Investigation Of High Temperature Water Corrosion Behaviour Of Fe-Cr Based Alloy Without And With Irradiation

From pressurized water reactor to the generation?supercritical water reactor,the materials are exposed to high dose irradiation and high-temperature water corrosion.The higher coolant temperature leads to a sharp increase in the oxidation rate of the material,and the radiation damage caused by the interaction between the material and the high-energy particles inside the reactor further leads to the deterioration of the material performance in all aspects and even the failure of the material.The service behavior of water-cooled reactor materials,especially cladding materials,in the environment of strong irradiation and high temperature corrosion affects the safety and development of water-cooled reactor.Therefore,it is necessary to conduct related studies to simulate the service environment of reactor materials and evaluate the corrosion resistance and irradiation performance of relevant materials,further understanding the degradation mechanism of materials in the corrosive and irradiated environment,and predicting the service behavior of materials.In the highly corrosive environment of water-cooled reactor,the evaluation and investigation on the corrosion resistance of materials is a problem that needs to be paid more attention.The high temperature and high pressure water as the coolant of the water-cooled reactor is in a state of rapid flow(the coolant velocity in pressurized water reactor can reach about 3 m/s,and the core velocity of the supercritical water-cooled reactor is also above 1 m/s).In order to simulate the high temperature water environment of water cooled reactor,we designed and built the high temperature and high pressure water dynamic corrosion test circuit.The loop has a maximum operating temperature of 700?and a flow rate of 10 m/s.In this paper,the corrosion behavior of ferrite/martensitic steel(SIMP,T91)and austenitic stainless steel(316L,15-15Ti),two candidate structural materials for supercritical water-cooled reactor(SCWR),in fast-flowing high temperature and high pressure water has been studied by using the test circuit.The corrosion behavior of Fe-Cr alloy and related mechanism were investigated by means of high-precision electronic balance,optical microscope(OM),X-ray diffraction(XRD),Raman spectroscopy(Raman),atomic force microscope(AFM),scanning electron microscope(SEM)and transmission electron microscope(TEM)from the perspective of dynamics and microstructure.The effects of temperature,time,dissolved oxygen content and flow rate on the oxidation corrosion behavior of Fe-Cr alloy were systematically studied,with emphasis on the initial corrosion behavior of materials and the related oxidation corrosion mechanism.At the initial stage of oxidation,the oxidation kinetic curves of austenitic steels followed near-linear rate equation,while those of ferrite/martensite steels followed near-parabolic rate equation.The oxide film structure of ferritic/martensitic steels and austenitic steels were similar.The oxide film was a double-layer structure,the outer layer was riched in Fe and composed of the columnar crystal oxide of the magnetite,and the inner layer was riched in Cr and composed of the porous Fe-Cr spinel oxides.The difference was that Ni was enriched in the oxide film/matrix interface of the austenitic steel.The high temperature water corrosion resistance of austenitic steels with higher Cr content was better than that of ferrite/martensite steels.The addition of Si element promoted the selective oxidation of Cr and Si at the oxide film/matrix interface,and a better protective(Cr,Si)O_x formed in SIMP.In the temperature range of 350?to 500?,the phase structure of oxide film of ferrite/martensitic steel and austenitic steel did not change,but the weight gains increased exponentially with the increase of temperature.The oxidation rate of ferrite/martensitic steel was controlled by the diffusion rate of O along the grain boundaries and pores of the oxide film,while the oxidation rate of austenitic steel was controlled by the interface reaction process.Dissolved oxygen had an important effect on the structure and growth rate of the oxide film.The increase of dissolved oxygen caused the exfoliation of oxide film and the formation of Fe₂O₃ on the oxide film surface of ferrite/martensitic steel and austenitic steel.The formation of Fe₂O₃ was not only related to the dissolved oxygen,but also related to the flow rate and oxidation rate.The increase of dissolved oxygen content and flow rate,and the decrease of oxidation rate contributed to the formation of Fe₂O₃ layer.In high temperature water with high flow rate,the increase of flow rate caused the decrease of hydrogen content and the increase of oxygen partial pressure in high temperature steam,which leaded to the increase of oxidation rate,and caused the stable formation of Fe₂O₃ in high temperature steam with low oxygen content.At the same time,the degradation mechanism of the corrosion resistance of structural materials caused by irradiation damage is another scientific problem that needs to be paid more attention in the highly irradiated and corrosive environment of water-cooled reactor.We further studied the change of the corrosion properties of the materials under irradiation damage.The effects of displacement damage and elemental doping on the corrosion resistance of ferrite/martensitic steels have been studied by using Fe and He ions.It was found that neither Fe ion irradiation nor He ion irradiation affected the phase structure and elemental composition of the oxide film,but the irradiation damage significantly promoted the corrosion of the materials.The high temperature water corrosion induced the migration and growth of He bubbles,and the evolution of He bubbles during the corrosion process further leaded to the increase of pores in the oxide film,which provided a fast path for the internal diffusion of oxygen and a space for the growth of internal oxides,thus accelerating the oxidation corrosion of the materials.By studying the degradation of the corrosion resistance of ferrite/martensite stainless steels under irradiation and the related mechanism,it provides an important data reference for the corrosion resistance research of candidate structural materials for advanced water-cooled reactor.