A process for producing a diffusion barrier or blocking layer includes providing aluminum oxide on a metal sheet formed of a base material including at least iron, chromium and aluminum. A process for producing an exhaust gas treatment unit includes providing a honeycomb body and a housing in which at least one of the honeycomb body or the housing is formed of the metal sheet and the metal sheet is formed of a base material including at least iron, chromium and aluminum. The metal sheet thus includes, at least in a subregion, a surface layer including at least aluminum oxide and a metal from the group including cobalt and nickel. An exhaust gas treatment unit and a motor vehicle are also provided.