Microstructural Investigation of the Initial Oxidation of the FeCrAlRE Alloy Kanthal AF in Dry and Wet O-2 at 600 and 800 degrees C

• Published in: Journal of the Electrochemical Society Vol. 157; no. 6; p. C223
• Main Authors: Canovic, Sead, Engkvist, Josefin, Liu, Fang, Lai, Haiping, Götlind, Helena, Hellström, Kristina M, Svensson, Jan-Erik, Johansson, Lars-Gunnar, Olsson, M., Halvarsson, Mats
• Format: Journal Article
• Language: English
• Published: 2010
• Subjects: 40-percent h2o; 900 degrees-c; alumina scales; cyclic oxidation; fecraire alloys; growth; high-temperature oxidation; mg-incorporation; oxide scales; water-vapor
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1.3391447

The FeCrAlRE (where RE is reactive element) alloy Kanthal AF was exposed isothermally at 600 and 800 degrees C for 72 h in dry O-2 and in O-2 with 10 vol % H2O. The mass gains were 3-5 times higher at the higher temperature. The presence of water vapor increased the oxidation rate at 800 degrees C, while no significant effect was observed at 600 degrees C. A thin two-layered oxide formed at 600 degrees C: an outer (Fe, Cr)(2)O-3 corundum-type oxide, containing some Al, and an inner, probably amorphous, Al-rich oxide. At 800 degrees C a two-layered oxide formed in both environments. The inner layer consisted of inward grown alpha-Al2O3. In dry O-2 the originally formed outward grown gamma-Al2O3 had transformed to alpha-Al2O3 after 72 h. Water vapor stabilized the outward grown gamma-Al2O3 and hence no transformation occurred after 72 h in humid environment. RE-rich oxide particles with varying composition (Y, Zr, and Ti) were distributed in the base oxide at both temperatures and in both environments. The RE-rich particles were separated from the alloy substrate by a layer of Al-rich oxide. At 800 degrees C the Y-rich RE particles were surrounded by thick oxide patches in both dry and humid O-2.