Enhancing the Surface Hydrophilicity of an Aluminum Alloy Using Two-Step Anodizing and the Effect on Inkjet Printing Characteristics

Aluminum alloy anodizing is widely used in the surface treatment industry to provide surface protection and decoration. The resulting anodic aluminum oxide film enables durable printing and dyeing of metals owing to its nanoporous structure, which easily absorbs ink. Conventional one-step anodizing of Al 1050 using sulfuric acid was observed to form a surface with small pore diameters less than 10 nm and lead to an average contact angle of 30°, whereas two-step anodizing yielded a regular pore pattern with significantly larger pores, reducing the contact angle to less than 20°. This change in pore structure and the corresponding enhanced hydrophilicity directly impacted inkjet printing characteristics; inkjet printing of 13 pL droplets on the one-step and two-step anodized surfaces showed that the average dot diameter varied from 72.2 μm to 48.0 μm according to applied voltage and anodizing time. The ink dot diameters on the two-step film were smaller than those on the one-step film produced under the same conditions, and the dot diameters decreased as the average pore diameter increased under an increasing anodizing voltage up to 20 V, indicating improved hydrophilicity. The pore volumes produced by two-step anodizing were larger, facilitating ink droplet absorption during spreading, which was examined by elemental analysis of cross-sections of the ink-filled porous specimen.