Study On Electroplating Aluminum Of Magnesium Alloy

Magnesium alloys have many excellent properties,and in various fields have broad application prospect.However the high chemical activity and poor corrosion resistance of magnesium alloy,have a serious impact on the wider use of magnesium alloy.In this paper,AZ31 magnesium alloy was chosen to study the effects of zincate solution,the temperature of zinc immersing,and the time of processing parameters on zinc immersing.And the current density of electroplating aluminum of magnesium alloy was studied at the same time.Optical microscope,SEM,EDS and XRD phase analysis was used to observe zinc immersing layer and electroplating aluminum coating composition and combination of performance.The corrosion resistance of zinc immersing layer was measured by NaCl corrosion test and the polarization curves.The adhesive force between the aluminum coating and the magnesium substrate has been estimated by using scribe test.Immersing zinc of pre-treatment has important influence on the electroplating process.Immersing zinc on magnesium alloy is mainly to reduce the chemical activity, to improve corrosion resistance and cover up magnesium alloy substrate surface defects,strengthen the adhesive force between aluminum coating and the magnesium alloy substrate.Different zincate solution,various zinc immersing temperature and time parameters of the process of pretreatment have a great impact on the immersing zinc layer.The result shows that:.When immersing zinc in ZnSOa-K₄P₂O₇ system zincate solution,at 80℃temperature,for 15 minutes,we could obtain uniformly dense layer of zinc.Current density has a great impact on the coating of aluminum.If current density was especially small,cathode would have a low potential,also nucleation conditions was poor,coating was loose.As well as crystals were coarse and irregular,further more magnesium alloy substrate was not fully covered,exposed parts of magnesium alloy substrate directly reacted with the plating bath,to cause damage to the poor combining force of the aluminum coating and magnesium alloy substrate;With the current density gradually increasing,the nucleation conditions was improved,the crystal of aluminum electroplating coating changed compactly,and the combining force was strengthened;If current density was too large,aluminum crystal would abnormal growth,caused non-uniform crystal size;With current density further increasing,crystal presented dendritic or bracteoid.The microstructure of surface and cross-section of electroplating layer of aluminum were analyzed by SEM and XRD: Electroplate aluminum under the current density of 3.5 A/dm²～4.0 A/dm² could obtain uniformly dense,small and regular crystal,also good adhesive force and shiny silver-white surface,further more high purity electroplating coating.