Effects of Temperature and Thiosulfate on Chloride Pitting of Austenitic Stainless Steels

Potentiodynamic polarization curves were measured for types 304 (UNS S30400), 316L (UNS S31603), and 904L (UNS N08904) stainless steels (SS) in 1 M sodium chloride (NaCl) solutions with various thiosulfate additions at temperatures from 20°C to 90°C. A minimum concentration of thiosulfate was required for activation of pitting corrosion in each alloy, and this critical value was shown to increase with increasing molybdenum content and to decrease with increasing temperature. Above the critical thiosulfate concentration for type 904L SS, stable pitting occurred at temperatures up to 30°C below the chloride-only critical pitting temperature. In addition, polarization data at high thiosulfate levels exhibited an anodic peak thought to represent the oxidation of elemental sulfur, produced by prior reduction of thiosulfate at potentials < –450 mV_Ag-AgCl.