Electrochemical Deposition Of Al-Mg Alloy Thin Film At Room Temperature

Al-Mg alloy is attracting more and more attention because of its lightness and excellent corrosion resistance. It is an ideal component protective layer and a fine hydrogen storage material. At present, the main methods of preparing Al-Mg alloy include the mixed method, mechanical alloying method and electrolysis method. However, these methods have some problems and disadvantages, especially in the preparation of Al-Mg alloy thin film with the function of hydrogen storage. The electrolytic method for preparing Al-Mg alloy is widely used because the products have characteristics of high purity and homogeneous quality.In this article, Al-Mg alloy was prepared by electro-deposition in C4H8O-C6H6-LiAlH4-AlCl3-MgX2(X=Cl, Br) organic solvent system. The electrochemical behavior and reaction mechanism of electro-deposition process were investigated. In addition, the optimum parameters of preparing Al-Mg alloy were determined, and products obtained by electro-deposition were characterized by SEM, EDS, XRD and XPS.Using Pt as working electrode, Al as reference electrode, the results analyzed by means of cyclic voltammetry at scan rate of0.01V/s show that:reduction reaction in C4H8O-C6H6-LiAlH4system, namely Al3++3e-→Al, starts at-0.76V; reduction reaction in C4H8O-C6H6-LiAlH4-AlCl3system, namely AlHCl2+3e-→H-+2Cl-+Al, starts at-0.36V; reduction reaction in C4H8O-C6H6-LiAlH4-MgCl2system, namely Mg(AlH4)2+8e-→Mg+2Al+8e-, starts at-0.54V; reduction reaction in C4H8O-C6H6-LiAlH4-AlCl3-MgCl2system, namely Mg(AlH4)2+8e-→Mg+2Al+8e-, starts at-0.45V; reduction reaction in C4H8O-C6H6-LiAlH4-MgBr2system, namely Mg(AlH4)2+8e-→Mg+2Al+8e-, starts at-0.62V; reduction reaction in C4H8O-C6H6-LiAlH4-AlCl3-MgBr2system, namely Mg(AlH4)2+8e-→Mg+2Al+8e-, starts at-0.48V, and another reduction reaction, namely MgAl2Cl8+8e-→Mg+2Al+8Cl-starts at-0.82V. The cathodic process in C4H8O-C6H6-LiAlH4-AlCl3-MgX2(X=Cl, Br) system is one step codeposition of Al and Mg. Using Pt as working electrode, the chronopotentiometry curves of C4H8O-C6H6-LiAlH4-AlCl3-MgCl2system were obtained, and the platform at-0.5V is connected with the electro-deposition of Al-Mg alloy. Two platforms were observed at-0.85V and-0.45V in chronopotentiometry curves of C4H8O-C6H6-LiAlH4-AlCl3-MgBr2system, which corresponds to reduction reactions at-0.48V and-0.82V in cyclic voltammogram curves. In C4H8O-C6H6-LiAlH4-AlCl3-MgX2(X=Cl, Br) system, the reduction of aluminium-magnesium ions are quasi reversible process and controlled by diffusion. The nucleation process of Al-Mg alloy on Pt electrode obeys instantaneous nucleation mechanism.The Al-Mg alloy coating was obtained on the Cu substrate by electro-deposition in C4H8O-C6H6-LiAlH4-AlCl3-MgCl2system, and the optimum parameters are as follows:the current density range from20mA/cm2to23mA/cm2; the optimum time is between2700s and5400s; the mass fraction of MgCl2should be larger than2.6%. In C4H8O-C6H6-LiAlH4-AlCl3-MgBr2organic solvent system, the optimum current density range from15mA/cm2to21mA/cm2,and the optimum time is between3600s and5400s.Electro-deposition products were characterized by XPS, SEM and EDS. The results from SEM show that as the increase of current density, the size of the sediment particles increase, and the roughness of the Al-Mg alloy coating also improved correspondingly. The existence of aluminum and magnesium in the coating was proved by EDS analysis. The highest content of magnesium in Al-Mg alloy, which was obtained by electro-deposition in C4H8O-C6H6-LiAlH4-AlCl3-MgCl2system, was4.3%, while that obtained by electro-deposition in C4H8O-C6H6-LiAlH4-AlCl3-MgBr2system was13.3%. The elemental distribution of cross section indicated that the distribution of aluminium and magnesium is overlapped. The thickness of Al-Mg alloy, which was obtained by electro-deposition in-1.0V for1h, is about60μm. The results from XPS confirmed that the products contain metallic magnesium element.