Issues to consider regarding heat insulation on the surface of stainless steel equipment

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I. Issues of corrosion and protection of metal surfaces: Generally, when applying insulating layers to metal surfaces, not enough attention is paid to the problem of corrosion on those surfaces. In some cases, no effort is made to protect the metals from corrosion during insulation work, and as a result, the metals corrode and develop holes after a few years of use, thereby reducing the equipment’s service life. 1. Analysis of corrosion causes: When polyurethane foam is used for thermal insulation. It is primarily 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. This has been proven in practical use. 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 insulating materials come into contact with metal surfaces, corrosion occurs, and 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 insulating 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 corrosion resistance of stainless steel surfaces should be considered before insulation is applied. 2. Recommended protective methods: For such corrosion issues, epoxy coatings can be used as a solution. It is important to note that before applying the topcoat, a phosphating primer must be applied to the stainless steel surface first. Since the surface of stainless steel belongs to non-ferrous metals, its oxide film is relatively dense. Generally, the primer of anti-corrosion coatings does not provide good adhesion to non-ferrous metals. Therefore, a layer of interface agent should be applied to improve the adhesion between the primer and the metal surface. The main function of a phosphating primer is to allow phosphoric acid to react with the metal surface to form a passivating film of new phosphates. The function of this membrane is to merge with the metal, forming chemical bonds. It also has excellent adhesion to non-metallic coatings. Additionally, a primer with good adhesion to non-ferrous metals can be considered. II. Regarding moisture prevention outside the insulation layer: There is an issue of temperature difference effects in insulated equipment and pipelines during use. In other words, moisture often exists on the outer surface of the insulation layer, which also has a corrosive effect on the protective layer used. Therefore, the common practice is to apply 5 millimeters of asphalt mastic on the surface of the insulation layer as a moisture barrier.