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+β_Nb phase and at 640 °C for 10 h to get the α+β_Zr 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 β_Zr phase, while retarded in the sample having β_Nb phase. This means that the formation of β_Nb phase resulted in the reduction of Nb concentration in the α matrix, thus the corrosion resistance was enhanced with the formation of β_Nb 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 (β_Nb or β_Zr).