Effect of water vapor on the oxidation behavior of Fe-1.5Si in air at 1073 and 1273 K

The oxidation behavior of Fe-1.5Si was investigated at 1073 and 1273 K in air, air-H2O, Ar-H2O, O-2-H2O, and O-2 atmospheres. The extent of corrosion in atmospheres containing H2O increased rapidly after an incubation period of slow oxidation, the incubation period becoming shorter in the order, O-2-H2O, air-H2O, and Ar-H2O. With increasing H2O contents in air-H2O, the incubation time decreased. During the incubation period, oxidation was slow, because of the formation of an inner Si-rich oxide layer and a Pt marker uas located between the external Fe2O3 (Fe3O4 included) and an inner Si-rich oxide layer. During the rapid oxidation, the inner FeO + Fe2SiO4 layer thickened and a Pt maker was at the interface between an external Fe-oxide and an inner FeO + Fe2SiO4 layer. Observations of scale cross sections indicated that voids made channels along the boundaries of columnar FeO crystals, suggesting transport of water molecules. The Si-rich oxide layer changed into an FeO + Fe2SiO4 mixture due to penetration of water molecules. A combined process of perforating dissociation and transport of water molecules is suggested to be the cause of the rapid growth of the inner Fe + Fe2SiO4 layer.