Corrosion mechanism of iron-oxidizing bacteria (IOB)

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IOB obtains energy through the oxidation of Fe(II) to Fe(III), a process that is often considered to be closely related to the formation of rust tumors in pipes. The deposition of IOB biofilms and Fe(III) corrosion products on the metal surface reduces the dissolved oxygen concentration in the covered areas, turning them into oxygen-poor zones, while the uncovered areas have high dissolved oxygen levels and function as cathodic zones. The structure of small anodes and large cathodes results in a high current density in the covered areas. The metal ions generated by anodic dissolution undergo hydrolysis to produce H+, which lowers the local pH. Meanwhile, the accumulation of a large amount of H+ causes corrosive anions (such as Cl-) in the bulk solution to migrate toward the anode region, thereby inducing pitting corrosion. At the same time, the rust nodules formed by the deposition of IOB biofilms and Fe(III) corrosion products provide a favorable environment for the growth of anaerobic microorganisms such as SRB; the combined action of these various microorganisms further increases the corrosion rate.