Basic Corrosion CP

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Jeff Schramuk Jeff Schramuk NACE CP Specialist #7695 NACE CP Specialist #7695 www.cpsolutionsinc.net www.cpsolutionsinc.net

Basic Corrosion

and

Cathodic Protection

Basic Corrosion & Cathodic Protection

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Topics to be Covered

Why Should We Be Concerned about Corrosion? Why Should We Be Concerned about Corrosion? Definitions and Terminology

Definitions and Terminology Forms of Corrosion

Forms of Corrosion

Pipe Coatings and Cathodic Protection Pipe Coatings and Cathodic Protection

Cathodic Protection using Magnesium Anodes

Advantages & Limitations of Galvanic Anode CP Systems Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection

Impressed Current Cathodic Protection Measurement and Testing of CP Systems Measurement and Testing of CP Systems Field Test Equipment

Field Test Equipment

Cathodic Protection Criteria. Cathodic Protection Criteria.

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Topics to be Covered

Definitions and Terminology Forms of Corrosion

Forms of Corrosion

Pipe Coatings and Cathodic Protection Pipe Coatings and Cathodic Protection

Advantages & Limitations of Galvanic Anode CP Systems Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection

Impressed Current Cathodic Protection Measurement and Testing of CP Systems Measurement and Testing of CP Systems Field Test Equipment

Cathodic Protection Criteria. Cathodic Protection Criteria.

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Definitions and Terminology

Forms of Corrosion

Pipe Coatings and Cathodic Protection

Advantages & Limitations of Galvanic Anode CP Systems

Impressed Current Cathodic Protection

Measurement and Testing of CP Systems

Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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Effects of Infrastructure Corrosion

Life Safety Life Safety Regulatory Regulatory Compliance Compliance

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Why Should We Be Concerned about Corrosion? Definitions and Terminology

Forms of Corrosion

Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection

Measurement and Testing of CP Systems Field Test Equipment

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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

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Corrosion - A Natural Process

IRON OXIDE REFINING MILLING

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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

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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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Anodic Area

DC Current

Cathodic Area (Protected)

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C o p p e r a t - 2 0 0 m V S t e e l a t - 6 0 0 m V The Simplified Corrosion Cell

C o p p e r a t - 2 0 0 m V S t e e l a t - 6 0 0 m V 1. Anode 2. Cathode 3. Electrolyte 4. Metal Path

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Components of a Familiar Corrosion Cell CARBON ROD (Cathode) ZINC CASE (Anode) NH₄ and Cl-_Paste (Electrolyte) WIRE I I I I I e

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-Material

Potential*

Pure Magnesium -1.75

Magnesium Alloy -1.60

Zinc -1.10

Aluminum Alloy -1.00

Mild Steel (New) -0.70 Mild Steel (Old) -0.50 Cast / Ductile Iron -0.50 Stainless Steel -0.50 to + 0.10 Copper, Brass, Bronze -0.20

Gold +0.20

Carbon, Graphite, Coke +0.40

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

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Corrosion Reaction and Ohm’s Law

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

∆E = Driving Potential (E_A minus E_C)

E_A = Anode Potential (measured in volts) E_C = Cathode Potential (measured in volts) I = Current Flow (measured in amperes)

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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:

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Corrosion Cell - Anodic Reactions C o p p e r a t - 2 0 0 m V S t e e l a t - 6 0 0 m V C a t h o d e A n o d e I e-Fe++ Fe++ Fe++ OH -OH -OH -I I

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Corrosion Cell - Cathodic Reactions C o p p e r a t - 2 0 0 m V S t e e l a t - 6 0 0 m V C a t h o d e A n o d e I e-H+ H+ H+ H+ e-I

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Corrosion Cell – Combined Reactions C o p p e r a t - 2 0 0 m V S t e e l a t - 6 0 0 m V C a t h o d e A n o d e I e-H₂ H₂ e-H₂ H₂ Fe₂(OH)₃ Fe₂(OH)₃ Fe₂(OH)₃ I

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Forms of Corrosion

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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

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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

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Material

Potential*

Zinc -1.10

Gold +0.20

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

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Old Pipe (Cathode)

New Pipe (Anode)

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Steel in Concrete-Soil

Cathodic

Zone Anodic_Zone

Concrete Encasement

Pipe in Soil Corrodes

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Dissimilar Surface Conditions

Pipe

(Cathode) _Threads

Scratches (Anode)

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Concentration Cell Corrosion

Due to differences in the environment

Dif

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Aerated Soil Aerated Soil

Differential Soil Aeration

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

C

Cllaayy ssooiill CCllaayy ssooiill

Anodic Zone Anodic Zone Cathodic Zone Cathodic Zone O O₂₂ OO₂₂

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Differential Aeration on Cast Iron Pipe

Cathodic Zone

Anodic Zone

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Clay (moist Clay (moist low oxygen) low oxygen) Sandy Loam Sandy Loam (well drained, (well drained, high oxygen) high oxygen) A Annooddee CCaatthhooddee Cathode Cathode

Differential Soil Aeration

Pavement Pavement Sandy Loam Sandy Loam (well drained, (well drained, high oxygen) high oxygen)

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Pitting Corrosion

Random and highly localized

Depth greater than area of attack Most destructive form of corrosion

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Selective Leaching Corrosion

Selective Leaching

Graphitization (Gray Cast Iron) Dezincification (Brass)

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Forms of Corrosion

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Eliminating the Corrosion Cell A n o d e C a t h o d e

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Coat the Structure & Electrically Isolate It

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Forms of Corrosion

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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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Current is obtained from a metal of a higher

energy level.

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Material

Potential*

Zinc -1.10

Gold +0.20

* Potentials With Respect to Saturated Cu-CuSO₄ Electrode L e s s A c t i v e M o r e

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C o p p e r - 2 0 0 m V S t e e l - 6 0 0 m V M a g n e s i u m - 1 . 7 V

1. Anode

2. Cathode

3. Electrolyte

4. Metal Path

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C a t h o d e C a t h o d e A n o d e

1. Anode

2. Cathode

3. Electrolyte

4. Metal Path

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Sacrificial Anode on a Buried Pipeline Sacrificial Anode Coating Defect Connection to Pipe Grade

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Coating Defect

Connection to Pipe Grade

Sacrificial Anode

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CP Test Station - Terminal Board structure lead wire anode lead wire insulated

terminal board _calibrated shunt resistor

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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.

Packaged Magnesium Anode Natural Gas PL

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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

Packaged Magnesium Anode

Natural Gas PL (cont.)

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Packaged Magnesium Anode Natural Gas PL (cont.)

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Forms of Corrosion

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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.

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Limited driving potential

 ∆

E = (E

_a

– E

_c

)

Limited current output



I =

∆

E / R

_t

Large number of anodes will be required on

bare or poorly coated structures

Ineffective in high-resistivity soil

environments



(R

).

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Forms of Corrosion

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Rectifier

Anode

Groundbed ( - ) ( + )

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Forms of Corrosion

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Potential Profile Survey Technique

Test Station

Voltmeter-Computer Wire Dispenser & Distance Chainer

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Forms of Corrosion

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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)

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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 - 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

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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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Forms of Corrosion

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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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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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