4-electrochemical kinetics of corrosion

8/12/2019 4-Electrochemical Kinetics of Corrosion

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CH.3.CH.3. EELECTROCHEMICALLECTROCHEMICAL KKINETICSINETICS OFOF CCORROSIONORROSION

1

Image Source: Corrosion Doctors, www.corrosion-doctors.org


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IntroductionIntroduction Corrosion is thermodynamically possible for most environments.

Thus, it is of primary important to know how fast corrosion occurs.

Methods Weight loss measurements.

A laboratory study and measurements

What should be measured??

Objective

An understanding of the fundamental laws of electrochemical reaction

kinetics.

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Thermodynamics and Kinetics of Corrosion ReactionThermodynamics and Kinetics of Corrosion Reaction

A steel pipe protected

by an organic coating

buried in a corrosive soil

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Kinetics of A ueous CorrosionKinetics of A ueous Corrosion

Anodic and cathodic reactions are coupled at a corroding metalsurface.

5

.

(a) The corrosion process M + O Mn+ + R showing the separation of anodic and cathodic sites.(b) The corrosion process involving two cathodic reactions.


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ElectroneutralityElectroneutrality

There may be more than one cathodic reaction, i.e., more than oneI and more than one anodic reaction i.e. more than one I e.g., for alloy).

== cacorr III

Because of anodic regions, Aa, are generally different from areas of, c, .

Ia

= - Ic

AaAciaAa = - icAc

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Faradays LawFaradays Law

Charge (or corrosion current) is related to mass of material reacted(corroded) in an electrochemical reaction.

nF

Ita

nF

Qam ==

= Ceq mol

g)sA(

[kg]

Q = charge (C)I = current (amperes, A) (1 A = 1 C/s)F = Faradays constant (96500 C/eq)=

m = mass of metal corroded (g)a = molecular (atomic) weight of metal (g/mole)

Corrosion rate, (mg/dm2/day; mdd)

Or

nF

a

tA

na

tA

mr ===

7

A = surface area (mm/y)i = current density, I/A (icorr)

nFtAtA'r ===


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Electrical DoubleElectrical Double La er ELa er EDLDL

A. Helmholtz Model C. Stern ModelB. Gouy-Chapman Model

Charge transfer reactions occur across the compact double layer and the

8

influence of the diffuse layer is usually neglected.


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Electrical Analo of EDLElectrical Analo of EDL

The electrical double layer is characterized by two layers of opposite charge facing each other,as in a capacitor. The electrical current can, however, pass across the metal-solution interface

.analogue composed of a capacitor parallel to a resistance RF called Faradaic resistance. The

RF is called also the polarization resistance or charge-transfer resistance.

Cdl

e-

e-

e--

M+z

+z

e-

Electrical double layer Equivalent Circuit

F

When an electrical current is impressed on the electrode, the RF must be overcome. Thisgenerates additional voltage and causes a shift in the electrode potential. At rest (open circuit),the electrode has a charged layer; in the absence of an electric current, the capacitor Cdl ischarged. The current impressed on the electrode, it, is divided into two parts.

9

it = iF + inF


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Electrical Analo of EDLElectrical Analo of EDL

Where iF : Faradaic currentinF : current of charge accumulated in the capacitor (Non-faradaic current)

sua y F nF .

The electrode potential is proportional to the charge Q of the double layer. Thus, theelectrode potential changes under an electric current across the double layer ;

E = Eeq + (i)

(i)= the additional voltage due to the current flow (overpotential; ).

CdlinF

RF

it

iF

10

Electrode-Electric Analogue


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General Scheme of a Faradaic ProcessGeneral Scheme of a Faradaic Process

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Activation FreeActivation Free EnerEner for Chemical Reactionfor Chemical Reaction

Chemical reaction: AB + C A + BC

ic

G

ic ckech

kTv

RT/0

==

12


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Activation ControlActivation Control

Activation control is when the corrosion is controlled by chargetransfer reactions.

