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

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

Revised Pages

16.5 Passivity • 679

Log current density, i

Electrochemical potential,

V

Transpassive

Passive

Active

V (M/M2+)

M2+

+ 2e

M

i 0 (M/M2+)

Figure 16.12 Schematic

polarization curve for a metal

that displays an active–passive

transition.

termed the “passive” region. Finally, at even higher potential values, the current

density again increases with potential in the “transpassive” region.

Figure 16.13 illustrates how a metal can experience both active and passive behav-

ior depending on the corrosion environment. Included in this figure are the S-shaped

oxidation polarization curve for an active–passive metal M and, in addition, reduc-

tion polarization curves for two different solutions, which are labeled 1 and 2. Curve

1 intersects the oxidation polarization curve in the active region at point A, yielding

a corrosion current densityiC(A). The intersection of curve 2 at point B is in the pas-

sive region and at current densityiC(B). The corrosion rate of metal M in solution 1

is greater than in solution 2 sinceiC(A) is greater thaniC(B) and rate is proportional

to current density according to Equation 16.24. This difference in corrosion rate

Electrochemical potential,

V

Log current density, i

i 0 (1) i 0 (2)

i 0 (M/M2+)

iC (B) iC (A)

B

A

1

2

Figure 16.13 Demonstration

of how an active–passive metal

can exhibit both active and

passive corrosion behaviors.