預長氧化層HR-224超合金於超臨界水環境下之腐蝕研究

Corrosion Behavior of Pre-oxidized HR-224 superalloy in Supercritical Water

劉宇晨(Yu-Chen Liu)

指導教授 ： 開執中

國立清華大學/原子科學院/工程與系統科學系/碩士(2014年)

摘要

超臨界水核反應器(Supercritical Water-Cooled Reactor, SCWR)主要特點在於其冷卻劑運轉溫度和壓力高於水的臨界點（374℃、22.1MPa）且為第四代核反應器設計中唯一的輕水式反應器。相較於現行輕水式反應器，其優點包括(1)熱轉換效率高於40%；(2)組件更簡潔；(3)結合既有輕水式核能反應器及超臨界態火力電廠；(4)更好的安全性。然而，由於爐心中的輻射以及水化學的變化將影響爐心核能結構材及護套材料的完整性。且在此嚴苛的水化學條件下，至今仍未找到合適的護套材料。
基於此理由，本論文中將新型鎳基超合金HR-224經982℃在流動的空氣環境中氧化100小時，預長一層連續且緻密的氧化鋁保護層後，將試片置於實驗溫度為700℃、壓力為 24.8MPa和溶氧量為8.3ppm的超臨界水環境下且腐蝕時間1300小時後，量測腐蝕後質量之改變，並透過電子顯微鏡技術探討其腐蝕機制。經高溫預長氧化層HR-224 鎳基超合金於高溶氧量 8.3 ppm、700℃的超臨界水中具有良好的抗腐蝕性。根據氧化層厚度而計算出的腐蝕後增重(w)與腐蝕時間(t)之關係遵守Δw=2.72×〖10〗^(-3)×t^0.71之趨勢。經高溫預長氧化層處理後，其氧化層分層為外層富結晶性之 spinel鐵、鉻和鎳氧化物混和物 ，中層為Cr2O3，內層為連續緻密且熱力學穩定的氧化鋁氧化層之三層氧化層結構。經超臨界水實驗後，氧化層結構並無明顯變化。氧化鋁層在100-1300小時腐蝕實驗後，厚度變化(Δd)與腐蝕時間(t)之關係遵守Δd = 3.26×〖10〗^(-2)×t^0.55之趨勢，接近拋物線率。而腐蝕時間1000小時以上，外層Spinel及Cr2O3層微幅成長。在預長氧化層處理後的所產生的氧化鋁有足夠的厚度，故在長時間腐蝕後，其連續性、完整性及附著性皆良好，使得長時間後氧化速率趨緩。

關鍵字

超臨界水 ； 鎳基超合金 ； 腐蝕 ； 氧化

並列摘要

Abstract
Supercritical Water-cooled Reactor(SCWR) whose operating environment exceeds the critical pressure and temperature of water (22.4MPa and 374℃) has been considered by the Gen-IV Nuclear Energy System International Forum as an option for future light water reactor(LWR). The main advantages over the current LWR include: (1) higher thermal efficiency (2) more compact thermal component (3) combination of existing LWR plant and SCP fossil power plant technologies and (4) higher safety. However, the irradiation and change of water chemistry in the core of SCWR will affect the structural integrity and cladding materials. In this study, nickel-based superalloy HR-224 was pre-oxidized in flowing air at 982℃ for 100hr to establish a continuous and dense α-Al2O3 layer. Then it exposed in supercritical water (SCW) environment with 8.3 ppm oxygen content at 700℃ and 24.8MPa for cyclic oxidation testing up to 1300h.
According the result of oxidation thickness, calculated mass gains (w) in the samples as a function of test duration (t) could be fitted by an equation of Δw=2.72×〖10〗^(-3) t^0.7089.
In addition, after pre-oxidizing process, triple scales were observed, consisting of an outer layer of Ni(Cr, Fe)2O4 spinel, a middle layer was Cr2O3 and an inner layer of α-Al2O3. This scale structure remained unchanged after it was exposed in SCW environment after 1300hr. The thickness of α-Al2O3 layer increased with increasing duration of time. The thickness change (Δd) with exposure time (t) can be described as a function of Δd = 3.26×〖10〗^(-2)×t^0.55. The results show that the pre-oxidizedα-Al2O3 layer is thick enough to maintain good adherence and provides a better corrosion resistance for long exposure time in SCW.

並列關鍵字

supercritical water ； Ni-based superalloy ； corrosion ； oxidation

參考文獻

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