Either the anodic charge transfer or the cathodic charge can control.

studied INDIVIDUALLY by electrochemical methods.

e.g., the changes in potential of an electrode caused by changes in the

measure the POLARIZATION.

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Charge (Electron) Transfer Reaction at Interface:Charge (Electron) Transfer Reaction at Interface:

The metal atoms on the electrode surface are in energy wells associated with the latticestructure, and in order to pass into the solution they have to overcome the activationenergy .

Gch G = Gchem + Gechema

ElectrolyteMetal

G

c

G

a*

M+

MG

Gechem +

+++

10IHP OHP

M M+

GG**

+

++

++

M

S

Helmholtz Gouy-Chapman LayerIHP OHP

X

14

E = M -s


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Char e Transfer Activation Over otentialChar e Transfer Activation Over otential

G* can be changed in electrochemical reactions by externally applied potential (Eapp). Thechange in electrode potential from the equilibrium value to acquire a net current (i.e.,

.

iepp = f(Eapp-Eeq) = f()(1) Anodic polarization

M Mn+ + ne- at Eapp > Eeq

iapp = ia ic = f(a)where Eeq = reversible potential or equil. potential MM+z

e-

e,a

iaic

a = Eapp-Eeq >0 : ano ic overpotentia oranodic overvoltage

(2) Cathodic polarization

: Eapp > Eeq

M Mn+ + ne- ............. at Eapp < Eeq

iapp = ic ia = f(c)

e-

ie,cicia

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c

=app

-eq

: ca o c overpo en a orcathodic overvoltage. M M

+z

: Eapp < Eeq


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Char e Transfer Activation Over otentialChar e Transfer Activation Over otential

If anodic polarization is applied to the metal electrode, what happens to the energy wellcurve? The energy of M(metal) increases by (nF) and the metal ions become moreuns a e g energy s a e .

G

M M+

,ia = ic = io

G*

G

1- nFAfter anodic

1-

nF

Anodic Polarization

Ga*

M Gc*

ie,a = ia - ic > 0

nF

(1-)nF

a

M M+0

nF

-

G*

16IHP OHP

1-0 1

Metal SolutionIHP OHP

0 1 SolutionMetal


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For anodic reaction, G* = G* +(1-)nF - nF = G* -nF

For cathodic reaction, G*c = G* + (1-)nF

ia = Ka exp (- G*a/RT) (K: reaction rate constant; )

- *-a

= Ka exp -(G*/RT) exp (nF/RT)

=io exp (nF/RT) (at equilibrium (=0), ia = -ic = i0)

ic = Kc exp - G c RT

= Kc exp (- G*/RT) exp [-((1-)nF)/RT]

= io exp [-(1-)nF/RT]

iapp = iaic = io{exp (nF/RT) - exp [-(1-)nF/RT]} ............ Butler-Volmer equation

*

ai ,

=RT

expKr ff

G*

nFrf ==

=

=G

ex'KG

ex'Ki*

r

*

f

17

= RTexpr rr RTRT


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Activation PolarizationActivation Polarization

A plot of the Butler-Volmer equation for the metal dissolution/deposition gives thepolarization curve:

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If the symmetric coefficient is 0.5, the curve is symmetrical and has a sinh form.


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A roximation of ActivationA roximation of Activation PolarizationPolarization

(1) At large enough , the reaction is essentially all in one direction (high field approx.)

for > ~0.03 V ...........high field approximation.

iapp ia = io exp (nF/RT)ailo= RT303.2=,

for < ~ -0.03 V .......cathodic polarization

iapp ic = io exp [-(1-)nF/RT]

0

,

nFa

or (where, )0

ccc,act

i

ilog =

nF)1(

RT303.2c

=

At sufficiently large overpotential, the [ i] relationship becomes exponential.

