The behavior of magnesium in anodic films formed on the surfaces of annealed aluminum foil with MgAl_2O_4 spinel particles at sizes of 0.1 to 0.2 mum was studied on the basis of TEM observation and EDX analysis. Anodic films were formed at a current density of 50mA/cm~2 in a boric acid solution at 90 deg C for 2min. Anodic films were stripped from the aluminum foil, and the extractive carbon replicas and cross sections of the anodic films were used for TEM observation. Crystalline oxide gamma ft -A1_2O_3 with a round shape was formed around MgAl_2O_4 particles and grew over the surface with an increase in the forming voltage up to 60V. Magnesium disappeared in gamma ft -A1_2O_3 above 100V, and voids and cracks were observed around crystalline oxide particles in the anodic films formed at 150V. At 300V, the 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 Mg~(2+) than that for Al~(3+) in a high electric field.
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