Effect of Annealing Temperature on Corrosion Behavior and ZrO2 Microstructure of Zircaloy-4 Cladding Tube

Abstract

This paper describes the corrosion behavior and the ZrO2 microstructure of Zircaloy-4 (Zry-4) cladding tubes that were intermediate annealed at various temperatures. The corrosion behavior of the cladding tubes was studied by autoclave tests performed under 633 K water condition and 673 K steam condition. A TEM examination shows that the microstructure of ZrO2 formed on the Zry-4 matrix consisted of both the columnar structure and the equiaxed grain. The grain size of the columnar grain was approximately 30 by 200 nm, while that of the equiaxed grain was less than 20 nm. The equiaxed grain was dominantly observed near lateral cracks and around intermetallic compounds that were incorporated into the ZrO2 film. An analysis of the HR-SEM images indicated that the equiaxed grain to columnar grain volume ratio increased with increasing weight gain, especially after the first transition. The equiaxed grain to the columnar grain volume fraction decreased with increasing annealing temperature, which corresponded to decreasing weight gain. It was suggested that grain boundary diffusion of oxygen ions was accelerated by grain-size change of the oxide owing to the ZrO2 microstructure transformation from the large columnar grains to the fine equiaxed grains. The ZrO2 microstructure transformation might be caused dominantly by the oxidation of the intermetallic precipitates. The intermetallic precipitates were fine and uniformly distributed in the low-temperature TREX annealed Zry-4. This resulted in high-temperature TREX annealing being beneficial for improving corrosion resistance of the Zry-4 tube in PWR environments.

Effects of Sn on the oxidation and oxide characteristics of Zr alloys in water and a LiOH solution

“…Many researchers have attempted to identify the relationships between the corrosion behaviors and the oxide microstructures, thereby proposing that a columnar structure enhanced the corrosion resistance while an equiaxed struc- ture did not [6,16,23,24]. In order to determine the variation of oxide microstructure, TEM studies were performed on corroded samples having a thin oxide formed at pre-transition in water, as shown in Fig.…”

Effects of precipitation characteristics on the out-of-pile corrosion behavior of niobium-containing zirconium alloys

“…It was suggested that the grain boundary diffusion of the oxygen ion was accelerated by the transformation of the ZrO 2 microstructure from the large columnar grain to the fine equiaxed grain. The equiaxed grain was dominantly observed around the precipitate that was incorporated into the ZrO 2 oxide and the transformation of the ZrO 2 microstructure might be caused dominantly by oxidation of the intermetallic precipitates [15]. The present study showed that the effect of precipitate size was more responsible for the difference in corrosion behavior between Al and A2 alloys.…”

“…Oxide characteristics such as oxide morphology and precipitates incorporated into the oxide are known to play an important role in the oxidation process of Zircaloys. It was shown that the equiaxed grain to columnar grain volume ratio in ZrO 2 oxide increased with increasing weight gain, especially after the first transition [15]. It was suggested that the grain boundary diffusion of the oxygen ion was accelerated by the transformation of the ZrO 2 microstructure from the large columnar grain to the fine equiaxed grain.…”