Electrodeposition of aluminum on magnesium from ionic liquid (EMIM)Br-AlCl₃

ZHANG Li-peng(张丽鹏), YU Xian-jin(于先进), DONG Yun-hui(董云会),

LI De-gang(李德刚), ZHANG Ya-li(张亚莉), LI Zhong-fang(李忠芳)

School of Chemical Engineering, Shandong University of Technology, Zibo 255091, China

Received 6 July 2009; accepted 30 December 2009

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Abstract: An ionic liquid was prepared by mixing AlCl₃ and 1-methyl-3-ethylimidazolum bromide (EMIM)Br under dry argon atmosphere. Electrodeposition of aluminum on magnesium was conducted at 298.15 K for 1 h by the ionic liquid and the electrochemical behavior was discussed. The results show that the aluminium deposition occurs at a potential about -0.1 V. Cathode surface was analyzed by SEM, XRD and EDS. Aluminum is successfully electrodeposited on magnesium from (EMIM)Br-AlC1₃ ionic liquid, and the crystal grain quality of the deposit at 15 mA/cm² is ideal with the perfect boundary of crystal grain.

Key words: aluminum; ionic liquid; (EMIM)Br-AlC1₃; electrodeposition

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Magnesium is a very important metal because of its excellent mechanical properties and lowest density among all of the metals. So far, the use of magnesium has been already extended into civil industry department. Although magnesium is very stable at room temperature under dry atmosphere, it gets eroded when exposed to wet environment. Furthermore, aluminum film shows advantageous anti-corrosion performance. Hence, the electrodeposition of aluminum on magnesium can protect this metal against corrosion.

Ionic liquid, also known as room temperature ionic liquid (RTIL), is an organic salt that consists entirely of anion and cation. Due to the large nonsymmetrical organic cations the melting point is less than 100 ℃[1-3]. The room temperature ionic liquids such as BPC/AlCl₃ and EMIC/AlCl₃ that are considered the most promising electrolytes are studied widely. They have many excellent properties: wide electrochemical window, good conductivity, negligible vapor pressure (at high temperature the vapor pressure is small), good solubility for inorganic and organic materials, high thermal stability and non-flammability, and adjustable acidity[4-9]. In recent years, there are many new reports[10-16] about electrodeposition of aluminum and its alloy from ionic liquids. The morphology, crystalline size and compactness of the aluminum coating are usually dependent on the electroplating parameters, such as current density. In this work, ionic liquid (EMIM)Br-AlCl₃ was prepared by AlCl₃ and 1-methyl-3-ethylimidazolum bromide (EMIM)Br, and the aluminum electrorefining with three-electrode system was studied. The effects of current density on morphology and crystalline size of aluminum coating were also investigated.

2 Experimental

The preparation of electrolyte and following electrodeposition experiments were conducted under inert gas condition. (EMIM)Br was synthesized by adding certain amount of bromoethane(＞99%) and N-methylimidazolum(>99%) into a conventional three- neck flask, and the mixture was continuously stirred by a slow magnetic bar for 4 h under dry purified argon atmosphere. To ensure complete reaction, it needs excessive amount of bromoethane. After drying in vacuum at 343 K for 12 h to remove residual moisture, the intermediate was moved into an indigenous glove box (Nanjing University Instrument Plant, Model NDZKX, China) under nitrogen atmosphere. An acidic ionic liquid of AlCl₃-EMIMBr with molar ratio of 2?1 was prepared by slow addition of AlCl₃.

All electrochemical measurements were conducted by IM6e electrochemical workstation. Constant current depositions were performed with a three-electrode electrolytic cell in a glove box filled with nitrogen. Magnesium was used as the cathode, aluminum was employed as the anode, and platinum was used as reference electrode. After electrodepositon the samples were washed with ethanol and dried in air, and then the surface appearance of sample was examined with a scanning electron microscope (SEM, Veeco Multimode NS3a) and the elements of the sample were analyzed by X-ray energy diffraction spectrometer (EDS, Oxford INCA Energy), and the crystal phase structures were examined with D8 advance type X-ray diffractometer (XRD, Brucker.axs Company, German).

