Effect of Pore Depth on Thickening the Barrier Layer of Porous Anodic Aluminum Oxide

The insulating properties of anodic layers of aluminum are one of their most important parameters, as they determine not only corrosion resistance, but also the possibility of using anodized aluminum for metal printed circuit boards. In the present work, nanoporous anodic aluminum oxide with different pore depth obtained by anodization in 1 M sulfuric acid at 20 V was reanodized in boric acid up to 300 V for barrier layer thickening. The change in thickness, composition, and impedance of barrier anodic layers with increasing pore depth were investigated by means of scanning electron microscopy, energy-dispersive X-ray analysis, and electrochemical impedance spectroscopy, respectively. The obtained results demonstrate that after 15 min reanodization at 300 V the increasing of the porous layer thickness from 2 to 12 μm induces increase in the steady-state current density by almost 500% and thickening of the barrier layer from 250 to 630 nm. As a results of these findings, we have formulated a hypothesis according to which increasing the depth of the pores favors the electrolyte species incorporation at the bottom of the pores and within the pore walls vicinity, which leads to lowering the impedance of the complex anodic layers.