Dissolution Behavior of Hydrated Tungsten Chlorides in Chloroaluminate Ionic Liquids and It's Application in Aluminum–Tungsten Alloy Electrodeposition

Abstract

Introduction The demand for aluminum (Al) alloys as coating materials have increased owing to their high corrosion and oxidation resistance. In addition, electrodeposition technique has attracted attention for obtaining Al alloy coatings, and various non-aqueous electrolytic baths for Al alloy electrodeposition, including ionic liquids, have been developed. Alloying Al with refractory metals, such as tungsten (W) and molybdenum (Mo), significantly improves the pitting-corrosion resistance of Al. However, the salts of these metals have relatively low solubility in Al electrodeposition baths, and thus, it is difficult to obtain Al alloys with a high content of these metals.In our previous study, we reported that a W(II) salt, namely W6Cl12, which is dissolved in 1-ethyl-3-methylimidazolium (EMIC)–AlCl3 ionic liquids at a higher concentration than other salts such as WCl4, enables the electrodeposition of Al–W alloy films with a high W content[1]. However, the W6Cl12 used previously turned out to contain water due to the synthesis process, which may have increased the solubility of W6Cl12.In this study, we investigated the effect of water content on the solubility of W precursors in ionic liquids. Herein, three W precursors with different water contents were employed, namely anhydrous W6Cl12 and hydrated complexes W6Cl12·2H2O and (H3O)2[W6Cl14]·7H2O (Figure 1). Al–W alloy was electrodeposited using baths containing different W precursors. In addition, the chemical speciation of the hydrated complexes in the ionic liquids was conducted using, Raman, and Fourier transform infrared (FT-IR), ultraviolet-visible (UV-Vis) spectroscopy. Experimental Herein, three electrolytic baths were prepared by adding anhydrous salt W6Cl12, hydrated complex W6Cl12·2H2O, and (H3O)2[W6Cl14]·7H2O to an EMIC–AlCl3 ionic liquid with EMIC:AlCl3 molar ratio of 1:2. The saturation concentrations of W precursors were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Potentiostatic electrodeposition was performed on copper substrates at a bath temperature of 80 °C under an Ar atmosphere in a glove box. The W content, current efficiency, and impurity content derived from the hydration water in the electrodeposited material were evaluated by energy dispersive X-ray spectroscopy (EDX), ICP-AES, and X-ray photoelectron spectroscopy (XPS), respectively. The dissolved species in the baths were analyzed by Raman, FT-IR, and UV-Vis spectroscopy. Results and Discussion The saturation concentration of each W ion source in the EMIC–AlCl3 ionic liquid was less than 1 mM for anhydrous W6Cl12; however, it increased to ~80 and 120 mM for W6Cl12·2H2O and (H3O)2[W6Cl14]·7H2O, respectively. Potentiostatic electrodeposition at potentials lower than 0 V (vs. Al/Al(III)) produced films consisting of Al and W. The W content increased to ~1 and 15 at.% from the anhydrous salt bath and hydrated complex bath, respectively. The current efficiency for electrodeposition was close to 100%, even in baths with hydrated precursors. EDX and XPS measurements of Al–15 at.% W alloy film obtained from the hydrated complex bath showed that Al and W existed in metallic states in the film.The Raman spectra of each bath showed that the Al2Cl7 − concentration decreased and AlCl4 − concentration increased in the hydrated complex bath compared to the anhydrous salt bath. This suggests that the hydrated W complex reacted with AlCl3 to form other Al species, thereby decreasing the apparent AlCl3/EMIC ratio in the bath. The FT-IR spectra of baths with the addition of hydrated complex or liquid water showed that both the hydrated water and liquid water reacted with Al anions to produce Al–OH–Cl species[2]. The UV-Vis spectra of the anhydrous salt bath and hydrated complex baths showed two prominent peaks: one at 240 nm (λ1) and the other at ~270–300 nm (λ2). λ1 was almost constant for all the W precursors, whereas λ2 shifted to a lower wavelength for the hydrated complex baths. The dissolved W species estimated from these spectroscopic data and their relationship with solubility are discussed at the meeting.