Abstract
A molten salt reactor (MSR) is a class of the Generation IV nuclear reactors. Owing to its desirable property of burning the minor actinides in spent fuel, it shows a good potential to be developed in Taiwan in order to solve the nuclear waste problem. However, the corrosion of materials is severe in high temperature molten salt environments. Therefore, searching for suitable structural materials has become an issue for MSR development. The presence of impurities leads to the high corrosivity of molten salt, causing a severe selective corrosion on specific alloying elements, especially Cr. Therefore, a low Cr-containing Ni-based alloy was chosen as a structural material for MSR generally. Nevertheless, some previous literatures indicated that Mo was expected to exhibit good corrosion resistance to molten salt, however, there are only few correlated research studies. Three Ni-based alloys with different Cr and Mo contents, Hastelloy-N, Hastelloy-B3 and X750, and one Mo-based alloy, TZM, were selected in this study. They were immersed in a static corrosion experimental system, equipped with a home-designed salt purification system, at 600℃ for different durations. The retrieved specimens were measured of their mass losses, and their surface and cross-sectional microstructures with corresponding element distribution analyses were examined by scanning electron microscopy and energy dispersive X-ray. The purpose is to investigate the corrosion resistance of Mo element and the influence of Mo addition on the corrosion behaviors of Cr-containing Ni-based alloys as well. The results indicated that both Cr and Mo were selectively corroded, leading to the formations of elements depletion zones in the three Ni-based alloys and causing the thickness of the TZM specimens to decrease. The major features of corrosion attacks observed in these three Ni-based alloys and TZM were intergranular corrosion and general corrosion, respectively. The long-term changes of mass loss of the three Ni-based alloys and TZM were dominated by the diffusion of Cr and Mo through the depletion zones and the diffusion of oxidizers to the metallic surface in the molten salt, respectively. At the meantime, it was found that a higher Mo but lower Cr content in the Ni-based alloys tended to induce decreases in diffusivities of Cr and Mo in alloys, rendering these Ni-based alloys to exhibit better long-term corrosion resistances.