Chapter 16

Corrosion and Degradation of

Materials

CORROSION AND DEGRADATION OF MATERIALS

Cost of Corrosion

Fundamentals of Corrosion

Electrochemical reactions

EMF and Galvanic Series

Concentration and Temperature (Nernst)

Corrosion rate

Corrosion prediction (likelihood)

Polarization

Protection Methods2

What is the….

Cost of Corrosion?

4

The Cost of Corrosion

Significance of Corrosion on Infrastructure

Engineer finds corrosion in collapsed bridge at North Carolina speedway (2000)

Corrosion & Catastrophic Failure.

A Concrete bridge failure

Fundamental Components Corrosion can be defined as the deterioration of material

by reaction to its environment. Corrosion occurs because of the natural tendency for

most metals to return to their natural state; e.g., iron in the presence of moist air will revert to its natural state, iron oxide.

4 required components in an electrochemical corrosion cell: 1) An anode; 2) A cathode; 3) A conducting environment for ionic movement (electrolyte); 4) An electrical connection between the anode and cathode for the flow of electron current.

If any of the above components is missing or disabled, the electrochemical corrosion process will be stopped.

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11

• Two reactions are necessary: -- oxidation reaction: -- reduction reaction:

Zn Zn2 2e

2H 2e H2(gas)

• Other reduction reactions in solutions with dissolved oxygen:

-- acidic solution -- neutral or basic solution

O2 4H 4e 2H2O

O2 2H2O 4e 4(OH)

Electrochemical Corrosion

Zinc

Oxidation reactionZn Zn2+

2e-Acid solution

reduction reaction

H+H+

H2(gas)

H+

H+

H+

H+

H+

flow of e-

in the metal

Corrosion of zinc in an acid solution

12

Standard Hydrogen Electrode• Two outcomes:

0ometal V (relative to Pt)

Standard Electrode Potential

-- Electrodeposition

-- Metal is the cathode (+)

Mn+ ions

ne-

e- e-

25°C1M Mn+ sol’n 1M H+ sol’n

Pla

tinum

met

al,

M

H+H+

2e-

0ometal V (relative to Pt)

-- Corrosion

-- Metal is the anode (-)

Pla

tinum

met

al,

M

Mn+ ions

ne- H2(gas)

25°C 1M Mn+ sol’n 1M H+ sol’n

2e-

e-e-

H+

H+

13

Standard EMF Series

metalo

• Metal with smaller V corrodes.

• EMF series

AuCuPbSnNiCoCdFeCrZnAlMgNaK

+1.420 V+0.340- 0.126- 0.136- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.363- 2.714- 2.924

metal Vmetalo

mor

e an

odic

mor

e ca

thod

ic

V = 0.153V

o

-

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

25°C NiCd

Cdo

Nio

• Ex: Cd-Ni cell V < V Cd corrodes

Driving force

A driving force is necessary for electrons to flow between the anodes and the cathodes.

The driving force is the difference in potential between the anodic and cathodic sites.

This difference exists because each oxidation or reduction reaction has associated with it a potential determined by the tendency for the reaction to take place spontaneously. The potential is a measure of this tendency.

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16

Galvanic Series• Ranking the reactivity of metals/alloys in seawater

PlatinumGoldGraphiteTitaniumSilver316 Stainless Steel (passive)Nickel (passive)CopperNickel (active)TinLead316 Stainless Steel (active)Iron/SteelAluminum AlloysCadmiumZincMagnesium

mor

e an

odic

(act

ive)

mor

e ca

thod

ic(in

ert)

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Solution Concentration and Temperature• Ex: Cd-Ni cell with standard 1 M solutions

VNio VCd

o 0.153 V-

Ni

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

Cd 25°C

• Ex: Cd-Ni cell with non-standard solutions

Y

Xln

nF

RTVVVV o

Cdo

NiCdNi

n = #e-

per unitoxid/redreaction(= 2 here)F = Faraday'sconstant= 96,500C/mol.

- +

Ni

Y M

Ni2+ solution

X M

Cd2+ solution

Cd T

Kinetics, Polarization, Corrosion Rates While it is necessary to determine corrosion tendencies by

measuring potentials, it will not be sufficient to determine whether a given metal or alloy will suffer corrosion under a given set of environmental conditions.

