Environmental Engineering Reference
In-Depth Information
and nitrates belong to the first group. They are effective even in the absence of
oxygen, as they themselves are readily reduced. Tungstates, molybdates, phos-
phates, silicates, borates, and benzoates are examples of nonoxidizing passivators.
They require dissolved oxygen in order to inhibit corrosion.
Passivators are essentially anodic inhibitors. The oxidizing passivators get ad-
sorbed and the nonoxidizing passivators facilitate the adsorption of dissolved
oxygen on the metal surface. The redox potential of the reduction system shifts
progressively in the nobler direction with the increase in passivator concentration.
At the same time, the rapid reduction of passivators or oxygen accelerates the
anodic dissolution of steel (an active-passive metal) to a value above the
i
critical
,
taking the anodic polarization to the passive region. The intersection of the ca-
thodic polarization curve with the anodic polarization curve in this region gives
a corrosion rate equal to
i
passive
, which is of insignificantly small value (Section
2.4). To ensure this situation, the passivator should be used in sufficient concen-
tration; otherwise, the intersection of the polarization curves will be in the active
region giving a higher corrosion rate. Since the passivators accelerate corrosion
when added in insufficient amount, they are also known as ''dangerous inhibi-
tors.'' The situations are shown in Fig. 4.6. A shift from 1 to 2 increases the
corrosion rate from A to B, whereas for situation 3, the cathodic polarization
curve clears the nose of the anodic polarization curve and passivity is achieved.
The critical concentration of passivators is usually of the order of a few ppm
Figure 4.6
Effect of passivator concentration on corrosion of iron.
