Causes of external corrosion in insulated pipes and preventive measures

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This post was last edited by Wang Wei2 on 2023-8-17 at 10:02. Causes of external corrosion in insulated pipes and preventive measures. I. Corrosion analysis 1. Failure of the anti-corrosion layer and metal corrosion. Corrosion under the insulation layer (CUI) not only leads to equipment failures and shutdowns, product leaks, and increased maintenance costs in the industrial sector; more seriously, it poses significant risks to safety in production. Most insulation materials are rock wool or ultra-fine glass fibers; due to the presence of large amounts of inorganic salts in these minerals, the insulation materials produced after processing still contain harmful substances such as chlorides, fluorides, and sulfides. When these insulation materials come into contact with metal surfaces, corrosion damage occurs. 2. Analysis of corrosion causes: The insulation materials for most tanks and pipes are rock wool or ultra-fine glass fiber, and these materials are made by calcining and bonding minerals. 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) Material-related factors: Since the insulation materials contain water-absorbing substances such as K2O and Na2O, these substances undergo chemical reactions with the moisture in the air, absorbing large amounts of water. Moreover, because these materials are composed of columnar fibers, a siphon effect easily occurs, leading to wetting of the insulation materials. 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, facilitating the electrochemical corrosion of metals. Furthermore, the presence of chloride ions in the insulation material can accelerate corrosion of metals. ⑵Oxygen concentration cell corrosion: Due to varying degrees of coverage by insulation materials, oxygen supply is difficult in the intact areas, resulting in oxygen-deficient zones. In the areas where the insulation material is damaged (those with severe corrosion), there is an ample supply of oxygen; this leads to electrochemical corrosion in the form of a concentration cell between the two adjacent areas where there is a continuous layer of electrolyte. ⑶Defects in the anti-corrosion coating: When there are issues with the anti-corrosion coating applied to the metal surface that is to be protected, the permeability of the coating allows corrosive substances to penetrate through it and spread at the metal interface, thereby destroying the adhesion between the coating and the metal surface and leading to damage to the paint film. In particular, when the bottom corners 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. When the metal surface is wet, 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. 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. II. Protection of metal surfaces with insulating layers (for heat retention and insulation) 1. Methods for protecting metals under insulating materials. Generally, epoxy coatings are used to prevent corrosion on metal surfaces; two coats of primer are sufficient. The key is to ensure proper surface preparation, which should meet at least the standard of St3 for manual or mechanical rust removal as specified in GB8923-88, or Sa2.5 for mechanical sandblasting (equivalent to level 2 of mechanical sandblasting). During the production process, the corroded areas on containers and tanks are mainly found at the bottom corners, where corrosion is quite severe, making it difficult to address this issue. Corrosion on the outer wall of the tank bottom corner is caused by damaged insulation material and water absorption. Therefore, by selecting an appropriate coating protective layer in combination with a heat-insulating material that has low corrosivity, excellent protective effects can be achieved. 2. Selection of anti-corrosion and insulation materials: By selecting appropriate protective materials, the use of the following materials as anti-corrosion barriers can achieve excellent protective effects. ⑴Highly corrosion-resistant and high-temperature resistant anti-corrosion coating. ⑵Choose qualified insulation materials.