Direct Observation of Pit Initiation Process on Type 304 Stainless Steel

Abstract

In situ and real-time optical microscopic observations of pit initiation process on a commercial Type 304 stainless steel with low-sulfur content (0.004mass%) were performed in 3M NaCl solution at 298K. MnS inclusions with diameters of ca. 1μm were found to act as the initiation sites of pitting, as was the case in a re-sulfurized Type 304 stainless steel (0.027mass%). The pit was initiated at the boundary between the MnS inclusion and the steel matrix, and grew with time. After a few seconds, no visible change was observed for 1 s, even though the anodic current was measured, suggesting that the steel dissolution proceeded in the depth direction. After that, a small hole suddenly appeared on the steel surface. The hole widened with time, steadily becoming a large stable pit. This pit initiation process in the low-sulfur stainless steel is much like that found in the re-sulfurized stainless steel. [doi:10.2320/matertrans.M2014028]