Microbiologically Influenced Corrosion (MIC) of stainless steel can be caused by the action of Iron Oxidizing Bacteria (lOB) in relatively low chloride waters. One effect of the IOB can be simulated by oxidising ferrous sulphate solution to cause precipitation of a ferric oxide (or hydroxide) deposit, and then drying the precipitated film on to a stainless steel surface. This process may also replicate cooling water treatment of iron-containing waters. In-situ Raman spectroscopy has been used to characterise the deposited film, which is shown to be mainly FeOOH at open-circuit, but is transformed by cathodic polarisation, firstly to Fe2O3 and then to Fe3O4 at lower potential. The presence of this film can reduce
the pitting potential of 304L SS by approximately 200 mV in 0.1 M NaCl, but it has a lesser effect in 0.01 M NaCI and does not lower the repassivation potential in either solution. Samples of 304L exposed
to iron-rich natural spring water reached open-circuit potentials of +250 mV (SCE), about 150 mV higher than achieved by samples exposed to sterile solutions in the laboratory. However, factors other than potential ennoblement and formation of iron-rich surface films are most probably involved in MIC of stainless steels. Possibilities include a synergistic effect of sulphate reducing bacteria (SRB), or the crevice-forming effects of gel-like extra-cellular material. Keywords: ennoblement, pitting corrosion, stainless steel, biofilm, Raman spectroscopy