Effect of Complex Inclusions on Pit Initiation in 18% Chromium-8% Nickel Stainless Steel Types 303, 304, and 321

Several grades of 18% chromium-8% nickel stainless steel (SS) with varying sulfur contents and immersed in 3% sodium chloride (NaCl) demonstrated essentially passive behaviors, with local depassivation events monitored as sharp negative changes in the free corrosion potential. Electrochemical desulfurization or trapping of the inclusion sulfides (with 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 experimental heat with high purity and few pinpoint inclusions showed an almost complete absence of transients. Precipitation of sulfides (as their cadmium salts) orginating from inclusions showed the corrosion behavior of type 303 SS (UNS S30300) was governed by the sulfur content, but grades with lower sulfur contents (e.g., type 304 SS [UNS S30400]) were more controlled by crystallized oxides of aluminum, calcium, and chromium that appeared to determine the frequency and duration of the depassivation transients. An exponential decrease in transients over time was observed with all steels containing > 0.003% sulfur. Introduction of titanium into the solid steel matrix (type 321 SS [UNS S32100]) trapped available sulfur and produced an almost transient-free characteristic. A relationship between the number of transients and the initial number of pit initiation sites was described.