Indirect oxidation of aluminum under an AC electric field

Anodic porous alumina films have attracted substantial attention and have found various applications because of their unique nanometric-scale porous structure. Though direct oxidation to form anodic porous alumina films has been studied for the protection or design of aluminum surfaces, the development of a practical process for the fabrication of porous alumina on a large number of small aluminum products, such as fine particles, has not yet been achieved. In this paper, we report the successful formation of porous alumina films by indirect oxidation under an alternating-current electric field without a direct electrical connection. Importantly, despite the indirect wireless oxidation, porous alumina films grew uniformly even in the case of aluminum particles used as an object. As with conventional direct-current anodization, film thickness increased with increasing delivered electricity. These results open a new route to technological and scientific applications of aluminum-based products.