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

本文研究了稀土元素对耐热合金高温腐蚀的影响，发现在Ni-20 Cr-1 Si合金中添加Y、Ce、La、Er和表面添加Y_2O_3、La_2O_3和CeO_2，分别降低了合金在等温和循环氧化过程中的增重，增加了Cr_2O_3膜的附着。根据稀土氧化物的高氧扩散系数，提出了一个机理模型，用以解释稀土对Cr_2O_3生长机制和粘附性的影响。在Ni-20 Cr-3Al合金上用真空蒸发法沉积CeO_2薄膜，也能改善氧化膜的粘附性。研究了SUS 310和SUS 430耐热合金在含硫、溴和水蒸气环境中的高温腐蚀行为。在含硫和溴的气体中，发现有必要形成厚而致密的氧化皮，以防止在氧化皮/合金界面形成硫酸盐或气态溴化物而对合金造成大范围腐蚀。在900 K左右的温度下，观察到环境中高浓度的水蒸气增加SUS 430的等温氧化速率。

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.