Biocorrosion studies on borated and non-borated 304 L stainless steel using Bacillus subtilis SNF-1, a bacterial isolate from SNF pool

Abstract

This study investigates microbiologically-influenced corrosion (MIC) aspects of borated and non-borated 304 L- stainless-steel using Bacillus subtilis SNF-1, which was isolated from the spent nuclear fuel pool (SNF). Over 28 days, electrochemical analyses revealed distinct corrosion behaviours: borated 304 L SS exhibited a more pronounced decrease in open circuit potential (from 0.03 to -0.35 V vs. Ag/AgCl) as compared to non-borated 304 L SS (from 0.05 to -0.10 V vs. Ag/AgCl) indicating higher susceptibility to MIC. Potentiodynamic polarization studies revealed an increase in passive current density (from 1.5 to 2.4 μA.cm^-2 for non-borated 304 L SS and from 2.4 to 3.4 μA.cm^-2 for borated 304 L SS), along with a lower pitting potential indicating the role of B. subtilis SNF-1 in MIC. Electrochemical impedance spectroscopy confirmed accelerated degradation, with polarization resistance (R_p) dropping by 69 % in borated 304 L SS and 86 % in non-borated 304 L SS. Despite higher absolute corrosion rates in borated 304 L SS, non-borated 304 L SS experienced a greater relative increase in corrosion (3.8-fold vs. 2.3-fold) due to denser biofilm coverage (95 % vs. 74 %). Surface analysis identified localized pitting beneath biofilms, exacerbated by boride-induced micro-galvanic effects. These findings underscore the dual role of alloy microstructure and biofilm dynamics in MIC severity.

Keywords: Biocorrosion; Biofilm; Electrochemical impedance spectroscopy (EIS); Microbiologically-influenced corrosion; Open circuit potential (OCP); Potentiodynamic polarization.