Callister - An introduction - 8th edition

17.4 Prediction of Corrosion Rates • 687
Overvoltage, c
+
0
–
i L
Figure 17.9 For reduction reactions,
schematic plots of overvoltage versus
logarithm of current density for
(a) concentration polarization, and
(b) combined activation–concentration
polarization.
Log current density, i
(a)
+
0
i 0
Overvoltage,
–
Activation
polarization
Concentration
polarization
Log current density, i
(b)
i L
of current density until i approaches i L ; at this point, c decreases abruptly in
magnitude.
Both concentration and activation polarization are possible for reduction reactions.
Under these circumstances, the total overvoltage is just the sum of both
overvoltage contributions. Figure 17.9b shows such a schematic -versus-log i plot.
Concept Check 17.3
Briefly explain why concentration polarization is not normally rate controlling for
oxidation reactions.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
Corrosion Rates from Polarization Data
Let us now apply the concepts just developed to the determination of corrosion rates.
Two types of systems will be discussed. In the first case, both oxidation and reduction
reactions are rate limited by activation polarization. In the second case, both
concentration and activation polarization control the reduction reaction, whereas
only activation polarization is important for oxidation. Case one will be illustrated
by considering the corrosion of zinc immersed in an acid solution (see Figure 17.1).