The initial stage of oxidation (up to 600s) in air and air-H
2O atmospheres was investigated for Fe and an Fe-1.5mass%Si alloy at 1373 and 1473K. The oxidation kinetics of Fe was parabolic at both 1373 and 1473K and the parabolic rate constants are very similar in both air and air-H
2O. For the Fe-1.5Si, at 1373K the oxidation amounts increased rapidly after an incubation period (up to 400s) in air-H
2O, while at 1473K oxidation obeyed a linear rate law in both air and air-H
2O, because a liquid phase was formed with FeO and Fe
2SiO
4. The linear rate constants were very similar both in air and in air-H
2O. In Pt-marker experiments in air-H
2O for Fe-1.5Si it was found that the Pt-marker located between external Fe-oxide and inner FeO+Fe
2SiO
4 layers at 1373K, while at 1473K the Pt-marker located on the alloy surface. The thickness of each layer was measured as a function of time at 1373K in air-H
2O. It was found that after an Si-rich oxide (SiO
2+Fe
2SiO
4) layer at the initial stage of oxidation disappeared, a thick inner FeO+Fe
2SiO
4 layer formed, accompanied by the formation of Fe
3O
4inside the outer Fe
2O
3 scale. Rapid oxidation after 400s proceeded with the growth of an FeO layer in the surface scale. The change of the Si-rich oxide layer to an FeO+Fe
2SiO
4 mixture is due to penetration of water molecules. A combined process of perforating dissociation and transport of water molecules was suggested to be the cause of the rapid growth of the inner layer.
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