Inconel 625超合金於超臨界水環境下氧化層結構變化之研究

Study on Scale Constitution and Oxidation Mechanism of Inconel 625 in 〝SCWR〞

顏存濱

指導教授 ： 開執中 ； 陳福榮 ； 葉宗洸

國立清華大學/原子科學院/工程與系統科學系/碩士(2010年)

摘要

摘要

超臨界水反應器(supercritical water-cooled reactor, SCWR)為第四代核能反應器之一，其操作環境高於水的臨界點，溫度必須超過374℃且壓力高於22.1 MPa。相對於現今輕水式反應器，超臨界水反應器的設計簡化使得安全性提高且降低成本，且超臨界水反應器操作在相當高溫的環境，因而提供相當高的熱效率，大約為45%(現今輕水式反應器熱效率約為33%)。
Inconel 625 鎳基超合金在8.3ppm的溶氧量之超臨界水環境中，600℃之實驗結論推測孔蝕與原有的(Nb,Ti)C析出相有關，並且利用300、600及1000小時的單位面積質量變化量得到一趨勢線，w2.21=1.4×10-5 t，符合拋物線律。而氧化層分析呈現兩層結構外層為spinel Ni(Cr,Fe)2O4，內層為spinel Ni(Cr,Fe)2O4加上Cr2O3組成緻密連續的氧化層。利用SEM量測平均氧化層厚度，得到一趨勢線，log(x)=0.432log(t)-1.2334，與 xn=kt 做比較，可得到Reaction order n=2.32。
本實驗接續600℃，進行較低溫400℃與500℃之實驗，未發現有γ”相的存在，更進一步證實孔蝕與原有的(Nb,Ti)C析出相有關。氧化層分析：在400℃之氧化層可分為兩層結構，外層為奈米級尺寸的NiO顆粒，NiO氧化物平均尺寸 d值趨勢線log(d) = 0.3378t +1.4873，方程式與 wn=kt 比較，得到n=2.96；內層則是由緻密且連續的spinel Ni(Cr,Fe)2O4，內層氧化層趨勢線log(L) = 0.5647t + 0.0673與 wn=kt 比較，得到n=1.77，趨近於n=2，符合拋物線率。而500℃之氧化層也分為兩層結構，表面有大約0.5μm大小的氧化鉻顆粒存在；而600℃之氧化層厚度相對於500℃較厚，鎳與鉻無法擴散至表面形成單一氧化物之情況下，氧化層則僅形成單一spinel Ni(Cr,Fe)2O4，且隨著氧往內擴散在內層形成spinel Ni(Cr,Fe)2O4加上Cr2O3組成緻密連續的氧化層。

關鍵字

Inconel 625 ； supercritical water ； oxide

並列摘要

Supercritical water-cooled reactor (SCWR) is one of the various fourth-generation nuclear reactors. The temperature of its operating environment must exceed 374 ℃ and the pressure is higher than 22.1 MPa which was above the critical point of water. Compared with the light-water reactor, SCWR has some advantages such like its design for safe improvement, reduce costs and so on. In addition, supercritical water reactor operates at very high temperature, according to this, it can provide a very high thermal efficiency, about 45% (nowadays, the light water reactor thermal efficiency is about 33%).
Inconel 625 Ni-base superalloy was set in the environment of supercritical water which dissolved oxygen 8.3ppm and 600℃. According to experimental results, suggesting that pitting was related to the original (Nb, Ti)C precipitates phase. And the change in mass per unit area of 300,600 and 1000 hours showed a trend line, w2.21 = 1.4 × 10-5 t, consistent with the parabolic law. The result of oxide layer analysis experiment result showed a continuous and compact oxide layer structure which was composed of spinel Ni (Cr, Fe)2O4 in the outer layer and spinel Ni(Cr, Fe)2O4 with Cr2O3 in the inner layer. SEM measures the average oxide layer thickness, we obtained a trend line, log (x) = 0.432log (t) -1.2334. Comparing our result with xn = kt, Reaction order n will be 2.32.
experiment showed γ "phase would not exist at 400 and 500℃, further evidence revealed great agreement for pitting with the original (Nb, Ti) C precipitates phase.
Oxide layer analysis of the parameter of 400 ℃ experiment, the oxide layer can be divided into two layers. Outer layer was composed of NiO nano-size particles, its average size d fitted the following formula: (d) = 0.3378t +1.4873. The equation compares with wn = kt, and obtained n = 2.96. Inner layer was composed of a continuous spinel Ni (Cr, Fe)2O4, which that average size L fitted following formula: (L) = 0.5647t 0.0673 compared with wn = kt. We also obtained n = 1.77 (when n = 2, the above equation was a parabolic line).
In the experiment of oxide layer analysis under 500 ℃, the oxide layer also can be divided into two layers. However, there were chromium oxide particles on its surface, which average size was about 0.5 μm. Because the thickness of the oxide layer at 600 ℃ is thicker than its at 500 ℃, nickel and chromium can’t diffuse to the surface and form single oxide layer. The single oxide layer is spinel Ni(Cr,Fe)2O4, and it forms a continuous compact oxide layer structure that composed of spinel Ni(Cr, Fe)2O4 and Cr2O3, due to the oxygen diffusion.

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