Description of thickness of hot-dip galvanized 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: heat balance and zinc-iron exchange balance. Surface. As the workpiece temperature gradually approaches the zinc liquid temperature, the alloy layer with different zinc-iron ratio forms on the surface of the workpiece, forming a layer structure of zinc coating. With the time prolonging, different alloy layers in the coating show different growth rates. From a macroscopic point of view, the above-mentioned process shows that the workpiece is immersed in zinc liquid, the zinc liquid surface appears boiling, when zinc. The iron reaction is gradually balanced and the zinc level is gradually calm. When the workpiece is raised to the zinc level, and the temperature of the workpiece is gradually reduced to below 200 degrees Celsius, the zinc-iron reaction stops, the hot-dip galvanizing coating is formed and the thickness is determined.
Thickness requirements for hot dip galvanizing coating
The main factors affecting the thickness of zinc coating are: the composition of substrate metal, the surface roughness of steel, the content and distribution of active elements silicon and phosphorus in steel, the internal stress of steel, the geometric size of workpiece, and the hot dip galvanizing process. The average thickness of the coating in the standard is based on the industrial production experience of the galvanizing mechanism mentioned above, and the local thickness takes into account the galvanizing process. Therefore, ISO standard, ASTM standard of USA, JIS standard of Japan and Chinese standard are slightly different in zinc coating thickness requirements.
Effect and influence of coating thickness on hot dip galvanizing
The thickness of hot-dip galvanized coatings determines the corrosion resistance of the plated parts. For a detailed discussion, see the attached data provided by the American Hot-dip Galvanizing Association. Users can choose the thickness of zinc coatings above or below the standard. It is difficult to obtain a thicker coating in industrial production for thin sheets with a smooth surface of less than 3 mm. In addition, it is difficult to obtain a thicker coating in industrial production with steel. Uneven thickness of zinc coating will affect the adhesion between coating and substrate and the appearance quality of coating. Over-thick coating will cause the appearance of coating rough, easy to peel off, the plating parts can not withstand the collision during transportation and installation. If there are more active elements silicon and phosphorus in steel, it is very difficult to obtain 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, which makes the_-phase Zn-Fe alloy layer grow rapidly and pushes the_-phase to the surface of the coating, resulting in the rough and matt surface of the coating and the formation of a gray coating with poor adhesion.