Basic Corrosion CP

Basic Corrosion & Cathodic Protection

Basic Corrosion and Cathodic Protection Jeff Schramuk NACE CP Specialist #7695  www.cpsolutionsinc.net

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Topics to be Covered

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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Topics to be Covered

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

2

Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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Effects of Infrastructure Corrosion

Life Safety

Economics

Regulatory Compliance

Environmental

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Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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Corrosion Can be Defined as:

Practical Definition Scientific Definition

The Tendency of a Metal to Revert to its Native State Electrochemical Degradation of Metal as a Result of a Reaction with its Environment 6

Corrosion - A Natural Process

IRON OXIDE

IRON

REFINING

CORROSION

MILLING

IRON OXIDE

7

Four Basic Parts of a Corrosion Cell

Anode – A metal electrode in contact with the electrolyte which corrodes Cathode - A metal electrode in contact with the electrolyte which is protected against corrosion Electrolyte – A solution or conducting medium such as soil, water or concrete which contains oxygen and dissolved chemicals Metal Path – An external circuit that connects the anode and the cathode 8

Electron Flow vs. Conventional Current

Flow of conventional current is from positive (+) to negative (-) Conventional current flow from (+) to (-) will be from the cathode to the anode in the metal path Conventional current flow from (+) to (-) will be from the anode to the cathode in the electrolyte.

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Definitions - Anodes & Cathodes

Cathodic Area (Protected)

DC Current

Anodic Area (Metal Loss) 10

The Simplified Corrosion Cell

1. Anode 2. Cathode 3. Electrolyte 4. Metal Path

   V   m    0    V    0    2   m      0    t    0   a    2   r     e    t   p   a   p   o   r   e    C   p   p   o    C

   V   m    0    0    V    6     m    t   a    0    0    l    6   -  e   e    t    t   a    S    l   e   e    t    S

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Components of a Familiar Corrosion Cell

CARBON ROD (Cathode) ZINC CASE (Anode)

I I

NH4 and Cl- Paste (Electrolyte)

I

I WIRE (Metallic Path)

I

e12

Practical Galvanic Series*

    e     r     o      M   e   v    i    t   c    A     s     s     e      L

Material

Potential*

Pure Magnesium Magnesium Alloy Zinc Aluminum Alloy Mild Steel (New) Mild Steel (Old) Cast / Ductile Iron Stainless Steel Copper, Brass, Bronze Gold Carbon, Graphite, Coke

-1.75 -1.60 -1.10 -1.00 -0.70 -0.50 -0.50 -0.50 to + 0.10 -0.20 +0.20 +0.40

* Potentials With Respect to Saturated Cu-CuSO 4 Electrode 13

Corrosion Reaction and Ohm’s Law

Ohm’s Law States that: I =  ∆E/R where:  ∆E

= Driving Potential (EA minus EC)

EA = Anode Potential (measured in volts) EC = Cathode Potential (measured in volts) I = Current Flow (measured in amperes) R = Resistance (measured in ohms) 14

Some Common Electrical Quantities

Current Flow: 1 ampere (A) = 1000 milliamps (mA) Examples: A sacrificial anode’s output is measured in mA A CP rectifier’s output is can be up 100 A

Voltage: 1 volt (V) = 1000 millivolts (mV) Examples: A magnesium anode’s potential is ~1.6 V (1600 mV) A CP rectifier can have a DC voltage of up to 100 V15

Corrosion Cell - Anodic Reactions

I I    V   m    0    0    2      t   e   a    d   r   o   e    h   p    t   p   a   o    C    C

Fe++ OHFe++ OH-

   I

e-    V

  m    0    0    6      t   a    l   e   e    d   e    t   o   n    S    A

OHFe++ 16

Corrosion Cell - Cathodic Reactions

   I

   V e  me- H+    0

   0    2      t   e   ae   d   r   o   e    h   p    t   p   a   o    C    C

H+

I

   V   m    0    0    6      t   a    l   e   e    d   e    t   o   n    S    A

e- H+ e- H+

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Corrosion Cell – Combined Reactions

   I

   V e-

  m    0    0    2      t   e   a    d   r   o   e    h   p    t   p   a   o    C    C

e-    V

H2 H2

I H2 H2

  m    0 Fe2(OH)3   0    6      t   a    l   e   e    d   e    t   o   n    S    A

Fe2(OH)3 Fe2(OH)3 18

Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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General Corrosion

