Magnesium Alloy Surface Aluminum Pre-treatment Study

The application trend of magnesium is very wide, because magnesium alloys have many superior performances. Its poor corrosion resisting property has seriously influenced the application prospect of the magnesium alloy. The AZ61 magnesium alloy is chose as the research object. The zincate and fluoride pretreatment and electroplate aluminum on the surface after pretreatment against its shortcoming of poor corrosion resisting property are preliminary studied.The zincate pretreatment processes and electroplating aluminum on the surface after pretreatment are studied. The phase and morphology on surface and cross-section of the magnesium alloy after zincate pretreatment are analysised by the XRD analysis and SEM. Different acid pickling and activation time have great impact on the corrosion of the magnesium alloy substrate and surface finish. Ultrasonic not only provides the additional energy for zinc deposition, strengthens the capacity of the ion transport, but also reduces the thickness of the washcoat and the concentration gradient, accelerates the replacement reaction. Meanwhile, it removes the poor bonding granule of zinc, gets uniform layer of zinc. In a word, it optimizes the operation condition of plating zinc. Then the aluminum is electroplated on the layer of zinc. The certain thickness, purity aluminum coating is got. The layer is analysised by the microscopy on surface and cross-section. The layer is very rough and loose. The aluminum grains are big. The bonding with the substrate is not very well. These have to be improved.Meanwhile, the fluorinated pretreatment processes and electroplating aluminum on the surface after pretreatment are studied. The fluorinated film after NH₄HF₂ treatment is incomplete and some oxide film is not removed. The fluorinated film is not protected the substrate very well and the substrate is oxidized again. And the NH₄HF₂ treatment causes the serious corrosion and is not consistent with the surface pretreatment requirements of the magnesium alloy. So it can not be used. The fluorinated film after HF treatment is skin rolling, clean and compact. It has good corrosion resistance. High concentration of hydrofluoric acid shows passivation properties on magnesium alloy substrate. The reaction of the hydrofluoric acid with magnesium substrate is diffused reaction and the reaction is inhibited by the grain boundary. The film protects the magnesium substrate very well. It is easily removed and replaced by the aluminum coating in the AlCl₃ molten salt. There is an interlayer between the coating and the substrate. Aluminum atoms diffuse along the grain boundaries to the substrate in the process of the electroplating aluminum. That is a typical short circuit diffusion. The coating combines well with the substrate of the magnesium alloy, because this interlayer helps to strengthen the combining performance between the coating and the substrate of magnesium alloy. It is entirely feasible that electroplating aluminum on the surface of the magnesium alloy after treatment with hydrofluoric acid.