Even though the tendency for corrosion may be high, the rate of corrosion may be very low, so corrosion may not be a problem.

Corrosion rates are determined by applying a current to produce a polarization curve (the degree of potential change as a function of the amount of current applied) for the metal surface whose corrosion rate is being determined.

The variation of potential as a function of current (a polarization curve) enables the study of concentration and activation processes on the rate at which anodic or cathodic reactions can transfer electrons.

Polarization measurements can thereby determine the rate of the reactions that are involved in the corrosion process (the corrosion rate).

19

Anodic Polarization Curve -1

Types of TestsANODIC POLARIZATION CURVE

•this curve is usually scanned from 20mV below the Eoc upwards

•by scanning at a slow rate (.2mV/s) this curve can be used to identify several corrosion mechanisms shown below

ip - passive current density

Epp - primary passivation potential

icrit - critical current density

Etrans - transpassive potential

• This curve is usually scanned from 20 mV below the Eoc (open circuit potential) upward.•The curve can be used to identify the following corrosion regions:

The degree of polarization is a measure of how the rates for anodic and cathodic reactions are slowed by various environmental factors (concentration of metal ions, dissolved oxygen in solution, diffusion limitations; referred to as concentration polarization) and/or surface process (activation polarization).

All electrochemical reactions consist of a sequence of steps that occur in series at the interface between the metal electrode and the solution.

Activation polarization is where the reaction is limited (controlled) by the slowest rate reaction of the steps (adsorption H+, film formation, ease of release of electrons, called the activation polarization).

Types of Corrosion

Uniform Attack – General Corrosion Galvanic Corrosion Crevice Corrosion Pitting Intergranular Corrosion Selective Leaching Erosion Corrosion Stress Corrosion

Uniform Corrosion

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Formerly a ship

Dissimilar metals are physically joined in the presence of an electrolyte.

The more anodic metal corrodes.

Galvanic

Bilge pump - Magnesium shell cast around a steel core.

Aluminum Alloys

Traditionally, structural aluminum alloys in aircraft have been 2024-T3 in damage critical areas and 7075-T6 in strength critical areas.

As aircraft structures became more complex, skin materials became an integral part of the structure and SCC became more prevalent.

The high performance aircraft designed since 1945 have made extensive use of skin structures machined from thick plates and extrusions. The residual stresses induced by heat treatment in conjunction with those from machining made these materials sensitive to SCC.

Stress Corrosion Cracking, SCC

A structure that has SCC sensitivity, if subjected to stresses and then exposed to a corrosive environment, may initiate cracks and crack growth well below the yield strength of the metal.

Consequently, no corrosion products are visible, making it difficult to detect or prevent; fine cracks can penetrate deeply into the part.

Narrow and confined spaces.

Crevice Corrosion

PittingPitting is a localized form of corrosive attack.  Pitting corrosion is typified by the formation of holes or pits on the metal surface.  Pitting can cause failure, yet the total corrosion, as measured by weight loss, may be minimal.

5th Century sword

Boiler tube

304 stainless steel / acid chloride solution

Corrosion alonggrain boundaries,often where precipitateparticles form.

Intergranular

Combined chemical attack and mechanical wear (e.g., pipeelbows).

Erosion-corrosion

Brass water pump

Selective Leaching

Preferred corrosion ofone element/constituent[e.g., Zn from brass (Cu-Zn)]. Dezincification.

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Energy Technology Developments Using Gamry Electrochemical Instrumentation Electrochemical cells used in energy technology include:

BatteriesFuel CellsSupercapacitorsSolar Cells

Batteries are the ultimate electrochemical device, so typically, battery scientists understand and use electrochemistry as a routine tool to develop and improve their products. 

The challenge for these engineers is to higher energy densities at lower prices. 

A battery is a very active electrochemical device, so safety is an important issue. 

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35

Types of Tests3. Electrochemical Tests

•because corrosion of metals is an electrochemical process, electrochemical measurements are especially suited to it 8

•they are extremely fast compared to other corrosion tests and the results are easily analyzed to find corrosion rates

•Open Circuit Potential Scans

•Polarization Resistance

•Potentiodynamic Curves

•Galvanic Corrosion Tests

•Electrochemical ImpedenceSpectroscopy (EIS)

Types

Zn+2 HCl solution

H+

H+

H+ H+

H +H2 Cl-

Cl-

Zinc

e-e-

Corrosion Test Methods

1: The measurement of the open circuit potential is very easy and inexpensive, but is not considered to be very reliable, since the potential tells nothing about the kinetics of the process.

