Investigation On Microsturcture And Corrosion Resistance Of Nickel Based Alloy 690 Of Nuclear Pressure Vessel And Safe End

Nickel based alloy 690 is applied for the third generation Pressurized Water Reactor?PWR?nuclear power plant due to its excellent high-temperature strength,thermal stability and corrosion resistance.The filler metal 52M of Inconel 690 is used for the dissimilar metal welded joints between pipe nozzles of nuclear pressure vessels and safe ends in the primary water system.However,some defects like stress corrosion cracking?SCC?,intergranular corrosion?IGS?will occur for the combined effects of longer nuclear power operation time,the reaction with reactor coolant,thermal degradation of the material and the residual stress in welded joint,etc.Therefore,investigation on the as-welded microstructure and corrosion resistance of nickel based alloy at nuclear power plant safe end has great significance to understand the safety and stability of the low alloy steel-nickel based alloy joint in primary water environment.In this study,the as-deposited nickel based alloy overlay are fabricated through different welding processes.And post-welding heat treatment are carried out on the as-deposited nickel based alloy.Then the microstructure including crystal orientations,grain boundary characters and precipitates are characterized with optical microscope?OM?,scanning electron microscope?SEM?,electron backscattered diffraction?EBSD?.The mechanical properties of nickel based overlays under different welding conditions are evaluated.According to the results,the influence of the welding processes and heat treatment on the microstructure,like high angle grain boundary distribution ratio and the morphology of grain boundary M₂₃C₆ precipitates of nickel based overlays are explained.Corrosion resistance of the nickel based overlays are studied by electrochemical tafel polarization curves,double loop electrochemical potentiokinetic reactivation?DL-EPR?,electrochemical impedance spectroscopy?EIS?and slow stain rate test?SSRT?.The results show that,welding processes will affect the corrosion potential,corrosion current density,intergranular corrosion and stress corrosion cracking sensitivity of nickel based overlays.The nickel based overlays processed by high heat input cold wire TIG exhibit worse corrosion resistance to Cl^-,higher corrosion speed,larger IGS and SSC sensitivity compared that processed by hot wire TIG.And heat treatment under 600?of overlays leads to decrease in corrosion resisstance.The local corrosion resistance of nickel based alloy 690 overlay on low alloy steel-alloy 690 weldment-alloy 690 pipe butt joint are tested by tafel polarization curves in chloride ion solution and alkaline solution,DL-EPR,EIS and SSRT.The results show that,the alloy 690 overlay has the worst corrosion resistance,the largest corrosion speed and the highest sensitivity to IGS in the whole joint.By contrast,the butt joint zone has the best corrosion,IGS and SSC resistances.