the pipeline as a viscous liquid before it hardens off due to chemical reactions. Coating thickness is usually about 70 μm (3mils) dry film thickness for indoor metal furniture and up to 800μm for severe immersion exposure. (32 mils). The equipment required to apply the coating is complex and costly and almost all fusion-bonded coatings are applied to pipe lengths, at a pipe coating mill, before despatch to the site.
Page 35
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 36
Marine Corrision & Coatings
Metallic Coatings - Galvanising Dipping steel articles in a bath of molten zinc forms galvanised coatings. The zinc layer formed on the surface is sacrificial to steel and protects it by galvanic action, even if there is a scratch in the coating. Although zinc corrodes in air or seawater it does so at a fairly slow rate. Galvanised coatings are used in industrial and marine atmospheres for steel protection. Heavy duty galvanized coatings are also suitable for full immersion service. They are not satisfactory in acid or alkaline conditions.
6.4
Effect of Coatings on Cathodic Protection Design
Coatings are usually the principle corrosion protection on submerged or buried structures. However the coating always contains defects or damage areas and deteriorates further with time. Cathodic protection (CP) provides the protection required for the damaged coating areas. The coating also reduces the current demand on the hull or other component to be protected by up to 98% with a very high-grade coating.
6.5
Coating Evaluation and Inspection Measurements
It is often necessary to monitor the condition of a surface as it is being prepared for a coating, as the coating is being applied, and after the coating has dried and weathered. The different monitoring conditions need the use of different inspection instruments. The coating thickness criteria being measured include the following: a. surface profile b. surface cleanness c. wet film thickness d. climatic conditions e. destructive thickness f. dry film thickness g. porosity or holiday detection h. adhesion quality Surface Profile Monitoring The blasting surface profile can easily be measured in the field by visual comparison with a special standard set of profiles or by a profile gauge. The latter method uses a plastic film that is pressed on the surface and then peeled off to provide a replica of the surface. The replica is then measured for a change in thickness from its original condition, by a special micrometer. This thickness change corresponds to the peak to valley height of the actual surface. Surface Cleanliness Surfaces must be properly cleaned, by using blasting, grinding, wire brushing, mechanical sanding and chipping, or solvent washing techniques. Dry Film Thickness Once a coating has been applied and has dried, it is necessary to monitor the thickness to ensure that the specified amount has been applied to the surface. Magnetic dry film gauges are able to measure the thickness of non magnetic coatings such as paint, epoxy resin, glass, zinc and plating substances, and of non-conductive coatings such as glass fibre,
Page 36
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 37
Marine Corrision & Coatings
Module K
rubber, plastic, and polyurethane sheeting on magnetic surfaces (carbon steel, but not stainless steel). These gauges work on the principle that magnetic forces are reduced as the magnet is moved further away from the steel. Another type of thickness measuring devise is the Eddy Current Gauge that can be used on magnetic or non-magnetic substrates. Porosity or Holiday Detection Ideally, the finishing coat should provide a nonporous protective shield of durable thickness and quality that will resist penetration by moisture to any undercoats. The coating may contain pores due to solvent bubbles trapped inside it or due to areas of contaminated metal surfaces that prevent the coating from bonding. Holiday detectors are non-destructive test instruments which show the position of pores or very thin coatings. The technique is used mainly on high performance coatings for water immersion or buried service. The painted item to be examined has to be electrically earthed in order to carry out the test. After earthing the coated area is traversed by a metal brush or metal loop (various designs exist) that is supplied with a very high voltage input. The voltage can be varied depending on the thickness of the coating. When a holiday or thin area is located the electrical insulation of the coating breaks down and a spark passes from the holiday detector to the suspect area that is marked out for rectification. Adhesion Testing Adhesion Testing is a form of destructive testing that determines the adhesive or bonding quality of a coating system. The technique used is based upon the principle of pulling off the coat from the protected surface material. Two such methods are; the loaded spring tool that exerts a specific pull on a test dolly that has itself been glued to the coating surface, and the crosshatch cutting tool.
Page 37
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 38
Marine Corrision & Coatings
7. CATHODIC PROTECTION Cathodic protection (CP.) is one of the most important methods of corrosion control on Ships, jetties, sheet piling, tanks and pipelines.
7.1
Theory
Some of the theory of CP has already been covered in the Sections on General Corrosion and Electrochemistry. The Battery Cell as shown in Figure 8 is a simple example of an anode and cathode For cathodic protection to work there must be an electric circuit for the transport of electrons. This circuit is usually the metal to be protected, a suitable metal anode, a connecting wire and the soil or water (electrolyte) in which the system is buried. Seawater has a very low electrical resistance of 30 ohm cm. Soils have a relatively high electrical resistance value. Typical values may be 300 - 1,000 ohm cms and 2,000 - 21,000 ohm cms for low and high resistivity soils respectively. The composition of the metal surface, such as the presence of mill scale and variations in chemistry determine where anodes and cathodes will be present in the corrosion risk areas of the structure. When corrosion is taking place electrons flow through the metal circuits that exist in the different compositions of metal grains in the plate metal. The electrons flow through the metal from the anodes to cathodic areas of the pipe surface, the anode areas corrode as iron ions are released into the water or soil whereas the cathodic areas are protected. Pitting occurs at the anode areas and eventually the pitting undercuts some cathodic grains that then fall out of the body of the metal even though they are not corroded. Corrosion then is a continuous process of actual dissolution of anode areas and undermining and breakdown of cathodic areas. The electrical potential established between a steel surface and adjacent water or soil is generally in the range of -0.4V to –650mV when measured against a standard copper/copper sulphate reference cell. This is the natural corrosion potential of the steel. (Reference cells are discussed later) If a new metal could be introduced into the corrosion circuit and controlled at a potential that causes the current flow to be reversed on all of the most negative area found on the metal structure that requires protection, it follows that the new metal would become anodic to the whole of the pipeline. In this case there would be a flow of positive current from the new metal, through the water or soil onto the metal that requires protection. The new metal would become the anode and the whole of the system would be cathodic to it, with the result that the corrosion of the structure would cease. This condition can be obtained if the structure metal to water or soil potential can be changed to be equal to, or more -ve than –850mV (-0.85V) with reference to the standard Cu/CuSO4 electrode. Highly electro-negative metal sacrificial anodes are made from zinc, aluminium or magnesium. Aluminium is the most popular seawater anode, If zinc is used the weight of nodes required is much higher than for aluminium. Magnesium produces a higher potential against steel ( - 1.4 to -1.7V) and is generally only used in soil with a high resistivity. Positive current flows from the protected structure through insulated wiring to the anode. From the anode the current flows back through the conductive electrolyte (sea water or soil) onto the surface that is protected.
Page 38
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 39
Module K
Marine Corrision & Coatings
The current can be masked by coatings or by corners in the structure. In the following illustration no current would reach the back of the structure and a separate anode, or set of anodes would be needed to protect the back. Also the current only protects the face of the metal that “sees” the current. The inside face is not protected all. The sacrificial anodes are consumed by corrosion instead of the protected structure and the require replacement at calculated time intervals.
7.2
Impressed Current Cathodic Protection (ICCP)
This is the system used for large boats or large structures. In this case the anode may be graphite, cast iron, coated titanium or metal oxide. Applying a DC current to the system, which pushes the electrons around the circuit, creates the flow of electrons. The anode material is not consumed by corrosion and ca have a long life. Also it is possible to use much smaller and lighter anodes than are required for sacrificial protection. Typical CP installation schematics are shown as follows:
SCHEMATIC DIAGRAM OF SACRIFICIAL CP +ve current flow
Boat or Structure
Zn or Al Anode
Sea Water
Note: Only the outer surface is protected as shown. Also the protective current will not flow around the back faces of the boat or structure. FIGURE 12 SCHEMATIC DIAGRAM OF SACRIFICIAL CP
Page 39
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 40
Module K
Marine Corrision & Coatings
A number of rectangular aluminium anodes are distributed around the hull, below the water line and attached to the hull by bolting or through a welded doubler plate. Several anodes concentrated around the stern of the boat can be used to protect the rudder and propeller. Special connections have to used to ensure a complete electrical circuit. The propeller requires conductive slip rings on the shaft to ensure a good connection.
IMPRESSED CURRENT CP - Plan view Impressed current CP for ships hull From ships Power Supply Transforma Recliner -ve return path
-ve return path +ve
+ve current flow onto hull
+ve
Anode
Anode
+ve current flow onto hull
+ve current flow onto hull
Anode installation detail Conductive path to return current to TR
Anode bolts insulated from hull DC from TR
ships hull steel plates Anode shield - Fibreglass or paint
Anode shield - Fibreglass or paint
Anode
FIGURE 13 IMPRESSED CURRENT CP Plan view A range of reactive metals such as zinc, aluminium and magnesium can be used to provide sacrificial CP systems while silicon iron, platinised titanium or mixed metal oxides are typical low corrosion rate impressed current anodes. Impressed current CP is applied to the hulls of most large ships. Two to six anodes are bolted on the outside of the hull at carefully selected points. The connecting cables are fed through watertight insulating glands in the hull to an adjustable direct current source (Transformer /Rectifier or TR) inside the ship. The anodes are usually bars, plates or discs of platinised titanium or mixed metal oxide construction. Relatively small anodes can carry the amp current loading necessary to protect the hull and the anodes breakdown very slowly by self corrosion. Mechanical damage is one of the main risks and designs must take this into consideration. Anode shields, as shown in Figure 13 are important in the design. The dielectric insulators are 3 to 5 times the anode length and their purpose is to prevent the majority of current flow taking the shortest route to the metal and causing and depleting the flow to points further along the hull.
Page 40
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 41
Module K
Marine Corrision & Coatings
LARGE CURRENT FLOW VARIATION WITHOUT ANODE SHIELD IMPRESSED CURRENT CP reduced current density
high current density
high current density
reduced current density
FIGURE 14 For details of anode assemblies see BS7631 Cathodic Protection. Part 1. 1991.
7.3
Protective Potentials and Potential Measurements
To achieve corrosion protection on steel the specified potential between the ship or structure to be protected and the surrounding water is generally accepted as being between -800mV and -1250mV as measured with a silver / silver chloride reference electrode, also known as a half-cell. The design objective is to try and get all areas of the protected structure to meet these requirements. If the structure is in soil a copper / copper sulphate electrode is used instead of the silver chloride type. This electrode is more stable in soil use. The potential required against the copper / copper sulphate electrode is between -850 and -1300 mV.
The reference electrodes are used to measure the structure to soil potential. The structure / water or earth junction forms one half cell and the reference electrode/ water or earth junction is the other half of the complete cell. To achieve accurate results a high impedance 10 or 20 meg ohm digital Voltmeter must be used and the connecting cables must have a low resistance. The Structure is always connected to the +ve connection of the voltmeter.
Page 41
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 42
Marine Corrision & Coatings
MEASURING CP POTENTIALS IN SEA WATER Reference Cell
Method of Measuring Potential
Voltmeter
CABLE
HULL
Anode
Silver Rod Silver/ Silver Chloride ref. cel
Silver Chloride solution Porous plug
FIGURE 15 Measuring CP Potentials in Sea Water Achieving the Potential A suitable anode has to be selected to achieve the potential. The type depends on mainly on the water or soil resistivity and the area or length of structure to be protected. High soil resistivity may require use of Magnesium anodes with a potential of -1.5V with reference to a C/CS electrode. For lower resistivity soils and salt water Zinc anodes with a potential of -1.1 V may be used. Protection of large areas such as the hull of a +1000 ton deadweight ship or a long length of pipeline may be by inert silicon or metal oxide anodes with the DC current supplied from a mains rectifier / transformer.
7.4
Cathode Current Density
In order to achieve the correct potential the current flow onto the structure must be of an adequate current density in mA/m2. Values have been given to different types of waters and soils some of which are as follows: (Typical values from the National Association of Corrosion Engineers, NACE Handbook) NACE current density values for coated steel are: Soils: 50 to 500 ohm cm resistivity Soils: 500 to 1500 ohm cm resistivity Soils: 1500 to 4000 ohm cm resistivity Sea water (quiet) Sea water (fast flowing) Sea mud
mA /m2 (WELL COATED) 0.5 to 1.0 mA /m2 (WELL COATED 0.1 to 0.5 mA /m2 (WELL COATED) 1 to 2
mA/m2 (WELL COATED) 250 to 1000 mA/m2 (NOT COATED) 25 to 50 mA/m2 (NOT COATED) 2
to 5
Cathode Area A large cathode area in low resistivity water or soil can require a lot of current (current density x bare area. Large numbers of sacrificial anodes may be required or it may be necessary t use impressed current to provide the current. The total area of cathode to be protected is massively reduced by application of high duty coatings.
Page 42
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 43
Marine Corrision & Coatings
7.5
Module K
Importance of Coatings for CP
Coatings are usually the principle corrosion protection on ships, submerged or buried structures. However the coating always contains defects or damage areas and deteriorates further with time. CP provides the protection required for the damaged coating areas. Steel ships usually have their hulls cathodically protected. For ships the current required is usually 250mA / m2 of bare metal. The area of bare metal on a well painted ship is taken to be about 0.5% to allow for damage and porosity. Therefore the total current required would be: Submerged area of hull x 250 x 0.,5/100 = Total immersed area x .125mA. However this requirement increases as the water temperature increases and also as the average ships speed increases. Loss of paint also increases the current demand and has to be taken into account. Ships propellers that are moving very fast can require 400mA /m2. Example: A bare metal ships hull requires 250mA / m2 for protection. The hull is well coated with a new epoxy that is assumed to have 0.5% holidays. (holes in the coating) The area of “bare” pipe is now reduced to 0.5/100th of the original area and the current requirement is also reduced to 0.5/100 of the original demand = 125mA/m2 based on the total area. Note. The CP Engineer must always check and state whether current density calculations are based on bare metal or based on the coated pipe. A good high performance coating is a requirement in conjunction with CP. As time passes an increased % of coating breakdown is expected and has to be accounted for in the CP. calculations.
7.6
Over Protection / Under Protection
Under protection occurs where the cathodically protected structure or ships hull potential to the surrounding soil or water is more +ve than -850mV when measured with suitable reference electrode. There is also a potential limit where if the potential is too negative it can cause damage to the pipeline coating and even to the pipeline steel. At potentials more -ve than -1500mV evolution of hydrogen at the cathode can cause damage to coatings and may also cause steel embrittlement. Over protection is only a problem with impressed current systems and occasionally with sacrificial magnesium anodes.
Page 43
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 44
Marine Corrision & Coatings
POTENTIAL VARIATIONS ON A STRUCTURE Potential v Distance Caternary Plots TR Ships hull Anode
Anode
Anode
-1250mV
Low protection
-800mV
TR
Ships hull Anode
Anode
-1250mV
-800mV
Low protection
FIGURE 16 POTENTIAL VARIATIONS ON A STRUCTURE Highest -ve potentials occur at the points where the pipeline, structure or ships hull is closest to the anode. At these points on an impressed current system, or where high voltage magnesium anodes are used, the potential may be high enough to create damage risk.
As the distance from the anode increases so the potential falls (becomes more +ve) until a point is reached at which it is no longer protective. Another anode may then be needed to boost it back to 850mV. The typical curve produced by a series of anodes is a Catenary Curve. (This is the type of curve a chain forms when hanging from two points)
Page 44
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 45
Module K
Marine Corrision & Coatings
7.7
Types and Properties of Anodes. Anode Weight, Shape and Life
When sacrificial anodes are selected the type of anode chosen also depends on the driving force. Driving force = (Potential output of Anode ref. C/CS cell - 850mV) Material* Mg (high Pot) Mg (Low Pot} Zn Al (in water)
1.7V 1,5V 1.15V 1.1V
Potential Driving Force 850mV 650mV 300mV 250mV **
> 1500 ohm cm > 1000 ohm cm < 1500 ohm cm Water only
Materials are actually proprietary alloys rather than pure metals. Appendix.
Typical alloys are shown in the
Shape and Size. The larger the surface area of an anode, the lower the electrical resistance to the environment and the greater the current the anode can supply. Typical formulae indicate that the anode resistance is inversely proportional to the square root of the anode area. The geometry at an individual anode is selected to ensure that each anode has sufficient surface area to provide the required current for the section of the pipe or structure that is protected by the individual anode. Anode sizing is taken from tables provided by the anode manufacturer or from calculation of a readily available resistance formula. The weight of each anode must be sufficient for the anode to last the required design Life. This also determines the geometry and size of the anode. Anode types Ships hull Anodes On shore Pipeline Anodes; Off-shore Pipeline Anodes; Tank Internals Large Plate Structures; Tank Bottoms Deep Ground Beds
Usually rectangular or trapezoidal Usually short cylindrical type. Bracelet type Long cylindrical type Bolt on Anodes, often rectangular Bolt on Anodes rectangular or spherical Long cylindrical
Anode Life The life is based on the number of amps per Kg of material that the anode can produce. This is derived from Faraday’s Law that:The Electrolytic Dissolution of one Equivalent Weight of a Metal will produce 96,500 coulombs of Electricity. One Coulomb = one Amp flowing for one second. Example. Zinc. Equiv. Wt. = Atomic Wt /2 65.4/2 = 32.7g Zn produces 96,500 coulombs. Therefore 1kg produces (1000/32.7) X 96,500 = 2,951,070 coulombs.. This is 2,951,070 / 3600 = 819Amp hours per kg of Zn. In practise the proprietary alloys are not pure zinc and may produce rather less than this amount. A typical minimum output is 780 amp hrs / kg of Zinc This is known as the Anode Capacity.
Page 45
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 46
Marine Corrision & Coatings
Anode Weight The weight and number of the individual anodes for protection is determined by the amount of current that each anode can put out. The current depends on the internal resistance of the anode and that in turn depends on the anode weight, size and shape. The subject is covered in detail in a later Section.
7.8 Calculations for Design The first calculation is to determine the current demand for the CP system. This is generally done by estimating the current demand on the basis of well-established values that have been determined for various soil resistivities. The current demand is determined on the amount of bare steel estimated to be present at various stages in the pipe or structure life. Estimates for coating breakdown vary tremendously. There is no precise figure, however the following date is often used: Initial Coating Damage 0.5% for a modern coating, 2% for a coat and wrap or fusion bonded epoxy and 5% for poorer coatings. Damage after 20-year life 1% for 3 layer, 5% for Coat and wrap or FBE. 20% for poorer coatings. Coating damage is increased where pipes / tubing have been forced through the earth in crossings or wells. The next step is to multiply the bare area by the current density required, say 25mA/m2 for a corrosive Subkha area, 5mA/m2 for a fairly dry high resistance desert area. This gives a total current value, ( I) Sacrificial anodes are selected on the basis of a well known formula:W = I x 8766 x Y / Z x U Where
W= Total Weight of Anodes in kg I = Total Current in Amps Y= Required Design Life of the Anodes Z= Anode Capacity in Amp hrs / kg U= Utilisation Factor (a decimal fraction) 8766 = No.of hours in a year.
I is known from the initial calculation, Y is the design life. Z = can be found in proprietary data for any type of anode. Z = See Appendix. U is usually taken as 0.9.
The basic design procedure is shown on the next page,
Page 46
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 47
Marine Corrision & Coatings
Module K
Courtesy of Impalloy. (Now Corrpro Europe)
Page 47
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 48
Module K
Marine Corrision & Coatings
The number of anodes are selected to satisfy two criteria: 1. Total Anode weight / Individual anode weight
= Number
2. Total Current required / Individual Anode output = Number Once the total weight has been obtained an individual type anode type is selected for material, shape and weight. The individual output is calculated from Ohm’s Law I = E / R and the internal resistance ( R ) is found from the Dwight formula as shown on the next page.
7.9
Impressed current cathodic protection
Criteria for Selection The size / length of the sructure or hull will indicate whether impressed current should be selected. Other factors are the driving force to obtain the required potential on the pipeline, and the availability of electrical power .
