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

1. A process for producing a diffusion barrier layer including aluminum oxide on a metal sheet formed of a base material containing at least iron, chromium and aluminum, the process comprising the following steps:

a) providing the metal sheet with a surface having at least one subregion;

b) applying at least one metal oxide selected from the group consisting of cobalt oxide and nickel oxide, to the at least one 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, to the at least one subregion as a surface layer,

the metal oxide or the material compound in the surface layer in step b) being present in a proportion of at least 40% by weight; and

c) carrying out a heat treatment above 650° C. on the metal sheet having the surface layer causing formation of a diffusion barrier layer including aluminum oxide only in the at least one subregion of the surface layer, with reduction of the metal oxide to a metal and oxidation of aluminum diffusing out of the base material to aluminum oxide.

2. The process according to claim 1 , wherein the aluminum oxide is homogeneously distributed in the surface layer after carrying out step c).

3. The process according to claim 1 , wherein a proportion of the metal of the metal oxide or of the material compound being present in elemental form in the surface layer has increased by least 5% by weight after carrying out step c).

4. The process according to claim 1 , wherein a proportion of aluminum in the base material in step a) is in a range from 1 to 7% by weight and a proportion of aluminum in the metal oxide or in the material compound in step b) is at most 2% by weight.

5. The process according to claim 4 , wherein a proportion of aluminum in the diffusion barrier layer after carrying out step c) is at least 15% by weight.

6. The process according to claim 1 , wherein the surface layer has a thickness of at most 1 μm [micron] after carrying out step b).

7. The process according to claim 1 , wherein the base material in step a) has a maximum of 20% of the metal of the metal oxide or the material compound from step b).

8. 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.

9. The process according to claim 8 , which further comprises, after step v), carrying out a further coating process applying an exhaust gas treatment layer completely covering the surface layer in the at least one subregion.