Surface oxidation of heating resistors made from Kanthal AF: Increasing the lifetime of glow plugs

Abstract

Glow plugs in the automotive industry are based on heating resistors made from FeCrAl alloys, often referred to as Kanthal. Metals and alloys exposed to high temperatures tend to react with the surrounding atmosphere, often resulting in high-temperature corrosion. Some high-temperature alloys use aluminium to form an Al2O3 protective layer in order to improve the oxidation resistance. In the temperature range above 1200 °C, SiO2 and Al2O3 are the two oxides capable of forming an efficient, protective barrier against further oxidation. The alloy nominally requires more than 4 wt % Al to form a continuous protective layer. In this paper we have investigated the Al2O3 formation and growth on the surface of iron aluminides prepared using different heat-treatment parameters to achieve a continuous protective layer on the surface of the heating resistors. SEM/EDX mapping analyses were performed to confirm the elemental distribution on the cross-section of the resistors heat treated in air or pure oxygen, or in a humid H2/Ar atmosphere, for different times and temperatures to achieve a continuous coverage of the oxide layer. The results of different heat treatments and their influences on the oxide growth are compared.