National standard for the use of anti-corrosion coatings on metal surfaces under insulation
Corrosion Under Insulation (CUI) refers to a corrosion phenomenon that occurs on the outer surface of a pipe or equipment to which an insulation material is applied. Due to the existence of the insulation layer, the CUI has strong concealability and is difficult to detect, and it is often difficult to detect at the first time, which is likely to cause a sudden serious leakage accident. In the petrochemical industry, more than 60% of pipeline failures are triggered by CUI. Each year, global losses caused by CUI-induced equipment and pipeline failures, dangerous equipment leakage, equipment abnormal parking and even casualties are billions of dollars. The equipment or pipeline to which the insulation structure is applied will greatly increase the probability of CUI occurring after 5 years of operation, and 60% of the insulation layer after 10 years of use contains corrosive condensed water, which greatly increases the probability of occurrence of CUI. In China, CUI has not been paid attention to. The field of research and insulation is only concerned with the insulation layer. The field of anti-corrosion research only pays attention to corrosion and fails to evaluate its whole. Due to the needs of the production environment, many equipment and pipelines in the petrochemical industry need to apply insulation measures, which not only need to be insulated, but also need to be kept cold. Since the outer surface is covered by the heat insulating layer, under normal operating conditions, especially thermal cycling, the moisture under the insulating layer is condensed, thereby causing local electrolyte solution to aggregate, thereby causing corrosion of the metal material. The seriousness of CUI is that it cannot be discovered in time. Generally for the aesthetic effect, after finishing the insulation layer, a layer of stainless steel or aluminum foil is often coated on the outside of the insulation material. Therefore, it is often too late to see corrosion under the insulation layer, often leading to various failure accidents. Especially when the operating temperature is lower than 150 ° C, a certain amount of condensed water tends to exist under the insulating layer. In addition, the under-insulation equipment will also form a micro-environment where corrosion occurs during the construction period and regular maintenance time. For the petrochemical industry, the corrosion under the insulation layer has its particularity. Because the petrochemical industry involves many different processes, the steel structure under the insulation layer often has to undergo thermal cycling. For traditional high-temperature coatings, due to the difference in thermal expansion coefficient and steel, the experience of thermal cycling often leads to an increase in the internal stress of the coating, which ultimately leads to early failure of the coating. For corrosion under the insulation layer, conventional high-temperature coatings, such as silicone aluminum powder paint, have low temperature resistance and limited corrosion resistance due to low film thickness. Therefore, it is not suitable for designing anticorrosion of low temperature insulation layer with long corrosion protection. In the past, many petrochemical industries used inorganic zinc silicate anticorrosion under the thermal insulation layer. In recent years, studies have shown that metal zinc may reverse polarity with iron at 60-120 °C, and zinc reacts rapidly in water at 50-150 °C. Therefore, NACERP0198 stipulates that the use of inorganic zinc silicate coatings for corrosion protection is prohibited under the insulation layer of 50-150 °C. The metal surface outer protective coating physically blocks the wet corrosive environment and the direct contact of the contaminant with the metal surface, which can prevent the corrosion of the base metal to a certain extent. The environmental conditions in which CUI occurs determine the high film-forming property, corrosion resistance, oxidation resistance, high temperature resistance, temperature and cycle resistance, chloride ion permeability resistance, and cathodic disbonding resistance. It does not degrade under conditions, can withstand the stress caused by thermal expansion and shrinkage; it can be sprayed quickly and efficiently under the condition of low surface pretreatment and can be well combined with the substrate; it can provide enough barrier for the substrate material to resist the corrosion Corrosion of pollutants and hot water and water vapor. Inappropriate choice of coating or unacceptable coating is prone to chemical degradation, resulting in increased water permeability, in which case even if the coating does not break, it will increase water penetration and CUI. In view of the relatively closed characteristics of the undercoat layer, the coating needs to have longer weather resistance and maintenance period; for the use environment, some anti-corrosion coatings should also have certain reflection and heat insulation functions. At present, there are no relevant standards for anti-corrosion coatings under insulation. At the China High Temperature and Fire Retardant Coating Technology Annual Meeting held in Qingdao on September 6-8, 2015, experts will discuss the current international NACESP0198-10 "Control System for Corrosion Insulation and Fireproof Materials", Sinopec Qingdao Safety Supervision Institute The anti-corrosion construction specifications under the insulation layer and the national standard for the anti-corrosion coating for metal surfaces under the insulation layer are being developed. The annual meeting is the global official meeting of the National Association of Anti-Corrosion Engineers (NACE). 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