Al–W alloys are known to exhibit high corrosion resistance and mechanical strength. Thus, Al–W alloys have attracted much research attention as corrosion-protective coatings for reactive materials such as Mg alloys and steels.^1–3 Recently, our research group reported that dense Al–W alloy films can be electrodeposited from 1-ethyl-3-methylimidazolium chloride (EMIC)–AlCl₃ ionic liquids containing W₆Cl₁₂ as the tungsten precursor.³

In this study, we explored the feasibility of anodization of electrodeposited Al–W alloy films with the aim of providing the films with additional functionalities. Anodization of Al metal yields a porous Al₂O₃ layer on its surface. Hence, the anodization of Al–W alloys should potentially yield a porous Al₂O₃ layer containing WO₃. WO₃is a photocatalytic material sensitive to visible light and decomposes pollutant organic compounds under visible light illumination. Therefore, the electrodeposition and anodization of Al–W alloy films can be a facile process to produce corrosion-protective coatings with additional functionalities such as water purification and self-cleaning.

Only a few reports on the anodization of Al–W alloy films are available in literature,⁴and the anodization behavior of Al–W alloy films under different anodization conditions has not been elucidated. In addition, the properties of anodized films have not been investigated. In this study, we investigated the effect of anodization conditions on the microstructure of the porous layer obtained after the anodization of the Al–W alloy films. The photocatalytic activity of the anodized films was also evaluated.

Al–W alloy films containing ~12 at% W were electrodeposited from an EMIC–AlCl₃ ionic liquid containing W₆Cl₁₂. The anodization of the Al–W alloy films was carried out by applying 40 V between them and a platinum cathode in an oxalic acid solution for up to 40 min. The cross-sectional SEM images of the films revealed that pores with diameters of 10–30 nm were formed on their surface. These pores extended up to ~2 μm in the direction toward the substrate. The anodized films displayed bright interference colors, which varied with the anodization conditions. The photocatalytic activity of the anodized films was evaluated by measuring the photo-decomposition rate of Congo Red (CR) in aqueous solution. The anodized film was immersed in a 1 μM solution of CR and was illuminated with a xenon lamp (wave length = 350–750 nm). The anodized films decomposed more than 30% of CR in 180 min. The decomposition performance of the anodized films was higher than that of the as-deposited film.

1. T. Tsuda, Y. Ikeda, T. Arimura, M. Hirogaki, A. Imanishi, S. Kuwabata, G. R. Stafford, and C. L. Hussey, J. Electrochem. Soc., 161, D405 (2014).

2. K. Sato, H. Matsushima, and M. Ueda, ECS Trans., 75, 305 (2016).

3. S. Higashino, M. Miyake, H. Fujii, A. Takahashi, and T. Hirato, J. Electrochem. Soc., 164, D120 (2017).

4. S. J. Garcia-Vergara, P. Skeldon, G. E. Thompson, and H. Habazaki, Surf. Coat. Technol., 201, 9506 (2007).