Content and Distribution of Boron in a Barrier Type Anodic Oxide Film on Aluminum

In anodization of alminum, electrolyte anions are incorporated into the oxide film from the liquid electrolyte. The anion distribution is related to physical and chemical properties and the mode of film formation. Prompt Gamma-ray Analysis (PGA) has higt sensitivity (2000cps·mg-1) and a low detection limit (0.002μg) for boron, so it is applicable to the determination of boron in aluminum oxide films.It was found that the average boron content was 0.25wt% with electrolysis at 25°C in a 0.5mol·dm-3 H3BO3-0.05mol·dm-3 Na2B4O7 solution. Differences of the electrolytes and in formation voltages, i.e. in the thickness of the barrier films, had little effect on this value, but it varied in proportion to the boron concentration in the electrolyte and decreased at higher temperatures.Boron was distributed to a distance of half the depth from the surface, and was not detected near the oxide/metal interface. This result can be explained on the basis of the transport number (0.45-0.6) of Al(III) ion and by the absorption of borate ions into hydrous aluminum oxides.