Fe-1.5mass%Si合金の高温初期酸化に対する温度と水蒸気の影響

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The initial stage of oxidation (up to 600 s) 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 400 s) 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_4 inside the outer Fe_2O_3 scale. Rapid oxidation after 400 s 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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Fe-1.5mass%Si合金の高温初期酸化に対する温度と水蒸気の影響

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