Electron Microscopy of Anodic Films Formed on Aluminium in Sulphuric Acid

Abstract

Porous anodic films have been formed on aluminium in sulphuric acid at constant voltage and examined by the usual techniques, using carbon replicas taken from the outer surfaces, sections and cell base patterns. Generally, resolution of the major anodic film parameters from carbon replicas of films formed at low voltages is restricted and transmission electron micrographs of stripped thin films are more informative, particularly with respect to the early film development and pare initiation stages. Ion beam thinned films have been examined in order to obtain further information on the film substructure and exposure under the electron microscope beam causes remarkable changes in the film morphology. The changes may relate to a drying out process in the electron microscope, or to some type of crysallisation phenomenon, but they appear to throw light on the film formation process. Electronoptical examination of ultramicrotomed sections of relatively thick anodic films improves the resolution of the porous structure and substructure of the anodic film but, because of the non-uniform nature of the section, precise determination of the major anodic film parameters is impaired. The problems are minimised on examination of thinner anodic films, which has proved illuminating in evaluating film development and pore initiation at low voltages in sulphuric acid.