Growth and characterization of oxide films on zirconium-niobium alloys
- Atomic Energy of Canada Ltd., Chalk River, Ontario (Canada). Chalk River Labs.
Pressure tubes for CANDU reactors are made from extruded and cold-drawn Zr-2.5Nb alloy. Their microstructure consists of elongated {alpha}-Zr grains containing about 1 atom % Nb, surrounded by a thin network of metastable {beta}-Zr phase, containing about 20 atom % Nb. Alloys of Zr-1Nb an 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. Oxidation of Zr-20Nb ({beta}-Zr phase) was more rapid than that for the Zr-1Nb (predominantly {alpha}-Zr phase) but, despite this, the hydrogen absorption was considerably lower. During corrosion testing, the metastable {beta}-Zr undergoes partial decomposition to omega phase. The oxides show contrasting morphologies in terms of crystallite size (20 to 60 nm for oxides on {alpha}-Zr versus about 15 nm for oxides on {beta}-Zr). In addition to monoclinic ZrO{sub 2}, there is evidence for either tetragonal ZrO{sub 2} or the mixed oxide, 6ZrO{sub 2}Nb{sub 2}O{sub 5} in the {beta}-Zr oxide. Scanning transmission electron microscopy (STEM) imaging shows niobium associated with the oxide formed over the {beta}-Zr phase in oxidized pressure tube material. Hydrogen depth profiling by {sup 15}N 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. 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 tubes.
- OSTI ID:
- 55650
- Report Number(s):
- CONF-930611-; ISBN 0-8031-2011-7; TRN: 95:012895
- Resource Relation:
- Conference: 10. international symposium on zirconium in the nuclear industry, Baltimore, MD (United States), 21-24 Jun 1993; Other Information: PBD: 1994; Related Information: Is Part Of Zirconium in the nuclear industry: Tenth international symposium. ASTM STP 1245; Garde, A.M.; Bradley, E.R. [eds.]; PB: 818 p.
- Country of Publication:
- United States
- Language:
- English
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