Abstract
Zr-2.5Nb alloy is used as a pressure tube material in CANDU reactors. Tests in high temperature water and steam have shown that corrosion of Zr-2.5Nb is very sensitive to changes in microstructure. Highest corrosion rates are generally seen on martensitic structures in β-quenched material, however, lower rates are obtainable with Widmanstatten structures produced by cooling more slowly from the β region. Corrosion resistance can be substantially improved by ageing β-quenched structures at temperatures below the monotectoid temperature of 610°C. Further improvements can be realized by quenching from the two-phase α+β phase field and introducing cold work between the quenching and ageing treatments. The presence of equilibrium α-Zr and the precipitation of β-Nb from the supersaturated a prime martensitic structure both appear to contribute to lowering the corrosion rate. Improvements in corrosion resistance are also observed when extruded and cold worked pressure tubes are thermally aged at 400-500°C. The improvements seem to correlate with the eventual decomposition of the β-Zr phase to the more stable α-Zr plus β-Nb phases.
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