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 1.0MKNO3 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 1.0MKNO3 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.

Barrier‐Type Aluminum Oxide Films Formed under Prolonged Anodizing: II . Dielectric Characteristics

“…2. Similar problems have been encountered using alternate correction methods (6). These problems did not occur with the 1M electrolyte.…”

“…The electrode assembly, comprised of two Teflon gaskets compressed against the parallel faces of a prepared specimen, and the electrochemical cell used for anodizing and testing have been described previously (6). The cells were maintained at a constant temperature of 30.0 ~ • 0.3~ during both anodizing and testing by a conventional water bath.…”

On the impedance of the silicon dioxide/electrolyte interface

“…The possrbrhty of electroruc conductton through StO, layers has been discussed by Schnudt [36], Momson and co-workers [37,38] and Revesz [39]. A few expenmental results concerning the influence of pH on the impedance of the oxide/electrolyte interface for other oxides can be found in the literature of the anodic oxidation of metals such as aluminium and tantalum [30,40]. Libsch and Devereux [40] (2) The interfacial capacitance depends on the charge originating from the mechanism of pH response, and is different from the double-layer capacitance C,,.…”

“…A few expenmental results concerning the influence of pH on the impedance of the oxide/electrolyte interface for other oxides can be found in the literature of the anodic oxidation of metals such as aluminium and tantalum [30,40]. Libsch and Devereux [40] (2) The interfacial capacitance depends on the charge originating from the mechanism of pH response, and is different from the double-layer capacitance C,,. This means the oxide capacitance C,, 1s the capacitance of the SIO, layer from the SI/SIO~ interface up to the region where the pH response mechanism is located.…”

On the impedance of the silicon dioxide/electrolyte interface

“…The possrbrhty of electroruc conductton through StO, layers has been discussed by Schnudt [36], Momson and co-workers [37,38] and Revesz [39]. A few expenmental results concerning the influence of pH on the impedance of the oxide/electrolyte interface for other oxides can be found in the literature of the anodic oxidation of metals such as aluminium and tantalum [30,40]. Libsch and Devereux [40] and CDL = C',,,, = 20 PF cmm2 at high ionic strengths, the maximum C,;: expected m this case can be seen from eqn.…”