Effect of Water Vapor on High-Temperature Oxidation Behavior of Fe–10 mass% Ni Alloy

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.