Page 110 - Corrosion Engineering Principles and Practice

CHAPTER 5

Corrosion Kinetics

and Applications of

Electrochemistry to Corrosion

5.1 What Is Overpotential?
Thermodynamic principles can explain a corrosion situation in terms
of the stability of chemical species and reactions associated with
corrosion processes. However, thermodynamic principles cannot be
used to predict corrosion currents or corrosion rates. In reality,
polarization effects control the cathodic and anodic currents that are
integral components of corrosion processes.
When two or more complementary processes such as those
illustrated in Chap. 3 occur over a single metallic surface, the corrosion
potential that results from such situations is a compromise between
the various equilibrium potentials of all the anodic and cathodic
reactions involved. The difference between the resultant potential (E)
and each individual reaction equilibrium potential (E ) is called
eq
polarization and is quantified in terms of overpotential (h ) described
in Eq. (5.1):

−
h = E E (5.1)
eq
The polarization is said to be anodic when the anodic processes
on the electrode are accelerated by moving the potential in the positive
(noble) direction or cathodic when the cathodic processes are
accelerated by moving the potential in the negative (active) direction.
There are three distinct types of polarization and these are additive,
as expressed in Eq. (5.2):
h total = h act + h conc + iR (5.2)

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