核電廠沸水式反應器( Boiling Water Reactor, BWRs )在長時間運轉下組件材料容易發生沿晶應力腐蝕龜裂(Intergranular Stress Corrosion Cracking, IGSCC)的問題。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。在1980年代，核能工業多採用加氫水化學(Hydrogen Water Chemistry, HWC)技術，降低材料的電化學腐蝕電位，減緩沿晶應力腐蝕龜裂的發生，但HWC技術在較高的飼水注氫量下(高於0.6ppm)，會伴隨管路輻射劑量增加的副作用。
為了減少HWC伴隨的副作用，本研究利用抑制性被覆防蝕技術，針對304不□鋼以100 nm氧化鋯(ZrO2)進行被覆處理，在90 °C與150 °C兩種溫度條件下採用動態循環的熱水沉積法(hydrothermal deposition)維持7天，再置入模擬BWR高溫純水環境中進行動態電位極化掃瞄及ECP監測。試片在被覆前先經由敏化熱處理，並分別在300 ppb溶氧及50 ppb溶氫288 °C純水環境中預長氧化膜。研究中以SEM、EDX對試片覆膜前後進行表面成分分析，並由拉曼能譜辨別hematite (□-Fe2O3)及magnetite (Fe3O4)氧化膜結構。
本研究主要目的為研究比較304不□鋼不同氧化膜表面氧化鋯被覆的防蝕效益，模擬BWR爐心中因輻射水解產生溶氧與溶氫的高溫純水環境，在兩種氧化還原劑濃度變化下進行極化掃瞄及ECP監測，以了解氧化鋯被覆在不同氧化膜結構的腐蝕特性差異。由SEM影像觀察，溶氧環境中形成的氧化物分佈緻密，且顆粒大小平均；溶氫環境中形成的氧化物分佈稀疏，且顆粒大小不均。經氧化鋯被覆試片的SEM影像中，氧化鋯可被覆在兩種不同氧化結構上，但是以在□-Fe2O3結構上90 °C時被覆條件效果最佳。極化曲線結果顯示經氧化鋯被覆處理試片，在腐蝕電流密度、腐蝕電位及交換電流密度皆小於未被覆處理的試片，以90 °C條件被覆在□-Fe2O3結構上試片，腐蝕電流密度可降低二分之一，而交換電流密度在不同溶氧濃度中可降低達10~30倍最佳。另外，被覆試片經過10天浸泡在288 °C含溶氧純水環境中，測試前後的ECP及SEM影像並無明顯改變，仍維持良好的被覆防蝕效果；經過10天浸泡在溶氫環境後，SEM影像中觀察到，部分區域在沒有氧化鋯被覆下，會有金屬氧化物溶解現象。整體而言，實驗結果驗證了氧化鋯被覆可以有效降低304不□鋼在BWR環境的腐蝕速率。

Incidents of intergranular stress corrosion cracking (IGSCC) have been readily observed in boiling water reactors (BWRs) for many years. The electrochemical corrosion potential (ECP) is a major indicator for IGSCC susceptibility of stainless steel components in 288 °C pure water. In the early 1980s, the technology of hydrogen water chemistry (HWC) was developed to mitigate the IGSCC problems in BWRs. However, HWC has side effects of exposing operators to elevated level of radiation and high hydrogen cost. To reduce these side effects, a novel technique of zirconium oxide (ZrO2) coating that eventually requires no hydrogen addition has been proposed.
In this study, electrochemical polarization analyses and ECP response monitoring were conducted to investigate the impact on ZrO2 treated specimens in simulated BWR environments. Prior to the electrochemical tests all specimens were thermally sensitized and pre-oxidized in high temperature water containing either 300 ppb O2 or 50 ppb H2. Afterwards, the specimens were treated with 100 nm ZrO2 by hydrothermal deposition at 90 °C and 150 °C for one week. The morphologies of the specimens were examined by scanning electron microscopy and energy dispersive X-ray spectroscopy. The hematite (α-Fe2O3) and magnetite (Fe3O4) structures of oxides were identified by Raman spectrum.
The objective of this study was to evaluate the effectiveness of ZrO2 treatment on specimens with different oxide structures. Differences between the sizes and the packing densities of the oxide particles produced from different water chemistry condition could be clearly observed on the SEM images. Based on observation, the coating density of ZrO2 on the oxides with a hematite structure was greater than on those with a magnetite structure. From the electrochemical potentiodynamic polarization results, we observed lower electrochemical corrosion potentials, corrosion current densities, and exchange current densities on the treated specimens than on the untreated ones in high temperature water with either dissolved oxygen or dissolved hydrogen only. Furthermore, the ECPs and surface morphologies of the ZrO2 treated specimens showed almost no changes after ten days of immersion in 288 °C water. The overall results indicated that the ZrO2 treatment could effectively reduce the corrosion rate of Type 304 stainless steel in simulated BWR environments.

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