High-Temperature Oxidation Resistance of Al_2O_3- and Cr_2O_3-Forming Heat-Resisting Alloys with Noble Metals and Rare Earths

摘要

The high-temperature oxidation resistance of heat-resisting alloys is in practice provided by scales consisting of alumina or chromia continuous layers. Such scales are achieved by preferential or selective oxidation of aluminum or chromium present as components in alloys. In spite of the importance of these scales in innumerable practical applications, there are still considerable gaps in our understanding of their properties and growth mechanisms. Al_2O_3-forming heat-resisting alloys have industrially served in oxidizing atmospheres at temperatures up to 1573K because of the formation of protective α-Al_2O_3 continuous layer, which exhibit excellent oxidation resistance. However, the α-Al_2O_3 scale often spalls during cooling after oxidation. Various hypotheses have been offered to explain the spalling of oxide scale, and it has been focused on segregation of sulfur at the oxide/alloy interface (1-12). On the other hand, to improve the oxide adherence, many studies have been conducted with regard to the effect of small additions of reactive elements such as rare earths, zirconium, and hafnium (13-24). Recently, the effects of small noble metals (palladium, rhodium, platinum) additions on the oxidation behavior of the Al_2O_3-forming heat-resistant alloys have been also studied (25-31). On the other hand, water vapor usually has adverse effects on the oxidation resistance of Al_2O_3-forming alloy (32-35). Cr_2O_3-forming heat-resisting alloys have also served in oxidizing atmospheres at temperatures up to 1473K (36-45), however, the Cr_2O_3 have volatile property during high-temperature oxidation. In the present study, high-temperature oxidation behavior of Al_2O_3- and Cr_2O_3-forming heat-resisting alloys with noble metals (palladium, platinum) and rare earths (yttrium, lutetium) is considered in oxygen and oxygen-water vapor.