A comparative study of anodic oxide films on titanium, niobium and tantalum

Abstract

Anodic oxide films were formed on titanium, niobium and tantalum, mostly up to about 40 volts, using three electrolytes of quite different composition and a particular electrical program intended to prevent uncontrolled transient phenomena. The existence of several different anodic oxides was observed for each metal, depending upon the electrolyte used. The growth of the oxides occurs at the oxide/electrolyte interface by a mechanism of cation migration through a stationary oxygen sub-lattice. At this interface, a transition region similar to an electrochemical double layer appeared to behave as a dielectric layer added to the anodic oxide layer during anodization. Data were compared concerning the differential thickness, the specific capacitance, the dielectric constant and the apparent electronic charge on the anions electrodeposited into the oxides. From the latter it was deduced that water is the main oxidizing agent and that protons are incorporated into the anodic oxides as well as some sulfuric anions when certain electrolytes are used. Special attention was devoted to data on titanium anodic oxides; fluctuations in thickness were observed from one sample to another, and their effect was eliminated by the use of an interference colour scale to estimate the actual thickness; the dielectric constant was found to remain constant throughout the thickness of the film, and the resistivity was also found constant except in a layer of about 50 Å situated next to the growth interface. From data about crystalline oxides of the same metals the concept of a quasi-compact oxygen sub-lattice with octahedral sites for metallic cations was deduced and applied to the amorphous anodic oxides as well as to the transition layer at the growth interface; the validity of this hypothesis could be tested on one particular titanium anodic oxide, for which interferometric thickness measurements could be checked against oxygen determination by nuclear reaction.