硫酸中で生成したポーラス型アルミニウムアノード酸化皮膜の構造と密度

抄録

Electropolished 99.99% Al foil coupons were anodized in H₂SO₄ solutions of 0.5-4.0mol·dm^-3 at 10-40°C, by applying a constant current in a range of 0.5-50mA·cm^-2, Morphology of the oxide films was examined using electronmicroscopic and electrochemical techniques. For the accurate estimation of the number of pores, N, from electronmicrographs, the pores were widened by chemical dissolution of the oxide in a H₂SO₄ solution before the film was stripped from the substrate metal. The total film thickness, h, was estimated from the sections of films under a scanning electronmicroscope, and the pore radius, r, was calculated from the porosity of the oxide, α(=Nπr²), measured by the pore-filling method proposed by Middelhoek. The estimation of the barrier layer thickness, δ_b, was made according to Hunter's principle by measureing the voltage jump, E_j, when applying a small anodic current in a neutral borate solution. The thickness/voltage ratio, δ_b/E_j, was determined from the time-variations in the pore-wall thickness, δ_p, and E_j during chemical dissolution of the oxide, on an assumption that δ_b and δ_p thin at equal rates. In agreement with the results reported previously, N decreases and r and δ_b increase as the anodizing voltage, E_a, increases, but the results of this investigation clearly show that these quantities are solely the function of E_a although E_a is strongly affected by the solution condition and anodizing c.d. It is demonstrated that δ_b/E_a ratio is not constant but decreases with increasing E_a. For films formed with a fixed number of coulombs, the value of h somewhat increases with increasing E_a, solution concentration and temperature. The density of oxide ρ, is estimated to be 3.0-3.5 by dividing the weight of the film by the oxide volume calculated from the values of h, N, r and the chemical dissolution rate of the pore-wall during anodizing. It is interesting to note that ρ decreases with increasing E_a, solution concentration and temperature. The significance of the techniques of determining the quantities describing the film structure is discussed.