Corrosion of metals caused by insulation materials and its reasons

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This post was last edited by Wang Wei2 on 2019-7-22 09:04. Corrosion of metals caused by insulation materials and its reasons: 1. Corrosion conditions. Metal containers, towers, and oil tanks are the main equipment used in petrochemical production and oil storage. The insulation materials used on the outer walls of general chemical plant equipment are mostly rock wool or ultra-fine glass wool, while the outer protective layer is made of cement asbestos ash or galvanized iron sheeting. Due to long-term exposure to the atmosphere, the outer protective layer has cracked; in particular, the wall panels at the bottom corners, about half a meter away from the outer walls of towers, containers, and tanks are severely damaged. It is manifested as large areas of the asbestos-gray protective layer peeling off, leaving the insulation completely exposed to the atmosphere. After about 5 years of use due to severe corrosion, perforations appeared in the tank walls. Based on the surface corrosion condition, the thickness of the rust product is approximately 3–5 mm; beneath the rust layer are corrosion pitting or pitted areas of varying sizes. 2. Causes of corrosion: Most insulation materials used for tanks are rock wool or ultra-fine glass fibers; these materials are produced by calcining and bonding minerals together. Since minerals contain large amounts of inorganic salts, the insulation materials produced after treatment (or sometimes without treatment) still contain harmful components such as chlorides, fluorides, and sulfides. When these insulation materials come into contact with metal surfaces, corrosion occurs as a result of the following reasons: (1) Since the insulation materials contain water-absorbing substances such as K2O and Na2O, they undergo chemical reactions with the moisture in the air, absorbing large amounts of water. Moreover, because these materials are composed of columnar fibers, a siphoning effect easily occurs, which keeps the insulation materials moist. The presence of soluble salts in these materials also enhances the water absorption process. The adsorption and condensation of water make the insulation material damp, forming a water film on the metal surface. The dissolution of soluble substances turns the water film into a strong electrolyte solution, creating the necessary conditions for the electrochemical corrosion of metals. Analysis shows that the chloride ion content in the glass wool in close contact with the metal surface is as high as 1.6%, while the chloride ion concentration in the glass wool itself is 1800 mg/L. ⑵Due to the varying degrees of coverage by insulation materials, it is difficult to ensure adequate oxygen supply, resulting in oxygen-deficient areas. In the areas where the insulation material is damaged (those with severe corrosion), there is an adequate supply of oxygen. This results in the formation of oxygen concentration cells in the two adjacent areas, where there is a continuous layer of electrolyte, thereby leading to electrochemical corrosion. When the protected metal surface is coated with alkyd paint, due to the permeability of the coating, corrosive substances can penetrate through it and spread to the metal surface, compromising the adhesion between the paint and the metal surface and resulting in damage to the paint film. Especially when the corners at the bottom of the tank remain wet or contain moisture for a long time, it reduces the bonding strength between the paint film and the substrate, leading to peeling or bubbling of the paint film. In humid conditions, there is an excess on the metal surface, and the corrosion rate is often controlled by the supply rate of oxygen. Due to the presence of the electrolyte film layer, electrochemical corrosion occurs at the defects on the metal surface. The reactions are as follows: Anodic reaction: Fe → Fe2+ + 2e-. Cathodic reaction: 1/2O2 + H2O + 2e- → 2OH-. In the solution: Fe2+ + 2OH- → Fe(OH)2; Fe(OH)2 + O2 → Fe2O3·H2O. Oxygen comes into contact with the metal on its surface; areas with high oxygen concentration act as cathodes, while areas with low oxygen concentration act as anodes and are subject to corrosion. When the coating is damaged, the metal surface is prone to corrosion, resulting in rust tumors that form a hemispherical-like cover, allowing the oxygen-deprived areas of the metal beneath it to continue to corrode. Depending on the degree of oxygen supply, the surface layer of the rust nodules is reddish-brown ferric oxide, while the inner layer consists of black magnetic iron oxide or a mixture of gray-green ferrous and ferric oxide compounds. When there is a rust layer on the metal surface, it acts as a reservoir for water and oxygen; under certain conditions, the corrosion products can affect the electrode reactions involved in atmospheric corrosion. Evans believes that the rust layer resulting from atmospheric corrosion, in wet conditions, can act as a strong oxidizing agent. Therefore, once a rust layer forms on the metal surface, the corrosion of that surface will worsen further as the gas conditions alternate between dry and wet.