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This study focuses on the effect of tungsten as a Laves phase forming element on the steam oxidation behavior of the Fe-9Cr alloy. Steam oxidation tests of the Fe-9Cr alloys with different tungsten concentrations were conducted in an Ar-15%H2O gas mixture at 923 K for up to 345.6 ks. The results showed that tungsten addition significantly improved the oxidation resistance of the Fe-9Cr alloy. By forming FeWO4 in the internal oxidation zone (IOZ) during oxidation, the oxygen partial pressure in this zone corresponded to Fe(W)/Fe2W/FeWO4 equilibrium, which is slightly lower than the Fe/FeO equilibrium. Therefore, the matrix of the IOZ was not being oxidized to form the inner scale, resulting in an improvement in oxidation resistance. A finer and higher fraction of the Fe2W phase produced by rapid cooling provided better Cr enrichment and improved the oxidation resistance of the alloys. [ABSTRACT FROM AUTHOR]

The effect of water vapor on the transition from internal to external oxidation of austenitic alloys has been conducted at 1,073 K under the equilibrium oxygen partial pressure for the coexistence of Fe and FeO. Critical Cr concentrations in the Fe-Cr-30Ni (at.%) austenitic alloys were determined to be 30 at.% in dry atmosphere and 37 at.% in humid atmosphere. Thus, water vapor significantly affected the transition from internal to external oxidation of austenitic alloys. Two oxides of CrO and FeCrO precipitated in the Fe-5Cr-30Ni (at.%) alloy and solid state reaction for the formation of FeCrO may be influenced by water vapor. Oxygen permeability, which was estimated by considering the effective stoichiometric ratio, was also enhanced by water vapor. [ABSTRACT FROM AUTHOR]