Electronic Current Leads to the Rapid Growth of Porous Anodic Alumina with Cavity Structures

In this paper, anodization of aluminum strips was carried out in a mixed electrolyte of oxalic acid and ammonium hypophosphite, and a unique porous anodic alumina (PAA) with cavity structures was discovered. By dividing the current–time curve during its anodizing process into five stages and conducting anodization step by step, the growth process of PAA with cavity structures was restored. The samples obtained from the above experiments were characterized by FESEM and analyzed by EDS. The FADT cannot effectively analyze the formation process and causes of PAA with cavity structures. By analyzing it with the help of the viscous flow model, the electronic current theory and the oxygen bubble model effect, the formation mechanism of this special structure can be well explained. The complex anodizing current–time curve was explained by the electron current theory rather than the field-assisted dissolution theory. We have discovered that the main reason for the formation of this structure is that the balance between the electronic current and the ionic current is disrupted, leading to the rate of oxygen generation being much higher than the rate of oxide growth.