Behavior of interfacial micro-defects in aluminum and its oxide film

High-purity aluminum foils were investigated using Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM), after dissolution for various times in 1 M NaOH at room temperature and anodic oxidation for different voltages in borate buffer solution. The characteristics of circular voids presented in TEM images are found to be similar to those of cavities revealed by oxide stripping and viewed with AFM. It is concluded that c avities are voids. Voids in anodized films with NaOH treatment are very different from those in anodized films of as-received foils. It indicates that pre-conditions of subsurface voids in the metal determine the behavior of void formation during oxidation. The vertical profiles of voids in the oxide film are obtained and voids are hardly found in the outer 40% part of the oxide films. Void concentrations show small variation in the inner 60% part. This result demonstrated that voids are apparently formed at the metal-film interface during anodic oxide growth, likely as a result of the oxidation of metal atoms. Void concentration was also found to change with NaOH pretreatment time. The behavior was the same as that for cavities on the metal prior to anodization. All the above results strongly suggested a mechanism of void regeneration at the interface between metal and oxide.