Callister - An introduction - 8th edition

17.4 Prediction of Corrosion Rates • 685
4. The coalescence of many hydrogen molecules to form a bubble
For activation
polarization,
relationship between
overvoltage and
current density
The slowest of these steps determines the rate of the overall reaction.
For activation polarization, the relationship between overvoltage a and current
density i is
h a ; b log i
i 0
(17.25)
where and i 0 are constants for the particular half-cell. The parameter i 0 is termed
the exchange current density, which deserves a brief explanation. Equilibrium for
some particular half-cell reaction is really a dynamic state on the atomic level. That
is, oxidation and reduction processes are occurring, but both at the same rate, so
that there is no net reaction. For example, for the standard hydrogen cell (Figure 17.4),
reduction of hydrogen ions in solution will take place at the surface of the platinum
electrode according to
with a corresponding rate r red . Similarly, hydrogen gas in the solution will experience
oxidation as
at rate r oxid . Equilibrium exists when
2H 2e ¡ H 2
H 2 ¡ 2H 2e
At equilibrium,
equality of rates of
oxidation and
reduction, and their
relationship to the
exchange current
density
r red r oxid
This exchange current density is just the current density from Equation 17.24 at
equilibrium, or
r red r oxid i 0
nf
(17.26)
Use of the term current density for i 0 is a little misleading inasmuch as there is no
net current. Furthermore, the value for i 0 is determined experimentally and will vary
from system to system.
According to Equation 17.25, when overvoltage is plotted as a function of the logarithm
of current density, straight-line segments result; these are shown in Figure 17.7
Overvoltage, a (V)
+0.2
+0.1
0
–0.1
i 0 (H 2 /H + )
H 2 2H + + 2e –
2H + + 2e –
–
+
H 2
Figure 17.7 For a hydrogen
electrode, plot of activation
polarization overvoltage versus
logarithm of current density for
both oxidation and reduction
reactions. (Adapted from M. G.
Fontana, Corrosion Engineering,
3rd edition. Copyright © 1986 by
McGraw-Hill Book Company.
Reproduced with permission.)
–0.2
–0.3
0.001
0.01 0.1 1 10 100 1000
Current density
(logarithmic scale)