Effects of sulfidation of passive film in the presence of SRB on the pitting corrosion behaviors of stainless steels

Pitting corrosion of stainless steels is a very complex process in the media with sulfate-reducing bacteria (SRB). Bacterial activities not only instantaneously affect the pit-initiation and the pit-growth, but also induce the changes of passive film in structure/properties and in turn affect the pitting corrosion behaviors of stainless steels as well. In this work, sulfidation of passive film in the cultures of SRB and its effects on the pitting corrosion behaviors of stainless steels were investigated by electrochemical techniques and X-ray photoelectron spectroscopy (XPS). As results showed, anodic cyclic polarization curves demonstrated that sulfidation caused a loss of passivity of stainless steels, but sulfidation happening in the short term is more detrimental than that in the long term; from standing point of cathodic depolarization of corrosion, sulfidation polarized the cathodic reactions to a greater extent, which means that sulfidation helps impede the pit-growth; XPS revealed that almost only iron oxides in the passive film were converted into sulfides in forms of FeS and FeS2, while elements of Cr and Ni still remained essentially as oxides/hydroxides. So, in understanding the mechanisms of pitting corrosion of stainless steels in the media with SRB, effects of sulfidation of passive film must be taken into consideration.