(Tafel behavior)

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A roximation of ActivationA roximation of Activation PolarizationPolarization

(2) when is very small, ..... ||


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Polarization Dia ram Evans Dia ramPolarization Dia ram Evans Dia ram

(1) For anodic polarization

a

Anodic current

a0

a

aa,act i

i

log =

(2) For cathodic polarization

c

EeqEc

cilo=

log io

log |i|

Cathodic current0

,

RT303.2a =

where, io = exchange current density

Eeq = equil. potential, or rest potentialnF)1(

RT303.2c

=

21

a, c : a e cons anNormally, -0.05V < |a, c| < 0.15VFor =0.5, a or c is 0.12 V.


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Activation PolarizationActivation Polarization -- SummarSummar

An electrode reaction is described by i0 and .

How one can determine i0? From charge transfer resistance

0 = n ct ct = at


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One of the assumptions in the derivation of the B-V equation is the uniformity of concentrationnear the electrode. This assumption fails at high current densities because migration of ionstowards the electrode from the bulk is slow and ma become rate determinin . A lar eroverpotential is then needed to produce a given current because the supply of reducible or

oxidizable species has been depleted. This effect is called concentration polarization.

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Activation control Mass Transfer control


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For reduction reaction:

Mn+ + ne- M

eq = B .................

When a passage of external current is made through the cell, the interfacial conc. changes to avalue of CS, resulting in change of electrode potential.

ep = e+ RT/nF ln CS ..................(2)

Concentration overvoltage, conc

conc = ep - eeq = RT/nF ln Cs/CB ...(3)

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Concentration PolarizationConcentration Polarization

From the Ficks 1st law, a flux of cathodicreactant to surface, J, is:

J= -D C/x = -D (CS - CB)/ = i/nF

= Nernst layer, ~0.1mm.

So, i = DnF(CBCS)/

In the limit,

CS 0, i iL: limiting current density,

Thus, iL = DnFCB/

i = iL(1 - CS/CB), CS/CB = (1 - i/iL) c = Ep - Eeq =RT/nF ln Cs/CB

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Finally, = Lconc

i1lognF

.


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Polarization curve

for the cathodic process

Point 1: Small shift from equilibrium. No limitation on reactant supply activation control.

=

26

o a ac conc

Point 3: Large shift from equilibrium reaction rate maximum, conc infinite.


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During corrosion, concentration polarization for anodic dissolution can be ignoredbecause an unlimited supply of metal atoms is available at the interface. But, at highcorros on ra es, e concen ra on o n+ on n e ano y e s s gn can y ncrease anexert a back emf which results in anodic conc. polarization.

iL

A

+

+

Tafel (Linear Kinetics)

afel Region

++

+ ++++

+++

+M

27

log ia Conc. polarization


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Combined PolarizationCombined Polarization

Total cathodic polarization

+=+=

L

c

0

ccc,concc,actc

i

i1log

nF

RT303.2

i

ilog

Total anodic polarization

0

aaa,acta

ilog ==

29


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Summar : TafelsSummar : Tafels LawLaw

ten

tial

b=2.303

P

E0a

Slope b

i

30

,


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SummarSummar

[O2]

Cathodic reaction -rate increases withRate with constantsurface concentrationRate with surface

[O2]

tia

l

eq,c of oxygenconcentration ofoxygen varying

[O2]

Pote

ln |i|i0,c log |i| iL

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Parameters (or a, c), i0, iL can be used to describe virtually all electrochemical corrosion systems.


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Resistance Overvolta eResistance Overvolta e

Resistance Overvoltage (R) arises from the passage of electriccurrent throu h an electrol te solution with low conductivitsurrounding the electrode.

Significant when surface oxide films forms on the electrode surfaceas a result of electrochemical reaction.

R = i(Relectrolyte + Rfilm)

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Mixed PotentialMixed Potential TheorTheor

Presented by Wagner and Traud in 1938.

Based on the following hypotheses

All free corrosion reactions involve at least one anodic and one cathodic.

