アルミニウム陽極皮膜の光学的研究 (1)

αアルミナ皮膜生成機構の光学的解析

抄録

The authors found that α-alumina film is produced on aluminium by anodic oxidation in the low temperature melts of bisulphates.^{1, 2, 3)} In this paper, in order to clarify the mechanism of formation, optical studies were made for such films as well as the films formed under certain other conditions which are likely to cause the thermal transition of amorphous alumina to crystalline structure. The films were then observed under polarized light and the existence of double refraction was checked. At the same time, refractive indices were determined for these films. The results were compared with those of X ray analysis.
Main constituents of the films produced in sodium and potassium bisulphate melts or their mixtures were confirmed, optically too, to be α-Al₂O₃, The film formed in ammonium bisulphate melt contained an appreciable amount of γ-alumina. The films formed in conc. H₂SO₄ or in conc. H₂SO₄+oleum, consist mainly of γ-Al₂O₃, but by longer formation, it was partly converted to α-Al₂O₃. Even by aqueous ammonium borate solution, if anodic oxidation is continued for a long time under such high voltage at which sparking is noticed on the anode surface due to breakdown of the film, the film is gradually converted from amorphous, to γ-, and finally to α-Al₂O₃, though the transition may not be complete. From these experiments, it was confirmed that the local formation of α-Al₂O₃ by anodic oxidation of aluminium is not rare, though it is realized only by barrier type films and not by duplex (barrier plus porous) films.
The thermal transition from amorphous, through γ-, to α-Al₂O₃, is caused by local heating of the anode surface due to remarkable exothermic reaction 2Al⁺⁺⁺+O^--→Al₂O₃, Joule's heating effect through the film and by sparking.
Further, when Al-Ti alloy is anodized in bisulphate melt, the film consists of α-Al₂O₃ and TiO₂ in Anatase form (not in Rutile form).