不同矽含量熱浸鋁化低碳鋼其高溫氧化行為之研究

Study of High-temperature Oxidation Behavior of Hot-dipped Aluminide Coating on Mild Steel with various Silicon Contents

本研究採用低碳鋼熱浸於700℃之純鋁、鋁-2.5矽、鋁-5矽與鋁-10矽熔湯（wt.%）中180秒。熱浸試片於750℃進行高溫恆溫與循環氧化實驗，以探討不同矽含量之熱浸鋁化低碳鋼其高溫氧化行為。恆溫氧化結果顯示，不同矽含量之熱浸鋁化試片的重量增重皆遵守拋物線率。熱浸鋁化試片的抗恆溫氧化能力與鋁湯矽含量成反比。造成高矽鋁化試片具有較高的恆溫氧化速率可歸因於鋁湯中矽的添加降低鋁化層的厚度以及促使相變化誘發體積變化的孔洞生成。循環氧化結果顯示，各熱浸鋁化試片的重量增重皆大於恆溫氧化之結果。由於低矽鋁化試片具有較厚的鋁化層，加上其主要由硬脆之Fe2Al5相組成，造成鋁化層易於循環氧化過程因熱應力的累積而形成垂直鍍層之裂縫，進而大幅提高鋁化試片的重量增重趨勢。因此，低碳鋼熱浸純鋁試片具有最差之抗循環氧化能力。

Mild steel was coated by hot-dipping into molten baths containing pure aluminum, Al-2.5Si, Al-5Si and Al-10Si (wt.%) at 700 oC for 180 seconds. Isothermal and cyclic oxidations were carried out at 750 oC in a static air to study the oxidation behavior of the hot-dipped aluminide steels with various silicon contents. The results of the isothermal oxidation show the weight gains of all the aluminide steels followed a parabolic law. The isothermal oxidation rates of the aluminide steels were directly proportional to the silicon content in the aluminide layers. The reason for the aluminide steel with high silicon has a high isothermal oxidation rate is the reduction in the aluminide layer thickness and the formation of phase transformation induced voids in the aluminide layer, which happened due to the silicon addition in the molten bath. The results after cyclic oxidation show the weight gains of the aluminide steels were larger than those after isothermal oxidation. Because the aluminide layer with low silicon was mainly composed of a thick brittle Fe2Al5 phase, thermal stress was easily generated in the aluminide layer and caused the formation of vertical cracks in aluminide steel suffering from the cyclic oxidation. Once cracks appeared, the weight gains of the aluminide steels were accelerated. Thus, mild steel after hot-dipping in pure aluminum with the thickest aluminide layer possessed the worst resistance to the cyclic oxidation.