Inconel 625鎳基超合金於不同溶氧量超臨界水環境下之腐蝕行為研究

Effect of dissolved oxygen concentration on the oxide structure of Inconel 625 Ni-based superalloy in supercritical water environment

張凱翔(Kai-Hsaing Chang)

指導教授 ： 開執中

國立清華大學/原子科學院/核子工程與科學研究所/碩士(2012年)

摘要

超臨界水核反應器(Supercritical Water-Cooled Reactor, SCWR)為第四代核反應器設計之一，其運轉溫度和壓力高於水的臨界點（374℃、22.1MPa），具有比輕水式反應器更高的熱轉換效率。超臨界水反應器之設計不但簡化爐心的複雜度且縮小反應槽體積，因而更能有效提升安全性且降低建廠成本。然而，超臨界水具有可無限溶解無極性氣體的特性、高運轉溫度以及高功率密度加劇了輻射水解的效應，使爐心水化學環境之腐蝕性比輕水式反應器來的更加嚴重。因此，發展可適用於超臨界水反應器中高溫、高腐蝕性和高應力爐心環境中的金屬材料，即顯得相當重要。
本論文中，將Inconel 625 鎳基超合金置於模擬超臨界水環境迴路中進行測試，在實驗溫度為700℃、壓力為24.8MPa 和溶氧量分別為150ppb、1ppm 和8.3ppm 的環境下，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。在8.3ppm 溶氧量環境下，腐蝕時間(t)在70 至600 小時間，質量增加(w)遵守w^2.095 = 2.60 × 10^-5t趨勢，符合拋物線律。從橫截面的分析上發現氧化層呈現雙層的結構：外層為spinel Ni(Cr, Fe)2O4，內層由緻密的Cr2O3 組成。而在不同溶氧量環境下，在150ppb 時其外層spinel 厚度明顯較8.3ppm 來的薄，而內層Cr2O3 之厚度大致相同，代表於低溶氧環境下，Inconel 625 表面以氧化鉻為主，高溶氧則以含鎳氧化物spinel 為主。Inconel 625 在700℃下於母材中生成大量γ”強化析出相，而在1000 小時後γ”相密度大幅下降取而代之的是δ 相。

關鍵字

超臨界水 ； 鎳基超合金 ； 腐蝕 ； 氧化 ； 溶氧量

並列摘要

Supercritical Water-Cooled Reactor (SCWR) is one of the Generation-IV nuclear reactor designs which operates above the thermodynamic critical point of the water (374℃ and 22.1MPa), and exhibits a higher thermal efficiency and a simpler system configuration with respect to a conventional light water reactor (LWR). However, the water radiolysis effect in the core of SCWR is much severer than that in LWR because the supercritical water could dissolve the non-polar gas, such as oxygen, without the solubility limit, much higher temperatures and higher power density. This phenomenon makes the environment in SCWR much corrosive so that evaluating the feasibility of materials is a significant issue for developing SCWR.
Ni-based superalloy Inconel 625 was exposed in supercritical water environment with 150 ppb, 1 ppm and 8.3 ppm dissolved oxygen [DO] at 700℃ and 24.8MPa for various periods of time up to 1000h. Before 600h, the mass gains (w) in the samples
as a function of test duration (t) could be fitted by an equation of w^2.095 = 2.60 × 10^-5t approximately followed the parabolic law. In addition, oxides with a double-layer structure were observed. The outer layer of the scales was constructed by spinels Ni(Cr, Fe)2O4 and the compact inner layer was Cr2O3. Nevertheless, the thickness of outer layer in 150ppb low [DO] was smaller than in 8.3ppm high [DO] environment but the thickness of inner layer was in the same order, indicating that the Ni-containing oxides, such as NiO and Ni(Cr, Fe)2O4 were stable in high [DO] and Cr-containing oxides were favorable in low [DO]. After oxidation, large amount of γ” precipitates were formed in the matrix of Inconel 625 alloy. Extending the exposure time to 1000 h, the density of γ” precipitates decreased and the γ” phase was replaced by the δ phase precipitate.

並列關鍵字

supercritical water ； Ni-based superalloy ； corrosion ； oxidation ； dissloved oxygen concentration

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