The Influence Of Surface Modification On Corrosion Behavior Of 304 Stainless Steel In Simulated Nuclear Power Environment

Stainless steel,as a kind of key materials of light water reactor nuclear power plant,is prone to localized attack such as pitting corrosion and crevice corrosion after long-term service.It seriously affects the safe operation of nuclear power plants.Because of the complexity of the experiment simulating the nuclear power environment,there are few studies on how to improve the corrosion resistance of structural materials in high temperature pressurized water.Therefore,it is of great significance for the safe operation of nuclear power plants to explore the methods of the surface modification and processes about improving the corrosion resistance of structural materials effectively.In this article,nuclear grade 304 stainless steel?304 SS?was used as the matrix.And the remelted layers with or without Y₂O₃,the nitrided layers at different time,the coatings of pure Cr with plasma nitriding and N/CrN composite coatings were respectively prepared by laser remelting,plasma nitriding and magnetron sputtering.The corrosion resistance in normal temperature containing Cl^-solution and the oxidation process in high temperature pressurized water were investigated using scanning electron microscopy?SEM?,energy-dispersive spectrometer?EDS?,optical metallurgical microscope?OM?,X-ray diffraction?XRD?,Raman,XPS,electrochemical workstation,etc.Moreover,the corrosion mechanisms of different modified layers in two environments were discussed.The main progress is as follows.The effects of addition of Y₂O₃ on the pitting corrosion behavior of remelted layes have been investigated.The pitting corrosion resistances of 304 SS were improved by laser remelting with and without Y₂O₃.The defects and inclusions in the remelted layers were decreased after the addition of Y₂O₃ and the structure of the remelted layer with Y₂O₃ was compact.Therefore,the pitting corrosion resistance of the remelted layer was further improved.The effects of addition of Y₂O₃ on oxidation behavior of remelted layes in high temperature pressurized water have been investigated.The diffusion of nearby Fe atoms was hindered by the strengthening phase formed around the Y₂O₃.The corrosion products were mainly Cr₂O₃ hematite.A small amount of Fe₃O₄ spinel oxide was formed on the surface outside the Mn/Si enriched region.The effects of nitriding time on the corrosion behavior of 304 SS in the containing Cl^-solution have been investigated.After plasma nitriding,a uniform and dense nitriding layer was formed on the surface of the matrix,and the corrosion resistance was improved.In this work,the nitrided layer after nitriding for 2 h had the highest corrosion potential and the lowest corrosion current density.The corrosion loss rate in artificial seawater was the smallest and the corrosion resistance was the excellent.The effects of nitriding time on oxidation behavior of 304 SS in high temperature pressurized water have been investaged.The oxide formed on the surface of the nitrided layer after exposuring in high temperature and high pressure water was mainly hematite,and the amount was decreased with the nitriding time prolonged.Moreover,the oxide size was changed from large size to fine circular particles.The further oxidation of the matrix was hindered by N atom in the nitrided layers.As the nitriding time was extended,the ability of N atoms to hinder the oxidation behavior was stronger.The corrosion behaviors of the coatings of pure Cr with plasma nitriding have been investaged.With the nitriding time prolonged,the corrosion resistance of the coating was increased.After exposuring in high temperature pressurized water,the oxide types formed on the surfaces of pure Cr coating and the coatings of pure Cr with plasma nitriding at different time were Cr₂O₃ hematite and a small amount of spinel.The amount of oxide on the surfaces of the coatings was decreased with the nitriding time increasing.Moreover,the corrosion behaviors of N/CrN composite coatings have been investaged.The corrosion resistance of the N/CrN composite coatings in the containing Cl^-solution were first increased and then decreased with the nitriding time prolonged.