2: Linear polarization measurements are encumbered by “IR” effects from the concrete; there is so much potential drop in the concrete, that an accurate determination of the potential of the rebar surface is very difficult.

3: Electrochemical impedance spectroscopy (EIS) can overcome the difficulties of the concrete resistance.

Electrochemical Basics Corrosion is an electrochemical phenomena

The simultaneous combination of electrical & chemical processes Techniques involve either or both of:

Measuring voltage difference (thermodynamic) Measuring current flow (kinetic)

Working electrode Equipment material

Reference electrode Maintains constant potential

• Even at large currents

Counter (Secondary) electrode Allows infinite current

E

I

Ref

eren

ce E

lect

rode

Wor

king

Ele

ctro

de

Sec

onda

ry E

lect

rode

E

I

Ref

eren

ce E

lect

rode

Wor

king

Ele

ctro

de

Sec

onda

ry E

lect

rode

Test Samples

EG&G Instruments: Potentiostat/Galvanostat Model 273A

Comparison of Corrosion Potential vs Time for MCI Treated Concrete with Untreated Samples

-300

-250

-200

-150

-100

-50

0

50

100

0 50 100 150 200 250

Time of Submersion (Days)

Pot

entia

l, (m

V)

RF1-untreated MR1-rebar treated

MS1-surface treated MM1-mortar coated surface

Corrosion Potential vs Time, ASTM C876-91, Cortec MCI 2022 & 2021 Compared with Unprotected Concrete (Various Concrete Densities)L=low density, H=high density

-600

-500

-400

-300

-200

-100

0

0 50 100 150 200 250 300 350 400 450

Time of Submersion (Days)

Pot

entia

l, (m

V)

L untreated

L2021

L2022

H untreated

H2021

H2022

EIS Results, Bode PlotsLD=untreated low density concrete, 2S=MCI 2022/high density, 2L=MCI 2022/low density;

Concrete densities: low = 130 lbs/ft3, high = 150 lbs/ft3.

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05

FREQUENCY (Hz)

l Z l

(O

hm

s)

2L-Day 1 2L-Day 238 2L-Day 325

L-Day 1 L-Day 233 L-Day 332

2S-Day 1 2S-Day 236 2S-Day 327

Concrete Exterior & Interior

Concrete Interior (untreated)

The potential, polarization resistance and current density data can provide useful information about:

• Corrosion state of the metal (active or passive).

• Estimates of the Tafel constants for input into LPR analysis, corrosion rate measurement or cathodic protection criteria.

Useful Parameters

Open Circuit Potentials

46

Types of TestsOpen Circuit Potentials

•is a summation of the half-cell reaction potentials in a specific electrolyte

•using a potentiostat it measures the potential of the working electrode with respect to the reference electrode potential vs. time

•these measurements are made under equilibrium conditions (ie. absence of current flow ) to describe the thermodynamic equilibrium of an electrochemical system 8

Aluminum (FS)

Zinc (BHD)

This electrochemical technique enables the measurement of the instantaneous corrosion rate. It quantifies the amount of metal per unit of area being corroding in a particular instant. The method is based on the observation of the linearity of the polarization curves near the potential (Ecorr). The slope expresses the value of the polarization resistance (Rp) if the increment is close to zero. This Rp value is related to the corrosion current (Icorr) by means of the expression:

Where A is the area of metal surface evenly polarized and B is a constant that may vary from 13 to 52 mV. For steel embedded in concrete, the best fit with parallel gravimetric losses results in B= 26 mV for actively corroding steel , and a value of B= 52 mV, when the steel is passivated.

