All Types of Galvanized Coil Coating

Formation mechanism of hot-dip galvanized coating

Hot dip galvanizing is a metallurgical reaction process. From a microscopic point of view, the process of hot dip galvanizing is two dynamic balances: thermal balance and zinc-iron exchange balance.

When the iron and steel workpiece is immersed in molten zinc solution at about 450°C, the workpiece at room temperature absorbs the heat of the zinc solution, and when it reaches above 200°C, the interaction between zinc and iron is gradually obvious, and zinc penetrates into the surface of the iron workpiece.

As the temperature of the workpiece gradually approaches the temperature of the molten zinc, alloy layers with different ratios of zinc and iron are formed on the surface of the workpiece, forming a layered structure of the zinc coating. As time goes on, different alloy layers in the coating show different growth rates.

From a macroscopic point of view, the above process shows that the workpiece is immersed in the zinc liquid, and the zinc liquid surface boils. When the zinc-iron reaction gradually balances, the zinc liquid surface gradually calms down.

The workpiece is lifted out of the zinc liquid level, and when the temperature of the workpiece gradually drops below 200 °C, the zinc-iron reaction stops, and the hot-dip galvanized coating is formed and the thickness is determined.
Hot-dip galvanized coating thickness requirements

The main factors affecting the thickness of the zinc coating are: the composition of the base metal, the surface roughness of the steel, the content and distribution of the active elements silicon and phosphorus in the steel, the internal stress of the steel, the geometric size of the workpiece, and the hot-dip galvanizing process.

The current international and Chinese hot-dip galvanizing standards are divided into sections according to the thickness of the steel, and the flat thickness and local thickness of the zinc coating should reach the corresponding thickness to determine the corrosion resistance of the zinc coating.

For workpieces with different steel thicknesses, the time required to achieve thermal equilibrium and zinc-iron exchange equilibrium is different, and the thickness of the formed coating is also different.

The average thickness of the coating in the standard is based on the industrial production experience value of the above-mentioned galvanizing mechanism, and the local thickness is the empirical value required to take into account the unevenness of the thickness distribution of the zinc coating and the requirements for the corrosion resistance of the coating.

Therefore, the ISO standard, the American ASTM standard, the Japanese JIS standard and the Chinese standard have slightly different requirements on the thickness of the zinc coating, which are similar.

Next, several different hot-dip galvanizing and coating characteristics are explained.
Hot-dip pure zinc-coated steel sheet (GI sheet)

At present, 0.2% Al element is added to the zinc solution used in the production of GI plates. The effect of adding AI is to improve the fluidity of the zinc solution. Aluminum and iron elements react preferentially to form an iron-aluminum alloy phase layer, which inhibits the reaction between zinc and the steel plate substrate, thereby improving the adhesion of the coating.

At the same time, a small amount of aluminum in the zinc solution is added to zinc. The aluminum oxide film is formed on the surface of the liquid, which prevents the oxidation reaction of zinc on the surface of the zinc liquid and reduces the consumption of zinc.

On the surface of the coating, aluminum can first react with oxygen to form an aluminum oxide protective film, which prevents the surface of the coating from oxidizing and increases the surface brightness.

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