Pitting corrosion of Alloy 690 in thiosulfate-containing chloride solutions

The effects of thiosulfate ion and solution pH on pitting corrosion of Alloy 690 in chloride solution were explored. Potentiodynamic polarization measurements were conducted to evaluate pitting corrosion susceptibility of Alloy 690 in these environments. The results showed that pitting corrosion occurred in the mill-annealed (1050 °C/5 min) Alloy 690 in 1 wt% NaCl solution but not in 0.1 M Na₂S₂O₃ solution. The value of pitting nucleation potential (E_np) determined in 1 wt% NaCl solution (without Na₂S₂O₃) increased with increasing solution pH value in the range of 2-10. The addition of Na₂S₂O₃ to 1 wt% NaCl solution greatly affected the pitting corrosion behavior, which was dependent on concentration. The preformed nickel sulfide surface film due to the presence of Na₂S₂O₃ caused Alloy 690 to become more susceptible to pitting corrosion in 1 wt% NaCl solution.