A Study on Corrosion Behaviors of Ferritic/Martensitic Steels in Liquid Sodium Condition for Sodium-cooled Fast Reactor Application
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- A Study on Corrosion Behaviors of Ferritic/Martensitic Steels in Liquid Sodium Condition for Sodium-cooled Fast Reactor Application
- Shin, Sang Hun
- Kim, Ji Hyun
- Issue Date
- Graduate School of UNIST
- Research purpose is the study on corrosion of Gr. 92 steel (ferritic/martensitic steel) with liquid sodium for advanced nuclear reactors application. Moreover, development of new measurement methods of oxygen and carbon dissolved in sodium has been carried in this study.
In this study, ASTM A182 Grade 92 steel was exposed to liquid sodium at 650°C in order to investigate surface oxidation and corrosion behavior as well as microstructure changes, especially production of precipitates. Comparisons were made to the same material exposed to argon gas at 650°C. In sodium-exposed specimens, intergranular oxidation and chromium depletion were observed at the near surface by an electron probe micro analyzer (EPMA) used for elemental analysis. The chromium, oxygen, and sodium X-ray images overlapped, indicating NaCrO2 oxidation. Since, the carbon image overlapped on NaCrO2 oxide, the carbon was trapped in the oxide. The chromium image shows the chromium-depleted zone. With the argon-exposed specimen, however, this behavior at the surface was not observed. Different types of precipitates in the Gr.92 steel matrix were observed. At the interface between the Cr-depleted zone and the bulk steel matrix in specimens exposed to 650°C liquid sodium showed mainly M23C6 carbide, while specimens in the bulk steel matrixes which exposed to 650°C sodium as well as argon showed mainly a M2(W,Mo) phase. These differences may be mainly a result of oxidation (NaCrO2) and chromium depletion, and presumably decarburization observed in the sodium-exposed specimens, which are different from the precipitate behavior in the case of the bulk steel matrix in sodium as well as argon-exposed specimens.
Non-metallic elements, such as carbon, oxygen, and nitrogen in sodium have a large effect on the mechanical behavior and corrosion rates of materials in the primary and secondary coolant heat transport system in sodium-cooled fast reactors (SFRs). Austenitic Fe-18%Cr-8%Ni foil and ferritic Fe foil were equilibrated at 550°C sodium. After the equilibration, the concentration of carbon in the foils was analyzed by carbon analyzer (Leco). The carbon activity in liquid sodium was calculated. Also the thermodynamic activity of carbon in ferrite iron (Fe-C alloy) were calculated in the temperature range of 682 - 848°C. An equilibrium method using ferrite iron steel has been developed to measure the activity of carbon in liquid sodium. The results obtained are discussed and a new expression is proposed relating carbon activity with the composition of steel. The thermodynamics of carbide equilibrium in the steel are discussed by characterizing the carbide phase. These data have been analyzed using regular solution model for the carbide phase, Fe3C and based on this observed composition of Fe3C as well as the variation of the carbon potential of steel with temperature have been explained.
- Department of Nuclear Engineering
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