Abstract
Ni-Cr-Fe alloys such as UNS N08800 (alloy 800) suffer pitting corrosion in chloride (Cl−) plus thiosulfate (S2O32−) solutions. This occurs due to a synergistic effect between both ions in a certain range of solution concentration and electrochemical potential. Potentiodynamic tests in 1 M NaCl + 10−3 M S2O32− solutions evidenced thiosulfate pitting by a peak in anodic current centered near -0.15 VSCE. The maximum anodic current of this peak increased with a decrease in potential scanning rate, reflecting the autocatalytic nature of the pitting process and the increased available time for pit growth. A wire of alloy UNS N08800 was embedded in epoxy to study the kinetics of anodic dissolution using the one-dimensional artificial pit electrode. Pits were grown at -0.16 VSCE, near the anodic peak observed in potentiodynamic tests, and at 0.4 VSCE, i.e. above the conventional chloride pitting potential. At -0.16 VSCE, the one-dimensional pit was under ohmic drop control. When the wire was polarized to 0.4 VSCE, the usual transitions from activation, ohmic drop and diffusion control regimes were observed. Thiosulfate increased the rate of anodic dissolution during the activation-controlled regime of pit growth and decreased the transition potential (ET), the potential where pit growth shifts from activation/ohmic drop control to diffusion control.
