Zr-1Nb-0.7Sn-0.03Fe-xGe合金在360℃LiOH水溶液中耐腐蚀性能的研究

STUDY ON THE CORROSION RESISTANCE OF Zr-1Nb-0.7Sn-0.03Fe-xGe ALLOY IN LITHIATED WATER AT 360℃

对添加微量合金元素Ge的Zr-1Nb-0.7Sn-0.03Fe-xGe(x=0,0.05,0.1,0.2,质量分数,%)合金在360℃,18.6 MPa和0.01 mol/L LiOH水溶液中进行静态高压釜腐蚀实验.利用TEM和SEM研究了合金和氧化膜的显微组织.结果表明:添加适量Ge可以显著提高Zr-1Nb-0.7Sn-0.03Fe合金在360℃,18.6 MPa和0.01 mol/L LiOH水溶液中的耐腐蚀性能;在Zr-1Nb-0.7Sn-0.03Fe-xGe合金中,除了存在bcc结构的β-Nb型第二相和四方结构的Zr-Nb-Fe-Cr第二相,还存在四方结构的Zr-Nb-Fe-Cr-Ge和四方结构的Zr_3Ge型第二相;这些第二相的氧化速率比α-Zr基体慢.腐蚀190 d后,Zr-1Nb-0.7Sn-0.03Fe-0.1Ge合金氧化膜中微裂纹较少,并且存在较多的Zr_2O柱状晶;添加Ge既可以有效延缓氧化膜中的缺陷形成微孔隙和微裂纹的过程,又可以延迟Zr_2O柱状晶向等轴晶的演化,因而可以提高合金的耐腐蚀性能.

Zirconium alloys have low thermal neutron absorption cross-section, good corrosion resistance and adequate mechanical properties. They have been successfully developed as fuel cladding materials in pressurized water reactors. It＇s well known that the corrosion resistance of Zr-Sn-Nb alloys is significantly superior to that of Zircaloy-4 alloy when corroded in lithiated water. The corrosion resistance of zirconium alloys is controlled by their chemical compositions, characteristics of second phase particles （SPPs） and microstructure evolution of the oxide in them. The corrosion tests of Zr- 1Nb-0.7Sn-0.03Fe-xGe （x=0, 0.05, 0.1, 0.2, mass fraction, %） alloys were investigated by means of an autoclave test in lithiated water with 0.01 mol/L LiOH at 360 ~ under a pressure of 18.6 MPa. The microstructures of the alloys and oxide films on the corroded specimens were examed by using TEM and SEM. The sample for the oxide microstructure observation was prepared by a HELIOS- 600I focused ion beam. The results reveal that the corrosion resistance of Zr-lNb-0.TSn-0.03Fe-xGe（x=0.05, 0.1, 0.2） alloys was remarkably superior to that of Zr-lNb-0.7Sn-0.03Fe alloy. The corrosion resistance of Zr-lNb 0.7Sn-0.03Fe alloys is markedly improved by the addition of （0.05%--0.2%）Ge. In addition to Zr-Nb-Fe-Cr SPPs with the tetragonal crystal structure （tet） and /^-Nb SPPs with the bcc crystal structure, the Zr Nb-Fe-Cr-Ge SPPs with the tet structure and Zr3Ge SPPs with the tet structure were detected out in Zr 1Nb-0.7Sn-0.03Fe-xGe alloys. The oxidation of SPPs was found to be slower than that of c^-Zr matrix. There exist a few micro-cracks and more ZrO2 columnar grains in the oxide film formed on Zr 1Nb-0.7Sn-0.03Fe-0.1Ge alloys corroded for 190 d. However, more micro-cracks and ZrO2 equiaxed grains appear in the oxide film formed on Zr-lNb-0.7Sn-0.03Fe alloys corroded for 130 d. Because the P. B. ratio of Ge is smaller than those of Zr, Nb, Fe and Cr, it is likely that the volume expansion of the oxide on Zr 1Nb-0.7Sn-0.03Fe xGe （x--0.05, 0.1, 0.2） alloys is smaller than that on Zr-lNb-0.7Sn-0.03Fe alloy, and the compressive stress can be reduced and the micro-cracks can be effectively decreased in the oxide on Zr-lNb-0.7Sn-0.03Fe-xGe （x=0.05, 0.1, 0.2） alloys. The addition of Ge can not only delay the developing process of the defects in oxide films to form micro pores and micro-cracks, but also retard the microstructural evolution from columnar grains to equiaxed grains. Therefore, it is concluded that the addition of Ge can improve the corrosion resistance of alloy.