Electrodeposition Of Aluminum On Magnesium Substrate In Ionic Liquids Based On Quaternary Onium Salts

The goal of this research is to obtain a good aluminum coating as a protecting forcorrosion electrodeposited on magnesium substrate from ionic liquids based onquaternary onium salts at ambient temperature. The properties of ionic liquid, thepossibility of the aluminum electrodeposition, the mechanism of the electrodepositionand the electric parameters were investigated. A satisfied aluminum coating wasobtained on magnesium substrate after a proper pretreatments finally.First, an ionic liquid consisted by benzyltriethylamine chloride（BTEAC）-AlCl₃-toluene in1:2:4molar ratio was choosed as the electrolyte for thealuminum electrodeposition. Compared with the mechanism of aluminumelectrodeposition, the price and properties of the ionic liquids, the ionic liquids basedon quaternary onium salts were choosed as electrolyte for the electrodeposition.Theproperties and conductivity of the benzyltrimethylamine chloride（BTMAC）-AlCl₃andBTEAC-AlCl₃ionic liquids were compared,1:2mole ratio of BTEAC-AlCl₃waschoosed as electrolyte. The influence of the diluents on the conductivity of the ionicliquid was considered. As the diluents of toluene and benzene increased, theconductivity of the1:2mole ratio of BTEAC-AlCl₃ionic liquid increased first andthen decreased. Taken the effect of holding time on conductivity of the ionic liquidand the toxicity of the diluents together, the1:2:4mole ratio of BTEAC-AlCl₃-toluenewas the right electrolyte for the aluminum electrodeposition.The possibility of the aluminum electrodeposition in choosed electrolyte wasinvestigated on copper substrate. The results show that aluminum electrodeposition inchoosed BTEAC-AlCl₃-toluene electrolyt was practicable at ambient temperature.Analyses of the cyclic voltammograms and chronoamperograms indicated that thedeposition process of aluminum on copper substrates was controlled bythree-dimensional instantaneous nucleation with diffusion-controlled growth, thediffusion coefficient D is10-7cm2/s and the nucleation number density of form nucleiN is105/cm2. At ambient temperature in the1:2:4mole ratio ofBTEAC-AlCl₃-toluene electrolyte, under the condition of3hours²4hours and0.5V².0V, an uniform, silvery lustre, compact aluminum coating with good adhesion oncopper matrix can be obtained with1μm partical size, mass fraction being over90%. A satisfied aluminum coating was obtained on magnesium substrate after aproper pretreatments in choosed ionic liquid at ambient temperature. The aluminumcoating with a bad deposit adhesion and appearance which was obtained onmagnesium substrate after a mechanical grinding directly in choosed electrolyt wasnot satisfied. Analyses of the eletrochemistry and the energy-dispersive X-rayspectroscopy（EDS） indicated that the eletrochemical behavior of magnesium in bothionic liquid and NaCl aqueous solution are completely different, an active to passivetransition behavior was observed for magnesium in ionic liquid. The reason for thebad deposit adhesion was that the passive film formed in active to passiveeletrochemical process and the oxide formed in atmosphere were between themagnesium substrate and the aluminum coating. In order to change this condition, thepretreatment of HF, H₃PO₄and zinc immersing before aluminum deposition wereconsidered. Compared with the appearance of aluminum coating after differentpretreatment, zinc immersion was the best treatment for aluminum deposition onmagnesium substrate in choosed electrolyt. Under the condition of1.0V to2.0V and6hours¹2hours, an uniform, silvery, compact aluminum coating with goodadhesion on magnesium matrix can be obtained with1μm³Î¼m partical size,10μm²0μm thickness, mass fraction being over90%. The corrosion resistance of thesamples after zinc immersion and aluminum electrodeposition was better thanmagnesium matrix. Analyses of the cyclic voltammograms indicated that thedeposition process of aluminum on zinc immersion magnesium substrates was airreversible process.