Hot dip galvanizing layer formation process
The formation process of hot-dip galvanizing coating is the process of forming an iron-zinc alloy between the iron matrix and the outermost pure zinc layer. The iron-zinc alloy layer is formed on the surface of the workpiece during hot-dip plating, which makes the iron and pure zinc layers very close. Good combination, the process can be simply described as: when the iron workpiece is immersed in the molten zinc liquid, a solid solution of zinc and α-iron (body center) is first formed on the interface. This is a crystal formed by dissolving zinc atoms in the base metal iron in a solid state. The atoms of the two metals are fused, and the gravitational force between the atoms is relatively small. Therefore, when zinc reaches saturation in the solid solution, atoms of zinc and iron diffuse into each other. The zinc atoms that diffuse into (or penetrate into) the iron matrix migrate in the matrix lattice and gradually form an alloy with iron. The iron in the molten zinc liquid forms an intermetallic compound FeZn13 with zinc, which sinks to the bottom of the hot-dip galvanizing pot and becomes zinc slag. When the workpiece is removed from the zinc immersion solution, a pure zinc layer is formed on the surface, which is a hexagonal crystal. Its iron content is no more than 0.003%.
Protective properties of hot-dip galvanized coating
Usually the thickness of the electro-galvanized layer is 5 to 15 μm, while the hot-dip galvanized layer is generally above 65 μm or even as high as 100 μm. Hot-dip galvanizing has good covering ability, dense coating and no organic inclusions. As we all know, the mechanism of zinc's resistance to atmospheric corrosion includes mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, there are protective films of ZnO, Zn(OH)2 and basic zinc carbonate on the surface of the zinc layer, which can slow down the corrosion of zinc to a certain extent. This protective film (also called white rust) will form a new film layer if it is damaged. When the zinc layer is severely damaged and endangers the iron matrix, zinc will electrochemically protect the matrix. The standard potential of zinc is -0.76V and the standard potential of iron is -0.44V. When zinc and iron form a micro-battery, zinc is dissolved as an anode and iron is used as an anode. The cathode is protected. Obviously, hot-dip galvanizing has better resistance to atmospheric corrosion of base metal iron than electro-galvanizing.