What are Microbiologically Influenced
Corrosion (MIC) Mechanisms and Processes?
- 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
- 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.
- 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
- 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.