鉑覆膜處理後304不銹鋼於高溫純水環境中的裂縫成長研究

Crack Growth Behavior of Platinum Treated Type 304 Stainless Steels in High Temperature Pure Water

張峻毓(Chun-Yu Chang) ； 葉宗洸(Tsung-Kuang Yeh) ； 蔡春鴻(Chuen-Horng Tsai)

《防蝕工程》 18卷4期 (2004/12) Pp. 325-336

https://doi.org/10.6376/JCCE.200412.0325

摘要

爲解決服役中電廠壓力槽內部組件發生沿晶應力腐蝕龜裂(Intergranular Stress Corrosion Cracking, IGSCC)現象且不易更換組件的問題，沸水式反應器(Boiling Water Reactor, BWR)採用了加氫水化學技術(Hydrogen Water Chemistry,HWC)，降低組件材料的電代學腐蝕電位，逹到防蝕的目的。但HWC技術的採用會大幅增加管路輻射劑量，有增加人員輻身劑量的作用；對於部份壓力槽內組件，如爐心上方究竟和爐心側析上部等過氧化氫濃度偏高的區域，抑制IGSCC的效果不彰。因此過去十年間又有催化性貴重金屬被覆技術(Noble Metal Chemical Addition,NMCA)的開發，利用貴重金的催化性，促進HWC的防蝕效益，而在低注氫量下即可達到保護組件的效果，並避免前述的副作用。
本研究主要針對經鉑覆膜被覆處理後之304不銹 於高溫純水環境中之裂縫成長行爲進行分析。在實驗過程中，首先建立一模擬BWR爐心高溫高壓環境（溫度288℃，壓力8.3MPa，志電度0.06μS/cm）的純水循環系統，并授權用經熱敏化處理與疲勞預裂後的Compact Type(CT)試片做為觀察裂縫成長的標的。CT試片的鉑被覆處理係以熱水沈積法進行，處理的條件爲150℃的100ppb Na2Pt(OH)6容液中分別浸泡12與24小時。CT試片於實驗進行期間持續承受一固定荷重，以維持試片裂口的起始應力強度因子爲27.5MPa·m^1/2，並且採用往復式直流電位降(Reversing Direct-Current Potential Drop)測量法，監測試片上的裂縫成長情況。純水循 的水化學條件設定爲300ppb溶氧量搭配不同溶氫量，以使量氫氧莫爾比(Mg/O)分別成爲0、0.5與2.7。
實驗結果顯示，在貴重金屬催化效應下，Mh/o爲0時，經NMCA處理試片之裂縫成長深度分別較未處理試片增加3-7倍左右，這量力而爲鉑覆膜加速催化了溶氧之還原速率間接提高金屬之氧化。MH/O爲0.5時，三種試片均有IGSCC産生，雖然經處理後試片其裂縫成長這起始點較未處理試片早子許多，但裂縫卻緩慢生長，證實在此條件下HWC加上NMCA有助於減緩鉑覆膜試片裂縫成長速率。在Mh/O爲0.5時，兩種鉑覆試片均出現了明顯的IGSCC，而未處理試片則無IGSCC，推測在高溶氧(300ppb)條件下，即使溶氫量高達50ppb，貴重金屬仍能有效促進氧化劑的還原，致使鉑覆膜試片裂縫依然成長。因此爲了防制鉑覆膜304不銹鋼的IGSCC，除了一般認知的Mh/o值提昇外，絕對溶氧量的降低也是必須同時攷慮的要素。

關鍵字

沿晶應力腐蝕龜裂 ； 催化性貴重金屬被覆 ； CT試片 ； 主復式直流電位降

並列摘要

More frequent incidents of intergranular stress corrosion cracking (IGSCC) have been seen in the vessel internal components of boiling water reactors (BWRs) for the past decades. The technology of hydrogen water chemistry (HWC) was developed to mitigate the IGSCC problems in BWRs in early 1980s. The principle of HWC is that the amounts of oxidizing species in a reactor coolant may be radiolytically reduced with the addition of hydrogen and the electrochemical corrosion potential (ECP) of structural components can thus be lowered. However, the ECP reduction effectiveness of HWC could not be extended to all areas in a BWR and the shortcoming of raising exposure dose rate inherently comes with this technology. Therefore, the catalytic coating technique that exploits the catalysis nature of noble metals was proposed to enhance the effectiveness of HWC and lower the required dissolved hydrogen level.
Crack growth tests was conducted to investigate the intergranular stress corresion cracking (IGSCC) characteristics of platinum treated Type 304 stainless steels in 288 oC pure water. Compact Tension (CT) specimens were thermally sensitized and pre-oxidized before the tests, and some of them were additionally treated with platinum via hydrothermal deposition. Test environments were specifically designed in a circulation loop to create water chemistry conditions of hydrogen to oxygen molar ratios (M (subscript H/O)s) of 0, 0.5, or 2.7in a coolant. Crack growth test results showed that the the corrosion potentials of the platinum treated specimens could be lower or higher than those of the untreated ones in the absence of hydrogen. In particular, IGSCC cracks were observed on the platinum treated specimens but not on the treated one at 2.7 M(subscript H/O). It was therefore suggested that one must exercise extra caution to use corrosion potential as a sole indicator in evaluating the influence of platinum treatment on the IGSCC susceptibility of Type 304 stainless steels, especially when the M (subscript H/O) in the coolant is relatively low or the dissolved oxygen concentration remains comparatively high.

並列關鍵字

Intergranular stress corrosion cracking ； Noble metal chemical addition ； Compact tension specimen ； Reversing direct-current potential drop