Effect of β phase, precipitate and Nb-concentration in matrix on corrosion and oxide characteristics of Zr–xNb alloys

The corrosion test and oxide characterization were performed on the specimens having different Nb-content in the range of 0–5 wt%. The specimens were heat-treated at 570 °C for 500 h to get the α+βNb phase and at 640 °C for 10 h to get the α+βZr phase after β-quenching. The corrosion tests were carried out at 360 °C. In the low Nb-contents of 0.1–0.2 wt% where Nb was soluble in the matrix without the formation of Nb-containing precipitates or β phase, the samples showed the excellent corrosion resistance and their corrosion resistance was not affected by heat-treatment. The corrosion resistance was improved by the stabilization of tetragonal ZrO2 and columnar oxide structure when all added Nb was soluble in the matrix to equilibrium concentration. In the high Nb-contents of 1.0–5.0 wt%, the corrosion rate was very sensitive to the annealing condition. The transformation of oxide crystal structure from tetragonal ZrO2 to monoclinic ZrO2 and oxide microstructure from columnar to equiaxed structure was accelerated in the samples having βZr phase, while retarded in the sample having βNb phase. This means that the formation of βNb phase resulted in the reduction of Nb concentration in the α matrix, thus the corrosion resistance was enhanced with the formation of βNb phase. From the corrosion test and oxide characterization, it is suggested that the equilibrium concentration of Nb in the α matrix would be a more dominant factor to enhance the corrosion resistance than the Nb-containing precipitates, supersaturated Nb, and β phase (βNb or βZr).