臺灣博碩士論文加值系統

　　本研究選用SB450、STPA24及STPA26三種原材，經單純空氣氧化及噴覆三式2mg/cm2 NaCl/Na2SO4混合鹽於750℃、850℃進行高溫腐蝕後，藉由動態極化曲線、腐蝕形態之觀察及生成物鑑別分析等方式來討探在常溫3.5%NaCl電解液環境下之腐蝕行為。
實驗結果顯示：合金原材於3.5wt%NaCl電解液之icorr隨鉻的含量增加而減少，腐蝕電位則隨鉻含量增加呈現線性上升並呈鈍態(noble)之趨勢。其中STPA26有最小之icorr及明顯鈍化(passivation)，整體有最佳的抗蝕性。
經高溫腐蝕後，影響腐蝕電位之因素主要為電極之種類，氧化數愈高的氧化物，其腐蝕電位愈高，愈傾向於鈍態（noble）。影響icorr及循環極化曲線再現性之主要因素則為 氧化皮膜之形態，顆粒狀的氧化物或皮膜愈鬆散，愈易導致間隙腐蝕，所以icorr愈高且循環極化曲線之再現性愈差，並因電解液可透過氧化物之間隙接觸到內層低價氧化物甚至合金原材，使腐蝕電位降低而呈較活性之狀態。
當沈積鹽尚含有NaCl時，由於氯氧化反應，氧化皮膜呈多孔狀而易產生間隙腐蝕，以致icorr較原材高2~3倍、循環極化曲線再現性變差，腐蝕電位偏低，接近基材之電位。於單純空氣氧化之氧化皮膜較為緻密，故不易發生間隙腐蝕，以致icorr與原材相近，循環極化曲線再現性僅次於原材，抗蝕性較前者佳。
隨腐蝕溫度及時間之增加，高溫腐蝕氣氛轉變為單純空氣氧化或完全Na2SO4之沈積，反應趨向氧化反應而有較緻密之氧化皮膜，以致icorr降低，腐蝕電位上升。此外，當試片有Fe1-XO殘留時，由於Fe1-XO為高溫殘留相，且具高缺陷結構，以致icorr明顯高於無Fe1-XO之試片。

　　The room-temperature corrosion behavior of three Fe-Cr alloys(containing SB450 and STPA24, and STPA26) was studied in 3.5% NaCl aqueous electrolyte after hot corrosion with 2mg/cm2 NaCl/Na2SO4 deposit at 750℃ and 850℃. The aqueous-corrosion mechanism was evaluated in terms of scale morphology by OM and SEM, XRD, and dynamic polarization curve measurements.
The results showed that the higher the Cr content the lower icorr and the higher Ecorr values of the alloy which tends to be more noble. STPA 26 has the best corrosion-resistant ability among the alloys studied, having the lower icorr values and forming a passive zone.
The main reason to affect the Ecorr after hot corrosion is sort of electrodes, in which the higher valence of oxides the higher Ecorr values and the more noble of noble alloy. In addition, the main reason to the icorr and the repeatability of cyclic polarization curve is strongly dependent on the scale morphology, in which particle-like or loose scale are much easier to induce crevice corrosion. The higher the icorr values, the worse the repeatability of cyclic polarization curve present. In addition, electrolyte may also contact with lower valence oxides or substrate, which decrease Ecorr values, thereby resulting in a more active state for the alloy.
Because the oxychloridation taken place when hot corrosion with NaCl deposits, scales become more porous and easy to corrode, leading to an increase of icorr value by a factor of 2 as compared to the alloy. Thus, the repeatability of cyclic polarization curve gets worse, and decreases Ecorr values. On the other hand, the scales are more dense in pure oxidation, so that crevice corrosion is more difficult to occur. As a result, icorr will be close to that of the raw alloy, and the repeatability of cyclic polarization curve is second to that alloy, in which the corrosion resistance of scales is much improved.
The hot-corrosion atmosphere tend to transfer to air-oxidation or complete Na2SO4-deposit environments as increase temperature and exposure duration. As a result, the overall reaction is favorable to form dense oxide-scales, which reduces the icorr but increases the Ecorr values. However, the high-defective Fe1-XO scale retained on the alloy cause a significant increase in the icorr values as compare to that without Fe1-XO.

第一章 前言 1
第二章 文獻回顧 3
2.1常溫腐蝕和電化學反應 3
2.1.1電化學反應及熱力學 3
2.1.2腐蝕動力學 6
2.1.2.1法拉第定律 6
2.1.2.2極化現象 6
2.1.2.3混合電位原理及腐蝕電位、電流、塔弗斜率 8
2.1.2.4腐蝕速率測量及循環極化曲線 9
2.1.3影響腐蝕速率之因素 14
2.2高溫腐蝕及熱力學 20
2.2.1熱力學資料 20
2.2.2高溫腐蝕機構 23
2.2.2.1 NaCl之熱腐蝕 23
2.2.2.2 Na2SO4之熱腐蝕 24
2.2.2.3 NaCl/Na2SO4混合鹽之熱腐蝕 25
第三章 實驗方法 27
3.1 實驗流程 27
3.2 試片準備 28
3.3 高溫腐蝕實驗 29
3.3.1 氯化鈉/硫酸鈉混合鹽噴覆作業 29
3.3.2 高溫腐蝕實驗作業 29
3.4 高溫腐蝕實驗後分析設備與方法 30
3.4.1 分析設備 30
3.4.2 分析方法 30
第四章 實驗結果 36
4.1 沈積鹽揮發試驗 36
4.2 試片腐蝕外觀 38
4.3 X光繞射分析 43
4.4 截面金相觀察 52
4.5 動電位極化曲線量測 59
4.5.1 塔弗曲線量測 59
4.5.2 循環極化曲線量測 62
4.6 腐蝕電位之趨勢 67
4.7 表面SEM之觀察 70
第五章 討論 74
5.1 高溫腐蝕後之皮膜形態 74
5.1.1 單純空氣氧化 74
5.1.2 100%NaCl沈積熱腐蝕 75
5.1.3 100%Na2SO4沈積熱腐蝕 78
5.1.4 50%NaCl/50%Na2SO4混合鹽沈積熱腐蝕 79
5.2 電化學腐蝕行為 82
5.2.1 原始素材 82
5.2.2 單純空氣氧化 82
5.2.3 100%NaCl沈積熱腐蝕 84
5.2.4 100%Na2SO4沈積熱腐蝕 84
5.2.5 50% NaCl/50%Na2SO4混合鹽沈積熱腐蝕 84
第六章 結論 87
參考文獻 89
附錄1 X-ray繞射分析 93

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