The ennoblement of stainless steel by manganic oxide biofouling

Abstract

Twenty-three 316L stainless steel (SS) coupons were exposed in situ to fresh river-water for periods of up to 35 days. All samples developed steady-state corrosion potentials (Ecorr) near + 350 mV (SCE) and polarization measurements showed enhanced cathodic current density characteristic of passive metal ennoblement. Epifluorescence and scanning electron microscopy of the attached biofilm showed numerous 10–20μm diameter Mn-rich annular deposits, associated clusters of bacterial cells, and abundant sheathed bacteria. Dissolution of the Mn deposits using Na2SO3 shifted Ecorr to pre-exposure values. SS coated with MnO2paste displayed electrochemical behavior nearly identical to that of ennobled samples. A mechanism of ennoblement by MnO2 biofouling is proposed which explains a variety of findings on the electrochemical behavior of microbially colonized SS.