7.10
Power Sources
Mains electricity is always the preferred power source. The mains supply is connected to a Transformer / Rectifier (TR) which reduces the voltage and provides a direct current (DC) output to the anodes. The DC output may be from 5 volts to 50 volts depending on the driving potential which in turn depends on soil resistivity and the length of pipe to be protected. The amp output is also based on the required current density and the area to be protected. A 20% contingency allowance is usually added to the ratings. Cables DC cables shall be single core multi-strand copper, double insulated and sheathed for protection against the aggressive soil environment. Minimum cable size shall be 10mm2. Bonding cables and main DC current carrying cables shall generally be 50mm2.
7.11
Types and Selection of Anodes
Anode materials that are used for the impressed direct current methods of cathodic protection include: a.
Graphite
b.
Magnetite
c.
Silicon alloy iron
d.
High Chromium Silicon Iron
e.
Platinised titanium
f.
Metal oxide coated metals
Graphite anodes are less commonly used as they are brittle and will tend to flake. Silicon alloy iron anodes are rather less brittle than graphite and are more expensive. In salt water conditions deep pits develop to severely shorten their design life and they are only suitable for buried conditions. In soils with high chloride the high chromium type of silicon iron should be specified for resistance to chloride attack. ( in salt marsh / subkha areas) or wherever high chloride content exists.
Page 48
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 49
Marine Corrision & Coatings
Module K
Platinised titanium anodes are less brittle than either the graphite or silicon alloy iron anodes. The basic titanium metal is electrically plated with an extremely thin layer of platinum, about 2.5 microns (0.000 1 inch) thick. These anodes are expensive but have low weight and good long term current output characteristics. One anode or a cluster of anodes in a ground bed can protect long lengths of pipeline from a kilometre up 50Km in ideal conditions. Mixed metal oxide anodes are more recent developments and are capable of carrying high current loadings with very low self corrosion rates. Anode sizes and types are selected on capability to carry the current required for the selected life and cost. A major advantage of impressed current systems is that because they are mains driven the driving potential is not limited to 250mV as for zinc, or 700mV as for magnesium sacrificial anodes. In areas where no mains supply is available thermoelectric generators or solar powered systems can be used. It is important to site the inert anodes in moist soils that have low resistivity levels and, in consequence, less potential is required for the needs of the cathodic protection system.
7.12
Calculations for Design
Detailed calculations are outside the scope of this course but can obtained from books referenced at the end of the Course notes (1) (2)
7.13
Interference Current
Current from a protected structure can flow onto other structures in the area and set up corrosion cells as shown in Figure 17 when it tries to return to the protected section of the structure by the shortest low resistance return route to the source supply. In bad cases, the corrosion is quite severe in positions close to the point where the current leaves the foreign structure.
Page 49
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 50
Module K
Marine Corrision & Coatings
STRAY CURRENT INTERFERENCE Current leaving unprotected ship
Current leaving unprotected ship
Unprotected ship moored close to protected ship
Corrosion
Corrosion Anode current flowing on to unprotected ship
Anode
Anode
Anode
Anode
Use chain or cable to connect the ships hulls and carry the current back through a metal path to stop corrosion on the unprotected hull FIGURE 17 STRAY CURRENT INTERFERENCE Foreign CP interference is a major concern on ships as follows: A Two ships moored together, one protected by CP, the other unprotected or only partly protected. Current from the anodes on the protected ship flows on to the unprotected ship and helps to provide it with some degree of cathodic protection. However because the current from one ship is now trying to protect two ships the current supply is probably insufficient to protect either ship properly and corrosion may occur on the “protected vessel. Also where the current leaves the “unprotected” ship (foreign structure) this point becomes an anode area and corrosion cells can be set up. To prevent the setting up of a corrosion cell by stray current flow between two ships an electrical resistance bond is connected between them. This can be a heavy duty cable or chain well earthed to the two hulls. B Ship moored to a pier or jetty with cathodically protected piling or structure. Current from the piling can pass onto the ship hull and then find a return path from the ship through the water back to the piling. At the point where it leaves the ship the current can cause concentrated corrosion. The problem is generally only serious if the ship is going to be moored for days rather than hours. Here again prevention is by bonding the ship to the protected area (piling etc.) by a chain or cable. Note. Normal mooring bollards may not be electrically connected to the piling and may not provide the type of connection required.
Page 50
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 51
Module K
Marine Corrision & Coatings
Stray current and foreign structure problems have been a source of considerable pipe corrosion on land. The situation occurs when a new pipeline with C.P. is laid in the vicinity of an existing line or other underground structure such as a tank. Current flows onto the old structure and causes it to corrode at the newly created anode sites.
7.14
Transformer / rectifiers
The transformer rectifier (TR) is a vital part of any impressed current cathodic protection system with the function of reducing mains voltage, usually 240V, to a usable level for the CP system and changing the AC sine wave amplitude to full or half wave rectification. .17CP monitoring and maintenance
7.15
Potential Surveys
The pipe to water or soil potential is the most important method on monitoring CP. to ensure the - 0.85 to -0.90V is attained. This reference electrode for buried pipelines and structures is the Copper / Copper Sulphate half cell that has already been described. The half cell is connected to the buried steel structure through a high resistance voltmeter that indicates the potential level. For ships and marine structure a silver / silver chloride cell is used. The positive side of the voltmeter is connected to the reference electrode. The connection to the structure is made through a special test point. This may be special box that contains spade terminals that can be disconnected and reconnected through a test meter. If the observed potential is less negative than -650 mV, corrosion attack may be freely occurring. Between -650mV and 750mV some degree of protection would normally be expected and full protection would be expected at –850mV.
If the observed potential is more -ve than –1500mV, as can occur with impressed current, then a condition called over potential may occur leading to hydrogen embrittlement of steel and / or disbonding of coatings. Close interval potentials along the length of a marine structure
-850
V -750 -650 Unprotected area
20
40
60
80
100
120
Distance m FIGURE 18. POTENTIAL SURVEY The above short survey shows an area with a sharp drop in potential to a level that is close to the natural corrosion potential. This indicates a problem that needs investigating. If there are sacrificial anodes along the structure it may be that local anodes are not connected. Alternatively there may be a serious
Page 51
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 52
Marine Corrision & Coatings
coating defect or a current drainage point in this area. If the whole structure is below the protection level then the number of anodes may need to be increased or if it is an impressed current system the TR output may not be sufficient. Additional possibilities are that the coating is much poorer than allowed for in the calculations or the pipeline is contacting another structure that increases the area beyond the calculated area. (Foreign interference) If the whole structure is more -ve than -1.5V (only possible on impressed current) then it may be necessary to reduce the TR output or introduce resistors into the system.
Instant Off Potentials The potential measured at the ground surface is not accurate because the reference electrode is remote from the metallic item The reference electrode is measuring the potential gradient at the earth surface and this includes an Ir component. The measured value can be calculated if the soil resistivity is known and the current flux estimated, but this is cumbersome and usually inaccurate. Instead the TR is switched off. The potential falls within milliseconds to the actual structure potential and then decays slowly as the capacitance charge across the interface leaks away. The off potential is recorded immediately after the TR is switched off. This can be done manually but nowadays the switching is made automatically by a quartz-based timer which is synchronised with a similar clock on a recording voltmeter package. The meter records the on and off potential automatically. An instant off potential shift of 100mV in the +ve direction indicates that a structure is adequately protected. (Ref. NACE Standard RPO 169-96) CP switched off
-0.9V
Protected potential Ir drop (>100mV)
-0.5V
Corrosion potential
Time (hrs)
FIGURE 19 POTENTIAL OFF
7.16
C.P. Maintenance Factors
Monitoring and maintenance of the coating is important in corrosion management and the operation of a protective CP system. Excessive loss of coating will cause a fall in the current density and a consequent +ve potential shift to an unprotected condition. Coating loss can occur through poor substrate preparation or poor application procedure. Mechanical damage may occur and cathodic disbonding of the coating is a risk at high -ve potentials.
Page 52
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 53
Marine Corrision & Coatings
Module K
Tape applications often fail due to embrittlement or blistering of the material that might not have been the correct choice for the temperatures involved. The survey methods described above provide the indications of coating breakdown. When a problem area has been identified it is necessary to investigate for signs of coating failure, anode failure, poor circuit or inadequate power source output. Prompt repair is required.
7.17
CP SAFETY
Safety factors are always a primary requirement in any activity related to corrosion control and cathodic protection. There do not appear to be any National or International specifications or procedures on the subject. Some practical considerations are given below. CP Safety in fire risk area. (Zones 1 and 2) Care has to be taken to avoid sparks or flames during construction and installation of all systems. In general zinc and aluminium sacrificial anode systems do not present a risk of sparking during operation. However magnesium and aluminium anodes may spark if dropped. Zinc should be specified for high risk areas . Magnesium anodes may create sparks under adverse conditions due to their higher potentials. All test points, electrical equipment and isolation joints should be positioned outside the high risk zone where possible. Impressed current systems present much higher spark risk. If such a system has to be used a detailed risk assessment should be carried out. The TR’s, test points and terminals should be outside the risk area. All equipment within the area should be flame proofed, CP Safety near parallel high voltage lines Installers should ensure pipe lengths are grounded. Tests should be made on the pipes to check induced or stray potentials. CP Safety near known or suspected buried electrical cables. Pipelines or structures should be checked for current pick up and should be grounded. Bonding Bonding of a CP system to any structure associated with the grounding system of an electricity supply network should be avoided.
Loading Lines and Ship to Shore Lines in hazardous area. Connecting and disconnecting loading lines to tankers, barges or gantries in fire risk areas can be hazardous. Gantry supply pipeline that connected to CP should be fitted with isolating joints before the Gantry. Ship to shore bonding of the tanker and jetty CP systems is not good practise because the differences in the current demand of the two systems can allow substantial currents to flow between them. Isolating joints should be installed in the loading lines.
Page 53
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:12
Page 54
Marine Corrision & Coatings
In order to provide protection against arcing during connection and disconnection of cargo hoses it is necessary to fit an isolating joint or flange or a single length of non-conducting hose in each of the metal arms or cargo hose strings to ensure electrical isolation between the ship and shore. All metal on the seaward side of the isolation should he electrically continuous with the ship and all metal on the land side continuous with the jetty grounding system. Isolating joints or flanges should be designed to avoid accidental short circuits. Where the loading line is wholly flexible the isolation should be fitted at the jetty manifold. Where the line is partly flexible and partly a metal loading arm, the isolation should be inserted between the flexible hose and the metal arm. For an all metal arm care should be taken to ensure that the isolation cannot be short circuited by guy wires or tools. For tankers at submarine Line berths at least two sections of non-conducting hose should be inserted into the string of flexible hoses at the end of the rigid line. These should preferably be the second and third hoses from the tanker manifold. Switching off the CP systems is not considered advisable due to loss of protection and the difference in polarisation times might create bigger potential differences than leaving the system operating.
Page 54
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 55
Marine Corrision & Coatings
8.0
Module K
CORROSION PREVENTION MAINTENANCE
Preventive maintenance is very important in all marine operations. Some of it is relatively simple, such as keeping decks free from corrosive dusts and cargo residues. Carrying out regular re-painting schedules and greasing or cleaning proprietary equipment to the manufacturers recommendations. On large vessels and particularly on tankers one of the problems is knowing where to look for corrosion and structural damage. Corrosion will occur where water or wet deposits can collect. The effects can be particularly bad if the area is poorly painted, if is a warm location or if the water or deposits are allowed to remain for a long period. Ship’s hulls are relatively easy to inspect and maintain at dry dock periods. The hulls suffer from general corrosion and may have localised pitting but do not suffer from external cracking unless a mistake has been in material and welding specifications. The main difficulties occur inside the ship, on internal hull support structure and in storage tanks and cargo bays. Ballast tanks, frequently holding salt water, are high risk areas requiring regular inspection and maintenance. Cargo holds for corrosive or abrasive chemicals are metal ores are exceptionally high-risk locations. The movement of heavy loading vehicles and cranes inside the holds damages coatings. The pouring and movement of abrasive ores also cause damage. Once damage has occurred to the protective or to the metal then corrosion occurs rapidly in the cargo becomes at all wet. These are areas requiring very high standards of inspection and maintenance.
Page 55
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Module K
Page 56
Marine Corrision & Coatings
9.0
IDENTIFICATION OF DEFECTS
Visual inspection is the most important aid to identifying defects. Experience and intelligent use of simple devises such as a strong knife, a coating thickness gauge, a pocket lens, pH papers and a pit depth gauge can reveal a vast amount of valuable information. Coating Defects: Visual Inspection or simple Instruments required Poor surface preparation prior to blasting. Use Profile Test Gauge Coating below specification thickness Use Thickness Gauge General Loss of adhesion Visual Blistering Visual Metal Defects Deformation of Structure Loosening of Rivets and Bolts General corrosion Pitting Large weld defects Macro -cracking of metal
Visual Visual Visual Visual and Pit Depth Gauge Visual and Hand Lens Visual and Hand Lens
Defects requiring specialised Instrumental Techniques Coating cure condition Visual indication and then Laboratory test. Coating Adhesion.
Test with knife, then Specialised Adhesion Tester Kit
Confirmation of Correct Material Specification
Sample required for spectrographic analysis
Identification of micro cracks or internal cracks
Ultrasonic Testing
Identification of Laps, Inclusions or Pores
Ultrasonic Testing (or X ray)
Identification of Structure Failure as it Occurs
Acoustic Emission Monitoring
Identification of Damage under Marine Fouling
High power water jet clean, Visual Inspection and Pit Gauge
For detailed identification of paint defects and marine fouling see Fitz’s Atlas of Coating Defects.
Page 56
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 57
Module K
Marine Corrision & Coatings
10.0 SURVEY METHODS AND EQUIPMENT 10.1
Non-Destructive Inspection Methods.
Ultrasonic Methods Ultrasonic sound waves generated by a transducer can travel through very thick sections of metal. The ultrasonic sound wave is a mechanical energy wave and is transmitted, via a liquid interface, into the metal. It then travels through the metal until it reaches a discontinuity that may be a crack, void or the other side of the metal. At this point the wave is reflected and travels back to the transducer that, in some designs, can be made to operate as a receiver and detects the reflected wave. The principle of ultrasonic techniques is shown in Figure 20. Here, part of the signal generated is reflected at a crack in the metal. The remainder of the signal waves are reflected at the back surface. The time taken from emitting the signal to receiving the reflected waves is obviously less for the waves returning from the crack than for the wave penetrating to the back surface. This time difference due to defects in the path of the signal can be shown up on cathode ray tube. Ultrasonic measuring equipment is used regularly in oil producing operations to monitor the quality of welds and determine the presence of cracks, voids, corrosion or stress corrosion cracks. The simplest ultrasonic meter is the D-meter, a hand-held portable instrument that does not use a cathode ray tube, but simply records metal thickness on a digital display. It is useful to confirm specification thickness, detect large amounts of general corrosion or large size pits on the inside of a pipeline or vessel.
Specimen
Back wall Transmitter receiver
Flaw Flaw echo
Impulse
Flaw echo d1
Zero line d2 Back wall FIGURE 20 PRINCIPLE OF PULSE-ECHO ULTRASONICS
Page 57
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 58
Marine Corrision & Coatings
X ray Methods X-rays are high energy electromagnetic radiation. X-rays will penetrate metals, but are gradually absorbed as they pass through. The amount of absorption is proportional to the thickness of the metal and to its density. (A thick lead foil will absorb most X-rays because it is very dense). The determination of corrosion or the presence of cracks in metal by X-rays is based on the use of a photographic film on the opposite side of the metal to the X-ray source. This plate, after development, shows the intensity of the X-rays. If the metal has a local thin area, less radiation is absorbed than in the thicker area. Cracks voids and corrosion spots show up as dark areas on the plates. Acoustic Emission Acoustic emission is a new technique being used experimentally on the main structural components of large bulk tankers. The technique involves installing a large number (+100) sensitive microphone systems on critical structures. Sounds created by strain in the structure are channelled to a data logger and computer. Variations in noise output can be interpreted as plastic strain or cracking and can alert the Inspector to the problem. The large number of instrument heads enables the location of the suspect noise to be found. Additional NDT tests can then be used to define the risk .
Further studies recommended / Practical exercises Sections 5 to 10 Appendix B
Page 58
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 59
Marine Corrision & Coatings
11
Module K
CASE HISTORIES
Service History A medium sized bulk carrier spends its time plying up and down a large overseas river carrying iron from a terminal 300 miles upstream to a large storage ship permanently moored at sea near the river estuary, ore from this ship is then unloaded onto ships for destinations all over the World. The river carries a lot of iron ore sediment and has one 50km shallow stretch where the flat-bottomed bulk carrier stirs up mud, creates turbulence and sometimes scuffs the bottom of the ship on the riverbed. The Carrier ship is cathodically protected but has had a number of severe hull corrosion problems. Corrosion has also occurred on the cargo deck and deck fittings.
Principle problems identified with the CP system have been as follows: 1
Under Protection. Potentials on some parts of the submerged hull have been measured as more +ve than -600mV to a silver / silver chloride electrode in seawater.
2
Frequent complete shut down of the CP system by the automatic overload sensor. This problem occurred particularly while under way in seawater.
3
Failure of the CP survey procedure with a silver / silver chloride reference cell to measure true potentials when the ship was moored at the river dock. Readings of 200mV were often obtained which appears false because this value is more +ve than the natural corrosion potential of steel in fresh water, which is about -400 to -500mV.
Cause of CP Problems. The CP system appeared to be designed for 10 - 12% coating failure which would be quite normal. However below water coating failure is estimated as considerably more than this due to damage by turbulent water and scuffing on the riverbed. When anchored the protection requirement is less than 100mV / square metre bare metal. Under way in seawater this could increase to 300mV / square metre and the transformer rectifiers could not provide enough current to achieve this because of the amount of bare metal on the hull. The hull was under protected. The system then cut out due to overload and all protection stopped until it was manually re-started. At the dock side there appeared to be stray current interference from the DC power supply to a large conveyor. However this was not considered a major problem. The problem with the silver chloride reference cells appeared to be that they became contaminated with iron from the iron ore in the water and gave false readings. In these conditions use of pure zinc reference cells are more likely to give true readings. When using zinc the readings appeared to be satisfactory.
Page 59
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 60
Marine Corrision & Coatings
Deck Corrosion Considerable exterior pitting of Deck piping. Holes in thin wall pipe. Corrosion of mechanical equipment.
Some pitting of hatch covers.
Cause of Deck Corrosion Problems. Iron ore -Iron oxides are very cathodic to steel and when wet can set up galvanic corrosion cells and oxygen concentration cells. Considerable ore spillage occurred on the decks and pipe surfaces which were often wet and pitting corrosion started. Iron Ore in Holds. The iron ore in the holds could be a severe corrosion problem if wet, but fortunately the product is always stored dry.
Possible Solutions to the Problems
CP 1
Increase the Amp output on the T /Rs.
2
Improved damage resistant coating on the lower hull. (Glass flake coating could be a suitable coating to be applied at refit).
3
Alarm system on CP cut out with easy access reset switch to ensure minimum down time if overloaded.
4
Use of zinc reference cells when at or near the Ore Port.
5
Rigorous cleaning and washing programme on deck after loading and unloading to remove ore dust.
6
Improved coatings on deck fittings.
Deck
Cargo Holds 7.
Ensure good maintenance and safety procedures keep the cargo hatches operational, watertight and closed during voyages.
Considerations on the Storage Ship permanently moored at sea. This ship is always stationary. It has been subject to the same deck corrosion problems as the Carrier but the hull corrosion and CP performance is quite different. The ships hull is well protected.
Reasons for good CP Protection on the Stage Ship. 1
No river movement to cause excessive coating damage.
2
Stationary ships are protected by lower voltages than moving ships and therefore draw far less current. The requirement is easily supplied by the TRs.
Page 60
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 61
Module K
Marine Corrision & Coatings
3
Polarisation effect. This is the build up of chemical species, hydrogen gas, chalk scale or even marine growths that occur on stationary cathodically protected objects that block out bare metal areas and reduce the current demand.
In the above case a high level of polarisation occurred, reducing current demand to a very low level and providing full protection at very low amperage output.
BULK CARRIER
IRON ORE CARRIER, EXTENSIVE PAINT LOSS ON BOTTOM DUE TO FREQUENT GROUNDING REDUCED EFFECTIVENESS OF CP PROTECTION.
