Effect of Bi Addition on Corrosion Resistance of Zr-1Nb Alloy at 400 ℃ in Superheated Steam

Abstract:

In order to explore the the effect of Bi addition on the corrosion resistance of zirconium alloys, the corrosion behavior of Zr-1Nb-xBi alloys (x=0, 0.35, and 0.85, wt%) in superheated steam at 400 ℃/10.3 MPa was studied. Results reveal that the addition of a small amount of Bi (0.35wt%) can significantly improve the corrosion resistance of Zr-1Nb alloy, but further increase in Bi content (0.85wt%) can deteriorate the corrosion resistance. TEM observation and EDS analysis on the microstructures of the alloys indicate that 0.35wt% Bi has little effect on the type and size of the second phases in Zr-1Nb alloy, while adding 0.85wt% Bi can promote the precipitation of h-Zr₆Bi₂Fe and inhibit the precipitation of hcp-Zr(Nb,Fe)₂. Based on the microstructure observation and elemental valence state analysis of the oxide films, the deterioration mechanism of corrosion resistance of the high-Bi-content zirconium alloy primarily involves two aspects. On the one hand, Bi³⁺ increases the concentration of anionic vacancies in the oxide film, which deteriorates the corrosion resistance; on the other hand, the diffusion rate difference between Bi³⁺ and Fe³⁺ and the accumulation of local stress generated by the oxidation of h-Zr₆Bi₂Fe can make it easier to form micropores and microcracks around the oxidation product m-Bi₂O₃, thereby accelerating the corrosion process.