Microscopic behavior of single corrosion pits: The effect of thiosulfate on corrosion of stainless steel in NaCl

Pitting corrosion of Type 304 and 316 stainless steels was investigated in neutral chloride solutions containing various concentrations of thiosulfate (0.01 to 500 mM) and chloride (1 to 1000 mM). The effect of thiosulfate on pitting corrosion of stainless steel 304 was quantitatively studied at the microscopic level by initiating single pits on small wires in neutral chloride solutions containing thiosulfate. Analysis of single pit data indicated that thiosulfate may promote stability through either a cathodic side-reaction or through the existence of a salt film. In galvanostatic experiments, the potential response exhibited six types of behavior that depended on the concentrations of chloride and thiosulfate. Thiosulfate was found to promote the sustained growth, or stability, of pits within a concentration range that depended upon the concentration of chloride ion. An excess of thiosulfate prevented pit formation. Type 316 stainless steel required higher concentrations of thiosulfate to produce stable pits, as compared to Type 304. The effect of surface adsorption of thiosulfate on pitting of 316 stainless steel was investigated by an in situ radiotracer method. It was found that the extent of thiosulfate surface adsorption differed for the three levels of chloride in 0.1 mM Na2S2O3 and that an increase in the surface concentration of thiosulfate from the small pit region caused formation of large pits. (C) 1997 Published by Elsevier Science Ltd.