Iron-aluminum alloys are currently under investigation for use as corrosion protective weld overlay coatings in reducing environments. These materials are relatively inexpensive, do not exhibit macro- or microsegregation, and have better corrosion resistance compared to conventional Ni base and stainless steel-type compositions presently in use. However, their use is limited due to weldability issues and their lack of corrosion characterization in very aggressive environments. Therefore, the objective of this research was to examine the sulfidation behavior of weldable Fe-Al compositions in highly aggressive reducing atmospheres. The high temperature corrosion behavior in environments containing oxygen and sulfur was characterized by thermogravimetric techniques. As-solidified Fe-Al alloys, with 0-20 wt% Al, were isothermally held at temperatures between 500-700 °C for up to 100 hours in a reducing environment. Specially tailored gases maintained the partial pressures of oxygen and sulfur at each temperature [p(O_2) = 10~(-25) atm, p(S_2) = 10~(-4) atm]. Post-exposure characterization of the corrosion reaction products consisted of surface and cross-sectional microscopy in combination with energy dispersive spectroscopy and electron probe microanalysis. The corrosion behavior was found to be directly related to the aluminum content of the alloy. For high aluminum compositions (above 7.5 wt% Al), protection was afforded due to the development of a thin, continuous alumina scale that inhibited rapid degradation of the alloy. Increasing the aluminum content of the alloy was found to promote the formation and maintenance of this scale. For lower aluminum alloys, the ability to form and/or maintain the alumina scale was not observed. Instead, thick growing sulfide phases were found to develop either in the form of localized nodules (at 7.5 wt%Al) or a continuous surface scale (below 7.5 wt% Al). The results from this study indicate that weldable compositions of Fe-Al alloys (10 wt% Al) show excellent corrosion resistance to aggressive reducing environments. With the potential promise for applications requiring a combination of weldability and corrosion resistance in moderately reducing environments, these alloys are viable candidates for further evaluation for use as sulfidation resistant weld overlay coatings.
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