The Role of Inclusions on Initiation of Crevice Corrosion of Stainless Steel: I . Experimental Studies

Electrochemical dissolution of in deaerated was found to produce thiosulfate ions. In addition, the presence of both chloride and thiosulfate ions above critical levels of concentration was found to cause rapid depassivation of 304 stainless steel in well‐stirred deaerated solutions. Therefore, the hypothesis was explored that initiation of crevice corrosion in this system is caused by entrapped thiosulfate ions produced by electrochemical dissolution of inclusions. The hypothesis differs from the view that breakdown is caused by acidification owing to Cr dissolution and hydrolysis. An artificial crevice cell, constructed from two optically flat surfaces, was used to measure the initiation time under controlled variation of crevice gap, applied potential, and sulfide inclusion density. It was found that crevice corrosion did not initiate for sufficiently large crevice gaps, negative (cathodic) applied potentials, and low inclusion densities. In Part II of this series of papers, data are compared to predictions based on a mathematical model of the proposed "thiosulfate entrapment" mechanism.