Effect Of Nb On The Corrosion Anisotropy Behavior Of Zr-Sn Series Zirconium Alloys

Zirconium alloys,a crucial structural material,have been widely used for fuel cladding in water-cooled nuclear power plants for decades due to their low thermal neutron cross-section,proper corrosion resistance and mechanical properties.Corrosion resistance of zirconium alloys is of anisotropy as well as their other characteristic due to their hexagonal close-packed crystal structure.Also because of crystal structural characteristic of zirconium alloys,a strong texture is inevitably produced in sheet or tubular materials during the fabrication process.The anisotropic characteristic is bound to be reflected on the corrosion behavior and the texture will influence the corrosion resistance of zirconium alloys.Based on the oxides thickness on the rolled surface?S_N?,the surface?S_R?perpendicular to rolling direction and the surface?S_T?perpendicular to transversal direction as a function of exposure time for specimens,the effect of Nb on the corrosion anisotropy of Zr-0.75Sn-0.35Fe-0.15Cr alloy by auto clave tests in aqueous solution with various LiOH concentration has been investigated.The influence of Nb on the microstructure of Zr-0.75Sn-0.35Fe-0.15Cr-xNb alloys was studied by using EBSD,HRSEM and HRTEM;Hydrogen produced by the corrosion process was studied by comprehensive analysis methods of using O,N and H analyzer,metallographic microscopes and SIMS;The concentration of C,Sn,Fe,Cr in the oxide film was analyzed with the help of SIMS;Interactions between Nb and C,Sn,Fe,Cr in Zr-0.75Sn-0.35Fe-0.15Cr alloy were studied by using SIMS and XPS.All these studies were to seek for the reason why Nb can inhibit the anisotropic corrosion of Zr-0.75Sn-0.35Fe-0.15Cr alloy.Finally,mechanism of Nb inhibition effects on the anisotropic corrosion of Zr-0.75Sn-0.35Fe-0.15Cr alloy was proposed from the perspective of hydrogen absorption and interactions between Nb and other elements in the alloy.The main experimental results and conclusions are as follows:?1?When corroded in deionized water,lithiated water with 0.01 M and 0.04 M LiOH aqueous solution at 360?and 18.6 MPa respectively,the growth of oxide film on S_N surfaces is faster than that on S_R and S_T surfaces for Zr-0.75Sn-0.35Fe-0.15Cr alloy,showing obvious anisotropic corrosion characteristics.LiOH aqueous solutioon can accelerate the corrosion of Zr-0.75Sn-0.35Fe-0.15Cr-xNb alloy.The required Nb contents adding into Zr-0.75Sn-0.35Fe-0.15Cr alloy to inhibit the anisotropic corrosion are 0.15%,0.50%and 1.0%corressponding to the above corrosive media,positively correlated with the LiOH concentration in corrosive media.The addition of Nb can inhibit the growth rate of the oxide film on the S_N surface but can also increase the growth rate of the oxide film on the S_R and S_T surfaces.?2?Typical rolling texture exists in the four types of alloy,that is,?0001?plane of grains mainly distributed on S_N surface,?1-210?and?10-10?planes of grains are mainly distributed on S_R and S_T surfaces,respectively.The addition of Nb has no significant influence on the texture of Zr-0.75Sn-0.35Fe-0.15Cr alloy.?3?The crystal structure of second phase particles transforms from fcc-Zr?Fe,Cr?₂ to hcp-Zr?Fe,Cr,Nb?₂ with the Nb content changing from 0%to 1.0%.Nb/Fe ratio increases and Fe/Cr ratio changes little with the increasing Nb content.Fe diffuses outward the oxide film and diffuses faster than Cr when Zr?Fe,Cr?₂ is oxidized.Nb can inhibit the Fe diffusion,whose inhibition effect becomes more obvious with the increase of Nb content in the Zr?Fe,Cr,Nb?₂.?4?Oxide films on Zr-0.75Sn-0.35Fe-0.15Cr-xNb alloys all consist of equiaxial crystal grains with small fraction near the outer surface and columnar crystal grains with large fraction.Micropores and microcracks are common in all oxide films and Nb lead more micropores in oxide films.?5?Hydrogen absorbing fraction of Zr-0.75Sn-0.35Fe-0.15Cr-xNb alloys corroded in deionized water and 0.01 M LiOH aqueous solution at 360?and 18.6MPa declines with the exposure time.The Hydrogen uptake fraction of Nb containing alloys is smaller than that of Zr-0.75Sn-0.35Fe-0.15Cr alloy for a relative long time exposure,which indicates that Nb is benefit to reducing the hydrogen uptake of the zirconium alloys.?6?The concentration,distribution and existing form of alloying elements in oxide films have significant influence on the defects concentration,stress level and mass transportation in oxide films.The distribution of Sn,Fe,Cr,C and H in oxide film on S_N surface and S_T surface of Zr-0.75Sn-0.35Fe-0.15Cr alloy is anisotropic.Nb addition can reduce C concentration and the concentration gap in oxide film between S_N surface and S_T surface,produce an oxide layer with a stable hydrogen concentration in the oxide film and reduce the gap of hydrogen diffusion coefficient in the oxide film between S_N and S_T surfaces,change the surface segregation trends of Sn,Fe or Cr on S_N and S_T surfaces.This inevitably has effect on defects concentration,stress level and mass transportation in oxide films to impact the corrosion rate of S_N and S_T surfaces,achieving an inhibitation effect on anisotropic corrosion of Zr-0.75Sn-0.35Fe-0.15Cr alloy...