What Causes Corrosion Under Insulation (CUI)

The problem of CUI of process plant and pipework is associated mainly with steel components. Failures have occurred, with loss of containment, due to localised corrosion progressing undetected beneath insulation.

Steel corrodes when it is in contact with water and has a free supply of oxygen. When plant and pipework are insulated there is usually a space in which water can collect on the metal surface with access to air. The ingress of water into the insulation is often caused by one or more of the following:

1. poorly designed and/or installed protective finish or cladding;
2. cladding joint sealant breakdown;
3. mechanical damage to the protective finish; or
4. cladding removed and not properly replaced (common around valve boxes).

When water penetrates the insulation it tends to collect at low-lying sections of plant and pipework and around discontinuities, e.g.-

1. the base of vessels;
2. pipework supports;
3. the intersection between nozzles and vessels;
4. the underside of elbows and horizontal pipe runs; and
5. drain legs.

CUI may be influenced by the chemical nature of the insulation material. Some cladding materials contain free chlorides which may promote CUI. Chloride stress corrosion cracking should be considered for austenitic stainless steel at temperatures of 65oC and above. This is of particular importance for coastal sites and offshore installations where chloride contamination will be enhanced. For high carbon stainless steel grades at temperatures of 50oC and above, intergranular attack and chloride stress corrosion cracking cannot be ruled out. Insulating materials applied on zinc-rich coatings should preferably have neutral pH values as the coating may deteriorate in the presence of water with low or high pH values.

Surface coatings may provide protection against corrosion if the insulation becomes wetted, though proper selection and application are particularly important. Coatings should be compatible with the insulating material and suitable for use at the anticipated temperature. Coatings applied to poorly prepared or hot surfaces are much more likely to break down. Particular attention should be given to proper application at welds

Experience indicates that many factors influence the risk of CUI including whether trace heating is installed (a high risk factor). In particular, operating temperature greatly affects the risk of CUI. The following table indicates the likely risk of CUI for carbon steel pipework, without trace heating, under various operating regimes:

At low temperatures the corrosion mechanism is suppressed and at elevated temperatures moisture in the insulation material should evaporate during start-up. However, even where the risk of CUI is generally considered low, the increased risk during periods when the plant is shut down should not be discounted. The problem may be exacerbated by frequent plant cycling between operating and shutdown conditions.

In general terms, experience has shown that the most critical temperature range for CUI is 30oC-120oC. US data for carbon-manganese steel indicates typical corrosion rates of 0.5 mm/year at 80oC under insulation. 

For more information about CUI, contact us as www.leopad.com

Source:http://www.hse.gov.uk/foi/internalops/hid_circs/technical_general/spc_tech_gen_18.htm#detection