熔鹽式反應器 (Molten Salt Rector，MSR) 為第四代核反應器的一種，其可轉化用過核燃料中的錒系元素，有助解決現今用過核燃料的問題，適合作為台灣核能發展的反應器類型，但材料於高溫熔鹽的腐蝕相當嚴重，所以找出合適的結構材料成為開發MSR必須研究的題目。熔鹽的高腐蝕性源自於雜質的存在，並會對合金內特定元素造成嚴重腐蝕，尤其是Cr，所以一般挑選用於MSR的結構用材料皆採用低Cr的鎳基合金為主，不過亦有文獻指出Mo的抗熔鹽腐蝕能力是備受期待的，但相關的腐蝕研究仍不多。
本實驗選定Hastelloy-N、Hastelloy-B3以及X-750這三款不同Mo與Cr含量的鎳基合金、以及一款鉬基合金TZM進行靜態浸沒腐蝕實驗，配合自行設計的簡易熔鹽純化系統以求除去熔鹽雜質。實驗溫度為600℃，實驗時間為100、200、500、750以及1000小時，並於實驗後量測試片的質量損失，且同時利用掃描式電子顯微鏡觀察試片表面與橫截面的形貌，以探討Mo元素的抗蝕能力、與其添加到含Cr之鎳基合金的影響。
結果顯示Cr與Mo皆受到選擇性腐蝕，導致鎳基合金出現沿晶腐蝕和元素耗乏層的特徵，也導致鉬基合金因均勻腐蝕而試片厚度變薄。長時間浸沒後，三款鎳基合金的腐蝕行為由Cr與Mo在合金內部的擴散所主導，而TZM則是受熔鹽內氧化劑擴散至金屬表面的機制所主導。實驗結果顯示Cr與Mo在高Mo且少Cr含量的鎳基合金中表現較慢的擴散速率，使此類鎳基合金具更好的長期抗蝕表現。

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.

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