Corrosion issues to be considered for heat insulation on the surface of stainless steel equipment
Thread Content
Corrosion issues that need to be considered when insulating the surface of stainless steel equipment 1. Corrosion and protection of metal surfaces: Generally, when applying insulation layers to metal surfaces, not enough attention is paid to the issue of corrosion on those surfaces. In some cases, no effort is made to protect the metals from corrosion during the insulation process, and as a result, corrosion and perforations occur after a few years of use, thereby reducing the equipment’s service life. ⑴ Analysis of the cause of corrosion: Polyurethane foam plastic was used in this case. 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 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 to 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. ⑵ It is recommended to use protective measures. For such corrosion issues, epoxy coatings can be used as a solution; however, it is important to apply a phosphating primer to the stainless steel surface before applying the topcoat. 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 must 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 one with the metal, forming a chemical bond. It also has excellent adhesion to non-metallic coatings. 2. Regarding moisture prevention outside the insulation layer: There is an issue of temperature differences arising during the use of insulation equipment and pipelines. 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.
The main corrosion issues that need to be considered regarding the surface insulation of stainless steel equipment are two: corrosion and protection of the metal surface, as well as moisture prevention outside the insulation layer. 1. Corrosion and protection of metal surfaces: When providing thermal insulation on the surface of stainless steel equipment, attention must be paid to preventing corrosion of the metal surface. During some construction processes, proper protection of the metal surface against corrosion may not have been given attention to, resulting in metal corrosion and perforations after a few years of use, which shortens the equipment’s service life. Analysis of corrosion causes: The commonly used insulation material is polyurethane foam plastic, which is an organic synthetic material composed of polyether, polyisocyanate esters, as well as various additives and flame retardants. When the insulation material comes into contact with the metal surface, it is dry and does not cause corrosion to the metal surface. However, over time and due to temperature differences, moisture can accumulate between the insulation layer and the metal surface. When these insulation materials come into contact with metal surfaces, they release acidic water with a pH value of 3 to 5, which also contains harmful substances such as chlorides, potassium oxide, and sodium oxide. These substances can cause corrosion and damage to metal surfaces; stainless steel is particularly prone to pitting corrosion. Recommended protection method: To address such corrosion issues, epoxy coatings can be applied; however, before applying the topcoat, a phosphating primer must first be applied to the stainless steel surface. Since the surface of stainless steel is considered a non-ferrous metal, its oxide film is quite dense, and therefore conventional anti-corrosion coating primers have poor adhesion to non-ferrous metals. Therefore, a layer of interface agent needs to be applied to improve the adhesion between the primer and the metal surface. The function of the phosphating primer is to react with the metal surface to form a new phosphate passivation film that becomes integrated with the metal through chemical bonds; it also provides good adhesion to non-metallic coatings. 2. Moisture prevention issue outside the insulation layer: Due to temperature differences that occur during the use of insulation equipment and pipes, moisture often accumulates on the outer surface of the insulation layer, which in turn causes corrosion to the protective layer used. The common practice is to apply 5 millimeters of asphalt mastic on the surface of the insulation layer as a moisture barrier, in order to prevent moisture from corroding the protective layer. .