Element redistribution in the oxide scale during air oxidation of Kanthal AF alloy

Abstract

The oxidation mechanism of Kanthal AF (FeCrAl alloy) was studied in the temperature range of 800–1200 °C through isothermal and thermo-cyclic oxidation tests. At 800 °C, tests involved 12-h cycles (up to 36 cycles), while at 1000 °C and 1200 °C, they were conducted isothermally (up to 432 h). Oxidation constants, oxide scale thickness, surface microstructure, and elemental composition were analyzed using TGA, SEM-EDXS, TEM, AES, XPS, GIXRD, and ToF-SIMS. The results show that the Al2O3 layer has a multilayer structure, defined by a naturally formed thin Cr-rich oxide layer on the alloy surface, which exhibits temperature-dependent selective permeability of metal cations, influencing the oxide scale growth. At 800 °C, inward oxide diffusion dominates due to limited cation permeability in the Cr-rich layer, while at higher temperatures, outward growth of the alumina scale is facilitated by increased permeability to metal cations. Despite the low Mg concentration in the alloy (0.01 wt%), Mg diffuses into the alumina rapidly, forming Mg-Al-Fe oxides that transform into MgAl2O4 spinel, impacting scale integrity and adhesion due to associated porosity. Consequently, Mg is considered an undesirable impurity, even at trace levels. Titanium forms separate oxides in the alumina scale, while zirconium and yttrium incorporate uniformly into the alumina scale.

Keywords

• FeCrAl alloys
• Oxidation tests
• Protective alumina scale
• High temperature oxidation
• Element distribution in alumina scale