The Relation Between Environmental pH, Porosity, and the Impedance of Anodic Aluminum Oxide Films

Abstract

The capacitance and resistance of an electrochemical cell have been measured to determine the effect of the pH (3.6–8.2) of aqueous potassium nitrate and perchlorate electrolytes upon 360Aå thick anodic oxide films formed on polycrystalline 99.99% aluminum. While the film has been found to be quite stable upon exposure to electrolytes having pH values in the range 5.1–8.2, frequency dependent capacitance increases and resistance decreases have been observed during a 3000 min test period for cells having electrolytes of pH 3.6. The data for electrochemical cells having electrolytes of pH 3.6 have been corrected for the presence of the electrolyte by two electrical analogs. This corrected aluminum cell data represent average anodic oxide film properties. The authors have interpreted the large capacitance increases and resistance decreases of electrochemical cells having an electrolyte of pH 3.6 in terms of the growth of pores in the anodic oxide film. The data for cells having electrolytes of pH 3.6 have been quantitatively analyzed as a function of exposure time and frequency by modeling the electrochemical cell as a parallel combination of two parallel RC circuits in series with a third parallel RC circuit. Transmission electron microscopy of films before and after testing supports the interpretation of the data in terms of a porous film structure. Some comments are forwarded on the defect structure of the film before and after environmental exposure.