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 investigat ed using X-ray photoelectron spectroscopy (XPS) and electrochemical te chniques. Samples were exposed to a SRB culture, and then resultant su rface states were analyzed by XPS and corrosion resistance by cyclic p olarization in deaerated 0.1 M HCl. Passivity of the SRB-exposed sampl es was further studied by XPS after polarization at a passive potentia l (-160 mV(SCE)) in 0.1 M HCl. The samples were characterized under tw o surface conditions: unrinsed and rinsed with deaerated alcohol and d eaerated 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, MoS 2, Cr2S3, NiS, and possibly Fe1-chiS. These sulfides were mainly in th e outer layers of the surface so that the passive film was not markedl y deteriorated. In addition, a Mo(V) species was formed in the biofilm and remained insoluble in 0.1 M HCl, thereby protecting the surface f rom the ingress of chloride. The results revealed that Mo increased th e passivity by: (i) reducing the bacterial activity and sulfate reduct ion through formation of molybdate and (ii) forming a Mo(V) species wh ich is insoluble in 0.1 M HCl.