Steel must meet three basic conditions before entering the zinc liquid: first, the saponified grease and phosphating film on the steel surface must be cleaned; second, the oxides on the steel surface must be thoroughly pickled; third, the steel surface must be thoroughly pickled before entering the zinc liquid. If it cannot be micro-oxidized twice before, it is necessary to coat the surface of the steel with a layer of plating agent and dry the steel to reduce the temperature difference between the steel itself and the zinc liquid. After that, the steel enters the molten zinc and begins a series of reactions. The formation process of its galvanized layer is as follows:
1. Solvents containing zinc chloride and ammonium chloride interact with the steel surface to further remove ferrous salts and other oxides on the steel surface, making the steel surface clean. The solvent adhered to the surface of the steel quickly decomposes in the zinc liquid and a series of chemical reactions occur. At the same time, the temperature of the steel itself gradually rises. When the zinc liquid around the steel meets the cold steel, it solidifies and wraps the steel, forming a zinc shell. The steel and zinc shell continue to absorb heat and increase the temperature until the melting point of zinc (419.5°C) is reached, and the zinc shell begins to melt. During this process, the alloy layer and intermediate layer will not be formed.
2. After the steel enters the zinc liquid, heat exchange occurs with the zinc liquid quickly, and eventually the temperature of the steel itself gradually rises to basically the same temperature as the entire zinc liquid in the zinc pot.
3. At this time, the trace amount of metallic aluminum in the zinc liquid first reacts with the iron in the steel to form an Fe2Al5 intermediate compound layer, which gradually increases to a certain thickness.
4. The iron atoms in the steel and the liquid zinc begin to undergo a diffusion reaction, causing the iron concentration in the zinc liquid near the steel surface to increase. At the same time, the iron (Fe)-zinc (Zn) alloy layer begins to form and continues to grow. Due to the blocking effect of the Fe2Al5 intermediate layer, the diffusion speed is greatly restricted, and the diffusion process proceeds relatively slowly at this time.
5. When the steel leaves the zinc liquid surface, the temperature of the liquid zinc brought out from the surface drops, begins to cool, and continues to cool to the freezing point of zinc.
6. The liquid galvanizing layer solidifies and crystallizes at about 419.5°C, and forms a dense protective film mainly composed of aluminum oxide (Al2O3) on the surface.
7. The galvanized layer continues to cool in the solid state, and becomes the galvanized layer at room temperature after being air-cooled and water-cooled.
