抄録
Porous oxide films with different thicknesses were formed on aluminum at a constant c.d. of 10mA/cm2 in an oxalic acid solution. They were treated with hot water at 99.5°C, and the time-variation in the film structure due to hydration was followed by impedance measurements in a boric acid-borate solution. The frequency range examined was 0.1-10, 000Hz and the results were analyzed dusing a Bode diagram method in which the log of the absolute value of impedance was plotted against the log of the frequency. The equivalent circuit of the hydrated film was found to consist of the capacitance, Cb, of the barrier oxide layer, combined in series with a parallel combination of the capacitance and resistance components, Ch, and Rh, of the hydrated oxide in the pores. The values of these components were determined using the fact that Cb, Rh, and Ch are decisively responsible for the film impedance in the frequency range of<10, 10-500, and 500-10, 000Hz, respectively. For films thicker than 0.8μm (or anodizing time ta>3min), 1/Cb, initially 6μF-1·cm2, decreases with the hydration time, th, and reaches a steady value of about 4.5μF-1·cm2 at th=10-20min. This indicates that the barrier layer thickness decreases with th due to hydration occur.ing from the outside and that the hydration reaction becomes extremely slow when the pores are filled up with hydrous oxide at time beyond th=10-20min. The 1/Ch value, corresponding to the thickness of the hydrous oxide phase increases with th for 10-20min to reach a steady value which is roughly propotional to the initial film thickness. The value of Rh is roughly propotional to ta at any th but it continues to increase with th even after 1/Ch becomes steady, suggesting that the specific resistance of the formed hydrous oxide increases with time by an aging effect. For thin films formed for ta=1min, oxide is completely hydrated within th=20min and thereafter the substrate metal gradually reacts with hot water. These findings are in agreement with the results of previous investigation, in which the weight gain of the films due to hydration was examined.
