Effects of temperature and thiosulfate on chloride pitting of austenitic stainless steels

Potentiodynamic polarization curves were measured for types 304 (UNS S30400 ), 316L (TUNS S31603), and 904L (UNS N08904) stainless steels (SS) in 1 M s odium chloride (NaCl) solutions with various thiosulfate additions at tempe ratures from 20 degrees C to 90 degrees C. A minimum concentration of thios ulfate was required for activation of pitting corrosion in each alloy, and this critical value was shown to increase with increasing molybdenum conten t and to decrease with increasing: temperature. Above the critical thiosulf ate concentration for type 904L SS, stable pitting occurred at temperatures up to 30 degrees C below the chloride-only critical pitting temperature, I n addition, polarization data at high thiosulfate levels:exhibited an anodi c peak thought to represent the oxidation of elemental sulfur, produced by prior reduction of thiosulfate at potentials < -450 mV(Ag-AgCl).