Discuss the corrosion and protection issues related to heat insulation on the surfaces of equipment and pipelines

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1. Issues related to corrosion and protection of metal surfaces: Generally, insufficient attention is paid to the problem of metal surface corrosion when applying thermal insulation layers to metal surfaces. In some cases, during cryogenic insulation work, no emphasis is placed on preventing metal surface corrosion. As a result, after several years of use, the equipment develops corroded holes in the metal, thereby shortening its service life. 1.1 Analysis of corrosion causes: Polyurethane foam plastic is generally used for insulation. It is mainly an organosynthetic material that is foamed on-site using polyethers and polyisocyanate esters along with various additives and flame retardants. Since the insulation material is dry when in contact with the metal surface during initial use, it does not corrode the metal surface. However, over time, due to temperature differences, moisture inevitably accumulates between the insulation layer and the metal surface. Due to the presence of water in the interface layer between polyurethane and metal, the polyurethane material releases substances in the presence of this water, resulting in acidic water with a pH of 3-5. At the same time, it still contains harmful components such as certain chlorides, potassium oxide, and sodium oxide in water. When these insulation materials come into contact with metal surfaces, corrosion occurs and causes damage. The reason for this is as follows: the insulation materials contain water-absorbing substances such as K2O and Na2O, which undergo chemical reactions with moisture. The presence of soluble salts also enhances the condensation of water in the insulation material. Furthermore, under the action of acidic aqueous solutions, the dissolution of soluble substances turns the water film into a strong electrolyte solution, creating the necessary conditions for the electrochemical corrosion of metals. Therefore, not only carbon steel is prone to corrosion, but stainless steel as well. The main characteristic of corrosion in stainless steel is pitting corrosion as its primary feature. Therefore, the issue of corrosion prevention should be considered for metal surfaces before insulation is applied. 1.2 Recommended protective methods: For such corrosion problems, highly corrosion-resistant coatings can be used as a solution. This can be addressed using coatings such as epoxy and vinyl. Additionally, a composite insulation method can be employed: for the surface in contact with metal, an inorganic material such as foam glass or a corrosion-resistant insulation material is used as the inner layer, while polyurethane foam is used as the outer layer. This reduces the corrosion of the metal surface by the insulation material, resulting in only a minor increase in the overall cost of insulation. Improve the cost-effectiveness of cold insulation projects. 2. Regarding moisture prevention outside the insulation layer: There is an issue of temperature differences arising from the use of insulation equipment and pipelines. In other words, condensation water often forms on the outer surface of the insulation layer, and moisture also has a corrosive effect on the protective layer used. Therefore, it is common practice to apply 3–5 millimeters of asphalt mastic on the surface of the insulation layer as a moisture barrier. It prevents condensation water from accumulating on the insulation material, thereby avoiding the formation of unnecessary acidic water. First, it prevents a reduction in the effectiveness of the insulation layer ； Second, it prevents acidic water from corroding the protective layer.