The incorporation of electrolyte byproducts into barrier anodic Al 2O 3 coatings

Abstract

We have observed that substantial amounts of electrolyte byproducts are incorporated into barrier anodic Al 2O 3 coatings during deposition. The electrolyte used for the process studied consisted of a solution of ammonium tartrate and water which was diluted to 1 part in 40 with ethanol. The tartrate concentration in this electrolyte decreases as a function of the amount of aluminum anodized. Fourier transform IR analysis indicates that the coatings produced by the process contain substantial amounts of carboxylates. Auger electron spectroscopy, transmission electron spectroscopy and other indirect evidence, e.g. Fourier transform IR spectroscopy, strongly suggest that the concentration of carboxylates increases with proximity to the outer surface of the coatings. Our results also indicate that these byproducts play a substantial role in inhibiting the crystallization of amorphous material present in the coatings. The results of this study support the hypothesis that incorporated electrolyte byproducts beneficially contribute to the dielectric properties of barrier anodic coatings. The depletion of tartrate ions from the electrolyte in conjunction with anodic film formation is accompanied by the appearance of acetate and formate. The concentrations of these ions increase rapidly and are then depleted together with tartrate late in the bath's life. This depletion of the anion species appears to define a practical limit to the area of aluminum which can be anodized in a given volume of the electrolyte without replenishment.