DNV GL © 2013 SAFER, SMARTER, GREENER DNV GL © 2013

AEP BRO 2014

Brett Tossey

Fundamentals of Corrosion

1

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Introduction

Definition of Corrosion

Electrochemistry Fundamentals

– Theory (just a bit)

– Thermodynamics

– Kinetics

– Forms of Corrosion

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Corrosion Definition

What Is Corrosion??

– Degradation of a material through environmental interaction.

– Steel = thermodynamically unstable

– Steel is found in nature as an ore (iron oxides)

–We put energy into it to convert it to an alloy

– It wants to revert back to the oxide

– Lowest energy state

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Life Cycle of Steel

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Why Steel (or Iron-Based Alloys)

– Readily available

– Can be inexpensive

– Wide variety of processing techniques

– Strong

– Ductile

– Versatile

– Wide range of alloys to achieve desired properties

– Strength, ductility, hardenability

– But, susceptible to corrosion

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Corrosion

All forms of aqueous corrosion are electrochemical in nature

– Metal oxidation:

M M+ + e-

– Reduction reaction to consume electrons

O2 + 2H2O + 4e- 4OH-

2H+ + 2e- H2

– Charge transfer between locations of oxidation and reduction reactions

Both reactions required for corrosion to occur

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Corrosion

LEO says GER (Losing Electrons Oxidation, Gaining Electrons Reduction)

Oxidation Reaction (loss of e- ,) = Anodic Reaction

– Occurs at Anode

– M M+ + e-

Reduction Reaction (gain of e-) = Cathodic Reaction

– Occurs at Cathode

– O2 + 2H2O + 4e- 4OH-

Both reactions required for corrosion to occur

– A reduction reaction must be present to consume electrons liberated by

oxidation reaction

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Differential Corrosion Cell

Four Components

– Anode

– Cathode

– Electrically conductive metallic path

– Between anode and cathode

– Electrolyte

– Ionically conductive path

– Between anode and cathode

– e.g., soil

– Corrosion morphology related to spacing between anode and cathode

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Differential Corrosion Cell

Fe(s) Fe 2+ (aq) + 2e-

Anodic ReactionMetal loss from oxidation reaction

O2 (do) + H2O (aq) + 4e- 4OH-(aq)

e-Cathodic Reaction

Reduction of Oxygen

Alloy Substrate

ionic current flow(dissolved, charged species)Direct

current flow

e-

e-

e-

e-

e-

e-

e-e-

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How Do We Detect Ongoing Corrosion?

Monitor voltages and currents associated with corrosion

Voltage develops across metal- electrolyte interface when metal is placed in

electrolyte

– Referred to as corrosion potential

– Cannot measure it directly

– Can measure it with respect to a reference electrode (RE)

– Copper Sulfate Electrode (CSE) commonly used for soils

– Seawater RE = Silver / Silver Chloride

– Common laboratory RE = Calomel

– Solid RE = Platinum wire

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Test Methods: Potential Measurements

Does not yield information on corrosion rate

Provide means to measure oxidizing power of the environment

– The more positive (noble) the potential, the stronger the oxidizing power of the medium

Provide ranking of relative nobility of materials

– Used to predict galvanic couple activity

Measure Voltage (Potential)

Of Corrosion Cell

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Potential Measurements

Measure Voltage (Potential)

Of Corrosion Cell

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How Do We Detect Ongoing Corrosion?

– Potentials related to relative corrosion resistance of a metal in a given

environment

– Gold and Platinum very positive

– Magnesium and Zinc very negative

Metal or Alloy Potential (V,CSE)

Platinum 0.2 to -0.1

Copper -0.3

Mill Scale on Steel -0.3

Lead -0.5

Rusted Mild Steel -0.2 to -0.5

Clean Mild Steel -0.5 to -0.8

Aluminium -0.8

Zinc -1.1

Magnesium -1.75

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How Do We Detect Ongoing Corrosion?

Current measurements

– Measure potential gradients in soil or metal

– V = IR (where anode and cathode are separated)

– Other electrochemical techniques

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Thermodynamics

Tells us whether a reaction will occur

One principle of Thermodynamics

– Material seeks lowest energy state

– Gibbs free energy (G)

– ∆G < 0 for process to be thermodynamically favored

∆G related to electromotive force (E)

∆G = - zFE

– z = valence change for reaction

– F = Faraday’s Constant

A positive value of E indicates that a reaction (combination of two half reactions)

