Change in plating microstructure after addition of aluminum to zinc bath
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The ratio of aluminum to zinc is a more chemically more active metal. It is reflected in the fact that aluminum is more easily reacted with iron to form metal compounds. The metal compounds of aluminum and iron are mainly FeAl, FeAl2, FeAl3, and Fe2Al5, of which Fe2Al5 is the most. stable.
This characteristic of aluminum causes a fundamental change in the reaction of the metal atoms during the galvanization of the aluminum-containing zinc liquid. Although the concentration of aluminum is very low, after the steel strip enters into the zinc pot, aluminum first reacts with the iron on the surface of the steel strip quickly, forming a layer of Fe2Al5 film on the surface of the steel strip. This dense layer of film contains iron and zinc. Segregation hinders the diffusion of iron into the zinc liquid, which also hinders the formation of iron-zinc compounds. Experiments show that when the content of aluminum is 0.10%, the palisade layer in the compound layer is greatly reduced. When the content of aluminum is 0.20%, the palisade layer can be completely eliminated, and only the drift layer and the pure zinc layer can be formed. organization. Like other chemical reactions with poor concentration, although aluminum can preferentially react with iron, the absolute high concentration of zinc will quickly displace aluminum from the iron-aluminum alloy, causing the iron-aluminum alloy to disappear, which will lose the iron barrier. The role of zinc reaction. Usually the delay time that aluminum can inhibit the formation of iron-zinc compound layer is called the incubation period. In actual production, it is necessary to ensure that the plating layer is cooled and solidified within the incubation period in order to exert its proper function. Therefore, a qualified product can only be produced if the production line speed is within a certain range.
Facts have proved that the thin and homogeneous intermediate layer formed of aluminum and iron has a particularly important effect on the plating layer. It can firmly adhere to the surface of the steel strip, and the fork can be compared with the outer iron zinc compound layer and the pure zinc layer. Well combined together, plays a mediating role between the entire plating and the steel belt. The adhesion of the coating to the thickness of the iron-zinc compound layer is not too large but depends only on the iron-aluminum compound interlayer. Thus, the morphology and thickness of the iron-aluminum compound layer serve as an indicator of the adhesion of the plating layer, and a great deal of research has been conducted on it, and various measures have been taken to study and measure the thickness of this intermediate compound layer.