EFFECT OF COMPLEX INCLUSIONS ON PIT INITIATION IN 18-PERCENT CHROMIUM-8-PERCENT NICKEL STAINLESS-STEEL TYPE-303, TYPE-304, AND TYPE-321

Several grades of 18% chromium-8% nickel stainless steel (SS) with var ying sulfur contents and immersed in 3% sodium chloride (NaCl) demonst rated essentially passive behaviors, with local depassivation events m onitored as sharp negative changes in the free corrosion potential. El ectrochemical desulfurization or trapping of the inclusion sulfides (w ith cadmium in the electrolyte or titanium in the solid metal matrix) reduced the initiation rates associated with depassivation transients and changed the properties of the alloys markedly. A control experimen tal heat with high purity and few pinpoint inclusions showed an almost complete absence of transients. Precipitation of sulfides (as their c admium salts) orginating from inclusions showed the corrosion behavior of type 303 SS (UNS S30300) was governed by the sulfur content, but g rades with lower sulfur contents (e.g., type 304 SS[UNS S30400]) were more controlled by crystallized oxides of aluminum, calcium, and chrom ium that appeared to determine the frequency and duration of the depas sivation transients. An exponential decrease in transients over time w as observed with all steels containing >0.003% sulfur. introduction of titanium into the solid steel matrix (type 321 SS [UNS S32100]) trapp ed available sulfur and produced an almost transient-free characterist ic. A relationship between the number of transients and the initial nu mber of pit initiation sites was described.