In the hot-dip galvanizing industry, the application of plating aids is a very important process in the pretreatment process, which not only directly affects the appearance quality of the coating, but also affects the corrosion resistance of the coating.
Author: Marisa
In the hot-dip galvanizing industry, the application of plating aids is a very important process in the pretreatment process, which not only directly affects the appearance quality of the coating, but also affects the corrosion resistance of the coating. It plays a role in cleaning and activating the surface of the steel substrate. Therefore, the plating aid must have good performance, such as stable composition. It is generally believed that the role of the plating aid is to: strip (dissolve) the residual iron oxide film on the steel substrate and improve the surface activity; reduce the surface tension of the alloy solution, improve the wettability of the alloy solution to the steel substrate, and prevent the clean steel substrate surface obtained by degreasing and rust removal from being oxidized again.
The main components of the traditional plating aid are a double salt solution of zinc chloride (ZnCl2) and ammonium chloride (NH4Cl). After the workpiece is pickled, the water is not thoroughly cleaned, and the iron salt is brought into the zinc pot. 1 kg of iron can produce 20-50 kg of zinc slag, which not only increases the zinc consumption and the cost of galvanizing, but also increases the oxides and other impurities in the zinc liquid, which will cause the zinc liquid to age and reduce the quality of hot-dip galvanizing. Therefore, the plating agent must be treated to remove iron.
The main purpose of removing iron from the plating agent is to remove the divalent iron ions in the plating agent. The divalent iron ions are oxidized to trivalent iron ions, and the trivalent iron ions are hydrolyzed to form iron hydroxide. Iron hydroxide is insoluble in water and is separated by precipitation in the plating agent. There are many ways to remove iron. There are oxidation with compressed air, oxidation with ozone, oxidation with chemical oxidants, etc. After long-term field operation practice, it is more reasonable to select the chemical oxidation iron removal process. The process basis is: adjust pH value - oxidation iron removal - filtration (separation).
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