ANION IMPURITIES IN ANODIC OXIDE FILMS ON ZIRCONIUM

The carbon, sulphur and boron contents of anodic ZrO2 films formed in 0·1 M sodium carbonate, 0·1 M sulphuric acid and saturated ammonium borate solutions respectively have been determined. These impurity elements are distributed uniformily on a 2-3 μm scale in the plane of the film. Over-all impurity contents are determined by the ionic formation cd. The variation of content with cd is different for each anion. Carbon and sulphur are not distributed uniformly in depth but concentrate towards the oxide/solution interface. Sulphur contents are an order of magnitude greater than those of tantalum and tungsten films formed under similar conditions. Carbon sulphur and boron effect the ionic conductivity of the film similarly. Carbon and boron are equally effective in increasing formation current efficiency; sulphur is considerably less effective. The presence of carbon or sulphur reduces the porosity of the film. The films formed in all three electrolyte solutions are sufficiently impure that models for anodization which assume that a pure oxide is formed cannot be applied to the anodization of zirconium. © 1968.