Passive film degradation on TC4 titanium alloy driven by FAD-mediated extracellular electron transfer of Pseudomonas aeruginosa

This study investigates the effect of the exogenous electron shuttle flavin adenine dinucleotide (FAD) on the corrosion behavior of TC4 titanium alloy in the presence of Pseudomonas aeruginosa ( P. aeruginosa ) biofilms. Results show that FAD addition enhances the localized corrosion of TC4 in the biotic system. After 14 d of immersion, the maximum pit depth, corrosion current density, and donor density of the passive film in the biotic + FAD system increase to 1.17, 1.22, and 1.20 times those of the biotic system without FAD, respectively. This indicates that FAD promotes pit propagation, anodic dissolution, and donor-type defect accumulation. Electrochemical measurements and Mott-Schottky analysis demonstrate that FAD functions as an electron shuttle, enhancing extracellular electron transfer at the biofilm/passive-film interface, accelerating interfacial charge transfer, and reducing passive-film stability. Further characterization reveals that P. aeruginosa preferentially colonizes grain-boundary regions and, through FAD-mediated electron shuttling, promotes valence-state reduction, localized dissolution, and thinning of TiO 2 passive film, ultimately leading to pit initiation and propagation. This study provides new insights into the role of electron shuttles in regulating the microbiologically influenced corrosion (MIC) of titanium alloys and offers guidance for the development of corresponding protection strategies.