IRON ORE DOCKS
Page 61
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:12
Page 62
Marine Corrision & Coatings
DECK PIPING PERFORATED DUE TO EXTERIOR IRON ORE CORROSION
HEAVY DECK PIPE PITTING DUE TO IRON ORE CORROSION
Page 62
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 63
Module K
Marine Corrision & Coatings
LOADING THE IRON ORE
Page 63
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 64
Marine Corrision & Coatings
12
METHODS OF CORROSION CONTROL - COATINGS
12.0
Minimising Corrosion Effects
Although corrosion cannot be totally prevented, it can be limited. By attempting to control some of the major factors that influence corrosion, we can slow the rate at which it occurs. Relative Humidity. If the relative humidity can be kept below approximately 50%, the corrosion rate will be reduced to insignificant levels. The use of de-humidification equipment is particularly useful in an enclosed environment such as the hold of a ship undergoing maintenance, or simply laid up. Material Choice The design life of a structure can be significantly varied by the choice of construction materials. Corrosion potential of materials is only one factor in their choice, alongside strength, weight, cost etc. It is desirable, however, that sufficient consideration is given to the possible rates of corrosion. Construction materials may be corrosion resistant alloys at relatively little extra cost. Consistency and homogeneity within a metal are important, and different metals (even different ‘similar’ grades of steel) should not be used together without consideration of the corrosion possibilities. Coating Corrosion will occur when the metal substrate comes into contact with the environment. A coating applied to the metal protects that substrate in the following ways: Barrier Prevents or restricts contact between the environment and the substrate. Inhibitive Releases substances, (inhibitors) which repel attack from the environment. Sacrificial Produces an electric current that protects the substrate (in a similar manner as a sacrificial anode). Cathodic Protection. ‘Zinc-Rich’ paint coatings are primers that operate on the principle of cathodic protection, protecting the substrate metal by sacrificial corrosion.
12.1
Corrosion Prevention using Protective Coatings
12.2
What are Coatings?
Paint is a fluid suspension, which is spread in thin coats to decorate and/or protect a surface. It consists of pigment, or colouring matter, and the binder in which the pigment is suspended. Usually, the binder is diluted by the addition of a solvent and these two together become the vehicle that carries the pigment to a surface. It is the function of the vehicle to change its nature (i.e. solidify) and bind the pigment to the chosen surface. Binder The binder consists of oils or resins, or a blend of oils and resins. Its essential function is film formation, the ability to change from a liquid film that flows to a more or less hard plastic film. The choice of binder plays the major part in deciding the properties of the paint film. Among the most important properties of the paint film are the following:•
Adhesion to the surface which is painted (usually called the ‘substrate’).
•
Gloss: different binders show a different degree of ‘glossiness’.
Page 64
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 65
Marine Corrision & Coatings
•
Module K
Mechanical Properties: these include such things as hardness, flexibility, resistance to abrasion, impact, or expansion and contraction due to temperature changes. It is important to note here that the mechanical properties of a paint film are much influenced by the nature of the substrate, and that a coating with ideal characteristics for a steel substrate may perform very poorly on, say, a wooden substrate.
Pigments Pigments consist of small crystalline particles that are insoluble in the solvents that may be used. They are added to a paint film for many reasons, among which may be:• • • • • • •
To provide colour and to hide the surface. To protect the film and the surface from the effects of Ultra-violet light and weather. To decrease the permeability of the film. To provide rust-inhibiting properties. To add body, i.e. to thicken the paint so that higher film can be achieved. To decrease gloss. To aid storage properties.
Many pigments are metallic salts (e.g. Iron Oxide, Titanium Dioxide, Zinc Chromate), which were initially chosen by artists and decorators for their colouring ability. Pigments that do not have this colouring ability are usually called ‘extenders’, and are added to paints because of their ability to add thickness and solids to paint films at relatively low cost. Some have other properties; for example, mica is a transparent material that breaks up into flat plate-like particles that will lie flat in a paint film and decrease the permeability of the film. Other examples of extenders are chalk, china clay and talc. Types of Paint Paint consists of pigment distributed in a binder. Most types of pigment are compatible with most types of binder, but it is usually from the binder that we take the name used to describe a type of paint. Hence we may recognise the following types of paint:• • • • • • • • • • •
Acrylic Phenolic Alkyd Polyester Chlorinated Rubber (currently being phased out) Polyurethane Coal Tar Silicone Epoxy Inorganic Silicate Vinyl
Although it is the binder that has the greatest influence on the ultimate characteristics of a paint film, it is important to realise that the pigments also play their part. For example, a pigment such as Micaceous Iron Oxide has a very distinctive and characteristic effect upon a paint film to which it is added, and for this reason, it is usually named in conjunction with the film binder (e.g. paints known as Chlorinated Rubber M.I.O. or Epoxy M.I.O.). Barrier Coatings Industrial coatings are mainly employed to protect a surface rather than to decorate. The major function of a protective coating is to form a barrier between the surface being protected and the environment. These coatings, therefore, are designed to prevent the passage of water and oxygen, together with other specific contaminants. In this way, the surface is protected from the corrosion influence, and maintained in good condition.
Page 65
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 66
Module K
Marine Corrision & Coatings
Unfortunately, no paint material is able to completely exclude water or air. As a result, even the best paints fail to protect steel by eventually failing to prevent the passage of these corrosive agents. In practise, it is necessary to ensure that the barrier coatings are firmly adhered to the metal surface, and that the coatings are as impermeable as possible. This can be achieved by:• • •
Selection of pigments Increasing thickness of coating Ensuring that coating does not have physical flaws (e.g. pinholes, voids).
Inhibitive Coatings. If corrosion enhanced conditions exist beneath a barrier coating of the type described above, the general tendency will be for corrosion to take place at the steel surface, travelling sideways along the surface. In this process, the barrier coatings become progressively disbonded from the steel surface, despite the apparent integrity of the coating. For this reason, it is common to include, as part of a multi-coat system, a coating which will ‘inhibit’ the corrosion process. These coatings are commonly known as primers, since they are mainly effective when used as the first or priming coat against the steel surface. Inhibitive primers, in addition to having good adhesion and good resistance to the passage of corrosive agents such as water, will contain rust inhibitive chemicals. Temporary Coatings Many coatings are employed to give temporary or short-term protection to a structure. Amongst these are:•
Pre-fabrication primers, designed to allow large quantities of steel to be cleaned and coated by machine prior to erection of the structure concerned.
•
Rust converters, which transform rust into a more chemically inert material on a partially corroded surface, and are able to protect the surface over a limited period of time.
•
Wax based coatings, which are often used in relatively enclosed and static corrosion situations, such as ship’s tanks, but require frequent renewal if they are to remain effective.
•
Anti-fouling paints, which have a limited life by design and need to be renewed at regular intervals.
Solvents Solvents are volatile organic liquids used in paints to reduce the viscosity or consistency of the material and so facilitate the application for the oil or resin present. After application, the solvent is no longer required and should evaporate completely from the film. Solvents are readily organised into chemical groupings, and some of the more common solvents are listed below as part of their relevant chemical grouping.
•
Hydrocarbons: Include Turpentine, White Spirit, Benzene, Toluene and Xylene
•
Alcohols: Include Methyl Alcohol, Ethyl Alcohol and Isopropyl Alcohol
•
Ketones: Include Acetone, Methyl Ethyl Ketone (M.E.K.)
•
Esters: Include Methyl Acetate, Amyl Acetate
Page 66
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:12
Page 67
Marine Corrision & Coatings
Module K
Other Important Features of the Solvent. •
The evaporation rate affects the process of application, the film formation and fluidity of the paint. E.g. Cellulose enamel, if sprayed with Acetone as the solvent, can cool the surface so rapidly that the dew point is reached and condensation on the surface causes ‘blushing’. Too slow evaporation can cause running or sags. Too fast evaporation can cause dry spray.
•
The Flash Point - is the temperature at which a liquid produces a combustible vapour. Solvents whose Flash Point is below 21?C are ‘highly inflammable’. The risk of fire is usually small, but naked lights and sparks should be avoided when a solvent of low Flash Point is being used.
•
Most solvent vapours are toxic, but if sufficient ventilation is maintained then it should not constitute a health hazard. When extremely toxic vapours are present, then masks and external air supplies should be utilised. Solvents can also cause Dermatitis and skin rashes. Gloves should be worn when dealing with most solvents.
•
The cost of most solvents is high as they are derived from the petroleum industry.
Paint Curing For the great majority of paints, one or more of the following processes are involved in the drying of the applied film:•
Evaporation of a solvent from the vehicle, leaving behind a film of solid material.
•
Conversion of constituents of the vehicle (e.g. linseed oil) to the solid state by chemical changes involving mainly oxidation by atmospheric oxygen. Paints often contain additives known as driers to hasten these changes.
•
Polymerisation, or reaction between components of the vehicle, brought about by putting a curing agent in the paint, or by the application of heat.
Very few paints contain no solvent at all, and many will therefore combine solvent evaporation with the chemical change through oxidation or polymerisation. Hence it becomes apparent that the timing of the drying process is critical, and this must be an important consideration during the formulation of the paint. Some chemically cured resins are capable of use as coatings material, without the addition of solvent. In general, these will be used either in their natural paste form, applied by hand, or applied in the usual way after heating the components to reduce viscosity. One of the major values of solvent free materials is their ability to cure in situations where solvent would not evaporate readily, e.g. underwater. These materials are capable of application at very high film thickness. They do not change their volume on curing and cannot suffer from solvent entrapment. Chemically cured resins can also be applied to a surface in the form of a powder. The powders that are in general use have been developed in such a form that polymerization reaction (chemical cure) is triggered by heating.
Page 67
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:12
Page 68
Marine Corrision & Coatings
Simplified Schematic of the Polymerisation Process
Stage 1 - Material unmixed and as delivered in can. Individual chemical constituents are un-bonded or unlinked.
Stage 2 - Cure agent is added and mixed to base material. Cross- linkage or chemical bonding starts to take place.
Page 68
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 69
Marine Corrision & Coatings
Module K
Stage 3 - Completion of the chemical bonding that takes place in Polymerisation. A highly chemical resistant matrix is formed.
Polymerisation is affected by the following factors. 1)
Temperature - The higher the ambient temperature the faster or more rapid the polymerisation process. Lower ambient temperatures retard the process. Always check with the manufacturer or material data sheet mixing temperatures.
2)
Potlife - refers to the time and temperature that the materials are at a workable consistency.
3)
Cureing Agent addition - Always refer to the data sheet regarding mixing ratios.
4)
Induction or Sweat in time - Some products require a short time after mixing to start the chemical reaction process - usually this is stated in the manufacturers product data sheets.
Coating Systems There are many types of coating materials and many variations within a single ‘type’. Each coating is formulated for a specific purpose, and it has been discovered that the best coating system for long-term protection is a multi-coat system in which the individual coatings are selected for their ability to fulfil a certain ‘role’.
Page 69
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 70
Marine Corrision & Coatings
Some of these roles are: Priming Paints, which secure good ‘wetting’ and adhesion for the entire system, form a suitable ground for, and to hold, the remainder of the system, and play a special role in controlling the corrosion of steel. Undercoats, which form a suitable ground for, and hold, the remainder of the system, and act as barrier to corrosive elements. Finishing Paints, which adhere to and must be compatible with preceding coats, protect the preceding coats against the effects of the environment (i.e. sunlight, atmospheric pollution), and give the system required physical and chemical properties, such as abrasion resistance, water impermeability, chemical resistance, colour, and gloss. The complete system usually comprises priming, undercoating and finishing paints, although, to an increased extent, undercoating and finishing paints are interchangeable and even priming paints are omitted; but, where this happens, care must be taken to ensure that the paint performing the dual function is really able to do so satisfactorily.
Compatibility of Coatings Many paint failures are due to the incompatibility of the individual costs, each of which may be good of its kind, but unsuitable for use with the remaining coats in the system. This makes a powerful argument for making sure that any coating system is regarded as a unit and that consideration is given to any possible interaction between the coating materials used within the system. Problems associated with compatibility (or lack of it) are generally derived from the solvents that are used in a coating. In addition to fulfilling its prime function as a solvent, the volatile content of a coating must be strong enough to soften the underlying coating without being so strong as to destroy it. Some coatings have little or no problems in this direction, others are well known for their adhesion faults, or generally poor compatibility. One way of minimising compatibility problems is, wherever possible, to adapt a coatings system of a single manufacturer’s products, in line with recommendations made by the manufacturer. This course of action has the additional benefit of allowing only one source of redress should the system fail due to a coating material defect. Where different coating types are used as part of a coating system (e.g. Epoxy coatings over a zinc silicate primer) great attention must be given to the potential problems of incompatibility.
Page 70
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 71
Marine Corrision & Coatings
13
Module K
CHOOSING A PAINT SYSTEM
Exposure Testing of Paint Films To account for structural variables, service tests of organic coatings are sometimes conducted on actual structures in service. More often, test panels are prepared, since greater control of surface preparation and exposure conditions can be exercised. Critical parts of the test programme are the preparation of test specimens, the selection of the exposure conditions, and the selection of significant coating properties to be evaluated as a measure of deterioration with time.
Performance Expectation of Coatings Practical coatings are a compromise between the maximum protection that can be extracted from a system and how much money is available to pay for protection. As the effective life of a coating system increases, its cost usually increases also. How long the coating will last depends upon:•
Which coating is chosen - different situations require different coatings, so selection is important.
•
The correct surface preparation chosen and achieved.
•
Method and standard of coating application.
•
Inspection is carried out at all times.
When long term performance is desired, it is often more economical to choose a high initial cost coating rather than a complex maintenance programme which uses an initially cheaper coating. Since the coatings industry is continually developing new and ‘improved’ coatings, it is necessary to decide whether the new coatings are suitable for use. Performance testing may be by:• •
Natural exposure Accelerated weathering tests
In either case, it is important to realise that accurate prediction of coatings performance is not generally possible.
Coating Systems and their Selection Environmental Considerations The ultimate performance of a properly applied coating system will depend to a large extent on the environment to which it is exposed. Five environmental conditions are generally recognised:Rural: Industrial: Chemical: Coastal: Marine:
Open country - remote from industrial areas or from sea Areas where industry is concentrated Areas within or immediately surrounding chemical or heavy engineering works Fringe areas near coast Areas where air carries sea-water spray, usually restricted to the vicinity of high water mark
Of these environments, it is sometimes very obvious which applies. For example, an oil production platform will undoubtedly encounter a marine environment, although certain parts will also encounter chemical pollution.
Page 71
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Module K
Page 72
Marine Corrision & Coatings
Long Term Performance The long-term performance capability of a coating system is often not thoroughly tested. New and relatively untried materials are produced at regular intervals, and few coating systems have become accepted through many years of trials. Unfortunately, fashion in the paint industry often dictates the selection of a coating system. There is no doubt, however, that certain types of coating material offer the potential of a substantially longer life as a protection against corrosion. It is well known that flame-sprayed metal coatings or zinc-galvanised coatings offer good long-term protection. Used in conjunction with a follow-up coating system, extremely long life can be achieved. Set against this fact must be the difficulty, often, of applying metallic coatings, and the relative difficulty of proper maintenance. Alternatively, a coating system may be chosen for its ease of application, with long term performance as a secondary consideration. In this case, it should be recognised that low initial costs may well be offset against more regular requirements for maintenance (and therefore costs) in the future. Each coating system, properly applied has a maximum potential life span, during which time it will give adequate protection against corrosion. The many factors which affect the life span of a coating system may help either to achieve that maximum or may contribute to the premature failure of the system. Selection of a coatings system must consider:• • • •
Design life of a structure Ease of maintenance Quality of initial application Economics of coating operations
Factors Affecting Coating Life The life expectancy of a properly applied coating system will depend upon:• •
The type and quality of the coatings The environment to which the coatings are exposed
Choosing a Coating System. Many attempts have been made to compare the various coating systems that are available. One of such comparisons of three coating types is made here:-
Page 72
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 73
Module K
Marine Corrision & Coatings
Alkyd
Chlorinated Rubber
Epoxy
Plus Features
Minus Features
Excellent Exterior durability Good Flexibility Good Re-coatability Excellent adhesion to most surfacesEase of Application
Poor Chemical and Solvent Resistance Fair Water Resistance Poor Heat Resistance
Fast Drying Good Chemical resistance Good Water Resistance Good Exterior durability
Limited Heat resistance Poor solvent resistance Blast-cleaned surface required
Excellent chemical and solvent resistance Excellent adhesion Good high-build capabilities Good long-term performance
Poor gloss in exterior exposure Difficult to over-coat Mistakes possible in mixing and application
Note - Chlorinated Rubber coatings are now being phased out and replaced with water based or Acrylic binder type materials. Thes materials are VOC (Volatile Organic Compound) compliant. It can quickly be seen, however, that a listing of this type can only be of limited value. Very few paint manufacturers will wish to list the minus features of their product. There can be a great many different ways of evaluating a particular product, and a certain quality may be either an advantage or a disadvantage, depending on the requirements of the engineer who is making the decision! However, once basic decisions relating to the above factors have been made, it is the quality of the application process that is important. It has been estimated that the quality of a coatings application process depends upon the following factors and in noted percentages of importance:Surface Preparation: Coatings Application: Quality of Coatings:
55% 25% 20%
* Indicates that at the most recent inspections, the painting scheme had not failed on all the surfaces concerned. Naturally, it is also important that high quality is sought from both the application procedures and the coating materials. Simple though it may sound, the rules of good painting practice are often ignored. One major corrosion report published in the U.K. suggested that the cost of corrosion to the nation (many millions of £ per year) could be reduced by up to 20% simply by ensuring that painting work was supervised with the aim of achieving a quality standard of workmanship. How then, does an engineer select a coating system? Experience The experience of an engineer, or his department, or his company, will often give a good indication of whether a coating system is suitable for the engineer’s purposes. This naturally supposes that similar coating work has been undertaken before, and that accurate and detailed analysis of coating performance has been made. Often, an engineer will gather information from his colleagues in a different company who may have experience that he does not have.
Page 73
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:13
Page 74
Marine Corrision & Coatings
Research Surprising results can often be achieved from apparently familiar coatings, if a detailed approach is made to the evaluation of performance. A systematic appraisal of premature failures will often indicate how similar failure can be avoided. In this way, consistent results can be achieved and the engineer becomes familiar with specific coatings and learns to achieve long term protection with those coatings. Expert Advice Advice is available from many sources, including the paint manufacturing companies. In the main, the advice will be good, provided that the advisor takes the time to assess both the requirements of the engineer and the reality of the situation. Local knowledge and experience is useful. It is wise for the engineer to evaluate advice and to ask questions. Coatings on Zinc Surfaces When coatings are applied to non-ferrous substrates, such as sheet zinc and zinc-galvanised steel, special precautions are necessary. When new, the surfaces should be degreased before painting, to ensure removal of foundry acids and adequate adhesion of the paint film. Adhesion would be improved if the surface were modified by chemical treatment or by the action of the weather, or use of a recommended priming paints. Zinc may react with many types of conventional paint to form water sensitive, brittle compounds at the paint/metal interface, and the various forms of pretreatment prevent these interactions taking place. In some cases, calcium plumbate primer may be used without surface treatments. The most satisfactory pretreatment for zinc is by using, in the factory, specially formulated phosphating and/or chromating solutions, followed without delay, by a suitable priming coat. Further coats can be applied onsite after cleaning down the primer and touching up any damaged parts. If this is not possible, then the following methods of treatment can be adopted: •
By the use of wash or etch primers.
•
By treatment with an etching solution, after which the surface should be rinsed with clean water to remove acids and salts.
•
If the zinc or galvanised steel is allowed to weather up to three months before painting it creates a roughened surface, through corrosion, which is suitable to receive paint without having to degrease or etch, although dirt and loose corrosion products do have to be removed.
•
The surface can be abraded with emery cloth (lubricated with a suitable grease solvent) to produce a uniform, fine, matt surface, and subsequently wash with clean solvent. This process is only suitable for small areas as it is rather slow.
