The influence of sulfate-reducing bacteria on the passivity of type 317L austenitic stainless steel

The influence of sulfate-reducing bacteria (SRB) on the passivity of a Mo-bearing austenitic stainless steel (SS), type 317L, was investigated using X-ray photoelectron spectroscopy (XPS) and electrochemical techniques. Samples were exposed to a SRB culture, and then resultant surface states were analyzed by XPS and corrosion resistance by cyclic polarization in deaerated 0.1 M HCl. Passivity of the SRB-exposed samples was further studied by XPS after polarization at a passive potential (-160 mV(SCE)) in 0.1 M HCl. The samples were characterized under two surface conditions: unrinsed and rinsed with deaerated alcohol and deaerated deionized water. Control samples were exposed to uninoculated medium and "as-polished" coupons provided baseline for comparison. The following sulfides were formed during the exposure: FeS, FeS2, MoS2, Cr2S3, NiS, and possibly Fe1-chiS. These sulfides were mainly in the outer layers of the surface so that the passive film was not markedly deteriorated. In addition, a Mo(V) species was formed in the biofilm and remained insoluble in 0.1 M HCl, thereby protecting the surface from the ingress of chloride. The results revealed that Mo increased the passivity by: (i) reducing the bacterial activity and sulfate reduction through formation of molybdate and (ii) forming a Mo(V) species which is insoluble in 0.1 M HCl.