The structure is then heated in a vacuum furnace so that the brazing material forms a strong joint without any oxidation. To make a Fecralloy substrate, alternating layers of corrugated and flat metal foil are pressed together, with a housing or mantle around the perimeter, and a brazing material is introduced at the points where they make contact. Therefore this type of metal substrate is hygienic enough to use in cooking appliances. The main advantage of Fecralloy is that, when heated in air, a layer of aluminium oxide forms on the surface which protects the iron from corrosion and also provides a good key for the catalytic coating. There are various ways of making metal substrates but the top-quality versions utilize Fecralloy foil which consists of 74% iron, 21% chromium and 5% aluminium. The latter option is known as ‘herringbone’ and gives rise to turbulence which increases the activity of the catalytic converter but at the expense of an increased pressure drop. Another variable is the pattern of the corrugations in the foil, which can either be straight through or involve changes of direction. Metal substrates can be made in a variety of profiles, with round, square and rectangular being most common.

Most metal substrates used in the cooking and heating sectors have between 100 and 200 cells per square inch (cpsi). Since greater cell density results in increased surface area, this means that a metal substrate of a given volume will be more catalytically active than a ceramic one.

Cathalitic convertos honeycomb