Download - 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/mole)metalofweightatomic
(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:
(g/mole)metalofweightatomic
)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