Highly pure aluminum was anodized at a constant current density of 25Am−2 at 293 Kin 0.5 M boric acid/0, 0.005, or 0.05 M sodium tetraborate solutions, to examine the effect of sodium tetraborate concentration on the formation and breakdown characteristics of barrier oxide films by using inductively coupled plasma atomic emission spectrometry, electroluminescence/photoluminescence measurements, scanning electron microscopy, transmission electron microscopy, and electrochemical impedance spectroscopy. In boric acid/borate solutions, a crystalline alumina formed locally in the middle of the amorphous oxide film. Above the crystalline alumina, a void is formed and may lead to a breakdown of the oxide film at 420 to 540 V. In boric acid solution, an amorphous oxide film grew until 1180 V with the formation and development of imperfections and with enhancement of electroluminescence and gas evolution. At imperfections, the oxide/solution interface was convex and the oxide/metal interface curved in the opposite direction. This deformation is attributed to high‐pressure O2 evolved in the pores of imperfections and to the local formation and dissolution of oxide. The breakdown of the oxide film started when the O2 evolution and oxide dissolution at imperfections become predominant. The mechanism of formation and breakdown of the anodic film in the boric acid/borate solutions is discussed in terms of pH buffering of the anodizing solution, and the electronic structure of anodic oxide films is correlated with electroluminescence and photoluminescence spectrum results.

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Formation and Breakdown of Anodic Oxide Films on Aluminum in Boric Acid/Borate Solutions.-The effect of borate concentration on the formation and breakdown of anodic films on Al at a current density of 25 A/m2 at 293 K is studied in 0.5 M H3BO3/0, 0.005, or 0.05 M Na2B4O7·10H2O solutions by ICP-AES, EL, PL, FE-SEM, TEM, and EIS. Differences in the film formation and breakdown between boric acid/borate and boric acid solutions are discussed in terms of the pH-buffering ability of the solutions. The electronic structure of the anodic oxide films is correlated with the electroand photoluminescence spectra. -(LI, Y.; SHIMADA, H.; SAKAIRI, M.; SHIGYO, K.; TAKAHASHI, H.; SEO, M.; J.

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