What is the change in the plating structure after adding aluminum to the zinc bath?

The steel strip has a thicker compound layer between the pure zinc layer and the steel strip obtained after the pure zinc plating, because the metal compound is hard and brittle as compared with the pure zinc, which deteriorates the performance of the coating. In particular, the binding force between the compound layer and the steel base is very weak, as if there is a fracture layer between the zinc layer and the steel strip, which cuts the connection between the steel base and the pure zinc layer. Therefore, the adhesion of the coating is very unfavorable. In order to improve this situation, a certain amount of aluminum is added to the zinc liquid in the production to improve the performance of the coating. Aluminum-to-zinc ratio is a more chemically more active metal. It is reflected in metallurgy that aluminum reacts more easily with iron to form metal compounds. The main metal compounds of aluminum and iron are FeAl, FeAl2, FeAl3, Fe2Al5, among which Fe2Al5 is more 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 combines iron and zinc. Segregation hinders the diffusion of iron into the zinc liquid and thus hinders the formation of iron-zinc compounds. Tests have shown 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 preferentially reacts with iron, high concentration of zinc quickly displaces aluminum from the iron-aluminum alloy, causing the iron-aluminum alloy to disappear, and thus preventing iron and zinc from being hindered. The role of the 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 coating is cooled and solidified within the gestation period in order to play its due role. 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 index 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.

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