The Influence of Sealing Time on the Morphology of Anodic Oxide Films Formed on Aluminium in Sulphuric Acid

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Abstract
Morphological changes produced by the sealing in boiling water of porous anodic films, formed on aluminium in sulphuric acid, have been followed at various stages by replica electron microscopy of film surfaces and fracture sections. The outer surface becomes covered by crystalline oxide which largely obliterates the pores after sealing for about 3 min, the crystals presenting a more fibrous appearance after about 30 min, and eventually becoming acicular or plate-like with increased sealing up to 24 h. In section, there is little obvious pore blocking in depth after 3 min but after 10 min the pores are at least partially filled, probably by initial precipitation and subsequent agglomeration processes, which partly eliminate and partly redistribute the porosity. The fracture morphology prevents detailed study of slower, longer-term sealing changes.