Effect Of Irradiation On The Mechanical And Corrosion Properties Of Nuclear Stainless Steel Studied By SPM

During the 40-60 years's operation of the reactor,the metal materials used in the primary circuit have been damaged by the coolant at high temperature and pressure and intense radiation of the radioactive particles from the core of reactor.The pressure vessel(Reactor Pressure Vessel,RPV)could not be replaced once it has been damaged to produce local corrosion by irradiation due to its relatively large volume,which could lead to the shutdown of the entire reactor.In order to prevent corrosion of RPV inner side,the inner wall of the RPV have been protected by overlay welding of 308 L austenitic stainless steel material.The 308 L exhibited excellent mechanical and welding properties due to many limited size of ?-ferrites at the austenite phase's boundary.The mechanical and corrosion behavior of 308 L's different phases have not been yet distinguished.Therefore,it was particularly important to investigate the behavior of mechanical and corrosion of 308 L after irradiation.Similiarly,it was also of great significance for keeping the safe operation and life extension of nuclear power reactors in the future,as well as the exploration and development of advanced radiation-resistant materials should be investigated.The proton irradiation was used for simulating neutron irradiation to form certain degree damage.The simulation software The Stopping and Range of Ions in Matter.(SRIM)was conducted to systematically divide the regions affected by irradiation damage in the irradiated cross-section.The indentation mechanics scanning probe technology was performed to characterize the micro-mechanical properties of austenite and ferrite phases in different radiation damage regions.Scanning Kelvin probe force microscope(SKPFM)was conducted to study the Volta potential changes of the two phases in various damaged regions.By the favor of correlation between the electronic work function and the Volta potential,(Electron Probe Micro-analyzer,EPMA)was performed to characterize two phases'evolution in the composition of the different damaged regions.By the favor of combination of scanning electron microscope(SEM)and Time-of-flight secondary ion mass spectrometer(ToF-SIMS),a series of characterizations were performed on the corrosion morphology,and the composition and thickness of the air formed oxide film in different damaged regions.Finally,the electrical resistivity of the oxide film on two phases in different damaged regions of 308 L and 2507 super duplex steels was characterized by the current-sensing mode of atomic force microscope(CS-AFM).According to the effective characerizations of various behaviors above,the following conclusions have been drawn:The effect of irradiation on the hardening of the austenite and ferrite phases on the irradiated surface was different from that inside the irradiated bulk(uniform damage and peak damage region).In the entire irradiation damaged region,ferrite was suffered from the effects of decomposition hardening and irradiation-induced hardening at the irradiation temperature(360?)for more than 100 hours.However,the austenite phase was only affected by irradiation hardening in the entire irradiation damaged regions,and it had no influence on the hardning of the austenite phase by thermal aging.The different results of two phases' hardening of the irradiated surface and inside the bulk was caused by the different damage degree of the two regions by irradiation.After that,it caused the austenite be more easily susceptible to radiation damage on the irradiated surface than ferrite.Scanning Kelvin probe force microscope(SKPFM)test results showed that,the electronic work functions of two phases in different damaged regions were quite different.Two-phase's corrosion tendency of the irradiated surface had a significant flip phenomenon before and afrer proton irradiation.The irradiated ferrite phase become more stable and less prone to corrosion,while the austenite phase underwent opposite changes.The two phases in the irradiated cross-section(uniform damage region)had different phenomenon with that on the irradiated surface.It was found that the electronic work function of the two phases was reduced at the uniform damaged region,and it coud be seen that the corrosion tendency of both phases was significantly enhanced.The composition test of the irradiated surface and the inner phases was analyzed by EPMA,the result also proved that the Ni and Cr contents of the ferrite were increased on the irradiated surface,while the Ni content of austenite phase was increased and the Cr content was reduced;however,Ni content of the austenite phase and ferrite phase were both reduced in uniform damaged region.The corrosion morphology and the varies of the oxide film thickness and composition were characterized by SEM and ToF-SIMS,respectively.It showed that the thickness of austenite's oxide film in the peak damage region had reached the maximum value,which was caused by the more content of vacancies and a large number of implanted protons stayed here,and significantly to promote the growth of the oxide film.The thickness of the austenite phase oxide film in uniform damage region increased significantly but with no changing of ferrite phase,which was proved that the irradiation had promoted the growth of austenite phase's oxide film,while no major influence was found on the ferrite phase.After calculating the electrical resistivity of the oxide film,it was known that the resistivity of the two-phase oxide film decreased in the uniformly damaged region and the values reached the lowest in the peak damaged region.However,the resistivity of the ferrite-phase' s oxide film reduced in the uniform damaged region and the electrical resistivity was still larger than the austenite-phase.It was proved that the ferrite phase oxide film still had a certain degree of protection in this region.Further testing and analysis of the oxide film's composition change had been performed,it had found that the electrical resistivity of the two-phase's oxide film was reduced in the uniform damaged region,and which was caused by different kinds of defects were formed in the two phases,and the content of Cr oxidation had changed in the oxide film.The reason for the decrease of oxide film's the electrical resistivity of the two-phase in the peak damage region was that the presence of hydrogen and the increased formation of voids lead to the content of the protective Cr oxide component inside the oxide film decreased.Moreover,the content of the hydroxide component with poor protection was also increased.