During the corrosion of an electrically isolated metal, the total partialanodic current must equal to the total partial cathodic current :

Aaia = Acic at E = Ecorr (mixed equilibrium)

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We plot the cathodic reaction on the same diagram as the anodicreaction (Butler-Volmer expression)

Current-potentialrelationshi s for a metaldissolution/depositionand an accompanying

redox reaction showing

couple together at thecorrosion potential, Ecorr


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Mixed Potential TheorMixed Potential Theor

Note: ia = -ic (=icorr) at one spot on the diagram the corrosion potential Ecorr

Ecorr is the mixed potential

eeq,a < Ecorr < eeq,c

act,a = Ecorr - eeq,a

And the cathodic reaction is driven by the cathodic activation overpotential:

act,c = eeq,c - Ecorr

Note: the thermodynamic driving force for corrosion, Etherm

therm = eeq,c eeq,a

Usually, Etherm is large enough to put Ecorr in the Tafel regions for both reactions(i.e., the reverse reactions are negligible) unless oxide films interfere.

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Evans DiagramEvans Diagram

The coupled portions of the curves for the anodic and cathodic reactions (i.e., ia +ve, icve) are usually plotted as potential vs. logarithm of the current, with the negative sign ofthe cathodic curve neglected

Both curves appear in thepositive quadrant.

This is the Evans Diagram.

Evans diagram for the corrosion

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process + n+ +


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Mixed Potential TheoryMixed Potential Theory--

ial

Ta el slo e ex ressed as mV

Eo and iofor the cathodic reaction

Mixed equilibrium occurs when sumof all currents is zero

e

Potent

per decade of current

mV

,

positiveEcorrand icorr

for the corrosion reaction

Electro

log (-i2) - log (-i1)

Cathodic reaction,

Tafel slope is negative

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Eo and iofor the anodic reaction


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Mixed Potential Theor :Mixed Potential Theor : Corrosion of Zn in AcidCorrosion of Zn in Acid

Consider Zn undergoing active corrosion in a deaerated HCl solution at 25oC.

Anodic reaction: Zn2+ + 2e- Zn

eeq,Zn = e0Zn2+/Zn + 0.059/2 log aZn2+= - 0.763 + 0.059/2 log 10-6

= - . - . = - . .

Cathodic reaction : 2H+

+ 2e-

H2= 0

Zn2+

2e-

2H+ H2 H+

Cl-eq, 2 . 2

= - 0.059 V (at PH2= 1 atm and pH =1)

For hydrogen reduction reaction,

Zn

eeq,H2 = 0.059 V, io,H+/H2(on Zn) = 10-11A/cm2, c = 0.12 V/decade

For Zn oxidation reaction,

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eeq,Zn

= . ,o,Zn

2+

/Zn= - cm ,

a= . eca e


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Reduction Reactions Involvin OReduction Reactions Involvin O and Hand H OO

2

2 2 + + e- = 2 2 + 2 + e

- = - 2 + 2 + e- = -

Without O2 2H+ + 2e- = H2 2H2O + 2e

- = H2 + 2OH- 2H2O + 2e

- = H2 + 2OH-

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Mixed Potential Theor :Mixed Potential Theor : Corrosion of Zn in AcidCorrosion of Zn in Acid

+0.2

eeq,H2 = 0.059

io,H+/H2 (Zn) 2H+ +2e- H2

0

-0.2-0.4

c

SHE)V

Ecorr

eeq,Zn = 0.94

n n + + e--0.6

-0.8

-

a

E

corr

io,Zn2+/Zn10-10 10-8 10-6 10-4 10-2 10-0

log |i| A/cm2

Corrosion potential (Ecorr) and corrosion current density (icorr) aredetermined at the point where the total rates of oxidation and reductionare equal.

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icorr (the rate of Zn dissolution) = iH2at Ecorr (the rate of hydrogen evolution at Zn surface)


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Mixed Potential TheorMixed Potential Theor

Ecorr

Rest potential

Open circuit potential

.