3 Results and discussion

3.1 Cyclic voltammetry behavior during electro- deposition process

The electrodeposition process of aluminium on magnesium substrate from (EMIM)Br-AlCl₃ ionic liquid with molar ratio of 2?1 was investigated. Fig.1 shows a typical cyclic voltammogram recorded on magnesium electrode at a scan rate of 50 mV/s. It is apparent that the cathodic process in curve a corresponds to the deposition of aluminium, and aluminium deposition occurs at the potential about -0.1 V, whereas the anodic peak in curve b is ascribed to the subsequent stripping of the deposited aluminium. The anodic oxidation current decreases to almost zero at 0.5 V due to passivation of the aluminium. The passivation behaviour of aluminium was previously found to occur in AlCl₃-based molten salts containing KCl-NaCl, LiCl, or TMPAC ionic liquids[17]. Although the anodic passivation behaviour of aluminium in chloroaluminate electrolytes has not been satisfactorily elucidated, it could be attributed to AlCl₃ precipitation on the electrode surface, or to the adsorbed monovalent to trivalent aluminium intermediates[17].

Fig.1 Typical voltammogram recorded at scan rate of 50 mV/s

3.2 Analysis of electrode after electrodeposition

After electrodeposition, electrodes were removed and cleaned carefully. SEM was used to observe the surface appearance of anode, and the new cathode aluminum was analyzed by EDS and XRD.

After electrodeposition, it can be seen that aluminum can be electrodeposited by using ionic liquids at room temperature, and the cathode becomes silver-gray. The morphologies and crystalline sizes of aluminum electrodeposit coating microstructures are different at different current densities. The surface morphologies of the deposits obtained at current densities of 5, 10 and 15 mA/cm² were further inspected by SEM, as shown in Fig.2. The deposits at a current density of 5 mA/cm² are not continuous, consisting of polygon crystals with many porosities on the surface. The deposits at a current density of 10 mA/cm² are relatively dense and glossy, and have polygon crystals with some porosities on the surface. The deposits at a current density of 15 mA/cm² are denser and have metallic luster, composing of 1-2 ?m cuboidal crystals, but the deposits have less porosities on the surface. The aluminum crystal grain size is uniform, crystal grain growth integrates, and the crystals grain boundary is clear and perfect.

Fig.2 SEM images of cathode surface at different current densities: (a) 5 mA/cm²; (b) 10 mA/cm²; (c) 15 mA/cm²

In order to further analyze the new aluminum in the experiment, the cathode surface of fresh aluminium was analyzed by X-ray emission spectrum analysis. The spectrum of the new aluminium on the cathode by X-ray energy dispersive spectrum (EDS) is shown in Fig.3. It can be seen that there are no impurities on the cathode surface. Strong peaks indicate aluminum elements, and weak peaks denote oxygen elements with a low content. Because after electrodeposition the new aluminum is passivated during cleaning electrode, oxide films form on the surface of the grain aluminium, and as a result the weak peaks of oxygen elements occur on the spectrum.

Fig.3 Energy spectrum of cathode surface

Cyrstal structure for the above deposits at 15 mA/cm² was analyzed by XRD. Fig.4 shows the acquired diffraction pattern. As seen from Fig.4, the diffraction peaks attributed to pure aluminum with face-centred cubic structure (FCC) are clearly detected from the sample. The diffraction patterns for deposits are composed of (111), (200), (220) and (311) faces. Peaks of (111) and (311) faces have high intensity, revealing the preferred stacking orientation of the electrodeposit.

Fig.4 XRD pattern of cathode surface

4 Conclusions

1) Aluminum is successfully electrodeposited on magnesium from (EMIM)Br-AlC1₃ ionic liquid.

2) The deposits at 15 mA/cm² are denser and have metallic luster, composing of 1-2 ?m cuboidal crystals, but the deposits have few porosities on the surface.

3) Aluminum crystal grain size is uniform, crystal grain growth is integrated, and the crystals grain boundary is clear and perfect.

4) The deposits at 15 mA/cm² are composed of (111), (200), (220), and (311) faces, presenting preferred stacking orientation of the electrodeposit.

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Foundation item: Project(50374045) supported by the National Natural Science Foundation of China; Project(Y2007F60) supported by the Natural Science Foundation of Shandong Province, China; Project(J09LB59) supported by the High Education Science Technology of Shandong Province, China; Project(BS2009NJ007) supported by the Midlife-Youth Scientists Research Encouraging Foundation of Shandong Province, China; Project(2008) supported by the Zibo Science Technology Development of Shandong Province, China

Corresponding author: YU Xian-jin; Tel: +86-533-2781681; E-mail: xjy@sdut.edu.cn

(Edited by CHEN Wei-ping)