A comparative study of the anodic alumina film thickness measured via SEM and evaluated using Faraday’s Law

Abstract

Five Al specimens (99.301 wt% pure) were anodized in 0.3 M oxalic acid at 0 °C and 40 V for five different anodization times in the range 5–30 min. The thickness of the anodic alumina films was measured by SEM as well as calculated by Faraday’s Law. It was found that the average current (4.71 mA), average current density (2.14 mA cm−2), and average growth rate (57.37 nm min−1) of the anodic alumina films remain independent of anodization time. The total charge transferred (1.42–8.16 C) and thickness of alumina film calculated by Faraday’s Law (0.29–1.66 μm) increase linearly with the increase in anodization time from 5 to 30 min. It was established that the value of film thickness measured by SEM (5–79 μm) also increases linearly with the anodization time. However, the value of was substantially greater than that of The relation between and can be described by = R (t, V, T) × where the value of multiplying factor R depends on the anodization time (t), voltage (V), and temperature (T). Analysis of the SEM top-view image micrographs of the anodic alumina films revealed that the pore diameter, interpore distance, and porosity of the anodic alumina film increase linearly with the increase in the anodization time. On the contrary, opposite behavior was observed in the case of pore circularity and pore density. The peak intensity of preferentially oriented (311) plane of Al substrate decreased whereas the FWHM increased on anodization, which means that the crystallographic nature of (311) plane was rather deteriorated on anodization.