In cold regions or low-temperature working conditions, galvanized profiles frequently suffer from cold brittle fracture and aging and peeling of the zinc layer. When the ambient temperature is lower than the brittle transition temperature of zinc (about -25°C), the lattice structure of the zinc layer changes, and the ductility drops sharply, resulting in cracks on the surface, which in turn accelerates the corrosion of the base steel. To improve the cold brittleness resistance and weather resistance of galvanized profiles in low-temperature environments, it is necessary to conduct systematic research from multiple dimensions such as material modification, process optimization, and structural design.
Traditional pure zinc coatings are prone to intergranular fracture at low temperatures, while zinc alloy coatings can significantly improve the low-temperature toughness of the coating by adding elements such as aluminum, magnesium, and nickel. Studies have shown that when the aluminum content in the zinc layer reaches 5%, the Zn-Al alloy phase formed can refine the grains and improve the impact resistance of the coating; when the magnesium content is 1-3%, the generated MgZn₂ phase can enhance the corrosion resistance and wear resistance of the coating. The temperature of the zinc-aluminum alloy melt is controlled at 450-480℃ through hot-dip plating process, and rare earth elements are added as refiners to make the coating structure more uniform and dense, effectively inhibiting the expansion of low-temperature cracks.
The protection ability of a single zinc layer is limited at extremely low temperatures, and the use of composite coating technology can significantly improve weather resistance. Epoxy zinc-rich primer is applied on the surface of the galvanized layer to form a transition layer with strong chemical bonding, and the thickness is controlled at 30-50μm; then polyurethane topcoat is sprayed to use its excellent UV resistance and flexibility to resist freeze-thaw cycle damage at low temperatures. For coastal low-temperature environments, fluorocarbon coating can be added. The high bond energy of its C-F bond gives the coating excellent salt spray resistance, effectively preventing zinc layer corrosion caused by chloride ion penetration.
The low-temperature toughness of the base steel directly affects the cold brittleness resistance of the galvanized profile. Through quenching and tempering treatment, the steel is heated to 30-50℃ above the Ac3 temperature, kept warm, quenched, and then tempered at 550-650℃ to obtain tempered troostite structure, which significantly improves the low-temperature impact toughness of the steel. For Q235 steel, after quenching and tempering treatment, its -40℃ impact energy can be increased from 15J to more than 40J. In addition, the controlled rolling and controlled cooling process (TMCP) is used to refine the grains and precipitate fine and dispersed second-phase particles by precisely controlling the rolling temperature and cooling rate, further enhancing the low-temperature toughness of the steel.
In the design of profile structures, stress concentration areas such as sharp angles and notches should be avoided. Changing the right-angle connection of the profile to a rounded transition with R≥5mm can make the stress distribution uniform and reduce the probability of crack initiation at low temperatures. For structures that bear dynamic loads, the use of arc-shaped reinforcement ribs instead of straight reinforcement ribs can effectively disperse the impact force and reduce local stress concentration. When splicing profiles, butt welding and post-weld heat treatment are used to eliminate welding residual stress and prevent brittle fracture at the weld in low temperature environments.
Micro-nano structures are constructed on the surface of steel by chemical etching or laser processing, which can significantly improve the bonding strength between the coating and the substrate. Studies have shown that when the surface roughness Ra reaches 5-10μm, the mechanical bite between the zinc layer and the substrate is enhanced, and the bonding strength can be increased by more than 30%. Further deposition of a graphene intermediate layer on the surface of the micro-nano structure can effectively alleviate the interface stress caused by the difference in thermal expansion coefficient at low temperature and reduce the risk of zinc layer peeling by utilizing its good thermal conductivity and chemical stability.
To improve the cold brittleness and weather resistance of galvanized profiles in low temperature environments, it is necessary to comprehensively adopt zinc alloy coating technology, composite coating system, low temperature heat treatment process, structural optimization design, and surface micro-nano structure regulation. By improving the toughness of the coating through alloying, constructing multiple protective barriers, optimizing the performance of the matrix organization, reducing the risk of stress concentration, and enhancing the interface bonding strength, the service life of the galvanized profile in low temperature environments can be significantly improved. Future research can further explore intelligent responsive coating materials so that they can automatically repair tiny cracks in low-temperature environments and continue to provide protection.
