High Temperature Corrosion Behavior of Iron Aluminide Alloys and Coatings

A multi-year effort has been focused on optimizing the long-term oxidation performance of ingot-processed (IP) and oxide-dispersion strengthened (ODS) Fe{sub 3}Al and iron aluminide-based coatings. Based on results from several composition iterations, a Hf-doped alloy (Fe-28Al-2Cr-0.05at.%Hf) has been developed with significantly better high temperature oxidation resistance than other iron aluminides. The scale adhesion is not significantly better; however, the {alpha}-Al{sub 2}O{sub 3} scale grows at a slower rate, approximately a factor of 10 less than undoped iron aluminide. The benefit of Hf is greatest at 1100-1200 C. Long-term oxidation resistance of commercially fabricated ODS Fe{sub 3}Al has been determined and compared to commercially available ODS FeCrAl. Scale spallation rates for ODS Fe{sub 3}Al are higher than for ODS FeCrAl. To complement studies of iron-aluminide weld-overlay coatings, carbon steel was coated with Fe-Al-Cr by thermal spraying. These specimens were then exposed in air at 900 and 1000 C and in air-1%SO{sub 2} at 800 C. Most likely due to an inadequate aluminum concentration in the coatings, continuous protective Al{sub 2}O{sub 3} could not be maintained and, consequently, the corrosion performance was significantly worse than what is normally observed for Fe{sub 3}Al.