In situ and real-time optical microscopic observations of pit initiation process on a commercial Type 304 stainless steel with low-sulfur content (0.004 mass%) were performed in 3M NaCl solution at 298 K. MnS inclusions with diameters of ca. 1 μm were found to act as the initiation sites of pitting, as was the case in a re-sulfurized Type 304 stainless steel (0.027 mass%). 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.
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