2021 Volume 62 Issue 11 Pages 1639-1646
In Fe–Ni alloys, protective oxide scales are not formed in a dry atmosphere as in an atmosphere containing water vapor; however, oxide scales consisting of duplexes with outer and inner layers are formed. This oxide scale structure is similar to that formed in an atmosphere containing water vapor. This enables accurate evaluation of the effect of water vapor on the oxidation behavior of alloys. In this study, we focused on the effect of water vapor on the growth kinetics and microstructure of the inner layer, and tried to verify a model according to which the growth of the inner oxide scale is caused by the dissociative mechanism of the outer oxide layer.
Fe–10 mass%Ni alloys were oxidized at 1200°C in N2–10 O2 or N2–10 O2–20 H2O (vol%, unless state otherwise) at 1200°C for 5, 15, 30, 60, and 180 min, and the effect of water vapor on the microstructure of the oxide scale was investigated. The thickness of the outer layer was not considerably different in the N2–10 O2 atmosphere compared to that in the N2–10 O2–20 H2O atmosphere. The thickness of the inner layer was significantly greater in the N2–10 O2–20 H2O atmosphere than that in the N2–10 O2 atmosphere. The inner layer was of dense FeO in N2–10 O2–20 H2O, but contained many voids in N2–10 O2. The oxidation rate was higher in N2–10 O2–20 H2O than in N2–10 O2. This could be caused by the dissociation of FeO in the atmosphere containing water vapor, which additionally supplies Fe to the surface.
