Environmental Engineering Reference
In-Depth Information
Figure 2.22
Effect of temperature and hydrogen ion concentration on anodic polariza-
tion of an active-passive metal.
situation marked by D, beyond which the rate assumes a steady value. In
contrast, for an active-passive metal, an increase in
i
L
up to
i
cr
increases
the corrosion rate and beyond
i
cr
the corrosion rate falls drastically, as the
intersection of the cathodic polarization curve with the anodic polarization
curve takes place only in the passive region (Fig. 2.23).
3.
Addition of oxidizers
. The effect of addition of oxidizing agents such as fer-
ric, cupric, or chromate ions in the electrolyte on the corrosion behavior of
active-passive metals has been illustrated in Fig. 2.24. Additions of increas-
ing amounts of oxidizer shifts the reversible electrode potential in more posi-
tive direction in accordance with the Nernst equation:
RT
nF
a
oxid
a
red
E
E
0
2.3
log
(2.23)
For the sake of simplicity, it has been assumed that the exchange current
density of the redox system and the Tafel slope remain unchanged. The ca-
thodic polarization curves shift progressively upward and in the case of a
normal metal an increase in corrosion rate is encountered. In the case of an
active-passive metal, however, the corrosion rate increases initially, de-
creases drastically as the cathodic polarization curves intersect the anodic
polarization curve in the passive region, and increases again if the intersec-
tion takes place in the transpassive region.
