Characteristics of several different hot-dip galvanized and zinc alloy coatings

Mechanism of formation of hot-dip galvanized coating

Hot-dip galvanizing is a metallurgical reaction process. From a microscopic point of view, the hot dip galvanizing process is a dynamic equilibrium of two kinds: thermal equilibrium and zinc-iron exchange equilibrium. When the steel workpiece is immersed in molten zinc at about 450℃, the workpiece at room temperature absorbs the heat of the zinc liquid, and when it reaches above 200℃, the interaction between zinc and iron becomes obvious, and the zinc penetrates into the surface of the iron workpiece.

With the workpiece temperature gradually close to the temperature of the zinc liquid, the workpiece surface to form an alloy layer containing a different proportion of zinc and iron, constituting the layered structure of the zinc plating layer, with the extension of time, the different alloy layers in the plating layer show different growth rates. From the macroscopic point of view, the above process is manifested as the workpiece is immersed into the zinc liquid, boiling occurs on the surface of the zinc liquid, and when the zinc-iron reaction is gradually balanced, the surface of the zinc liquid is gradually calmed down. The workpiece is raised to the surface of the zinc liquid, the workpiece temperature is gradually reduced to below 200 ℃, the zinc and iron reaction stops, hot-dip galvanizing coating formation, thickness determination.

Requirements for hot-dip galvanized coating thickness

Factors affecting the thickness of the zinc coating are mainly: the base metal composition, the surface roughness of the steel, the active elements in the steel silicon and phosphorus content and distribution status, the internal stress of the steel, the geometry of the workpiece, the hot dip galvanizing process.

Current international and Chinese hot-dip galvanizing standards are based on the thickness of the steel to divide the zones, zinc coating global thickness as well as the local thickness should be the corresponding thickness to determine the corrosion resistance of the zinc coating. Workpieces with different steel thicknesses take different amounts of time to reach thermal and zinc-iron exchange equilibrium and form different thicknesses of coating. The average thickness of the coating in the standard is an empirical value for industrial production based on the galvanizing mechanism described above, and the local thickness is an empirical value needed to take into account the non-uniformity of the thickness distribution of the zinc coating, as well as the requirements for 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 for the thickness of the zinc coating, which are more or less the same.

Several different hot-dip galvanizing and coating characteristics are explained next.

Hot-dip pure zinc-coated steel sheet (GI sheet)

The zinc solution used in the current production of GI plates has 0.2% Al added. The role of adding AI is to improve the fluidity of the zinc solution, aluminum and iron elements through the preferential reaction to form a ferro-aluminum alloy phase layer, inhibit the reaction between zinc and the steel plate substrate, thus improving the adhesion of the plating layer, and at the same time, a small amount of aluminum in the zinc solution in the zinc liquid surface of the formation of an aluminum oxide film, preventing the oxidation reaction of the zinc on the surface of the zinc liquid, reducing the zinc consumption. And in the plating surface aluminum can first react with oxygen to form a layer of alumina protective film, preventing the plating surface oxidation, increase surface brightness.

GI plate is a typical use of sacrificial anode (zinc) cathodic protection mechanism to protect the steel substrate from corrosion, and GI plate is the most common and widely used coated steel, in construction, home appliances, automotive, transportation, agriculture and other industries in a large number of applications.