Role of Sulfide Inclusions on Localized Corrosion of Ni200 in NaCl Solutions

Early stages of pitting corrosion in the vicinity of sulfide inclusions on Ni200 were observed by several techniques including scanning electron microscopy, Auger electron spectroscopy, microelectrochemical cell, and scanning electrochemical microscopy (SECM). These data provide information on the evolution of shape during early stages of dissolution along with elemental distribution over the surface, potentiodynamic behavior associated with individual inclusion sites, and the spatial/temporal distribution of local solution composition. It was found that pits were most likely to form near sulfur-rich inclusions, that a sulfur-containing layer was formed on the walls of pits grown around such inclusions, and that the spatial distribution of the sulfur-containing layer was dependent on fluid flow. The pitting potential exhibited a dependence upon the concentration of chloride and thiosulfate, as well as upon the presence of fluid flow. SECM data demonstrated that dissolved sulfur-containing species accumulated locally near sulfide inclusions before pitting began as well as during pit growth. In the presence of fluid flow, sulfide inclusions exhibited sulfur-containing region (tail) on the downstream side. As flow rate increased, the pit tail became slimmer and longer. Pits with diameters less than about 5 μm did not exhibit a pit tail.