•
Light blast cleaning is possible, and produces a good key for coating. Some engineers allow application of a regular coating system, others like to use an etch primer prior to application of the remainder of the system.
Coatings on Aluminium Surfaces If the aluminium has a smooth polished surface, it does not provide a good key for paint, and special treatment is essential to secure good and permanent adhesion. Etching primers are essential. Factory pretreatment and priming is preferable, involving chromic, sulphuric or phosphoric solutions initially, followed rapidly by priming and possibly one coat of undercoat. Anodizing is an electrolytic method that results in the formation of a dense film of oxide on the surface of the metal. Although an anodized surface is an excellent key for paint, it is seldom used for this purpose owing to its cost. Anodizing is more often used for aluminium surfaces that are to be protected by clear finishes.
Page 74
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 75
Marine Corrision & Coatings
Module K
As with Zinc surfaces, light blast cleaning may be used to provide an anchor profile for subsequent coatings, although the use of an etch primer may still be beneficial to adhesion of the coating system. Specialist Coatings for Special Purposes There are a great number of coatings that have been developed for particular special purposes. Some of these coatings are discussed below. Thermoplastic Coatings The use of Plastics for corrosion resistant coatings is a relatively recent innovation. It allows the combination of the mechanical strength of metal and the corrosion resistance of plastic. Coatings are normally applied by dipping, spraying or by extrusion. Heavy Duty Bituminous Coatings for Pipeline The types of coating traditionally used are based on coal tar or asphalt bitumen enamels. To aid bonding to the pipe surface, a thin layer of primer coat is applied. This may be a solution of the coal tar or bitumen enamel, or based on chlorinated rubber. The enamel is applied to the pipe as a flood coating, with simultaneous application of a tape wrap, normally of glass fibre. This wrap sinks into the enamel, and should be neither in contact with the primer nor exposed to the air. A second tape is also applied at the same time, again normally of glass fibre, probably impregnated with enamel. This overlay is intended to function as a protective shield, resting more or less on the surface of the enamel coating, although bonded to it. Recent developments have seen the production of cold-applied, self-adhesive wrapping tapes. Based on PVC or polyethylene backing, with a rubber-based adhesive, they are applied over a primer to give thicknesses in excess of 500 microns.
The tapes are normally overlapped by 50% or more to give a double thickness. The same is true generally of the reinforcing tapes mentioned above. Cold-applied wrapping tapes may be applied over a wire-brushed surface (coated with primer) and are used primarily for protection of field-welded joints and for repair purposes where speed of repair is important. Fire Retardant Coatings For domestic purposes, a fire-retardant coating may be a coating that resists burning or the passage of flame for a certain period of time (normally 30 minutes or an hour). In the oil industry, the fire retardant coating has the same requirement, but would need to resist the higher temperatures of an oil/gas fire. Intumescent paint coatings provide protection by foaming under the effect of heat to form an insulating layer. In practise they are handled and look like an ordinary paint coating. Insulation coatings provide better long-term fire protection than paint coatings. This type of material is generally a concrete reinforced with an insulator (e.g. vermiculite). Spattered onto a surface, it provides a thick coating (up to 4 inches) held in place by riveted studs and re- inforcing wire. It is a difficult coating to clean, and has a tendency to crack, when its fire retardant properties are reduced. All fire retardant coatings are required to be tested and issued with a fire certificate showing a satisfactory performance. Anti-Fouling The growth of marine organisms (or fouling) is a hazard or nuisance on all underwater structures. Of particular interest are:•
Fouling on ships bottoms may significantly reduce the speed of the vessel through the water (or increase fuel usage).
Page 75
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 76
Marine Corrision & Coatings
•
Fouling on an offshore structure may severely affect the design calculations related to loading and wave action, etc.
In general, anti-fouling coatings are more commonly used on ships. These coatings are designed to resist marine growth by releasing a poisonous material (pigment) into the surrounding water at a controlled rate. This means, of course, that the coating will have a limited life and will need to be replaced at regular intervals. This service interval may be as little as 15 or 18 months in some climates, and requires expensive dry-docking for re-coating. A recent development is the production of ‘self- polishing’ coatings which gradually dissolve away whilst maintaining a smooth surface on the ships bottom. Non-Skid Deck Coatings On surfaces which are commonly used by personnel, and which may become slippery due to contamination by oil, water, etc., the use of non-slip coatings is required. In addition to their normal anticorrosion properties, suitable coatings are impregnated with sand or other sharp grit. This is only normally done with the topcoat of a multi-coat system. Typical applications are the decks of ships and oil production/drilling platforms. In the case of tankers or oil/gas platforms, it is important that the non-slip additive is non-sparking. Insulation Materials Insulating materials are often applied to pipes and vessels for one of two reasons:•
To maintain the temperature of the pipe/vessel when it is significantly different from the environment, e.g. to keep oil flowing in an Arctic pipeline.
•
To reduce noise levels.
In both cases, the cause of corrosion concern lies with the potential problems caused by a microenvironment within the insulated space. Insulation materials are either expanded foam or pre-formed sections of material, typically glass-fibre or mineral wools. Both have a low value for thermal transmission, and would normally be contained by a metallic (aluminium or stainless steel) sub-structure. The likely cause of corrosion is a wet or condensing environment inside the insulation barrier. It is important, therefore, that the spaces are sealed, and that attention is given to the vapour-seal as part of maintenance procedure. Glass-Fibre Reinforced Products When a coating is required to have considerable structural strength in its own right, such as when repairing a weakened steel structure, it is common to reinforce the coating material by incorporating woven glass-fibre matting. Suitable materials for this reinforcing technique are generally polymers such as epoxy and polyester resins. Coatings for Application on Wet Surfaces. The increased use of coatings for corrosion protection below sea-water level at maintenance, and the development of various techniques of blasting with water have led to the production of coatings suitable for application on wet surfaces. These are generally of two types:•
Moisture Tolerant - Coatings which can be applied under conditions where the steel surface is wet, but not dripping with water, and water condensation may occur. This does not include seawater.
•
Typical of these are coatings that will displace a certain quantity of water, but also absorb a proportion of water that is incorporated into the curing mechanism.
Page 76
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 77
Marine Corrision & Coatings
•
Module K
Water Tolerant - Coatings which can be applied when the steel is dripping with water, underwater, or alternately wetted. The water source may be condensation, fresh water or seawater.
Typical of these are two-pack epoxy coatings of 100% solids, whose cure is determined totally by their content at mixing, and virtually impervious to outside influence. Both types of coating need to have superior wetting properties in order to ensure displacement of the water from the steel surface. Thermal Sprayed Metal Coatings Many metallic or ceramic coatings can be sprayed onto steel, either to provide corrosion protection, to modify the surface characteristics, or to ‘rebuild’ a surface that has worn away. The most common corrosion resistant coatings applied by metallizing are aluminium or zinc; typically, zinc is chosen for atmospheric service, and aluminium for water immersion service. The process of metallizing projects a stream of melted metal (aluminium or zinc) onto the prepared surface that is to be coated. The conventional method of surface preparation is grit blasting, and a high standard - usually white metal - is required. The aluminium or zinc may be may be supplied as a wire or as a powder, and can be melted in a gas flame (wire), by creating an electric arc (two wires) or in a plasma flame (powder). Each of these methods creates different characteristics in the coating, and is generally chosen for convenience and/or portability when used to apply corrosion resistant coatings. The coatings created are likely to have a rough surface and may be somewhat porous, and should be ‘sealed’ using a penetrative organic coating to achieve maximum performance from the coating. The coatings are often selected and used as the ‘primer’ coat of a coating system, although aluminium metallizing has been used by some companies for its ‘cathodic capability when immersed in seawater. Extremely good performance has been reported using this kind of metal coating for corrosion prevention.
Page 77
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Module K
Page 78
Marine Corrision & Coatings
14
COATING SPECIFICATIONS
It is essential for the coating chosen for a particular purpose to be applied correctly, and in order to ensure this, a coating specification is used, which is usually part of the set of contract documents. Essential Considerations. What is a Specification? A specification is the means of communicating requirements regarding the quality of materials and standards of workmanship necessary to provide a good protection to ferrous construction materials and components against deterioration by corrosion. Why do we need a Specification? In order to ensure the potential life of the protective system, we must:•
Choose the correct system.
•
Make sure the materials used in the system can be supplied when required and with the properties attributed to them when making the choice.
•
Apply the materials in the correct conditions and with the required standard of workmanship.
•
Handle, store and transport the materials in such a way that their properties are unaffected.
•
Ensure that any erection procedures do not damage any coatings in such a way that the coatings cannot be repaired to the required standard.
Because every project is different, with many factors at work, the specification should always be included in the contract documents. Specification for Maintenance Coatings A complete set of schedules should be prepared if maintenance coatings are part of the specification. This is necessary as there may be fluctuation in conditions compared with the original first coatings application. Definition of Responsibilities A large number of people may be involved with the coating specification, each with their own responsibilities; some examples might be:Client/Engineer
Preparation of Specification
Contractor/Sub-Contractor
Carrying out the schedules correctly
Paint Inspector
Ensures that all the coating specifications are adhered to; maintenance of records etc.
Safety Officer
Control over dust, fumes, disposal of waste etc.
As well as these, there are a large number of other processes occurring on site and these all have to be taken into consideration for a successful operation.
Paint Guarantees Many clients ask for guarantees that the protective coating system will successfully protect the steel for a specified period. There are various ways of setting up guarantees. •
Ask for successful performance over a long period of time, e.g. 5 years. This is often combined with some method of limiting contractors’ liability. i.e. It may be stated that in the event of coating failure, the guarantor would be liable for the:-
Page 78
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 79
Marine Corrision & Coatings
Module K
Full cost of repair after 1 year 90% cost of repair after 2 years 65% cost of repair after 3 years •
An alternative form is to ask for a 100% perfect job after 1 year, with the guarantor responsible for repair should repair prove necessary at the 1 year inspection period.
Both of these methods and many others suffer from the same problems.: •
In the event of coating failure, it is difficult to determine liability as contributory factors to failure may include:• Defective paint materials • Weather conditions at application • Poor application techniques • Mishandling of materials - poor storage conditions • Neglect of good working practises • In-service conditions to which the coatings were subjected
•
In order to institute the guarantee, it is necessary to establish the reasons for coating failure
Paint Inspection In order to ensure that a coating project meets the specification, it is essential to use a form of quality control. However, painting and metal coating differ from many other industrial processes in that they are susceptible to operator abuse or adverse environmental influences throughout all stages of the work. Furthermore, it is generally difficult to deduce from examination of the completed work what has occurred. Consequently, the coating may fail prematurely, but more often the effect is a reduction in longterm durability. Inspection is not a substitute for adequate supervision and proper specification. Its primary purpose is to check that the coating specification is being properly followed, and where, for any reason, this is not so, the inspection should lead to the instigation of immediate remedial action. The specification should state the type and degree of inspection that is to be carried out so that the contractor is fully aware of its implications before the tendering stage. The duties of a Painting Inspector are difficult to define; he should, however, be in no doubt as to what those duties are. Duties may include: • • • • • • •
Understand specifications Know his authority Enforce specification Check materials Maintain required records Make sure all items are completed Have all test instruments (in good working order) and standards required
Page 79
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Module K
Page 80
Marine Corrision & Coatings
15
PRACTICAL PAINTING CONSIDERATIONS
Painting Before Fabrication When steel is stored for any length of time prior to fabrication, it should be protected against corrosion. A typical example of this situation is found in the shipbuilding industry where ship’s plate is bought in large quantities, and delivered to the shipyard for storage prior to use. Unless corrosion is prevented, long periods of storage may lead to deterioration of the plate, perhaps to the point where it steel structure, such as an oil/gas platform. It has become common practise to prepare the surface of such steel and apply a protective coating as it is received at the place of storage, and prior to the storage period. In particular, fixed shot-blasting machines (often known as ‘wheelabrators’) may be used, followed by the application of a ‘pre-fabrication’ primer, or even a substantial part of the final coating system. Any coatings that are used should be: (a) Tough, to withstand handling during the fabrication stage (b) Capable of being overcoated after fabrication (c) Nontoxic when welded, if welding is to be used as the method of fabrication It is also useful if the coatings are rapid drying, in order to permit handling fairly soon after the coatings are applied. The two main advantages of coating prior to fabrication are: 1.
Steel is protected during storage
2.
Surface preparation and coating methods are easier and faster (therefore more economical) whilst the steel is in relatively small and simple units
The main disadvantages of coating prior to fabrication are: •
Coatings may be damaged during fabrication
•
Deterioration of coatings and/or surface contamination acquired during storage or fabrication may mean that the coatings have to be fully or partially removed before subsequent painting operations
Page 80
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 81
Marine Corrision & Coatings
Module K
Storage and Handling Steel to be stored on site in a partially painted condition, should be dealt with so as to avoid deterioration in handling, transport and storage by employing the following rules: •
Allow adequate time for full hardening of coating.
•
Use nylon slings and ropes for lifting (not metal)
•
Plan to reduce movements to a minimum.
•
Use special supports and lashings on vehicles.
•
Allow good ventilation of components when stacked.
•
Inspect at monthly intervals and deal with any deterioration as it arises.
•
Maintain good discipline during erection to avoid damage and contamination.
Painting at Site In a situation where structural steel arrives at site in a partially painted condition, consideration should be given to several factors: •
Is the coating clean and in good condition?
•
Is there any handling damage?
•
Is the steel stored properly prior to fabrication.
The same consideration apply to the steel after fabrication has taken place, but prior to full painting. It is essential also at this stage to remove all traces of weld flux and spatter, and to effectively clean the weld areas. Coatings used for this twostage type of coating process should necessarily have an indefinite overcoating period. Access (via scaffolding/cradles etc) A major coating consideration, particularly at the maintenance stage, is the provision of access for the surface preparation and coating application procedures. This can be a major cost factor in any coating project, and can also be a major hindrance if the design of the access is poor. A particular problem arises with the preparation and painting of the scaffold support points, which may be numerous. Another problem may be caused by mechanical damage whilst the scaffold is removed. If cradles are used for painting suitable areas, problems may be generated by difficulties with coatings overlapping (or not overlapping) or by lack of access to some areas. The safety of all access points is also a major consideration. For the first time in 1982 in the North Sea, full time scaffolding Inspectors were employed on major coatings projects.
Page 81
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 82
Marine Corrision & Coatings
16 SURFACE PREPARATION - ALTERNATIVE METHODS Steel Surface Defects It is not widely appreciated that ‘surface preparation’ does not just mean the removal of all millscale, rust and contaminants, but suitable ‘dressing’ of the steelwork to remove all surface defects that could break through the paint film or prove difficult to protect adequately by painting. It should be remembered that where defects are exposed by blastcleaning and subsequently removed by grinding, it is necessary to reprepare the immediate area in order to retain the surface profile. Surface Laminations and Shelling The commonest type of surface defect on steelwork is surface laminations, generally caused by the rolling process. It is important that all such defects are removed by grinding as no painting system, however, thick, can effectively protect them. In the case of small shelling and surface laminations, even if these do not project above the surface, they may later curl upwards and penetrate the painting system. Cracks and Crevices Any form of crack or deep crevice will form a danger to the protective treatment, as it cannot be effectively filled by the painting system. It will contain impurities, and gather entrapped moisture and air, then form a galvanic cell leading to a painting system failure. All such cracks and crevices should be ground out, unless too deep for such treatment, in which case, they should be filled by welding and then ground smooth. Inclusions All forms of surface inclusions, such as rolled in millscale should be removed by chipping and the surface ground (with weld filling if necessary). The Painting Specification To obviate any possible dispute as to whether surface defects should be removed, it is strongly recommended that a clause be included in the painting specification covering the dressing of surface defects. Manual Cleaning Manual cleaning is the slowest and least satisfactory method of surface preparation. Normal tools used are wire brushes or scrapers or chipping hammers. The process is slow, laborious and costly, with the end result still far from satisfactory. It is impossible to remove all rust and millscale by this method. A further complication these days can be the reluctance of labour to engage in arduous manual tasks. Manual cleaning should only be used when weather or some other factor precludes the use of any other process. Power Tool Cleaning The principal tools used are the grinder and the rotary wire brush. Whilst quicker than hand tools, the work is very laborious and expensive. The biggest drawback to the use of power tools is the burnishing effect that arises on the metal. This polishing effect, inherent with power tools, is the most unsatisfactory feature of this method. A polished surface seriously affects paint adhesion and should be avoided at all costs. In addition, no amount of polishing will remove the rust from the bottom of pits etc. A more sophisticated type of power tool is the needle gun. This tool consists of a number of hardened steel rods that are vibrated against the surface. It is slow in operation and suffers from the same defect as other power tools as it has a burnishing effect when producing a relatively clean surface.
Page 82
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 83
Marine Corrision & Coatings
Module K
Blast Cleaning: Equipment and Materials Abrasives The degree of surface roughness and the rate of cleaning depend primarily on the characteristics of the abrasive grit used. Although the blasting abrasives in general use range widely from crushed walnut shells, glass and crushed slag, to various metallic shots and grits, and even ceramic grits, there are only three main types of grit that find general acceptance for the gritblast preparation for painting. These are: • • •
Chilled Iron Grit Crushed Slag Ceramic Grits
Attention is particularly directed to the fact that, despite the widespread use of the term ‘sand blasting’, sand is not listed as a grit blasting abrasive. Sharp sand or flint is indeed a cheap and highly effective abrasive, but increasingly cannot be used (nor any abrasive containing free silica) on factory premises throughout the world, due to the very real danger of Silicosis. Permission to use sand is very occasionally given for ‘site work’ in the open air, but only when the operators and other personnel are carefully protected from the dust created and the Local Factory Inspector approves the site and blasting conditions. Chilled Iron Grit This is by far the most widely used abrasive for surface preparation in a coatings application facility. Chilled iron grit is available in a variety of grades and to a specific minimum hardness. It is an excellent general purpose abrasive, due to its relatively high density which gives high particle energy, and its slow but effective rate of breakdown which maintains sharp cutting edges on the grit particles. Crushed Slag While chilled iron grit is used extensively for grit blast preparation in works or on site where grit reclamation and recirculation can be practised, it is too expensive an abrasive to be used where grit reclamation is not possible, as on many site jobs. With the nonavailability of sand, certain crushed slag from metallurgical processes have been made available as relatively cheap expendable abrasives. Copper slag and Aluminium slag are common. While quite effective grits for 'once only' use, by reason of their rapid breakdown to dust, they are not generally suitable for grit reclamation and reuse. Ceramic Grits. (Aluminous Oxides and Silicon Carbides) These are relatively expensive grits, but their use is often justified by special considerations. Due to the retention of sharp cutting edges on the particles in use, their cutting action is particularly effective, especially on hard base materials which may resist effective blasting by chilled cast iron grit. Additionally, this effective cutting action is shown at blasting pressures considerably lower than normally employed for other abrasives. These ceramic grits are particularly well suited to blast preparation of thin metal surfaces, which show ‘buckling’ or distortion if blasted with chilled iron grit at conventional blast pressures. Finally, as these ceramic grits are essentially inert to normal corrosive influences, they can be safely used to grit blast stainless steel or nonferrous material surfaces, without causing rust staining or discolouration. Shot Types of blasting abrasive that are rounded in shape are known as shot. Their common use during the development of blastcleaning techniques leads to the common misuse of the term 'shotblasting'. The use of shot will prolong the life of blasting equipment and machinery because it has less cutting effect. The disadvantage here is that less cutting effect gives a less rough surface, often below the
Page 83
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:13
Page 84
Marine Corrision & Coatings
requirements of today's high performance coatings. One use of shotblasting may be to 'workharden' a metal surface by 'peening', a process which can reduce the incidence of stresscorrosion cracking. Chemical Analysis of typical Copper Slag Abrasive A typical analysis of Copper Slag abrasive may show the chemical content to be similar to the following table: SiO2 Al2O3 TiO2 FeO4 Fe2O3 MnO CaO MgO K2O Na2O CuO PbO ZnO S
38.40% 3.35% 0.35% 1.55% 3.15% 0.27% 5.86% 2.15% 0.53% 0.40% 0.47% 0.04% 1.68% 0.96%
Total
98.84%
Silicon Oxide Aluminium Oxide Titanium Oxide Iron Oxide (II) Iron Oxide (III) Manganese Oxide Calcium Oxide Magnesium Oxide Potassium Oxide Sodium Oxide Copper Oxide Lead Oxide Zinc Oxide Sulphur
Notice that there is very little copper in any form, since the slag is the by-product of copper extraction from ore. Notice also that most of the contents are oxides of one metal or another. Compressors Compressed air is a common source of power for blasting machinery, paint spray equipment, power tools etc. It is favoured on site because it is relatively safe, being less dangerous than, say, electricity. In order to produce quantities of compressed air, it is necessary to use a compressor. Normally driven by a diesel motor, a compressor draws in atmospheric air, pressurizes it, and feeds the air into a pressure vessel (known as a receiver). The air is then held in the receiver until demanded by the equipment in use. The production of compressed air gives two problems to the surface preparation process. These are: •
Any change in atmospheric pressure may result in the release of water vapour from the air.
