In general, industrial anodization based on direct current or alternating current (AC) electrolysis can be applied to the surface treatment of large-sized and long aluminum products such as building materials. However, conventional anodization is unsuitable for the simultaneous surface treatment of a large number of small components including fine particles because direct conduction between the aluminum anode and the cathode is essential for completing the electrical circuit.

Recently, we proposed new surface treatment method, i.e., indirect oxidation and investigated the influences of electrolysis conditions such as voltage, frequency, reaction time, concentration of electrolyte, and temperature on the growth of porous oxide film [1]. In our previous study, high-purity (99.99 %) aluminum sheets with a working area of 10 cm² and aluminum particles (99.5 %) of 3 mm diameter were used as processing objects. In the case of AC electrolysis in 8 mmol dm⁻³ oxalic acid at 20 °C, 60 V_M, and 150 Hz, which were the base condition in this study, the current density was approximately 100 A m⁻². When AC electrolysis was conducted at higher temperatures and voltages using a high concentration of oxalic acid compared with that in the base condition, a high current density in the 120–200 A m⁻²range was observed. Frequency had relatively small effect on current density. Even in the indirect oxidation, we observed that the relationship between the film thickness and the AC electrolysis time was approximately linear. We also used aluminum particles as a processed material to demonstrate the versatility of the indirect-oxidation-based surface treatment [1]. After indirect oxidation for 240 min under the base condition, the treated aluminum particles were immersed in dye solution. The dye was absorbed in porous alumina films over the entire surface of the aluminum particles. This result indicates that the porous films were uniformly and simultaneously formed on aluminum particles, i.e., the present technique can accommodate a wide variety of particle shapes and sizes with high throughput and low costs.

In this study, we carry out a preliminary study to investigate the effect of frequency on the structure of porous alumina formed by indirect oxidation. We mainly focus on the difference between the proposed method and conventional AC anodization. The correlation between the cell dimensions and applied voltage will be also discussed.

[1] H. Asoh, M. Ishino, H. Hashimoto, RSC Adv., 6, 90318 (2016).