Electrochemistry of stainless steel colonized by manganese-oxidizing bacteria

Microbial colonization of stainless steel (SS) surfaces can modify the electrochemical properties of the metal and increase the risk of corrosion damage. Two dominant effects of colonization--a several hundred millivolt noble shift in electrical potential and a two to three decade increase in cathodic current density--are implicated in the corrosion processes. These effects, collectively known as ennoblement, are attributed to the metabolic activity of attached microorganisms, however the mechanism by which microbial activity modifies cathodic reactions at the metal surface remains unclear. The present dissertation investigates the role of microbially generated oxidants in ennoblement and demonstrates that ennoblement is caused by manganic oxide biomineralization.; Microelectrode measurements of dissolved oxidants within biofilms on SS indicated that redox potential in the bulk biofilm did not change during ennoblement. A proposed mechanism involving oxygen electroreduction was also eliminated by showing that cathodic rates were independent of oxygen concentration. The findings directed attention to microbial production of surface-bound oxidants. Quantification of metal capacitance and surface-bound reactants suggested that metal oxides were involved in ennoblement and led to the hypothesis that ennoblement is caused by manganic oxide biodeposition.; Epifluorescence microscopy and bacterial culture methods confirmed the presence of manganese-oxidizing bacteria on ennobled SS. Manganese-rich surface deposits were confirmed by wet chemical and energy-dispersive x-ray analysis. SS coated with MnO{dollar}sb2{dollar} paste exhibited electrochemical properties that closely matched those of ennobled SS and elevated electrical potential decayed to pre-exposure values when bisulfite ion was used to reductively dissolve the surface manganese deposits. The biological mechanism of ennoblement was validated by inducing the effect using pure cultures of the manganese-oxidizing bacterium Leptothrix discophora. Coulometric titration and wet chemical analysis demonstrated that 15-75 nmoles cm{dollar}sp{lcub}-2{rcub}{dollar} of manganic oxide surface deposit shifts the potential of SS to a value near +350 mV versus the saturated calomel electrode.; Manganese-oxidizing bacteria have been widely reported at sites of SS corrosion. The present dissertation unifies this observation with the separate issue of ennoblement by linking both issues to a common cause, manganic oxide biomineralization. The finding provides a plausible explanation for the corrosive effects of manganese-oxidizing bacteria on SS.