Active-passive corrosion behavior controlled potential

When the potential of a metallic component is controlled and shifted in the anodic (positive) direction, the current required to cause that shift will vary. If the current required for the shift has the general polarization behavior illustrated in Fig. 5.40, the metal has an active-passive transition and can be anodically protected. Only a few systems exhibit this behavior in an appreciable and usable way. The corrosion rate of a metal with an active-passive behavior can be significantly reduced by shifting the potential of the metal so that it is at a value in the passive range shown in Fig. 5.40. 

Second, the critical potential may be a repassivation potential. It has been shown in artificial active crevices that lowering the potential of the free surfaces causes the local environment to become less aggressive (see, for example, Pourbaix [36]). Thus, at some point, the environment becomes not aggressive enough for active dissolution to be sustained and the metal surface in the crevice becomes passive. In this case, a subsequent increase of the corrosion potential does not produce immediate reactivation inside the crevice. Starr et al. [82] observed this situation on 12% Cr stainless steels in near-neutral environments Dunn and Sridhar [83] observed the same behavior on alloy 825. However, the repassivation may be attributed to different environment changes an increase of local pH in the crevice [82,84], a destabilization of the salt film that controls the... 

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