High Temperature Oxidation of Fe-14Cr Alloys with Small Additions of Al in Wet Oxygen

Abstract

A study has been made on the isothermal oxidation behaviour of Fe-14Cr-Al alloys containing up to 2.0%Al in 1 atm O₂/H₂O vapour mixtures in the temperature range 900 to 1200°C. Compared with the previously reported behaviour in dry O₂, little change has been observed on 0.3%Al alloy at and below 1100°C. The good oxidation resistance of the alloy is explained to be due mainly to the stability against moisture of the scale rich in Cr₂O₃, the nucleation and growth of which being brought about preferentially by an internal oxidation zone of dispersed α-Al₂O₃ particles. At 1200°C water vapour begins to have effects on the alloy in such a way that it accelerates the initiation and growth of warts formed exclusively along the specimen edges. The structure of warts formed on the alloy after breakaway is double-layered and has been characterized by accumulation of Al just beneath the outermost part of the inner layer. This accumulation of Al has been considered to be the result of agglomeration of the dispersed α-Al₂O₃ particles during the growth of the warts. Two types of breakaway are found to occur in 0.1 and 0.8%Al alloys. One appears predominantly along the specimen edges at high temperatures, with the Al accumulation as in the case of 0.3%Al alloy, and the other, with no significant accumulation of Al, occurs uniformly on the specimen surface at relatively low temperatures and a high level of water vapour contents. Under the oxidizing condition where the latter type of breakaway occurs, development of the internal oxidation zone of well-developed and dispersed α-Al₂O₃ particles is not sufficient in these two alloys so that the preferential formation of the Cr₂O₃-rich scale cannot be fully established. The alloys are thus subject to an intense effect of moisture as observed. The oxidation resistance of 2.0%Al alloy is high at temperatures below 1000°C, beyond which it decreases more rapidly than in dry O₂.