Probing Barrier Oxide Layer of Porous Anodic Alumina by In Situ Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is a unique tool for both ex situ and in situ studies of the barrier layer thickness of anodic aluminium oxide (AAO). Here EIS is applied to study porous AAO formed by anodizing of high-purity Al foils in 0.3 M sulphuric acid and 0.3 M oxalic acid electrolytes. It is shown that in the high-frequency range, in situ EIS spectra coincide with the ex situ ones. Thus, the thickness of the barrier layer at the pore bases does not change after the end of the anodic polarization. The barrier layer thickness (h) is governed by the anodizing potential (U) according to the equation h = 1.03(2) nm·V−1·U + 2.2(3) nm. Features of the current transients observed during potentiostatic anodizing are discussed. It has been proved that a local maximum of current at the initial stage of anodizing corresponds to the maximum of the aluminium electroactive surface area arising due to the rearrangement of pores.