Shigeo Maruno1, Osami Kasuya2 and Shigeyuki Hikita2
1Material Research Laboratory, Nagoya Institute of Technology, Nagoya
Direct observation and X-ray analysis of oxide films formed on Fe-Cr-Al alloys with high aluminum content were made by SEM with an energy dispersive X-ray spectrometer and by EPMA. The alloys were heated in air at 1150, 1200, 1250 and 1300°C for 1 hr, respectively. SEM observation of the oxide films shows that a large number of fine granular crystals of aluminum oxide ( α -Al2O3) formed at temperatures lower than 1200°C grow to rectangular α -Al2O3 crystals with about 0.6∼1.0 μ in size at 1300°C and that the oxide film structure becomes porous with increase in heating temperature. The formation of the alumina film is not essentially connected with alloy grain boundaries but is pronounced at the parts of the sharp microridges on the alloy surface. The crystals of the anomalous oxides produced on the alloy surface mainly consist of α -(Cr, Fe)2O3 which is a solid solution of chromium in α -Fe2O3. However, the formation of chromium oxides in which iron is occluded was recognizable within the considerable anomalous oxides, where the spalling of alumina film had occurred. A needle-shaped-alumina, in which calcium is occluded, was also observed at around the anomalous oxides formed on the specimen that had a calcium oxide adhesive firmly to the surface.
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