Basic Corrosion and Cathodic Protection -...

Jeff SchramukNACE CP Specialist #7695

www.cpsolutionsinc.net

Basic Corrosion Basic Corrosion andand

Cathodic ProtectionCathodic Protection

Basic Corrosion & Cathodic Protection

Topics to be Covered

Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection Measurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.

Effects of Infrastructure Corrosion

Life Safety

Economics Environmental

Regulatory Compliance

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

Corrosion - A Natural Process

IRON OXIDE REFINING MILLING

IRON CORROSION IRON OXIDE

Four Basic Parts of a Corrosion Cell

Anode – A metal electrode in contact with the electrolyte which corrodesCathode - A metal electrode in contact with the electrolyte which is protected against corrosionElectrolyte – A solution or conducting medium such as soil, water or concrete which contains oxygen and dissolved chemicalsMetal Path – An external circuit that connects the anode and the cathode

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.

Anodic Area(Metal Loss)

DC Current

Cathodic Area

(Protected)

Definitions - Anodes & Cathodes

The Simplified Corrosion Cell

1. Anode

2. Cathode

3. Electrolyte

4. Metal Path

Components of a Familiar Corrosion Cell

CARBON ROD(Cathode)

ZINC CASE(Anode)

NH4 and Cl- Paste(Electrolyte)

WIRE(Metallic Path)

Material Potential*Pure Magnesium -1.75Magnesium Alloy -1.60Zinc -1.10Aluminum Alloy -1.00Mild Steel (New) -0.70Mild Steel (Old) -0.50Cast / Ductile Iron -0.50Stainless Steel -0.50 to + 0.10Copper, Brass, Bronze -0.20Gold +0.20Carbon, Graphite, Coke +0.40* Potentials With Respect to Saturated Cu-CuSO4 Electrode

Practical Galvanic Series*

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)

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 V

Corrosion Cell - Anodic Reactions

e-Fe++

Corrosion Cell - Cathodic Reactions

Corrosion Cell – Combined Reactions

Fe2(OH)3

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

True Uniform Corrosive Attack

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 metalsArea Effect

Avoid small anode connected to a large cathode Distance Effect

Area closest to anode will have the greatest corrosion

Galvanic Corrosion Bimetallic Connection

Old Pipe (Cathode)

New Pipe (Anode)

Old-New Pipe Corrosion Cell

Steel in Concrete-Soil

Cathodic Zone

Anodic Zone

Concrete Encasement

Pipe in Soil Corrodes

Note: Arrows Indicate Direction of DC Current Flow

Dissimilar Surface Conditions

Pipe(Cathode) Threads

Bright Metal (Anode)

Scratches (Anode)

Concentration Cell Corrosion

Due to differences in the environment

Differential Soil Aeration – Very common

Aerated Soil

Differential Soil Aeration

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

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

Clay soil Clay soil

Anodic Zone

Cathodic Zone

Differential Aeration on Cast Iron Pipe

Cathodic Zone

Anodic Zone

Clay (moist low oxygen)

Sandy Loam (well drained, high oxygen)

Anode CathodeCathode

Differential Soil Aeration

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

Pavement

Sandy Loam (well drained, high oxygen)

Pitting Corrosion

Random and highly localized

Depth greater than area of attack

Most destructive form of corrosion

Pit location and growth difficult to predict

Pitting of Coated Carbon Steel in Soil

External Pitting: Ductile Iron Water Main

Selective Leaching Corrosion

Selective LeachingGraphitization (Gray Cast Iron)

Dezincification (Brass)

Dealloying Corrosion (Graphitization)

Eliminating the Corrosion Cell

Apply a Bonded Tape Wrapping

Pitting at a Coating Defect

Coat the Structure & Electrically Isolate It

What’s Wrong Here?

Encase the Pipe in a “Corrosion Barrier”

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.

How Cathodic Protection Works

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

Galvanic Anode Cathodic Protection

1. Anode

2. Cathode

3. Electrolyte

4. Metal Path

Galvanic Corrosion – No C.P. Benefit

1. Anode

2. Cathode

3. Electrolyte

4. Metal Path

Galvanic Corrosion - Mitigated w/CP

CP Performance - Can Be Verified

Sacrificial Anode on a Buried Pipeline

Sacrificial Anode

Coating Defect

Connection to Pipe

Coating Defect

Connection to Pipe

Sacrificial Anode

Sacrificial Anode w/Test Station

CP Test Station - Terminal Board

structure lead wire

anode lead wire

insulated terminal board calibrated

shunt resistor

Magnesium Anodes

Proper distance of anode from pipeAt 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

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*Detail courtesy of Midwest Energy Association

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.

Galvanic Anode CP Advantages

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 ).

Galvanic Anode CP Limitations

Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic ProtectionMeasurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.

Rectifier

Anode Groundbed

( - ) ( + )

Pipeline(Structure)

Surface (Horizontal) Anode System

Deep Anode (Vertical) Anode System

Continuous Linear Anode System

Impressed Current Transformer Rectifier

Why Should We Be Concerned about Corrosion?Definitions and TerminologyForms of CorrosionPipe Coatings and Cathodic ProtectionCathodic Protection using Magnesium AnodesAdvantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic ProtectionMeasurement and Testing of CP SystemsField Test EquipmentCathodic Protection Criteria.

Have you checked your rectifier lately?

Monitoring Data for a CP Rectifier

Can you locate your test stations?

Potential Profile Survey Technique

Reference Cells

Test StationVoltmeter-Computer

Wire Dispenser & Distance Chainer

Pipeline

CP Test Equipment - Multi-Meters

Multi-Meter Characteristics

Basic FunctionsReads AC & DC Volts

Reads Ohms (optional diode checker)

Reads AC and DC Amps (be careful here!)

Performance CriteriaField rugged, water/drop resistant

High input impedance (min. 20 M-Ω)

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

Reference Electrode Basic Components

Reference Electrode - Maintenance

Periodically verify cell against a known standardKeep porous plug covered when not usedClean and refill the reference cell annually

Clean copper rod with a non-metallic abrasive padReplace w/fresh Cu/CuSO4 solution (½ full at all times)Some Cu/CuSO4 crystals should always remain in suspensionWash hands after using – Cu/CuSO4 solution is hazardous

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.

Meter Connections

Cathodic Protection Criteria-0.85 V (w/IR-drop consideration)-0.85 V Instant-Off100 mV polarization decayOther criteria determined to be “appropriate” by regulatory authority

DOT Standard – Part 192.463

NACE International – CP Criteria

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.

Do We Have a Good Reading?

Questions?Questions?

Jeff SchramukNACE CP Specialist #7695

www.cpsolutionsinc.net

Basic Corrosion and Cathodic Protection -...

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