The impedance characteristics of barrier-type oxide layers on aluminum was
studied using impedance spectroscopy. Since anodic films on Al have a varia
ble stoichiometry with a gradual reduction of oxygen deficiency towards the
oxide-electrolyte interface, the interpretation of impedance spectra for o
xide layers is complex and the impedance of surface layers differs from tho
se of ideal capacitors. This frequency response of the layer with conductan
ce gradients cannot be described by a single resistance-capacitance (RC) el
ement. The oxide layers of Al are properly described by the Young model of
dielectric constant with a vertical decay of conductivity.