kinetics of corrosion

7/22/2019 Kinetics of Corrosion

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K inet ics of Corros ion

Bob Cottis

This presentation is copyright Bob Cottis 1997. All rights reserved. It may not be copiedas a whole or in part in any form without prior written permission of the author.


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Recommended Books

Corrosion Engineering, Fontana

Corrosion, Shreir


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What is Corros ion?

Reaction of a metal with its environment

Aqueous corrosion

reaction with water (usually containing dissolvedions)

High temperature oxidation

reaction with oxygen at high temperature

High temperature corrosion

reaction with other gases


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Examples o f Corros ion

Rusting of steel

corrosion product (rust) is solid but not protective

Reaction of aluminium with water corrosion product is insoluble in water, so may be

protective

Burning of magnesium in air

high temperature oxidation


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Corros ion of Zinc in Acid


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Acids and Bases

An acid is a substance that produces excesshydrogen ions (H+) when dissolved in water

examples are HCl, H2SO4A base is a substance that produces excess

hydroxyl ions (OH-) when dissolved in water

examples are NaOH, KOH


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Corros ion of Zinc in Acid

Zinc dissolves with hydrogen evolution

Zn + 2HCl ZnCl2+ H2

Zinc known as a base or active metal

One atom of zinc metalplus two molecules of hydrogen

chloride (hydrochloric acid)

reacts to form

goes to

one molecule of zinc chlorideplus one molecule of hydrogen gas


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Corros ion o f Plat inum in

Ac id

Platinum does not react with acids

Platinum is known as a noblemetal


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Connect ion of Plat inum to

Zinc

Zn Pt

HCl

Zinc and platinum notconnected, no reactionon platinum

Zinc and platinumconnected, current flowsand hydrogen is evolved

on platinum

Zn + 2HCl ZnCl2+ H2

metal + acidsalt + hydrogen

Zn Zn2++ 2e-

metalmetal ions + electrons

2H++ 2e- H2

hydrogen ions + electronshydrogen gas

electrons


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Connect ion of Plat inum to

Zinc

Zn + 2HCl ZnCl2+ H2

But we can separate metal dissolution andhydrogen evolution

Zn Zn2++ 2e-

Reactions that involve both

chemical change and the

transfer of charge

2H++ 2e- H2

These are known as electrochemicalreactionsOne atom of zinc metal

one zinc ion in solutiontwo electrons in the metal


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External Curren t App l ied to

Plat inum in Acid

Pt Pt

HCl

+-

Oxygen evolved on

positive electrode

2H2OO2+ 4H++ 4e-

Hydrogen evolved on

negative electrode

2H++ 2e-H2

Overall reaction

2H2O2H2+ O2

Acid - chemical species that

produces hydrogen ions in water


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Plat inum in A lkal i

Pt Pt

NaOH

+-Oxygen evolved on

positive electrode

4OH-O2+ 2H2O + 4e-

Hydrogen evolved on

negative electrode

2H2O + 2e-H2+ 2OH

-

Overall reaction

2H2O2H2+ O2

Alkali - chemical species that

produces hydroxyl ions (OH-)

in water

Note that H+and OH-are in

equilibrium in water:

H2O

H+

+ OH-

The product of [H+] times [OH-]

is 10-14, so in pure water both

[H+] and [OH-] are 10-7. Thisleads to the concept ofpH,

which is defined as -log[H+]

Hence pH = 0 is strong acid, 7is neutral, and 14 is strong

alkali


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Plat inum

Hydrogen evolution at one electrode

2H++ 2e- H2 (acids)

or 2H2O + 2e- H2+ 2OH- (alkalis)

A piece of metalin the solution

Oxygen evolution at the other electrode

2H2O O

2+ 4H++ 4e- (acids)

or 4OH- O2+ 2H2O + 4e- (alkalis)


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Faradays Law

Charge is related to mass of material reactedin and electrochemical reaction:

2H+

+ 2e-

H2

Two hydrogen

ions

React with twoelectrons

To produce one molecule

of hydrogen gas


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Faradays Constant

One mole of hydrogen ions (1 g) containsAvogadros number (6 1023) ions

Hence electrons will react with each mole ofhydrogen ions

Charge on the electron is 1.6 10-19CHence one mole of ions requires 96500 C

This is known as Faradays constant


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Faradays Law

(g/mole)metalofweightatomic

(g)oxidisedmetalofmassedtransferrelectronsofnumber

C/mole)(96500constantsFaraday'

