Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-16 GTBL042-Callister-v2 September 13, 2007 13:10

Revised Pages

16.4 Prediction of Corrosion Rates • 675

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 16.9bshows such a schematicη-versus-logi

plot.

Concept Check 16.3

Briefly explain why concentration polarization is not normally rate controlling for

oxidation reactions.

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

Corrosion Rates from Polarization Data

Let us now apply the concepts developed above to the determination of corrosion

rates. Two types of systems will be discussed. In the first case, both oxidation and re-

duction 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 16.1).

The reduction of H+ions to form H 2 gas bubbles occurs at the surface of the zinc

according to

2H++ 2 e−→H 2 (16.3)

and the zinc oxidizes as

Zn→Zn^2 ++ 2 e− (16.8)

No net charge accumulation may result from these two reactions; that is, all electrons

generated by reaction 16.8 must be consumed by reaction 16.3, which is to say that

rates of oxidation and reduction must be equal.

Activation polarization for both reactions is expressed graphically in Figure

16.10 as cell potential referenced to the standard hydrogen electrode (not over-

voltage) versus the logarithm of current density. The potentials of the uncoupled

hydrogen and zinc half-cells,V(H+/H 2 ) andV(Zn/Zn^2 +), respectively, are indicated,

along with their respective exchange current densities,i 0 (H+/H 2 ) andi 0 (Zn/Zn^2 +).

Straight line segments are shown for hydrogen reduction and zinc oxidation. Upon

immersion, both hydrogen and zinc experience activation polarization along their

respective lines. Also, oxidation and reduction rates must be equal as explained

above, which is only possible at the intersection of the two line segments; this inter-

section occurs at the corrosion potential, designatedVC, and the corrosion current

densityiC. The corrosion rate of zinc (which also corresponds to the rate of hy-

drogen evolution) may thus be computed by insertion of thisiCvalue into Equa-

tion 16.24.

The second corrosion case (combined activation and concentration polariza-

tion for hydrogen reduction and activation polarization for oxidation of metal M)

is treated in a like manner. Figure 16.11 shows both polarization curves; as above,

corrosion potential and corrosion current density correspond to the point at which

the oxidation and reduction lines intersect.