Microbiologically Influenced Corrosion of Stainless Steel by Sulfate Reducing Bacteria: A Tale of Caution

The influence of different experimental media composition and air purging on the potential for microbiologically influenced corrosion (MIC) of Type 304 stainless steel with sulfate-reducing bacteria (SRB) was investigated. Modified Baar's (MB) medium, MB medium without iron ions and supplemented with sodium chloride (MBN), and air purged MBN medium (MBO) were used. Pitting corrosion attack was found on the surface of the coupons for all of the conditions tested including the abiotic tests, and detailed statistical analysis showed no significant difference between the pitting results. General corrosion and maximum pit penetration rates also showed no difference between the coupons exposed to different test conditions. Interestingly, the pits found on the surface of the coupons in all of the tested conditions were comparable in size/shape and depth to that of the inclusions present on the surface of the stainless steel coupons. These findings suggest that (i) the test conditions studied do not lead to increased corrosion rates of stainless steel with SRBs and (ii) care needs to be taken to avoid the pitfall of misinterpreting the corrosion of inclusions present on the surface of stainless steels, which can occur as a result of cleaning of the coupons, as MIC pits.