Growth Kinetics and Morphology of Porous Anodic Alumina Films Formed Using Phosphoric Acid

Abstract

The growth kinetics of porous alumina films formed by anodic oxidation of aluminum in phosphoric acid under galvanostatic conditions was studied. Scanning electron microscope measurements, Faraday's law, and oxide film mass measurements was used to analyze the growth kinetics and obtain film growth rates, pore density, and porosity. Current efficiency was also determined from these measurements. The effect of current density and solution temperature on the oxide film growth rate and morphology was examined. The rate of growth of the alumina film was found to increase with an increase in current density. The rate of growth was observed to increase with temperature at low current densities (7.5 mA/cm2), but was found to exhibit a maximum as temperature was varied at high current densities (17.5 mA/cm2). The pore density was found to decrease with an increase in current density and with a decrease in temperature. The porosity and the average cross‐sectional pore area of the films were found to increase with anodization time and decrease with an increase of current density and temperature. These results can be explained by considering the interplay between film growth and oxide dissolution that controls the resulting film morphology. The understanding of the effect of process variables on film morphology that is gained by this study provides the information that is needed to prepare alumina films with well‐defined and well‐characterized morphology.