Preparation of zinc alloy die castings before electroplating

For zinc alloy die-casting parts, zinc alloy materials with an aluminum content of about 4% should usually be selected to improve the pass rate of electroplated products.

When electroplating zinc alloy die-cast parts, please pay attention to the following characteristics of zinc alloy die-cast parts and take appropriate measures.

(1) The surface of the die casting is a dense surface layer, about 0.05mm~0.1mm. Below the surface layer is a porous structure. Therefore, when polishing, do not throw away the surface layer to expose the loose bottom layer. Otherwise, electroplating is very difficult and will reduce the corrosion resistance of the product.

(2) During the die-casting process, the die-cast part changes from a molten state to a solid state. Since the freezing point during cooling is different, segregation is likely to occur on the surface of the die-cast part, and therefore an aluminum-rich phase or a zinc-rich phase is generated in a part of the surface. Therefore, do not use strong alkalis and strong acids for degreasing and etching during pretreatment. Because the strong alkali can first dissolve the aluminum-rich phase, the strong acid can first dissolve the zinc-rich phase, thereby forming pinholes and micropores on the surface of the die-casting part, and the strong alkali solution and strong acid solution will remain, so when electroplating after electroplating, it is very Easy to cause peeling and air bubbles.

(3) The shape of zinc alloy die-casting parts is usually more complicated, and solutions with better dispersion and coverage should be used when electroplating. In order to prevent zinc from substituting positive-potential metal in recesses or masking places, and thus make the bonding force poor, the pre-plating layer should choose a plating solution with good dispersion and covering ability.

(4) The coating used is preferably a bright coating, please try to avoid the polishing process or reduce the polishing workload. On the one hand, because the shape is complex, it is not easy to polish, on the other hand, the thickness of the coating can be guaranteed to ensure the quality.

(5) If the layer of copper is copper, the thickness should be slightly thicker, because when copper is plated on the surface of the zinc alloy, the copper will diffuse into zinc and form a layer of brittle copper-zinc alloy. In the intermediate layer, the thinner the copper layer, the faster the diffusion, so the thickness of the copper must be at least 7μm or thicker.

(6) Multi-layer chromium plating is a cathodic protective coating of zinc alloy, so the coating must have a certain thickness to ensure that the coating is non-porous. Otherwise, since the electrode potential of zinc alloy is relatively negative, white powdery corrosion products of alkaline zinc carbonate may be generated in humid air. Therefore, the appropriate coating thickness must be selected according to the use conditions of the product.

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