The effect of aluminum content on the corrosion behavior of Fe-Al alloys in reducing environments at 700 degrees 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. Postexpos ure characterization of the corrosion reaction products consisted of surfac e and cross-sectional microscopy, in combination with energy dispersive spe ctroscopy, electron probe microanalysis, and quantitative image analysis. F rom the kinetic data, three stages of corrosion behavior (i.e., inhibition, breakdown, and steady state) were found with the observance and/or duratio n 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 t he 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 nod ules, containing iron sulfide (Fe1-xS) and the spinel-type tau phase (FeAl2 S4), developed through the alumina scale due to sulfur short-circuit diffus ion. These growths were accompanied by relatively high corrosion rates. Fur ther decreasing the aluminum content to 5 wt pet and below lead to the form ation 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 p revious two regions.