1.What are the coating structure and composition characteristics of alloyed galvanized sheet?
Coating composition: Typical alloyed galvanized layers contain about 90%-95% zinc (Zn), about 5%-10% iron (Fe), and may contain trace amounts of aluminum (Al, used to control the composition of the galvanizing solution). The coating structure from substrate to surface is: iron substrate → Fe-Zn alloy layer (dense columnar crystals) → surface oxide layer (containing ZnO, FeO, etc.).
Microstructure: The alloyed layer has no zinc flowers (different from ordinary hot-dip galvanized sheets), the surface is silver-gray or dark gray, and the glossiness is low. Because the alloy phase is evenly distributed, the surface flatness is better than that of unalloyed galvanized sheets.
2.What is the corrosion resistance of alloyed galvanized sheet?
Electrochemical protection mechanism: The electrode potential of zinc in the zinc-iron alloy layer is lower than that of iron, and sacrificial anode protection can still be formed; but due to the increase in iron content, the corrosion potential is slightly higher than that of the pure zinc layer, the initial corrosion rate is slightly faster, and the long-term corrosion products (such as basic zinc carbonate and iron hydroxide) form a dense protective film, and the corrosion resistance tends to be stable.
Corrosion resistance data: In the neutral salt spray test, the white rust resistance time of 50μm alloyed zinc coating can reach 500-1000 hours, and the red rust resistance time exceeds 2000 hours, which is better than ordinary cold-rolled sheet and equivalent to or slightly better than pure zinc coating (depending on the degree of alloying).
3.What are the mechanical and processing properties of alloyed galvanized sheet?
Formability: The hardness of the alloyed coating is higher than that of the pure zinc layer, but due to the strong bonding between the coating and the substrate, it is not easy to fall off during stamping and is suitable for deep drawing.
Welding performance: When welding the pure zinc layer, zinc vapor is easy to cause pores, while the melting point of the zinc-iron alloy in the alloyed coating is higher, the amount of zinc evaporation in the welding heat-affected zone is reduced, and the electrode burnout is reduced, which is suitable for resistance welding (such as spot welding, seam welding) and laser welding.
Coating adhesion: The surface oxide layer of the alloyed layer has a stronger chemical bonding with the paint, and the salt spray resistance after coating (such as scratch test) is better than that of the pure zinc coating, and it is often used in scenes with high coating requirements such as automotive exterior panels.
4.How are the surface characteristics of alloy galvanized sheet compatible with its uses?
Surface treatment compatibility: Phosphating treatment can be directly performed to improve the pre-treatment effect of coating; however, since there is no zinc flower on the surface, the appearance is slightly less decorative and is more suitable for functional purposes.
High temperature resistance: The alloying layer has stable performance at temperatures below 200°C, which is higher than the pure zinc layer and is suitable for parts that require subsequent heat treatment.
5.What are the application scenarios of alloy galvanized sheet?
Automobile industry: accounting for more than 70%, used for body floors, door inner panels, chassis parts, etc. For example, car companies such as Volkswagen and Toyota use alloyed galvanized sheets to improve the corrosion resistance of welded parts and coating adhesion;
Home appliances and steel structures: parts that need to be welded, such as refrigerator back panels and air conditioning brackets, as well as corrosion-resistant structural parts such as bridges and containers;
Special fields: marine engineering (need to take into account both corrosion resistance and welding), agricultural machinery parts (anti-soil corrosion).