Q: What are Microbiologically Influenced Corrosion (MIC) Mechanisms and Processes? A: MIC MECHANISMS: Usual mode of MIC is pitting because biological colonies are dispersed/separated over a surface. Shown below is pitting of austenitic stainless steel weld due to MIC. Base metal 316L, metal, ER308 5X. (Ref: Borenstein, Ibid) Tubercles Form - Term refers to a mound on the metal surface which contain biological species, probably in an adherent, slimy material. Eventually the mounds contain corrosion products resulting from their electrochemical activity. MIC PROCESS: A schematic diagram of electrochemical and microbial processes involved in tuberculation is shown below. Not all of these processes may be active in any given situation. Cl-, chlorides. Deposit of slime forming, Fe oxidizing bacteria where there is low velocity or stagnation of water. Oxygen concentration cell forms. Dissolves Fe2 from metal surface, which moves outward through the tubercle, oxidizing further to Fe3+. Gallionella (Fe oxidizing bacteria) can do this forming Fe (Oh)3. Wall of tubercle becomes Fe(OH)3 + slime + other bacterial species, if present. Exterior is cathodic,metal pit surface is highly anodic. As tubercle matures, slime/biomass begins to decompose, forming sulfates that attract SRB (sulfate reducing bacteria), which then produce H2S in the interior. FeS is also possible. Finally, if chlorides are present with the Gallionella, Fe chlorides may form which are highly acidic. In an extreme, unmonitored situation, tubercles grow together, forming a coating with severe pitting underneath, and possibility of severely limiting or shutting down flow if in a pipe/tube. Tuberculation can result from non biomass materials e.g., carbonates, silicates, phosphates, greases, mud, road debris. The result is pitting or even crevice corrosion underneath.