Electrodeposition of Al-W Alloys in AlCl3-NaCl-KCl Molten Salt Containing WCl4

Abstract

Introduction Aluminum and aluminum alloys are widely used in a variety of industrial products like cars and electric appliances due to the lightweight and excellent electrical and thermal conductivity. Furthermore, aluminum oxide which is very thin and chemically stable forms on the surface. The oxide film contributes the corrosion resistant property of the surface. However, in the presence of halogen ions (ex. Cl-, F-, I- and Br- ions), corrosion takes place for the oxide surface. Al‐transition metal alloys have better corrosion resistance for chloride solution. Al-W alloys especially show the good corrosion resistance for pitting corrosion of the surface. Al-W alloys are also expected the increase of the hardness compared with pure Al. One of the available method to obtain Al alloys is an electrodeposition. Tsuda et al. previously reported the electrodeposition of Al-W alloy in the Lewis acidic AlCl3-[EtMeIm]Cl room temperature ionic liquid1). When higher temperature molten salts are used for electrodeposition of Al-W alloys, rapid electrodeposition may be achieved. AlCl3-NaCl-KCl molten salt is useful melt because the eutectic point of this system is 366 K at the composition of AlCl3 : NaCl : KCl = 61 : 26 : 13 (molar ratio). In this study, we report the mechanism of electrodeposition of Al-W alloys in the AlCl3-NaCl-KCl melt and discuss the relationship between the composition of the electrodeposit and the electrolysis potential. Experimental Electrodeposition of Al-W alloys was carried out in AlCl3-NaCl-KCl melt containing 10 mM WCl4 at 423 K. The electrochemical cell was used three-electrode system. A glassy carbon plate or a pure copper plate was used as working electrode at the voltammogram measurements or constant current electrolysis, respectively. An aluminum plate or a tungsten plate was used as counter electrode. Pure aluminum wire placed in a Pyrex glass tube filled with the AlCl3-NaCl-KCl melt was used as a reference electrode. All electrochemical measurements were performed under air atmosphere. The composition, surface morphology and identification of the electrodeposits were used X-ray fluorescence spectrometer (XRF), scanning electron microscope (SEM) and X-ray diffraction (XRD) instrument. Results and discussion A voltammogram was measured between 1.5 V and -0.2 V vs. Al/Al (III) at a sweep rate of 0.01 Vs-1 on a glassy carbon electrode in the AlCl3-NaCl-KCl melt with or without WCl4. In the melt without WCl4, the voltammogram showed a cathodic current at lower than -0.03 V with steep increase. An anodic current was observed from -0.02 V in the reverse scan. The cathodic and anodic currents correspond to the Al deposition and dissolution reactions, respectively. In the melt containing WCl4, two small cathodic currents were observed from 1.05 V and 0.45 V in the cathodic scan. The two cathodic currents correspond to a reduction of tungsten ions. When the potential is reversed at -0.2 V, an anodic current appeared from 0 V. The anodic current corresponds to dissolution of Al and Al-W alloy formed by the previous cathodic scan. In the results, the reduction steps of W ions lower than 1.05 V may occur from W (IV) to W (III), and from W (III) to W (0) below 0.45 V. To examine the relation between the electrolysis potential and the composition of the electrodeposits, constant potential electrolysis of Al-W alloys was carried out on Cu substrate at the potential range from -0.05 V to 0.4 V with electric charge of -30 Ccm-2(theoretical film thickness, 10 μm). The results suggested that when the electrolysis potential becomes higher, the W content in the electrodeposits increases. SEM images of the Al-W alloy prepared at -0.05 V and -0.025 V showed size of particle of about 5 μm, and cover the entire surface uniformly. On the other hand, it was found that a relatively small electrodeposit was formed on the substrate at higher than 0 V. The results of XRD showed the peaks corresponding to Al when the W content is less than 1 at%. When the W content is higher than 19 at%, Al peaks disappeared. However, broad peak showing the presence of an amorphous phase of W was observed at 0.4 V. Reference 1) Tetsuya Tsuda et al, J. Electrochem.Soc. 161, D405 (2014). Acknowledgment This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).