奈米光觸媒被覆對高溫純水環境中304不銹鋼的防蝕效益研究

An Investigation into the Corrosion Mitigation Effect of Nano-photocatalytic Coatings on Type 304 Stainless Steels in High Temperature Pure Water

早期奈米光觸媒主要應用於潔淨與自淨環境上。近幾年此光觸媒技術已被應用於保護沸水式反應器（Boiling Water Reactor, BWR）的組件，研究發現二氧化鈦被覆在BWR組件表面後，可抑制304不銹鋼組件的沿晶應力腐蝕龜裂（Intergranular Stress Corrosion Cracking, IGSCC）。其原理是利用BWR運轉時爐心產生的Cherenkov輻射，激發具n型半導體特性的二氧化鈦產生自由電子，促使水化學環境中的氧化劑進行還原反應，因而降低氧化劑濃度並抑制金屬組件的腐蝕。目前運轉中的BWR多採用加氫水化學（Hydrogen Water Chemistry, HWC）技術，來降低組件材料的電化學腐蝕電位，防制IGSCC的發生。然而在較高飼水注氫量（高於0.5 ppm）的HWC狀態下，會伴隨著輻射劑量率增加的副作用。若採用具備抑制特性的二氧化鈦被覆，理論上應可在不施行HWC情況下，亦能有效降低組件材料的腐蝕電位與腐蝕速率。
本研究採用動態循環熱水沉積法，在150℃溫度條件下進行已預長氧化膜304不銹鋼試片的二氧化鈦被覆處理，並觀察被覆處理前後試片的表面形貌。我們亦於模擬沸水式反應器水化學環境下，改變不同的溶氧條件及紫外光照射強度，對試片進行電化學極化掃描（electrochemical polarization）分析，以瞭解不同水化學環境下304不銹鋼施行氧化鈦被覆前後的電化學特性差異。結果顯示，預長氧化膜試片經被覆處理後，二氧化鈦在試片表面呈不均勻分佈。在不同溶氧濃度且紫外光照射的環境下（50、150、300 ppb），二氧化鈦處理之試片有明顯較低的陰極電流密度及交換電流密度；在紫外光關閉的情況下，經被覆處理與未被覆試片的陰極電流密度則無顯著差異，顯示具光觸媒特性的二氧化鈦被覆搭配紫外光照射確實具有腐蝕抑制的效果。

Nano-photocatalysts have been widely used for cleaning or self-cleaning applications for the past few decades. This photocatalysis process was adopted to protect the structural components in a boiling water reactor (BWR) from corrosion in recent years. It was found that TiO2 coatings deposited on the surfaces of Type 304 stainless steels were able to mitigate intergranular stress corrosion cracking (IGSCC) in the stainless steels. The principle is to generate sufficient free electrons on the TiO2 coated components upon the absorption of Cherenkov radiation in a BWR. The TiO2 coating bears the n-type semiconductor characteristics, and the induced free electrons would enhance the reduction of the oxidizing species and thus lessen the corrosion of the coated components. Hydrogen water chemistry (HWC) has been the most commonly used techniques for corrosion potential reduction and IGSCC mitigation in BWRs around the world. However, this technique would cause elevated radiation fields when the feedwater hydrogen concentration exceeds 0.5 ppm. A successful application of TiO2 coatings in a BWR is expected to effectively reduce the corrosion potential and the corrosion rate of the structural components even without the presence of HWC.
TiO2 nanoparticles were deposited on pre-oxidized Type 304 stainless steel specimens by hydrothermal deposition at 150 oC , and the surface morphologies of the specimens were examined. Electrochemical polarization analyses were conducted to investigate the impact of ultraviolet (UV) radiation on the electrochemical behavior of oxygen and the TiO2 treated specimens in simulated BWR environments of various oxygen concentrations (50, 150, and 300 ppb). The results showed that the distribution of TiO2 deposited on the specimen surface was not uniform and continuous. It was also observed that the corrosion current densities of the treated specimens and the exchange current densities of the oxygen reduction reactions were comparatively lower in the presence of UV radiation. Without UV radiation, however, no significant differences were observed between the TiO2 treated and untreated specimens. These results indicate that the TiO2 treatment in combination with UV radiation would effectively reduce the corrosion rate of Type 304 stainless steels in high temperature oxygenated environments.