Micro-Characterization of Corrosion Resistant Zirconium-Based Alloys

On the basis of experimental results using button ingots, three zirconium, tin, niobium, iron, and chromium (Zr-Sn-Nb-Fe-Cr) alloy compositions (VAZ series) with superior corrosion resistance and equivalent mechanical properties to those of conventional Zircaloy-4 were determined in a previous work. The alloys of these compositions were vacuum arc melted in 5-kg ingots and were fabricated into sheets with various annealing parameters. They were subjected to long-term corrosion tests in water at 633 K and tension tests at room and elevated (658 K) temperatures. Results showed that the corrosion weight gains and the hydrogen uptakes of VAZ alloys were about 50% and 20 to 60% of that of Zircaloy-4, respectively, and that the tensile strengths were equivalent to those of Zircaloy-4. Transmission electron microscopy/energy dispersive X-ray (TEM/EDX) analysis showed that niobium precipitated in VAZ alloys containing only 0.2% by weight niobium. In order to clarify the effects of niobium at low concentration on the corrosion property, zirconium-tin-niobium (Zr-Sn-Nb) alloys (iron/chromium precipitate-free) were prepared from a zirconium X-bar and subjected to corrosion test in 633 K water. Results showed that the corrosion resistance remarkably increased with increasing niobium content from zero up to 300 ppm and was approaching the level of VAZ alloys. It is thought that a very small amount of niobium, almost dissolved in the matrix, leads to the superior corrosion resistance of VAZ alloys in pure water.