Pre-plating treatment of zinc alloy die castings

- Aug 29, 2018 -

Zinc alloy die-casting parts are mainly zinc-aluminum alloys, and zinc and aluminum are both active amphoteric metals, both soluble in acid and alkali. Therefore, strong alkali degreasing and strong acid etching cannot be used in the pre-plating treatment. . Moreover, due to the reasons of die-casting process and mold design, the surface of zinc alloy die-casting parts often has surface defects such as rough unevenness, cold grain, burrs, parting lines, flash edges and shrinkage holes, and the surface layer is a dense layer, and the inside is a loose porous structure. Therefore, it is necessary to perform mechanical cleaning, polishing and fear of light, but it should not be excessively polished to avoid exposing a large number of internal defects, causing plating difficulties and affecting the quality of the plating.

After the zinc alloy die-casting parts are polished and polished, a large amount of oil stains, polishing pastes, etc. on the surface must be pre-cleaned. The pre-cleaning can be performed by organic solvent or chemical degreasing, and the degreasing process should be carried out as soon as possible after polishing, so as to prevent the polishing paste from hardening for a long time. After pre-degreasing oil, electrochemical degreasing should be carried out. As far as possible, low-temperature, low-alkalinity solution (pH <10, generally no NaOH) is used for degreasing. Try not to use the anode to remove the oil to avoid corrosion or dissolution of the zinc alloy surface to produce corrosion or white gummy corrosives and pitting. After the zinc alloy die-casting part is degreased, there is a very thin oxide film on the surface. In order to completely remove the oxide film and ensure the bonding strength of the plating layer, it is usually selected by hydrofluoric acid impregnation activation or fluoroboric acid corrosion activation treatment. The electrode potential of the zinc alloy is relatively negative. In order to prevent the displacement reaction from occurring and affect the bonding strength of the plating layer, the zinc alloy die-casting material is usually subjected to cyanidation pre-plating.

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