SURFACE OXIDE ON ETCHED ALUMINUM

The formation and properties of the surface oxide formed on aluminum etched in a sulfuric acid-sodium dichromate solution have been investigated using electrical impedance, electron microscopy, and contact angle measurements. The frequency dependence of the impedance measurements is shown to follow the theory of Gevers and DuPre for amorphous dielectrics. By anodizing in neutral solutions, known oxide layers are added. From this and literature data, the dielectric constant at any frequency, the roughness factor, and the thickness of the oxide layer can be estimated. From the roughness factor, a porous cell structure is derived which is confirmed by electron micrographs. From the effects of drying the oxide, the presence of a hydrous layer which has a high dielectric loss and poor wettability is indicated. Upon heating to high temperatures, the loss decrases and the surface becomes more wettable. It is shown that this hydrous layer occurs, except when the surface is polarized anodically during etching. When the surface is polarized anodically, the anodic cell structure is prominent and little or no hydrous layer forms. The addition of copper ion to the etch causes a surface layer to form as under anodic polarization.