Corrosion prevention measures for steel structures

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Corrosion prevention measures for steel structures (1) Weathering steel: Steel with superior corrosion resistance compared to ordinary structural steel is known as weathering steel. It generally contains metals such as phosphorus, copper, nickel, chromium, and titanium, which form a protective layer on the metal surface to enhance its corrosion resistance. Its low-temperature impact toughness is also better than that of ordinary structural steels. The standard is \"Weathering Steel for Welded Structures\" (GB4172-84). (2) Hot-dip galvanizing: Hot-dip galvanizing involves immersing rust-removed steel components in molten zinc at a temperature of around 600°C, so that a zinc layer is formed on the surface of the steel components. The thickness of this zinc layer must be no less than 65 μm for thin plates with a thickness of 5 mm or less, and no less than 86 μm for thicker plates. Thereby achieving the purpose of corrosion prevention. The advantages of this method are a long service life, a high degree of industrialization in production, and stable quality. Therefore, it is widely used in outdoor steel structures that are heavily affected by atmospheric corrosion and are difficult to maintain. Such as a large number of transmission towers, communication towers, etc. Profiled steel sheets and similar components used in lightweight steel structure systems, which have become increasingly common in recent years. Hot-dip galvanizing is also widely used for corrosion prevention. The first step in hot-dip galvanizing is acid washing to remove rust, followed by cleaning. If these two processes are not thorough, it will create risks for corrosion prevention. So it must be dealt with thoroughly. For steel structure designers, it is necessary to avoid designing components with mating surfaces, so as to prevent incomplete pickling or residual acid in the gaps between those surfaces. This causes yellow liquid to flow from the galvanized surface. Hot-dip galvanizing is carried out at high temperatures. For tubular members, their ends should be left open. If both ends are sealed, the air inside the tube will expand, causing the end plates to burst and resulting in safety accidents. If one end is closed, the flow of zinc liquid is hindered, and it tends to accumulate inside the tube. (3) Thermal spray aluminum (zinc) composite coating: This is a long-lasting corrosion protection method that offers comparable corrosion resistance to hot-dip galvanizing. The specific procedure is to first perform sandblasting on the surface of the steel components to remove rust, revealing a metallic shine and creating a rough surface. The continuously fed aluminum (zinc) wire is then melted using an acetylene-oxygen flame, and blown onto the surface of the steel components with compressed air to form a honeycomb-shaped aluminum (zinc) coating layer (with a thickness of approximately 80μm to 100μm). Finally, the capillaries are filled with coatings such as epoxy resin or neoprene paint to form a composite coating. This method cannot be used for coating the inner walls of tubular components; therefore, both ends of such components must be hermetically sealed to prevent corrosion of their inner walls. The advantage of this process is its strong adaptability to component dimensions, with almost no restrictions on the shape and size of the components. Locks as large as those at Gezhouba are also constructed using this method. Another advantage is that the thermal impact of this process is localized and confined, thus preventing thermal deformation. Compared to hot-dip galvanizing, this method has a lower level of industrialization; sandblasting and aluminum (zinc) coating require heavy labor, and the quality is also prone to being affected by the operator’s mood. (4) Coating method: The corrosion resistance of the coating method is generally inferior to that of long-term corrosion prevention methods (although currently, fluorocarbon coatings can provide corrosion protection for up to 50 years). Therefore, it is more commonly used in indoor steel structures or in outdoor steel structures that are relatively easy to maintain. Its cost per use is low, but the maintenance cost is high when used outdoors. The first step in applying the coating method is rust removal. A high-quality coating depends on thorough rust removal. Therefore, for coatings with high requirements, sandblasting or shot blasting is generally used to remove rust, reveal the metallic luster, and eliminate all rust and oil stains. The coating applied on-site can be manually derusted. The choice of coating should take the surrounding environment into account. Different coatings have varying tolerances to different corrosion conditions. Coatings generally consist of a primer layer and a topcoat layer. The primer contains a high amount of powder and little binder. The film formed is rough, with strong adhesion to steel and good compatibility with topcoats. The topcoat contains a high amount of binder, forms a glossy film, protects the primer from atmospheric corrosion, and resists weathering. There is an issue of compatibility between different coatings; when using different coatings one after another, it is important to pay attention to their compatibility. The application of the coating requires appropriate temperature (between 5 and 38°C) and humidity (relative humidity not exceeding 85%). The coating should be applied in an environment with low dust levels, and there must be no dew on the surface of the components. It must not be exposed to rain within 4 hours after painting. The coating is usually applied 4 to 5 times. The total thickness of the dry paint film is 150 μm for outdoor projects and 125 μm for indoor projects, with an allowable deviation of 25 μm. On the seaside, at sea, or in an atmospherically highly corrosive environment, the total thickness of the dry paint film can be increased to 200–220 μm. (5) Cathodic protection: Applying a more reactive metal to the surface of steel structures to replace the steel and prevent corrosion. It is commonly used in underwater or underground structures. Last edited by Lailai on 2009-3-14 01:22]