•
Because compressed air in the receiver is stored by pressurizing an oil reservoir, there is a possibility of oil vapour being retained by the air as it is released.
Both of these factors require that adequate vapour traps are fitted to blastcleaning equipment in order to remove the contaminating oil and water. Compressors are rated for: •
Air pressure, measured in pounds per square inch (p.s.i.) or bar.
Air pressure is normally set at a maximum of 100 p.s.i. For portable compressors (in the U.K.) and this pressure, if successfully maintained, is capable of producing an efficient blastcleaning operation. •
Capacity, measured in cubic feet per minute (c.f.m.) or litres per minute.
The capacity of a compressor will determine the quantity of air it is able to deliver at its working pressure. For blastcleaning purposes, it is better to have a large capacity compressor working below its maximum level rather than a smaller compressor that is working at or near to its maximum level.
Page 84
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 85
Marine Corrision & Coatings
Module K
Blast Cleaning Cabinets It is sometimes desirable to blastclean individual items in an enclosed space so that other trades can continue to work in the immediate vicinity. If this is a regular requirement, many factories will buy or build a blastcleaning cabinet. The size of typical cabinets may vary from the very small 'cupboard' where blasting is done from outside the cabinet, with hands inserted through holes in the side to the relatively large blasting room. The more sophisticated blast rooms may have a rail system to transport large items into the room, and will have grit recovery and recycling systems. In general, the blast cleaning apparatus is similar to that used for on site blasting. The most complex of the blastcleaning cabinets are designed for large quantities of steel to be blastcleaned on a regular basis, such as all plate received by a shipbuilding yard. These machines, often known as 'Wheelabrators', are designed to work on a continuous basis, and include a conveying system that will carry items through the cabinet continuously. It is usual for these cabinets to use a system of rotating vanes to propel the abrasive, from which the term wheel-abrator has been adapted for general use. These cabinets too have an abrasive recovery and recycling system, and are capable of very high rates of cleaning.
Blast Pots/Hoses/Nozzles Grit Blasting Equipment Particles of abrasive may be projected by direct feed of the particles from a pressurised container into a high-pressure air stream 'Pressure Blasting'; or by centrifugal projection from rapidly rotating impellers 'Centrifugal Blasting' or 'Airless Blasting'. Hoses and Nozzles Owing to the inevitable pressure drop within air lines, pressure hoses should be kept as short as possible, the blast hose preferably not more than 20 ft. Restrictions in internal diameters at couplings should also be carefully avoided. Hose diameters should also be of an internal diameter at least three or four times that of the nozzle bore. All joints should be carefully maintained fully pressuretight and any leaks quickly rectified. Nozzle Size Other factors being constant, the speed of blasting is directly related to the size of the nozzle used. So also is the air consumption. Accordingly, the maximum nozzle size that can be used must depend upon the capacity of the compressor feeding it. The following table gives guidance on this, showing the volume of air required at the various pressures to feed different sizes of nozzles. Nozzle Qualities Maintenance of nozzle size is of considerable importance and can be a problem if normal cast iron nozzles are employed, as these wear oversize quite rapidly. More efficient blasting can be obtained by the use of nozzles produced in special wearresisting alloys or in such materials as tungstencarbide or ceramics. Although of higher initial cost, these nozzles are more economical in practice. Nozzle Design The internal profile of the blast nozzle is also an important factor. Venturi nozzles are generally preferred to the parallel bore nozzles formerly used, as they last longer, give higher grit velocity at more economic air consumption, and result in an overall increase in blasting efficiency.
Page 85
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:13
Page 86
Marine Corrision & Coatings
Operating Clothing/Air Supply For the safety and comfort of the blastcleaning operator, it is essential that good quality working clothes are used. Typically these would include: • • • •
Safety Boots (with steel insert toecap). Overalls. Strong Leather Gloves. Airfed Blasting Helmet, incorporating a replaceable visor and leather cape.
It is important that the operator has a good supply of clean, fresh air for breathing. Two common ways of achieving this are: •
A supply of air at low pressure is delivered from the blastpot via a filter. This method has the disadvantage of the air being the same as that used for blasting.
•
A separate supply of air, also at relatively low pressure, is fed from a remote airdriven pump, well away from any contaminated or dust laden atmosphere.
Water Blasting A significant recent development in the area of surface preparation has been the adoption of blastcleaning methods using water. This usually takes one of three different forms: • • •
High-pressure waterblasting, using water only Gritblasting with a water shroud Slurry blasting, with a water/abrasive mix
High Pressure Water Blasting Commonly used for the cleaning of buildings, pure water blasting is seldom used for surface preparation prior to painting. It is, however, in common use underwater for the removal of marine growth on ships and oil related structures. The cleaning of marine growth from a ship’s hull or offshore structure is normally achieved at pressures between 200 bar and 600 bar. The performance depends upon two main factors: • •
The maneuvrability of the diver and his visibility The density of the fouling and degree of barnacle or shell growth
On ships’ hulls in dry dock, a cleaning rate of up to 200m2/hour can be achieved with high pressure water jetting. However, many other factors must be taken into consideration underwater and the performance is usually reduced as a result. To give an indication of underwater jetting rates, the legs of a drilling platform in the North Sea were covered with weed and mussel growth up to 600mm thick, were cleaned at the rate of 20m2/hour this of course was exceptionally heavy fouling. However, as there is no reaction force with an underwater gun, the operation is carried out with considerably less physical effort compared with work above the surface. Underwater concrete cutting is another application for water jetting and equipment has been employed for this purpose at a depth of 450 ft. A section of damaged concrete coating on a 30 inch steel pipeline beneath a rig in the North Sea was successfully cut and jet blasted away without damage to the underlying steel pipe. Pressures of 760 Bar were employed on this occasion and one diver at a time from the team in saturation was employed to handle the gun and to perform the cutting. Where steel must be brought to a ‘white’ finish, a special underwater gun has been developed using abrasives.
Page 86
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 87
Marine Corrision & Coatings
Module K
Grit Blast with Water Shroud One of the advantages of the use of water in a blastcleaning operation is the reduction of the dust hazard. For this reason, equipment has been developed which will provide a shroud of water around a normal flow of grit carried in compressed air. Other advantages of this system include the ability to remove soluble contaminants from the surface, and to use sand as an abrasive. Disadvantages include the problem of removal of the spend abrasive (in its wet form) and the necessity of using an inhibitor in the water to prevent surface rusting. Slurry Blasting, with Water/Abrasive Mix Many of the comments made above relate also to the use of slurry blasting equipment, although these specialised units have certain extra advantages. Because the slurry is pumped as slurry, the pressure can be easily contained. This means that the cutting effect of the abrasive can be increased or reduced at will, to allow special effects such as removal of only the top coating, or feathering back the coating edges. The use of water blasting has created a great deal of controversy. There is no doubt that the control of the system (with its ability to reduce pressure etc.) is valuable, and that reduction of contamination of the surface is important. On the other hand, the need to use inhibitors and thereby set deposits on the steel surface, and the problems of disposal of the spend abrasive may be matters which require considerable justification. There is also, as yet, some doubt about the quality of the new coatings which have been developed for application to wet surfaces.
Blast-Cleaning on Site Operator Safety Abrasive blastcleaning at high pressure is a dangerous operation. It is essential that steps are taken to protect both the operators and any spectators or other site personnel. Some considerations would be: • • • • •
No-one but the operator should be allowed within the vicinity of the blastcleaning operation Warning notices should be displayed A lookout (or ‘potman’) should be on the alert All equipment should be tested for safety in operation A ‘Dead-Man’s Handle’ cut out device should be fitted and used
Practical Methods of BlastCleaning For practically every coating process, such as phosphating, galvanising, electroplating or painting the initial cleaning and preparation of the surface to be coated is a process step on which the subsequent success of the actual coating application virtually depends. The most generally established method of surface preparation for the application of coatings is by 'GritBlasting' as defined below. Indeed, when modern sophisticated coatings are applied for surface protection, there is no truly satisfactory alternative process. The fundamental principle of the gritblasting process is the removal of rust, millscale, or other surface contaminant (and obtaining a suitably roughened surface) by projecting a highly concentrated stream of relatively small abrasive particles at high velocity against the surface to be cleaned. Blasting Technique Manual blasting should systematically cover the entire surface to be cleaned, by moving the nozzle at fairly constant speeds in straight paths, each succeeding pass overlapping the preceding one and exposing clean mental without any evidence of patchiness. The nozzle should be held at an angle close to 90° to the surface, though not so as to allow the abrasive to bounce right back at the operator. Speed of nozzle traverse is dictated by rate of cleaning and should be as fast as will clean thoroughly without unnecessary dwell.
Page 87
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 88
Marine Corrision & Coatings
Masking Off Where areas have to be left unblasted for any reason, these can be masked off by suitable templates of metal or rubber, or by suitably tough masking tapes firmly secured to the surface requiring protection from the blast stream. Surface Cleaning B.S. 7079 outlines the surface preparation requirement that 'the surface shall be thoroughly cleaned and roughened by compressed air or centrifugal blasting with a suitable abrasive grit, or by other suitable means. Immediately before spraying, it shall be free from grease, scale, rust, moisture or other foreign matter'. It is also essential that surface cleanliness should not be impaired during the actual grit blasting process. This can arise from oil vapour or droplets carried over in the air from the compressor, or from moisture entrapped in the high-pressure air, or again, from residual dust arising during blast cleaning. Accordingly, precautions should be taken to ensure that compressed air supplies are oil and moisturefree, by the installation and adequate maintenance of suitable aftercoolers, moisture traps and filters in the airlines. Moist air can also cause clogging of the abrasive lines or blastpot, and rust contamination of the blast cleaned surface. To remove residual dust, on completion of blasting, the blasted surface should be blown over with a high pressure jet of clean dry air, or even better, vacuum cleaned or brushed over with a clean dry brush until thorough inspection ensures no traces of residual dust or grit. Care of Prepared Surface Obviously, a surface prepared for coating should not be allowed to deteriorate or to be contaminated in any way between grit blasting and spraying. A clean grit blasted surface should never be handled or touched unless hands are protected with clean gloves. It should also not be exposed for prolonged periods nor, indeed, exposed at all to atmospheric or storage conditions of high humidity, as under such conditions spontaneous oxidation and rusting proceed very rapidly. As far as practicable, any storage should be in a warm, dry environment and, ideally, spraying should follow grit blasting as quickly as possible. These points are very adequately covered in B.S. 5493, which indicates a maximum delay of 4 hours and requires that “If visible deterioration has occurred, the surface preparation shall be repeated”. Blasting Efficiency The condition of the material to be blasted very significantly influences both the rate at which blasting can be done and the quality of blast finish obtained. Heavy rusted or corrosionpitted steel is difficult to clean thoroughly because of the extra blasting time and attention necessary to remove deep-seated scale and injurious corrosion products. Somewhat heavier grades of grit and highblasting pressures are advantageous for such surfaces. Certain heat-treated alloy steels also acquire a particularly tenacious oxide scale, again requiring heavier grit and highblasting pressures for most efficient cleaning. Air Pressure In general, air pressure for grit blasting depends on such factors as grit quality and grade, type of material being blasted and overall production requirements. Expendable grits are better blasted at high pressures above 90 p.s.i. to obtain maximum cleaning rates, as grit breakdown after impact is of little consequence other than creating something of a dust problem. Increased pressure materially increases rates of blast cleaning, due to greater grit throughputs and higher particle velocities. Labour costs per areas blasted, can be considerably reduced, although grit losses tend to increase significantly above about 70/75 p.s.i.
Page 88
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 89
Marine Corrision & Coatings
Module K
Also, at high blasting pressures - above 80/85 p.s.i. - Operator fatigue can limit productivity on long blasting "stints". Nevertheless, it is usually recommended that blast pressure as measured at the nozzle be kept as high as is possible, with the provision that it cannot normally exceed 100 p.s.i. for safety regulations. The reasons are related to efficiency and production rates: “Work done is directly proportional to the air pressure at the nozzle”. Quantity of air is measured in “c.f.m.” (cubic feet per minute). Pressure of air is measured in “p.s.i.” (pounds per square inch). Note: Nozzle pressure may be measured with hypodermic needle gauge. 100 p.s.i. at the nozzle gives 100% efficiency. 80 p.s.i. at the nozzle gives 66% efficiency. 66 p.s.i. at the nozzle gives 50% efficiency.
e.g. At 60 p.s.i. nozzle pressure, only half of the area would be cleaned, compared to the area cleaned at 100 p.s.i. (in the same time of course). The main reasons for inability to maintain high nozzle pressures are: •
Inadequate air supply. Theoretically, a 3/8” nozzle (one of the popular sizes) required 196 c.f.m. Normally a compressor of at least 240 c.f.m. would be employed. For a larger nozzle more air would be required, hence a bigger compressor would be needed.
•
Too small air hoses; friction losses are expensive.
•
Internal couplings can cause 15% loss of efficiency . External couplings and nozzle holders are a must.
•
Badly designed machines have a significant pressure loss through the machine.
•
Too small piping on the machine again causes friction losses.
•
Air lines should be kept straight and as short as possible.
Other important factors affecting blasting performances are: •
Correct choice of nozzles. The Venturi types are twice as efficient as straight bore nozzles.
•
Air must be dry.
Rates of cleaning cannot be quoted definitely. There are so many variables affecting a blasting operation, e.g. air availability, nozzle size and type, type of equipment, state of surface to be cleaned, surface cleanliness required, degree of manoeuvrability, lighting standard, distance of nozzle from job, skill of operator, type and size of abrasive being used etc., that it is difficult to quote definite figures. As a very general guide only, high cleaning rates can reach 30 square metres per hour, low rates can fall to as low as 4 square metres per hour. It must be stressed that these speeds are indications only and should not be applied to any specific job. Similarly, the consumption of abrasive can be variable, and calculation of quantities used, etc. is very much a matter or trial or experience. As a general guide, a consumption of 50 kilos of abrasive per square metre is common for irregular structures.
Quality Control of Blast-Cleaning Blast Cleaning Standards A major advantage of blast cleaning is that it is the only economical process whereby a surface can be cleaned adequately and at the same time, etch the surface to provide a key for the subsequent coating.
Page 89
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Module K
Page 90
Marine Corrision & Coatings
Blast cleaning for painting ranges from 'brush off' blasting merely superficial removal of the loose millscale, rust and dirt, through 'Commercial Blasting' and 'Second Quality Blasting' to 'First Quality' or 'White Metal' finish. It is apparent that in order to be both economical and effective, both contractor and client must agree on the standard of surface cleanliness required. For the purpose of judging these standards, the industry has adapted one of three major international standards for blastcleaning. These are:
•
Structural Steels Painting Council of America S.S.P.C.
•
NACE Standards for surface preparation by Abrasive Blast Cleaning.
•
Swedish Standard: Pictorial Surface Preparation Standards for Painting Steel Surfaces. SIS 05 5900.
•
British Standard Specification for Surface Finish of Blast Cleaned Steel for Painting. B.S. 7079.
The standards produced by these four bodies have an equivalence which is expressed in the table below:
Description
SSPC
Swedish Standard ISO 8501-1 & BS 7079
Nace
White Metal
SP 5
Sa3
Nace 1
Near White Metal
SP 10
Sa2
Nace 2
Commercial
SP 6
Sa2
Nace 3
Brush-off Blast
SP 7
Sa1
Nace 4
It should be remembered that these preparation standards refer to surface cleanliness only, and not to surface roughness or amplitude. Surface Profile and Measurement. The surface profile of a blast cleaned surface is the shape of the actual peaks round or angular. The surface amplitude is the size of the peaks, measured from the peak to the trough. Some surface roughness is a great advantage in providing a physical key for the primer to adhere to. However, the degree of roughness should not be excessive or peaks of steel (rogue peaks) may project through the paint film and rust very quickly. The surface amplitude can be measured in various ways, either by direct reading or by using Testex Tape, a new method of measuring surface profile after abrasive blasting. Press-O-Film® is a replica tape composed of an incompressible Mylar backing of known thickness (2 mils) coated with a compressible material. The tape is placed against the metal surface and the Mylar backing is rubbed with a blunt instrument. The pressure applied to the tape causes the compressible material to replicate the blasted surface. By measuring the resultant tape thickness with a specially adapted spring micrometer, the maximum peaktovalley height can be obtained by subtracting the thickness of the Mylar backing from the total micrometer reading. Press-O-Film offers some advantages over alternative methods. It was found that when PressOFilm was compared with other available site methods of measuring blast profile, the results obtained corresponded more closely with those of the standard optical microscope than any other. PressOFilm will also work accurately on convex and concave surfaces, e.g. inside and outside pipes, where other methods do not. It may also be convenient to retain the tape and have a permanent record of the blast profile.
Page 90
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 91
Marine Corrision & Coatings
Module K
Inspection of Surface Cleanliness. The rust which forms on steel surfaces exposed to either industrial or marine atmospheres may contain significant quantities of sulphate and chlorides. If these compounds are not removed, they can give rise to premature paint failure. Spot tests have been developed to detect the presence of soluble iron, sulphates and chlorides on blastcleaned shipplate. In these spot tests, colour reactions are used to indicate the sites of contaminated rust. Each method employs pieces of chemically treated filter paper that can be dried after use to provide permanent records. The papers are pressed against the steel surface that has been wetted with distilled water. After a short time in contact with the surface, the papers are removed and examined for colour changes. Test for Soluble Iron. A filter paper is saturated with potassium ferricyanide solution (orange) and dried. When pressed against a wet steel surface it will show a blue colour wherever soluble The water has dissolved iron salts. Test for Soluble Sulphates. For this test, papers are prepared by being soaked in a barium chloride dehydrate (6%w) solution and dried. The dry paper is then pressed against the test surface, backed by a second paper soaked in saturated potassium permanganate. ‘Contamination’ is absorbed and held in the barium sulphate lattice, imparting a pink colour to those parts of the paper that contacted sulphates. Test for Soluble Chlorides. In this test, a filter paper is wetted with silver nitrate (2%w) solution and then pressed against the steel surface for about 20 seconds. It is then peeled off and thoroughly washed with chloride freewater. Any silver chlorides formed remain in the paper and can be detected by immersion in photographic developer. Chloride sites show as brownblack areas. The paper can be washed and dried for record purposes. ISO8502-1 also defines a suitable method for the determination of soluble iron salts. Test for Millscale. An acidic solution of copper sulphate, applied to a blastcleaned surface, will deposit bright copper on clean steel, but will show a black colour on millscale. Quantitative Test for Soluble Iron. The Merckoquant test provides an indicator paper that reacts to the quantity of dissolved iron in water. A measured quantity of distilled water is used to wash 15 cm square of blastcleaned steel. The water is retained, and tested with the indicator paper, which changes colour according to the quantity of iron. Test for Oil Contamination. Oil that is present in small quantities on blast cleaned surface may be detected by: •
Shining an ultraviolet light source on the surface, causing the oil to fluoresce.
•
Pouring solvent across the surface. The solvent should form a continuous flow and not ‘break’ into droplets.
