Studies were made of the effects of aluminum purity and individual impurity elements contained in it upon the formation and corrosion resistance of boehmite films formed on aluminum in boiling aqueous soulutions.
The following results were obtained:
(1) The thickness and corrosion resistance of the boehmite film formed on aluminum depended on the purity of aluminum. The film formed in 0.5% triethanolamine solution after boiled for 1hr. had larger thickness and higher corrosion resistance with the increase in aluminum purity.
(2) The electron microscopic examination and electron diffraction showed that the micro-structure of the film was independent of aluminum purity.
(3) As for impurity elements contained in aluminum, Fe and Cu increased the film thickness at the initial stage of boehmite formation, but decreased it with the lapse of time. These elements served to decrease the corrosion resistance of boehmite film, the effect of which was particularly more remarkable in Fe.
(4) Among impurity elements, Zn increased the thickness of boehmite film, but sharply decreased its corrosion resistance. Whereas, Si and Ti decreased the thickness of boehmite film, but had little effects on its corrosion resistance, except for cases of their high contents.
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