本研究以（1）試片現場懸掛試驗及（2）電化學模擬實驗來評估FGD（Flue gas desulfurization）系統內襯候用合金，做為內襯材料選用之參考依據。試片現場懸掛試驗選用三組合金共20個試片，懸掛於FGD系統中之冷卻器（cooler）、除塵塔（prescruber）、吸收塔（absorber）、加熱器（heater）等四個位置，經歷271天後取出評估腐蝕速率。電化學模擬實驗以綠色溶液為腐蝕液，以極化阻抗、Tafel極化、循環陽極動態極化法及交流阻抗等方法來評估高抗局部腐蝕指數（Local corrosuin equivalent, LCE）類合金（包含C276、5923、Ti、317LM、654SMO及254SMO）之耐蝕性。 現場試驗之結果顯示：冷卻器及除塵塔中之試片有一層覆蓋物，若此覆蓋物質地厚而脆，當覆蓋物產生破裂或是剝落時，腐蝕因子會經由這些裂縫，滲入而侵蝕基底金屬，當覆蓋物結構完整時，則具有保護效果，而使腐蝕有所緩和。在吸收塔及加熱器由於煙氣已經過脫硫程序，故試片的腐蝕情形並不嚴重。本試驗選用高LCE合金中，以5923合金表現最佳，而以317LM最差。 電化學模擬實驗結果顯示：所有的候選合金在通空氣的環境下，皆表現出良好的耐蝕性。5923合金在除氧的環境下，其腐蝕速率和在通空氣環境下很接近，且當環境溫度升高時依然保持良好鈍化效果，其鈍化膜的組成主要為Cr2O3。C276在除氧的環境下，試片表面在極化後出現微孔，當環境溫度升高，鈍化膜的保護性降低，即使在通空氣環境下亦出現腐蝕的痕跡。鈦在通空氣的環境下，表現出良好的耐蝕性，環境溫度升高，鈦之腐蝕速率亦增加且鈍化膜成長的速率降低，但其鈍化膜對溫度的穩定性良好，鈍化膜的組成主要為TiO2。654SMO合金的鈍化效果明顯，且鈍化膜對溫度有良好的安定性。254SMO在腐蝕溶液中，鈍化膜成長的速率隨溫度上升而下降，其鈍化膜會因溫度上升而有被破壞的趨勢。317LM在通空氣環境下鈍化膜成長快速，但其鈍化膜防蝕效果不佳。 由以上實驗得知，候選合金中以5923、Ti及654SMO耐蝕性最佳，而C276、254SMO和317LM則表現較差。 Coupon test and electrochemical techniques were employed in this work for selecting proper alloys in lining use of the flue gas desulfurization (FGD) system. In-situ coupon test was conducted in different locations, such as cooler, prescruber, absorber and heater for 271 days. Measurement of weight loss is used to calculate the corrosion rate of the coupons. Electrochemical study was carried out in green solution at various temperatures to simulate the corrosion condition in FGD. Twenty kinds of alloys were explored in coupon test. Six high corrosion-resistant alloys were compared in electrochemical study. The results of the coupon test indicated that the corrosivity in various locations decrease in the order cooler>prescruber>absorber>heater. A heavy deposit was coated on the coupons, which have been put in cooler and prescruber. 5923 is the most favor alloys for lining the FGD. The electrochemical study demonstrated that the result of DC polarization (including polarization resistance, Tafel plot) is in agreement with that of electrochemical impedance spectroscopy (EIS). The corrosion-resistance alloys show the active-passive character and the passivity is better for 5923 than C276. The corrosion resistance for the stainless steels decrease in the order 654SMO>254SMO>317LM. Based on the result of coupon test and electrochemical study, it is suggested that 5923 and 654SMO are suitable for lining in FGD system.