Relative Humidity Dew Point. Relative humidity is defined as the amount of moisture (water vapour) in air, compared with the maximum possible in the air. In other words, as the amount of water vapour which air will support approaches the saturation level, relative humidity approaches 100%. If R.H. reaches 100%, then the air will not support any more water vapour, and a surplus would appear as condensation. For that reason, most coating specifications have a requirement that coating is not carried out if R.H. exceeds a certain limit, usually 85% or 90%. Dew point is defined as the temperature at which condensation occurs.
Page 91
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 92
Marine Corrision & Coatings
As air temperature falls, its ability to carry water vapour also falls, and R.H. rises. Sooner or later, a temperature is reached at which the air is effectively saturated. This temperature is dew point. If the temperature falls below dew point, or if a piece of structural steel has a temperature below dew point, then condensation will occur. Coatings applied over a wet surface will generally not achieve adequate adhesion to the substrate. For this reason, coating specifications normally have a requirement that coatings should not be applied if the temperature of the steel or the surrounding air is less than 3?C above dew point. Relative humidity and dew point are both calculated by making temperature measurements with a hygrometer. Dehumidifiers One way to ensure that ambient conditions are suitable for painting is to dehumidify the air. This is naturally only possible in an enclosed space. A dehumidifier works by drawing ambient air across a desiccant such as Silica Gel, and delivering the air at a lower R.H. to the desired space. A separate stream of air is heated and passed over the desiccant to remove the moisture and revitalise it. This second stream of air is vented to the atmosphere. The attainment of very low humidity is possible in large spaces. This technique is commonly used in ships’ tanks and crude oil storage tanks.
Page 92
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 93
Marine Corrision & Coatings
Module K
17 PAINT APPLICATION: QUALITY CONTROL Paint testing on Site The range of paint tests that may be successfully carried out on site is limited by both the difficulty of handling complex laboratory equipment, and by the lack of laboratory experience of most site personnel. In practise, the common tests that are performed are: •
Viscosity Check to check suitability of flow characteristics.
•
Density Check to test quantity of added thinners.
•
Wet Film Thickness Check to ensure that the film will achieve the desired DFT.
•
Visual Checks on: condition of paint, batch numbers, storage time, condition of paint container, label details, etc.
Paint Mixing. In most paints, especially heavily pigmented primers, red lead, etc., there is a tendency for settlement to occur on the bottom of the container and, unless the paint is thoroughly mixed before use, it will not perform satisfactorily. Poor opacity, slow drying, poor gloss and other troubles often result from failure to mix the paint properly before use. When mixing the paint, a properlymade stirrer should be used, long enough to reach the bottom of the container and with a broad, chisellike edge and a comfortable handle. Stirring, using a beating and lifting action, should mix all settlement. If large quantities of paint are being mixed, it is advisable to us a mechanical mixer, preferably of the type which is driven by compressed air. Many spray pumps are equipped with a builtin mechanical stirrer. If thinning of the paint is required, this should be done under supervision, and with the correct solvent. Where the paints are to be applied by spray, a certain amount of thinning may be required. It will not normally be necessary to add more than 10% of the recommended thinner to bring the paint to a suitable consistency; overthinning should be avoided because the thin coating which will result will have poorer durability and a lower resistance to corrosion than one applied at the correct thickness. In general, thinning of paints should be avoided unless necessary In the case of two-pack (or multipack) paints, it is important that materials are mixed in the correct ratio. Manufacturer's instructions should be followed with respect to induction period of pot-life requirements. It is generally not recommended that part quantities of supplied materials (e.g. 2_ litres from a 5 litre tin) be mixed. If this method is adopted, it is important that the materials are stirred before being measured out, and important that the measuring is done accurately. Only the required amount of paint should be mixed and any remaining over after the job is completed should be thrown away and not returned to the tin. Brushes, spray equipment etc. should be cleaned out with the recommended solvent immediately after use, and at work breaks, because the paint may quickly harden causing unnecessary damage and delays, especially to airless spray equipment. Wet Film Thickness Checks Measurement of wet paint film thickness provides a useful guide for inspectors or paint sprayers to ensure that a correct and even film thickness is being applied to the article being coated. Use of a wet film comb at this stage of a paint application operation, helps to prevent ‘reject work’ which in itself is time consuming and therefore costly. Weather Conditions Application of Coatings during poor weather conditions is likely to lead to a reduced quality of protection. Quality Control on site should monitor Relative Humidity, Dewpoint Temperature and the temperature of the surface being coated, to avoid problems with :
Page 93
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Module K
Page 94
Marine Corrision & Coatings
•
Coatings being applied to a damp surface
•
Freshly applied coatings becoming affected by moisture soon after application
•
Coatings being applied when the temperature is too low for the curing reaction to proceed
•
Coating solvents being unable to evaporate, due to high partial vapour pressure (i.e. water vapour in the air)
As a general guide, a specification is likely to require that the air temperature is greater than 5oC, that the surface be dry, and that the Relative Humidity is lower than 90%.
Page 94
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 95
Marine Corrision & Coatings
18
Module K
PAINT FAULTS
There is a great variety of paint faults, which may occur before, during or after application. The analysis of paint faults and their cause is, perhaps, best left to the experts. Some common faults are described below:
Bleeding This term describes the discoloration or staining of newlyapplied paint by a constituent of the old paint, or other material, over which it is applied. It is commonly encountered when painting over bituminous or tar products. The new paint develops brown stains and its drying may be affected.
Blistering Blistering is usually caused by the evaporation of moisture or solvent trapped beneath or in the paint film. Soluble iron salts on the substrate prior to painting can be a major cause of blister formation. If surfaces are painted shortly after washing down, even if time is allowed for surface drying, blistering may occur as a result of moisture absorbed into the old paint.
Painting in the direct heat of the sun may result in rapid ‘skin drying’, causing solvent to be trapped in the film. Recoating before the previous coat is hard dry may cause blistering for the same reason.
Page 95
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 96
Marine Corrision & Coatings
Blooming (Haze, Milkiness) Blooming is the formation of a surface mist or haze considered to be due to the presence of moisture or impurities in the atmosphere or on the newly painted surface. Epoxy coatings often suffer a ‘blush’ if moisture is allowed to affect the surface in the early stages of the drying and curing process.
Brush marks Brush marks result from the failure of the paint to flow out after application.
Chalking This term is used to describe surface powdering of the paint film, leaving a fine, white, powdery film. It can occur when paint deficient in binder or vehicle is used on surfaces exposed to the attack of weather, and is common in epoxy coatings. The powdery layer can be easily removed, leaving the colour and gloss of the paint film unimpaired, although the thickness of the film will be reduced. The process will continue throughout the life of the paint coating. Some paints are formulated to chalk in this way, thus continually presenting a clean surface and minimizing the amount of preparation required before repainting.
Page 96
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 97
Marine Corrision & Coatings
Module K
Cissing (Fish-eyes) Cissing is the shrinkage or drawingaway of the new coating from large or small areas of the work. It may become apparent as small 'craters' or circles, or it may result in almost complete lack of adhesion over a large area, where the paint gathers in blobs. It is nearly always due to the presence of oil or grease on the surface.
Crazing (Cracking) Crazing is the breakdown of a paint or varnish, usually due to ageing. The first sign can be fine checking or hair crazing of the surface coat that develops to a coarser ‘crocodiling’. It may then develop further into peeling or flaking. Curtaining, Running or Sagging This defect is due to careless or inefficient workmanship resulting in an uneven coat. Over application on moulded or riveted surfaces will often result in 'runs' or 'tears' developing.
Page 97
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Module K
Page 98
Marine Corrision & Coatings
Discoloration Rapid discoloration is nearly always caused by agencies outside the paint itself, and may be due to a variety of causes, a few of which are mentioned here. Discoloration is often the result of attack by salts in solution. Usually, the oil of vehicle in the paint is affected and this gives rise to an apparent change of colour, even when the pigments are unaffected. Pigments such as Prussian blue, yellow chromes, Brunswick greens and some reds, can be discoloured by alkali. Dry Spray (Over Spray) Dry spray gives a rough, gritty finish with a dust of dried paint on the surface. It is caused by atomised paint particles being dry before they strike the surface, either because the spray gun is being held too far away from the surface, or because the solvent balance in the paint material is incorrect. It is frequently found on windy days, due to wind diverting the spray. Drying Problems Certain types of paint naturally take longer to dry than others. Straight oil paints, for example, usually require longer drying periods than modern gloss paints. If paint dries more slowly than it should, the cause will usually be one or more of the following: •
Application in unsuitable weather, such as frost, low temperature, fog, rain or excessive humidity, or in a fumeladen atmosphere.
•
Application over bituminous products or wax polish.
•
Application over dirt, oil or grease, or over an undercoat or primer which has not been allowed to harden sufficiently.
•
Using unsuitable thinners or too much driers.
Gelling (Livering) Gelling is the changing of paint, partially or completely, into a jellylike state. It should not be confused with thickening resulting from partial evaporation of the thinner or volatile content.
Grinning (Poor Opacity) This is the term used when the undercoat or substrate shows through the finished coat, and may be due to the use of an undercoat of an unsuitable colour, poor workmanship, or an endeavour to carry out a wide colour change with too few coats. Lifting (Picking Up, Working Up) Lifting denotes the disturbance of the previous coat on the application of new material. It can be due to insufficient drying of the earlier coat, particularly with paints drying by evaporation. It can also occur when strong solvents in the new material attack the old paint underneath; this is usually manifested by wrinkling. In such cases it may be necessary to remove the old material entirely before proceeding.
Page 98
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 99
Marine Corrision & Coatings
Module K
Loss of Gloss (Sleepiness, Matting) When loss of gloss occurs immediately or soon after application, it is seldom due to a fault in the paint itself. It is most likely to arise form atmospheric conditions at the time of application frost, fog, moisture, lack of ventilation, etc. or because of a porous undercoat, or the omission of an undercoat. It can sometimes result from painting over greasy or waxy surfaces, or from the use of unsuitable thinners, such as paraffin. Loss of gloss after long exposure may be due to these causes, to the start of ‘chalking’ or to normal ageing. A very fine surface ‘shrivelling’ can also give the appearance of low gloss.
Misses (Holidays) These are gaps in the coating caused by careless workmanship, which may be exaggerated by the use of too thick a paint, or application over greasy or moist surfaces.
Mud-Cracking Mud-cracking is the splitting of a coating (particularly inorganic zinc silicate) that has been applied at too great a film thickness. The dried film has the appearance of a driedup river bed, and does not adhere to the substrate. It is particularly common at welds and in corners. Opacity - Lack of Lack of 'body' or opacity in a paint (that is, its ability to obliterate or hide) is usually due to underpigmentation. This may be caused by failure to stir the paint properly before use or by overthinning. (see Grinning). Orange Peel Sprayed coating which dried to a textured finish resembling the skin of an orange. This effect is caused by inadequate paint flow.
Page 99
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 100
Marine Corrision & Coatings
Peeling This is a defect similar to flaking. Whereas flaking is normally confined to relatively small individual areas, peeling may be described as the detachment of large sheets of paint from the surface. Saponification Saponification means the formation of soap, and it occurs when alkali attacks the oil content of paint in the presence of moisture. The paint film loses gloss and colour; becomes soft and sticky. Affected areas often exude, in drops or runs, a sticky brown liquid that looks like oil. Settling This is the separation of pigment from the paint vehicle on standing or storage. Certain pigments are particularly prone to settling, as are some heavy extenders such as barytes. The effect is most noticeable in heavily pigmented or flat paints, particularly when stored for a long period. When storage of these paints is necessary it is advisable to invert the containers at regular intervals, in order to counteract this tendency. Special attention should be paid to the stirring of such paints.
Wrinkling The development of wrinkles inn paint causes swelling, blisters and lifting.
The pictures contained within this section are protected by copyright. All rights reserved. No part of these pictures may be reproduced, stored in a retrieval system, or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior permission of MPI Group.
Page 100
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 101
Marine Corrision & Coatings
Module K
19 FURTHER INFORMATION Corrosion Societies Several Corrosion Societies exist which the user can join to keep up-to-date with developments in the Industry; these include: The Institute of Corrosion (UK) - ICorr: Based in Leighton Buzzard, the Institute focuses on all aspects of Corrosion, and has more than one thousand members in the UK, and approximately 100 members outside the UK. The bi-monthly magazine of the Institute is called ‘Industrial Corrosion’, and is supplied either to subscribers or to members of ICorr. Contact ICorr, telephone 01525-851771, fax 01525-376690 The National Association of Corrosion Engineers - International - NACE: Based in Houston, Texas, NACE caters for all aspects of Corrosion, and has several thousand members mostly in the USA, but also in other parts of the world. There are NACE ‘chapters’ in South-East Asia, in England, in the Middle-East and in Europe. NACE publishes a monthly magazine called ‘Materials Performance’, which has one section specifically devoted to Coatings. NACE publishes the results of its many committees, either as Standards, Test Methods or Recommended Practises. NACE has been operating a Certification scheme for Coating Inspectors since 1982, called the NACE International Coating Inspector Training and Certification Program (NICITCP), and more than 2,000 people have participated at one level or another (1993). NACE also operates other training qualifications and offers professional recognition for various grades of Corrosion Technicians, Specialists and Engineers. Contact NACE at PO Box 218340. Houston, Texas, 77218, telephone 713-492-0535. Steel Structures Painting Council - SSPC: Based in Pittsburgh, USA, SSPC caters only for coatings-related aspects of corrosion, and has several thousand members with an interest in the coatings industry. SSPC publishes books and standards related to coatings, and has a monthly journal devoted to Coatings called ‘The Journal of Protective Coatings and Linings’. Contact SSPC at telephone 412-687-1113. Oil & Colour Chemists Association (UK) - OCCA: Based in Middlesex, England British Standards Institute - BSI: Based in London The National Corrosion Service is funded by the DTI (Department of Trade and Industry) the NCS Helpline offers UK Industry free and impartial advice on all matters relating to Corrosion. The National Corrosion Service is based at the National Physical Laboratory Teddington UK. The function of the NCS Helpline is as follows: To support UK industry in its efforts to control corrosion. The helpline staff provide a fast technical response to UK companies with questions relating to corrosion. Examples of the support offered are: • Solutions to corrosion problems that have already occurred • Advice on how to prevent and control corrosion • Guidance on sources of information • Provision of good practice guides Advice can also be provided on a consultancy basis when appropriate. The NCS website provides guidance that you can use to control corrosion. It is set out in a checklist format to enable you to better consider how you can control corrosion. www.nationalcorrosionservice.org Fitz’s Atlas™ of Coating Defects - ISBN 0 9513940 2 9: This publication illustrates the range of coating and surface defects likely to be encountered, and gives advice on probable causes, prevention and repair. Contact MPI Group on telephone +44 (0)1252 732220 (see inside back cover for full contact details).
Page 101
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:13
Page 102
Marine Corrision & Coatings
APPENDIX A Glossary of Corrosion and Paint Terms ABRASION RESISTANCE: resistance to being worn away by rubbing or friction; related to toughness, rather than the hardness of a paint film. ABRASIVE: material used for abrasive blast cleaning, such as mineral grit, steel shot or steel grit. ACCELERATED CORROSION TEST: Corrosion test, carried out under more severe conditions that will yield results in a shorter time than in service. ACTIVE-PASSIVE CELL: Corrosion cell, with anode and cathode formed by active and passive surface areas of the same metal. ADHESION: degree of attachment between a paint film and the underlying material, either substrate or another coating film. AGITATE: to stir or shake. AIR COMPRESSOR: a machine which draws in air and compresses it, thus providing high pressure and volume of air required for abrasive blasting and spray painting. AIRLESS SPRAY: method of paint application using fluid pressure to atomise paint by forcing it through a fine orifice (spray tip). ALKYD (PAINT): alkyd resin modified by combination with a vegetable oil or fatty acid, then pigmented. ALKALI: substance such as caustic soda, or lime with pH greater than 7, that can be highly destructive to paint films; opposite of acid; caustic. ANODE: one electrical component of corrosion cell, the point at which positive current enters an electrolyte, and metal loss occurs. ANODIC OXIDE COATING: Protective, decorative or functional coating, formed by conversion of the surface of a metal in an electrolytic oxidation process (see ISO 2080 : 1981). ANODIC PROTECTION: Electrochemical protection by increasing the corrosion potential to a value corresponding to the passive state. AMBIENT TEMPERATURE: temperature in immediate vicinity, natural temperature or temperature of surroundings. ANCHOR PATTERN: rough pattern of peaks and valleys created by abrasive blasting; surface profile. ANTIFOULING: coating applied to ship bottoms or other immersed structures to prevent marine growth. ATMOSPHERIC CORROSION: Corrosion with the earth’s atmosphere at ambient temperature as the corrosive environment. ATOMIZE: to reduce a liquid to a mist of fine droplets; to break a stream of paint into small particles. BIMETALLIC CORROSION: CONTACT CORROSION (DEPRECATED): Galvanic corrosion, where dissimilar metals or other electronic conductors form the electrodes. BINDER: film-forming material; the nonvolatile material forming a paint film; it binds pigment particles together and holds them in place on a surface.
Page 102
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 103
Module K
Marine Corrision & Coatings
BITUMINOUS (PAINTS): class of coating material consisting of natural bitumen dissolved inorganic solvents - often contains softening agents, pigments and inorganic fillers. Usually black or dark in colour. Within recent years the term ‘bituminous’ has, by common usage, come to include bitumen like products, such as petroleum asphalt; the term is not used for paints based on coal tar or coal tar pitch. BLAST PATTERN: the surface area hit by the abrasives; during blast cleaning. BLISTERING: the formation of swellings on the surface of an unbroken paint film, cause by moisture, gases or the development of corrosion products between the substrate and the paint film. BONDING: attraction between two surfaces, such as two coats of paint or paint and a substrate. see ‘adhesion’ BOUNCEBACK: spray paint rebound from the surface. BOXING: manual mixing of paint by pouring back and forth from one container to the other. BREAKDOWN CHARACTERISTICS: the extent to which an abrasive pellet is damaged after striking the work surface; related to the recyclability of an abrasive and degree of dusting. BRONCHITIS: an inflammation of the mucous lining of the bronchial tubes. BURNISH: make shiny by rubbing or polishing - Burnished metal is a poor surface for paint bonding. BURR: rough edge, generally caused by mechanical damage or impact - which will interfere with coating adhesion. CATALYST: substance that promotes a chemical reaction; often called curing agent; hardener. CATHODE: part of a corrosion cell at which positive current leaves the electrolyte and enters the metal; electrically opposite to anode. CATHODIC PROTECTION: Electrochemical protection by decreasing the corrosion potential. CAVITATION CORROSION: A process involving conjoint corrosion and cavitation. corrosion may occur, for example, in rotary pumps and on ships’ propellers.
NOTE -Cavitation
CFM: cubic feet per minute; the capacity, or the air volume, of an air compressor is measured in CFM units. CHALKING: friable, powdery coating on the surface of a paint film caused by disintegration of the binder due to action of ultra-violet light or weather. CHECKING: hair line cracking of a dry coating film, generally due to the effects of aging or poor flexibility characteristics. see also ‘crazing’, ‘crocodiling’. CHLORINATED RUBBER (PAINT): a pigment solution of plasticised (chlorinated) rubber which cures (dries) mainly by evaporation of the solvent; convertible coating. COAL TAR/EPOXIDE: combination of coal tar pitch and epoxide resin with an amine or polyamide curing agent; coal tar improves the water resistance (permeability). COBWEBBING: stringy spray pattern, with the appearance of a spider’s web. This effect is frequently seen in hot weather when a fast solvent is used in chlorinated rubber or vinyl paints. The solvent evaporates before the paint is deposited resulting in ‘fingers’ or ‘cobwebs’ of paint being delivered to the surface. CONCENTRATION CORROSION CELL: Corrosion cell in which the potential difference arises from a difference in concentration of the corrosive agent(s) near its electrodes.
