低碳鋼於氯化鈉之熱腐蝕機制及熱浸鋁之防制改善

The Mechanism of NaCl-induced Hot Corrosion of Low Carbon Steel and the Effects of Hot-dipped Aluminum Coatings

選用低碳鋼、經熱浸鍍鋁及再作預氧化處理之三式表面狀態，於覆氯化鈉900℃靜態空氣之熱腐蝕環境，探討低碳鋼於氯化鈉之熱腐蝕反應機制，以及熱浸鋁塗層對鋼材之耐熱腐蝕性改善作用。經由形態觀察，以及動力學數據、生成相、反應熱力學的分析，低碳鋼於有限量被覆氯化鈉之熱腐蝕，可區分為當NaCl仍存在時，由氯氧化所主導之初始階段，以及NaCl已耗盡後由氯化／再氧化循環反應所主導之成長期。低碳鋼經熱浸鋁處理後，由於試片有較多的Al2O3 形成，得以阻隔NaCl與基材金屬之氯氧化反應，減少了成長期氯化／再氧化循環反應之氯氣氛源，因此對於氯化鈉熱腐蝕之耐蝕性可獲得明顯的改善。熱浸鋁處理後之再預氧化，提早了鐵、鋁擴散反應的發生，造成合金化層的過度成長與鋁元素的稀釋，並於冷卻時產生許多表面裂縫，提供液態氯化鈉積存的位置及滲入路徑，因而對鋼材長期之抗熱腐蝕特性有不良的影響。

The mechanism of NaCl-induced hot corrosion of low carbon steel with an initial coating of 9mg/cm2 NaCl was studied at 900℃ in static air. The effects of the hot-dipped aluminum coatings and its post preoxidation treatment on the corrosion resistance for low carbon steel were also investigated. Through the morphological studies, kinetics data, phases identification and the thermodynamic analysis, two kinds of mechanisms were identified in the hot corrosion process with the limited thickness of NaCl coating. The first stage, or the initial stage, was dominated by the oxychlorination reaction when the NaCl still existed. The second stage, or the propagation stage, was predominated by the chloridation and reoxidation cyclic reactions when the NaCl was depleted. The hot-dipped aluminum coating enhanced 'the formation of Al2O3. The Al2O3 then separated the contact of metal from the molten salt, thus inhibited the formation of chlorine from the oxychlorination reactions during the initial stage. Therefore, the reaction rate during the propagation stage was also reduced for the lack of chlorine. As a result, the aluminum coating improved the hot corrosion resistance for low carbon steel. However, the post preoxidation of aluminum coating caused the dilution of aluminum content and the over growth of the alloying layer by means of Al/Fe interdiffusion. Microcracks were formed in the brittle alloying layer during the cooling from the preoxidation temperature. The microcracks provided deposition position and short cut diffusion paths for the molten salt and the metals. The post preoxidation of the aluminum coating showed detrimental effect on the NaCl-induced hot corrosion.