氯化鈉熱腐蝕誘發之鐵錳鋁合金脫錳內氧化

NaCl-Induced Manganese Depletion and Internal Oxidation on Fe-Mn-Al Alloys during Hot Corrosion

以Fe-31. 1Mn-9.07Al-0.89C合金於750℃、800℃及850℃進行初始沈積2 mg/cm2氯化鈉之熱腐蝕試驗，藉以評估合金於氯化鈉熱腐蝕環境中之抗蝕性。實驗結果顯示熱腐蝕反應所生成的揮發性物質會促進內侵蝕孔洞內發生內氧化現象。於750℃及800℃之熱腐蝕試驗，由於氯化鈉作用時間長及保護性皮膜無法生成，使合金基材遭受嚴重的內侵蝕。於內侵蝕層中，外側近皮膜處相連孔洞之形成乃由於初始錳的選擇性氧化流失留下的空孔聚集成孔洞，再經由揮發性的物質-金屬氯化物之揮發而成長。由於錳、鋁受到氯及氧的選擇性侵蝕使得鐵在內侵蝕層之內側近基材處之活性相對昇高，以致在孔洞內填滿細小之Fe3O4微粒。於850℃之熱腐蝕，溫度的提昇使氯化鈉揮發速率增加，進而減少氯化鈉實際腐蝕合金之作用時間及作用量，加上鋁在合金內之擴散速率提高，有利保護性皮膜的生成，因而使得合金在此溫度受到最少的熱腐蝕，合金基材只形成量較少但較為粗大的內侵蝕孔洞。

The internal oxides formed on Fe-31.1Mn-9.07Al-0.89C alloy at 750-850℃ in air with 2mg/cm2 NaCl deposits initially were investigated by means of transmission electron microscopy. The results showed that the volatile species generated by hot corrosion accelerated internal oxidation in the internal voids. The alloys suffered severe subscale attack at 750℃ and 800℃because the protective alumina scale was not formed. The morphology of attacked subscale can be divided into three layers. The interconnecting voids in the outer subscale are finer and denser than those in the inner subscale. The products inside the voids of the outer subscale are metal oxides, while the oxides inside the voids of the inner subscale are filled with fine Fe3O4 particles. However, due to the formation of aluminum-rich oxide scale at 850 , only a small amount of internal oxidation was generated in subscale, which showed the best corrosion resistance in this study.