Page 103
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 104
Marine Corrision & Coatings
CONDENSATION: moisture resulting from temperature or atmospheric pressure changes; changing from a gaseous (vapour) to a liquid state. CONTAMINANT: dirt, oil, grease, loose rust, paint, or millscale or any other matter which is considered detrimental to good coating adhesion. CONVERSION COATING: Coating produced by the reaction of a metal or of its corrosion products with components of a selected environment (see ISO 2080 : 1981). CORROSION: Deterioration of a metal due to an electro-chemical reaction with its environment. CORROSION CELL: Short-circuited galvanic cell in a corrosion system, the corroding metal forming one of its electrodes. CORROSION CURRENT: Current due to an electrode reaction, directly causing corrosion. NOTE - The corrosion current density is equivalent to the rate of electrochemical corrosion. CORROSION DEPTH: Perpendicular distance between a point on the surface of a metal affected by corrosion and the original surface of the metal. CORROSION FATIGUE: A process involving conjoint corrosion and alternating straining of the metal. NOTE - Corrosion fatigue may occur when a metal is subjected to cyclic straining in a corrosive environment. Corrosion fatigue may lead to cracking. CORROSION INHIBITOR: Chemical substance which decreases the corrosion rate when present in the corrosion system at a suitable concentration, without significantly changing the concentration of any other corrosive agent. NOTE - A corrosion inhibitor is generally effective in a small concentration. In commercial applications additives are sometimes named as inhibitors. CORROSION POTENTIAL: Electrode potential of a metal in a given corrosion system. NOTE - The term is used whether or not there is a net electrical current flowing to or from the metal surface under consideration. CORROSION PRODUCT: Substance formed as a result of corrosion. CORROSION PROTECTION: Modification of a corrosion system so that corrosion damage is mitigated. CORROSION RATE: Corrosion effect on a metal per unit of time. NOTE - The type of corrosion rate to be used will depend on the technical system and on the type of corrosion effect. Thus corrosion rate may be expressed as an increase in corrosion depth per unit of time, or the mass of metal turned into corrosion products per unit area of surface or per unit of time, etc. The corrosion effect may vary with time and may not be the same at all points of the corroding surface. Therefore, reports of corrosion rates should be accompanied by information on the type, time dependency and location of the corrosion effect. CORROSION RESISTANCE: Ability of a metal to withstand corrosion in a given corrosion system. CORROSION SYSTEM: System consisting of one or more metals and all parts of the environment that influence corrosion. NOTE - Part of the environment may be coating, surface layer, additional electrode, etc. CORROSIVE AGENT: Substance that when in contact with a given metal will react with it. CORROSIVITY: Ability of an environment to cause corrosion in a given corrosion system. CRACKING: generally the splitting of a dry paint or varnish film, usually as the result of ageing. CRAZING: resembles checking but the cracks are deeper and broader.
Page 104
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 105
Module K
Marine Corrision & Coatings
CREVICE CORROSION: Corrosion associated with, and taking place in, or immediately around, a narrow aperture or clearance. CROCODILE OR ALLIGATORING: advanced type of crazing producing a pattern like the hide of a crocodile. CURING: chemical reaction to produce a dry paint film; the process of polymerisation of resins by heat or chemical means. CURING AGENT: chemical additive which promotes curing of paint films. DE-AERATION: Removal of air from environment. “deoxygenation” is more appropriate.
NOTE - If only oxygen is removed the term
DEADMAN VALVE: shutoff valve at the blast nozzle, which allows the blaster to start or stop the abrasive flow. DEHYDRATION: loss of moisture or fluid; to dry out. DERMATITIS: skin irritation or rash. DETERIORATION: gradual decay of a material; worsening of physical or chemical properties. DEW POINT: temperature at which atmospheric moisture condenses; when air is cooled below a certain temperature, some of the atmospheric moisture forms as dew on any surface below this temperature. The actual temperature at which this happens depends mostly on the relative humidity of the air. DFT: Dry Film Thickness - see film thickness DIFFERENTIAL AERATION CELL: Corrosion cell, in which the potential difference arises from a difference in the concentration of oxygen near its electrodes. NOTE - In some cases the differential aeration cell may result in an active-passive cell. DRIERS: substances which, when incorporated (in relatively small proportions) in drying oils, paints or varnishes, reduce their drying times. Driers are usually compounds of lead, manganese or cobalt. DRYING: the process of change of a coat of paint from the liquid to the solid state, due to evaporation of solvent, physiochemical reactions of the binding medium, or a combination of these causes. When the drying process takes place during exposure to air at normal temperatures, it is called ‘air drying’. If it is accelerated by the application of a moderate degree of heat (normally not exceeding 65 degrees C.) it is termed ‘forced drying’ as distinct from ‘stoving’ which involves higher temperatures up to 300 degrees C. DRY TO HANDLE: time interval between application and ability to receive next coat satisfactorily. EFFLORESCENCE: a deposit of soluble white salt found on the surface of brick, concrete, plaster and other masonry; caused by passage of water through the porous material, bringing salts to the surface, particularly during the initial drying phase. ELECTROCHEMICAL PROTECTION: Corrosion protection achieved by electrical control of corrosion potential. EPOXIDE RESIN: polymer resin, frequently used in coatings for high performance applications, particularly when good chemical resistance is required. Epoxy coatings are amongst the most common protective coatings in general use. EPOXY ESTER: a pigmented solution of an epoxide resin that has been modified by combination with a fatty acid, to allow curing by combination with atmospheric oxygen. This type of epoxy is a single pack material. EROSION: wearing away of a surface, generally due to friction and impact, as in a pipe carrying slurry materials.
Page 105
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Module K
Page 106
Marine Corrision & Coatings
EROSION-CORROSION: A process involving conjoint corrosion and erosion. NOTE -Erosion-corrosion may occur in,, for example,- pipes with high fluid flow velocity;- pumps and pipe lines carrying fluid containing abrasive particles in suspension. ETCH PRIMERS: Priming paints which contain a proportion of acid to aid adhesion to smooth surfaces and non-ferrous substrates, such as zinc galvanised or metal-sprayed surfaces. They are generally based on phosphoric acid, and can be very sensitive to water during and immediately after application. also known as wash-primers. EVAPORATION: change from liquid to a gas (vapour); the release of solvents from a paint fluid into the atmosphere. FILM THICKNESS: depth of applied coating film, usually expressed in microns (metric system) or mils (USA and related countries) FLAMMABILITY: ability to burn. ‘FOOTNER’ PROCESS: acid pickling process involving the successive immersions of steel in three seperate solutions; sulphuric acid at 50-60?C (120-140?F), Hot rinsing water at 60-65?C (140-150?F), phosphoric acid at not less than 85?C (185?F). FOULING: growth of attachments, such as weeds or barnacles, to hulls of ships or other marine structures. FRETTING CORROSION: A process involving conjoint corrosion and oscillatory slip between two surfaces in contact. NOTE - Fretting corrosion may occur, for example, at mechanical joints in vibrating structures. GALVANIC CORROSION: Corrosion due to the action of a corrosion cell. NOTE - The term has often been restricted to the action of bimetallic corrosion cells, i.e. bimetallic corrosion. GENERIC: a general class or group; generic paint types most often take their name from the kind of resin in the formulation alkyd, epoxy, etc. GLOSS: ability to reflect light; shininess; lustre. GROUND BED: buried or submerged conducting mass which is connected to the positive terminal of an independent source of DC current to pass Cathodic Protection Current into soil or water. GROUNDING: dissipation of electrical or electrostatic charge. HARD DRY: paint film that has reached the condition when the surface is not easily marked by physical contact. HEAT REACTIONS: Chemical reactions, usually taking one of two forms: •
Exothermic Heat is generated (given out) during reaction, this takes place when curing agent is mixed with epoxy base.
•
Endothermic Heat is absorbed (taken in) during reaction, this takes place as solvents evaporate.
HIGH BUILD (PAINT): modified coatings with thixotropic qualities, which can be applied at a higher thickness, typically with single coat DFT in the range of 120 microns upwards. HOLIDAYS: areas left uncoated during painting operations; flaws detected by testing with holiday detector. HYDRO-BLASTING: water blasting; surface cleaning by the use of water pumped to extremely high pressures (5,000-20,000 p.s.i).
Page 106
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 107
Marine Corrision & Coatings
Module K
HYDROGEN EMBRITTLEMENT: A process resulting in a decrease of the toughness or ductility of a metal due to absorption of hydrogen. NOTE - Hydrogen embrittlement often accompanies hydrogen formation, for example by corrosion or electrolysis, and may lead to cracking. HYPODERMIC NEEDLE: a gauge inserted in to the blast hose GAUGE close to the nozzle to measure air pressure at the nozzle. IMPERMEABLE: not permitting the passage or penetration of moisture, air, vapour or other substance. IMPRESSED CURRENT PROTECTION: Electrochemical protection in which the protecting current is supplied by an external source of electric energy. INDUCTION TIME: the length of time that a mixed catalysed paint must stand before it is ready to apply; also called ‘setting up’ or ‘sweat-in’ time. IRON OXIDE: a combination of iron (Fe) and oxygen O2); rust is a common form of iron oxide. INHIBITIVE PIGMENTS: pigments mixed into priming paints that enable them to retard or prevent the corrosion of metals by chemical (electrochemical) means. Red lead, zinc phosphate and zinc chromate are examples of inhibitive pigments. INTERGRANULAR CORROSION: Corrosion in or adjacent to the grain boundaries of a metal. LOCALISED CORROSION: Corrosion at discrete sites of the metal surface exposed to the corrosive environment. NOTE - Localised corrosion may result in, for example, pits, cracks, grooves, etc. MEDIUM: in paint the continuous phase in which pigment is dispersed. When applied to liquid paint, the term includes the solvent and is synonymous with ‘vehicle’: when applied to the dry film it is synonymous with ‘binder’. MICROBIAL CORROSION: Corrosion associated with the action of micro-organisms present in the corrosion system. MICROCLIMATE: the collective physical conditions, such as temperature and humidity, in a contained, specific environment MIL: one onethousandth of an inch; .001”; 1/000 inch. MILD STEEL: steel with carbon content not exceeding 0.25%, most commonly used for structural fabrication. MILLSCALE: layer of iron oxide, bluish in appearance, formed on the surface of steel while still hot during manufacture. MIST-COAT: a heavily thinned coat of paint applied in a thin film to ‘reflow’ the previous coat of paint; also used on porous films such as inorganic zinc to seal porosity prior to over-coating. ORIFICE: opening or hole, as in a spray gun fluid tip. OLEORESINOUS: generally refers to varnishes composed of vegetable drying oils in conjunction with hardening resins. OVERSPRAY: fluid that is lost by missing the surface to be painted. PASS: motion of the spray gun in one direction only; one stroke. PASSIVATION: Decrease of corrosion rate by the formation of a corrosion product on the metal surface.
Page 107
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:13
Page 108
Marine Corrision & Coatings
PEEN (PEIN): to beat a surface with a rounded tool, leaving rounded indentations; in blast-cleaning refers to the results of shot-blasting, providing a rounded anchor pattern. PERMEABLE: allowing passage or penetration. pH: index of the acidity or alkalinity of aqueous solution, using a scale of 0 to 14. A value of 7 towards 0 denotes increasing acidity; values ranging from 7 to 14 denote increasing alkalinity. PHENOLIC RESINS: Phenolic resins are the products of condensation reactions between phenols and aliphatic aldehydes, usually formaldehyde. Used as paint resins, Phenolics provide excellent chemical resistance, and are widely used for protection of inside of chemical containing vessels. Widely used as ‘stoving’ paints PHENOLIC (PAINT): a paint containing as binder a phenolic resin, either alone or modified by combination with for example, drying oils, alkyd resins or epoxide resins. PIGMENT: insoluble solid particles dispersed in a paint, which give the dried film its characteristic properties of colour, gloss and opacity and influences durability in various environments. PITCH: black or darkcoloured residue that results from the distillation of tar and similar materials. PITTING CORROSION: Corrosion resulting in pits, i.e. cavities extending from the surface into the metal. PLASTICISER: nonvolatile substance, incorporated with film forming materials in a paint, or lacquer, to improve the flexibility of the dried film. POT LIFE: the period after mixing, during which a twopack paint remains usable and fully effective. POLYURETHANE (RESIN): polymer used in coatings for its high gloss and colour retention. Often used with di-isocyanate or a poly-isocyanate curing agent, use is restricted in some cases by concern over the carcinogenic effects of the curing agents. POLY-VINYL-CHLORIDE (PVC): a colourless thermoplastic material composed of polymers of vinyl chloride. POTABLE WATER: drinkable water. PRE-FABRICATION PRIMER: fast-drying primer applied to steel prior to fabrication, which will not emit toxic fumes when welded and does not have to be removed prior to welding. Although intended only to provide temporary protection it should be compatible with the permanent protective systems. PRIMER: the first complete coat of a painting system, often using inhibitive or sacrificial pigments to actively combat corrosion. PROFILE: surface contour of a blastcleaned surface as viewed from the edge; improves the ability of the paint to bond to the surface. PROTECTIVE COATING: Layer(s) of material applied to a metal surface to provide corrosion protection. p.s.i: pounds per square inch; a measure of force, such as the air pressure at the blast nozzle. PSYCHROMETER: instrument with wet and dry bulb thermometers for measuring relative humidity and dewpoint; also called a hygrometer. RELATIVE HUMIDITY: amount of water vapour in air, compared to the maximum quantity the air could hold if saturated (at same temperature and atmospheric pressure), expressed as a percentage. RESIN: major ingredient of paint which binds the other ingredients together and creates the protective film; usually a polymer.
Page 108
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 109
Marine Corrision & Coatings
Module K
RESPIRATOR: a device worn over the mouth and nose to prevent inhaling harmful substances such as dust, solvent vapours, fumes, etc. Respirators may be cartridge, filter, or aired. RUST: Visible corrosion products consisting mainly of hydrated iron oxides. SAPONIFICATION: decomposition of an organic compound by reaction with alkali - typically affects alkyd coatings in alkaline exposure. SCALE: Solid layer of corrosion products formed on a metal at high temperature. NOTE -The term scale is also used in some countries for deposits of hardness salt from water. SHELF LIFE: period that a paint will stay in good condition in storage. SHERARDISING: process of coating iron or steel with zinc by diffusion. The material is packed in zinc dust and heated at about 3700C (7000C) SILICONE RESIN: a class of synthetic resin produced from organic compounds containing silicone, generally used as a binder for coatings used in high temperature applications. SILICOSIS: type of respiratory disease caused by ‘free’ silica particles lodged in the lungs. SIMULATED CORROSION TEST: Corrosion test conducted under simulated service conditions. SKIDDING: a paint roller sliding across a surface leaving roller tracks; caused by too little or too much paint on the roller cover. SOLIDS (IN PAINT): the nonvolatile components in a coating formulation that, after drying, constitute the dry film. see also Volume Solids. SOLVENTLESS PAINTS: term used to describe organic coating materials which contain little or no volatile thinner. SOLVENTS: liquids which are used in the manufacture of paint to dissolve or disperse the film forming constituents, but which mostly evaporate during drying. Also used to modify viscosity and regulate application properties. SPRAY FAN: shape of the spray pattern. SPRAY PATTERN: description of the shape and size of the paint mist when it strikes the surface; varies from a circle to a long narrow oval. SPREADING RATE: surface area covered per unit volume of paint applied. May be used to calculate paint requirements, and depends on volume solids and film thickness applied STRAY-CURRENT: Current flowing through paths other than the intended circuits (see ISO 2080 : 1981). SUBSTRATE: surface to be painted. SURFACE PROFILE: surface contour of a blast cleaned surface improves the ability of the paint to bond to the surface. see Anchor Pattern STAINLESS STEEL: a steel containing sufficient chromium, or chromium and nickel, to render it highly resistant to corrosion and give a characteristic bright, shiny finish. Prevents corrosion by formation of an ultra-thin surface layer of oxide STRESS CORROSION: A process involving conjoint corrosion and straining of the metal due to residual or applied stresses. STRESS CORROSION CRACKING: Cracking due to stress corrosion.
Page 109
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 110
Marine Corrision & Coatings
STOVING: the process of curing paint film by heating, usually at temperatures over 65°C (150?F) in an oven or by exposure to radiant heat. SURFACE PREPARATION: cleaning a surface prior to painting; all operations necessary to prepare a surface to receive a paint coating. TEMPORARY PROTECTIVES: materials used to protect metallic surfaces during fabrication, transport and storage, and which are easily removable, by stripping or cold application of common solvents. The significance of the term ‘temporary’ lies in the easy removal of the protective and not in the duration of its protection efficiency. THERMIT REACTION: Explosive effect which occurs when aluminium and iron oxide are heated. Aluminium pigmented paints associated with rusty steel are susceptible to the effect - leading to potential problems in hazardous environments such as live gas, fuel oils, etc. THINNER: volatile liquid which may be added to paints and varnishes to facilitate application or aid penetration by lowering the viscosity. Thinners should only be added by the user in limited quantities and in accordance with the manufacturer’s instructions. THIXOTROPIC (PAINT): materials with modified viscosity; paint with a modified (high) viscosity or ‘body’ that undergoes a temporary reduction in consistency during application. This allows the paint to be spread easily but to resist the formation of runs, drips or sags when heavy films are applied; may be called ‘nondrip’. TIE-COAT: low viscosity paint materials, applied to aid adhesion; used with zinc silicate primers to seal the porosity and provide a base for application of further coatings. TOXIC: poisonous. TWO PACK PAINT: paint for which the materials are supplied in two parts, usually in the correct relative proportions. UNDERCOAT PAINT: the coat or coats applied to a surface after primer, filling, etc., or after the preparation of a previously painted surface, and before the application of a finishing coat. An undercoat should possess good covering ability and a colour leading up to that of the finishing coat, and should be compatible with the other paints in the system. UNDERGROUND CORROSION: Corrosion of buried metals, soil being the corrosive environment. NOTE The term soil includes not only the naturally occurring material but also any other material, ballast, backfill, etc., used to cover a structure. UNIFORM CORROSION: Corrosion proceeding at almost the same rate over the whole surface of the metal exposed to the corrosive environment. VEHICLE: liquid portion of paint in which the pigment is dispersed; it is composed of binder, solvent (thinners) and other liquid additives such as plasticizers. VENTURI NOZZLE: nozzle with a tapered lining shape having a ‘waist’- it accelerates through-flow, increases abrasive speed and creates a more consistant blast pattern. VINYL RESIN: class of synthetic resin formed by the polymerisation of chemical compounds containing the vinyl group; used to make convertible coatings. VISCOSITY: internal resistance to flow possessed by a liquid; measure of the shear strength within a liquid; liquids with high viscosity have low flow rates and appear to be ‘thick’. VOLUME SOLIDS: percentage of the total volume of paint occupied by nonvolatiles (paint solids).
Page 110
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 111
Marine Corrision & Coatings
Module K
WEATHERING: the deliberate exposure of new steelwork to weather, allowing the corrosion process to ‘release’ millscale and other contaminants prior to painting - now rarely used as a form of surface preparation. WEATHERING STEEL: A low-alloy structural steel which develops a rust layer with improved protective properties, when freely exposed in certain natural atmospheres. WELD FLUX: acidic fluxing (flowing) agent used to aid the welding process; thought to be responsible for early coating failure on the welds, through failure to remove all traces. WELD SPATTER: beads of metal scattered next to a weld seam by the welding process. WHITE SPIRIT: commonly used thinner for alkyd or oleo-resinous paints and varnishes. It consists of straightrun or blended petroleum hydrocarbons. ZINCRICH PAINT: priming paint in which the predominant pigment is finely divided metallic zinc, often having a total zinc content up to 95% by weight. ZINC SILICATE: paint in which the predominant pigment is metallic zinc, and the binder is inorganic silicate.
Page 111
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
Module K
11:13
Page 112
Marine Corrision & Coatings
APPENDIX B Marine Terminology and Construction Naval Vessels Harbour craft Tugs Rope Handling Pilot Cutters Harbour masters Bunker Barges Water Barges River Barges Naval Pinnaces – Admirals Barge Harbour Ferries Offshore Supply Vessels Standby Boats Lifeboats Ferries Anchor Handling Heavy Lift Cranes Pipelay Barges Patrol Craft Military Protection – Fast Craft and Patrol Coastal Cargo Chemicals Bulk Tankers Containers Multi Purpose RoRo Ferries – Passenger, Freight, Railway. Minesweepers FPB’s Fishery Protection RFA Deep Sea Cargo Chemicals Bulk Tankers Containers Multi Purpose Ro-Ro Container Lash Frigates, Destroyers, Corvettes, Aircraft Carriers Submarines etc.
