The water vapor oxidation behaviors of Fe and Fe-Cr alloys under isothermal and heat-cyclic conditions in an Ar-12 vol% H₂O atmosphere were investigated by measuring both the hydrogen partial pressure using a hydrogen sensor and the oxidizing current of hydrogen using an oxygen pump-sensor, which were installed at the back section of the reactor. First, the performance of the oxygen pump-sensor was evaluated using Ar-1000 ppmH₂ gas. As a result, it was found that the hydrogen concentration in the gas can be accurately measured by the oxidizing current of hydrogen. The isothermal oxidation test at 1173 K showed that the mass gain of the Fe-20 mass% Cr alloy was the highest, while the mass gain of the Fe-30 mass% Cr alloy was the lowest. For the Fe-20 mass% Cr alloy showing the highest mass gain, a rapid increase in the oxidation rate was observed during the oxidation test. The cyclic oxidation test indicated that the Fe and Fe-30 mass% Cr alloy had a lower oxidation rate in the second cycle than in the first cycle. On the contrary, for the Fe-20 mass% Cr alloy, the oxidation rate was higher in the second cycle than in the first cycle. For the Fe-30 mass% Cr alloy, the oxidation rate and mass gain during the cyclic oxidation test were low. The oxidation rates measured by the hydrogen sensor and the oxygen pump-sensor were showed the same behavior during the isothermal oxidation test. Since the oxidation rate is measured in situ using the hydrogen sensor and oxygen pump-sensor methods, it was clarified that both methods are effective for studying the water vapor oxidation behavior.

High-temperature oxidation behavior of Fe-Ni-Cr alloys with different Ni contents in air and steam at 700°C was investigated. Oxidation mass gain in steam was higher than that in air. In both atmospheres oxidation mass gain tended to decrease with increasing Ni content. However, the Ni concentration dependence of oxidation mass gain in air was complex, i.e., oxidation mass gain increased with increase in Ni content up to 35% then decreased with higher Ni content. But oxidation mass gain of binary Ni-20Cr significantly increased. Such a complex oxidation behavior was not observed when alloys were oxidized in steam. The internal oxidation zone was locally formed in air, and a Cr₂O₃ scale was found to develop on the areas without internal oxidation. The size of area, where an internal oxidation zone was developed, changed depending on Ni content. In steam, most of the surface area of alloys was covered by an internal oxidation zone. Cr content, 20%, was considered to be insufficient but close to the critical Cr content for exclusive Cr₂O₃ scale formation in air. The critical Cr content for exclusive Cr₂O₃ scale formation was found to depend on alloy Ni content due to lower oxygen permeability in alloys with higher Ni content and the cross-term effect for Cr outward diffusion.

Steam oxidation resistance of Fe-20at.%Cr-30at.%Ni austenitic stainless steels with each of Nb, Mo, Ta and W added was respectively investigated by steam oxidation experiments at 700°C. Cr₂O₃ layer was uniformly formed at the boundary between an inner oxide scale and the metal substrate of the alloy with Nb, Mo and W addition, and inhibited the growth of the oxide scale and thus improved steam oxidation resistance. It was revealed that Cr₂O₃ layer formation is promoted as the precipitation of intermetallic compounds including Fe increases. It was assumed that the precipitation changed the chemical composition both of the substrate of the alloy and below the inner oxide scale, and thereby increased the Cr activity gradient. As a result, it was suggested that an increase in the Cr flux accelerated the formation of Cr₂O₃ layer.

High-temperature steam oxidation causes accelerated oxidation damages for stainless steel tubes in boilers. A duplex oxide scale made of the outer scale of mainly Fe₃O₄, and the inner scale of mainly (Fe, Cr)₃O₄, was formed on stainless steels in steam oxidation conditions. In this study, microstructure of the inner scale in the oxide scale formed by stream oxidation of 18Cr-8Ni stainless steel was analyzed by using the Auger electron spectroscopy. The inner scale consisted of three areas: Area I made of Cr₂O₃ on the inner scale/ alloy interface, Area II made of (Fe, Cr)₃O₄ neighbor with Area I, and Area III consisted of (Fe, Cr)₃O₄ matrix with dispersed Ni nano-particles. Continuity of Cr₂O₃ scale. Area I, was sometimes lost, resulting in the formation of the interface between (Fe, Cr)₃O₄ and the stainless steel. Graphite is sometimes detected in only Area I, not in Area II and III, even after Ar sputtering for a long time. The existence of graphite is the evidence of permeation of CH₄ gas into the scale/ alloy interface. Formation of fine Ni in Area III can be also explained with gas penetration through the cracks and post-healing of the outer scale.

High temperature corrosion of nickel aluminum alloys were investigated in air with a trace amount of NaCl vapor at 1273 K. Mass gains of Ni-5, -8 and -10 at%Al alloys were reduced in air with 7.2 ppm and 32.1 ppm NaCl vapor against those in air without NaCl vapor. Cross sectional investigation using SEM and EPMA revealed that there was a continuous Al₂O₃ layer at the bottom of the scale when nickel aluminum alloys oxidized in air with NaCl vapor. That effect was obvious for Ni-10 at%Al and the slight tendency was recognized for Ni-8 at%Al. A mechanism of Al₂O₃ protective layer formation with NaCl vapor was considered from the view point of thermodynamics. Under the oxygen potential of the atmosphere, Ni-Al alloy is oxidized into NiO. By the scale formation, oxygen potential gets down at the metal/scale interface maintaining the small chlorine potential induced by NaCl and O₂, volatile AlCl₃ formation becomes dominant instead of NiO formation. This AlCl₃ is oxidized into Al₂O₃ if oxygen potential rises even a little at the outer part of the scale. In those manners, protective alumina scale is considered to form on Ni-Al alloys in air with NaCl vapor, representing a relatively higher corrosion resistance than that in air without NaCl vapor.