Studies were carried out on the effect of mild steel on fermentation by sulphate‐reducing bacteria (SRB) and on the effect of sulphate and ferric ions on metal corrosion. Laboratory scale fermenters of 1–21 capacity containing American Petroleum Institute (API) medium were inoculated with enriched SRB cultures from crude oil‐seawater admixtures obtained from Trengganu Offshore Oil Fields, Malaysia. Fermentations were maintained at a 10 day hydraulic retention time (HRT) at 28°C, and mixed twice daily. The parameters analysed were percentage corrosion, pH, total alkalinity and sulphate concentration. The presence of metal pieces accelerated SRB fermentation, as indicated by a rapid uptake of sulphate and a more alkaline system compared with SRB fermentation in the absence of metal pieces. Sulphate and ferric ions generally increase the rate of metal corrosion. It is believed that SRB play a dominant role in microbial corrosion in the presence of sulphate. However, in the presence of ferric ions other mechanisms of metal corrosion could be more important in SRB fermentation.