Corrosion Under Insulation (CUI)

Definition - What does Corrosion Under Insulation (CUI) mean?

This is a localized external corrosion occurring in equipment made of insulated carbon and low-carbon steels. For this corrosion to occur, water must collect in the insulation and oxygen must be present. It is most common in processing plants and refineries that operate at very high temperatures.

Corrosionpedia explains Corrosion Under Insulation (CUI)

Water and oxygen are the main drivers for this type of corrosion. The water entering into the insulation reaches a point of dry out at the equipment wall or the hot pipe. Another zone next to the dry out has pores of insulation, but they are filled with saturated salt solution. The saturated salt solution zones experience drying and wetting cycles, which results in stress corrosion cracking.


Inspection and prevention of CUI is possible. Inspection is done through ultrasonic testing, x-ray, eddy currents, electro-magnetic devices and remote monitoring. Prevention is mainly through ensuring that water does not intrude into the system. The conditions that might cause wet insulation include abuse from people, vibration, and weathering. High-humidity areas require a special consideration to the insulation.


The five factors considered during prevention include:


· Insulation selection


· Protective paints and coatings


· Equipment design


· Maintenance practices


· Weather barriers


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