Corrosion of zirconium alloys in concentrated lithium hydroxide solutions

Abstract

The accelerated corrosion of two alloys used as fuel cladding material, such as Zircaloy-4 and Zr–1%Nb has been studied in concentrated lithium hydroxide (LiOH) solutions at high temperature and pressure. Zr–2.5%Nb pressure tube material (PT) was also tested in order to analyze the influence of the amount of β-Zr phase on the accelerated corrosion of the Zr–Nb alloys. The microstructure of Zircaloy-4 consisted of α-Zr equiaxed grains whereas that of Zr–2.5%Nb (PT) and Zr-1%Nb alloys showed a two phase (α-Zr+β-Zr) microstructure.Autoclaving tests were performed in LiOH solutions with concentrations ranging from 0.1M to 1M for 16h at 343°C, and also in steam at 400°C. The extent of corrosion was evaluated through the weight gain and visual appearance of the oxide. Measurements included hydrogen uptake and oxide thickness. Optical microscopy observations of the hydride distribution were also made on the specimens.Results showed that Zircaloy-4 suffered accelerated corrosion at 0.7M and 1MLiOH concentrations with high hydrogen uptakes (∼50%). For these LiOH concentrations, although the Zr–2.5%Nb (PT) and Zr–1%Nb alloys showed weight gains higher than the threshold value established for the “transition”, their hydrogen uptakes were low (∼1%). This behavior indicates that at the early stages of the corrosion process in these solutions, an oxide barrier layer may be present at the metal/oxide interface of these Zr–Nb alloys. The lower amount of β-Zr phase in Zr–1%Nb improves the characteristics of the oxide layer in relation to that formed on Zr–2.5%Nb and does not affect the hydrogen uptake.