Branching Colony Pore Structure in Anodic Alumina Films

A LARGE cellular structure may be present underneath thick porous anodized films grown on aluminium in chromic acid and an alkaline borate electrolyte1. We termed these "secondary" cells because their population was much smaller than the number of pores in the anodic film as counted from electron micrographs of either surface replicas of thick films or transmission through very thin ones. It appears that these cells are not related to the pretreatment or subgrain structure of the base metal. Their presence has now been confirmed by electron-optical examination of cross-sections of anodic films grown on super-pure aluminium (99.999%) in a 10% chromic acid solution at 30 V, 40° C. The method for the preparation and taking replicas of film sections was similar to that described by Wood and O'Sullivan2. An A.E.I. 'E.M.6.G.' electron microscope was used for the examination of the replicas.