basic corrosion cp

8/6/2019 Basic Corrosion CP

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Jeff SchramukNACE CP Specialist #7695

www.cpsolutionsinc.net

Basic CorrosionBasic Corrosion

andandCathodic ProtectionCathodic Protection

Basic Corrosion & Cathodic Protection


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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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Forms of Corrosion






Field Test EquipmentCathodic Protection Criteria.



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Effects of Infrastructure Corrosion

Life Safety

Economics Environmental

Regulatory

Compliance


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Forms of Corrosion









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Corrosion Can be Defined as:

Practical

Definition

Scientific

Definition

The Tendencyof a Metal to

Revert to its

Native State

Electrochemical

Degradation of Metal

as a Result of aReaction with its

Environment


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Corrosion - A Natural Process

IRON OXIDE REFINING MILLING

IRON CORROSION IRON OXIDE


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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 (+) tonegative (-)

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

(Metal Loss)

DC Current

Cathodic

Area

(Protected)

Definitions - Anodes & Cathodes


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Copperat-2

00mV

Steelat-600mV

The Simplified Corrosion Cell

Cop

perat-200m

V

Ste

elat-600mV

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

(Metallic Path)

I

I

I

I

I

e-


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Material Potential*Pure Magnesium -1.75Magnesium Alloy -1.60Zinc -1.10Aluminum Alloy -1.00

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

Gold +0.20Carbon, Graphite, Coke +0.40

* Potentials With Respect to Saturated Cu-CuSO4 Electrode

L

ess

Active

More

Practical Galvanic Series*


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Corrosion Reaction and Ohms Law

Ohms 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)


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Some Common Electrical Quantities

Current Flow: 1 ampere (A) = 1000 milliamps (mA)

Examples:

A sacrificial anodes output is measured in mA

A CP rectifiers output is can be up 100 A

Voltage: 1 volt (V) = 1000 millivolts (mV)

Examples:

A magnesium anodes potential is ~1.6 V (1600 mV)

A CP rectifier can have a DC voltage of up to 100 V


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Corrosion Cell - Anodic Reactions

Copperat-200mV

Steelat-60

0mV

Cathode

AnodeI

e-Fe++

Fe++

Fe++

OH-

OH-

OH-

I

I


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Corrosion Cell - Cathodic Reactions

Copperat-20

0mV

Steelat-60

0mV

Cathode

Anode

I

e-H+

H+

H+

H+

e-

e-

e-

e-

I


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Corrosion Cell Combined Reactions

Copperat-20

0mV

Steelat-600mV

Cathode

Anode

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

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


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Material Potential*Pure Magnesium -1.75Magnesium Alloy -1.60Zinc -1.10Aluminum Alloy -1.00

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



Less

Active

More



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Galvanic Corrosion Bimetallic Connection


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Old Pipe(Cathode)

New Pipe(Anode)

Old-New Pipe Corrosion Cell


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Steel in Concrete-Soil

Cathodic

ZoneAnodic

Zone

Concrete

Encasement

Pipe in Soil

Corrodes

Note: Arrows Indicate Direction of DC Current Flow


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Dissimilar Surface Conditions

Pipe(Cathode) Threads

Bright Metal

(Anode)

Scratches(Anode)


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Concentration Cell Corrosion

Due to differences in the environment

Differential Soil Aeration Very common


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Aerated Soil

Differential Soil Aeration

Oxygen diffusing throughbackfill sustains corrosion tocathodic (top) area of pipe

Lack of oxygen at bottom ofpipe creates relative corrosioncell to (top) area of pipe

Clay soil Clay soil

Anodic Zone

Cathodic Zone

O2 O2


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Differential Aeration on Cast Iron Pipe

Cathodic Zone

Anodic Zone


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Clay (moistlow 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)


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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 LeachingGraphitization (Gray Cast Iron)

Dezincification (Brass)


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Dealloying Corrosion (Graphitization)


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Definitions and TerminologyForms of Corrosion









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Eliminating the Corrosion Cell

Anode

Cathode


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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

Whats WrongHere?


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Encase the Pipe in a Corrosion Barrier


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Corrosion occurs where current dischargesfrom metal to electrolyte

The objective of cathodic protection is toforce the entire surface to be cathodic to the

environment.

How Cathodic Protection Works

G l i A d C th di P t ti


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Current is obtained from a metal of a higher

energy level.

Galvanic Anode Cathodic Protection

P ti l G l i S i *


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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 -0.20



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Gal anic Corrosion No C P Benefit


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Copper-200mV

Steel-600mV

Magnesium

-1.7V

1. Anode2. Cathode

3. Electrolyte

4. Metal Path

Galvanic Corrosion No C.P. Benefit

Galvanic Corrosion Mitigated w/CP


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Cath

ode

Cathode

Anode

1. Anode

2. Cathode

3. Electrolyte

4. Metal Path

Galvanic Corrosion - Mitigated w/CP

CP Performance Can Be Verified


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CP Performance - Can Be Verified

Sacrificial Anode on a Buried Pipeline


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Sacrificial Anode on a Buried Pipeline

Sacrificial Anode

CoatingDefect

Connection to Pipe

Grade

Sacrificial Anode w/Test Station


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CoatingDefect

Connection to Pipe

Grade

Sacrificial Anode

Sacrificial Anode w/Test Station

CP Test Station - Terminal Board


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CP Test Station - Terminal Board

structurelead wireanodelead wire

insulated

terminal board calibratedshunt resistor

Magnesium Anodes


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Magnesium Anodes

Packaged Magnesium Anode

N l G PL


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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 dont lift by the lead wire

Tamp earth firmly around anode package.

Natural Gas PL


N t l G PL ( t )


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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

Do not allow any foreign pipeline contacts.

Natural Gas PL (cont.)


N t l G PL ( t )


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Natural Gas PL (cont.)

*Detail courtesy of Midwest Energy Association*Detail courtesy of Midwest Energy Association



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Galvanic Anode CP Advantages


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No external AC power is requiredEffective utilization of protective current

Simple and inexpensive to install on newunderground structures

Seldom cause stray DC interferenceMinimal maintenance requirements.

Galvanic Anode CP Limitations


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Limited driving potential

E = (Ea Ec)Limited current output I = E / Rt

Large number of anodes will be required onbare or poorly coated structures

Ineffective in high-resistivity soilenvironments (Rt ).



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Surface (Horizontal) Anode System


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Rectifier

AnodeGroundbed

( - ) ( + )

Pipeline

(Structure)

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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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


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Reference Cells

Test Station

Voltmeter-Computer

Wire Dispenser &Distance Chainer

Pipeline



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Forms of Corrosion



Advantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection




CP Test Equipment - Multi-Meters


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Multi-Meter Characteristics


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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-)

Test Equipment Quality Assurance


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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


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Reference Electrode - Maintenance


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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 ( full at all times)

Some Cu/CuSO4 crystals should always remain in

suspension

Wash hands after using Cu/CuSO4 solution is

hazardous

P/S Potential Readings


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Connect voltmeter to pipe and reference

Ensure reference cell plug has good contact with moist

soil not pavement

Place reference cell away from anodesRead P/S on DCV scale

Record P/S reading using standard forms

If polarity is positive, notify corrosion dept.

Meter Connections


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Forms of Corrosion



Advantages & Limitations of Galvanic Anode CP SystemsImpressed Current Cathodic Protection




DOT Standard Part 192.463


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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

NACE International CP Criteria


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DOT Standard Part 192.465


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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?


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