DEFECTS IN POROUS ANODIC FILMS FORMED ON HIGH-PURITY ALUMINUM

The morphology and the formation behavior of various types of defects in porous anodic films formed on high purity aluminum have been studied by electron microscopy. Circular voids were found within anion-free layers at the triple polar junctions where three cells met. The voids contained oxide particles which appeared darker in contrast. Crystallization induced by electron beam irradiation initiated around the voids. Long and narrow holes (crack-like voids) which passed through both anion-free and anion-incorporated layers from triple cell junctions to pore walls were also observed. These facts reveal that both circular and narrow voids are produced by certain effects which are preferentially developed between metal ridges of the aluminum substrate and pore bases. The narrow holes were more frequently observed than voids in a phosphoric acid film, whereas circular voids were prominent in an Ematal film. Circular voids were also detected in a barrier-type film on the metal ridges of preconditioned aluminum substrate. The cause of the void formation which is dependent on the geometry of the metal substrate has been discussed.