Effect of Final Annealing Temperature on Corrosion Resistance of SZA-6 Zirconium Alloy Cladding Tubes

The corrosion resistance of SZA-6 zirconium alloy(Zr-0.5Sn-0.5Nb-0.3Fe-0.015Si) cladding tubes finally annealed at 480°C, 510°C and 560°C were studied by static autoclave in 360°C/18.6 MPa pure water and 360°C/18.6 MPa/0.01 mol/L LiOH aqueous solution. The microstructure of the samples before and after corrosion were analyzed by EBSD, TEM and SEM. The results showed that the corrosion weight gains of the three SZA-6 alloy samples were lower than that of Zr-4 alloy after 500 days corrosion in both hydrochemical mediums. After long-term corrosion, the corrosion weight gains of SZA-6 alloy in pure water and LiOH aqueous solution increased obviously with the final annealing temperature, while the corrosion weight gain of unstressed Zr-4 alloy was higher than that of recrystallized under the same condition. With the increase of the final annealing temperature, the high-angle grain boundaries in the alloy larger than 15° became more and recrystallization degree also increased. The Second Phase Precipitates (SPPs) were fine, uniform, and dispersively distributed with an average diameter of about 120 nm. Although the size and distribution of the SPPs were similar, the Nb/Fe ratio in the SPPs increased. The long-term corrosion weight gain of zirconium alloy was related to the number of parallel cracks in the oxide film and the uneven growth degree of the oxide film on the interface of the oxide film/matrix. The corrosion resistance of the alloy in two hydrochemical mediums was related to the degree of recrystallization and the content of Nb in the SPPs. Increasing the final annealing temperature would promote the formation of fine and uniform recrystallized grains, which was benefit to the corrosion resistance, but at the same time it would reduce the content of solid solution Nb in the αZr matrix, which in turn would be detrimental to the corrosion resistance.