Microelectrochemical Investigation on Pit Initiation at Sulfide and Oxide Inclusions in Type 304 Stainless Steel

MnS and MnO inclusions with a small amount of chromium were obtained by heat-treatments of Type 304 stainless steel at 1353, 1573, and , and the anodic dissolution behavior of the inclusions was investigated using a microelectrochemical technique. In the case of the sulfide inclusions, those with a high dissolution potential were found to provide high pitting potentials in the macroscopic measurements in NaCl. The initiation sites of metastable and stable pits were at the boundaries of the inclusions and the matrix in NaCl, , and LiCl. The dissolution current densities on the inclusion surface at the moment of the initiation of a stable pit decreased with increasing chloride-ion concentration. The synergistic effect of chloride ions and sulfur-containing species released from the inclusions was thought to play an important role in the pit-initiation process. The oxide inclusions did not dissolve in the passive region of Type 304 stainless steel. In the solutions with chloride-ion concentrations of up to , pitting did not occur at the oxide inclusions in microscopic polarization measurements. The oxide inclusions exhibited a comparatively inert characteristic as pit-initiation sites.