Microbes/microorganisms 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 of MIC involved, which may operate at the same time or in succession.

These mechanisms of MIC may be direct or indirect.

Examples of direct mechanisms are:

• Corrosion by hydrogen sulphide (H2S) produced by anaerobic Sulphate Reducing Bacteria (SRB)
• Corrosion by weak organic acids produced by many aerobic microbes
• Corrosion by strong acids (sulphuric or nitric) produced by a few aerobic microbes

• Oxygen gradient corrosion associated with oxygen depletion beneath microbial growth
• Microbial attack on protective paints and coatings
• Microbial destruction of chemical corrosion inhibitors

Examples of severe microbial corrosion of steel, largely caused by sulphate reducing bacteria, 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.

In aircraft fuel tanks, corrosion of aluminium alloys is promoted by local aggression from microbially produced organic acids and by the oxygen gradients which become established at aggregations of microbial growth.

Information from- www.echamicrobiology.co.uk