Opaque White Anodic Oxide Film Formed on Aluminum

Anodic porous alumina film, which is formed by anodizing of aluminum in an appropriate acidic solution, has been studied on the protection or design of aluminum surfaces owing to commercial applications. The appearance and surface quality of aluminum products (e.g., architectural materials, electronic devices) are key points as with surface properties such as a corrosion resistance, hardness and wear resistance. Although dyeing, painting and electrolytic coloring are well known as industrial methods to provide color variation for aluminum surface, integral coloring, which produces color by anodizing step itself without use any pigment or metallic salt, is also applied in terms of the simplicity and production cost. In our previous work, it was reported that alumina films with irregular porous structure formed in Ematal bath or chromic acid showed opaque white appearance [1, 2]. Irregular porous structure in the direction of pore depth, which contained branching, curving and distortion of pores in anodic alumina, produced the roughening of the metal-oxide interface and made the appearance of the film more opaque [1, 2]. However, Ematal bath containing expensive potassium titanium oxalate deteriorates easily. In terms of reduction of environmental burdens, it is required to avoid using harmful chemical compound such as chromic acid.  Under such background, we studied a low-cost alternative anodizing method to form opaque white oxide film. In addition, we also investigated in detail the relation between the appearance and porous structure of oxide film.

Anodizing was conducted using high purity (99.99 %) aluminum sheets in various electrolytes (phosphoric acid, sulfuric acid, critic acid, and Ematal bath). Whiteness of the obtained samples was evaluated by spectrophotometry (KONICA MINOLTA, CM-5) based on standard of American Society for Testing Materials (ASTM E313-73).

When anodizing was carried out in phosphoric acid at 60 °C at a constant current density of 200 Am⁻², anodized specimen indicated whiteness value of 39. Surface appearance was gray in color. The aluminum specimen anodized in Ematal bath showed whitish appearance compared with that treated in phosphoric acid. Whiteness value of aluminum specimens anodized in Ematal bath increased from 35 to 47 with increasing final arrival voltage from 150 V to 350 V during constant current anodizing. On the other hand, whiteness value increased effectively using two-step anodizing; long first anodizing and short second anodizing were conducted in phosphoric acid and subsequently Ematal bath. Under optimized conditions, whiteness value increased to 58. The more increase of the thickness of total oxide film and barrier layer and the degree of branching, the more white appearance indicated. Thus, it is confirmed that the appearance of anodized specimen was strongly affected by these factors.

[1] S. Ono, S. Chiaki, T. Sato; J. Met. Finish. Soc. Jpn., 26, (10), 456-460 (1975).

[2] S. Ono, T. Sato; J. Met. Finish. Soc. Jpn., 31, (3), 134-139 (1980).