The Effect of Temperature and Water Vapor on the Initial Stage of High Temperature Oxidation of an Fe-1.5mass% Si Alloy

The initial stage of oxidation (up to 600s) in air and air-H2O 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-H2O. For the Fe-1.5Si, at 1373K the oxidation amounts increased rapidly after an incubation period (up to 400s) in air-H2O, while at 1473K oxidation obeyed a linear rate law in both air and air-H2O, because a liquid phase was formed with FeO and Fe2SiO4. The linear rate constants were very similar both in air and in air-H2O. In Pt-marker experiments in air-H2O for Fe-1.5Si it was found that the Pt-marker located between external Fe-oxide and inner FeO+Fe2SiO4 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-H2O. It was found that after an Si-rich oxide (SiO2+Fe2SiO4) layer at the initial stage of oxidation disappeared, a thick inner FeO+Fe2SiO4 layer formed, accompanied by the formation of Fe3O4inside the outer Fe2O3 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+Fe2SiO4 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