The Influence of Sulfate‐Reducing Bacteria on the Passivity of Type 317L Austenitic Stainless Steel

Abstract

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, , NiS, and possibly . 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.