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Mixed Potential TheoryMixed Potential Theory

eeq,c

eeq,c

Ecorr

Ecorr

act,a

act,a

eeq,a

Generally, Etherm [= (eeq,c - eeq,a)] < Etherm [= (eeq,c - eeq,a)]

So, icorr < icorr

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, act,a act,a

Thermodynamics are controlling


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Mixed Potential TheoryMixed Potential Theory

eeq,c eeq,c

eeq,c

Ecorr

eeq,c

act,c

act,c

eeq,a

Ecorr

act,a

act,a

icorr < icorr icorr > icorr

For both cases E = e - e < E = e - e

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, , , ,

Kinetics are controlling


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Effect of Exchan e Current DensitEffect of Exchan e Current Densit

i0,H+/H2(on Zn) = ~10-11 A/cm2 i0,H+/H2(on Fe) = ~10

-6 A/cm2

oten

tial

io,H2 (on Fe)io,H2 (on Zn)

io,Fe

Ecorr(Fe)

corr n

io,Zn

44

log |i|icorr(Zn) icorr(Fe)


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Effect of Exchan e Current DensitEffect of Exchan e Current Densit

Er,Zn = 0.94V < Er,Fe = 0.62 V

i > i, ,This is due to the lower exchange current density for hydrogen evolution on

Zn compared to that on Fe. i,e io,H+/H2(Zn) < io,H+/H2(Fe)

The exchange current density ( io,H+/H2) for hydrogen evolution reaction ishighly sensitive to the nature of the metal substrate on which the reaction

occurs, and are markedly reduced by the presence of trace impurities such, , , .

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Effect of OxidizerEffect of Oxidizer

The driving force for corrosion isincreased by the addition of a strongerox zer, a s, a re ox sys em w ahalf cell electrode potential much more

noble than that of any others present.Consider the corrosion of metal M inan acid containing Fe2+-Fe3+.

Fe +

Fe3+

Fe2+

Fe3+

H2 H+

H2O

46

2H+2e-


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Effect of OxidizerEffect of Oxidizer

Ecorr is determined by the point at which the total rate of oxidation equals the total rate ofreduction.icorr= iMM+ = iFe3+Fe2+ + iH+H2

(1) shifts corrosion potential in the noble direction,

(2) increases the corrosion rate from i'corr to icorr,(3) decreases hydrogen evolution from i'corr to iH+H2.

The effect of an oxidizer on the corrosion rate is dependent on its redox potential and itsexchange current density.

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Effect of Dissolved Ion ConcentrationEffect of Dissolved Ion Concentration

Drawing appropriate polarization diagrams, determine the effect ofincreasin the concentration of dissolved H+ on E and i of ametal M corroding to dissolved M+ in a deaerated acid solution under

activation control with all other parameters constant.

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Exam le ProblemsExam le Problems

The corrosion potential of mild steel in a deaerated solution of pH = 2 is -0.324V vs.SHE. Determine the corrosion rate in mm/y taking the exchange current density forhydrogen evolution on the steel, i0 = 10

-7A/cm2, the Tafel constant for thehydrogen evolution reaction c = -0.1 V and the molecular weight of steelmwFe=55.85 and density of the steel Fe = 7.89 g/cm3.

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Effect of Concentration PolarizationEffect of Concentration Polarization

At low cathodic polarization the reduction process is activation controlled, but at highpolarization it is diffusion or concentration controlled.

a = a log i/io

c=

clog i/i

o+ 2.3RT/nF log(1 - i/i

L) [3.14]

in dilute aerated seawater. The cathodic process is reduction of dissolved oxygen (DO).

The maximum solubility of dissolved oxygen in water is relatively low, about 8 ppm at

ambient temperature.

O2 + 2H2O + 4e- 4OH-

iL = 100A/cm2

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Oxygen reduction is often affected by concentration polarization

Rate of cathodic oxygen reductionwithout concentration polarization

ten

tial

l

ectrode

P Rate of cathodic oxygen reduction withconcentration polarization

-

lo current densit

E rate of reaction limited by availability of

oxygen at the metal surface

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Ecorr,B

Ecorr,CioC+/C

oH+/H2(M)

Ecorr,A ioB+/B

ioA+/A

In this system, icorr=iL,

depending on conc., temperature and

L

For different metals; A, B and C,

icorr is equal to iL..

