What Is Corrosion Under Insulation (CUI)

Corrosion under insulation (CUI) is a severe form of localized external corrosion that occurs in carbon and low alloy steel equipment that has been insulated. This form of corrosion occurs when water is absorbed by or collected in the insulation. The equipment begins to corrode as it is exposed to water and oxygen. CUI is common in refineries and process plants that typically operate equipment at high temperatures.

Types of Corrosion Under Insulation

By understanding the types of corrosion that can occur under insulation, the proper materials and construction can be employed to prevent them. Intruding water is the key problem in CUI. Special care must be taken during design not to promote corrosion by permitting water to enter a system either directly or indirectly by capillary action. Moisture may be external or may be present in insulation.

What is the Mechanism of Corrosion Under Insulation?

The mechanism of corrosion under insulation involves three requirements

1. Availability of oxygen.
2. High temperature.
3. Concentration of dissolved species.

Normally, as the temperature increases, the amount of oxygen dissolved in solution decreases as the boiling point is reached resulting in reduced corrosion rates. However, on the surface covered by insulation, a poultice effect is created which holds in the moisture which essentially makes it s closed system. In fact the measured corrosion rates associated with corrosion under insulation follow trends to higher corrosion rates commonly associated with only pressurized systems. Furthermore, in cases where precipitation becomes trapped on the metal surface by insulation, corrosive atmospheric constituents such as chlorides and sulfuric acid can concentration to also accelerate corrosion. In some cases, chlorides are present in the insulation which greatly promotes corrosion of the underlying surface which it becomes laden with moisture.

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