The role of sulfides in iron activation in chloride-containing solutions

The mechanism of iron activation in sulfide-containing solutions was studie d. It was shown that sulfide did not induce the activation of a prepassivat ed electrode in solutions free of chloride, however, a significant reductio n of the pitting potential occurs in the presence of chloride. The effect w as associated with sulfide salt layer formation over activated sites during metastable pit growth, which prevented their repassivation. Due to a low-s olubility product, ferrous sulfide can precipitate on iron surfacse at a mu ch lower ferrous ion concentration compared to ferrous chloride. Hence, the transition from metastable to stable pit growth could be expected at much lower pit growth current densities and as a result at potentials well below the breakdown potential as measured in chloride solutions. (C) 1999 The El ectrochemical Society. S1099-0062(99)01-053-6. All rights reserved.