ELECTRON-OPTICAL EXAMINATION OF SEALED ANODIC ALUMINA FILMS - SURFACE AND INTERIOR EFFECTS

Sealing of porous anodic films, formed on aluminum in an oxalic acid electrolyte, has been studied by direct replica electron microscopy and electron diffraction. Pore plugging and changes in pore-wall material occur throughout the film but most rapidly at its exterior, while crystalline material forms on the outer film surfaces. Sealing appears to occur in three distinct but overlapping steps, the first being precipitation of material in the pores, especially near the outer film surface, the second, formation of crystalline material at the surface,hich is a minor process, and finally further changes in the film depths involving aggregation of the oxide microcrystallites and a redistribution of porosity.