Microbially reduced flavins destabilize the TiO2 passive film of titanium under bulk aerobic conditions

Passive metals protected by surface oxide films dominate modern structural materials. Thus, understanding the mechanisms governing passive film breakdown is essential. The electronically insulating TiO2 passive film on titanium separates the underlying Ti° from environmental oxidants, but microbial biofilms have been reported to catalyze titanium corrosion. Here we demonstrate that the common marine microbe Shewanella algae growing on titanium reduces TiO2 to destabilize the passive film. The biofilm consumes oxygen, generating anaerobic conditions near the metal interface even when the bulk environment remains aerobic. High-resolution characterizations reveal thinning of the passive layer and accumulation of lower-valence titanium oxides. Electrochemical analyses show a six-fold increase in corrosion current from Ti0 accompanied by enhanced pitting. Mutant studies and other approaches indicate that extracellular flavins function as an intermediary electron carrier for S. algae TiO2 reduction. These results demonstrate how microbes create localized microenvironments that accelerate corrosion of otherwise highly corrosion-resistant metals.