| Magnesium alloys are widely used in electronic products and transportation industry because of its low density,high specific strength,excellent thermal conductivity and electromagnetic shielding.However,due to the poor corrosion resistance and high electrochemical activity of magnesium alloys,their industrial applications have been greatly limited.Improving the corrosion resistance of magnesium alloys has become the focus of many scholars.Electrodeposition can effectively improve the corrosion resistance of magnesium alloy.In this study,Al-coated AZ31 magnesium alloys with different deposition time were prepared by constant current deposition in an AlCl3-containing choline chloride-urea Reline ionic liquid.Subsequently,OM,SEM and EDS were used to observe and analyze the microstructure and phase composition of magnesium alloy samples before and after electrodeposition,compared the effect of deposition time on the surface morphology of magnesium alloy.The nucleation and growth behavior of aluminium ions in electrodeposition process were analyzed by cyclic voltammetry and chronoamperometry.The diffusion coefficients of aluminium ions in solution and aluminium atoms on the surface of magnesium alloys at the later stage of electrodeposition were calculated.The thickness of coating on magnesium alloy surface at different deposition time was compared.The corrosion behavior of aluminium-coated magnesium alloy in 3.5 wt.%NaCl solution were studied by open-circuit potential method,potentiodynamic scanning method and electrochemical impedance spectroscopy.The morphology of electrochemical corrosion was observed and the protection mechanism of aluminium coating on AZ31 magnesium alloy at different deposition time was analyzed.The conclusions of this study are as follows:1.Aluminium-coated magnesium alloy was successfully prepared at 70?and 1 mA/cm2 in AlCl3-chloride-urea ionic liquid.2.The cyclic voltammetry scanning in ionic liquid electrolyte at different scanning rates shows that the peak current of the cathode is linearly related to the square of the scanning rate,which indicates that the reduction process of aluminum ions is controlled by diffusion,and the diffusion coefficient is 1.773×10-1010 cm2/s.3.Analyzing the chronoamperometric curves at different potentials,it was found that the electrodeposition of aluminium ions agrees with the model of three-dimensional progressive nucleation and the later stage follows three-dimensional instantaneous nucleation.4.Observing the surface morphology of Al-coated magnesium alloys with different deposition time,it is found that there existed the thin-film Al layer attached to the surface of magnesium alloys after 1 h deposition.With the deposition time rised,the Al atoms in different locations diffuse to the appropriate growth point.When the deposition time is 2 h,the deposition morphology shows the granular distribution;When the deposition time is 3 h,the deposition morphology has became more biger,and there are lamellar aluminium produced by atom diffusion between particles.5.The particle size distribution on the surface of Mg alloy were collected.It was found that the particle size was uniform,showing a normal distribution.The diffusion kinetics of aluminum ion in the electrodeposition process was studied,and its growth model was established.The diffusion coefficients of aluminium atoms at 2 h and 3 h were 6.3×10-1111 cm2/s and 5.1×10-1111 cm2/s,respectively.6.The test results of open circuit and polarization curves in 3.5 wt.%NaCl solution show that the corrosion potential of Al-coated magnesium alloy is better than uncoated magnesium alloy,which indicates that the aluminum coating has a protective effect on the magnesium alloy.The polarization resistance of Al-coated magnesium alloy is larger than uncoated AZ31magnesium alloy matrix by impedance spectroscopy,and the polarization resistance of 3 h deposition is about 2.3 times higher than uncoated magnesium alloy,it indicated that Al layer provides a good corrosion resistance for magnesium alloy.7.According to the corrosion morphology,it can be inferred that lamellar-particle composite morphology has the best protective effect on magnesium alloy. |
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