Surface treatment characteristics of zinc alloy die castings:
1. The surface of die casting is a dense surface layer, about 0.05~0.1mm. Below the surface layer is a loose and porous structure. For this reason, when polishing and polishing, do not throw away all the surface layers and expose the loose bottom layer. Otherwise, electroplating is very difficult and will reduce the corrosion resistance of the product.
2. Die-casting parts change from a molten state to a solid-state during the die-casting process. Because of the different freezing points during cooling, segregation tends to occur on the surface of the die-casting part, causing some parts of the surface to produce an aluminum-rich phase or a zinc-rich phase. For this reason, do not use strong alkali and strong acid to remove oil and etch during pretreatment. Because the strong alkali can dissolve the aluminum-rich phase first, and the strong acid can dissolve the zinc-rich phase first, thus forming pinholes and micro-pores on the surface of the die-casting part, and will leave strong lye and strong acid solution, so that it will be plated on After that, it is easy to cause peeling and air bubbles.
3. The shape of zinc alloy die castings are generally more complicated, and electroplating should use a solution with better dispersion and coverage. In order to prevent the replacement of zinc with positive potential metal in the recessed or masked area, which will make the bonding force poor, the pre-plating layer should choose a plating solution with the good dispersing ability and covering ability.
4. The plating layer used is a bright plating layer, and the polishing process is avoided or the polishing workload is reduced as much as possible. On the one hand, it is not easy to polish because of the complicated shape, and on the other hand, the thickness of the coating can be guaranteed to ensure quality.
5. If a copper layer is used for the first plating layer, its thickness should be slightly thicker, because when copper is plated on the surface of the zinc alloy, the thinner the copper layer, the faster the diffusion will occur, so the thickness of the copper must be at least 7μm or thicker Some.
6. The multi-layer chromium plating layer is a cathodic protection plating layer of zinc alloy, so the plating layer must have a certain thickness to ensure that the plating layer has no pores. Otherwise, because the electrode potential of zinc alloy is relatively negative, it is easy to produce white powdery corrosion products of basic zinc carbonate in the humid air. Therefore, the appropriate coating thickness must be selected according to the use conditions of the product.
7. If the die-casting process is inappropriate, or the die-casting mold is designed unreasonably, causing cold lines, shrinkage, looseness or pinholes on the die-casting surface, then even if measures are taken in the electroplating process, a satisfactory coating is often not obtained.