The microbiologically influenced corrosion of pure iron was investigated in the presence of Shewanella oneidensis MR-1 with various levels of exogenous riboflavin (RF) serving as electron shuttles for extracellular electron transfer (EET). With more RF available, a larger and denser phosphate layer was formed on the surface of pure iron by the bacteria. The results of electrochemical impedance spectroscopy, linear polarization resistance and potentiodynamic polarization tests showed that the product layer provided good corrosion protection to the pure iron. Using electrochemical noise, we observed that the addition of RF accelerated the corrosion at the initial stage of immersion, thereby accelerating the deposition of products to form a protective layer subsequently.

Microbiologically influenced corrosion of metals is prevalent in both natural and industrial environments, causing enormous structural damage and economic loss. Exactly how microbes influence corrosion remains controversial. Here, we show that the pitting corrosion of stainless steel is accelerated in the presence of Shewanella oneidensis MR-1 biofilm by extracellular electron transfer between the bacterial cells and the steel electrode, mediated by a riboflavin electron shuttle. From pitting measurements, X-ray photoelectron spectroscopy and Mott-Schottky analyses, the addition of an increased amount of riboflavin is found to induce a more defective passive film on the stainless steel. Electrochemical impedance spectroscopy reveals that enhanced bioanodic and biocathodic process can both promote the corrosion of the stainless steel. Using in situ scanning electrochemical microscopy, we observe that extracellular electron transfer between the bacterium and the stainless steel is bidirectional in nature and switchable depending on the passive or active state of the steel surface.