The risk of Microbially Influenced Corrosion (MIC) can be virtually eliminated by control of water quality, sterilisation of the system, use of biocides (in non-drinking water systems) and improved system design and operation.

In both drinking water and closed recirculating heating and cooling systems, the most important microorganisms are sulphate reducing bacteria (SRB), which are anaerobic but can grow in aerated drinking water systems under deposits, where oxygen levels are very low.

SRB’s reduce sulphates in the water to sulphides. The bi-product is corrosive hydrogen sulphide and iron(II) sulphide or copper(I) sulphide, depending whether iron or copper is present. The presence of SRB’s, therefore, can usually be detected by the characteristic smell of hydrogen sulphide.

Microorganisms, which are implicated in microbially influenced corrosion , include the following groups:

• Sulphate reducing bacteria (SRB)
• Organic acid producing bacteria
• Iron or manganese oxidising bacteria
• Acid producing fungi
• Aerobic slime formers

Corrosion Damages caused by MIC in Water Systems
In copper pipes carrying drinking water, MIC due to sulphate reducing bacteria has been found beneath deposits. This has generally been confined to private supplies where little or no chlorination has been carried out and/or in little used pipework and dead-legs. The attack manifests itself in a group of small, steep sided pits and has thus been termed ‘pepper-pot’ corrosion. Within the pits is a layer of crystalline cuprous oxide and the pits are capped with sulphates, oxides and organic material. Damage on copper or copper alloy components due to MIC is not restricted though to wall perforation. The formation of copper sulphide layers due to the action of SRB has resulted in the seizing of precision control valves.

Under-deposit corrosion due to MIC can also result in wall perforation on steel and cast iron components in closed recirculating heating and cooling systems. This is more common in larger more complex systems, where it is more likely that temperatures and flows in parts of the system are more conducive to rapid growth of bacteria. The cause is mainly due to the presence of SRB’s, and can also involve iron or manganese oxidising bacteria and organic acid producing bacteria.

Aerobic slime formers are a potential problem in open recirculating cooling systems. Large colonies of bacteria with associated sticky polymers grow on metal surfaces and prevent oxygen reaching the underlying surface. This can often lead to under-deposit attack (a form of crevice corrosion) usually on steel surfaces. In addition, the anaerobic conditions produced are ideal sites for SRB growth, which can increase the rate of attack.

Prevention of MIC
The quality of the water supply has an important influence on the likelihood for MIC. Waters carrying high amounts of organic material and inorganic nutrients, such as phosphates and nitrates, promote the growth of microorganisms. The water quality can be improved by filtration and other water treatment methods.

Large hot and cold domestic water systems and open cooling systems should be sterilised on commissioning and regularly during service to prevent the risk of legionella. This has the added benefit of destroying the microorganisms responsible for MIC. Sterilisation (chlorination) should be carried out according to ACOP 2000.

If MIC can be found in a small system, this can also be sterilised in a controlled manner using chlorine release tablets. Shock doses of chlorine or bromine are often used in conjunction with non-oxidising biocides in cooling towers to control the growth of microorganisms.

Systems, which have accumulated large amounts of corrosion or organic debris, should first be cleaned using a suitable chemical cleaner. Afterwards, the system should be thoroughly flushed using fresh water. This should also prevent under-deposit corrosion occurring even in the absence of microorganisms.

Dead-legs and regions of low flow should be eliminated at much as practical by changing the design or valve settings. In addition, temperatures should be set to above 60°C for heating systems or less than 25°C for cooling systems.