Page 128 - Corrosion Engineering Principles and Practice

102 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 103

5.5 Graphical Presentation of
Kinetic Data (Evans Diagrams)
The use of polarization curves for the study of corrosion reactions can
be traced back to the 1930s with the work of Wagner and Traud [4].
However the representation of the mixed potential behavior is often
associated with Professor Evans who has popularized this represen-
tation of corrosion polarization measurements [5].
These polarization diagrams can be quite useful for describing or
explaining parallel corrosion processes. According to the mixed-
potential theory underlying these diagrams, any electrochemical
reaction can be algebraically divided into separate oxidation and
reduction reactions with no net accumulation of electrical charge.
Under these circumstances the net measurable current is zero and
the corroding metal is charge neutral, that is, all electrons produced
by the corrosion of a metal have to be consumed by one or more
cathodic processes.
In order to model a corrosion situation with mixed potential
diagrams, one must first gather the information concerning the
(1) activation overpotential for each corrosion process involved and
(2) any additional information for processes that could be affected by
concentration overpotential. The following sections present some
examples that illustrate how the mixed potential theory may be used
to explain simple cases where corrosion processes are purely activation
controlled or cases where concentration controls at least one of the
corrosion processes.

5.5.1 Activation Controlled Processes
For purely activation controlled processes, each reaction can be
described by a straight line on an E versus Log i plot, with positive
Tafel slopes for anodic processes and negative Tafel slopes for
cathodic processes.
The following example illustrates the polarization behavior of
carbon steel in a deaerated solution maintained at 25°C with a pH of
zero. The solid line in Fig. 5.14 is the polarization plot itself and the
dotted lines in this figure represent the anodic reaction in Eq. (5.19)
and the cathodic reaction in Eq. (5.20) that describe the corrosion
behavior of steel in these conditions. These lines are extrapolated
from the linear sections of the plot on either the anodic or cathodic
sides of the curve.

Fe → Fe 2+ + 2e (5.19)
+
(s)
+
+
2H + 2e → H g) (5.20)
(
2
While it is relatively easy to estimate the corrosion potential
(E ) from the sharp peak observed at −0.221 V vs. standard
corr