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 600 s) 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 nas parabolic at both 1373 and 1473K and the parab olic rate constants are very similar in both air and air-H2O. For the Fe-1. 5Si, at 1373K the oxidation amounts increased rapidly alter an incubation p eriod (up to 400 s) in air-H2O, while at 1473K oxidation obeyed a linear ra te law in both air and air-H2O, because a liquid phase uas 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 tha t the Pt-marker located between external Fe-oxide and inner FeO + Fe2SiO4 l ayers 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 a t the initial stage of oxidation disappeared a thick inner FeO + Fe2SiO4 la yer formed accompanied by the formation of Fe3O4 inside the outer Fe2O3 sca le. Rapid oxidation after 400 s proceeded with the growth of an FeO layer i n the surface scale. The change of the Si-rich oxide layer to an FeO + Fe2S iO4 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.