The Effect of crystal textures on the anodic oxidization of zirconium in a boiling nitric acid solution
Kato, Chiaki ; Ishijima, Yasuhiro ; Ueno, Fumiyoshi ; Yamamoto, Masahiro
The effects of crystal textures and the potentials in the anodic oxidation of zirconium in a boiling nitric acid solution were investigated to study the stress corrosion cracking of zirconium in nitric acid solutions. The growth of the zirconium oxide film dramatically changed depending on the applied potential at a closed depassivation potential (1.47 V vs. SSE). At 1.5 V, the zirconium oxide film rapidly grows, and its growth exhibits cyclic oxidation kinetics in accordance with a nearly cubic rate law. The zirconium oxide film grows according to the quantity of electric charge, and the growth rate does not depend on the crystal texture in the pretransition region before the cyclic oxidation kinetics. However, the growth and cracking under the thick oxide film depend on the crystal texture in the transition region. On the normal direction side, the oxide film thickness decreases on average since some areas of the thick oxide film are separated from the specimen surface owing to the cracks in the thick oxide. On the rolling direction side, cracks are found under the thick oxide film, which deeply propagate along the RD without an external stress. The cracks under the thick oxide film propagate to the center of the oxide layer. The cracks in the oxide layer propagate in the (0002)Zr plane in the zirconium matrix. The oxide layer consists of string-like zirconium oxide and zirconium hydride. The string-like zirconium oxide contains orthorhombic ZrO in addition to monoclinic ZrO. As one assumption for the mechanism of crack initiation and propagation without an external stress, it is considered that the oxidizing zirconium hydrides precipitate in the (0002)Zr and then the phase transformation from orthorhombic ZrO to monoclinic ZrO in the oxide layer causes the crack propagation in the (0002) plane.