Anodic Film Growth on Hafnium in Nitric Acid

Abstract

In 70% nitric acid at room temperature, an anodic film developed uniformly over single hafnium crystals with a thickness dependent upon the metal crystal orientation. At 0.025 ma/cm2 the cell voltage did not exceed 1.5 v. At 1.5 ma/cm2 the cell voltage rose to 185 v and the film was broken down by sparking. In each case, the anodizing produced more oxide on metal crystal planes which had shown greater oxide growth on zirconium under similar conditions. At 90°C and 0.025ma/cm2, the oxide developed discontinuously over single hafnium crystals with the formation of wedge‐like patches. The degree to which these patches covered the grains was not related to the metal crystal face in the same way as the thickness of the room temperature film. The discontinuous growth was attributed to nucleation at metal surface imperfections. An orientation effect in the electropolishing of hafnium was also noted.