ANALYSES OF SURFACE OXIDE FILMS ON ELECTROPOLISHED ALLOY 690TT AFTER IMMERSION FOR DIFFERENT TIMES

Abstract

The corrosion of Ni base alloys in high temperature and high pressure water is affected by samples surface statuses.The morphologies and structures of surface oxide films grown on electropolished(EP) alloy 690TT after immersion in the simulated hydrogenated primary water of pressured water reactors(PWRs) for different times were analyzed by AFM,SEM,TEM,EDS and XPS.After immersion for 15 and 35 h,the EP alloy 690TT samples were covered with columnar oxides. With the increase of the immersion time,the sample surfaces were covered with scattered big oxide particles and loose needle-like oxides.Regardless of the immersion time,the formed oxide films are composed of spinel oxides and metallic Ni.After immersion for 720,1440 and 2160 h,the oxide films are composed of three layers:the outmost layer is the separated big oxide particles which are rich in Fe and Ni:the intermediate layer is the loose needle-like oxides rich in Ni;the inner layer is the continuous and compact Cr oxides.The peak decomposition of the XPS revealed that the Cr oxides in the inner layer are probably Cr_2O_3.Only the inner layer in the oxide film could restrain the outward diffusion of metal atoms and also the inward diffusion of the oxygen atoms and then played the role of barrier layer well. Electropolishing treatment disadvantaged the fast growth of protective oxide film on alloy 690TT in the studied solution.The average corrosion rate of the inner layer does not decrease gradually with increasing the immersion time.After immersion for 2160 h,the oxide film still could not protect the matrix from further corrosion.