Pitting Corrosion of Inconel 600 in Chloride and Thiosulfate Anion Solutions at Low Temperature

Pitting corrosion of mill annealed Inconel 600 was studied in aqueous chloride and chloride-thiosulfate solutions at low temperature. Potentiodynamic and potentiostatic polarization techniques were used to determine the characteristic pitting potentials for Inconel^† 600 in chloride and chloride-thiosulfate media. The pitting potential, Enp, was evaluated in four different concentrations of sodium chloride between 0.1 and 0.0001 M. Enp decreased with increasing concentrations of the chloride ion. Immunity to pitting corrosion was evident at chloride levels below 0.0025 M without thiosulfate additions. Increasing thiosulfate concentrations resulted in improved chloride pitting resistance; however, low concentration (0.0001 M) thiosulfate additions reduced pitting resistance. The lowest concentrations of chloride ions with thiosulfate additions causing pitting were determined. The results of open circuit immersion test were consistent with that determined by use of the anodic polarization scan. The EDS data show that sulfur is enriched in pits; thiosulfate may play an important role in the processes of pit growth. Pit density and average pit depth increased with increasing concentrations of sodium chloride and temperature of solution. The probable mechanism of the processes leading to pitting of Inconel 600 in low temperature water is discussed.