Environmental Engineering Reference
In-Depth Information
Figure 2.20
Potentiostatic anodic polarization curve for an active-passive metal.
again shows an increase, and we have what is termed
transpassive region
. The
increase in current density may be indicative of oxygen evolution, as for iron in
1NH
2
SO
4
, or of increased anodic dissolution as for chromium or 18-8 stainless
steel in the same solution according to the reaction;
2Cr
7H
2
O
→
Cr
2
O
7
2
14H
12e
(2.44)
All active-passive metals exhibit such typical S-shaped anodic polarization curves
with the exception of titanium, which does not possess a transpassive region.
The corrosion behavior of an active-passive metal can be understood from
consideration of the mixed electrodes involving the cathodic reduction process.
This has been illustrated in Fig. 2.21. The cathodic polarization curve, depending
on its exchange current density and Tafel slope, will intersect the anodic polariza-
tion curve in any one of the three ways shown as 1, 2, and 3. For case 1,
i
corr
corresponds to A, which is in the active region and is obviously high. For case
2,
i
corr
corresponds to B, C, and D, but C being electrically unstable, the corrosion
current assumes either the low value at D or the high value at B depending on
whether the metal is in the passive or active state. The unstable passivity achieved
by exposing iron to fuming nitric acid gives rise to such a situation. For case 3,
where the cathodic curve clears the nose (
i
cr
),
i
corr
corresponds to
i
passive
and this
is the most desirable situation from the viewpoint of corrosion prevention.
The attainment of
i
cr
is an important criterion for achieving passivity. An in-
