kinetics lecture2011

8/12/2019 Kinetics Lecture2011

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Electrochemical Kinetics of

Corrosion


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Electrochemical Reactions

Characteristics of EC reactions that both

oxidation(produce e-)and reduction

( consume electrons) occur and electrons

transfer. eg : Zinc or iron in HCl


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Electrodes

Electrodes are pieces of metal on which an

electrochemical reaction is occurring

An anodeis an electrode on which an

anodicor oxidation reaction is occurring

A cathode is an electrode on which a

cathodicor reduction reaction is occurring


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Single and mixed electrodes

A copper electrode in contact with its own ions (single

electrode) and with an aerated solution (mixed electrode).

CuCu2+ + 2 e

1/2O2+2H++2eH2O


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Corrosion of zinc in acid

When zinc is placed in acid the metal will

start to dissolve and hydrogen will start to

be liberated according to the potential of themetal

Consider the anodic zinc dissolution

reactionZn Zn2++ 2e-


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8

Two reactions are necessary:

-- oxidation reaction:-- reductionreaction:

Zn Zn2 2e

2H2e H2(gas)

Other reductionreactions:

-- in an acid solution -- in a neutral or base solution

O2 4H4e 2H2O O2 2H2O4e

4(OH)

Adapted from Fig. 17.1, Callister 7e.

(Fig. 17.1 is from M.G. Fontana,

Corrosion Engineering, 3rd ed.,

McGraw-Hill Book Company, 1986.)

CORROSION OF ZINC IN ACID

Zinc

Oxidation reaction

Zn Zn2+

2e-Acidsolution

reduction reaction

H+H+

H2(gas)

H+

H+

H+

H+

H+

flow of e-in the metal


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Review of the Electrochemical Basis of

Corrosion

Corrosion normally occurs at a rate determined by anequilibrium between opposing electrochemical reactions.

The first is the anodic reaction, in which a metal is

oxidized, releasing electrons into the metal. The other is

the cathodic reaction, in which a solution species (often O2or H+) is reduced, removing electrons from the metal.

When these two reactions are in equilibrium, the flow of

electrons from each reaction is balanced, and no net

electron flow (electrical current) occurs.


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Corrosion of zinc in acid

Zn Zn2++ 2e-

Rate of ReactionElectroch

emicalPote

ntial

2H++ 2e- H2Corrosion Potential

Corrosion RateAt the CorrosionPotential, Ecorr, we have astable mixed equilibrium

Then the corrosionrate may be

expressed as the

corrosion currentdensity, icorr

Current densityicorr

Ecorr


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Graphical Representation of the Butler-Volmer Relationship between

Potential and Current in a Mixed Potential System

J. Scully & R. Kelly, ASM Handbook, Volume 13A,2003

M + 2H+ M2++ H2

An experiment like this is calleda Tafel Plot and is relatively

common in todays corrosionlaboratory.

Experimental result from thecorrosion measurement system.


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The equilibrium potential assumed by the metal in the

absence of electrical connections to the metal is called the

Open Circuit Potential, Eoc. The terms Eoc (Open Circuit

Potential) and Ecorr (Corrosion Potential) are usually

interchangeable, but Eoc is preferred.

The value of either the anodic or cathodic current at Eoc is

called the Corrosion Current, Icorr. If we could measure

Icorr, we could use it to calculate the corrosion rate of themetal.

Unfortunately, Icorr cannot be measured directly. However,

it can be estimated using electrochemical techniques. In any

real system, Icorr and Corrosion Rate are a function of manysystem variables including type of metal, solution

composition, temperature, solution movement, metal history,

and many other


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When the potential of a metal sample in solution is

forced away from Eoc, it is referred to as polarizing the

sample.

The response (current) of the metal sample is measuredas it is polarized. The response is used to develop a

model of the sample's corrosion behavior.

The polarization expresses the difference

between the potential of a mixed electrode

subjected to anodic or cathodic polarization andits corrosion potential.

=E Ecor


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Types of Polarization

Activation Polarization

The polarization necessary for the electrochemical

reaction to go at the given rate

Given by Tafels Law:

o

o

i

iEE log

E = potential at current i

Eo= potential at current io

= Tafel slope


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An electrochemical reaction under kinetic control obeys the

Tafel Equation.

I = I0e(2.3(E-E)/)

In this equation,

I is the current resulting from the reaction

I0 is a reaction dependent constant called the

Exchange Current

E is the electrode potential

Eo is the equilibrium potential (constant for a

given reaction) is the reaction's Tafel Constant (constant for

a given reaction).

Beta has units of volts/decade.


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Hydrogen Evolution

Consider hydrogen evolution in acid:

2 H++ 2 e- H2

Actually occurs in two steps:

1 H++ e- Hads

either 2a 2 Hads H2or 2b Hads+ H

++ e- H2


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Anodic Dissolution of Iron

Fe Fe2++ 2 e-

Rate of reaction is proportional to [OH-] in acid

solutions Reaction sequence is thought to be:

1 Fe + H2OFeOH + H++ e-

2 FeOH FeOH++ e-(rds)

3 FeOH++ H+Fe2++ H2O

The pH dependence comes from the equilibriumin step 1


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Rate Determining Step

With a multi-step reaction, one step will

typically go more slowly, and therefore

control the rate of reaction Known as rate determining step (rds)


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Concentration Polarization

Additional polarization caused by drop in

concentration of a reactant at the electrode

surface As concentration falls, more polarization is

needed to make the current flow

Eventually, no more current can flow becauseno more reactant can reach the metal, and a

limiting currentis reached


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Concentration Polarization

Oxygen reduction is often affected by

concentration polarization

log |current density|

Electrode

Potential

Rate of cathodic oxygen

reduction without


Rate of cathodic oxygen

reduction with


Limiting current density


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Resistance Polarization

If there is a resistance between the anode

and the cathode in a cell, then the current

flowing through that resistance will cause apotential drop given by Ohms Law:

V=IR

This is important for paint films and forhigh resistance solutions


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Resistance Polarization

log |current density|

Elec

trode

Poten

tial

Resistance Polarizationcauses potential of anode

and cathode to differ due

to potential drop across

solution, hence corrosion

current is reduced

E i t l d t i ti f i t


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Experimental determination of corrosion rates

Two types of experimental tests are commonly used to determine the

corrosion rate in solution:

Immersion tests;

Electrochemical tests.


