Reduced exhaust emissions in automotive catalytic converters through the use of high-temperature Fe-Cr-Al foils

New exhaust emission standards, such as EURO LEVEL IV and (S)ULEV, call for new catalytic converter designs and, especially for measures to reduce exhaust emissions during the cold-start phase. This can be achieved by using thinner metal support foils, which heat up faster and give a larger reaction surface due to their higher cell density. For further support, an additional preconverter can be installed. Conventional supports of automotive catalytic converters ate made using 50 pm thin foils of Fe-20Cr-5Al alloys with approximately 20% Cr, 5% Al, and additions of reactive elements. These materials owe their outstanding oxidation resistance at high temperatures to a slow-forming protective layer of alpha-Al2O3. However, this process gradually consumes the alloy's aluminium content as the wall thickness is further reduced. After extended service periods, the protective aluminium oxide layer will spall. It follows that the aluminium content must be raised to between 6% and 7%. As conventional processing of Fe-Cr-Al alloys with. aluminium contents of this order of magnitude is affected by high failure rates, a new process was developed which essentially comprises aluminium coating of an Fe-Cr base material and diffusion annealing at an advanced stage of production. Adherence of the oxide layer is influenced by the type and amount of reactive elements in the material. Accordingly, the oxidation behaviour in terms of weight loss and microstructural changes was tested at temperatures of between 900 and 1,300 degrees C. in addition, the kinematic pattern of oxide formation was examined with the aid of an X-ray diffractometer.