Study On Effect Of Ge,V And Pd Alloying Elements On The Corrosion Resistance Of Zirconium Alloys

Zirconium alloys have low thermal neutron absorption cross-section,good corrosion resistance,and adequate mechanical properties.They have been used as nuclear fuel cladding materials in PWRs of nuclear power plants.In order to meet the demand for higher burn-up and longer refueling period in PWRs,great efforts need to be made to improve the corrosion resistance of zirconium alloys.Alloying is one of the main methods to improve effectively the corrosion resistance of zirconium alloys.In this study,small amount of Ge,V or Pd was added to the zirconium alloys of Zr-4 and Zr-0.7Sn-1Nb,and the corrosion behavior was investigated in lithiated water with 0.01 M LiOH at 360 ? under a pressure of 18.6 MPa and in superheated steam at 400 ? under a pressure of 10.3 MPa,respectively.The microstructures of the alloys and oxide films,and the oxidation behavior of second phase particles were observed by HRTEM and HRSEM.It was discussed that the mechanism on the impact of these alloying elements on the corrosion resistance of zirconium alloys.The main experimental results and conclusions are as follows:(1)With increasing the amount of Ge,in Zr-4+xGe alloys,there exists orderly fine Zr(Fe,Cr)2 and Zr(Fe,Cr,Ge)2 second phase particles(SPPs)with hcp structure,Zr-Sn-Fe-Cr-Ge SPPs with larger size and coarse Zr3 Ge SPPs with tet structure;in Zr-0.7Sn-1Nb-xGe alloys,there exists orderly fine ?-Nb SPPs with bcc structure,(Zr,Nb)2(Fe,Cr)and(Zr,Nb)2(Fe,Cr,Ge)SPPs with tet structure,and coarse Zr3 Ge SPPs with tet structure.(2)The corrosion resistance of Zr-4 and Zr-0.7Sn-1Nb alloys in lithiated water with 0.01 M LiOH at 360 ? under a pressure of 18.6 MPa is remarkably improved by addition of 0.05%~0.1%Ge.When the Ge content further increases,the role of improving the corrosion resistance of alloys weakens.However,the corrosion resistance of Zr-4 and Zr-0.7Sn-1Nb alloys in superheated steam at 400 ? under a pressure of 10.3 MPa is degraded by addition of 0.05%~0.1%Ge.The more amount of Ge addition,the lower the corrosion resistance is.(3)Ge improves the surface free energy of Zr O2 by a primary principle of simulation after a small amount of Ge atoms to replace Zr atoms in the crystal lattice of ZrO2.(4)When the alloys are corroded in lithiated water,the solid solution of Ge in the ?-Zr matrix into ZrO2 by oxidation,would degrade the reducing extent of the surface free energy of ZrO2 because of Li+ and OH-incorporated in oxide films during corrosion,thus the process of vacancies developing to form micro-pores and micro-cracks can be delayed,and the process of the microstructural evolution of oxide films can be postponed.This is the main role for Ge addition to improve the corrosion resistance of zirconium alloy in the LiOH aqueous solution.The metastable phase(amorphous ZrO2,c-ZrO2 and t-ZrO2)is formed around oxide film of the fine Ge-containing SPPs during oxidation,that can reduce its surrounding compressive stress in oxide film,and the process of the microstructural evolution of oxide films can be retarded.This is the secondary role for Ge addition to improve the corrosion resistance of zirconium alloy in the LiOH aqueous solution.When the coarse Zr3 Ge SPPs are oxided,the local large additional stress is apt to be produced around Zr3 Ge.The stress concentration and larger micro cracks are produced easily.Thus,the microstructural evolution of oxide is speeded up and this is harmful effect of Ge on the corrosion resistance of zirconium alloys.(5)When the alloys are corroded in superheated steam at 400 ?,the delay effect of the solid solution of Ge in ?-Zr on the microstructural evolution of oxide film is not obvious because of higher corrosion temperature,and without the harmful effect of Li+.The influence of the SPPs on the corrosion resistance is larger.The volume expansion resulting from the oxidation of Ge-containing SPPs is not harmonious with that from the oxidation of its surrounding ?-Zr matrix.That will make the oxide film produce larger additional stress in oxide film around the SPPs,and accelerate the microstructural evolution of the oxide film,so that the corrosion resistance of the alloys is degraded.(6)In Zr-4+xV alloys,V is mainly precipitated as Zr(Fe,Cr,V)2 SPPs with hcp structure.In Zr-4+xPd alloys,it is precipitated in turn as fine Zr(Fe,Cr)2 SPPs with hcp structure,Zr2(Fe,Cr,Pd)SPPs with tet structure and coarse Zr2(Pd,Fe)SPPs with tet structure with increasing the amount of Pd.(7)The corrosion resistance of Zr-4 alloy in lithiated water is significantly improved by addition of 0.05%V or Pd.The role of improving the corrosion resistance of alloys weakens when the V or Pd content increases further.However,the corrosion resistance of Zr-4 alloy in super heated steam at 400 ? is degraded by addition of 0.05%V or Pd,and a downward trend for the corrosion resistance of alloys increases with further adding V or Pd content.(8)When Zr-4+xV(or Pd)alloys are corroded in lithiated water,the process of vacancies developing to form micro-pores and micro-cracks is delayed by small addition of V or Pd,and the microstructural evolution of oxide is put off.The corrosion resistance of the alloys corroded in the LiOH aqueous solution is improved.When Zr-4+xV(or Pd)alloys are corroded in superheated steam at 400 ?,the effect of the SPPs on the corrosion resistance is larger.Larger additional stress in oxide film around the SPPs is produced by the oxidation of SPPs containing V or Pd.The formation process of micro pores and micro cracks by diffusion and condensation of vacancies is promoted,and the microstructural evolution of oxide film is accelerated,so that the corrosion resistance of the alloy is degraded.