1.What role does aluminum in the zinc coating play in improving coating adhesion?
Inhibit the growth of brittle alloy layer: When the steel plate is immersed in zinc liquid, iron (Fe) reacts with zinc (Zn) to form Fe-Zn intermetallic compounds. If not controlled, an excessively thick Fe-Zn layer will cause the coating to become brittle and easy to fall off.
Form a protective barrier layer: Aluminum will react with iron first, quickly forming a uniform and dense Fe₂Al₅ alloy layer on the surface of the steel plate. This layer of compound has a stable structure and can block further reactions between iron and zinc, controlling the thickness of the Fe-Zn brittle phase within 5μm, significantly improving the bonding strength between the coating and the steel plate.
2.What effect does aluminum in the galvanized layer have on the size of zinc flowers?
Increase the number of crystal nuclei and inhibit excessive crystal growth: Aluminum will be adsorbed on the surface of zinc crystal nuclei in zinc liquid, reducing the anisotropic growth advantage of crystals, so that more crystal nuclei grow evenly at the same time, rather than a few crystal nuclei "dominant".
When the aluminum content is ≥0.15%, the diameter of the zinc flower can be reduced from several millimeters of pure zinc to less than 1mm; when the aluminum content is ≥0.2%, it can even form a "zinc flower-free" surface (zinc flower diameter <0.3mm, almost invisible to the naked eye).
Advantages: The surface of small zinc flowers or no zinc flowers is smoother, the primer adhesion is stronger when painting (reducing the uneven coating thickness caused by zinc flower protrusions), and the appearance is more suitable for scenes with high requirements for surface consistency such as home appliances and automobiles.
3.What role does aluminum in the galvanized layer play in the corrosion resistance of the coating?
Optimize the microstructure of the coating: The solid solution formed by aluminum and zinc can refine the coating grains, reduce grain boundary defects (such as micropores and impurities), and reduce the penetration rate of corrosive media (water, oxygen).
Form a dense oxide layer: When exposed to air, the aluminum on the surface of the coating will be preferentially oxidized to form an Al₂O₃ film (more dense than ZnO), covering the surface of the zinc layer and delaying the electrochemical corrosion of zinc (the rate at which zinc is consumed as a sacrificial anode is reduced).
4.How to suppress "zinc flow" and "leakage plating" in galvanized aluminum?
Aluminum can reduce the surface tension of zinc liquid and improve the wettability of zinc liquid to steel plate:
Reduce zinc streaks: After the wettability is improved, the flow of zinc liquid on the surface of the steel plate is more uniform, avoiding streaky defects caused by local accumulation.
Reduce the risk of missed plating: For tiny oxidation points or oil residues on the surface of the steel plate, aluminum can promote the spread of zinc liquid at the defects and reduce "missed plating" caused by insufficient wettability, especially for steel plates that are not thoroughly pretreated.
5.What are the differences in typical application scenarios for different aluminum contents?
Small zinc flower galvanized sheet: fine zinc flower + improve adhesion, ordinary structural parts, non-painted decorative parts
No-spangle galvanized sheet: No-spangle appearance + high adhesion + corrosion resistance; home appliance panels, automobile exterior panels (need to be painted)
Zinc-aluminum alloy coating: significantly improves corrosion resistance (3-5 times higher than pure zinc), high corrosion environment (chemical industry, coastal construction)