Abstract
High Temperature Gas-Cooled Reactor (HTGR) is one of the Generation-IV nuclear reactor designs. With the long experience of the traditional gas-cooled reactors, HTGR is currently the most promising reactor design to be commercialized. The reactor core uses graphite as neutron moderator, and helium as coolant. For the purpose of higher heat transfer efficiency, the core outlet temperature is set above 900℃, and the efficiency is expected to exceeds 50%. The high temperature and the great heat can also supply the application of gas reformer and hydrogen generation system. However, the structure materials used in the reactor core or the intermediate heat exchange (IHX) system will face great challenges with the elevated core outlet temperature. Therefore, materials with high corrosion resistance and superior mechanical strength should be studied for the application of an HTGR system. In this thesis, superalloys which were potential candidates for the structure materials of the IHX system were investigated in the simulated dynamic flow corrosion system. Four types of iron- and nickel-based superalloys including Incoloy 800H, Hastelloy X, Inconel 617 and Incoenl 625 were selected as test materials. The testing temperature was set from 650℃ to 950℃, and the testing time was 48 to 144 hr. Various coolant compositions of helium with impurities of 10% dry air, 10% oxygen, 10% relative humidity, and 50% relative humidity were selected as corrosion conditions. After the corrosion tests, the mass change of the specimens was measured with microbalance, and the morphology and the structure of the oxide scales was analyzed by scanning electron microscopy, grazing incident X-Ray diffraction and glow discharge spectrometer. Results show that the main influence on the corrosion rate of the alloys was the temperature. All the specimen tested at 650℃ and 750℃showed great corrosion resistance. The corrosion rates of the specimen were greatly increased under 850℃ and 950℃, and the mass gain at 950℃ was ten times higher with respect to that at 650℃. The alloys showed an order of mas gain with Incoloy 800H the highest followed by Inconel 617, Inconel 625 and Hastelloy X the lowest under all conditions at 950℃. The mass gain of all the alloys was increased with the presence of 10% oxygen. However, water vapor had different influences on the tested alloys. Water vapor greatly increased the mass gain of Inconel 617; on the other hand. The surface of all the tested specimen was covered by continuous chromium oxide which acted as an oxygen barrier to protect the alloys from further corrosion. Spinel particles such as MnCr2O4 was also observed on the outer layer, while the internal oxidation of Al2O3 was found beneath the Cr2O3 layer in both Incoloy 800H and Inconel 617. The internal oxidation formed along the grain boundary would result in the scaling of the external oxide. Therefore, gain boundary ridges were observed on the oxide surface in the SEM images. Furthermore, the specimens of Incoloy 800H and Hastelloy X oxidized in water vapor were covered by big nodules of SiO2, and these nodules could easily spall off upon cooling; accordingly, the life time of the material could be reduced. In conclusion, although the oxidation rate of Inconel 625 was slightly higher than that of Hastelloy X, there is no formation of deleterious oxide such as internal oxide and silicon oxide in Inconel 625 under all condition. Hence, Inconel 625 exhibited the best corrosion resistance on the alloys studied.