Aqueous Corrosion: Fundamentals - Environmental Degradation of Metals - page 29

Figure 2.14

Combined curve of activation and concentration polarizations for reduc-

tion process.

Zn → Zn 2 2e (2.2)

2H 2e → H 2 (2.3)

These reactions again are the anodic and cathodic partials, respectively, of the
equilibrium reactions:

Zn i Zn 2 2e (2.20)

2H 2e i H 2 (2.22)

These equilibria correspond to the potentials 0.76 V (SHE) and 0 V, respec-

tively, with their characteristic exchange current density values which can be
conveniently shown in a potential versus current density (usually log of current
density) plot. As the current flows, polarization sets in. Assuming the polarization
to be activation polarization only, straight line variation corresponding to the
Tafel equation will be obtained and the polarization plots for the relevant partial
equations will tend to intersect as shown in Fig. 2.15.
At the point of intersection the rate of anodic reaction is equal to the rate of
cathodic reaction, which corresponds to the corrosion current density, i corr :

i a i c i corr

as no accumulation of charge is observed in a piece of corroding metal. The
potential corresponding to this point of intersection is the potential of the corro-
ding metal, which is represented by E corr and is called corrosion potential .
It can be readily noticed, therefore, that the corrosion potential is not the equi-
librium potential of either of the constituent partial reactions, but rather some
intermediate potential determined by simultaneous occurrence of these partial