(C)chargewhere

M

m

n

F

Q

M

nFmQ


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Electrodes

Electrodes are pieces of metal on which anelectrochemical reaction is occurring

An anodeis an electrode on which an anodicor oxidation reaction is occurring

A cathodeis an electrode on which acathodicor reduction reaction is occurring


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Anod ic React ions

Examples

Zn Zn2++ 2e- zinc corrosion

Fe Fe2++ 2e- iron corrosion

Al Al3++ 3e- aluminium corrosion

Fe2+ Fe3++ e- ferrous ion oxidation

H2 2H+

+ 2e-

hydrogen oxidation

2H2O O2+ 4H++ 4e- oxygen evolution

Oxidationreactions

Produceelectrons


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Cathod ic React ions

Examples

O2+ 2H2O + 4e- 4OH- oxygen reduction

2H2O + 2e- H2+ 2OH-hydrogen evolution

Cu2++ 2e- Cu copper plating

Fe3++ e- Fe2+ ferric ion reduction

ReductionreactionsConsumeelectrons


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Metal Ion Hyd ro lys is

Note that metal ions may react with water (ahydrolysis reaction)

e.g. Al3+

+ 3H2O Al(OH)3+ 3H+

or 2Al3++ 3H2O Al2O3+ 6H

+

Note that in an electrochemical reaction, we

have the same number of each atom on eachside of the equation, and the same overallcharge


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Effec t o f Po ten t ial

Electrochemical reactions involve transfer ofcharge

Hence, we expect that the voltage of themetal with respect to the solution will affectelectrochemical reactions

Voltage of metal with respect to solution is

known as theelectrochemical potential


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Corros ion o f zinc in acid

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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Corros ion o f zinc in acid

Zn Zn2+

+ 2e-

Rate of ReactionElectroch

emicalPote

ntial

2H++ 2e- H2

Rate of ReactionAt the CorrosionPotential, Ecorr, we have astable mixed equilibrium

Then the corrosionrate may be

expressed as the

corrosion currentdensity, icorr

Current densityicorr

Ecorr

As the reaction involvestransfer of charge, the rate

of reaction may beexpressed as a current per

unit area, or current density

If the potential is belowthe Corrosion Potential,then it will rise, due to

consumption of electrons

If the potential is abovethe Corrosion Potential,then it will fall due to

production of electrons


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How Fast w il l Corros ion

Occur?

Corrosion kinetics

Concerned with the rates of corrosion reactions

Mixed potential theory: The corrosion potential will be that potential at

which the sum of all anodic (positive) and cathodic(negative) currents on the electrode is zero

Polarization The change in potential that is caused by the

passage of a current


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Types o f Po lar izat ion

Activation Polarization

The polarization necessary for the electrochemicalreaction to go at the given rate

Given by Tafels Law:

o

oi

ibEE log

E = potential at current i

Eo= potential at current io

b= Tafel slope


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E-log i and Evans Diagrams

Plot Eagainst log |i|, then activationpolarization gives a straight line

log |current|

ElectrodePotential Cathodic reaction,

Tafel slope is

negative

Tafel slope

expressed as

mV per decade

of current

mV

log (-i2) - log (-i1)

Anodic reaction,

Tafel slope ispositive

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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Concentrat ion Po lar izat ion

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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Concentrat ion Po lar izat ion

Oxygen reduction is often affected byconcentration polarization

log |current density|

Electrod

ePotential

Rate of cathodic oxygen

reduction without

concentration polarization

Rate of cathodic oxygen

reduction with

concentration polarizationLimiting current density -

rate of reaction limited by

availability of oxygen atthe metal surface


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Res is tance Polarizat ion

If there is a resistance between the anode andthe cathode in a cell, then the current flowingthrough that resistance will cause a potential

drop given by Ohms Law:

V= IR

This is important for paint films and for high

resistance solutions


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Res is tance Polarizat ion


Elec

trodePoten

tial

Resistance Polarizationcauses potential of anode

and cathode to differ due

to potential drop across

solution, hence corrosion

current is reduced


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Passivat ion

When a passive film is formed, this causes amarked drop in current density due to theresistance of the film and its effect as a

barrier to diffusion

This effect is seen on the anodic curve


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Passivat ion


Elec

trodePoten

tial

Active corrosion gives normal

activation polarization

Current falls as the passive

film starts to form - the

active-passive transition

When a stable passive film

has formed, the current has

a steady, low value - the

passive current density

The rate of corrosion will be

critically affected by the

cathodic curve

Rapid rate of cathodic

reaction leads to passivation,

and lowrate of corrosionLower rate of cathodic

reaction leads to activity, and

highrate of corrosion

But it may also lead to low

rate of corrosion?Very slow cathodic reactionleads to low rate of corrosion


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ElectrodePotenti

al

Po lar izat ion Curves

Iron in hydrochloric acid

Cathodic hydrogen evolutionAnodic iron dissolution


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ElectrodePotenti

al


Iron in sulphuric acid

Cathodic hydrogen evolution

Anodic iron dissolution (withactive-passive transition)Oxygen evolution on passive

film (or transpassivecorrosion

as metal is oxidised to a

higher oxidation state)


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ElectrodePotential


Iron in aerated neutral NaCl solution

Cathodic hydrogen evolutionAnodic iron dissolution

Cathodic oxygen reduction


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Effect o f pH on react ion rate

