沸水式反應器不銹鋼組件應用抑制性被覆之防蝕行爲研究

Corrosion Mitigation for Stainless Steels Treated with Inhibitive Coatings in Boiling Water Reactor Environments

李明墉(Ming-Yong Li) ； 葉宗洸(Tsung-Kuang Yeh) ； 蔡春鴻(Chuen-Horng Tsai)

《防蝕工程》 16卷1期 (2002/03) Pp. 51-64

https://doi.org/10.6376/JCCE.200203.0051

摘要

近年來，許多沸水式反應器均已採用加氫水化學 (Hydrogen Water Chemistry, HWC) 技術，藉以降低主冷卻水迴路中各組件的電化學腐蝕電位 (Electrochemical Corrosion Potential, ECP) 並進而防制延晶應力腐蝕龜裂(Intergranular Stress Corrosion Cracking, IGSCC) 與輻射促進應力腐蝕龜裂(Irradiation-Assisted Stress Corrosion Cracking) 的發生。文獻資料顯示，在較高飼水注氫量下（通常在0.6 ppm以上），HWC有提昇管路輻射劑量率的副作用，且其對於反應器壓力槽內部接近爐心出口附近組件的ECP降低與IGSCC抑制功效並不明顯。因此，為求減少HWC技術中的注氫量需求與管路輻射劑量率，且有效降低爐心出口附近組件的腐蝕發生機率，我們嘗試利用抑制性被覆的方式，進行反應器主冷卻水迴路組件防蝕之可行性研究。
本研究以氧化鋯(ZrO2)、氧化鈦(TiO2)及二硝酸基氧化錯(ZrO(NO3)2)等化合物為試藥，透過化學添加的方式進行試片的被覆處理，在模擬沸水式反應器的水化學循環迴路中，針對經被覆處理後的304不銹鋼進行動態極化掃描分析、電化學腐蝕電位量測與慢應變速率拉伸測試，藉以了解上述材料在不同被覆處理狀態下的腐蝕行為。透過動態極化掃描分析我們發現，304不銹鋼在經過氧化錯、二硝酸基氧化鋯與氧化鈦的化學添加被覆後，其腐蝕電流密度與鈍化電流密度皆比未被覆試片為低。在電化學腐蝕電位量測實驗中，經過抑制性被覆處理的試片，其ECP值並非如預期地較未被覆試片低，推測其原因可能是抑制性被覆同時降低氧還原與不銹鋼氧化的交換電流密度的結果。在慢應變速率拉伸實驗的部分，我們發現被覆與未被覆試片均出現嚴重的IGSCC，但未被覆試片出現較小的伸長量與較短的斷裂時間，顯示試片龜裂起始的時間確實因抑制性被覆處理而延長，但發生龜裂的程度並不因被覆處理而有明顯的改變。

並列摘要

Incidents of intergranular stress corrosion cracking (IGSCC) and irradiation-assisted stress corrosion cracking (IASCC) of stainless steel components in the primary coolant circuits of boiling water reactors (BWRs) are occurring with increasing frequency as the power reactors age. In the past decade, the hydrogen water chemistry (HWC) technique has been widely adopted as a measure for mitigating IGSCC and IASCC in BWR vessel internal components. However, this technique is not without problems. In general, at a feedwater hydrogen concentration higher than 0.6ppm, the radioactive Nitrogen-16 content in the main steam line is very likely to increase and the resulting radiation fields exert a high man-REM cost on the operator. Furthermore, it is not at all clear that HWC is effective in protecting some components against IGSCC and IASCC in terms of electrochemical corrosion potential (ECP) reduction, particularly for protecting near-core components. Therefore, new technologies, such as inhibitive coatings, were brought into consideration to enhance the effectiveness of HWC in the aspects of lower hydrogen consumption and more effective ECP reduction.
In the current study, surfaces of pre-oxidized Type 304 stainless steels (SS) were treated with various chemical compounds of TiO2, ZrO2, and ZrO(NO3)2 by chemical immersion at different temperatures. Electrochemical potentiodynamic polarization, ECP measurement, and slow strain rate tensile (SSRT) test were conducted to characterize the corrosion properties of the treated and untreated stainless steels. Test results showed that the treated SS specimens exhibited lower open circuit potentials, corrosion densities, and passive current densities than the preoxidized specimen. The ECPs of the treated and untreated specimens at 288℃ could vary by more than 200 mV at different dissolved oxygen concentrations, and the pre-oxidized specimen did not exhibit the highest ECP. According to the SSRT test results, all tested specimens showed severe IGSCC, but the pre-oxidized one had the lowest elongation and the shortest fracture time. The results indicated that inhibitive coatings did prolong the crack initiation times but failed to provide distinct protection against IGSCC once cracking started.