Abstract
Commercially pure aluminum specimens were anodically oxidized in a 13M H2SO4 solution at 20°C by applying a constant current density of 1A·dm-2, the voltage attained being 36-38V. The composition and properties of the oxide formed were investigated by chemical analysis, electrochemical techniques, thermogravimetric analysis, and transmission electron microscopy. The results were compared with those for oxide films formed in a 1.5M solution. (1) The content of SO42- and H2O were about 40% and 6.5% respectively for the 13M film and 15% and 5.5% for the 1.5M film. Thus, film composition is expressed by Al2O2.29 (SO4)0.71⋅;0.61H2O and Al2O2.81 (SO4)0.19⋅0.38H2O. (2) For the 13M films, the thickness of the barrier layer was 330A and that of the pore wall 270A, pore diameter was 200A. The number of pores was 19.6×109cm-2 for the 13M film and 72.1×109cm-2 for the 1.5M film. (3) The rate of chemical dissolution of both 13M and 1.5M films was higher in the 1.5M H2SO4 solution than that in the 13M solution. In both solutions, the 13M film dissolved faster than the 1.5M film, due to its larger SO42- content. (4) Microvoids were found to have been produced in the barrier layer and pore walls of the 13M film by the expansion of oxide due to the incorporation of a large number of SO42- ions. Because microvoids were present in both the barrier layer and the pore walls, density and abrasion resistance were lower for the 13M film than for the 1.5M film.
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