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Metallurgical reaction processes-galvanized coating

Metallurgical reaction processes-galvanized coating

Galvanizing is a metallurgical reaction process. From the microscopic point of view, the process of hot dip galvanizing is two dynamic equilibria: thermal equilibrium and zinc-iron exchange equilibrium. When the steel workpiece is immersed in molten zinc liquid about 450℃, the workpiece absorbs the heat of zinc liquid at normal temperature. When it reaches more than 200℃, the interaction between zinc and iron is gradually obvious, and zinc penetrates into the surface of iron workpiece. As the workpiece temperature gradually approached the temperature of zinc liquid, alloy layers containing different ratios of zinc and iron were formed on the workpiece surface, forming the layered structure of zinc coating. With the extension of time, different alloy layers in the coating showed different growth rates. From a macro point of view, the above process is manifested as the workpiece immersed in zinc liquid, zinc liquid level boiling, when the zinc iron reaction gradually balanced, zinc liquid level gradually calm. When the workpiece is raised to the zinc liquid level and the workpiece temperature gradually decreases below 200℃, the zinc-iron reaction stops, the hot-dip galvanized coating is formed and the thickness is determined.

The main factors of zinc coating thickness are: metal composition of substrate, surface roughness of steel, content and distribution of active elements silicon and phosphorus in steel, internal stress of steel, workpiece geometry, hot dip galvanizing process. The current international and Chinese standards for hot dip galvanizing are divided into sections according to the thickness of steel, and the thickness of the zinc coating should reach the corresponding thickness to determine the corrosion resistance of the zinc coating. For workpiece with different steel thickness, the time required to achieve thermal balance and zinc-iron exchange balance is different, and the coating thickness is different. The average thickness of the coating in the standard is the experience value of industrial production based on the above galvanizing mechanism, while the local thickness is the experience value required to take into account the uneven distribution of the thickness of the zinc coating and the requirements of corrosion protection of the coating. Therefore, the ISO standard, the American ASTM standard, the Japanese JIS standard and the Chinese standard have slightly different requirements on zinc coating thickness, but little difference.

The thickness of galvanized coating determines the corrosion resistance of plating parts. Please refer to the attached data provided by the American Hot-Dip Galvanizing Association for detailed discussion. Users can choose zinc plating thickness above or below the standard. For the smooth surface of 3mm or less thin steel plate, it is difficult to get thicker coating in industrial production. In addition, the thickness of zinc coating is not commensurate with the thickness of steel will affect the binding force between coating and substrate and the appearance quality of coating. Too thick coating will cause the coating appearance is rough, easy to flake, plated parts can not stand the collision in the process of handling and installation. If there are more active elements silicon and phosphorus in the steel, it is difficult to obtain a thin coating in industrial production. This is because the silicon content in the steel affects the growth mode of the alloy layer between zinc and iron, causing the rapid growth of the zinc-iron alloy layer and pushing the ζ phase towards the surface of the coating, resulting in rough and dull surface of the coating, forming a gray coating with poor adhesion.


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