Corrosive environment is uniform around the structure Anode area is uniformly distributed over the structure Corrosion rate is usually constant over the structure

Environments where uniform attack can occur  Atmospheric, Aqueous, Concrete

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True Uniform Corrosive Attack

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Galvanic Corrosion

When two different metals are connected and placed into a corrosive environment. Corrosion current is proportional to the difference in electrochemical energy between the two metals Area Effect Avoid small anode connected to a large cathode

Distance Effect Area closest to anode will have the greatest corrosion 22

Practical Galvanic Series*

    e     r     o      M   e   v    i    t   c    A     s     s     e      L

Material

Potential*

Pure Magnesium Magnesium Alloy Zinc Aluminum Alloy Mild Steel (New) Mild Steel (Old) Cast / Ductile Iron Stainless Steel Copper, Brass, Bronze Gold Carbon, Graphite, Coke

-1.75 -1.60 -1.10 -1.00 -0.70 -0.50 -0.50 -0.50 to + 0.10 -0.20 +0.20 +0.40

* Potentials With Respect to Saturated Cu-CuSO 4 Electrode 23

Galvanic Corrosion Bimetallic Connection

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Old-New Pipe Corrosion Cell

Old Pipe (Cathode)

New Pipe (Anode)

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Steel in Concrete-Soil

Concrete Encasement

Note: Arrows Indicate Direction of DC Current Flow

Pipe in Soil Corrodes

Cathodic Zone

Anodic Zone 26

Dissimilar Surface Conditions

Pipe (Cathode)

Threads Bright Metal (Anode)

Scratches (Anode)

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Concentration Cell Corrosion

Due to differences in the environment

Differ Dif ferent ential ial Soil Soil Aerat Aeration ion – Ver Very y commo common n

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Differential Soil Aeration

O2

Clay soil

Aerated Soil Oxygen diffusing through backfill sustains corrosion to cathodic (top) area of pipe

Cathodic Zone

O2

Clay soil

Anodic Zone

Lack of oxygen at bottom of  pipe creates relative corrosion cell to (top) area of pipe

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Differential Aeration on Cast Iron Pipe

Cathodic Zone

Anodic Zone

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Differential Soil Aeration Pavement

Sandy Loam (well drained, high oxygen)

Cathode

Clay (moist low oxygen)

Anode

Sandy Loam (well drained, high oxygen)

Cathode

Factors contributing to an increased corrosive attack are de-icing salts and agricultural fertilizers31

Pitting Corrosion

Random and highly localized Depth greater than area of attack Most destructive form of corrosion Pit location and growth difficult to predict

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Pitting of Coated Carbon Steel in Soil

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External Pitting: Ductile Iron Water Main

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Selective Leaching Corrosion

Selective Leaching Graphitization (Gray Cast Iron) Dezincification (Brass)

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Dealloying Corrosion (Graphitization)

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Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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Eliminating the Corrosion Cell

  e    d   o    h    t   a    C

  e    d   o   n    A

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Apply a Bonded Tape Wrapping

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Pitting at a Coating Defect

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Coat the Structure & Electrically Isolate It

What’s Wrong Here? 41

Encase the Pipe in a “Corrosion Barrier”

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Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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How Cathodic Protection Works

Corrosion occurs where current discharges from metal to electrolyte The objective of cathodic protection is to force the entire surface to be cathodic to the environment.

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Galvanic Anode Cathodic Protection

Current is obtained from a metal of a higher  energy level.

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Practical Galvanic Series*

    e     r     o      M   e   v    i    t   c    A     s     s     e      L

Material

Potential*

Pure Magnesium Magnesium Alloy Zinc Aluminum Alloy Mild Steel (New) Mild Steel (Old) Cast / Ductile Iron Stainless Steel Copper, Brass, Bronze Gold Carbon, Graphite, Coke

-1.75 -1.60 -1.10 -1.00 -0.70 -0.50 -0.50 -0.50 to + 0.10 -0.20 +0.20 +0.40

* Potentials With Respect to Saturated Cu-CuSO 4 Electrode 46

Galvanic Corrosion – No C.P. Benefit

1. Anode 2. Cathode 3. Electrolyte 4. Metal Path

   V   m    0    0    2     r   e   p   p   o    C

   V   m    0    0    6      l   e   e    t    S

   V    7  .    1     m   u    i   s   e   n   g   a    M

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Galvanic Corrosion - Mitigated w/CP

