The behavior of magnesium in the anodic films formed on the surfaces of aluminum foils containing l approx 78wt-ppm magnesium was studied based on TEM observations and EDX analyses. Specimens were annealed at 575 deg C for 5h in an Ar gas atmosphere (O_2 concentration <10ppm). Particles with MgAl_2O_4 spinel particles of 0.1 to 0.2 mum were distributed over the surfaces of the specimens. The ratio of the thickness of crystalline oxide layer in the anodic film layers formed up to 600V was increased with increasing magnesium content. The crystalline oxide gamma'-Al_2O_3 with round shapes were formed around MgAl_2O_4 particles and grew over the surface with an increasing forming voltage up to 60V. Magnesium disappeared in gamma'-Al_2O_3 above 100V, and cracks were observed in the crystalline oxide particles in the anodic films formed at 150V. The crystalline oxide developed at 300V was directly separated by a crack. It appears that the formations of the voids and cracks are associated with the greater migration rate of Mg~(2+) than that of Al~(3+) under the high electric field.
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