Consider hydrogen evolution reaction

2H+ + 2e- H2

The concentration of hydrogen ions willinfluence the rate of the reaction

As the hydrogen ion concentration isincreased (i.e. the solution made more acid),

so the rate of the reaction increasesSimilarly the potential will influence the

reaction - the more negative the potential thefaster the reaction


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Effect o f pH and po ten t ial on

rate o f hydrogen evo lu t ion

pH

Potential

Faster

Slower


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On platinum no metal dissolution will occur,but to balance the charge a reaction whichcreates electrons must occur

If the solution contains dissolved hydrogen,the reverse of the hydrogen evolutionreaction can occur:

H2

2H+ + 2e-


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H2 2H+ + 2e-

This reaction will go faster in alkaline solution(since H+will be removed by H++ OH- H2O)

This reaction will go faster at more positive

potentials (because electrons will be removedfrom metal)


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Effect o f pH and po ten t ial on

rate o f hydrogen ox idat ion

pH

Potential

Oxidation

Faster

Oxidation

SlowerReductionFaster

Reduction

Slower

Rates equalElectrochemical

Equi l ibr ium


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Thermodynamic

Equi l ibr ium

2H+ + 2e- H2The potential at which it occurs for a given

solution composition is known as theequilibrium potential.

The concentrations of reactants controls therates of the forward and reverse reactions

and hence the equilibrium potential


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The Pourbaix (E-pH)

Diagram

Potential

H2O is stable

H2is stable

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pH = - log [H+]

2H++ 2e-= H2

Equilibriumpotential falls as

pH increases

2.0

1.6

0.8

1.2

-0.4

0.4

0.0

-1.6

-0.8-1.2

0

2H2O = O2+ 4H++ 4e-Equilibrium potential

falls as pH increasesO2is stable


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Potential

7 14

2.0

1.6

0.8

1.2

-0.4

0.4

0.0

-1.6

-0.8-1.2

0

Pourbaix Diagram for Zinc

Equilibrium for

ZnZn2++ 2e-

Zn metal stable

Zn2+stable

in solution

Equilibrium for

Zn2++ 2OH-Zn(OH)2

Zn(OH)2

stable

solid Equilibrium for

Zn + 2OH-Zn(OH)2+ 2e-

Equilibrium forZn(OH)2+ 2OH

-ZnO2

2-+ 2H2O

ZnO22-

stable in

solution

Equilibrium forZn + 4OH-ZnO22-+ 2H2O + 2e

-


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Potential

7 14

2.0

1.6

0.8

1.2

-0.4

0.4

0.0

-1.6

-0.8-1.2

0

Pourbaix Diagram for Zinc

Zn metal stable

Zn2+stable

in solution

Zn(OH)2

stable

solidZnO2

2-

stable in

solution

Corrosion

Corrosion

Immunity

Passiv

ity

Corrosion

possible withoxygen

reductionCorrosion

possible with

hydrogenevolution

Corrosion requires

strong oxidising

agent

Corrosion is

thermodynamically

impossible

Corrosion is

possible, but likely

to be stifled by solidcorrosion product


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Pourbaix Diagram for Gold

Potential

7 14

2.0

1.6

0.8

1.2

-0.4

0.4

0.0

-1.6

-0.8-1.2

0

Gold metal stable

Immunity

C

CPassivity

Gold cant corrode

with oxygen reduction

or hydrogen evolution


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Pourbaix diag ram for

Copper

Potential

7 14

2.0

1.6

0.8

1.2

-0.4

0.4

0.0

-1.6

-0.8-1.2

0

Cu metal stable

Cu2+stablein solution

Cu oxides

stable

CuO2

2-stableinsoln.Will copper

corrode in

acid?

No - hydrogen

evolution only

occurs below the

potential for coppercorrosion

Will copper

corrode in

neutral waters?Usually it will just

passivate, but

corrosion can occurin slightly acid

solutions


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Pourbaix Diagram for Iron

Potential

7 14

2.0

1.6

0.8

1.2

-0.4

0.4

0.0

-1.6

-0.8-1.2

0

Fe metal stable

Fe3+

Fe oxides

stable

Will ironcorrode in

acid?

Fe2+stable

Yes - there is a

reasonably wide

range of potentialswhere hydrogen

can be evolved and

iron dissolved

Will iron

corrode in

neutral waters?Yes - although iron can

form an oxide in neutral

solution, it tends not to

form directly on themetal, as the potential

is too low, therefore it is

not protective.

Will iron corrode

in alkaline

solution?

No - iron forms a solidoxide at all potentials,

and will passivate


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Pourbaix diag ram for

A lumin ium

Potential

7 14

1.2

0.8

0.0

0.4

-1.2

-0.4

-0.8

-2.4

-1.6-2.0

0

Al

Al3+Al2O3

AlO2-


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L im itat ions o f Pourbaix

Diagrams

Tell us what canhappen, not necessarilywhat wi l lhappen

No information on rate of reaction

Can only be plotted for pure metals andsimple solutions, not for alloys


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kinetics of corrosion

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