1. Anode 2. Cathode 3. Electrolyte

  e    d   o    h    t   a    C

  e    d   o    h    t   a    C

  e    d   o   n    A

4. Metal Path

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CP Performance - Can Be Verified

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Sacrificial Anode on a Buried Pipeline

Grade

Sacrificial Anode

Connection to Pipe

Coating Defect

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Sacrificial Anode w/Test Station

Grade

Sacrificial Anode

Connection to Pipe

Coating Defect

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CP Test Station - Terminal Board

insulated terminal board

anode lead wire

calibrated shunt resistor 

structure lead wire

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Magnesium Anodes

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Packaged Magnesium Anode Natural Gas PL

Proper distance of anode from pipe At least 3’ from a coated pipe At least 6’ from bare steel At least 1’ deeper than pipeline Evaluate pipe coating

Install anode carefully – don’t lift by the lead wire Tamp earth firmly around anode package.

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Packaged Magnesium Anode Natural Gas PL (cont.)

Leave slack in the anode lead wire Wet area thoroughly around anode Make a secure electrical connection to the pipe (e.g. exothermic weld) Repair pipe coating to match original Place test box where it is protected from damage and can be easily located Do not allow any foreign pipeline contacts.

Packaged Magnesium Anode Natural Gas PL (cont.)

*Detail courtesy of Midwest Energy Association

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Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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Galvanic Anode CP Advantages

No external AC power is required Effective utilization of protective current Simple and inexpensive to install on new underground structures Seldom cause stray DC interference Minimal maintenance requirements. 58

Galvanic Anode CP Limitations

Limited driving potential   ∆E = (Ea – Ec) Limited current output  I =  ∆E / Rt Large number of anodes will be required on bare or poorly coated structures Ineffective in high-resistivity soil environments  (Rt ). 59

Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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Surface (Horizontal) Anode System

Rectifier 

(-)

(+) Anode Groundbed

Pipeline (Structure) 61

Deep Anode (Vertical) Anode System

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Continuous Linear Anode System

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Impressed Current Transformer Rectifier 

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Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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Have you checked your rectifier lately?

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Monitoring Data for a CP Rectifier 

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Can you locate your test stations?

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Potential Profile Survey Technique

Voltmeter-Computer  Test Station Wire Dispenser & Distance Chainer 

Pipeline Reference Cells 69

Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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CP Test Equipment - Multi-Meters

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Multi-Meter Characteristics

Basic Functions Reads AC & DC Volts Reads Ohms (optional diode checker) Reads AC and DC Amps (be careful here!)

Performance Criteria Field rugged, water/drop resistant High input impedance (min. 20 M-Ω) 72

Test Equipment Quality Assurance

Perform pre-test operational checks in accordance with the manufacturer instructions Verify the battery strength (if so equipped) Initiate corrective action for equipment out of specification Have the equipment calibrated each year 

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Reference Electrode Basic Components

74

Reference Electrode - Maintenance

Periodically verify cell against a known standard Keep porous plug covered when not used Clean and refill the reference cell annually Clean copper rod with a non-metallic abrasive pad Replace w/fresh Cu/CuSO4 solution (½ full at all times) Some Cu/CuSO4 crystals should always remain in suspension Wash hands after using – Cu/CuSO4 solution is hazardous 75

P/S Potential Readings

Connect voltmeter to pipe and reference Ensure reference cell plug has good contact with moist soil – not pavement Place reference cell away from anodes Read P/S on DCV scale Record P/S reading using standard forms If polarity is positive, notify corrosion dept.

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Meter Connections

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Basic Corrosion & Cathodic Protection

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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DOT Standard – Part 192.463

Cathodic Protection Criteria -0.85 V (w/IR-drop consideration) -0.85 V Instant-Off  100 mV polarization decay Other criteria determined to be “appropriate” by regulatory authority

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NACE International – CP Criteria

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DOT Standard – Part 192.465

Monitoring of Cathodic Protection Potentials tested every 12 months at intervals not exceeding 15 months, or  10% per year to sample entire line every 10 years Rectifiers and critical bonds checked every 2 months at intervals not exceeding 2-1/2 months.

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Do We Have a Good Reading?

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