鎳基超合金在含雜質氦氣環境下之氧化行為研究

Oxidation Behavior of Nickel Based Superalloys in Helium Environments Containing Impurities

莊凡誼(Fan-Yi. Chuang)

指導教授 ： 葉宗洸 ； 王美雅

國立清華大學/原子科學院/工程與系統科學系/碩士(2018年)

摘要

第四代核反應器之一的超高溫氣冷式反應器(VHTR)，其設計以氦氣作為工作流體，爐心出口溫度達700℃至950℃，且為了達到更高的發電效率(>50%)，最終目標為1000℃以上的操作溫度。在這樣的高溫環境下，冷卻劑中的微量雜質，或發生進氣事故中流入的氣體，都可能加速結構材料的腐蝕速率。被開發應用於飛機渦輪引擎元件的鎳基超合金，便因其在700℃以上高溫的熱阻抗性與抗腐蝕性，成為新型核電廠中間熱交換管(IHX)的候選結構材料。本研究選用以氧化鉻作為其防護層的Inconel 625與Hastelloy C4，以及以氧化鋁作為保護層的HR-224三種鎳基超合金作為待測材料，探討其在850與950℃的氦氣環境下，不同濃度之氧氣水氣雜質所造成的氧化行為影響。而氧化測試的結果顯示，溫度的提高使三種合金的氧化速率均上升，Inconel 625在950℃下表現出三合金中最大的單位質量變化。而在所有測試條件下Inconel 625、Hastelloy C4都在表面形成連續的氧化鉻層，其下則可見氧化鋁內氧化的出現。但僅有Hastelloy C4，在特定氧化條件中被發現有孔洞出現於氧化層與基材界面處。而以形成氧化鋁作為保護層的HR-224合金，則隨著氧化條件的差異在表面出現氧化鉻、氧化矽等其他氧化物，氧化層剝落的情形也在水氣條件中更為嚴重。本研究亦會利用分析所得結果與相關文獻討論三種合金的氧化機制。

關鍵字

鎳基超合金 ； 超高溫反應器 ； 高溫腐蝕行為 ； 氧化鋁保護層合金 ； 氧化鉻保護層合金

並列摘要

The Very-high-temperature Reactor (VHTR) is one of the Gen-IV reactors. Helium is used as the coolant of a VHTR and the core outlet temperature can achieve 700-950°C, which might reach up to 1000°C for higher efficiency in the future. At such high temperature, the impurities from original coolant or a loss-of-coolant accident (LOCA) may have a significant impact on the performance of structural materials. As the promising candidate materials for Intermediate Heat Exchanger (IHX), chromia formers Inconel 625 and Hastelloy C4 alloys and alumina former HR-224 alloy should be tested in further studies to prove their corrosion resistant ability. In this research, three alloys were tested at 850°C and 950°C in helium environments with various concentrations of O2 and H2O. In general, the oxidation rates of all the alloys increased with the increasing temperature, and Inconel 625 alloy showed the highest mass gain among the alloys at 950℃. Continuous chromium oxide layer with internal alumina was formed on the surface of Inconel 625 and Hastelloy C4 alloys in all conditions, while the alumina scale spalled and other oxides could be found on the surface of HR-224 alloy under specific conditions. Further analyses and oxidation mechanisms of these alloys would be presented in this study.

並列關鍵字

Nickel based superalloy ； Very-high-temperature reactor (VHTR) ； High temperature oxidation behavior ； Alumina former ； Chromia former

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