Growth Mechanism of Porous Anodic Films Formed on Aluminium in Sulphuric Acid

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Abstract

Abstract The present study is concerned with the mechanism of growth of porous anodic films formed on high purity aluminium and sputtering deposited aluminium over a wide range of current density between 5 to 50 mA/cm2 for times up to 5400 s in 24.5 wt % sulphuric acid and at temperatures of either 0 or 20 0C. The resultant films were examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA), optical interferometery, microhardness and nanoindentation.The RBS analysis enabled determination of the composition of the porous films, which was expressed as Al2O3.xAl2(SO4)3, with the sulphur species content increasing with increase in current density and decrease in temperature. The average expansion factor (expressing, the ratio of the film thickness to the oxidized aluminium thickness) increased between 5 mA/cm2 and 50 mA/cm2 for films formed at 0 0C, extending from 1.58 to 1.88 and from 1.57 to 1.78 according to SEM and TEM respectively. For films fabricated at 20 0C, the average expansion factor increased from 1.45 to 1.66 and from 1.42 to 1.67 derived from SEM and TEM respectively. The expansion factor increases as the current density increases for both temperatures, and decreases as electrolyte temperature for a given current density increases. The increase of expansion factor is also associated with a rise in the steady voltage during film growth. However, the film expansion does not depend on the anodizing time. The increase in expansion factor correlates with a small increase in the amount of sulphur in the film, which increases with rise of current density. The surface of the porous alumina revealed a network of furrows and ridges, reflecting the pattern of the cellular textures on the topography of the elecropolished aluminium. The retention of topography indicates that the thinning of the film due to chemical dissolution by the electrolyte is negligible, although softening of the film toward the film surface increases with rise of electrolyte temperature and anodizing time as determined by microhardness measurements on film cross-sections. For films fabricated at 0 0C, nodules appeared with a low population density on the film surfaces formed at 20 mA/cm2 for 5400 s, and a locally high population density, but non-uniform distribution, for films formed at 30 to 50 mA/cm2 for a wide range of anodizing times. NRA determined the oxygen concentrations in the films, from which the efficiency of the film grown was derived. The efficiency showed a correlation with the expansion factor, with values increasing with rise of current density and with decrease in the anodizing temperature, ranging from 72 % to 87 % between 5 mA/cm2 and 50 mA/cm2, for an electrolyte temperature of 0 0C, and between 66 % to 75 %, for the same range of current density, for an electrolyte temperature of 20 0C. The change in the relative film thickness with a change of the anodizing conditions might due to either a rise in the film porosity under a constant efficiency of film growth (assuming a flow model) or an increase in the efficiency of film growth for a constant film porosity (for either a flow or dissolution model), or a combination of the two factors. However, the film expansion appeared to be relatively little dependent on the change of the porosity over selected anodizing conditions. The dependence of the efficiency on the anodizing conditions is possibly associated with a change in the transport number of ion species in the film with a reducing contribution of cation migration to the total ionic current with an increase in the current density and in decrease of the anodizing temperature, which correspond to conditions of increasing electric field. The film porosity probably develops by flow of film material underneath the pore base toward the cell wall, as indicated by distribution of tungsten band through the film and distribution of electrolyte species from previous work, with the displaced material enhancing the thickness of the film.

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M. P. A. F. Al.

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