在模擬壓水式反應器一次側水環境中溶氫對鎳基合金與沃斯田鐵系列不銹鋼之電化學行為影響分析

Effect of Dissolved Hydrogen on the Electrochemical Characteristics of Nickel-based Alloy and Austenite Stainless Steel in Simulated PWR Primary Water Environment

葉育維(Yu-Wei Yeh)

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(2019年)

摘要

在壓水式反應器一次側水迴路與冷卻系統中，鎳基合金與沃斯田鐵系列不銹鋼為常見的的結構組件材料，而在一次側水迴路中的飼水端會注入氫氣，此設計的目的是用來抑制材料的均勻腐蝕、應力腐蝕龜裂現象和核反應時產生的輻射分解效應，但實際運轉經驗以及許多實驗結果顯示，在目前EPRI規範的注氫濃度25-50 cc H2/kg H2O與運轉溫度320-360 oC下，鎳基合金組件材料會有應力腐蝕龜裂的現象發生，其原因為在此注氫濃度與運轉溫度，鎳基合金會處在Ni/NiO的相轉換交界處，因此不論是鎳或是生成的氧化鎳層，都處在不穩定的狀態，而降低保護基材能力，從而使裂縫容易生成與成長。因此世界各國的核電廠決定調整注氫的濃度，歐美國家主張從原先規範的25-50 cc H2/kg H2O，提高至金屬鎳為穩定相的注氫濃度(> 75 cc H2/kg H2O)，而日本的學者則認為應該降低注氫濃度使氧化鎳為穩定相(< 5 cc H2/kg H2O)。這兩種方法相對於目前的條件有一定的優缺點，而本研究將會從材料腐蝕與電化學行為的角度，來分析與探討提高與降低注氫濃度所造成的影響。
在本研究中，主要會將鎳基合金Alloy 600、Alloy 690、Alloy X750與沃斯田鐵系列不銹鋼SS316、SS316L放置於320 oC的模擬PWR一次側水環境中進行實驗，並調整實驗環境中的溶氫濃度，由低到高分別為0, 5, 30, 75 cc H2/kg H2O，透過動態電位極化掃描分析不同材料在不同環境下的腐蝕電位與腐蝕電流密度變化，並透過拉曼光譜系統與場發射掃描電子顯微鏡分析材料在不同環境下，表面氧化層結構與形貌的變化。實驗結果顯示，不論在低或高的溶氫濃度(5或75 cc H2/kg H2O)，材料的腐蝕電位與腐蝕電流密度都有顯著的下降，而環境中越高的溶氫濃度會使材料有越低的腐蝕電位與腐蝕電流密度，代表氫氣對於抑制材料腐蝕行為有明顯的效果。而在表面分析的部分，SS 316與SS 316L的表面形貌與結構較為接近，Alloy 600則與Alloy X750有類似的表面形貌與結構，且在5種材料中，皆能觀察到尖晶石氧化物的比例隨著溶氫濃度的增加而增加。

關鍵字

鎳基合金 ； 不銹鋼 ； 溶氫濃度變化 ； PWR一次側水迴路 ； 腐蝕與電化學行為

並列摘要

Nickel-based alloys and Austenite stainless steels are major construction material used reactor coolant system of a PWR. To mitigate the problem of general corrosion and stress corrosion cracking, hydrogen is added to maintain the reducing conditions in PWR primary coolant to minimize general corrosion of material surface, and risk of stress corrosion cracking of stainless steels and nickel-based alloys. At the dissolved hydrogen concentration of 25-50 cc H2/kg H2O, the hydrogen contents may affect the nickel-based alloy surface stability due to the nickel/nickel oxide transition and lead to a higher crack growth rate. A change of hydrogen injection rate from 25-50 cc H2/kg H2O to <5 cc H2/kg H2O or to >75 cc H2/kg H2O is beneficial in avoiding hydrogen induced cracking in nickel-based alloy.
In the study, with four different dissolved hydrogen contents (0, 5, 30, 75 cc H2/kg H2O), the electrochemical behavior and the surface morphology of Alloy 600, Alloy 690, Alloy X750, SS316, and SS316L in simulated PWR primary water at 320 oC were investigated by potential dynamic test, scanning electron microscopy and Raman spectroscopy. Hydrogen injection affects the corrosion behavior significantly, the electrochemical potential become lower and the corrosion rate of alloy reduce to a smaller value. Increasing DH content results in a more negative ECP and a lower corrosion rate.

並列關鍵字

PWR primary water ； Ni-based alloy ； Dissolved hydrogen ； Electro-chemistry

參考文獻

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[3] EPRI-1007832, PWSCC of Alloy 600 Type Materials in Non-Steam Generator Tubing Applications-Survey Report through June 2002: Part 1: PWSCC in Components Other Than CRDM/CEDM Penetrations (MRP-87), EPRI, Palo Alto, CA, USA, 2003.

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[5] EPRI-103696, PWSCC of Alloy 600 Materials in PWR Primary System Penetrations, EPRI, Palo Alto, CA, USA, 1994.