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.