MECHANISM OF THE RARE EARTH EFFECT IN HIGH TEMPERATURE OXIDE SCALES FORMED ON HEAT-RESISTING ALLOYS

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In this research, high temperature corrosion of heat-resisting alloys was studied with an emphasis on the effect of rare earth elements.Alloying additions of Y, Ce, La, Er, and superficial application of Y_2O_3, La_2O_3 and CeO_2 to Ni-20Cr-lSi alloys were found to decrease the mass gain of the alloy and increase Cr_2O_3 scale adherence during isothermal and cyclic oxidation, respectively. A mechanistic model based on the high oxygen diffusivities of the rare earth-oxides, was proposed to explain the effect of these additions on the growth mechanism and adherence of Cr_2O_3. A similar improvement in scale adherence was found in the case of Ni-20Cr-3Al alloys on which thin CeO_2 layers were deposited by vacuum evaporation. This superficial layer was observed to change the growth mechanism of the Cr_2O_3 scale and promote internal oxidation of aluminum.High temperature corrosion of heat-resisting alloys such as SUS 310 and SUS 430 were studied in sulfur, bromine and water vapor-containing environments. In sulfur and bromine-containing gases, formation of a thick, compact oxide scale was found necessary to prevent extensive corrosion of the alloy by forming sulfates or gaseous bromides at the scale/alloy interface. A high concentration of water vapor in the environment was observed to increase the isothermal oxidation rate of SUS 430 at temperatures around 900 K.