Polarization curves pitting dissolution, metals

FIGURE 22.26 Schematic polarization curves for anodic metal dissolution, passivation, passivity breakdown, pitting corrosion, and transpassivation Eb = film-breakdown potential and Ep]l — pitting potential. 

Figure 23 Schematic polarization curve for metal that spontaneously passivates but pits upon anodic polarization. A hysteresis loop, which can appear during a reverse scan, is shown ending at Erp. One dotted line shows behavior for anodizing conditions, while the other shows transpassive dissolution.

In some cases, passivity should be maintained. Anodic protection can be used to shift from the range of active dissolution to the passive range of the polarization curve. The necessary anodic current densities to maintain passivity are usually very small, in the range of a few pA cm. If however pitting is a problem in the presence of halides, the potential has to be maintained below Ep j. The presence of oxygen may cause a positive potential shift of a passive metal within the passive or even the transpassive range. 

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