A Study Of The Mechanism For The Anisotropic Corrosion Of Zirconium Alloys

Generally speaking,some alloying elements can segregate on the surfaces of an alloy specimen during the heating.This phenomenon could affect the absorption of the oxygen and the corrosion resistance of the specimens.So the surface segregation of alloying elements could be an important factor in the effect of the corrosion resistance of zirconium alloys.In this work,Sn contents on the rolling surface?S_N?,the surface perpendicular to transversal direction?S_T?and the surface perpendicular to rolling direction?S_R?of plate specimens of Zircaloy-4?Zr-Sn alloy?and B alloy?Zr-Sn-Nb alloy?,which possess similar textures of?0001?[10-10]and?0001?[11-20],are gauged by X-ray photo-electron spectroscopy?XPS?at 298 K,773 K and 873 K.The results show that surface segregation of Sn in Zircaloy-4 specimens is anisotropic,the level of the Sn surface segregation on these surfaces is in the order of S_T?S_R>S_N.This characterisic is also well verified by the results obtained from a coarse-grained?0.2-0.8 mm in diameter?Zircaloy-4 specimen after heating to 873 K and the bond-breaking theory calculation.In addition,the level of Sn surface segregation of B alloy specimens is obviously higher than that of Zircaloy-4 specimens at 873 K although the Sn content of B alloy is lower than that of Zircaloy-4,but the anisotropy is slightly weaker than that of Zircaloy-4.The reason of this phenomena is that the alloying element Nb soluted in?-Zr matrix could prompt the diffusion of Sn.The effect of the alloying element Nb on the diffusion of Sn is verified by the calculation results of the heat of mixing of Zr₅₃Sn₁ and Zr₅₂Sn₁Nb₁ models calculated by density functional theory?DFT?.At 773 K,there is nearly no Sn segregation on the surfaces of B alloy specimens.It is reckoned that the number of second phase particles?SPPs?could be an important factor in this phenomenon since the Sn could also segregate at the interfaces between SPPs and matrix.In 773 K/10.3 MPa superheated steam,the corrosion resistance of Zircaloy-4plate specimens is anisotropic.The S_N surface mainly suffers uniform corrosion,while S_T and S_R surfaces mainly suffer nodular corrosion.The chemical compositions at O/M interfaces of the thicker oxide and the thinner oxide are detected by APT.The results show that the Sn segregation at the O/M interface of the thicker oxide area is obvious as compared with that at the O/M interface of the thinner oxide area.A novel mechanism of nodular corrosion related to Sn segregation at the interface between oxide and matrix is proposed.The anisotropic nodular corrosion results from the anisotropy of Sn segregation at the O/M interfaces formed on S_N,S_R and S_T surfaces with different grain orientations.But the corrosion resistance of B alloy specimens in773 K/10.3 MPa superheated steam is isotropic,i.e.all S_N,S_R and S_T surfaces are subjected to uniform corrosion.The isostropic corrosion of B alloy could results from the more of second phase particles precipitated in B alloy due to the addition of Nb.In 633 K/18.6 MPa 0.01 M LiOH aqueous solution,the corrosion resistance of Zircaloy-4 is anisotropic,i.e.the corrosion rate on S_N surface is obvious faster than those on S_R and S_T.The effect of the morphologies of fracture surface,inner and outside surface of the oxide film,the micro-cracks in oxide film,the distribution of Li⁺ion and alloying elements in the oxide film and the misorientation of oxide grains on the anisotropy of the growth of oxide layers in Zircaloy-4 is discussed.Finally,we consider the misorientation of oxide grains formed on the grain surfaces with different orientations of?-Zr is the key factor.However,the corrosion resistance of B alloy specimens corroded in 633 K/18.6 MPa 0.01 M LiOH aqueous solution is isotropic,and the results could be attributed to the segregation of Nb at the grain boundaries of oxide grains.But no useful experimental results have been got due to the limited experimental conditions,so a tentative discussion is only conducted.