Discuss the causes of corrosion under insulation (CUI)

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Corrosion of the metal beneath an insulation layer, which occurs in equipment or pipes equipped with such insulation structures due to the infiltration of moisture that cannot evaporate in time, is known as Corrosion Under Insulation (CUI). CUI is divided into two types: one is the uniform corrosion or pitting corrosion that occurs in carbon steel or low-alloy steel, with the corrosion products typically taking on a loose flaky structure ； Another type is stress corrosion cracking of austenitic stainless steel. Corrosion mechanism: During the operation of the device, the structure of the insulation layer is damaged, allowing water to enter. The porous structure of the insulating material retains some of this water, thereby creating an electrochemical corrosion environment beneath the insulation layer. At the same time, the insulation materials contain certain amounts of elements such as Cl and S, which exacerbate corrosion. The electrochemical corrosion reaction equations are as follows: The Fe2+ and OH- ions involved in the anodic and cathodic reactions form Fe(OH)2; under the action of oxygen, this substance further transforms into Fe(OH)3 and Fe3O4. These corrosion products are loose and prone to falling off, lacking protective properties, which thus further accelerates corrosion.