Effect of Thiosulfate on Crevice Corrosion of Stainless Steels

Crevice corrosion studies were conducted on AISI 304, AISI 316L, and UHB 904L stainless steels using Teflon/metal crevices exposed to aqueous solutions of 1M NaCl and 1M NaCl + 0.01 M Na2S2O3. Tests were conducted at 22 C, except for some additional tests at 55 C on the more crevice resistant 904L. Crevice potentials and crevice corrosion currents were monitored using a previously designed corrosion cell. Average crevice corrosion penetration rates were computed from the corrosion current and area of localized corrosion. Maximum crevice corrosion penetration rates were determined by optical microscopy techniques. The presence of thiosulfate increased the penetration rates by a factor of at least 10 but did not significantly influence crevice corrosion initiation. Analysis of corrosion products, together with thermodynamic and kinetic considerations, indicated that the enhanced penetration rates in the thiosulfate solution were due to the formation of a reduced sulfur species in the active crevice, probably H2S, which catalyzed the anodic dissolution process.