How microbes accelerate corrosion
Bacteria and other microorganisms attach to wetted surfaces and build a biofilm: a layer of cells and extracellular slime that traps deposits and isolates the metal from the bulk fluid. Under that layer the chemistry is set by the community, not by the pipe contents. Several groups contribute:
- Sulfate-reducing bacteria live in the oxygen-free zone at the base of the film and produce hydrogen sulfide, forming iron sulfide deposits and deep pits.
- Acid-producing bacteria generate organic acids that lower the local pH.
- Iron- and manganese-oxidizing bacteria build tubercles and oxide deposits that create oxygen concentration cells.
- Slime-forming organisms provide the shelter the others need.
The result is a combination of differential aeration cells, acid production and sulfide attack concentrated at discrete sites — which is why MIC presents as pitting rather than general wall loss.
Where it shows up in plants
MIC needs water, nutrients and time without flow. Typical locations are firewater systems, hydrotest water left in piping or vessels, tank bottoms under sludge or water layers, cooling water circuits, dead legs and low points, and buried structures in anaerobic soils. Stainless steel is not immune: weld zones in austenitic piping left wet after hydrotest are a classic case.
How it is diagnosed
No single test proves MIC. A defensible diagnosis combines several lines of evidence:
- microbiological sampling taken at the damage site, by culture methods and increasingly by molecular methods that identify and quantify organisms;
- deposit chemistry, for example sulfides or high iron and manganese levels;
- pit morphology and metallurgy;
- an operating history that includes stagnation, low flow or untreated water.
Bacteria are present in almost every water system, so their mere presence in a bulk water sample is not a diagnosis.
What to do about it
Control works on the conditions microbes need. Keep water moving or drain and dry it: hydrotest water should not be left standing. Remove deposits by mechanical cleaning or pigging, since biocides penetrate established biofilms poorly. Treat water with biocides selected and monitored for the system. Isolate the metal with a lining or coating where stagnation cannot be designed out, and apply cathodic protection to buried and submerged structures.