Study Of Oxidative Corrosion Behavior Of Fe-Cr Based Alloy In High-temperature CO₂

Supercritical CO₂is one of the important candidate cooling media in advanced nuclear energy systems.Compared with steam Rankine cycle,the supercritical CO₂Brayton cycle has higher energy conversion rate,compact power system and lower economic investment,which has great application prospects in advanced nuclear energy systems such as high temperature gas-cooled reactors.However,the core outlet temperature of nuclear reactors with the CO₂Brayton cycle is usually 450-650°C.The structural materials inside the reactor are subject to severe oxidative corrosion in the high-temperature CO₂,which greatly reduces the service life of the materials and even causes serious safety problems.Fe-Cr based alloys such as ferritic/martensitic steel and austenitic steel are candidates for advanced nuclear energy systems such as lead-cooled fast reactors（LFR）and supercritical water reactors（SCWR）,and are expected to be used in CO₂Brayton cycle systems of advanced reactors due to their good corrosion and irradiation resistance and high-temperature strength.Therefore,it is necessary to evaluate and study the oxidative corrosion behavior of Fe-Cr based alloys in high-temperature CO₂environment to provide more data support for the application of Brayton cycle on advanced nuclear energy devices.In this thesis,ferritic/martensitic steel T91 and austenitic steels 316L,15-15Ti and 321 were selected for the study,and the early-stage oxidative corrosion behavior of Fe-Cr alloys with different compositions in high-temperature CO₂was comparatively analyzed by means of optical microscopy（OM）,scanning electron microscopy（SEM）,X-ray diffraction（XRD）,laser confocal Raman spectroscopy（Raman）,and transmission electron microscopy（TEM）.Meanwhile,the effects of temperature on early-stage oxidative corrosion behavior and microstructure evolution of austenitic steels 316L and 15-15Ti were investigated systematically,and the evolution process and formation mechanism of the lamellar structure in the inner oxide film of 15-15Ti steel were discussed emphatically.The main research conclusions are as follows:（1）The kinetic curves of early-stage oxide film growth of Fe-Cr based alloys T91,316L,15-15Ti and 321 in high-temperature CO₂followed a near-parabolic law.After exposure to CO₂at 600°C for 100 h,the weight gain and oxide film thickness of different materials were as follows:T91>15-15Ti>316L≥321,which showed that austenitic steels 316L,321 and 15-15Ti had better oxidative corrosion resistance than ferritic/martensitic steel T91 in high-temperature CO₂.The oxide films of Fe-Cr based alloys with different compositions were a double-layer structure,the outer layer was porous and loose Fe₃O₄covered with a thin Fe₂O₃layer,and the inner layer was mainly composed of Fe_3-xCr_xO₄phase with spinel structure.Compared with austenitic steel,an internal oxidation zone composed of Fe-Cr spinel phase and Cr-rich oxides was formed below the inner oxide film of ferritic/martensitic steel.（2）The results of research on the oxidative corrosion behavior of austenitic steel at different temperatures indicated that,with the increase of temperature and time,the thickness of the oxide film formed in the high-temperature CO₂increased,the grain size of the oxide film gradually increased,a large number of surface pores gradually disappeared and cracks began to appear.The double-layer structure and phase composition of the oxide film did not change with temperature and exposure time.However,the Ni element distribution in the inner layer exhibited a specific evolutionary pattern with temperature and time.At the initial stage of oxidation,scattered and dotted Ni-poor zones were formed in the inner layer of 15-15Ti and316L steel.With the increase of temperature and time,the density of the Ni-poor zones increased and then the Ni-poor zones gradually accumulated until the lamellar structure of"Ni-rich layer,Ni-poor layer,Ni-rich layer"alternately formed in the inner oxide film.The TEM results showed that the Ni-poor layer was only Fe-Cr spinel phase and contained a large number of nano-pores,while the Ni-rich layer consisted of Fe-Cr spinel oxide phase and Ni-rich phase with austenite structure.The analysis revealed that the oxidation and migration behavior characteristics of Ni in the inner oxide film were the main reasons for the formation of Ni lamellar structure.