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Resistance PolarizationResistance Polarization

If there is a resistance between the anode and the cathode in a cell,then the current flowin throu h that resistance will cause apotential drop given by Ohms Law:

This is important for paint films and for high resistance solutions. .,

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Resistance PolarizationResistance Polarization

Resistance Polarization causes

potential of anode and cathode todiffer due to otential dro across

ot

ential solution, hence corrosion current is

reduced

l

ectrode

log |current density|

E

54

E i l P l i i CE i l P l i i C


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Experimental Polarization CurveExperimental Polarization Curve

corr 0 from polarization data (Ch.5).

Fig. 3.15. - The Tafel behavior is limited to only about one decade ofcurrent densit .

55

Fig. 3.16. Effect of pH (concentration polarization)

E i t l P l i ti CE i t l P l i ti C


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Experimental Polarization CurveExperimental Polarization Curve

Increase cathodic polarization by c (from c = Ecorr ec to c = E*-ec) and plotapplied current, iapp vs. potential, E.

i + i = i

56

Same principle is applied in cathodic protection which is to be covered later.


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Ex erimental Polarization CurveEx erimental Polarization Curve

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Iron in hydrochloric acid

en

tial

trode

Po

Cathodic hydrogen evolutionAnodic iron dissolution

Ele

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Iron in sulfuric acid

en

tial

Anodic iron dissolution (with active-

passive transition)

Oxygen evolution on passive film (or

transpassive corrosion as metal is

trode

Po

Cathodic hydrogen evolution

ox se o a g er ox a on s a e

Ele

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Iron in aerated neutral NaCl Solution

Anodic iron dissolution

Po

tentia

Electrode

Cathodic oxygen reduction

log |current density|

Cathodic hydrogen evolution

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NonNon--ideal Experimental Polarization Curvesideal Experimental Polarization Curves

Use of cathodic polarizationdata to form the anodicpolarization curve in the case ofnon-linear anodic data(distorted probably byaccumulated corrosion productsat high currents)

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InstrumentationInstrumentation

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InstrumentationInstrumentation

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Measurement MethodsMeasurement Methods

Potential control

Reference Electrode -

corresponding terminals on

potentiostat

AE

RE

Potentiostat reference connection forpotential measurement

Luggin Probe - allows potentialto be detected close to metal

surfaceWE

Working Electrode - metal

Counter Electrode (orAuxilliary Electrode orSecondary Electrode) -

Potentiostat controls

65

solutionpotential


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Measurement MethodsMeasurement Methods

Current control

Current path

AE

RE

Potentiostat

WECounter Electrode

R

Current controlled by control of

Reference Electrode -

66

Working Electrode

limit IR errorvoltage across resistor (I=V/R)

,

connected to potentiostat


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Workin ElectrodeWorkin Electrode

Requirements

representative

free of crevices

free of galvanic effects

free of water-line effects

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Epoxy-embedded electrode:Apply thin layer of epoxy tominimise stress and risk of

Pretreat specimen forood adhesion

Weld or solder connecting wireto specimen

Apply thick layer of epoxy toseal connecting tube and for

68

expose metal Clean surface - dont use acetone


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WorkinWorkin Electrode C lindricalElectrode C lindrical

Retaining nut

Washers

Heavy-walled

glass tube

PTFE Washer

Electrode

Lip seal between PTFE case andelectrode

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Avesta cell:

SolutionPureH2O feed

Filter paper

70

f l df l d


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Reference ElectrodeReference Electrode

Commonly use Saturated Calomel Electrode (SCE)

Check one against another (should not be more than 1 to 2 mV

difference)

connect to working or counter electrode)

Do not allow to dry out

Solution in SCE (or Ag/AgCl electrode) is saturated KCl

Beware of chloride contamination of test solution by Cl- leaking fromreference electrode

Make sure solution remains saturated

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Reactions on CE During Cathodic PolarizationReactions on CE During Cathodic Polarization

For the working electrode as an anode, metal dissolution reactions of the type

M Mn+ + ne- 1

are of interest in corrosion.

