Effect of iron-oxidizing bacteria on pitting of stainless steel

Microbiologically influenced corrosion (MIC) of stainless steel (SS) can be caused by the action of metal-oxidizing bacteria (MOB) in natural, relatively low-chloride waters. This type of corrosion is frequently associated with ennoblement of the open-circuit potential and involves iron-oxidizing bacteria (IOB) and manganese-oxidizing bacteria (MnOB). This work focuses of the role of IOB and associated inorganic water chemistry processes that may cause corrosion of SS. Laboratory and field studies have been conducted to investigate the influence of deposited ferric oxide (or oxyhydroxide) on the corrosion of UNS S30403. Samples exposed to a natural spring water reached open-circuit potentials of 250 mV vs saturated calomel electrode (SCE) and were covered by a biofilm containing a mixture of geothite (alpha-FeOOH), lepidocrocite (gamma-FeOOH). and magnetite (Fe3O4) or maghemite (gamma-Fe2O3), together with low levels of manganese-based deposits. In the laboratory, an iron-based film was formed on some samples by oxidation of ferrous sulfate (FeSO4) with calcium hypochlorite (Ca[ClO](2)), giving a surface film with a composition similar to that produced by microbial activity. These samples did not show ennoblement, but they did show lower pitting potentials than were measured for control samples. The results of this work are consistent with the idea that MIC of SS in potable water involves manganese-based deposits causing ennoblement (W.H. Dickinson, F CaccavoJr, Z. Lewandowski, Corros. Sci. 38 [1996], p. 1,407), combined with iron-based deposits simultaneously causing a decrease in the pitting potential (M. Suleiman, I. Ragault, R. Piewman, Corros. Sci. 36 [1994], p. 479).