Technical Note: A Mechanism of Microbial Pitting in Stainless Steel

Pitting of stainless steel (SS) is sometimes encountered in warm natural water of very low chloride content and modest oxidizing ability. Sulfate-reducing bacteria are common in natural water, and their activity may be inferred from the presence of hydrogen sulfide. It is well known that high concentrations of H₂S can increase the susceptibility of SSs to pitting, so it may be suspected that diluted bacterial products have caused pitting even in cases in which no bacteria are at the site of attack. However, laboratory testing generally fails to produce convincing evidence for this indirect effect of microbial action. For example, Holt and Cottis¹  measured pitting potentials at room temperature in 200 ppm (0.005 M) chloride solutions containing various sulfide additions. The lowest pitting potential was ~500 mV [SCE (saturated calomel electrode)], which is at least 300 mV more positive than typical long-term corrosion potentials in aerated solutions....