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 soluti ons (pH similar to 5.6) was studied using in situ techniques: electroc hemistry and radiochemistry, as well as by Auger electron spectroscopy depth profiling and angle-resolved x-ray photoelectron spectroscopy i n ultrahigh vacuum. It was found that thiosulfate accumulation is an i rreversible 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 surfac e concentration increases linearly with potential, reaches a maximum a t -0.30 V, and at even more positive potentials, decreases to a slight ly lower level. Ultrahigh vacuum spectroscopic measurements indicate t hat the irreversible surface behavior can be attributed to thiosulfate incorporation into the substrate passive film. The present data obtai ned with 304 stainless steel are compared to previous results publishe d from this laboratory on thiosulfate adsorption on 316 stainless stee l, and the role of molybdenum surface enrichment in the thiosulfate ac cumulation reversibility is discussed. The effect of chloride on thios ulfate accumulation was also investigated. At high concentrations of c hloride, thiosulfate is desorbed from the surface due to chloride-indu ced dissolution of the stainless steel. At very negative potentials, t he thiosulfate surface concentration increases upon chloride addition, most probably due to the surface microroughening caused by chloride a dsorption.