Corrosion of Metallic Coatings
259
η η η
= +
a
c
The activation overpotential·η w results from the potential energy barrier
to overcome for a charge to cross the electrical double layer at the interface (
that Is, M ↔ M n+ + ne − ) and is given as follows:
In the anodic reaction,
η
β
a
a
a
= log
i
i
a
oa
β a
RT
nF
= ∝
2 3
.
Log Current Density
i corr.
i L
i oa
i oc
E a
E corr.
Concentration
overpotential
Activation
overpotential
Activation
overpotential
M
M
n+ + n e
Electrode Potential
E corr.
E c
O
2 + 2 H
2 O + 4 e
4 O H –
η c
α
η a
a
η c
c
FigurE 8.3
Activation and. concentration overpotentials in a typical corrosion process.