Growth and Characterization of Oxide Films on Zirconium-Niobium Alloys

Abstract

Pressure tubes for CANDU reactors are made from extruded and cold-drawn Zr-2.5Nb alloy. Their microstructure consists of elongated α-Zr grains (0.3 to 0.5 μm thick), containing about 1 atom percent niobium, surrounded by a thin (30 to 50 nm) network of metastable β-Zr phase, containing about 20 atom percent niobium. Alloys of Zr-1Nb and Zr-20Nb were prepared, heat treated, and oxidized in 573 K water to produce bulk microstructures and oxides that would simulate those normally found on a much finer scale in pressure tubes. These were subsequently characterized by chemical analyses, scanning electron microscopy (SEM), X-ray diffraction (XRD), analytical electron microscopy (AEM), secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), and nuclear reaction analyses (NRA). Oxidation of Zr-20Nb (β-Zr phase) was more rapid than that for the Zr-1Nb (predominantly α-Zr phase) but, despite this, the hydrogen absorption was considerably lower. During corrosion testing, the metastable β-Zr undergoes partial decomposition to omega phase. The oxides show contrasting morphologies in terms of crystallite size (20 to 60 nm for oxides on α-Zr versus about 15 nm for oxides on β-Zr). In addition to monoclinic ZrO2, there is evidence for either tetragonal ZrO2 or the mixed oxide, 6ZrO2 ∙ Nb2O5 in the β-Zr oxide. Scanning transmission electron microscopy (STEM) imaging shows niobium associated with the oxide formed over the β-Zr phase in oxidized pressure tube material. Deuterium distributions obtained by SIMS depth profiling through the oxides are radically different for each oxide type. The concentration of deuterium in the β-Zr oxide was substantially less than that in the α-Zr oxide, which was consistent with the observed lower deuterium uptake measured for the Zr-20Nb alloy. Complementary XPS results also suggest that some unoxidized niobium is present in these water-formed oxides. Hydrogen depth profiling by 15N nuclear reaction analyses has been used to investigate the diffusion of hydrogen in these oxides. The oxide films were implanted with hydrogen and the progressive dispersion of the implanted hydrogen, as a result of annealing, was used to investigate hydrogen diffusion as a function of temperature. The nondispersive nature of the implanted hydrogen peaks in the Zr-1Nb oxide after annealing was suggestive of the presence of interconnected porosity in those oxides. However, the broadened peaks in the Zr-20Nb oxide after annealing are indicative of a normal diffusion process in a nonporous medium. The implications of these observations will be discussed in terms of corrosion and hydrogen uptake in Zr-2.5Nb pressure tube material.