is favored

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Thermodynamics

Steel is thermodynamically unstable in water

– water reduction can promote corrosion

2H2O + 2e- 2OH- + H2

– But rates are low

– Hydrogen reduction occurs in acid environments

2H+ + 2e- H2

– Oxygen reduction is most common reduction reaction associates with corrosion

O2 + 2H2O + 4e- 4OH-

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Kinetics

Tells us the rate of a reaction

Electrical current related to corrosion rate through Faraday’s Law

m/at = iM/zF – m = mass, t = time, a = exposed area

– i = current density, M = atomic weight, z = electrons transferred, F =

Faraday’s constant

Corrosion Rate = (m/at)1/ρ α i = Current Density

– ρ = metal density

– For steel, 1 mil per year (0.001 in/yr) ~ 2 µA/cm2

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Evans Diagram

Plot showing corrosion kinetics

– Potential vs log current

Polarization = deviation in potential as a result of passage of current

Reactions are linear on plot for simplest type of behavior

– Activation polarization

– Limiting reaction is e- transfer

Another common type of polarization

– Concentration polarization

– Diffusion controls corrosion rate

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Evans Diagram

Ecor (corrosion potential) occurs where sum of oxidation reactions = sum of

reduction reactions

– No net accumulation of charge

Net current is zero at Ecor

Slopes of lines = Tafel Slopes

Tafel slopes are not all the same for different reactions

– Typically 50 to 200 mV/decade

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20

Evans Diagram

Ecor

Fe0 a Fe2+ + 2e-

Pote

nti

al

Log Current Density

2O2 + H2O + 2e- a 4OH-

icor

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Evans Diagram

Can’t measure corrosion current directly

– Can’t measure icor

– Measure difference between anodic and cathodic reaction

How do we determine icor

– Tafel extrapolation

– Other calculations

– Linear polarization resistance (LPR)

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Kinetics (Polarization)

– Polarization Resistance (PR) measurements

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Morphological Forms of Corrosion

Eight common forms of corrosion (Fontana)

– General (Uniform) Corrosion

– Pitting Corrosion

– Crevice Corrosion

– Stress Corrosion Cracking

– Galvanic Corrosion

– Intergranular Corrosion

– Selective Leaching (Dealloying)

– Erosion Corrosion

Forms in Red affect underground piping

– Discussed further

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General (Uniform) Corrosion

Uniform metal loss

Oxidation and reduction reactions are spatially close on the metal surface

– Example: steel in hydrochloric acid

Greatest destruction of metal on a tonnage basis

Not usually an economic or integrity concern

– Predictable

– Can be readily mitigated

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Pitting Corrosion

Localized attack that results in holes or pits in a metal

– Cavity with diameter about the same or less than the depth

Can cause failure with only a small percentage weight loss for a metal

Commonly associated with passive alloys

– Stainless steels; protective oxide film

– Associated with local breakdown

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Pitting Corrosion

Separation of anodic and cathodic reactions on metal surface

– Anodic reaction in pit

– Reduction reaction outside pit

Autocatylic

– Reactions in pit promote continued corrosion

– Hydrolysis reactions that reduce pH

– Movement of chlorides into pit

– Chlorides promote passive film breakdown

Potentials required for hydrogen evolution and ion movement is effected by

electric fields.

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Pitting Corrosion

Carbon steel not considered to be a passive metal

But, pitting in carbon steel does occur

– Internal and external pitting associated with microbes

– Microbiologically influenced corrosion (MIC)

– Internal CO2 corrosion

– External stray current corrosion

– DC or AC induced

– Corrosion attack focused on holidays in coating

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Crevice Corrosion

Localized attack within crevices

Associated with small volumes of electrolyte within the crevices

– Environment within the crevice becomes more aggressive with time

– Limited oxygen access

– pH reduces with time

Similar process to pitting corrosion

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Pit and Crevice pH

Chemistry in pit / crevice vastly different then bulk environment

– Crevice pH ~ 1.5 @ [Cr] > 21%; Bulk sol’n pH = 7.0

29

pH

Chromium Content in Alloy (wt. %)

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Pit and Crevice Chemistry

30

Chemistry in pit / crevice vastly different then bulk environment

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Pit and Crevice Chemistry

Low pH in pit / crevice

Rapid metal dissolution

31

River Patterns; Gravity effect

Dissolved 22% chrome alloy

3:00

6:00

Absorber slurry

pH = 5.5 Horizontal weld seam

Gravity

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Stress Corrosion Cracking (SCC)

Cracking caused by simultaneous presence of

– Tensile stress

– Susceptible metal

– Potent environment

Two forms of SCC

– Intergranular

– between grains in metal

– transgranular cracking

– Through grains

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Galvanic Corrosion

Classical Definition - dissimilar metal corrosion

– Corrosion as a result of the electrical connection of dissimilar metals in an

electrolyte

More negative member of couple becomes anode

More positive member of couple becomes cathode

Corrosion rate of anode is increased

– Polarized in positive direction

Corrosion rate of cathode is decreased

– Polarized in negative direction

Stainless Steel (~ 22% Chrome)

Stainless

Steel (18% Cr)

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Galvanic Series in Neutral Soil and Water

Metal or Alloy Potential (V,CSE)

Platinum 0 to -0.1

Copper -0.2

Mill Scale on Steel -0.2

Lead -0.5

Rusted Mild Steel -0.2 to -0.5

Clean Mild Steel -0.5 to -0.8

Aluminum -0.8

Zinc -1.1

Magnesium -1.75

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Sources of stray current

– Utilities (Pipelines, Overhead Transmission)

– Commercial Activities (welding)

– Circulating Currents

– Unbalanced Transformers (3rd Harmonics)

– Long Line Effects

– Telluric

– Subways, Railroad lines, Mining

– Smelting Operations, Welding

– Other

Stray Current Corrosion

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Corrosion Control

Corrosion is inevitable

Goal of corrosion control

– Control rate and mode of corrosion to achieve useful life of a structure

Methods of corrosion control

– Alloy selection

– Compatible with service environment

– Component design

– Avoid geometries that promote corrosion

– Surface preparation

– inhibitors

– Cathodic protection

– Coatings