In order to investigate the effects of Nb content and accumulated annealing parameter on corrosion resistance in Nb-containing zirconium alloys, which were consisted of Zr-0.8Sn-xNb(x=0.2, 0.4, 0.8 wt.%) and Zr-0.8Sn-0.4Fe-0.2Cr-xNb(x=0.1, 0.2, 0.4, 0.8 wt.%), the corrosion tests were carried out using static autoclave in the conditions of 400℃ steam and 70 ppm LiOH solution at 360℃. The corrosion resistance of the alloys was evaluated from weight gain after corrosion tests. And the precipitates in Zr matrix were observed and identified by TEM with EDX. A small angle x-ray spectrometer was applied to examine the oxide crystal structure in regime of pre-transition. Also the hydrogen pick-up of zirconium matrix and the morphology and structure of post-transition oxide after the long-term corrosion were assessed and discussed with the variation of Nb content and accumulated annealing parameter.
As the accumulated annealing parameter increased, the corrosion resistance was rapidly increased in Zr-0.8Sn-0.2Nb alloys but decreased in Zr-0.8Sn-0.4Nb and Zr-0.8Sn-0.8Nb alloys in both corrosion conditions. The degradation of corrosion resistance in Zr-0.8Sn-0.4Nb was resulted from the decrease of Nb concentration in matrix and the increase of Nb concentration in precipitates due to the formation of Nb-containing precipitates when the accumulated annealing parameter increased. The increase of tetragonal $ZrO_2$ in the oxide of Zr-0.8Sn-0.4Nb gave rise to increase the corrosion resistance with decreasing the accumulated annealing parameter. It was thought that the corrosion resistance of Zr-0.8Sn-0.8Nb increased with the decrease of super-saturated Nb content in matrix caused by the formation of β-Nb.
In the case of Zr-0.8Sn-0.4Fe-0.2Cr-xNb alloys, the corrosion resistance was decrease with increasing the Nb content and accumulated annealing parameter. It was thought that the trend of corrosion resistance between Zr-0.8Sn-0.4Fe-0.2Cr-0.2Nb and Zr-0.8Sn-0.2Nb was conflicted with each other since the Nb solubility in Zr-0.8Sn-0.4Fe-0.2Cr-0.2Nb alloy was less than that in Zr-0.8Sn-0.2Nb alloy due to addition of Fe and Cr. The oxide of Zr-0.8Sn-0.4Fe-0.2Cr-0.8Nb alloy with $∑A=3.49×10^{-17}$ hr was spalled out due to the Li-induced acceleration corrosion in LiOH solution. It was concluded that the corrosion resistance of Zr-0.8Sn-0.4Fe-0.2Cr-xNb alloys could be controlled by the characteristics of (Zr,Nb)$(Fe,Cr)_2$ type precipitates with the variation in Nb content and accumulated annealing parameter.
And, the corrosion rate of Zr-0.5Nb-1.0Sn-0.5Fe-0.25Cr alloy increased with increasing the accumulated annealing parameter. The relative fraction of tetragonal $ZrO_2$ also decreased gradually with increasing accumulated annealing parameter. From the hydrogen analysis of the corroded samples for 300 days, it was observed that, with increasing the size of precipitates, the hydrogen pickup was enhanced. It was revealed from TEM observation of the oxide that the larger precipitates still remained to be oxidized in the oxide layer and had undergone a reduction of Fe/Cr ratio from 2.1 to 1.5. The oxidation of the precipitates in the oxide gave rise to a volume expansion at the precipitate-oxide interface. This volume change could lead to the transformation in the oxide phase from tetragonal $ZrO_2$ to monoclinic $ZrO_2$ and in oxide structure from columnar grain to equiaxed grain. The precipitate in a Zr-0.5Nb-1.0Sn-0.5Fe-0.25Cr alloy is composed of Nb, Fe, and Cr and the Nb content in the precipitate increase with increasing accumulated annealing parameter. It can be thought that Nb in precipitates plays a key role in the microstructural change of oxide.
In conclusion, the corrosion resistance of Zr-0.8Sn-xNb alloys in the case of less than solubility of Nb was affected by $Zr(Fe,Cr)_2$ type precipitates and the size and density of the precipitates increased with increasing the accumulated annealing parameter. When the Nb content was more than solubility, the Nb-containing precipitates were formed and coarsened with increasing the accumulated annealing parameter. This resulted in the decrease of corrosion resistance in the alloy of more than Nb solubility of Zr-0.8Sn-xNb alloys. In addition, as the accumulated annealing parameter increased, the corrosion resistance of Zr-0.8Sn-0.4Fe-0.2Cr-xNb alloys decreased but the size and density of Nb-containing precipitates in the alloys increased. These are contrary to the trends of commercial Zircaloy-4 cladding in same test conditions. It is thought that the main reason should be different from each alloy in composition of precipitates. When Nb is added in a zirconium-based alloy, the Nb concentration in matrix would be constant regardless of the accumulated annealing parameter and the corrosion resistance of Nb-containing zirconium alloy would be improved in comparison with a zirconium alloy without Nb. Additionally, the concentration of supersaturated Nb in matrix is reduced by coarsening of Nb-containing precipitates as the accumulated annealing parameter increased. These changes of Nb concentration in precipitates and matrix would affect the corrosion resistance of the Nb-containing zirconium alloys. It is concluded that the corrosion properties of the Nb-containing zirconium alloys be more improved than those of commercial Zircaloy-4 cladding by controlling the size and density of precipitates throughout the optimal thermal treatment.
