Effect of non-halide anions on repassivation of crevice corrosion on the austenitic stainless steel

Crevice corrosion is the most common and detrimental corrosion mode for stainless steel in various industries where the environment contains halide anions (such as Cl^-). When crevice corrosion occurs in Cl^--containing solutions, the Cl^- concentration in the crevice increases by several orders of magnitude due to the potential gradient. This phenomenon contributes to localized corrosion in crevices. If high concentrations of non-halide anions (such as MoO₄^2- and SO₄^2-) coexist with Cl^-, these ions compete with Cl^- in migrating into the crevices and help improve the crevice environment. The present study investigates the effect of nonhalide anions on the repassivation of growing crevice corrosion in austenitic stainless steel. The study specimens were held at 200 mV_SCE in a 1000 ppm Cl^- solution, and the anode current behavior was investigated after the addition of non-halide anions (MoO₄^2- and SO₄^2-) to the solution. The results indicate that adding MoO₄^2- anions to the solution at a molar concentration tenfold greater than the Cl^- concentration significantly enhances repassivation. Conversely, adding insufficient amounts of MoO₄^2- or SO₄^2- has a rather detrimental effect, accelerating crevice corrosion. (author)