The Anodizing of Zirconium and Other Transition Metals in Nitric Acid

Zirconium was anodized in solutions of nitric acid and compared with certain other transition metals. At 1 ma/cm2 and concentrations of nitric acid exceeding 14 weight per cent, zirconium was found to develop a low‐resistance oxide layer. Titanium, hafnium, columbium, tantalum, and tungsten showed a high resistance at this current even in 70 weight per cent acid. Hafnium, however, produced a low7hyphen;resistance film at lower current densities. Zirconium, in nitric acid above 14 weight per cent, showed periodic changes of resistance with time. The transition was dependent upon grain orientation and metal texture. As the concentration of nitric acid was increased, the anodic layer changed in appearance from white to gray to black. These products were all monoclinic . The white oxide was friable, and developed from a thin barrier layer. The black oxide was hard and coherent, and comprised the entire layer. It is suggested that these layers differed in the number of anion defects, and varied from stoichiometric (white) to anion‐ defect semiconducting (black).