The Effects of Water Vapor on the Oxidation Behavior of Alumina Forming Austenitic Stainless Steels

The isothermal oxidation behavior of three alumina forming austenitic (AFA) stainless steels with varying composition was studied at 650 and 800 A degrees C in dry air and gases which contained water vapor. The AFA alloys exhibited better oxidation resistance than a "good chromia former" at 650 A degrees C, particularly in H2O-containing atmospheres by virtue of alumina-scale formation. Although the AFA alloys were more resistant than chromia formers, their oxidation resistance was degraded at 650 A degrees C in the presence of water vapor. In dry air the AFA alloys formed, thin continuous alumina scales, whereas in Ar-4%H-2-3%H2O the areas of continuous alumina were reduced and Fe oxide-rich nodules and regions of Cr, Mn-rich oxides formed. In some regions internal oxidation of the aluminum occurred in the H2O-containing gas. The alloy OC8 had slightly better resistance than OC4 or OC5 in this atmosphere. The alumina-forming capability of the AFA alloys decreases with increasing temperature and, at 800 A degrees C, they are borderline alumina formers, even in dry air. The oxidation resistance of all three alloys was degraded at 800 A degrees C in atmospheres, which contained water vapor (Air-10%H2O, Ar-3%H2O and Ar-4%H-2-3%H2O). The areas, which formed continuous alumina, were reduced in these atmospheres and areas of internal oxidation occurred. However, as a result of the borderline alumina-forming capability of the AFA alloys it was not possible to determine which of the H2O-containing atmospheres was more severe or to rank the alloys in terms of their performance. The experimental results indicate that the initial microstructure of the AFA alloys also plays a role in their oxidation performance. Less protective oxides formed at 800 A degrees C when alloy OC8 was equilibrated before exposure rather than being exposed in the as-processed condition. The reason for this is the presence of different phases in the bulk of the two specimens.