Hot dip galvanizing is a metallurgical reaction process. From a microscopic perspective, the hot-dip galvanizing process is composed of two dynamic balances: thermal balance and zinc-iron exchange balance. When a steel workpiece is immersed in molten zinc liquid at about 450°C, the workpiece at room temperature absorbs the heat of the zinc liquid. When it reaches above 200°C, the interaction between zinc and iron gradually becomes obvious, and zinc penetrates the surface of the iron workpiece.

As the temperature of the workpiece gradually approaches the temperature of the zinc liquid, alloy layers containing different proportions of zinc and iron are formed on the surface of the workpiece, forming a layered structure of the zinc coating. As time goes by, different alloy layers in the coating show different growth rates. From a macro perspective, the above process is manifested as the workpiece being immersed in the zinc liquid and the zinc liquid surface boiling. When the zinc-iron reaction gradually becomes balanced, 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, the hot-dip galvanized coating is formed, and the thickness is determined.

Hot-dip galvanizing coating thickness requirements:

The factors that affect the thickness of the zinc coating mainly include base metal composition, the surface roughness of the steel, content, and distribution of active elements silicon and phosphorus in the steel, internal stress of the steel, geometric dimensions of the workpiece, and hot-dip galvanizing process.

The current international and Chinese hot-dip galvanizing standards are divided into sections according to the thickness of the steel. The average thickness and local thickness of the zinc coating should reach the corresponding thickness to determine the anti-corrosion performance of the zinc coating. Workpieces with different steel thicknesses require different times to achieve thermal balance and zinc-iron exchange balance, and the thickness of the coating formed 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 uneven thickness distribution of the zinc coating and the requirements for the corrosion resistance of the coating.

Therefore, ISO standards, American ASTM standards, Japanese JIS standards, and Chinese standards have slightly different zinc coating thickness requirements, but they are mostly the same.

The role and influence of hot-dip galvanizing coating thickness:

The thickness of the hot-dip galvanized coating determines the anti-corrosion performance of the plated parts. For a detailed discussion, please see the attached data provided by the American Hot-Dip Galvanizing Association. Users can choose a higher or lower zinc plating thickness than the standard.

For thin steel plates with smooth surfaces below 3mm, it is difficult to obtain thicker coatings in industrial production. In addition, the thickness of the zinc coating that is not commensurate with the thickness of the steel will affect the bonding force between the coating and the base material and the appearance quality of the coating. The excessively thick coating will cause the coating to have a rough appearance and peel easily, and the plated parts will not be able to withstand the collision during transportation and installation.

If there are more active elements silicon and phosphorus in the steel, it will be very difficult to obtain a thinner 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, which will cause the zinc-iron alloy layer to grow. It grows rapidly and pushes the ζ phase to the surface of the coating, causing the surface of the coating to be rough and dull, forming a dark coating with poor adhesion.

Therefore, as discussed above, there is uncertainty in the growth of the galvanized layer. It is often difficult to obtain a certain range of coating thickness in actual production. The thickness specified in the hot-dip galvanizing standard is an empirical value generated after a large number of experiments. It takes into account various factors and requirements and is more reasonable.