The effect of aluminum content on the corrosion behavior of Fe-Al alloys in reducing environments at 700°C

The high-temperature corrosion behavior of monolithic Fe-Al alloys, with 0 to 20 wt pet Al, was investigated at 700 degrees C in a reducing atmosphere (p(S-2) = 10(-4) atm, p(O-2) = 10(-25) atm) for up to 100 hours. Postexposure characterization of the corrosion reaction products consisted of surface and cross-sectional microscopy, in combination with energy dispersive spectroscopy, electron probe microanalysis, and quantitative image analysis. From the kinetic data, three stages of corrosion behavior (i.e., inhibition, breakdown, and steady state) were found with the observance and/or duration of each stage directly related to the aluminum content of the alloy. The first stage, labeled the inhibition stage, was characterized by low weight gains and the absence of rapid degradation of the alloy. Typically observed for compositions with 10 to 20 wt pet Al, protection was afforded due to the development of a thin, continuous alumina scale. For alloys with 7.5 wt pet Al, the ability to maintain the initially formed alumina scale was not observed, resulting in the breakdown stage. Localized corrosion product nodules, containing iron sulfide (Fe1-xS) and the spinel-type tau phase (FeAl2S4), developed through the alumina scale due to sulfur short-circuit diffusion. These growths were accompanied by relatively high corrosion rates. Further decreasing the aluminum content to 5 wt pet and below lead to the formation of a continuous sulfide scale whose growth was controlled by iron and sulfur diffusion through the previously formed product. The alloy wastage rates in the steady-state stage were relatively high when compared to the previous two regions.