2000 年 51 巻 7 号 p. 710-713
The behavior of magnesium in anodic films formed on the surfaces of annealed aluminum foil with MgAl2O4 spinel particles at sizes of 0.1 to 0.2μm was studied on the basis of TEM observation and EDX analysis. Anodic films were formed at a current density of 50mA/cm2 in a boric acid solution at 90°C for 2min. Anodic films were stripped from the aluminum foil, and the films extractive carbon replicas and cross sections of the anodic films were used for TEM observation. Crystalline oxide γ′-Al2O3 with a round shape was formed around MgAl2O4 particles and grew over the surface with an increase in the forming voltage up to 60V. Magnesium disappeared in γ′-Al2O3 above 100V, and voids and cracks were observed around crystalline oxide particles in the anodic films formed at 150V. At 300V, crystalline oxide was directly separated by a crack. It appears that the formation of the voids and cracks is associated with the greater migration rate for Mg2+ than that for Al3+ in a high electric field.