When the working electrode is polarized as cathode (auxiliary electrode as anode), M must beselected for the auxiliary electrode with a very noble eM/Mn+ to prevent anodic dissolution, which wouldcontaminate the electrolyte. Either platinum or carbon/graphite is the usual choice.

In the absence of anodic dissolution at the auxiliary electrode by reaction (1), other anodic oxidationreactions are possible to liberate electrons. These include oxidation in a redox reaction such as

Fe2+ Fe3+ + e- (2)

And oxygen evolution by

4OH- 2H2O + O2 + 4e- (3)

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Reactions (2) and (3) both must operate at potentials below eM/Mn+ (e.g. ePt/Pt3+) so that noble metalauxiliary electrode is not dissolved.

S l iS l i


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SolutionSolution

Requirements:

remain the same (pH, composition) throughout the experiment - ensure

that volume is adequate ox en concentration often critical - aerate b bubblin air or O or

deaerate with N2 or Ar

most reactions are temperature sensitive, so control, or at least record,

temperature

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SS


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SummarSummar

1.23

Eh

Cu2+

CuO

eq(c)

Tafel

Ecorr

c

u = u + e

0Cu Cu2O

CuO22-

eeq(a)

Tafela

1/2O2+ 2H+ +2e = H2O

ogo o(a)o o(c) log icorr

74

pH0 5 15

SS


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SummarSummarEh

total reduction current=

i O + i H+.1/2O2 + 2H

+ + 2e = H2O

0

2H+ + 2e = H2Zn2+ n 2

ZnO22-

pH0

Zn

7 14

Zn = Zn2+ + 2e

Log icorr

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SummarySummary


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SummarySummary

(1) Equilibrium potential, eeq

(2) io

(3)

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SS


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SummarSummar

77

R fR f


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ReferencesReferences

Textbook (D.A. Jones)rd -. . , , ., , .

Homepage for Prof. Kwons Laboratory, http://corrosion.kaist.ac.kr

Lecture Notes by Dr. D.H. Lister, and Dr. W. Cook, Department of, .

H.H. Uhlig and R.W. Revie, Corrosion and Corrosion Control, John

Wiley and Sons, 1985.

78

H k P blH k P bl


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Homework ProblemsHomework Problems

Problems 1, 6, 7, 8, 10, 11 of Chapter 3 in textbook.

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F d C t tF d C t t


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Farada s ConstantFarada s Constant

One mole of metal (MW g) contains Avogadros number (61023) ofmetal atoms

Hence each mole of metal will produce n times that many number

of electrons-19 .

Hence, one mole of metal will produce a charge of n96500 C

96,500 C/equivalent is known as Faradays constant onvers ons: = s, =

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O en Sol bilitO en Sol bilit


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Ox en SolubilitOx en Solubilit

-

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. . , , , , .

Effect of Diffusion Rate of OxidizerEffect of Diffusion Rate of Oxidizer


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Effect of Diffusion Rate of OxidizerEffect of Diffusion Rate of Oxidizer

O2 + 4H+ + 4e 2 H2O :

ic = 4Fv = 4FD(Cb-Ci)/

Limiting current density iL= 4FDCb/ Cb :

:

'

Ecorr

B

C

' A: ,

B: A noble A .

C: .

log ilog icorr

A

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Counter electrodeCounter electrode


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Counter electrodeCounter electrode

Counter electrode should allow current to pass with tolerableolarization

Often claimed that counter electrode should have much larger area

than working electrode, but this is not often necessary for corrosionstudies

Usually use platinum or graphite, although stainless steel can beused in some situations (e.g. where only anodic polarization of

s ecimen is used

4-electrochemical kinetics of corrosion

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