Effect of water vapor on the oxidation behavior of fe-0.5 and 5 mass%Ni alloys in air at 1273 K

Oxidation behavior of Fe-0.5 and 5 mass%Ni alloys(0.5Ni, 5Ni) as well as Fe for comparison, was investigated at 1273 K for up to 7.2 ks in air and air containing 10.5 vol%H2O (air-10.5H(2)O) . In the air-10.5H(2)O, the 0.5Ni and 5Ni were oxidized faster than in air. The scale was composed of a duplex structure, an outer Fe-oxides layer and an inner FeO + Ni layer containing voids. Marker experiments with a small Pt wire were carried out for the 5Ni oxidized in air and the Pt-marker was located between the inner and outer layers, suggesting that the inner layer grows due to inward migration of oxygen and the outer layer clue to outward iron diffusion. A similar scale structure was observed for the 5Ni oxidized in air but the inner layer thickness of the 5Ni oxidized in air-10.5H(2)O was thicker than in air. Oxygen in the inner FeO + Ni layer formed in air was supplied by the dissociation of outer Fe-oxides, leaving voids there, probably by the perforating dissociation mechanism. Both oxygen and water molecules could be diffusing species through the void in the atmosphere containing water vapor. Concentration profiles of each element indicated that concentration of Ni was increased at the scale/alloy interface. The Ni-concentration of the 0.5Ni oxidized in air was lower than that in air-10.5H(2)O, but the Ni-concentration of the 5Ni oxidized in air was higher than that in air-10.5H(2)O.