MechChem Africa

Corrosion under insulation, or CUI, is a common problem in many industries, including those in the petrochemical and power sectors. Evaluation of life-cycle savings has recently led to new, simple approaches to its prevention, such as the thermal sprayed aluminium (TSA) processes that form barrier coatings that are mechanically bonded to the substrate.

Flamespray smallCorrosion under insulation (CUI) is difficult to find because of the insulation cover that masks the corrosion problem until it is too late. This is a problem shared by the refining, petrochemical, power, industrial, onshore and offshore industries.

The problem occurs on all carbon steels as well as on 300 series austenitic stainless steels. On carbon steels it manifests as generalised or localised wall loss, while on stainless pipes it often causes pitting and corrosion-induced stress corrosion cracking.

Though failure can occur in a broad band of temperatures, corrosion becomes a significant concern in steel at temperatures between 0 and 149 °C and is most severe at about 93 °C. Corrosion and corrosion-induced stress corrosion cracking rarely occur when operating temperatures are routinely above 149 °C.

Corrosion under insulation is caused by the ingress of water into the insulation, which traps the water like a sponge in contact with the metal surface. The water can come from rain water, leakage, deluge system water, wash water or sweating from temperature cycling or low temperature operations, such as refrigeration units.

It is also widely known that the results of CUI are costly. CUI can account for as much as 40 to 60% of a company's piping maintenance costs; result in repairs in the millions; and it leads to significant downtime. Most studies on the topic involve all forms of corrosion and their associated costs, without breaking down the costs of CUI.

A study completed in 2001 by a research team of corrosion specialists in the USA reported the direct cost of corrosion under insulation to be US$276-billion per year, with that number potentially doubling when indirect costs are also considered.

In recent years, the CUI prevention philosophy of many large petrochemical companies has been an inspection-free, maintenance-free concept. Insulated systems, particularly piping systems, are expected to have a service life of 25 to 30 years.

Evaluation of life-cycle savings has led to the consideration of new, simple approaches to preventing CUI, such as the application of a thermal spray coating of aluminium onto the piping prior to insulating it.

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