Polarization Resistance, Rp

Types of Tests

•Now that we have found Rp we can sub it into the Stern-Gearyequation:

)(303.2

1

BcBa

BaBc

Rpicorr

- Ba and Bc are the Tafel

Slopes and are

approximated to both be

.1 V

•Using the icorr value, we can now solve for a corrosion rate as we did before

•it has been found that there is good correlation b/w corrosion rates obtained by polarization resistance and those obtained by weight loss10

•it is also non-destructive and little time is required

Polarization Resistance

Galvanic Corrosion Tests

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Types of TestsGalvanic Corrosion Tests

•when two different metals are electrically coupled in the presence of a conductive solution, one of them corrodes at an accelerated rate while the other is protected

•this is important in sacrificial coatings because the coating acts as the anode and the substrate acts as the cathode

GraphitePtTiStainless SteelNi-Cu alloysAgNi alloysCu-NiBronzeBrassPb-Sn solderCuSnLow alloys steelLow carbon steelAl alloysAlBeZnMg

•in order to correctly assess a sacrificial coating we must look at the galvanic couple that exists between the coating and the substrate11

In Seawater

More Noble

More Active

coating

substrate

Types of TestsGalvanic Corrosion Tests

•when two different metals are electrically coupled in the presence of a conductive solution, one of them corrodes at an accelerated rate while the other is protected

•this is important in sacrificial coatings because the coating acts as the anode and the substrate acts as the cathode

GraphitePtTiStainless SteelNi-Cu alloysAgNi alloysCu-NiBronzeBrassPb-Sn solderCuSnLow alloys steelLow carbon steelAl alloysAlBeZnMg

•in order to correctly assess a sacrificial coating we must look at the galvanic couple that exists between the coating and the substrate11

In Seawater

More Noble

More Active

coating

substrate

Types of TestsGalvanic Corrosion Tests

•when two different metals are electrically coupled in the presence of a conductive solution, one of them corrodes at an accelerated rate while the other is protected

•this is important in sacrificial coatings because the coating acts as the anode and the substrate acts as the cathode

GraphitePtTiStainless SteelNi-Cu alloysAgNi alloysCu-NiBronzeBrassPb-Sn solderCuSnLow alloys steelLow carbon steelAl alloysAlBeZnMg

•in order to correctly assess a sacrificial coating we must look at the galvanic couple that exists between the coating and the substrate11

In Seawater

More Noble

More Active

coating

substrate

Potentiodynamic Curves

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Types of TestsPotentiodynamic Curves

•An electrochemical test that measures the current response to anapplied potential over a large potential range

•this test is used to analyze the overall behavior of the coating’s

corrosion protection

Evan’s Diagram Polarization Curve

+

-

2

+

-

E

log i

Ecorr

icorr

net + current

net -current

log i

iapplied

E +

-

2

+

-

icorr

Ecorr

Tafel Extrapolation

50

Types of TestsANODIC POLARIZATION CURVE

•another use of the polarization curve is for Tafel extrapolation

•by extrapolating the straight portion of the curve back to the Ecorr we can calculate the icorr and then corrosion rate

•A straight portion of 1 order of magnitude is suggested for accuracy9

Anodic Tafel Slope (Ba)icorr

Ecorr

Electrochemical Impedance Spectroscopy (EIS)

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EIS has been successfully applied to the study of corrosion systems and been proven to be a powerful and accurate method for measuring corrosion rates. To access the charge transfer resistance or polarization resistance  that is proportional to the corrosion rate at the monitored interface, EIS results have to be interpreted with the help of a model (see simple circuit model) of the interface. An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. To make an EIS measurement, a small amplitude signal, usually a voltage between 5 to 50 mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000 Hz. The EIS instrument records the real (resistance) and imaginary (capacitance) components of the impedance response of the system.

Proposed Relationship between Corrosion Rate and Remaining Service Life

icorr (A/cm2) Severity of Damage

<0.5 no corrosion damage expected

0.5-2.7 corrosion damage possible in 10 to 15 years

2.7-27 corrosion damage expected in 2 to 10 years

>27 corrosion damage expected in 2 years or less

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Use metals that passivate, form a thin, adhering oxide layer that slows corrosion.Use metals that are relatively unreactive in the corrosion environment.Use inhibitors (substances added to solution that decrease reactivity); slow oxidation/reduction reactions by removing reactants like O2 gas by reacting it w/an inhibitor).Slow oxidation reaction by attaching species to the surface. Apply physical barriers: films and coatings, paintReduce T (slows kinetics of oxidation and reduction)Cathodic (or sacrificial) protection; attach a more anodic material to the one to be protected.

