SRB-mediated corrosion of marine submerged AISI 1020 steel under impressed current cathodic protection

[Display omitted] •Cathodic protection inhibits the microorganisms’ adhesion to metallic surface.•The potential of −1000 mVAg/AgCl stimulates the hydrogenase activity of SRB.•Low cathodic protection potential increases density and depth of pits. Metallic corrosion is a recurrent and costly problem to almost every industry; therefore, prevention strategies might be well-defined on a case-by-case basis. Commonly, cathodic protection (CP) is the world’s most widely-adopted technique to guarantee the integrity of buried or submerged structures from corrosion. However, as current potential values are dependent on metal-structure and environmental features, the target shall be well-identified; otherwise, the intended effect will not be reached. In seawater, a protective current potential of −800 mVAg/AgCl is recommended by technical standards, while a more negative potential (−900 mVAg/AgCl) is the suggested criterion for the control of corrosion induced by sulfate-reducing bacteria (SRB), even though without proper scientific support. Thus, this study focused on evaluating the efficiency of different cathodic protection potentials (−800, −900 and, −1000 mVAg/AgCl) on inhibiting SRB-mediated corrosion of AISI 1020 steel. Both unprotected and impressed current cathodically protected steel specimens were exposed to indigenous microorganisms in seawater for 7 days. The Most Probable Number (MPN) enumeration of sessile aerobic heterotrophic bacteria, acid-producing bacteria and, sulfate-reducing bacteria was performed at the beginning and at the end of the assays. Also, the reducing activity of hydrogenase-positive SRB strains was measured. Although the microbial colonization was greater on unprotected steel surfaces than on the cathodically protected ones, biofilm quantification of CP specimens did not show important differences regardless of the potential. However, hydrogenase-positive SRB counts increased with the reduction of CP potential value, promoting an increase in the number and depth of pits on specimens protected at −1000 mVAg/AgCl when compared with those protected at −800 mVAg/AgCl and unprotected ones.