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The oxidation behavior of alloy 625 and 120 foils was studied at 650, 700 and 800 $$^\circ$$ C in dry air and flowing air + 10% H $$_2$$ O up to 10,000 h (alloy 625) or 30,000 h (alloy 120). The effect of water vapor on Cr loss was investigated. Manganese and iron in the 120 foil induced faster Cr depletion in the foil and breakdown of the Cr $$_2$$ O $$_3$$ scale into Fe and Cr-rich oxide compared to the alloy 625 foil. In the latter case, the presence of Nb led to the formation of Nb and Cr-rich oxide after breakdown of the Cr $$_2$$ O $$_3$$ scale. Simultaneous Cr loss due to oxidation and volatilization of the Cr $$_2$$ O $$_3$$ oxide scale was predicted and compared to experimental Cr loss measurements for exposures up to 30,000 h.

The oxidation and hot corrosion performance of NiCoCrAlY coatings fabricated via electrolytic codeposition was investigated. The cyclic oxidation behavior of NiCoCrAlY coatings on Ni-based superalloys was evaluated at 1000 and 1100 °C. Preliminary Type I hot corrosion testing was conducted to assess the hot corrosion resistance of these coatings in a Dean rig with flowing O2 + 0.1% SO2 gas. NiAl coatings synthesized by a non-contact pack cementation method were included in the oxidation and corrosion tests for comparison. The NiAl coatings exhibited excellent oxidation resistance; however, they showed very poor hot corrosion performance at 900 °C. The electro-codeposited NiCoCrAlY coatings demonstrated good overall resistance to both cyclic oxidation and hot corrosion and also showed better chemical/microstructural compatibility with the superalloy substrates.

A new test rig was developed to conduct creep evaluation in steam. The isothermal creep performance for ferritic 9Cr and ferritic/martensitic Gr.91 steels was superior at 650 °C in steam than in air. Microstructure characterization revealed the presence of an adherent and continuous (Fe,Cr)-rich inner oxide layer because of crack healing by re-oxidation. Stress calculation based on a composite structure showed that the longer lifetimes in steam were likely due a transfer of load from the alloy to the inner scale. However, a significant decrease of the Gr91 creep lifetime was observed in steam at 650 °C when the specimens were thermally cycled every 10 h. This could be explained by localized cracking of the load-bearing inner layer.

A new, cast austenitic stainless steel, CF8C-Plus (CF8C-P), has been developed for a wide range of high temperature applications, including diesel exhaust components, turbine casings and turbocharger housings. CF8C-P offers significant improvements in creep rupture life and creep rupture strength over standard CF8C steel. However, at higher temperatures and in aggressive environments such as those containing significant water vapor, an oxidation-resistant protective coating will be necessary to extend service life. The oxidation behavior of alloys CF8C and CF8C-P with various aluminide coatings were compared at 800 °C in air plus 10 vol% water vapor. Due to their affordability, slurry aluminides were the primary coating system of interest, although chemical vapor deposition and pack cementation coatings were also compared. Additionally, a preliminary study of the low-cycle fatigue (LCF) behavior of aluminized CF8C-P was conducted at 800 °C. Each type of coating provided substantial improvements in oxidation behavior, with simple slurry aluminides exhibiting very good oxidation resistance after 3,000 h testing in water vapor. Preliminary LCF results indicated that thicker aluminide coatings degraded high temperature fatigue properties of CF8C-P, whereas thinner coatings did not. Results suggest that appropriately designed slurry aluminide coatings are a viable option for economical, long-term oxidation protection of austenitic stainless steels in water vapor.