What are the surface treatments of zinc alloy products?

Zinc-aluminum die casting is a kind of die casting with zinc as the main component. This part of the surface has a dense surface layer, and the interior is porous structure and active amphoteric metal. Therefore, only by adopting appropriate pretreatment method and electroplating process can the electroplating layer on zinc alloy have good adhesion and meet the requirements of qualified products.

Zinc alloy materials commonly used in electroplating are: 3.5% ~ 4.5% of aluminum, 0.75% ~ 1.25% of copper, 0.03% ~ 0.08% of magnesium, and the balance is zinc, and the sum of impurities is ≤0.2%.

925 brand zinc alloy has high copper content and is easy to electroplate. Generally, the density of zinc alloy is 6.4 ~ 6.5g/cm3. If the density is

Extended data

1. The surface of die casting is a dense surface layer, about 0.05 ~ 0. 1 mm ... The surface layer is porous and porous. Therefore, when polishing, don't throw away all the surface layers to expose the loose bottom layer. Otherwise, electroplating is very difficult and the corrosion resistance of products will be reduced.

2. Die casting changes from molten state to solid state in the process of die casting. Due to the different freezing points during cooling, the surface of die casting often segregates, resulting in aluminum-rich or zinc-rich phases in some parts of the surface.

Therefore, in the pretreatment process, do not use strong alkali and strong acid for degreasing and etching. Because strong alkali will dissolve aluminum-rich phase and strong acid will dissolve zinc-rich phase first, pinholes and micropores will be formed on the surface of die casting, and strong alkali solution and strong acid solution will remain, which is easy to peel and blister after plating.

3. The shape of zinc alloy die castings is generally complicated, and electroplating should use a solution with good dispersion and covering ability. In order to prevent zinc from replacing metal with positive potential in depressions or hidden places, so that the bonding force is not good, a plating solution with good dispersibility and covering ability should be selected for pre-plating.

4. The coating used should be bright, which can avoid the polishing process or reduce the polishing workload. On the one hand, it is difficult to polish because of the complex shape, on the other hand, it can ensure the thickness and quality of the coating.

5. If copper is used as the first coating, its thickness should be slightly thicker, because the thinner the copper layer, the faster the diffusion, so the thickness of copper should be at least above 7 μ m..

6. The multi-layer chromium plating layer is the cathodic protection coating of zinc alloy, so the coating must have a certain thickness to ensure that the coating has no pores. Otherwise, due to the negative electrode potential of zinc alloy, it is easy to produce white powdery corrosion products of basic zinc carbonate in humid air, so the appropriate coating thickness must be selected according to the service conditions of the products.

7. If the die casting process is improper or the die casting die design is unreasonable, resulting in cold lines, shrinkage cavities, air holes or pinholes on the surface of the die casting, then even if the electroplating process takes measures, it is often impossible to obtain satisfactory coatings.

Baidu encyclopedia-zinc alloy