Selective oxidation behavior of ferritic martensitic Fe-Cr base alloys, exposed in various atmospheres containing combinations of O 2 , CO 2 , and H 2 O, were studied at various temperatures relevant to oxy-fuel combustion. This paper begins with a discussion of the required Cr content to form a continuous external chromia scale on a simple binary Fe-Cr alloy exposed in oxygen or air based on experiments and calculations using the classic Wagner model. Then, the effects of the exposure environment and Cr content on the selective oxidation of Fe-Cr alloys are evaluated. Finally, the effects produced by alloying additions of Si, commonly present in various groups of commercially available ferritic steels, are described. The discussion compares the oxide scale formation on simple binary and ternary Fe-Cr base model alloys with that on several commercially available ferritic steels.

Eight model high entropy alloys (HEAs) in the CoCrFeMnNi family (including one alloy each in the CoCrFeNi and CoFeMnNi subfamilies) were made, prepared, and exposed to laboratory air for 1100 h at 650°C and 750°C. Two commercial alloys, nickel-base superalloy 230 (N06230) and austenitic stainless steel 304H (S30409), were simultaneously exposed for comparison. Mass change oxidation kinetics were measured and cross-sections of exposed samples were observed. Seven of these HEAs contained much more Mn (12-24 wt.%) than is found in commercial heat-resistant stainless steels and superalloys. The oxidation resistance of CoCrFeNi was excellent and comparable to 304H at 650°C and only slightly worse at 750°C. The thin oxide scale on CoCrFeNi was primarily Cr oxide (presumably Cr 2 O 3 ) with some Mn oxide at the outer part of the scale. The CoCrFeMnNi HEAs all experienced more rapid oxidation than CoCrFeNi and, especially at 750°C, experienced oxide scale spallation. The addition of Y in the alloy to lower S improved the oxidation resistance of these HEAs. Alloy CoFeMnNi, without Cr, experienced much higher oxidation rates and scale spallation than the Cr-containing alloys. A linear regression analysis of the log of the parabolic rate constant, log(k p ), as functions of wt.% Cr and Mn found a good correlation for the compositional dependence of the oxidation rate constant, especially at 650°C. Mn was found to be more detrimental increasing log(k p ) than Cr was helpful reducing log(k p ). If CoCrFeMnNi HEAs are to be used in high temperature oxidizing environments, then examining lower levels of Mn, while maintaining Cr levels, should be pursued.

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A methodology is developed to calculate Cr evaporation rates from Cr 2 O 3 with a flat planar geometry. Variables include temperature, total pressure, gas velocity, and gas composition. The methodology was applied to solid oxide fuel cell conditions for metallic interconnects and to advanced steam turbines conditions. The high velocities and pressures of the advanced steam turbine led to evaporation predictions as high as 5.18 × 10 -8 kg/m 2 /s of CrO 2 (OH) 2 (g) at 760°C and 34.5 MPa. This is equivalent to 0.080 mm per year of solid Cr loss. Chromium evaporation is expected to be an important oxidation mechanism with the types of nickel-base alloys proposed for use above 650°C in advanced steam boilers and turbines. It is shown that laboratory experiments, with much lower steam velocities and usually much lower total pressure than found in advanced steam turbines, would best reproduce chromium evaporation behavior with atmospheres that approach either O 2 +H 2 O or Air+H 2 O with 57% H 2 O.

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