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CUI is a particularly severe form of localized corrosion that has been plaguing chemical process industries. 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. Moisture may be external or may be present in the insulation material itself.For high temperature equipment, water entering an insulation material and diffusing inward will eventually reach a region of dry-out at the hot pipe or equipment wall. Next to this dry-out region is a zone in which the pores of the insulation are filled with a saturated salt solution.When a shutdown or process change occurs and the metal-wall temperature falls, the zone of saturated salt solution moves into the metal wall.Upon reheating, the wall will temporarily be in contact with the saturated solution, and stress-corrosion cracking may begin. The drying/wetting cycles in CUI associated problems are a strong accelerator of corrosion damage since they provoke the formation of an increasingly aggressive chemistry that can lead to the worst corrosion problems possible, e.g. stress corrosion cracking, and premature catastrophic equipment failures.The mechanism of corrosion under insulation involves three requirements:1.Availability of oxygen.2.High temperature.3.Concentration of dissolved species.In the following paper the mechanism of CUI and ways of control and monitoring are explained in detail...