Electrodeposition of Al–W Alloy Films Using W(II) Salts Synthesized Using Different Methods
Authors: Higashino Shota , Miyake Masao , Ikenoue Takumi , Hirato Tetsuji 
Published: February 2020

Introduction

Electrodeposition of Al and its alloys in molten salts, ionic liquids, and organic solvents has been extensively studied to obtain oxidation and corrosion-resistant coatings. Among Al-based binary alloys, Al–W alloys with a W content of ~10 at.% are known to show the highest resistance to chloride-induced pitting corrosion1
–
2. Several studies have been conducted on the electrodeposition of Al–W alloys using chloroaluminate baths containing WCl4 and K3W2Cl9
2–4. We reported on the electrodeposition of dense Al–W alloy films with a high W content exceeding ~12 at.% using 1-ethyl-3-methylimidazolium chloride (EMIC)–AlCl3 ionic liquid containing a W(II) salt, W6Cl12 (WCl2)5
–
7. The W6Cl12 used in our studies was more soluble in EMIC–AlCl3 baths than the other W salts used in the previous studies.
The W6Cl12 that we used in our previous studies was obtained from (H3O)2[W6Cl14]·7H2O by annealing. The resulting salt had a Cl/W atomic ratio of 2, and its X-ray diffraction pattern agreed with that of poorly crystallized W6Cl12 reported in the literature. 8 Accordingly, we determined that this W salt was W6Cl12. However, it was recently found that well-crystallized W6Cl12 obtained by annealing this W salt at a higher temperature had a much smaller solubility in the EMIC–AlCl3 bath. Because the W salt was obtained from (H3O)2[W6Cl14]·7H2O, it could contain water as a impurity, which could affect the solubility and electrochemical behavior of this salt.
W6Cl12 can also be synthesized by disproportionation of WCl4 in a vacuum. This synthesis method is considered to yield much purer W6Cl12 than that from (H3O)2[W6Cl14]·7H2O. In this study, we investigated the solubility and electrochemical behavior of W6Cl12 synthesized using these two different methods.

Experimental

An electrolytic bath was prepared by mixing EMIC, anhydrous AlCl3, and W6Cl12. The molar ratio of EMIC and AlCl3 was 1:2. W6Cl12 was prepared using two different methods. One was annealing of (H3O)2[W6Cl14]·7H2O, which was produced by recrystallization of W(II) chloride produced by the reduction of WCl6 with Bi. Annealing of (H3O)2[W6Cl14]·7H2O was conducted at 325–450°C. The other method involved the disproportionation of WCl4 to W6Cl12 and WCl5, which was performed by heating WCl4 at 350–500°C in a vacuum. The electrochemical behavior of W­6Cl12 treated under different conditions was investigated by cyclic voltammetry and potentiostatic/galvanostatic electrolysis in the EMIC–AlCl3 baths at 80°C. All electrochemical experiments were conducted in an argon-filled glove box. The electrodeposited films were analyzed by scanning electron microscopy, energy dispersive X-ray, and X-ray diffraction. Current efficiency for the electrodeposition was determined by inductively coupled plasma atomic emission spectroscopy.

Results and discussion

W6Cl12 obtained by annealing of (H3O)2[W6Cl14]·7H2O at 325°C dissolved in EMIC–AlCl3 baths up to 49 mM, but the solubility decreased with increasing annealing temperature. W6Cl12 obtained by the disproportionation of WCl4 hardly dissolved in an EMIC–AlCl3 bath, irrespective of the disproportionation temperature. Potentiostatic electrodeposition in a bath containing W6Cl12 produced by the disproportionation of WCl4 yielded Al film with a trace W content of <0.1 at.%. These results indicate the W6Cl12 obtained by annealing of (H3O)2[W6Cl14]·7H2O at a relatively low temperature was more suitable as a W precursor for Al–W alloy electrodeposition than that obtained by the disproportionation of WCl4.

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