The incorporation of a siliceous impurity during the anodic oxidation of aluminum in a sodium tartrate electrolyte

We have observed the occurrence of a high concentration of silicon as an impurity in thin films of amorphous alumina formed by the anodic oxidation of pure aluminum in a neutral sodium tartrate electrolyte. X-ray absorption analysis of the film by a total secondary electron emission method shows that the amount of silicon is proportional to the film thickness and is, therefore, thought to be incorporated in the oxide rather than being a deposit at the metal/oxide interface, or on the surface. An examination of the films by scanning electron microscopy and EDAX microanalysis shows that the impurity takes the form of discrete particles. It is suggested that these siliceous particles, probably colloidal silica, are deposited from the electrolyte during the growth of the oxide layer and are formed as a result of the action of the sodium tartrate solution on the Pyrex glass container. This phenomenon might be a more general occurrence in surface treatments.