Effects of the accumulated annealing parameter on the corrosion characteristics of a Zr–0.5Nb–1.0Sn–0.5Fe–0.25Cr alloy

Abstract

Corrosion behavior, hydrogen pickup, oxide microstructure, and precipitate characterization have been studied in order to investigate the effect of the accumulated annealing parameter on the corrosion characteristics in a Zr–Nb–Sn–Fe–Cr alloy. An autoclave corrosion test was carried out in 400°C steam for 300 days on the Zr–0.5Nb–1.0Sn–0.5Fe–0.25Cr alloy, which had been given 18 different accumulated annealing parameters. The corrosion rate increased with increasing the accumulated annealing parameter. To investigate the crystal structure of oxide layer, the corroded specimens were prepared to have an equal oxide thickness (∼1.6 μm) by controlling exposure time. The relative fraction of tetragonal ZrO 2 also decreased gradually with increasing accumulated annealing parameter. From the hydrogen analysis of the corroded samples for 300 days, it was observed that, with increasing the size of precipitates, the hydrogen pickup was enhanced. It was revealed from transmission electron microscope (TEM) observation of the oxide that the larger precipitates still remained to be oxidized in the oxide layer and had undergone a reduction of Fe/Cr ratio from 2.1 to 1.5. The oxidation of the precipitates in the oxide gave rise to a volume expansion at the precipitate–oxide interface. This volume change could lead to the transformation in the oxide phase from tetragonal ZrO 2 to monoclinic ZrO 2 and in oxide structure from columnar grain to equiaxed grain. The precipitate in a Zr–0.5Nb–1.0Sn–0.5Fe–0.25Cr alloy is composed of Nb, Fe, and Cr and the Nb content in the precipitate increase with increasing accumulated annealing parameter. Thus, it can be thought that Nb within precipitates plays a key role in the microstructural change of oxide.