Abstract
The isothermal oxidation behavior of Fe–20Cr alloys with a simultaneous addition of 0.7 mass% rare earths (La or Y) and 0.7% Si was investigated in air at 1273, 1323 and 1373 K .
Simultaneous addition of Si with La or Y, especially La, substantially improved the oxidation resistance of Fe–20Cr alloy. The improving effect of a simultaneous addition of rare earths with Si was found to be more effective than that of the previously reported rare earths with Al. The accumulation of Si in the internal oxidation layer was remarkably enhanced by the rare earth addition, which was supposed to act as a physical barrier to the outward diffusion of metals. In the case of the addition of Y with Si, some metallic regions formed between the scale and the internal oxidation layer, whereas the metallic region was hardly observed in the case of the addition of La with Si. The formation of the metallic regions probably resulted in the evidence that the improving effect of the addition of Y with Si was less effective than that of La with Si.
The internal oxidation experiments carried out under the equilibrium oxygen pressure of Cr–Cr2O3 revealed that the rare earth addition did not enhance the diffusion rate of Si in this alloy. Then the accumulation of Si in the internal oxidation layer was considered to be caused by the reduction of the oxygen dissociation pressure of the scale due to the incorporation of rare earths and Si into the scale, i.e. the stabilization effect of the scale. It was emphasized that the stabilization of the scale in combination with the formation of a dense internal oxidation layer resulted in remarkably good oxidation resistance. The reason why the improving effect of the simultaneous addition of rare earths with Si on the oxidation resistance was more effective than that of rare earths with Al was discussed thermochemically.
