Microbes can sometimes induce the corrosion of steel and non-ferrous metals or they can accelerate normal electro-chemical corrosion processes.
There are a number of mechanisms involved, which may operate at the same time or in succession. The mechanisms may be direct or indirect.
Examples of direct mechanisms are:
- Corrosion by weak organic acids produced by many aerobic microbes
- Corrosion by strong acids (sulphuric or nitric) produced by a few aerobic microbes
- Corrosion by hydrogen sulphide produced by anaerobic Sulphate Reducing Bacteria (SRB)
Examples of indirect mechanisms are:
- Oxygen gradient corrosion associated with oxygen depletion beneath microbial growth
- Microbial destruction of chemical corrosion inhibitors
- Microbial attack on protective paints and coatings
Examples of severe microbial corrosion of steel, largely caused by SRB, can be found in ships’ hull plate beneath bilge water, ballast water and oil cargoes and in tank bottom plate of fuel and crude oil storage tanks. Corrosion pits are formed which often have terraced edges and have a silvery grey colour when first exposed. Non-ferrous metals are stained black by SRB.
Corrosion of aluminium alloys in aircraft fuel tanks is promoted by local aggression from microbially produced organic acids and by the oxygen gradients which become established at aggregations of microbial growth.