Effect of the Nickel Content in the Pitting of Stainless Steels in Low Chloride and Thiosulfate Solutions Available to Purchase

The effect of the nickel content on the pitting resistance of AISI⁽¹⁾ 304 stainless steel (SS) and Alloys 800 and 600 in chloride-thiosulfate solutions was investigated using two techniques, scratching and slow-stepped potentiostatic polarization. Adverse effects of nickel were seen at both low and high thiosulfate concentrations. For a constant chloride concentration at 24 and 80 C the pitting potential (Ep) exhibited a minimum value with the change in thiosulfate concentration. At this minimum, a semi-logarithmic relation prevailed between the nickel content and the ratio of the chloride-thiosulfate concentrations. With the nickel addition, the alloy became susceptible to larger concentration ratios and a tenfold increase in ratio occurred between AISI 304 SS and Alloy 600. The maximum decrease in Ep recorded at 80 C going from chloride to chloride-thiosulfate solutions amounted to 550, 625, and 700 mV for AISI 304 SS, Alloy 800, and Alloy 600 respectively. This decrease suggests that the presence of thiosulfate may account for some of the failures that have occurred with AISI 304 SS and Alloy 800 when used as tubing materials in fresh water-cooled heat exchangers.