Pitting of Type 304 stainless steee in the presence of a biofilm containing sulphate reducing bacteria

Abstract

The effects of sulphate reducing bacteria (SRB) on pitting corrosion of Type 304 stainless steel in nearneutral solutions based on 0.1 M NaCl have been studied at ambient temperature. SRB are considered to have at least two potentially corrosive effects: they create a biofilm having a crevice like geometry on the metal surface and they produce hydrogen sulphide. Slow potentiodynamic scans were used to measure pitting potentials and pit current transients in 0.1 M NaCl solutions with and without H 2S, a modified Postgate growth medium inoculated with SRB, and an inorganic medium based on the Postgate medium. Measurements were made after an 8 day incubation or aging period. Results indicate that although H 2S enhances pitting corrosion, SRB have less effect on the pitting potential than would be expected from the levels of H 2S they produce. In fact, a decrease in the metastable pitting rate was observed suggesting that, rather than forming aggressive crevice environments in these tests, the biofilms had inhibitory properties. Analysis of current transients in terms of the pit stability product suggested that pits in commercial grades of stainless steel should possibly be considered as micro-crevices rather than ideal hemispherical cavities.