Page 111 - Corrosion Engineering Principles and Practice

86 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 87

where h is the activation overpotential, a complex function describing
act
the charge transfer kinetics of an electrochemical reaction.
h is always present and the main polarization component
act
at small polarization currents or voltages.
h is the concentration overpotential, a function describing the
conc
mass transport limitations associated with electrochemical
processes. h conc is dominant at larger polarization currents
or voltages.
iR is the ohmic drop. This function takes into account the elec-
trolytic resistivity of an environment when the anodic and
cathodic elements of a corrosion reaction are separated by
this environment while still electrically coupled.
Activation polarization is usually the controlling factor during
corrosion in strong acids since both h conc and iR are relatively small.
Concentration polarization usually predominates when the
concentration of the active species is low; for example, in dilute
acids or in aerated waters where the active component, dissolved
oxygen, is only present at very low levels. The ohmic drop will
become an extremely important factor when studying corrosion
phenomena for which there is a clear separation of the anodic and
cathodic corrosion sites, for example, crevice corrosion. The ohmic
drop is also an important variable in the application of protective
methods such as anodic and cathodic protection that forces
a potential shift of the protected structure by passing a current in
the environment.
Knowing the kind of polarization which is occurring can be very
helpful, since it allows an assessment of the determining characteristics
of a corroding system. For example, if corrosion is controlled by
concentration polarization, then any change that increases the
diffusion rate of the active species (e.g., oxygen) will also increase the
corrosion rate. In such a system, it would therefore be expected that
agitating the liquid or stirring it would tend to increase the corrosion
rate of the metal. However, if a corrosion reaction is activation
controlled then stirring or increasing the agitation will have no effect
on the corrosion rate.

5.2 Activation Polarization
Activation polarization is due to retarding factors that are an inherent
part of the kinetics of all electrochemical reactions. For example,
consider the evolution of hydrogen gas illustrated previously in
Chap. 3 and described by Eq. (5.3):
+
−
(
2H + 2e → H g) (5.3)
2
While this reaction seems to be relatively simple, the rate at which
hydrogen ions are transformed into hydrogen gas is in reality a