The Effect of Trace Amounts of Mg in FeCrAl Alloys on the Microstructure of the Protective Alumina Surface Scales

摘要

Minor alloying additions of reactive elements are of crucial importance for the protective properties of alumina scales on FeCrAl alloys used for manufacturing of modern car catalyst carriers. The effects of the reactive elements, such as Y, Zr, Hf and/or lanthanides etc on the high temperature oxidation behaviour of metallic materials have been studied in literature since at least 30 years. However, the commercial FeCrAl alloys often contain a number of additional, intentionally added or unavoidable impurities, which might significantly affect the oxidation behaviour. In the present study a range of analytical methods were applied to study the effect of magnesium impurity (80 ppm mass) in a commercial FeCrAl alloy. The isothermal oxidation kinetics were determined at 1200℃ in air using Thermo Gravimetric Analysis (TGA). The results of the TGA measurements have been correlated with SIMS (Secondary Ion Mass Spectrometry) and microstructural investigations with TEM/EDX (Transmission Electron Microscopy/Energy Dispersive X-ray-analysis) and Electron Energy Loss Spectroscopy (EELS). The scale cross-sections for the TEM investigations were manufactured with the Focused Ion Beam technique (FIB) to reach large area transparent samples which can be analysed from the top of the oxide layer to the alloy matrix. Comparison of the data from different analyses techniques reveals that the alumina scale grows primarily by inward oxygen grain boundary diffusion. In addition to this main oxidation mechanism, outward transport of Mg, in combination with Mn, occurs via the scale grain boundaries. Hence, magnesium becomes enriched in the outer part of the oxide scale forming a layer of a MgAl_2O_4 spinel. An important effect associated with the spinel formation is the development of porosity in the outer part of the scale.