Electroactive Microbes Short-Circuit the Passive Film to Corrode Stainless Steel

Electroactive microbes are uniquely capable of aggressively corroding metals like stainless steel that were once thought immune to microbial attack. This activity has been attributed to microbial destruction of the protective chromium oxide passive film on the stainless steel surface that protects the underlying Fe⁰ from corrosive agents, allowing the microbes to establish direct electrical contact with the Fe⁰ and extract electrons to support anaerobic respiration. We show here that the electroactive microbe Geobacter sulfurreducens, despite its high corrosive activity, is unable to physically breach the passive film. Instead, it enables biologically mediated electron transfer across an intact chromium oxide-rich layer that remains sufficiently insulating to block abiotic proton reduction. These findings challenge the prevailing assumption that electroactive microbes must directly contact Fe⁰ for corrosion and provide new guidance for the design of corrosion-resistant metals.