Hot-dip galvanizing is a metallurgical reaction process. From a microscopic perspective, the hot-dip galvanizing process involves two dynamic equilibria: thermal equilibrium and zinc-iron exchange equilibrium. When steel workpieces are immersed in molten zinc at approximately 450 degree , the workpieces at room temperature absorb heat from the zinc liquid. As the temperature of the workpieces rises above 200 degree , the interaction between zinc and iron gradually becomes apparent, with zinc infiltrating the surface of the iron workpieces.
As the temperature of the workpieces gradually approaches that of the molten zinc, an alloy layer with varying zinc-iron ratios forms on the surface of the workpieces, constituting the layered structure of the zinc coating. Over time, different alloy layers within the coating exhibit varying growth rates. From a macroscopic perspective, this process manifests as the immersion of workpieces into the molten zinc, causing the zinc surface to boil. When the zinc-iron reaction gradually reaches equilibrium, the zinc surface calms down. Once the workpieces are lifted out of the molten zinc and their temperature gradually decreases below 200 degree , the zinc-iron reaction stops, and the hot-dip galvanized coating is formed with a determined thickness.
The primary factors influencing the thickness of the zinc coating include the composition of the base metal, the surface roughness of the steel, the content and distribution of active elements such as silicon and phosphorus in the steel, internal stress within the steel, the geometric dimensions of the workpiece, and the hot-dip galvanizing process.
Both current international and Chinese standards for hot-dip galvanizing categorize steel thicknesses into intervals, specifying the average and local minimum thicknesses of the zinc coating that must be achieved to ensure corrosion resistance. Workpieces with different steel thicknesses require varying amounts of time to reach thermal equilibrium and zinc-iron exchange equilibrium, resulting in different coating thicknesses.
