Corrosion Conundrum

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Science  15 Jun 2012:
Vol. 336, Issue 6087, pp. 1362
DOI: 10.1126/science.336.6087.1362-c

Corrosion of iron leads to damage and failure of a variety of structures, including bridges and underground pipes. The electrochemical process occurs by the oxidation of metallic iron (Fe0) to ferrous iron (Fe2+), which creates iron oxide rust in the presence of oxygen. In anaerobic sediments, however, the purely chemical reaction is so slow that sulfate-reducing bacteria are mainly responsible for the corrosion of buried iron structures. In theory, the bacteria themselves could use the electrons generated during corrosion to drive their metabolism, but crusts precipitated on the iron surface would present a physical barrier to prevent the cells from coming in direct contact with the metallic iron. In laboratory and field experiments, Enning et al. found that the precipitating crusts on highly corroded iron surfaces were semiconducting, which provides an electrical path for electrons to travel between the iron surface and bacterial cells. To complete the electrochemical circuit, chimney-like structures in the crust allowed the passage of neutralizing ions. A sharp pH gradient inside the chimney promoted dissolution and pitting of the iron surface, and the released Fe2+ was incorporated into the chimney and crust. Inhibiting bacterial sulfate reduction in the subsurface may therefore reduce the economic costs associated with iron corrosion underground.

Environ. Microbiol. 10.1111/j.1462-2920.2012.02778.x (2012).

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