Effect Of Surface State On Corrosion Behavior Of Materials Of Steam Generator Tubes In High Temperature And High Pressure Water

The steam generator(SG)tubes of pressured water reactor(PWR)nuclear power plant are one of important structural components.During the long-term service,the initiation and development of corrosion usually originate from the surface or subsurface area,and SG tubes usually undergo the strong plastic deformation processes such as surface scratch,high speed shot-peening during the factory,transportation and installation.Thus,the materials surface conditions seriously affect their long-term service performance.It is important to investigate the surface state on long-term corrosion behavior and short-term electrochemical behavior of tubes in service environment.This provides data support for the safety of key components and design and manufacturing of core components in the future.In this paper,the effect of dissolved oxygen(DO),dissolved hydrogen(DH),surface scratch pressure,solution temperature and applied potential on the surface of Alloy 625 with deformation surface,Alloy 800 with shot-peening treatment and Alloy 690TT with scratch are investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),transmission Kikuchi diffraction(TKD),focused ion beam(FIB),transmission electron microscopy(TEM)and precession electron diffraction(PED),simulated PWR water corrosion experiment,quick scratching electrode technique and electrochemical methods.The matrix microstructure after deformation,the cause of cracks,oxide microstructure,preferential corrosion and electrochemical propertied are investigated in details.The microstructure of Alloy 625 with deformation surface,general corrosion and preferential corrosion behavior under two types of DO(DO<10 ppb and DO=2000 ppb)are investigated.The results show that there is a thin plastic deformation zone on the surface of Alloy 625.The gradient nanostructure is characterized by TEM from the surface to the inside of the sample.In high temperature and high pressure water,the double-layer oxide on the surface of Alloy 625 is the face-centered cubic structure.The preliminary determination of the oxide is Ni(FeCr)2O4.The oxide film initially grows along the matrix structure.In particular,preferential corrosion is found to grow along the Σ3 grain boundary.When Alloy 625 surface deforms severely,the mechanical twins,micro-deformation bands and other dislocation structure interact with each other,and the interface energy of Σ3 boundary increases,resulting in improve the chemical activity and diffusion rates of oxygen and metal ions.Finally,the preferential corrosion occurs here.The microstructure of Alloy 800 with shot-peening,cracks in the subsurface,general and preferential corrosion behavior under two types of DO environments(DO<10 ppb,DO=300 ppb)are investigated.The results show that there is about 80 μm deformation region in the cross section of Alloy 800 and the hardness values gradually decrease from surface to matrix.The gradient nanostructure is characterized by TEM from the surface to the inside of the sample.A bowl-like unstable crack is found in the subsurface before corrosion.In high temperature and high pressure water,the outer oxide is Fe2(NiCr)O4 and inner oxide is(FeNi)Cr2O4 when DO<10 ppb.When DO=300 ppb,the outer and inner oxide are Fe2(NiCr)O4.The oxide film initially grows along the matrix structure.In particular,a bowl-like preferential corrosion propagates into the matrix.The unstable crack and preferential corrosion which are located at 500 nm from surface are also located between the nanostructure layer and ultra-fine grain layer.The deformation and residual strain within the nanostructure layer are low above the preferential corrosion,while the residual strain and deformation within ultra-fine grain layer are high.There is a great deal of difference of the residual strain and deformation between two sides of interface,resulting in the accumulation of residual strain around interface,and the mechanical crack and preferential corrosion could occur due to the strain localization.The matrix dissolution and oxide growth behavior of Alloy 690TT with scratching surface is investigated in simulated primary water.The results show that the oxide growth of Alloy 690TT conforms to the place exchange model(PED)in the initial stage,and the oxide growth of alloys conforms to high field ion conduction model(HFM)in the final stage.cBV represents the oxide growth kinetics parameter and the larger the cBV values are,the faster the oxide growth rate is.The cBV decreases with scratching pressure increases under different scratching pressure.It indicates the oxide growth rate increases gradually.The scratching pressure can improve more micro defects in the matrix and promote the passive film growth.The cBV increases with solution temperature increases under different temperature.It indicates that the oxide growth rate decreases gradually.According to Avrami kinetics,PEM and HFM,the detailed kinetics processes are analyzed at each oxide growth period.The cBV increases with applied potential increases under different applied potentials.It indicates that the oxide growth rate decreases gradually.The micro-process of electrochemical reactions is elaborated by point defect model(PDM).The cBV increases with DH increases under different DH contents(0.5 ppm～1.6 ppm).It indicates that the oxide growth rate decreases gradually.