|Abstract: ||本研究主要探討9Cr-1Mo鍋爐用鋼(T91)與其經過Ni-Al改質(T91-NA)在650~850oC不同氧化環境(乾空氣、通水汽及熱循環)下之高溫氧化行為。研究結果顯示，T91在三種不同環境下的氧化動力學皆遵守拋物線律，但其氧化速率常數(kp值)卻有很大的差異，其中含水汽下最快，熱循環次之，而乾空氣下最慢。T91在650oC乾空氣氧化主要生成具有保護性的(Fe,Cr)2O3與(Fe,Cr)3O4，而在750及850oC主要生成外層的Fe2O3及Fe3O4，內層則為含Cr的(Fe,Cr)3O4為主，亦可由白金指標顯示出內外層，T91在水汽下氧化，皆生成外層的Fe2O3及內層的Fe3O4與(Fe,Cr)3O4，當850oC時，內層還有少量的FeO生成，且亦觀察出氧化層(Fe3O4/(Fe,Cr)3O4)中會有裂縫產生(但在乾空氣氧化下未觀察到裂縫)，其主要原因可能為水分子進入氧化層後轉換成氣相，以致產生張應力而使氧化層裂開，而使合金的氧化速率加快。T91在650~850oC乾空氣熱循環下氧化，其中，在850oC氧化速率相當快速，其主要氧化物為Fe2O3，並在氧化層發現垂直的裂縫產生，以致使氧化速率加速。 T91-NA在650~850oC乾空氣與通水汽下氧化，其氧化動力學皆呈現拋物線律，可見緻密的α-Al2O3層在水汽與乾空氣下皆保持良好的保護性，致使合金的氧化速率明顯降低。此外，可發現基材與Ni-Al鍍層中有許多孔洞產生，主要為金屬間的相互擴散造成Kirkdenall效應，且在氧化過後基材處會觀察到不連續黑色的析出物AlN。|
The high temperature oxidation behavior of T91 Boiler-used Steel (T91) and the same alloy with surface Ni-Al coating (T91-NA) was studied over the temperature range of 650~850oC in various environments (dry air, wet air, and cyclic oxidation). The results showed that oxidation kinetics of the T91 followed the parabolic-rate law at 650~850oC in the three environments. The oxidation rates of T91, depending on oxidant environment, followed by the fast-to-slow rank of wet air > cyclic-oxidation > dry air. The scales formed on the alloy consisted of (Fe,Cr)2O3 and (Fe,Cr)3O4 at 650oC, and consisted of an outer layer of Fe2O3 and Fe3O4 and an inner layer of (Fe,Cr)3O4 at 750~850oC in dry air. Duplex scales formed on the alloy consisted of an outer layer of Fe2O3 and Fe3O4 and an inner layer of (Fe,Cr)3O4 in wet air. In addition, minor amounts of FeO were also detected in the inner-layer at 850oC. The (Fe3O4/(Fe,Cr)3O4) scales had numerous cracks at 650~850oC in wet air, however this phenomenon has never been observed in dry air. Very likely, the water molecules penetrated into the scales, and formed water vapor, which resulted in the severe cracks, thereby leading to the faster oxidation rates. On the other hand, the oxidation-rate was slightly faster at 650~750 oC in cyclic oxidation than in dry air. Yet, the oxidation-rate became very fast at 850oC in the cyclic oxidation, some vertical cracks were observed in the Fe¬2O3 scale at 850oC. The oxidation kinetics of the T91-NA alloy also followed the parabolic-rate law at 650~850oC in dry air and wet air. The scales formed on the T91-NA consisted of an exclusive α-Al2O3 at 650850oC, whose formation is responsible for the reduction of the oxidation rate for the T91-NA alloy, as compared to the uncoated alloy. The Kirkendall effect is observed for the formation of voids at the interface between the Ni-Al coating layer and the substrate. Numerous discontinuous AlN precipitates formed in the substrate after the oxidation, indicating the occurrence of internal oxidation.