Electrode Luminescence of ac Anodized Aluminum: a Study of the Brightness Waves

Abstract

The brightness waves emitted during ac anodization of 99.7% pure aluminum (Sample 1) in aqueous solutions of oxalic acid (OA) or oxalic acid plus borax (OA+B) were investigated. Electrode luminescence in OA was observed from 5 cps to 350 kcps. The suppression of the cathode flash at 60 cps in OA+B appears to be a secondary phenomenon due to the fact that there is only a small potential drop across the oxide surface layer during a cathodic half-period. At higher frequencies a cathode flash can be observed in OA+B. A 99.995% pure aluminum sample (Sample 2) was observed to luminesce as brightly as the less pure sample, contrary to other reports in the literature. The long-wavelength “phosphorescence,” which appears as a tail on the brightness waves during ac anodization of Sample 1, was not discernible for Sample 2.