Focusing on the coulombic efficiency of film formation, we systematically investigated the effects of polyhydric alcohols added to a sulfuric-acid-based electrolyte on the anodization behavior of aluminum, the coating ratio, the porosity, and the maximum attainable film thickness of the anodic oxide film by comparison with the effects of monohydric alcohols. Anodization was conducted in sulfuric acid containing various alcohols with different physical properties at a constant current density of 100 A·m−2 and at 20 °C. The viscosity and conductivity of the mixed electrolyte were affected by the addition of alcohol, resulting in an increase of the steady-state voltage with increasing alcohol concentration because of the high viscosity and low conductivity of the mixed electrolyte. Although the coating ratio, which is an index value used to evaluate the efficiency of the film formation, and the maximum attainable film thickness also increased with the addition of any of the alcohols, the addition of a polyhydric alcohol was more effective than that of a monohydric alcohol. When sulfuric acid with 50 vol% ethylene glycol was used as an electrolyte, an alumina film with less than 10% porosity was formed with high coulombic efficiency of film formation.

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Previously, we studied the anodization of aluminum by adding alcohol to common acidic electrolytes with a focus on film formation efficiency and the hardness of the anodic oxide film. In this study, we focus on the difference in carbon number of monohydric alcohols and select propanol as an additive to confirm whether the effects of adding alcohol on anodization behavior and the growth rate of anodic film are universal regardless of the alcohol type. The tendency of propanol concentration dependence of conductivity and viscosity was generally consistent with results obtained using methanol and ethanol additives; however, unlike other alcohols, the steady state voltage during constant current anodization decreased with an increased amount of propanol. Unlike sulfuric acid only, the addition of propanol clearly improved film growth rate and current efficiency under mild conditions below 100 A‧m−2 and high current density conditions above 500 A‧m−2.

The properties of anodic porous alumina films, which are generally formed by the anodization of aluminum in an acid electrolyte, are well known to be affected by the dimensions of their structure (e.g., pore diameter, pore density, and film thickness). For instance, to increase the hardness of anodic films on aluminum, the anodization is industrially carried out at low-temperature to avoid chemical dissolution of the formed oxide film during anodization. Anodization in an electrolyte containing organic compounds is also one approach for improving the performance of anodic oxide film not only for aluminum but also for other light metals such as titanium and magnesium (1). One such initiative is anodization of aluminum at a relatively high current density or high voltage using a common acidic electrolyte (e.g., sulfuric acid, oxalic acid, or phosphoric acid) with an alcohol added. In such studies, the effects of adding an alcohol are summarized as follows: i) the alcohol functions as an anti-freeze agent in the electrolyte, enabling the anodization temperature to be lowered; ii) the alcohol acts as a cooling agent—specifically, the vaporization of alcohol at the bottom of the pore channels removes the heat generated at the aluminum/oxide interface; and iii) nonuniform growth of the anodic film (i.e., burning) is effectively prevented, even during high-field anodization. However, these effects are observed not only for monohydric alcohols with a boiling point less than 100 °C (e.g., methanol and ethanol) but also for polyhydric alcohols with a boiling point greater than 100 °C (e.g., ethylene glycol and glycerol). Therefore, the effect of alcohol addition cannot be reasonably explained solely on basis of the boiling point of the alcohol used as an additive. Understanding the effects of alcohol addition necessitates a systematic investigation comparing types of alcohols, in addition to further fundamental studies. In this study, monohydric alcohols or polyhydric alcohols were added to the sulfuric-acid-based electrolyte to examine their effects on the anodization behavior of aluminum, the coating ratio, the porosity, the maximum attainable film thickness, and the resulting etching ability. Anodization was conducted in sulfuric acid solutions containing various alcohols with different physical properties under mild anodization conditions with a constant current of 100 A·m−2 at 20 °C (2, 3). The influence of the type and concentration of alcohol on the coulombic efficiency of film formation and the structure of the porous alumina under a constant electric charge was also investigated. In terms of basic research, the potential impact of alcohol addition in electrolytes will be discussed. H. Asoh, K. Asakura, H. Hashimoto, RSC Advances, 10, 9026-9036 (2020). H. Asoh, M. Matsumoto, H. Hashimoto, Surface and Coatings Technology, 378, 124947 (2019). M. Matsumoto, H. Hashimoto, H. Asoh, Journal of The Electrochemical Society, 167, 041504 (2020).