Corrosion of equipment beneath the insulation layer – how to address this hidden problem?

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1. What is corrosion under insulation, and what are its characteristics? Corrosion Under Insulation (CUI) refers to a type of corrosion that occurs on the outer surface of pipes or equipment equipped with insulating materials. Due to the presence of the insulation layer, CUI has strong concealment properties, making it difficult to detect and hard to identify in a timely manner, which can lead to sudden leaks. 2. CUI on carbon steel surfaces: The direct cause of CUI on carbon steel surfaces is the contact between carbon steel and water containing air, which leads to oxygen absorption corrosion. And the insulation layer is the biggest culprit, as it creates annular spaces or gaps where water and other corrosive agents can accumulate; in particular, this applies to insulation materials with capillary siphonage properties or water-absorbing characteristics, or those that may generate pollutants and thus accelerate the corrosion rate. The risk of CUI is highest when carbon steel equipment operates at temperatures between -4 and 175 °C ； When the continuous operating temperature is < -4 °C, corrosion generally does not occur ； When the operating temperature is >175 °C, high temperatures help keep the carbon steel surface dry, thereby reducing corrosion. Additionally, the CUI for devices with shutdown, cyclic operation, and intermittent operation is provided in 1.1.2 of the references, so it is not reproduced here. 3. CUI on stainless steel surfaces: CUI can occur in carbon steel under any type of insulation layer. However, in the case of stainless steel equipment, the cause or mechanism is as follows: when the environment or the insulation material contains chlorides or other halides, these substances are carried to the surface of the stainless steel by moisture. Once the moisture evaporates, these substances become concentrated, and this leads to external stress corrosion cracking (ESCZ) in austenitic stainless steel and duplex stainless steel. Similar to carbon steel CUI, the insulation layer is not the direct cause of stainless steel CUI; its role is to serve as a medium for retaining and transporting moisture containing chlorides or other halides to the metal surface. 4. Countermeasures: To avoid or reduce CUI on the surfaces of carbon steel and stainless steel equipment, measures can be taken in terms of the selection of insulation materials, the design of insulation structures, and the application of protective coatings on the equipment surfaces. For example, it is advisable to use little or no insulation materials with siphonage properties or high water absorption; materials such as rock wool and calcium silicate are hydrophilic, whereas foam glass (or other materials with hydrophobic additives) are hydrophobic ； Use no or as little as possible insulating materials that contain water-soluble salts capable of promoting corrosion (such as chlorides and sulfates), as well as residual compounds that can react with water to produce hydrochloric acid or other acids ； If it is only for heat protection, using a protective mesh is better than using an insulating layer. Insulation materials used for austenitic stainless steel shall comply with the provisions of GB/T 17393-2008 \"Specifications for Insulation Materials for Cladding Austenitic Stainless Steel\" ； The selection of protective coating under insulation layers can be carried out in accordance with the standard HG/5178-2017 \"Anti-corrosion Coatings for Metal Surfaces Under Insulation Layers\". When designing insulation structures, it is necessary to avoid compromising the integrity of the insulation system as much as possible. For protruding elements on the equipment and container structures, such as insulation supports, reinforcement rings, and nameplate brackets, additional design measures should be taken to prevent water from entering, seeping in, remaining, or accumulating there. References: Su Chunhai, Zhang Lei. Anti-corrosion coatings for metal surfaces beneath insulation layers – A discussion on standard development. Coating Technology and Abstracts, 2015, 36(10). Guo Jinbiao. A brief overview of the corrosion protection system design beneath insulation layers in newly built coastal refineries. Material Protection, 2020, 53(10).