EFFECT OF SULFATE–REDUCING BACTERIA ON THE PITTING CORROSION BEHAVIOR OF 18–8 STAINLESS STEEL

Pitting corrosion of stainless steels is a very complex process in the chloride–containing media with sulfate–reducing bacteria (SRB). Sulfate reduction by SRB results in the production of sulfur–containing species, which can significantly change the media state and ultimately affect both the pit–initiation (breakdown of passive film) and the pit–growth. In the present study, the effect of SRB on the pitting corrosion behavior of 18–8 stainless steel (18–8SS) in seawater was investigated by atomic force microscopy (AFM) and electrochemical methods. As the results show, 18–8SS can be quickly activated in the media with SRB, indicating that bacterial activities can accelerate greatly the process of breakdown of passive film; Measurements with AFM probe demonstrate that the rate of micropit growth of 18–8SS in the media with SRB is conspicuously higher than that in the sterilized media. As for the pitting corrosion mechanism, bacterial metabolites make the pit potential (Epit) and the repassivation potential (Erep) both decrease distinctly; However, Epit of 18–8SS is still more positive than the redox potential (Eh) of the media state, Erep of 18–8SS more positive than the reversible hydrogen electrode potential, and thus it is proposed that the localized free oxygen resulting from sulfidation of passive film in the media with SRB is the main factor which causes the breakdown of passive film; Subsequently, sulfur element or polysulfide in the media with SRB is the main factor which sustains the pit–growth, and the current density for cathodic reduction of sulfur element or polysulfide can reach to a very high level (>10 μA/cm2).