STUDY ON THE CORROSION RESISTANCE OF Zr-lNb-0.7Sn-0.03Fe-xGe ALLOY IN LITHIATED WATER AT 360 °C

Abstract

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-1Nb-0.7Sn-0.03Fe-xGe (x=0.05,0.1,0.2) alloys was remarkably superior to that of Zr-1Nb-0.7Sn-0.03Fe alloy.The corrosion resistance of Zr-1Nb-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 /3-Nb SPPs with the bcc crystal structure,the Zr-Nb-Fe-Cr-Ge SPPs with the tet structure and Zr_3Ge SPPs with the tet structure were detected out in Zr-1Nb-0.7Sn-0.03Fe-.rGe alloys.The oxidation of SPPs was found to be slower than that of Q-Zr matrix.There exist a few micro-cracks and more ZrO_2 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 ZrO_2 equiaxed grains appear in the oxide film formed on Zr-1Nb-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-1Nb-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-1Nb-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.