添加Bi对Zr-1Nb合金在400 ℃过热蒸汽中耐腐蚀性能的影响

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

摘要:

为了探究添加Bi对锆合金耐腐蚀性能的影响，研究了Zr-1Nb-xBi合金（x=0、0.35和0.85，质量分数，%）在400 ℃/10.3 MPa过热蒸汽中的腐蚀行为。结果表明，添加0.35% Bi能明显改善Zr-1Nb合金的耐腐蚀性能，但进一步增加Bi含量（0.85%）则会恶化合金的耐腐蚀性能。TEM和EDS分析结果表明，添加0.35% Bi对Zr-1Nb合金中第二相的种类和尺寸没有明显影响，而添加0.85% Bi会使合金中析出h-Zr₆Bi₂Fe第二相，并且抑制hcp-Zr(Nb,Fe)₂的析出。氧化膜的显微组织观察和元素价态分析结果表明，高Bi含量的锆合金耐腐蚀性能恶化机制主要有2个方面，一是Bi³⁺导致氧化膜中参与反应的阴离子空位浓度增加，从而恶化耐腐蚀性能；二是h-Zr₆Bi₂Fe氧化时产生的局部附加应力和Bi/Fe的扩散速率差会使其氧化产物m-Bi₂O₃周围更容易形成微孔隙和微裂纹，从而加速腐蚀进程。

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.