摘要

沸水式反應器(Boiling Water Reactor, BWRs)長時間運轉，其內部隸屬壓力邊界(pressure boundary)的組件材料容易遭受沿晶應力腐蝕龜裂(Intergranular Stress Corrosion Cracking, IGSCC)劣化破壞。電化學腐蝕電位(electrochemical corrosion potential, ECP)為評估304不□鋼組件在288 ℃純水環境中是否發生沿晶應力腐蝕龜裂的重要指標。核能工業多採用加氫水化學( hydrogen chemistry, HWC )技術，降低組件材料的電化學腐蝕電位，防制IGSCC的發生。然而HWC在較高注氫量下(高於0.6ppm)，會伴隨著輻射劑量率增加的副作用。另一種防蝕技術─抑制性被覆( Inhibitive Protective Coatings , IPC)的發展逐漸盛行。IPC技術甚至在不施行HWC情況下，亦能有效降低材料的腐蝕電位與腐蝕電流密度。
本研究利用IPC技術，針對模擬BWR不同管路位置水化學環境下預長氧化膜的304不□鋼試片施以氧化鋯被覆處理，在90 ℃條件下採用動態循環熱水沉積法( hydrothermal deposition），再對試片進行各種表面分析。並模擬BWR爐心因輻射水解(water radiolysis)而產生溶氧與過氧化氫的高溫高壓純水環境，隨氧化劑濃度變化進行電化學動態電位極化掃描(electrochemical potentiodynamic polarization)以了解不同水化學環境下預長氧化膜施行抑制性被覆前後電化學特性差異。
結果顯示，溶氧環境下預長氧化膜試片經由抑制性被覆後(O-90)，SEM影像觀察到氧化鋯在試片表面呈局部較厚與較薄分佈。並由高溫極化掃描驗證了氧化鋯被覆對於溶氧有抑制的效果，能如預期的降低金屬的腐蝕電流密度、電化學腐蝕電位與氧化劑的交換電流密度，但是對低濃度的過氧化氫才有抑制效果。而過氧化氫環境下預長氧化膜試片施行抑制性被覆後(HP-90)，被覆效果優於 O-90試片，且對於各種濃度的過氧化氫水環境皆有抑制成效，雖然不能有效降低金屬的腐蝕電位，卻能大幅削減其的腐蝕電流密度與過氧化氫的交換電流密度。

Abstract
As the boiling water reactors (BWRs) age, Intergranular stress corrosion cracking (IGSCC) of structural materials used in boiling water reactor (BWR) piping systems and vessel internals is the major degradation problem. Research has demonstrated that below a critical electrochemical corrosion potential (ECP) of -230 mVSHE, the susceptibility of stainless steel to IGSCC is substantially reduced. In past decade, several approaches to mitigating IGSCC by lowering the ECP have been developed and investigated. In the early 1980s, technique of hydrogen water chemistry (HWC), which reduced the oxidizing power of the BWR coolant environment by hydrogen injection and subsequently lowered the susceptibility of stainless steel components to SCC, was widely developed to mitigate the IGSCC problems. However, several side effects of HWC have been reported, such as increased N16 carry - over to the main stream line , a higher Co60 deposition rate , high H2 cost, etc. In addition, the IGSCC critical potential (-230 mVSHE) was difficult to achieve in highly oxidizing, high fluid flow and / or high level irradiation regions.
Recently, a new approach was explored to lower the corrosion potential and the corrosion current densities at all locations of BWRs, even without the presence of H2. It is termed Inhibitive Protective Coating (IPC), based on the assumption that a dielectric coating will inhibit the redox reaction on the surface and, therefore, the dissolution of metal at the crack-tip to which they are coupled – without the addition of hydrogen.
An experiment has been conducted to investigate the effects of inhibitive coating with zirconia (ZrO2) by hydrothermal deposition on Type 304 SS. The primary task in this study was to determine significant electrochemical parameters such as electrochemical corrosion potential, corrosion current density, exchange current density and Tafel constant of the reduction reactions of oxygen and hydrogen peroxide before and after inhibitive protective coatings. Specimens before and after the zirconia treating process were examined by the scanning electron microscopy (SEM) , the energy dispersive X-ray spectroscopy (EDX), focus ion beam image (FIB) and laser Raman spectra (LRS) . Effects of inhibitive coating with zirconia on Type 304 were measured by electrochemical potentiodynamic polarization tests in simulated BWR environment. Test results showed that treated SS specimens (O-90) in dissolved oxygen exhibited lower ECP than pre-oxidized specimens (P-O), and that specimen exhibited lower ECP only in lower hydrogen peroxide concentration. In high hydrogen peroxide concentration, the IPC treatment is unable to reduce ECP. On the contrary, the IPC specimens (HP-90) in dissolved H2O2 condition revealed nearly the same ECP level as that of the untreated ones (P-HP) for a wide H2O2 range. Furthermore, the corrosion current densities and exchanged current densities of H2O2 on the HP-90 specimens were lower apparently than those on the untreated ones. The overall results indicated that the ZrO2 treatment could effectively reduce the corrosion rate of Type 304 stainless steel in simulated BWR environments in views of both corrosion current densities and exchanged current densities.

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