Investigations of thiosulfate accumulation on 304 stainless steel in neutral solutions by radioactive labeling, electrochemistry, auger electron and x-ray photoelectron spectroscopy methods

Thiosulfate accumulation on 304 stainless steel in near neutral solutions (pH similar to 5.6) was studied using in situ techniques: electrochemistry and radiochemistry, as well as by Auger electron spectroscopy depth profiling and angle-resolved x-ray photoelectron spectroscopy in ultrahigh vacuum. It was found that thiosulfate accumulation is an irreversible process and occurs over a broad electrode potential range. Thiosulfate surface concentration is very small, below -1.0 V vs. Ag/AgCl reference. In the potential range from -1.0 to -0.50 V the surface concentration increases linearly with potential, reaches a maximum at -0.30 V, and at even more positive potentials, decreases to a slightly lower level. Ultrahigh vacuum spectroscopic measurements indicate that the irreversible surface behavior can be attributed to thiosulfate incorporation into the substrate passive film. The present data obtained with 304 stainless steel are compared to previous results published from this laboratory on thiosulfate adsorption on 316 stainless steel, and the role of molybdenum surface enrichment in the thiosulfate accumulation reversibility is discussed. The effect of chloride on thiosulfate accumulation was also investigated. At high concentrations of chloride, thiosulfate is desorbed from the surface due to chloride-induced dissolution of the stainless steel. At very negative potentials, the thiosulfate surface concentration increases upon chloride addition, most probably due to the surface microroughening caused by chloride adsorption.