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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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External Curren t App l ied to
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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External Curren t App l ied to
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
log |current density|
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
log |current density|
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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log |current density|
ElectrodePotenti
al
Po lar izat ion Curves
Iron in hydrochloric acid
Cathodic hydrogen evolutionAnodic iron dissolution
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log |current density|
ElectrodePotenti
al
Po lar izat ion Curves
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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log |current density|
ElectrodePotential
Po lar izat ion Curves
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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Effect o f pH on react ion rate
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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Effect o f pH on react ion rate
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
7 14
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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