Influence of sulfate-reducing bacteria on the passivity of type 304 austenitic stainless steel

The influence of sulfate-reducing bacteria (SRB) on the passivity of type 304 austenitic stainless steel was investigated using x-ray photoelectron spectroscopy (XPS) and electrochemical techniques. Multiple samples were exposed to a SRB culture, Desulfovibrio desulfuricans, and then resultant changes in the surface states were determined by XPS and in the corrosion resistance by potentiodynamic polarization in deaerated 0.1 M HCl. Furthermore, the SRB-exposed samples were anodically polarized at a passive potential (-160 mV_SCE) in 0.1 M HCl and subsequently analyzed by XPS. The electrochemical and XPS analyses were performed under two surface conditions, unrinsed with the biofilm and rinsed. Formation of sulfides and thermodynamically unstable products during exposure to SRB caused loss of the steel passivity. The following sulfides were observed: FeS, FeS₂, NiS, Cr₂S₃, and possibly Fe_1-xS. The unstable products included FeO and hexavalent chromium (Cr⁶⁺) species. The sulfides unevenly distributed in the depth of the passive film so that undercutting took place, rendering a higher degree of hydration of the inner layers of the passive film. The loss of passivity, measured by the polarization current density, was proportional to the extent of undercutting of the passive film and the relative proportion of the Cr⁶⁺ species.