Abstract
Pure aluminum specimens were covered with hydrous oxide films by immersion in boiling distilled water and then anodized galvanostatically in a neutral borate solution at different temperatures to form composite oxide films. The formation behavior of the hydrous oxide and composite oxide films was examined by gravimetry, XPS, chemical analysis and electron microscopy.
It was found that the hydrous oxide films consist of two layers; a fibrous outer layer (thickness δh.o) and a dense inner layer (δh.i). The thickness of the outer layer, δh.o, increased rapidly with time to reach a steady value of ca. 0.3μm but that of the inner layer, δh.i, continued to increase gradually after a rapid increase. The hydrous oxide had a chemical composition of Al2O3⋅2.7H2O and a density of ca. 2.3g/cm3, both of which did not change with immersion time.
The composite oxide films consisted of two layers; an outer crystalline oxide layer (thickness δo) and an inner amorphous oxide layer (δi). During anodizing, δh.i decreased linearly, and the total thickness, δo+δi, increased linearly with ta. The δh.i vs. ta and δo+δi vs. ta curves were independent of anodizing temperature, Ta. The outer layer thickness, δo, increased with ta, at a rate increasing with ta. This behavior was pronounced at higher Ta. The rate of increase in δi decreased with ta, and this also became more pronounced at higher Ta. The electric field supported by δi+δo was found to increase with ta from 7.7 to 10.3×106V/cm. This is explained as being due to the formation of the crystalline oxide layer capable of supporting higher electrical field and to the increase in δo/(δi+δo).
