The stability of localized corrosion

Chloride pitting of iron group metals at noble potentials proceeds in the polishing state dissolution, provided that metal chloride in the pit solution is maintained above a critical concentration. It ceases to progress by pit repassivation if the pit is smaller than a critical size, or transforms into the active state pitting if the pit size is greater. The boundary potential between the polishing state and the active state pitting may be represented by the passivation-depassivation potential in the pit solution of the critical chloride concentration. Crevice corrosion is characterized by the crevice protection potential, at which the hydrogen ion concentration in the crevice solution is equivalent to pH_pd—the passivation-depassivation pH of the crevice metal. It continues to corrode at more noble potentials than the protection potential, where the crevice solution is more acidic than pH_pd, but is inhibited in the less acidic crevice solution at less noble potentials.