A combination of moisture, a low level of oxygen, concentration of impurities (e.g. chloride) and temperature beneath the thermal insulation can lead to corrosion under insulation (CUI). The most important influences are temperature (operating and ambient) and water contact (in liquid or vapour state). CUI is any type of corrosion that occurs on pipework due to the presence of moisture within or under the insulation system. CUI can occur under any type of thermal insulation.

Corrosion under insulation is particularly aggressive where operating temperatures cause frequent condensation and re-evaporation of atmospheric moisture.

Water ingress from any external source such as rainfall, condensate from cold service equipment, steam discharge, process liquid spillage, wash-downs, etc. pose a risk for CUI if external finishes do not provide adequate water-tight protection.

Systems such as cold water, chilled water, refrigeration and cool air duct systems which all operate below ambient, are highly susceptible to the formation of condensation on their surfaces.

Formicary corrosion is a well-documented problem of external corrosion which can occur on copper pipework under phenolic foam insulation (formic acid attack from wet insulation).

Contaminants left during installation and residual flux used in joint assembly can contribute to corrosion. Some fluxes are more aggressive than others, but all fluxes should be considered corrosive to some extent. In coastal environments saltwater or salt laden air can cause aggressive corrosion.

Stress corrosion cracking (SCC) may occur in brass compression fittings or stainless steel components due to manufacturing defects or installation fault in environments with contaminants such as ammonia.

All metals are prone to corrosion. Some considerations are highlighted below.

Heavyweight steel tube has an inherent advantage with a thick wall that will take longer to corrode than thin-wall solutions. Lightweight thin-wall carbon pipe systems have less material in the pipes' structure, so the rate of corrosion that leads to failure can be rapid.

Austenitic stainless steel (to include 304 and 316) piping systems can suffer from stress corrosion cracking in the temperature range 50°C to 150°C. BS 5970 recommends the protection of stainless steel pipe surfaces, especially if the plant is likely to be heated to a temperature above 50°C. To avoid any issues with corrosion, stainless steel pipe must not be heated for bending.

Copper pipe systems are discouraged by the NHS in Scotland in Domestic Hot and Cold Water (DHCW) services in hospitals and other healthcare premises.

Brasses have a high susceptibility to stress corrosion cracking; copper, bronze and gun-metal are resistant to this type of corrosion. Over tightening brass fittings during installation can increase the risk of stress corrosion cracking. Brass fittings and valves should be wrapped with an effective moisture barrier tape such as butyl self-amalgamating tape.

As an alternative, gun-metal or copper fittings should be used in place of brass. Gun metal which is an alloy of [c.88%] copper, [8-10%] tin and zinc is resistant to stress corrosion cracking and dezincification.

Bronze is an alloy of copper and tin with good corrosion resistant. Bronze is a good choice for marine environments.

Plastic pipework systems are non-corrodible, however, internally the oxygen concentration will remain higher for longer and therefore corrodible materials elsewhere in the system will corrode at a high rate without suitable treatment.

Wood blocks should not be used as BS 5970 (2012) advises that they "should not be used for below ambient applications", they will expand in the presence of water and the thermal properties are poor when compared to thermal insulation blocks.