不同抑制性被覆條件之304不銹鋼在高溫純水環境中的電化學行為研究

Electrochemical Behavior for Stainless Steels Treated with Inhibitive Coatings in High Temperature Water Environments

王佰揚(B. Y. Wang) ； 葉宗洸(T. K Yeh) ； 蔡春鴻(C. H. Tsai)

《防蝕工程》 20卷3期 (2006/09) Pp. 209-232

https://doi.org/10.6376/JCCE.200609.0209

摘要

由於世界各地反核人士的極力阻擾下，核能電廠的興建始終處於百廢待興的狀況，但隨著民眾的用電量不減反增，所以就必須仰賴現有的核能電廠延長其除役的時間，而一旦增長反應器的運轉時問，其內部組件材料就有可能遭受沿晶應力腐蝕龜裂(Intergranular Stress Corrosion Cracking, IGSCC)以及輻射促進應力腐蝕龜裂(Irradiation-Assisted Stress Corrosion Cracking IASCC)的破壞。近十幾年來核能界發展兩種技術：其中之一為加氫水化學(Hydrogen Water Chemistry, HWC)技術，以抑制冷卻水環境中氧化劑（氧及過氧化氫）的濃度，進而降低內部組件材料的電化學腐蝕電位(Electrochemical Corrosion Potential, ECP)，達到防制IGSCC的發生。然而HWC技術有它的瓶頸存在：根據文獻記載，當飼水注氫量達到3 ppm時，爐心上方的ECP還是無法飾令臨界腐蝕電位(-230 mV(下標 SHE))，但不幸的是當飼水注氫量達到0.6 ppm以上，就會帶來輻射劑量增加的副作用；另一種為HWC技術搭配貴重金屬化學添加(Noble Metal Chemical Addition, NMCA)技術，利用貴重金屬的催化效果，來降低組件材料的ECP，但仍需要少量注氫，且有促進燃料棒護套腐蝕之問題存在。
近年來，第三種技術：抑制性被覆(Inhibitive Protective Coatings, IPC)防蝕技術的研究逐漸盛行，雖然IPC技術目前尚未達到足以取代HWC及NMCA的具體進展，但由於採用IPC技術具有不須搭配HWC的優點，對注氫量的需求可能減少甚至完全免除，因此相對地運轉人員的輻射劑量亦能隨之降低，故其後續的研發成果極有可能優於NMCA技術。
本研究主要是模擬沸水式反應器(Boiling Water Reactor, BWRs)爐心高溫高壓環境中之電化學特性分析，並針對不同溶氧及溶氫的濃度狀態下，量測經熱敏化處理與預長氧化膜後304不銹鋼方形試片與經過不同IPC處理條件後之試片的電化學動態極化掃描及表面阻抗，以比較試片沒有被覆處理與經過不同被覆顆粒大小及時問處理後所呈現的腐蝕電位及電流彼此問特性的差異，以進一步探討採用熱水沉積法的抑制性被覆之完整性，並結合X-ray、SEM、AES等儀器來觀察試片表面的氧化鋯被覆結構，以相互驗證抑制性被覆處理是否對304不銹鋼產生一定的防蝕保護效果。
研究結果發現，整體而言證明抑制性的被覆效果能如預期的降低金屬的腐蝕電流密度跟氧化劑、還原劑的交換電流密度，其中又以對氧化劑的抑制效果較好。對照不同的被覆顆粒大小處理試片後，以100nm的效果最好，700nm的效果較差。比較被覆時問長短的差異，2週的被覆時問果然呈現出比較好的結果。

關鍵字

沸水式反應器 ； 沿晶應力腐蝕龜裂 ； 電化學腐蝕電位 ； 抑制性批覆 ； 氧化鋯 ； 動態極化掃描

並列摘要

Intergranular stress corrosion cracking (IGSCC) of sensitized stainless steel components in boiling water reactors (BWRs) has been a major concern to worldwide BWR operators. Research has demonstrated that below a critical electrochemical corrosion potential (ECP) of -230 mV(subscript SHE), the susceptibility of stainless steel to IGSCC is dramatically reduced. In past decade, several methods have been developed to mitigate IGSCC by lowering the ECP. One is Hydrogen Water Chemistry (HWC). Hydrogen is added to the feedwater of BWRs to reduce the dissolved oxidant concentrations produced by radiolysis of water in the core of BWRs. However, several drawback of H2 addition have been discovered gradually, such as increased N^16 carry-over to the turbine, higher Co^60 deposition rate, high H2 cost, etc. In addition, the IGSCC protection potential (-230 mV(subscript SHE)) is difficult to achieve in highly oxidizing or high fluid flow regions. Another one is termed Noble Metal Chemical Addition (NMCA). But it still needs lower H2 additions to achieve a low corrosion potential (~500 mV(subscript SHE)). Apart from the high cost of noble metals, the main drawback of this technology is that sufficient hydrogen level cannot be maintained in the certain locations of a BWR (e.g. in the vicinity of the core spray and top guide). Because the H2 is insufficient, NMCA may accelerate the speed of IGSCC on the contrary.
Recently, a new approach was explored to lower the corrosion potentials at all locations of BWRs, even without the presence of H2. This method is formed a dielectric films on a metal surface. It is termed Inhibitive Protective Coating (IPC).
In this study, an experiment will be conducted to investigate the effects of inhibitive coating with ZrO2 (different treating conditions) by hydrothermal deposition on Type 304 SS. The effect of ZrO2 coatings were investigated by electrochemical potentiodynamic polarization tests in simulated BWR environment. And surface analyses were conducted by X-ray、SEM、AES to identify the surface structure and components.
Test results showed that with IPC treated 304 SS specimens exhibited lower ECP, low corrosion current density and lower exchange current density (ECD) of oxidant or reductant than the untreated specimen. Comparison the treatments of different particle size were obtained: the effectiveness of 100nm was better than 700 nm. Comparison the treatments of different deposition time were obtained: 2 weeks is better than 1 week.

並列關鍵字

Boiling Water Reactor BWR ； Intergranular Stress Corrosion Cracking IGSCC ； Electrochemical Corrosion Potential ECP ； Inhibitive Protective coating IPC ； ZrO2 ； Potentiodynamic Polarization