The role of β-Zr in a Zr-2.5Nb alloy during aqueous corrosion: A multi-technique study

Abstract

The Zr–2.5Nb alloy is used as pressure tubes in Canadian Deuterium Uranium (CANDU) nuclear reactors, and the typical starting microstructure consists of α-Zr grains elongated in both transverse and longitudinal directions and thin layers of partially decomposed β-Zr lying between the α-Zr grains. In this study, we have used state-of-the-art microscopy techniques to characterise the long-term thermally decomposed β phase in this alloy, and the oxide scale formed on them in a reactor coolant loop with the aim of understanding the mechanisms underpinning the thermal decomposition behaviour at service temperatures and exploring the role of the decomposed β-Zr phase in controlling the microstructure and microchemistry of the zirconium oxide, and hence its influence on the general corrosion resistance of the alloy. We observe that these β-Zr layers are heavily decomposed even after the short stress stage at 400 °C at the end of the manufacturing cycle, with a closely packed array of β-Nb precipitates forming in an α-Zr matrix. We have shown that the oxidation of these bands is significantly slower than the surrounding α-Zr matrix and that zirconium oxide grains are re-nucleated under each band. We conclude that it is the combination of the Nb-rich remnants of the original β-Zr layers arising from the hot extrusion and drawing stages and this new dense oxide that offers a significant barrier to the oxidation front (and also to the penetration of hydrogenic species), so the characteristic layered microstructure arising from the original manufacturing process is very important in determining the overall oxidation behaviour.