Oxidation Limited Lifetime of Ni‐Base Metal Foams in the Temperature Range 700–900 °C

Abstract

Metal foams are potentially suitable for various high temperature applications in which low-density components with relatively high mechanical properties are required. Examples of such applications are materials for walls of combustion chambers (or furnaces), high temperature catalyst supports, heat-exchangers, and filters. [1][2][3] The high temperature oxidation behavior of metal foams is therefore of substantial technological as well as scientific interest. However, only a very limited number of publications are available which deal with the oxidation behavior of porous metallic components. [4,5] Due to their excellent mechanical properties and oxidation resistance NiCr-base alloys are attractive materials for the manufacturing of metallic foams. These chromium-rich heat-resistant alloys rely for their high temperature oxidation resistance on the selective oxidation of chromium to form a dense, well-adherent, and protective Cr 2 O 3 oxide scale on the component surface. The formation of the oxide layer is associated with a continuous depletion of Cr from the alloy which may, after long service times, eventually result in breakdown of the Cr 2 O 3 scale due to an insufficient flux of Cr toward the oxide/metal interface. [6][7][8] This breakaway type oxidation is the result of the formation of fast growing base-metal oxides after a critical Cr depletion in the alloy has occurred. The time t B at which the Cr concentration at the oxide/metal interface reaches this critical value of the Cr concentration C b at which breakaway oxidation occurs, is a very important technological parameter defining the service lifetime limit of the components. Numerous studies [6,7,9] have shown that at a given temperature the time to breakaway is a function of oxidation rate, specimen dimensions, and geometry: (i) faster oxidation leads to accelerated chromium depletion, i.e., shortening of the lifetime, (ii) a thinner component possesses a smaller chromium reservoir and thus t b tends to be smaller than for thicker components, and (iii) the overall specimen geometry affects the supply of chromium from the bulk alloy, cylindrical, and spherical components having lower chromium supply compared to a flat sample of identical thickness because of the larger surface to volume ratio. [10] COMMUNICATION [*] Dr. [**] This work has been funded by the German Research Foundation within the framework of the SFB 561 project. Mr. H. Cosler and Ms. A. Kick are gratefully acknowledged for their assistance in carrying out the oxidation tests, and Mr. E. Wessel for the SEM/ EDX analyses.INCONEL 625 metal foams produced from alloy powder by the slip-reaction-foam-sinter-process are tested in respect to cyclic oxidation behavior in air in the temperature range 700-900 8C. The structure of the oxide scales formed on the foam particles is characterized using optical microscopy and SEM/ EDX analysis. Main emphasis is put on studying the oxidation limited lifetimes of the foams as function of temperature and foam microstructure. It is shown that mechanical...

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