Electrochemical behaviour of stainless steels in media containing iron-oxidizing bacteria (IOB) by corrosion process modeling

Localized corrosion mechanism of stainless steel (SS) types UNS S30403 and UNS 31603 in the presence of iron-oxidizing bacteria Sphaerotilus spp. isolated from rust deposits was studied electrochemically. OCP transient, cyclic anodic and cathodic potentiodynamic polarization curves were measured on steel electrodes through their exposure to 3% NaCl solution supplemented with Sphaerotilus culture. The exposure period was composed of three parts: (a) 5 days incubation of steel electrodes in sterile 3% NaCl solution; (b) addition of 3 days-old Sphaerotilus culture to 3% NaCl at 3:2 v/v ratio with subsequent electrodes exposure for 11 days up to complete sedimentation of ferric oxides and (c) subsequent exposure of electrodes for 14 days in upper and bottom (sediments layer) fractions of the experimental medium. The results revealed an instantaneous gradual shift of the transient potential of both steels towards negative potentials from steady-state value of −0.15 V to −0.35 to −0.42 V (SCE) during the whole exposure interval since IOB culture addition into sterile 3% NaCl solution.No evidence of pitting corrosion was found on SS samples subsequent to their exposure to sterile 3% NaCl solution, though in the presence of IOB culture, numerous pits were revealed on 304 L steels specimens exposed to iron hydroxides sediments layer. Electrochemical characteristics (OCP or corrosion potential – ECORR, breakdown potential – EBD, repassivation potential – ERP, passivation current – iPASS) periodically measured by cyclic polarization method, allowed monitoring the electrochemical behavior changes of experimental SS and to establish the initiation of pitting corrosion in the presence of IOB, resulting in crevice effect caused by biogenic ferric oxides deposits precipitated on steel surface. Overall, steel 316L demonstrated higher resistance to pitting corrosion compared to 304L.