Short Course _Kinetics

8/12/2019 Short Course _Kinetics

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Electrodes are pieces of metal on which anelectrochemical 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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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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When zinc is placed in acid the metal willstart to dissolve and hydrogen will start to beliberated according to the potential of themetal

Consider the anodic zinc dissolution reactionZn Zn2++ 2e-


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8

Two reactions are necessary:

-- oxidationreaction:-- reductionreaction: Zn Zn

2 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.)

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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Corrosion normally occurs at a rate determined byan equilibrium between opposing electrochemicalreactions.

The first is the anodic reaction, in which a metal isoxidized, releasing electrons into the metal. The

other is the cathodic reaction, in which a solutionspecies (often O2 or H+) is reduced, removingelectrons from the metal.

When these two reactions are in equilibrium, the

flow of electrons from each reaction is balanced, andno net electron flow (electrical current) occurs.


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Zn Zn2++ 2e-

Rate of ReactionElectrochem

icalPotential

2H++ 2e- H2

Corrosion Potential

Corrosion RateAt the Corrosion Potential, Ecorr,

we have a stable mixedequilibrium

Then the corrosion ratemay be expressed as the

corrosion current density,icorr

Current densityicorr

Ecorr


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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 metalin the absence of electrical connections to themetal 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 currentat Eoc is called the Corrosion Current, Icorr. If we

could measure Icorr, we could use it to calculatethe corrosion rate of the metal.

Unfortunately, Icorr cannot be measureddirectly. However, it can be estimated usingelectrochemical techniques.

In any real system, Icorr and Corrosion Rate are afunction of many system variables including typeof metal, solution composition, temperature,solution movement, metal history, and many

other


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When the potential of a metal sample insolution is forced away from Eoc, it is referredto as polarizing the sample.

The response (current) of the metal sample ismeasured as it is polarized. The response isused to develop a model of the sample'scorrosion behavior.

The pol riz tion expresses the difference betweenthe potenti l of mixed electrode subjected to anodicor cathodic polarization and its corrosion potential.

= E Ecor


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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 = I0

e(2.3(E-E)/)

In this equation,I is the current resulting from the

reactionI0 is a reaction dependent constant called

the Exchange CurrentE is the electrode potentialEo 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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Consider hydrogen evolution in acid:2 H++ 2 e- H2

Actually occurs in two steps:

1 H+

+ e-

Hadseither 2a 2 Hads H2

or 2b Hads+ H++ e- H2


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Fe Fe2++ 2 e- Rate of reaction is proportional to [OH-] in

acid solutions Reaction sequence is thought to be:

1 Fe + H2O FeOH + H++ e-

2 FeOH FeOH++ e-(rds)3 FeOH++ H+Fe2++ H2O

The pH dependence comes from theequilibrium in step 1


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With a multi-step reaction, one step willtypically go more slowly, and thereforecontrol the rate of reaction

Known as rate determining step (rds)


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Additional polarization caused by drop inconcentration of a reactant at the electrodesurface

As concentration falls, more polarization isneeded to make the current flow

Eventually, no more current can flow because

no more reactant can reach the metal, and alimiting currentis reached


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Oxygen reduction is often affected byconcentration polarization

log |current density|

Electrod

e

Potential

Rate of cathodic oxygen reductionwithout concentration polarizationRate of cathodic oxygen reduction

withconcentration polarization

Limiting current density


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If there is a resistance between the anode andthe cathode in a cell, then the current flowingthrough that resistance will cause a potentialdrop given by Ohms Law:

V= IR

This is important for paint films and for highresistance solutions


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log |current density|

Electrode

Potentia

l

Resistance Polarization causes

potential of anode and cathode to

differ due to potential drop across

solution, hence corrosion current is

reduced


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Experimental determination of corrosion ratesTwo types of experimental tests are commonly used to determine the corrosionrate in solution: Immersion tests; Electrochemical tests.


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Corrosion Penetration Rate (CPR)CPR = K W/DAT

K= constant (534for mpy, 87 6for mm/yr)W= weight loss,mgD= density, g/cm3A = Area,in2or cm2

T = time,hr


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Consider the reactionFe Fe2++ 2 e-

For every atom of iron reacting, twoelectrons will be produced.

One mole contains Avogadros number(61023) atoms

The charge on each electron is 1.610-19C Hence each mole produces 296500 C Faradays constant (F) = 96500 C/mole

The atomic

weight ingrams, i.e.55.8 g for Fe


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(g/mole)metalofweightatomic

(g)oxidisedmetalofmass

(C/mole)constantsFaraday'atommetaleachforelectronsofnumber

(C)passedchargewhere

M

m

Fn

Q

M

nFmQ

More

accuratelyrelativeatomic mass,but still withunits g/mole

According to Faradays Law, when ni moles of a given substance

react, a proportional electric charge Q passes across the electrode-electrolyte interface


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(g/s)corrosionofrate

(C/mole)constantsFaraday'atommetaleachforelectronsofnumber

(A)currentcorrosionwhere

M

K

Fn

I

MnFKI

Faradays law thus states that the rate of an electrodereaction is proportional to the magnitude of the electricalcurrent that crosses the electrode-electrolyte interface.


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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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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)/dCorrosion 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 RPStern-Geary Equation:

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

RP= Slope at the origin of the Polarization ResistancePlot in ohms

icorr= corrosion current, Amperesa,c= Tafel Constants from a Tafel Curve,

volts/current decade.


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Electrochemical kinetics of a corroding metal can be characterized bydetermining at least three polarization parameters, such as corrosioncurrent density (icorr),corrosion potential( Ecorr) and Tafel slopes (aand/or c).

Then the corrosion behavior can be disclosed by a polarization curve (Evs.log i).

Evaluation of these parameters leads to the determination of thepolarization resistance Rp and the corrosion rate as icorr which is oftenconverted into Faradaic corrosion rate CR having units of mm/yr.


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The Butler-Volmer equation describes the relationshipbetween the potential and the current (kinetics) in amixed 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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Plot Eagainst log |i|, then activationpolarization gives a straight line

log |current|

Electrod

e

Potential Cathodic reaction, Tafel

slope is negative

Tafel slope expressedas mV per decade of

current

mV

log (-i2) - log (-i1)

Anodic reaction, Tafel

slope is positive

Mixed equilibrium occurswhen sum of all currents is

zero

Eoand iofor the cathodic

reaction

Eoand iofor the anodic

reaction

Ecorrand icorrfor thecorrosion reaction


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ccorr

a

corrcorrapp

EE3.2expEE3.2expii

caapp iii

!n

x

!2

xx1e

n2

x

RELATIONSHIP BETWEEN APPLIED ELECTROCHEMICAL CURRENT

DENSITY AND POTENTIAL FOR A CORRODING ELECTRODE

cacorrcacorrapp

E3.2E3.2i

E3.21

E3.21ii


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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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Polarization resistance, defined as the slope of thepolarization curve at the origin

The extent of linearity depends on the values of Tafel constants selected

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Short Course _Kinetics