European and Japanese developments of technologies for coal-fired power plants employing advanced steam conditions have resulted in Fe-Cr alloys that appear to have acceptable strength up to 620°C (1148°F) or possibly 650°C (1202°F). However, these alloys have relatively low Cr contents (up to 13 percent), which are marginal for the formation of protective oxide scales at these temperatures. Since there is no practical experience with alloys of this type in steam at elevated temperatures, information on their oxidation behavior in steam, in particular their rate of oxidation, is needed. This paper reports an assessment of the available information on steam oxidation of Fe-Cr alloys. It appears that the 9-12Cr and the 2-3Cr alloys oxidize at essentially the same rate at 550°C (1022°F), but the higher-Cr alloys provide somewhat better protection at the higher temperatures of interest (up to 700°C/1292°F). However, the oxidation rates are still fast, since the predominant scales formed are based on magnetite. It will be important for lifetime prediction to determine if the scale grows according to a parabolic or linear rate law; this point is not well resolved by examination of existing data. Routes for promoting the formation of more protective, Cr-rich scales on the more highly-alloyed are being explored and some show good promise.
Keywords: ferritic steels; steam oxidation; magnetite scales; parabolic oxidation; linear oxidation; advanced steam conditions