Pit initiation at single sulfide inclusions in stainless steel: II. Detection of local pH, sulfide, and thiosulfate

Small wires of W, Ag, and Pt were placed within an electrochemical microcell above single sulfide inclusions to detect the pH, sulfide, and thiosulfate, respectively. In chloride-free electrolytes the onset of sulfide dissolution was accompanied by a decrease in pH of 0.5 units to 4.2, a value that is not low enough to initiate active metal dissolution. In chloride-containing electrolytes, metastable pitting events were found to cause rapid temporary periods during which the pH decreased to values as low as zero. However, the results suggest that sustained pH decreases occur only with onset of stable pitting. Sulfide species were only detected during chemical dissolution in a pH 2 electrolyte at the rest potential and not as a result of anodic dissolution at neutral pH. A Pt wire along with an iodide/triiodide couple was used to detect thiosulfate as a result of anodic dissolution. The results support the mechanism of electrochemical dissolution of sulfide inclusions to produce thiosulfate at neutral pH.