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

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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.

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.

(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 ===

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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

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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.

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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

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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

==

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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

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

*

ai ,

=RT

expKr ff

G*

nFrf ==

=

=G

ex'KG

ex'Ki*

r

*

f

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= 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