THE PREDICTION OF BREAKAWAY OXIDATION FOR ALUMINA FORMING ODS ALLOYS USING OXIDATION DIAGRAMS

The oxidation induced loss in wall thickness of alumina forming high t emperature alloys is in most cases not a limiting factor for component life because of the slow growth rates of the alumina surface scales. However, scale growth and rehealing after spalling lead to depletion o f the scale forming element, aluminium, in the bulk alloy. If, during long time service at high temperatures, the concentration of aluminium has decreased beneath a critical level, the protective alumina scale can no longer be formed and oxidation of the base elements Fe, Ni or C o occurs, leading to a catastrophic breakaway oxidation of the compone nt. A model is presented which allows the calculation of the time at w hich this breakaway occurs using FeCrAl based ODS alloys as an example . The parameters which are necessary for the calculations, such as sca le growth rates, scale spalling and bulk aluminium diffusion can be de termined from relatively short time laboratory experiments.The calcula ted and measured results for the commercial ODS alloys ODM 751, PM 200 0 and MA 956 at 1100 and 1200-degrees-C are presented in form of Oxida tion Diagrams in which the time to breakaway oxidation is plotted as a function of component wall thickness. On the basis of the theoretical considerations possible measures for increasing the oxidation limited life of the mentioned alumina forming alloys are discussed.