Abstract—The objective of this paper is to investigate the formation and adhesion of a protective aluminum-oxide (Al 2 O 3 , alumina) layer on the surface of Iron-Chromium-Aluminum Alloy (Fe-Cr-Al) sintered-metal-fibers. The oxide-scale layer was developed via multi-stage thermal oxidation at 930 o C for 1 hour, followed by 1 hour at 960 o C, and finally at 990 o C for 2 hours. Scanning Electron Microscope (SEM) images show that the multi- stage thermal oxidation resulted in the formation of predominantly Al 2 O 3 platelets-like and whiskers. SEM images also reveal non- uniform oxide-scale growth on the surface of the fibers. Furthermore, peeling/spalling of the alumina protective layer occurred after minimum handling, which indicates weak adhesion forces between the protective layer and the base metal alloy. Energy Dispersive Spectroscopy (EDS) analysis of the heat-treated Fe-Cr-Al sintered- metal-fibers confirmed the high aluminum content on the surface of the protective layer, and the low aluminum content on the exposed base metal alloy surface. In conclusion, the failure of the oxide-scale protective layer exposes the base metal alloy to further oxidation, and the fragile non-uniform oxide-scale is not suitable as a support for catalysts. Keywords—High-temperature oxidation, alumina protective layer, iron-chromium-aluminum alloy, sintered-metal-fibers. I. INTRODUCTION LLOYS containing aluminum, such as Fe-Cr-Al, are designed to form an Al 2 O 3 layer to protect the base metal alloy surface from further oxidation at high temperatures. The oxide-scale layer is usually developed via single or multi-stage thermal oxidation. Fe-Cr-Al contains mainly iron, chromium, and aluminum. The relatively high concentrations of aluminum and chromium increase the thermal resistance at high temperature by forming a protective oxide-scale layer on the surface. In addition, a small fraction less than 0.5% of rare earth elements, such as yttrium, zirconium, and cerium, are commonly added to Fe-Cr-Al to enhance its oxidation characteristics at high temperature and improve the adhesion between the oxide-scale layer and the base metal alloy [1]-[3]. Strawbridge and Hou [4] investigated the effects of rare earth elements on the formation and adhesion of the oxide-scale layer. Their study cited that rare earth elements had a Mr. Ben Naji is an instructor at the Mechanical Power Department, The Higher Institute of Energy, Kuwait (e-mail: lm.binnaji@paaet.edu.kw). Dr. Ibrahim was a CTO at Rypos, Inc., Franklin, MA 02038 USA. He is now an Associate Professor at the Mechanical Engineering Department, Kuwait University (e-mail: Osama.ibrahim@ku.edu.kw). Dr. Al-Fadhalah is an Associate Professor the Mechanical Engineering Department, Kuwait University (e-mail: khaled.alfadhalah@ku.edu.kw). significant improvement in the oxide-scale adherence on aluminum-containing alloys during isothermal and cyclic oxidation. A similar study by Ishii et al. [5] investigated the effect of rare earth elements on high-temperature oxidation of metal foils made of Fe-Cr-Al. The study cited that the rare earth elements prevent oxygen grain boundary diffusion, and Al 2 O 3 scale growth rate can be decreased with increasing rare earth elements content. Thermal oxidation of Fe-Cr-Al can also lead to interesting Al 2 O 3 morphology formations with high surface area such as platelets or whiskers [6]-[8]. Thermal oxidation temperature and exposure time affect the formation of Al 2 O 3 phases on the metal surface, which depend on conditions such as thermal oxidation technique, impurities, and crystallinity that found on the alloy base-metal [9]. Fei et al. [10] reported that during thermal oxidation temperatures between 800 and 900 o C metastable alumina platelets and whiskers mainly arranged on the surface. They also reported that the rapid growth rate of the Al 2 O 3 could deplete the aluminum content from the base metal alloy. Pint et al. [11] concluded that the transformation of the metastable, which consists mainly of θ-Al 2 O 3 , into stable α-Al 2 O 3 , causes volume change. This volume change could result in microcracks leading to failure of the protective oxide-scale layer. In a related note, Kadiri et al. [7] studied single-stage thermal oxidation at 900 o C for 5 hours on metal foils made of Fe-Cr-Al, which resulted in the formation of oxide nodules and crater on the surface. At the oxide crater nodule boundary, there was a radial arrangement of the platelets or whiskers. Kadiri et al. [7] reported that the changing in the oxide volume causes tensile stresses, which led to the formation of cracks and affected the adhesion of the oxide layer to the surface of the base metal alloy. A technical paper by Samad et al. [8] presented results for multi-stage thermal oxidation of Fe-Cr-Al sintered-metal- fibers as a catalyst support. Their focus was to study the formation of α-Al 2 O 3 whiskers as support for palladium-based catalyst. The thermal oxidation process started at a lower temperature and later continued with higher temperatures to accelerate the growth rate of the alumina layer on the surface of the fibers. Explicitly, the fibers were heated at three temperature steps: at 930 o C for 1 hour, 960 o C for 1 hour and finally at 990 o C for 2 hours. Their results show that the surface of the oxide-scale is predominantly covered with α- Al 2 O 3 whiskers of 203±34 nm in height and 100–200 nm apart; where θ-Al 2 O 3 phase has also existed on the surface of the fibers. Furthermore, the multi-stage thermal oxidation was Loai Ben Naji, Osama M. Ibrahim, Khaled J. Al-Fadhalah Formation of Protective Aluminum-Oxide Layer on the Surface of Fe-Cr-Al Sintered-Metal-Fibers via Multi-Stage Thermal Oxidation A World Academy of Science, Engineering and Technology International Journal of Chemical and Materials Engineering Vol:12, No:11, 2018 596 International Scholarly and Scientific Research & Innovation 12(11) 2018 Digital Open Science Index, Chemical and Materials Engineering Vol:12, No:11, 2018 waset.org/Publication/10009752