Page 112
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 113
Marine Corrision & Coatings
Module K
Classification Inspection Intervals Usually in 5 year cycles from new 1st Special Survey – 5 yrs 2nd Special Survey- 10 yrs 3rd Special Survey –15 rs 4th Special Survey- 20 yrs 5th Special Survey 25yrs If conditions warrant a “Poor “- Classification an annual survey and restrictions imposed
New Building Issues Access Production Fabrication Environmental Issues Post Construction Modifications New World Construction New Technology Use of Alloys Maintenance Factors Drydocking -Operational Constraints Charter parties Environment Compatibility Time Engineering Pressures Regional Location
En Passage Conditional Restraints Environment & Weather Accommodation Restrictions Supplies Cost Safety and Vessel Operation
Offshore Construction and Operation Lack of bed space Weather logistics Safety/survival qualifications Interaction with other trades Operations Safety
Page 113
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 114
Module K
Marine Corrision & Coatings
TUTOR MARKED ASSIGNMENT You have a 25-year-old General Cargo Vessel 15000 tons DWT that trades up the Orinoco River and the Caribbean with at least two voyages annually across the Atlantic to Europe. The vessel is fitted with a sacrificial anode system of cathodic protection. The vessel is expected to stay in service for a further five years before either being sold or scrapped. The vessel will dry dock in Hamburg after three years of trading. You are responsible for inspecting he outer hull as soon as dry dock is drained. What would you look for and report to the owners on condition? What would be your approach to recommending the surface preparation and coating of the immersed areas of the hull?
Participants taking the full Diploma option and Certificate must complete the tutor marked assignments for each module. Each tutor marked assignments will attract 5 marks, giving a total of 30 marks. This leaves 70 marks for the examination A Pass is 75% A Merit is 85% A Distinction is 95%. Each assignment must reach a standard. Tutors may ask for you to re-submit an assignment if this standard is not met. The assignments must be typed and NOT hand written. It is preferred that they are sent in hard copy. In this way the presentation of the answers can be properly assessed. The tutor may accept fax or email if this is the most expedient method due to logistic/geographical problems. But remember if the tutor is not satisfied with the Assignment he may ask for this to be submitted in hard copy format. It is NOT necessary to submit tutor marked assignments in strict order. Although it is easier for the participant to co-ordinate their work if they are done in sequence. The main objective must be to complete ALL assignments BEFORE sitting the examination. ALL tutor marked assignments must be sent to the following :Diploma in Marine Surveying Course Administrator. c/o International Institute of Marine Surveying. The Administration Office Stone Lane, Gosport, Hampshire PO12 1SS UK. Fax: +44 (0) 2392 588 002 E-mail
[email protected] The participant will receive a TMA result sheet with comments as appropriate from the tutor and /or course director. This will be sent by ordinary mail.
Page 114
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 115
Module K
Marine Corrision & Coatings
TM
of coating defects
“
Fitz’s Atlas has been compiled by coating and corrosion specialists who understand coatings and their application. It provides a comprehensive
“
and invaluable visual
reference for anybody
who uses or encounters paint coatings, their
defects and failures. I I I I I I I I
welding faults pre-surface conditions surface preparation standards dry abrasive blasting high pressure water-jetting coating and application defects marine fouling formulae and reference charts
View sample pages at
www.fitzsatlas.com Published by: MPI Group Peel House, Upper South View, Farnham, Surrey, GU9 7JN
Tel: 01252 732220 Fax: +44 (0)1252 732221
Fitz’s Atlas of Coating Defects: Fitzsimons, Weatherhead and Morgan ISBN 0 9513940 2 9
Page 115
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Module K
20/04/2009
11:13
Page 116
Marine Corrision & Coatings
Page 116
Diploma in Marine Industry Surveying
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 117
ICCP HULL CORROSION PROTECTION SYSTEMS
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 118
C-Shield ICCP systems for vessels of every type C-Shield impressed current cathodic protection systems (ICCP) have now been fitted to more than 4,000 vessels around the world. Since its introduction by Cathelco Ltd in 1991, the scope of the system has been developed to include the option of modular or thyristor control panels and an unusually wide range of anode designs to suit vessels of every type:● Cruise Ships ● Container Vessels ● VLCCs ● Ferries ● FPSOs ● Ice Class Vessels The C-Shield modular control panel, based on state-of-the-art computerised electronics, remains at the forefront of ICCP technology. As a result it has been widely adopted by operators seeking to protect the hulls of their vessels against corrosion by the most effective and economical methods. In addition to innovative ICCP product design, Cathelco offers a comprehensive background of experience in marine engineering and equipment manufacture. The company is a world leader in pipework anti-fouling and corrosion suppression systems for ships and offers a wide range of other services through its Group members. Corrintec Ltd, produce ICCP systems for naval vessels, fast ferries and other high speed craft including highly advanced waterjet and bow thruster protection equipment. They have also developed the Minitek ICCP system which provides effective protection for smaller craft. Beyond this, the Group’s interests
encompass marine electronics and environmental services for ships. Together, this provides an exceptionally strong combination of expertise which can be applied to customers particular requirements. Around the world, Cathelco is represented by a network of over 50 agents/installers who can provide technical advice and immediate access to the Group’s complete range of services.
The Problem of Corrosion Although modern hull coatings can provide some protection against corrosion they seldom offer a complete solution. For this reason, most operators choose to protect their vessels with a purpose designed impressed current cathodic protection system. Using an arrangement of hull mounted anodes and reference cells connected to a control panel(s), the system produces a more powerful external current to suppress the natural electro-chemical activity on the wetted surface of the hull. This eliminates the formation of aggressive corrosion cells on the surface of plates and avoids the problems which can exist where dissimilar metals are introduced through welding or brought into proximity by other components such as propellers. An essential feature of ICCP systems is that they constantly monitor the electrical potential at the seawater/hull interface and carefully adjust the output to the anodes in relation to this. Therefore, the system is much more effective and reliable than sacrificial anode systems where the level of protection is unknown and uncontrollable.
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 119
Thyristor Controlled Cathelco have taken proven thyristor controlled technology and added another dimension by combining it with the latest computerised information systems. This provides a cost effective solution to the power requirements of the largest ICCP systems. The thyristor control panels can be used for systems of up to 800 amps on vessels such as cruise ships and VLCCs, as well as the larger types of container ships and cargo vessels. As with the modular units, there are clear digital output displays and data regarding the performance of the system can be fed directly to ship’s computers. ● For ICCP systems of up to 800 amps. ● Economical control for largest systems. ● Combines computerised output displays, alarms and information systems. ● Can be linked to ship’s computers.
A choice of control panels for high performance and reliability The C-Shield modular control panel, based on advanced computerised electronics, is designed for installations of up to 450 amps. As a breakthrough in ICCP technology, it is extremely lightweight and compact in design. It can be easily installed in an engine room and requires the minimum of attention from the crew. The outputs to the anodes can be instantly checked by referring to the digital read outs. One of the major advantages of the unit is that the modules are interchangeable and can be quickly removed and replaced if necessary, thereby offering greater reliability. The system can incorporate a variety of safety features including ‘under’ and ‘over’ protection alarms. In addition,
information regarding the status of the system can be fed direct to bridge computers for continuous monitoring purposes.
Modular ● For ICCP systems of up to 450 amps. ● Modular design for greater reliability and flexibility. ● Extremely lightweight and compact. ● Clear digital output displays. ● Incorporates ‘under’ and ‘over’ protection alarms. ● Can be linked to bridge information systems.
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 120
Impressed Current Cathodic Protection Systems
RUDDER BONDING
LINEAR LOOP ANODE
REMOTE MONITORING UNIT
SHAFT EARTHING ASSEMBLY CONTROL PANEL
Anodes to suit a wider range of hull profiles
Linear anodes Provide excellent current distribution on larger vessels where anode weight is not a major consideration.
Elliptical Anodes The elliptical shape enhances current distribution. Provides the flexibility to fit into complex hull profiles.
Circular Anodes Ideally suited to vessels where a smooth hull profile is required. Can be flush mounted in areas where space is limited.
a
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 121
Shaft earthing and rudder bonding equipment
Technologically advanced systems to protect your vessels against hull corrosion now and into the future Cathelco have developed an extensive range of anodes to suit vessels of every type. Recessed anodes have the advantage of maintaining a smooth hull profile with minimal drag and are therefore ideal for ferries and other fast commercial craft as well as vessels trading in icy waters. The introduction of linear loop anodes with powerful outputs mean that large vessels can be protected with a smaller number of anodes, requiring fewer hull penetrations. Their lightweight, semiflexible design also enables them to be fitted more easily to hull profiles. All of the anodes are supplied complete REFERENCE CELL with mountings and hull penetration fittings.
CONTROL PANEL
REFERENCE CELL
To bring the propeller and rudder within the scope of the C-Shield system they must, at all times, be electrically connected to the hull. Once the propeller rotates, electrical contact is lost and for this reason a slipring with brush gear is fitted to the shaft. To monitor performance the unit is provided as standard with its own self contained indicator. Bonding kits for the rudder consist of lugged flexible cables for attachment between the top of the rudders stock and the hull. A similar cable kit is also supplied for bonding the control panel negative connection to the hull.
Reference Cells
ELLIPTICAL ANODE
and a operating conditions Diver Change Anodes
Linear Loop Anodes Produce a powerful output from a relatively small surface area. Lightweight and easy to install.
These anodes were specifically developed for vessels such as FPSOs which have long intervals between scheduled drydocking. They can be changed from the outside of the hull by a diver, simplifying replacement. This is achieved by using an installation ‘insert’ which is embodied in the anode resin during manufacture and takes the place of conventional wiring. In addition, a watertight sealing plate and sealing ring system has been designed to enable the transfer to take place easily.
These are essential to measure the electrical ‘potential’ at the seawater/hull interface and provide the optimum degree of protection. The readings are fed back to the control panel which automatically adjusts the current output to the anodes. Made with elements of silver or zinc, reference cells are generally recessed and supplied with mountings and hull penetration fittings to make the throughhull cabling watertight.
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 122
C-Shield – the savings Over a ten year period, significant capital savings result from the use of C-Shield impressed current cathodic protection. Sacrificial anodes have to be renewed at periodic drydocking intervals as they become consumed, resulting in on-going replacement costs. In contrast, C-Shield anodes last for many years and achieve a much more reliable level of protection, without the extra weight or drag which is inevitably associated with sacrificial methods. In addition, major operational savings result from the use of C-Shield. A smooth hull, free from corrosion, ensures the lowest fuel consumption for the vessel. Data has shown that unprotected vessels, after as little as 2.5 years, can require an additional 30% increase in shaft power to maintain service speed. Throughout the design of the C-Shield system, every care has been taken to ensure the highest reliability commensurate with minimal installation time. The lowest cost system may leave the installer with the highest work load.
Capital Cost Comparison Equipment Installed
100
.00
0
00
50.0
00
25.0
00
AC BS
10 P CC
BI
AC AS P CC
AI
Cost in U.S. Dollars
75.0
Yea r
s
8Y ear
s
6Y ear
s
4Y ear
s
2Y ear
s
Notes 1. Ship Types:
A - 25,000 dwt General Cargo, B - 150,000 dwt Bulk Carrier 2. Design Basis: Sacrificial 15mA/m*m. Impressed 25mA/m*m 3. Escalation: Material & Labour 7% pa compound
Easier Installation The installation of C-Shield systems follows well established principles which have been approved by all of the leading classification societies. Depending on the type of anodes being fitted, work begins by cutting holes in the hull to accommodate bosses or doubler plates. These are welded in position ready to receive cofferdams, enabling watertight electrical connections to be made to the anode. For surface mounted anodes, a mounting plate or mounting studs are welded to the exterior hull surface in preparation for fitting the anode. At this stage, the surrounding area is carefully grit blasted to a white metal finish. Next, a layer of mastic is applied to the anode mounting surface in order to create a watertight seal when the anode is fitted and the studs are tightened. Finally, mastic is applied to the area surrounding the anode to produce a di-electric shield which is essential to ensuring the maximum spread of current over the hull surface.
Combined Systems Comparison of Additional Fuel Required to Maintain Service Speed
500
.00
0
.00
0
AC
AC AS P
CC
AI
Notes 1. Ship Types:
.00
10
P CC
BI
0
250 .00 125
BS
Cost in U.S. Dollars
375
0
Yea r
s
8Y ear
s
6Y ear
s
4Y ear
Cathelco can combine C-Shield with their marine pipework anti-fouling and corrosion suppression system to provide a complete protection package for vessels. Cathelco anti-fouling systems are based on the electrolytic principle and eliminate blockages in pipes, valves and condensers caused by the growth of barnacles and mussels.
Minitek ICCP Systems
s
2Y ear
s
A - 25,000 dwt General Cargo, B - 150,000 dwt Bulk Carrier 2. Costings based on data for increased drag contained in BS CP1021 3. Escalation: 4% pa compound
The Minitek system has been designed by Corrintec Ltd to provide effective and economical protection for smaller craft such as fishing boats, work boats and
luxury yachts. With an extremely compact control panel and anodes, Minitek is completely automatic and significantly reduces maintenance costs in comparison with sacrificial anode systems.
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
20/04/2009
11:13
Page 123
A system specifically designed to protect your ship C-Shield systems are specifically designed for the particular requirements of individual vessels and their operating conditions. They can be installed at newbuilding or retrofitted to existing tonnage during scheduled drydocking.
engineers. Reports with comments are then mailed back to the operator.
With comprehensive experience of ICCP system design and the factors which influence hull corrosion, Cathelco’s engineers will formulate the most effective arrangement of hull anodes, reference cells and control panel(s). An important part of the service is the preparation of detailed installation drawings and instructions covering every aspect of the system.
Comprehensive Quality Approvals
All systems are provided with comprehensive manuals as part of the standard package. These include sections on theory, installation, operation, maintenance, fault finding, drawings and spare part listing.
Worldwide Service Cathelco engineers based world-wide are available to assist the system installer, commission the equipment and provide on-site instruction for operating staff. To relieve operators of the need to evaluate the performance of the system, Cathelco offer a monthly review service. Log sheets with daily readings completed by the ship’s staff are analysed by our
Specialised engineers are available to attend service callouts as well as routine drydocking.
The manufacture of C-Shield systems conforms to the highest standards of quality. Cathelco Ltd has been assessed by Lloyd’s Register Quality Assurance and is registered under BS EN ISO 9002 (1994). The on-going commitment to these standards is verified at regular intervals and is reflected by the commercial success of the company in meeting the quality requirements of some of the world’s most prestigious fleets. The C-Shield system fully complies with all British and International Standards. In particular BS 7361: Part 1:1991 Cathodic Protection, and is approved by all leading ship classification societies.
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd
Other products from Cathelco Marine pipework anti-fouling systems The Cathelco marine pipework anti-fouling and corrosion suppression system has been fitted to over 6,000 ships ranging from cruise ships and container vessels to ferries and fishing craft. Recognised as world leaders in this field with more installations than any other individual manufacturer, Cathelco have continued to develop and enhance the system. Based on the electrolytic principle, it eliminates blockages in seawater cooling lines caused by the growth of barnacles and mussels. Easy to install, completely safe in operation and requiring minimum attention from the crew, the system provides complete and continuous protection. To cover the requirements of smaller vessels, luxury yachts and leisure craft, Cathelco have now produced miniaturised versions of the system with extremely compact control panels and anodes. These enable vessels of any size to benefit from effective anti-fouling protection at a very economical cost.
Replacement Anode Service C-Shield anodes can be used as direct replacements for any impressed current cathodic protection system currently used in the world. They are also designed to be totally compatible with the full range of manufacturers control systems. The service enables operators to benefit from the latest developments in anode technology, whilst providing considerable cost savings in comparison with replacement using conventional designs from the original equipment manufacturer.
20/04/2009
11:13
Page 124
International Service Network Our international network of sales and service centres can provide immediate advice and assistance on the complete range of Cathelco products. Argentina Australia Bahrain Brazil Brunei Canada (East and West Coast) China (Dalian, Guangzhou, Shanghai Hong Kong) Croatia Denmark Egypt Estonia Finland France (Atlantic & Mediterranean Coasts) Germany Greece Holland Iceland India Indonesia Iran Ireland Israel Italy Japan
Korea Latvia Lithuania Malaysia Mexico Norway Pakistan Philippines Poland (Gdansk & Szczecin) Portugal Romania Russia Singapore South Africa (Durban & Cape Town) Spain Sri Lanka Sweden Taiwan (Kaohsiung & Taipei) Turkey United Arab Emirates USA (East, West & Gulf Coasts) Vietnam Venezuela
Cathelco Group Companies Corrintec Ltd Specialists in ICCP hull protection systems for naval and high speed ships. Subsea pipeline cathodic protection surveys. Marine Equipment A/S Design and installation of monitoring and alarm systems for ships ballast tanks and cargo tanks. MTB Environmental Ltd Bilge, ballast and storage tank cleaning services for naval and commercial vessels. Environmental services for petrochemical and other industries. Proconics Ltd Design and production of electronic control systems for marine and general applications. Casting Repairs Ltd Specialists in the repair of cast ironwork using the cold metal stitching technique.
Marine House, 18 Hipper Street South Chesterfield S40 1SS United Kingdom Telephone 01246 207702 Fax 01246 206519
International Telephone +44 1246 207702 Fax +44 1246 206519 E-mail:
[email protected] Web site: http://www.cathelco.co.uk
**** NOA 10F
Buff
Buff Grey
Dark Grey Grey
Stripe coating is an essential part of good painting practice and should be carried out on plate edges, welds, behind bars, cut outs etc. Consult International for details of stripe coating requirements. ** Intergard 343 is currently available in Korea. *** Intergard 840 is currently available in Europe. **** NOA 10F is currently available in Japan. † ISO 3233:1998 (ASTM D-2697 - North America).
*
Tar free epoxy anticorrosive incorporating unique self-indicating (SI) technology
High solids tar free epoxy anticorrosive
Two pack epoxy anticorrosive containing micaceous iron oxide
Intergard 400 Intergard 400
Bronze Grey
Bronze Aluminium
Colour
63
82 82
65 65
68 68
60 60
VS% †
6.90 6.90
175
11.82
300 175
5.91 5.91
9.84
250 150 150
4.92 4.92
11.82
300 125 125
5.91 5.91
11.82
300 150 150
5.91 5.91
DFT (mils) 150 150
DFT (μm)
11:13
Intergard 840 Intergard 840
Tar free, aluminium pigmented pure epoxy anticorrosive
Intergard 343 Intergard 343
Light coloured, abrasion resistant aluminium pure epoxy offering excellent long term corrosion protection
Description
20/04/2009
** **
*** ***
Intershield 300 Intershield 300
Product Name
Typical Schemes – Cargo Holds*
674 Marine Mod K 08-09:674 Marine Mod K 07-08.qxd Page 125
– 60 months (application at pre-delivery drydock – consult International for detailed working procedures)
*Intersleek 700 System
SPC Antifouling Intersleek 717 Intersleek 737 Intersleek 757
†
* **
Depending upon specification and in service conditions. Use Intergard 264 in North America. ISO 3233:1998 (ASTM D-2697 - North America).
Two pack foul release system Linkcoat Silicone elastomer foul release system especially designed for deep sea scheduled ships
– 60 months (application prior to launch – consult International for detailed working procedures) Light coloured, abrasion resistant aluminium pure epoxy offering excellent long term corrosion protection Silicone elastomer foul release system especially designed for deep sea scheduled ships
*Intersleek 700 System
Mid Brown Light Grey Grey
54 57 72
60 60 57 72
40
Dark Red
Bronze Aluminium Light Grey Grey
40
Dark Brown
40
Dark Red
3.94 3.94 5.91 13.79
350
19.69
500
100 100 150
4.92 4.92 3.94 5.91
9.44
240
125 125 100 150
4.72
120
4.72
10.62
270 120
5.31
5.31
DFT (mils)
135
135
DFT (μm)
11:13
300 300 737 757
– 36 months
