Pit initiation at single sulfide inclusions in stainless steel: I. Electrochemical microcell measurements

Electrochemical measurements were performed on single sulfide inclusions within a microcell to study their dissolution behavior and role in the initiation of pitting corrosion of 304 stainless steel in NaCl solutions. It was found that chloride ions catalyzed inclusion dissolution and caused metastable pits, and that no metastable pits formed in thiosulfate solutions without chloride. The thiosulfate ion accelerated inclusion dissolution, and when the thiosulfate ion was present above a critical concentration it caused stable pitting, at low concentrations the chloride ion inhibited the aggressive nature of the thiosulfate ion and stifled stable pitting, while at high concentrations the thiosulfate ion inhibited the ability of the chloride ion to cause metastable pitting events. The addition of xanthate ion in combination with electrode activation in copper sulfate was found to inhibit the rate of inclusion dissolution and increase the pitting potential. The initiation of stable pitting was found to depend on the inclusion geometry.