The Structure of Oxide Films Formed on the Zr Surface

Abstract

The present authors have studied the structure of thin oxide films formed on the Zr surface under various conditions. The following results were obtained by electron microscopy and electron diffraction. (1) The oxide films formed by dry oxidation, e.g., heating in air at 300°∼500°C or dipping in a fused salt bath (NaNO₃ 50 g, KNO₃ 50 g, MnO₂ 1 g: 400°C), consisted of only the monoclinic ZrO₂. (2) A number of regularly aligned lines of about 2500 Å in length was observed in some oxide films formed on the surfaces smoothed by cloth-polishing. (3) The tetragonal ZrO₂ as well as the monoclinic ZrO₂ was detected in the Zr oxide films produced by wet oxidation such as immersing in boiling conc. HNO₃. (4) On the hydrogen-absorbed surfaces by either cathodizing in 5%H₂SO₄ or immersing in 35%HCl, the oxide films composed of the tetragonal and the monoclinic ZrO₂ crystals were produced by dry oxidation. (5) When the Zr specimen was quenched in water after heating in air at 400°C, the surface was covered with both the tetragonal and the monoclinic ZrO₂ crystals. (6) The tetragonal ZrO₂ in the oxide films produced by wet oxidation transformed to the monoclinic ZrO₂ by heating in air at above 1150°C. (7) These results indicate that the meta-stable tetragonal ZrO₂ crystals are produced in the oxide films partly due to the existence of water or hydrogen in oxidizing atmosphere.