A brief discussion on the issues to consider regarding surface insulation of stainless steel equipment

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A brief discussion on the issues to consider regarding surface insulation of stainless steel equipment 1. The problems of metal surface corrosion and protection: Generally, when applying insulating layers to metal surfaces, not enough attention is paid to the issue of corrosion of those surfaces. In some cases, no effort is made to protect the metal surfaces from corrosion during insulation work, and as a result, corrosion and perforations occur after a few years of use, thereby reducing the equipment’s service life. 2. Analysis of corrosion causes: When polyurethane foam plastic is used for insulation, it is an organic synthetic material that is produced on-site through the foaming of polyethers and polyisocyanates 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 forms 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, the water still contains some harmful components such as chlorides, potassium oxide, and sodium oxide. When these insulating 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 anti-corrosion issue should be considered for the stainless steel surface before insulation is applied. 3. 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 or a primer with good adhesion to stainless steel should be applied to its surface, such as an epoxy zinc yellow primer or anti-corrosion materials with low surface treatment requirements. 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 become integrated with the metal, forming a chemical bond. It also has excellent adhesion to non-metallic coatings. 4. Regarding moisture prevention outside the insulation layer: There is an issue of temperature differences arising from the use of insulation equipment and pipes. In other words, moisture often accumulates on the outer surface of the insulation layer, which in turn has a corrosive effect on the protective layer used. Therefore, it is common practice to apply 5 millimeters of asphalt mastic on the surface of the insulation layer as a moisture barrier.