Potential-Dimension Diagram for Pitting and Crevice Corrosion of Metals

Abstract

Chloride-pitting of iron group metals developed at noble potentials proceeds in the polishing state dissolution as far as hydrated metal chloride in the pit solution remains greater than 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 becomes 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 metal 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 if the crevice solution is more acidic than pH_pd, but is inhibited if the solution is less acidic, which is realized at potentials less noble than the protection potential. A potential-dimension diagram may be drown for the region of stalle pitting and crevice corrosion.