The Role of Rare Earths and Reactive Elements in High-Temperature Oxidation Behavior of Fe–20Cr Alloy

The isothermal oxidation behavior of Fe–20Cr alloys with various amounts of lanthanum and with constant amounts of various reactive elements (Y, Gd, La, Al, Zr, Ti and Si) was studied in air at 1273 K. The oxidation behavior of the alloys with various amounts of La suggested that the improving mechanism of oxidation resistance was different from those proposed previously. Seven alloying elements of 0.7% resulted in a remarkable improvement of oxidation resistance. The extent of beneficial effect of each element considerably depended on its affinity for oxygen, but the effect of La and Si additions deviated from this tendency. Internal oxidation layers were observed beneath the scale in all the oxidized alloys except for the La-containing alloys, which showed the least improving effect. As the result, it was found that these elements had two main beneficial effects on the improvement of oxidation resistance. The first was the stabilization of the Cr2O2 scale with dissolving the alloying element which was followed by a drop in dissociation oxygen pressure of the scale and the second was the formation of an internal oxidation layer which was enriched under the lowered dissociation oxygen pressure induced by the first effect. It was assumed from the thermochemical considerations that the alloying elements having strong affinity for oxygen and also high mobility in the base alloy would be more effective in the improvement of oxidation resistance.