HIGH-TEMPERATURE OXIDATION OF FE-CR-AL-SI ALLOYS EXTRUDED INTO HONEYCOMB STRUCTURES

The oxidation behavior of Fe-20Cr-5Al-(0.5-5)Si and Fe-(12-20)Cr-(5-7) Al-(1-2) Si alloys extruded into honeycomb structures has been investi gated at 1150 degrees C in air for up to 500 hr, The oxidation weight gains decrease with increasing Si and Cr contents in the 5-Al alloys. Si additions are more efficient than Cr additions to reduce the weight gain. Increasing Si content in the 5-Al alloys suppresses the formati on of an iron-chromium complex oxide, forming mullite and vitreous sil ica in the scale, although the location is not clearly indicated. The 5-Si alloy shows anisotropy in elongation of the honeycomb specimen du ring oxidation in the Fe-20 Cr-5Al-xSi alloys, whereas alloying with S i and Cr does not improve the oxidation resistance of the 7-Al alloys significantly. These results are explained by Wagner's theory of a sec ondary getter. However, we point out additionally that the difference between Si and Cr in the Pilling-Bedworth ratio and the solubility of their oxides in the Al2O3 scale may contribute to the significant effe ct of Si additions. Finally, this paper demonstrates that the selected Fe-Cr-Al-Si honeycombs having walls 200 mu m thick show excellent oxi dation resistance over 500 hr at 1150 degrees C in air. The time to ca tastrophic oxidation is roughly proportional to the wall thickness in extruded honeycombs.