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Corrosion Rate Expression

Corrosion Penetration Rate (CPR)

CPR = K W/DAT

K= constant (534for mpy, 87.6for mm/yr)

W= weight loss,mg

D= density, g/cm3

A = Area,in2or cm2

T = time,hr


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Faradays Law

Consider the reactionFe Fe2++ 2 e-

For every atom of iron reacting, two electrons willbe produced.

One mole contains Avogadros number (61023)atoms

The charge on each electron is 1.610-19

C Hence each mole produces 296500 C

Faradays constant (F) = 96500 C/mole

The atomic

weight in grams,

i.e. 55.8 g for Fe


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Faradays Law

(g/mole)metalofweightatomic

(g)oxidisedmetalofmass

(C/mole)constantsFaraday'atommetaleachforelectronsofnumber

(C)passedchargewhere

M

m

Fn

Q

M

nFmQ

More accurately

relative atomicmass, but still

with units g/mole

According to Faradays Law, when ni moles of a givensubstance react, aproportional electric charge Q passes across

the electrode-electrolyte interface


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Faradays Law Divide by time


(g/s)corrosionofrate

(C/mole)constantsFaraday'atommetaleachforelectronsofnumber

(A)currentcorrosionwhere

M

K

Fn

I

MnFKI

Faradays law thus states that the rate of an electrode reaction is

proportional to the magnitude of the electrical current that

crosses the electrode-electrolyte interface.


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Faradays Law Divide by area:


)m(g/scorrosionofrate

(C/mole)constantsFaraday'

atommetaleachforelectronsofnumber

)(A/cmdensitycurrentcorrosionwhere

2

2

M

k

F

n

i

nF

iM

k

M

nFki


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Calculation of Corrosion Rate from icorr

From the engineering standpoint, it is convenient to expressCorrosion Rate in units of penetration, mpy (milli-inches per year)

or mmpy (mm per year).

Divide both sides of the equation by area and density (g/cm3),

Corrosion Rate (mpy) = 0.13 icorr(M/n)/d

Corrosion Rate (mmpy) = 0.00327 icorr(M/n)/d

where icorr is corrosion current density in A/cm2.

M/n = equivalent weight, for Iron = 27.92


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Calculation of icorrfrom RP

Stern-Geary Equation:

RP= E/i = ac/2.3 icorr(a + c)

RP= Slope at the origin of the Polarization Resistance Plot inohms

icorr= corrosion current, Amperes

a,c= Tafel Constants from a Tafel Curve, volts/current decade.


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In Short

Electrochemical kinetics of a corroding metal can be characterized

by determining at least three polarization parameters, such as

corrosion current density (icorr),corrosion potential( Ecorr) and

Tafel slopes (a and/or c).

Then the corrosion behavior can be disclosed by a polarization

curve (E vs.log i).

Evaluation of these parameters leads to the determination of thepolarization resistance Rp and the corrosion rate as icorr which is

often converted into Faradaic corrosion rate CR having units of

mm/yr.


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Butler-Volmer EquationThe Butler-Volmer equation describes the relationship between the

potential and the current (kinetics) in a mixed potential system.

I= Ia+ Ic= ICORR(e(2.3(E-Eoc)/a)e(-2.3(E-Eoc)/c))

Where:I = cell current (A)

ICORR= corrosion current (A)

E = applied potential (V)

Eoc= corrosion potential (V)a= anodic Tafel constant (V/decade)

c= cathodic Tafel constant (V/decade)

Rate of anodic reaction Rate of cathodic reaction


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E-log i and Evans Diagrams

PlotEagainst log |i|, then activation

polarization gives a straight line

log |current|

Electrod

e

Potential Cathodic reaction,

Tafel slope is

negative

Tafel slope

expressed as

mV per decade

of current

mV

log (-i2) - log (-i1)

Anodic reaction,

Tafel slope is

positive

Mixed equilibrium

occurs when sum of

all currents is zero

Eoand iofor the

cathodic reaction

Eoand iofor the

anodic reaction

Ecorrand icorrfor thecorrosion reaction


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LPR

ccorr

a

corrcorrapp

EE3.2expEE3.2expii

caapp iii

!n

x

!2

xx1e

n2

x

RELATIONSHIP BETWEEN APPLIED

ELECTROCHEMICAL CURRENT DENSITY ANDPOTENTIAL FOR A CORRODING ELECTRODE

cacorrcacorrapp

E3.2E3.2i

E3.21

E3.21ii


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STERN-GEARY EQUATION

A potential, usually 10-20 mV is applied

to a freely corroding element and the

resulting linear current response is measured.

Ohms Law

I=E/R

Therefore

R=E/I

This R is inversely

related to the

Corrosion Rate

(CR)

pca

ca

p

corr

cacorr

ca

0EEapp

2p

R

B

R3.2

1i

i3.2i

EcmR

corr

cacorrapp 11E3.2ii


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TYPICAL LINEAR POLARIZATION RESISTANCE

CURVEPolarization resistance, defined as the slope of the

polarization curve at the origin

The extent of linearity depends on the values of Tafel constants

selected

kinetics lecture2011

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