Abstract
Zircaloy-4 is used as materials for nuclear fuel rods in PWR plants. Zircaloy-4 is oxidized by the inward diffusion of the oxygen ions through the oxide on the surface in high temperature water and/or steam. Therefore, the structure of the oxide films should affect the corrosion resistance of Zircaloy-4. However, the relationship between the crystal structure of the oxide films and the corrosion resistance of Zircaloy-4 has not been clear. Zircaloy-4 shows a rapid increase in corrosion rate during the corrosion process. This phenomenon was called “transition”. However, a relationship between the crystal structure of the oxide films and the transition has not been clear. A purpose of this paper is to clarify the relationships. The materials used for corrosion tests were Zircaloy-4 sheets with two different intermediate annealings at 903 and 1053 K. The corrosion tests were carried out in static steam at 673 K. The oxides in the vicinity of an oxide-to-metal interface were observed in the cross-section using transmission electron microscopy. The main results are as follows: (1) The oxide films just before the first transition consisted of a two-layered structure. The outer layer was the monoclinic structure and the inner layer adjacent to the oxide-to-metal interface was the tetragonal structure. (2) Zircaloy-4 with intermediate annealing at 1053 K had better corrosion resistance and a thicker tetragonal layer than that with intermediate annealing at 903 K. (3) The tetragonal layer of both materials disappeared just after the transition. The oxide films of both materials consisted of only the monoclinic structure. “transition” corresponded to this structure transformation from the tetragonal to the monoclinic structure. (4) The acceleration of the corrosion rate at the transition could be caused by the formation of diffusion paths for the oxygen ions in the monoclinic layer by the volume expansion with the transformation.
