A Mechanistic Approach to Understanding Microbiologically Influenced Corrosion by Metal-Depositing Bacteria

Iron (Fe)- and manganese (Mn)-oxidizing bacteria are often cited individually and collectively as putative microorganisms for microbiologically influenced corrosion (MIC). The two groups of microorganisms have in common the ability to attach to surfaces and produce macroscopic accumulations (deposits) of metal oxides/hydroxides/oxyhydroxides that can influence corrosion of some metals and alloys in some environments. In all cases, once initiated, the corrosion is independent of the activities of the colonizing species. Despite the phylogenetic diversity of Fe-oxidizing bacteria (FeOB), the following sections will deal with corrosion mechanisms attributed to neutrophilic, lithotrophic, microaerophilic FeOB. The mineralogy of biologically oxidized Fe is consistent over a wide range of environments. All FeOB produce dense deposits that can cause corrosion of low alloy stainless steels (SS) directly, i.e., under-deposit corrosion. Association of Mn-oxidizing bacteria (MnOB) and other microorganisms may stabilize the under-deposit corrosion on low alloy SS. The influence of FeOB on the corrosion of carbon steel may be related to deposition of metal ions, causing galvanic corrosion or association with other corrosion-causing microorganisms. All MnOB produce Mn oxides that are strong oxidizing agents that can cause ennoblement of low alloy SS and increase corrosion currents on carbon steel in fresh water. Corrosion associated with Mn deposits depends on the relationship between the mineral deposit and the substratum.