ch16 corrosion

8/13/2019 Ch16 Corrosion

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Chapter 16

Corrosion and

Degradation ofMaterials

http://en.wikipedia.org/wiki/File:IronInRocksMakeRiverRed.jpg



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 forthe flow of electron current.

If any of the above components is missing or disabled,

the electrochemical corrosion process will be stopped.

10



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 2H2O4e 4(OH)

Electrochemical Corrosion

Zinc

Oxidation reaction Zn Zn2+

2e - Acidsolution

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:

0o

metal 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

P l a t i n u m

m e t a l , M

H +

H + 2e -

0o

metal V (relative to Pt)

-- Corrosion

-- Metal is the anode (-)

P l a t i n u m

m e t a l , M

Mn+

ions

ne - H2(gas)

25°C1M Mn+ sol’n 1M H+ sol’n

2e -

e - e -

H +

H +



13

Standard EMF Series

metal

o• Metal with smaller

V corrodes.

• EMF series

Au

Cu

Pb

Sn

NiCo

Cd

Fe

Cr

Zn Al

Mg

Na

K

+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

metalV

metal

o

m o r e a n o

d i c

m o r e

c a t h o d i c

DV =

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 toflow 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. Thepotential is a measure of this tendency.

15



16

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

Platinum

Gold

Graphite

Titanium

Silver

316 Stainless Steel (passive)Nickel (passive)

Copper

Nickel (active)

Tin

Lead

316 Stainless Steel (active)Iron/Steel

Aluminum Alloys

Cadmium

Zinc

Magnesium m o r e a n o d i c

( a c t i v e )

m o r e c a t h o d i c

( i n e r t )



18

Solution Concentration and Temperature

• Ex: Cd-Ni cell with

standard 1 M solutions

V Ni

oV Cd

o 0.153 V

-

Ni

1.0 MNi2+ solution

1.0 MCd2 + solution

+

Cd 25°C

• Ex: Cd-Ni cell with

non-standard solutions

Y

X ln

nF

RT V V V V

o

Cd

o

NiCdNi

n = #e- per unit

oxid/red

reaction

(= 2 here)F =

Faraday'sconstant

= 96,500

C/mol.

- +

Ni

Y MNi2+ solution

X MCd2 + 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 producea 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

ip - passive current density

Epp - primary passivation potential

icrit

- critical current density

Etrans - transpassive potential

• This curve is usually

scanned from 20 mVbelow 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 surfaceprocess (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 CorrosionGalvanic Corrosion

Crevice Corrosion

Pitting Intergranular Corrosion

Selective Leaching

Erosion CorrosionStress Corrosion



Uniform Corrosion

23

Formerly a ship



Dissimilar metals are

physically joined in thepresence of an electrolyte.

The more anodic metal

corrodes.

Galvanic

Bilge pump -

Magnesium shell cast

around a steel core.



Aluminum Alloys

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

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

The high performance aircraft designed since1945 have made extensive use of skin

structures machined from thick plates andextrusions. The residual stresses induced byheat treatment in conjunction with those frommachining made these materials sensitive toSCC.



Stress Corrosion Cracking, SCC

A structure that has SCC sensitivity, if

subjected to stresses and then exposedto a corrosive environment, may initiate

cracks and crack growth well below the

yield strength of the metal.

Consequently, no corrosion productsare visible, making it difficult to detect or

prevent; fine cracks can penetrate

deeply into the part.



Narrow and confined spaces.

Crevice Corrosion

http://corrosion.ksc.nasa.gov/images/gal2.jpg



Pitting Pitting is a localized form of

corrosive attack. Pitting corrosionis typified by the formation of holes

or pits on the metal

surface. Pitting can cause failure,

yet the total corrosion, asmeasured by weight loss, may be

minimal.

5th Century sword

Boiler tube

304stainless

steel / acid

chloride

solution

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Corrosion along

grain boundaries,

often where precipitateparticles form.

