Oxidation behaviour and phase stability at and above 700°C in brazed areas of metal-supported automotive converters with honeycomb structures made of ultra-thin FeCrAlRE materials for diesel applications

Abstract

Metal-supported automotive catalytic converters for diesel applications are made of flat and corrugated thin foils, brazed together at their contacting points. Brazing is done with nickel based alloys. The foil alloy is of type Fe–20Cr–(4–7)Al with the addition of reactive elements RE. In the lower temperature range for diesel applications (500°C to 900°C), attention should be paid to high temperature properties of brazed areas. Unusually large polygonal phases can occur in the original brazed area beside the fine globular phases. Both are of type NiAl. Polygonal phases are less corrosion resistant, resulting in shorter product lifetime. The occurrence of such phases, their stability and oxidation resistance were analysed under isothermal test conditions in ambient air using brazed samples with honeycomb structure.Diffusion of Ni into ultra-thin foil materials and Fe as well as Al into the Ni-based brazing material (erosion) leads to different compositions in brazed areas. NiAl phases (polygonal or globular) are already present after brazing. They do not change their shape over time. No oxidation of these phases was detected.Further investigation is focused on the effects of metallurgical and production parameters as well as cyclic load and test atmospheres on the occurrence and oxidation behaviour of the undesired large polygonal phases.