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

本文研究了耐热合金的高温腐蚀，重点研究了稀土元素对高温腐蚀的影响。在Ni-20Cr-lSi合金中添加Y、Ce、La、Er合金，表面添加Y_2O_3、La_2O_3和CeO_2合金，在等温氧化和循环氧化过程中分别降低了合金的质量增益，增加了Cr_2O_3的粘垢性。基于稀土氧化物的高氧扩散率，提出了一个机制模型来解释这些添加物对Cr_2O_3生长机理和粘附性的影响。用真空蒸发法在Ni-20Cr-3Al合金表面沉积薄层CeO_2后，其粘垢性也有类似的改善。观察到这一浅层改变了Cr_2O_3垢的生长机制，促进了铝的内部氧化。研究了sus310和sus430等耐热合金在含硫、含溴和含水蒸气环境中的高温腐蚀。在含硫和含溴的气体中，发现有必要形成厚而致密的氧化垢，通过在垢/合金界面形成硫酸盐或气态溴化物来防止合金的广泛腐蚀。在900 K左右的温度下，观察到环境中高浓度的水蒸气增加了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.