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EXPERIMENTAL POLARIZATION CURVES
Dr. Ramazan Kahraman
Chemical Engineering Department
King Fahd University of Petroleum & MineralsDhahran, Saudi
Arabia
Reading Material: Chapter 3, Sections 3.4 and 3.5 in
Principles and Prevention of Corrosion, Denny Jones,
Prentice-Hall, 1996.
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Increase cathodic polarization by c (from c=Ecorr-ec to c=E*-ec)
and
plot applied current, iapp, vs potential, E.
(Same principle is applied in cathodic protection which is to be
studied later)
[Principles and Prevention of Corrosion, Denny Jones,
Prentice-Hall, 1996]
Cathodic Polarization(Polarization of cathodic rxn by shift of
potential to a more negative value)
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Anodic Polarization
(Polarization of anodic rxn by shift of potential to a more
positive value)
Increase anodic polarization by a (from a=Ecorr-ea to a=E*-ea)
and
plot applied current, iapp, vs potential, E.
(Same principle is applied in anodic protection (in the case
of
active-passive metals) which is to be studied later)
ic + iapp = iasupplies e- consumes e-*
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Experimental Polarization Curves
[Principles and Prevention of Corrosion, Denny Jones,
Prentice-Hall, 1996]
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Non-ideal Experimental Polarization Curves
Use of cathodic polarization data to form the anodic
polarization curve in the case ofnon-linear anodic data (distorted
probably by corrosion products at high currents).
ia = ic - iapp
Cathodic Polarization
[Principles and Prevention of Corrosion, Denny Jones,
Prentice-Hall, 1996]
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Instrumentation
Controlled current (galvanostatic) technique
[Principles and Prevention of Corrosion, Denny Jones,
Prentice-Hall, 1996]
Current is applied at a
pre-determined rateand the potential is
measured through a
reference electrode
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Instrumentation
Controlled potential (potentiostatic) technique
[Principles and Prevention of Corrosion, Denny Jones,
Prentice-Hall, 1996]
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Reactions on the Auxiliary Anode
During Cathodic Polarization
For the working electrode as an anode, metal dissolution
reactions of the type
M Mn+ + ne- (1)
are of interest in corrosion.
When the working electrode is polarized as cathode (auxiliary
electrode as anode),
M must be selected for the auxiliary electrode with a very noble
eM/Mn+ to prevent
anodic dissolution, which would contaminate the electrolyte.
Either platinum orcarbon/graphite is the usual choice.
In the absence of anodic dissolution at the auxiliary electrode
by reaction (1), other
anodic oxidation reactions are possible to liberate electrons.
These include
oxidation in a redox reaction such asFe2+ Fe3+ + e- (2)
And oxygen evolution by
4OH- 2H2O + O2 + 4e- (3)
Reactions (2) and (3) both must operate at potentials below
eM/Mn+ (e.g. ePt/Pt3+) so
that nobel-metal auxiliary electrode is not dissolved.
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Home Work Problems
Prbs. 10 and 11 of Ch.3in Principles and Prevention of
Corrosion, Denny Jones, Prentice-Hall, 1996.
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References
Principles and Prevention of Corrosion, Denny Jones,
Prentice-Hall, 1996.
