高溫氣冷式反應器(High Temperature Gas-Cooled Reactor，HTGR)為第四代核反應器之一，此種反應器由於傳承氣冷式反應器的設計，成為目前可能最快進入商轉的第四代核反應器類型。高溫氣冷式反應器設計上，爐心使用石墨作為中子減速劑，在冷卻劑方面則使用化學反應活性低的惰性氣體氦氣，為了提高熱轉換效率，將爐心出口溫度設置在900℃以上，相較於傳統的輕水式反應器33%的效率，高溫氣冷式反應器熱轉換效率可達50%以上，且可搭配熱化學產氫或氣體重組技術，更可有效利用核反應所產出的熱。爐心溫度的提升也代表著使用在反應器結構組件的材料必須在高溫維持良好的機械性質與強度，且必須有二十年甚至更長的壽命，因此材料的抗腐蝕能力也是在材料實際應用上非常關鍵的問題。
本論文即探討使用高溫氣冷式反應器的熱交換材料，在高溫的氦氣環境中，長時間的腐蝕行為。選用四種耐高溫超合金，分別為鎳鐵基的Incoloy 800H、鎳基的Hastelloy X、Inconel 617和Inconel 625，在高純度氦氣與1%氧氣、10%空氣、10%氧氣、10%相對溼度與50%相對溼度的四種雜質氣體環境，溫度設置在650℃至950℃下進行48到144小時的腐蝕測試。在實驗前後針對試片進行質量改變的量測，並利用掃描式電子顯微鏡、X光繞射分析儀和輝光放電分析儀，對於氧化物結構與成份進行表面和橫截面的分析。實驗結果顯示溫度為主要控制合金氧化速率的因素，在650℃和750℃的環境下，四種合金都有極佳的抗腐蝕能力，當溫度提高至850℃和950℃合金氧化速率大幅提升，在950℃環境下，試片的單位面積質量變化大約是650℃的10倍左右。在950℃的環境中，不管在何種氣體環境下，四種不同合金單位面積的質量變化皆呈現Incoloy 800H > Inconel 617 > Inconel 625 > Hastelloy X的關係。合金在純氦氣和10%氧氣的環境中，氧化速率隨著氧氣分壓上升而加速，但水氣則在不同合金上有不同的影響，在Incoloy 800H和Inconel 625合金上，水氣會略為使試片的單位面積質量增加上升，而Inconel 617則會有大幅提高的氧化速率，但對於Hastelloy X在水氣環境中的試片反而有增重下降的情形。四種合金在高溫腐蝕環境氧化後，表面皆會成長出連續的氧化鉻，隔絕外界氧氣保護合金，此外在四種合金表面還會出現MnCr2O4的尖晶石氧化物，而在Incoloy 800H和Inconel 617氧化鉻下層的基材位置，都會發現氧化鋁的內氧化物，由於內氧化通常會沿晶界向基材內部延伸，通常可以發現對應的外側氧化鉻有增厚的情形，而在表面出現沿晶界的山脊狀氧化物。在水氣環境中氧化的Incoloy 800H和Hastelloy X，表面會出現大顆粒團聚(nodule)的氧化矽，此氧化物顆粒凸起於連續的氧化物表面，在高溫長時間氧化下，或在降溫時受到熱應力的影響而較容易剝落，露出內側的氧化鉻，對合金長期使用可能有不良的影響。因此總結以上結果，雖然Inconel 625合金的增重情形略大於Hastelloy X，但因其沒有內氧化的效應，且在水氣環境中不會出現氧化矽的大顆粒氧化物，整體來說Inconel 625為四種合金表現最佳的。

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.

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