What are the features of surface treatment on zinc alloy?

Answer 1

1. There is a layer of compact surface layer on the surface of die castings, and the thickness is about 0.05~0.1mm. The substratum is a loose poriferous structure. Therefore, during polishing, the surface cannot be polished completely and expose loose substratum, otherwise electroplating will be very difficult and it will weaken the corrosion resistance of alloy.

2. Die castings change from the molten state to solid state during die casting. Due to the different freezing points during freezing, segregation is usually caused on the surface of die castings, making aluminum-rich and zinc-rich phases on parts of the surface. So, during pretreatment, strong base and strong acid should not be used to deoil and etch, because strong base firstly dissolves aluminum-rich areas and strong acid firstly dissolves zinc-rich areas so that pinhole and micro-pores are formed on the surface of die castings. In addition, strong base and strong acid remain which easily results in decrustation and bubbles after plating.

3. The shape of zinc alloy die casting is commonly quite complicated. Electroplating should use a solution having good dispersing power and covering power. This is also necessary to prevent positive metal replacement from occurring in recessions and shelters.

4. The plating layer used should be light to avoid polishing or reduce workload of polishing as much as possible. On the one hand, for complicated shapes, die castings are not easy to polish, on the other hand, the thickness of plating layer can be maintained so that quality can be ensured.

5. If the first plating layer uses copper, the thickness should be thicker. Because when copper is plated on the surface of zinc alloy, the thinner the copper layer is, the more quick diffusion is, so the thickness of copper should reach to 7 μm at least or thicker.

6. Multi-layer plating with chromium is a cathode protective coating of zinc alloy, so the layer should have definite thickness, ensuring that there are no pores existing in the coating. The electrode potential of zinc alloy is negative, it can easily produce white power corrosion products of zinc carbonate in moist air. Nevertheless, an appropriate thickness of layer must be chosen based on the conditions in which the products are to be used.