Hot dip galvanizing is a metallurgical reaction process. From the microscopic point of view, the hot dip galvanizing process is two dynamic equilibria: thermal equilibrium and zinc-iron exchange equilibrium. When the steel workpiece is immersed in the molten zinc liquid at about 450℃, the workpiece at normal temperature absorbs the heat of the zinc liquid. When it reaches more than 200℃, the interaction between zinc and iron is gradually obvious. Zinc penetrates the surface of the iron workpiece.
As the temperature of the workpiece gradually approached the temperature of the zinc liquid, alloy layers containing different ratios of zinc and iron were formed on the surface of the workpiece, forming the layered structure of the 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, and zinc liquid surface boiling, when the zinc-iron reaction is gradually balanced, the zinc liquid surface gradually calms. When the workpiece is raised to the zinc liquid surface 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.
Galvanised GI Steel Coil
Causes Of Surface Defects Of Hot Dip Galvanizing Layer
1. Surface pretreatment
Poor surface pretreatment is the main reason for missing plating (exposed iron) :
Steel in the factory, storage, transportation, processing process stained with paint or mineral grease, welding parts of the flaw detection surface need to apply difficult to clean special grease, etc., and no degreasing process, by pickling to remove surface impurities, resulting in leakage plating (exposed iron).
2. The zinc layer falls off
The main causes of zinc layer shedding are: Surface oxidation, silicon compounds, the cold binding emulsion being too dirty, NOF oxidation atmosphere and the protective gas dew point being too high, air-fuel ratio being unreasonable, the hydrogen flow rate being low, furnace aerobic infiltration, strip steel into the pot temperature is low, RWP section furnace pressure is low and furnace door suction, NOF section furnace temperature is low, Grease evaporation is not enough, the zinc pot aluminum content is low, the unit speed is too fast, the reduction is insufficient, the residence time in the zinc liquid is too short, and the coating is thick.
3. Other components in the zinc solution
Excessive inclusion of other metal components or harmful elements in the zinc liquid may cause defects such as zinc slag particles adhering to the surface of the coating and some abnormal patterns, turtle cracks, and other defects.