Process for producing a diffusion barrier layer and process for producing an exhaust gas treatment unit

A process for producing an exhaust gas treatment unit, the process comprising the following steps:

• i) providing at least one metal sheet formed of a base material containing at least iron, chromium and aluminum for the formation of at least a housing or a honeycomb body from the at least one metal sheet and providing the at least one metal sheet with a surface having a subregion;
• ii) applying at least one metal oxide selected from the group consisting of cobalt oxide and nickel oxide, at least to the subregion as a surface layer, or
• applying at least one material compound able to form at least one metal oxide selected from the group consisting of cobalt oxide and nickel oxide as a result of a temperature increase, at least to the subregion as a surface layer;
• iii) forming the honeycomb body and inserting the honeycomb body into the housing;
• iv) applying brazing material at least to the honeycomb body or the housing in at least one brazing section;
• v) carrying out a brazing process above 1000° C. on the at least one metal sheet including metal oxide under vacuum or protective gas, causing:
• formation of a diffusion barrier layer including aluminum oxide only in the at least one subregion in the surface layer, with reduction of the metal oxide to a metal and oxidation of the aluminum diffusing out of the base material to aluminum oxide, and
• formation of a brazed joint on the at least one metal sheet in the at least one brazing section.