Anodizing of 99.99% aluminum foil in 250g/
l maleic acid containing 0.8-4.4g/
l H
2SO
4 was carried out for 30 minutes at 30°C and a constant current density of 2A/dm
2. The oxide film was then isolated from the substrate by dissolving aluminum in Br
2 -CH
3OH. After having been heat-treated at 400-900°C for 20 minutes, the film was investigated by means of visible and infrared spectra, ESR spectrum, X-ray diffraction, and quantitative chemical analysis of SO
42-. The maleic-sulfuric acids film was decolored when heated at more than 500°C, and the films formed in the baths of higher concentrations of H
2SO
4 were more easily decolored. Heat decomposition of organic compounds and SO
42-ions in the film and structual change of the oxide film did not correlate to the change in decoloring temperature of the film. However, the change in relative intensity of ESR adsorption, which was supposed to be caused by carbon radical of -COO (-) in the film, corresponded quite well to the decoloring process. The decoloring of the film was thought, therefore, to be caused by the state change of -COO (-) with a decrease in the amount of carbon radical before the organic compounds were finally decomposed to CO
2 gas.
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