Corrosion under the insulation layer on the tank roof

The surface of the insulation layer should be covered with a galvanized sheet that provides waterproofing and radiation protection. Due to water leakage, coupled with the water-absorbing properties of the insulation material and the binding of acidic substances in the air, corrosion of the steel plates is inevitable under such conditions. The main reason is to address the water leakage issue on the surface of the insulation layer. For reference.

There is a large gap between the insulation layer and the insulation itself, so water must be getting in. It also depends on what materials are used for the insulation layer and the protective layer

Corrosion beneath the insulation layer is often found on the tops of storage tanks, and there are generally two main reasons for this phenomenon. First, there are areas of leakage in the external anti-corrosion and waterproof layer, and the insulation layer is saturated with water ； Secondly, changes in the temperature of the inner wall of the storage tank cause condensation to form on the tank top. In prolonged exposure to humidity, the roof of the tank can damage the anti-corrosion coating on the metal surface; once this coating is compromised, corrosion of the metal occurs rapidly. It is recommended to apply enhanced anti-corrosion treatment to the metal surface, as well as proper insulation and waterproofing layers.

Corrosion Under Insulation (CUI) refers to a type of corrosion that occurs on the outer surface of pipes or equipment covered with insulating material. It is generally caused by condensed water or rainwater entering the insulation system, leading to localized corrosion on the outer surface of pipes or equipment. When there is moisture beneath the insulation system, pipes made of materials such as carbon-manganese steel, low-alloy steel, and austenitic stainless steel are highly susceptible to electrocorrosion, pitting, stress cracking, and crevice corrosion. Since the corroded surface is covered by an insulating layer, corrosion beneath it is generally difficult to detect. An important characteristic of CUI is that the additional insulation layer creates a closed environment with high temperature and high humidity, thereby accelerating corrosion. The materials used in insulation layers are mostly inorganic substances, which contain large amounts of inorganic salts such as chlorides, fluorides, sulfides, etc., and these can cause corrosion of steel structures ； At the same time, since insulation materials typically have a loose and porous structure with a large specific surface area, they possess strong water absorption capabilities. Once the insulation layer is damaged, these materials will quickly absorb the moisture in the surrounding environment, creating a highly humid environment that accelerates corrosion. At the same time, these corrosive impurities and insulation materials form a highly conductive electrolyte solution; once such a solution comes into contact with metal equipment, it can trigger intense corrosion reactions. Especially in the petrochemical industry, corrosion beneath insulation layers presents its own particularities – since this industry involves many different processes, the steel structures under such insulation layers often have to endure heat cycles as well. For traditional high-temperature coatings, due to their different thermal expansion coefficients from steel, exposure to thermal cycles often leads to an increase in internal stresses within the coating, ultimately resulting in its premature failure. Protection can be approached from two aspects: first, strengthening the protection of the insulation layer to prevent the formation of high-temperature and high-humidity conditions caused by moisture intrusion. This includes avoiding the use of insulation materials that absorb water easily and can lead to the accumulation of corrosive substances, especially those with a hollow structure and high water absorption capacity (such as rock wool, glass wool, carbon fiber mats, etc.). Instead, insulation materials with low water content and rapid drying properties should be used (such as ZS-111). An important indicator for evaluating the performance of insulation materials is their water absorption. When insulation materials absorb water, their thermal conductivity increases, and they can also wet the surface of equipment, leading to corrosion on that surface. The second method is applying anti-corrosion coatings, that is, coating the outer surface of the equipment (below the insulation layer) with a coating that possesses excellent adhesion, mechanical properties, heat resistance, and anti-corrosion capabilities. The environment in which the equipment is located beneath the insulation layer determines the specific requirements for such anti-corrosion coatings; it is necessary to ensure that the coating remains stable in high-temperature environments, while at the same time providing anti-corrosion protection for the steel structure in low-temperature and high-humidity conditions. In such cases, ZS-1021 can be used.