Corrosion Prevention

steel

zinczincZn2+

2e- 2e-

e.g., zinc-coated nail

Galvanized Steel

Metal (examples: Al, stainless steel)

Metal oxide

Passivation Process Stainless steel was “discovered” around 1900–1915. A result of

multiple scientific efforts in England, France and Germany on alloys with compositions that would later be known as the 410, 420, 430, 442, 446 and 440C grades.

Stainless steels must have a very low level of carbon; difficult to obtain (low carbon) for many years, which explains the late arrival of good ferritic grades in the 1980s.

Chromium (Cr) is by far the most important alloying element in the production of stainless steel. It forms the “passive” surface film (chromium oxide) that makes stainless steel corrosion resistant and increases scaling resistance, wear resistance and tensile strength.

A minimum of 10.5% chromium content (by weight) is required for the protective, self-repairing surface layer of chromium oxide to form reliably. The higher the chromium content, the stronger the passive layer.If the stainless steel surface is machined or accidentally damaged, the passive layer quickly re-forms, in the presence of air or water.

Sacrificial Anodes

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This field is located in Viosca Knoll, block 786, southeast of New Orleans.  It lies in water depths of approximately 1754 feet (535 meters).  Petronius is the largest free-standing structure in the world.  Texaco's choice was Galvotec-CW-III Aluminum Sacrificial Anodes for their Petronius cathodic protection system.  http://www.galvotec.com/img/texaco.jpg

"Salt water isn't good for anything."

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A man blamed a low-flying pelican and a dropped cell phone for veering his million-dollar (French-built Bugatti Veyron) sports car off a road and into a salt marsh near Galveston. The car was half-submerged in the brine about 20 feet from the road when police arrived (Nov 11, 2009).

WORLD'S FASTEST: Bugatti Veyron Busts Out With 1,000-hp and $1.3 Million Price Tag The Veyron's 16-cylinder engine develops a shade over 1,000 horsepower, giving it a 0-60 time of fewer than 3 seconds and a 252-mph top speed. Those staggering stats make the Veyron the world's fastest production car. It's also the most expensive (2005 stats).

$1.95 Million (2009)

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• Metallic corrosion involves electrochemical reactions -- electrons are given up by metals in an oxidation reaction -- these electrons are consumed in a reduction reaction• Metals and alloys are ranked according to their corrosiveness in standard emf and galvanic series.• Temperature and solution composition affect corrosion rates. Increasing T, speeds up oxidation/reduction reactions.• Forms of corrosion are classified according to mechanism

• Corrosion may be prevented or controlled by: -- materials selection -- reducing the temperature -- applying physical barriers -- adding inhibitors -- cathodic protection

• using metals that form a protective oxide layer• Painting/coating

Summary

"Rust's A Must"

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Mighty ships upon the oceanSuffer from severe corrosion,Even those that stay at docksideAre rapidly becoming oxide.Alas, that piling in the seaIs mostly Fe2O3.And where the ocean meets the shore,You'll find there's Fe3O4.'Cause when the wind is salt and gusty,Things are getting awful rusty.

We can measure, we can test it,We can halt it or arrest it.We can gather it and weigh it,We can coat it, we can spray it.We examine and dissect it,We cathodically protect itWe can pick it up and drop it.But heaven knows we'll never stop it!So here's to rust, no doubt about it,Most of us would starve without it.

The origin of this epic poem is a bit fuzzy.  We have seen a reference to the late Mr. T. R. B Watson of Corrosion Services Co., Ltd. in Toronto and we believe that he is the author. 

More Information http://www.electrochem.org/ The Journal of The Electrochemical Society (JES) is the leader in the

field of solid-state and electrochemical science and technology. This peer-reviewed journal publishes an average of 450 pages of 70 articles each month. Articles are posted online, with a monthly paper edition following electronic publication in the following areas:

Batteries and Energy Storage Fuel Cells and Energy Conversion Corrosion, Passivation, and Anodic Films Electrochemical/Chemical Deposition and Etching Electrochemical Synthesis and Engineering Physical and Analytical Electrochemistry Dielectric Science and Materials Semiconductor Devices, Materials, and Processing Sensors and Displays: Principles, Materials, and Processing Nanostructured Materials, Carbon Nanotubes, and Fullerenes Interdisciplinary Topics

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