Intergranular



Combined chemical attack and

mechanical wear (e.g., pipeelbows).

Erosion-corrosion

Brass water pump

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Selective Leaching

Preferred corrosion ofone element/constituent

[e.g., Zn from brass (Cu-

Zn)]. Dezincification.



32



33



Energy Technology Developments

Using Gamry Electrochemical Instrumentation

Electrochemical cells used in energy technology include:

Batteries

Fuel Cells

Supercapacitors

Solar 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.

34



35

Zn+2 HCl solution

H+

H+

H+ H+

H +H2Cl-

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

R e f e r e n c e E l e c t r o d e

W o r k i n g E l e c t r o d e

S e c o n d a r y E l e c t r o d e

E

I

R e f e r e n c e E l e c t r o d e

W o r k i n g E l e c t r o d e

S e c o n d a r y E l e c t r o d e



Test Samples

EG&G Instruments: Potentiostat/Galvanostat



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)

P o t e n t i a 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)

P o t e n t i a l , ( m V

)

L untreated

L2021

L2022

H untreated

H2021

H2022



EIS Results, Bode Plots

LD=untreated low density concrete, 2S=MCI 2022/high density, 2L=MCI 2022/low density;

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

3.

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 h

m 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 andcurrent density data can provide useful

information about:

• Corrosion state of the metal (active or

passive).

• Estimates of the Tafel constants for input intoLPR analysis, corrosion rate measurement or

cathodic protection criteria.

Useful Parameters



Open Circuit Potentials

46

•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. absenceof 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 unitof 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

)(303.2

1

Bc Ba

BaBc

Rpicorr

- Ba and Bc are the Tafel

Slopes and are

approximated to both be

.1 V



Galvanic Corrosion Tests

48

coating

substrate

•when two different metals are electrically

coupled in the presence of a conductivesolution, one of them corrodes at anaccelerated 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 alloys

AgNi alloysCu-NiBronzeBrassPb-Sn solder CuSnLow alloys steelLow carbon steel

Al alloys AlBeZnMg

In Seawater

MoreNoble

More

Active



Potentiodynamic Curves

49

•An electrochemical test that measures the current response to an

applied 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 H + + 2 e -

H 2

M M

+ + e -

E

log i

Ecor r

icorr

net +current

net -

current

log i

iapplied

E2 H +

+ 2 e -

H 2

M M

+ + e

-

icorr

Ecorr



Tafel Extrapolation

50

•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

El t h i l I d S t (EIS)



Electrochemical Impedance Spectroscopy (EIS)

51

EIS has been successfully applied to the study

of corrosion systems and been proven to be a

powerful and accurate method for measuringcorrosion 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, isapplied 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 (mA/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

Corrosion PreventionMetal oxide



53

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, paint

Reduce 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

zinczinc

Zn2+

2e - 2e -

e.g., zinc-coated nail

Galvanized Steel

Metal (examples: Al,

stainless steel)



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 theproduction of stainless steel. It forms the “passive” surface film(chromium oxide) that makes stainless steel corrosion resistant andincreases 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

55

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 ater isn't good for an thing "

http://www.galvotec.com/img/texaco.jpg

http://www.galvotec.com/img/texaco.jpg



"Salt water isn't good for anything."

56

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)

http://www.insideline.com/bugatti/veyron-164/2006/worlds-fastest-bugatti-veyron-busts-out-with-1000-hp-and-13-million-price-tag.html

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57



58

• 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"

59

Mighty ships upon the ocean

Suffer from severe corrosion,

Even those that stay at dockside Are rapidly becoming oxide.

Alas, that piling in the sea

Is 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 it

We 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 lateMr. T. R. B Watson of

Corrosion Services

Co., Ltd. in Toronto

and we believe that he

is the author.

f



More Information http://www.electrochem.org/

The Journal of The Electrochemical Society (JES) is the leader in thefield 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

http://www.electrochem.org/

http://www.electrochem.org/

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