Protective surface on stainless steel

A method to make a surface on a steel substrate, the method comprising:

• in an oxidizing atmosphere:
• in a first heating step, heating the steel substrate from room temperature to a first target temperature from 220 to 240° C.;
• in a second heating step, heating the steel substrate to a second target temperature from 365 to 375° C.;
• in a third heating step, heating the steel substrate to a third target temperature from 1000 to 1100° C.;
• and
• in a cooling step, cooling the steel substrate to a fourth target temperature from 18 to 25° C.;
• wherein the surface comprises:
• an outer layer comprising a spinel of the formula MnxCr3-xO4, wherein x is from 0.5 to 2;
• and
• an intermediate layer between the outer layer and the steel substrate comprising Cr2O3, wherein the outer layer and the intermediate layer covers at least 85% of the surface of the steel substrate;
• and
• wherein the steel substrate comprises from 40 to 55 wt. % Ni, from 30 to 35 wt. % Cr, from 15 to 25 wt. % Fe, from 1.0 to 2.0 wt. % Mn, from 0.01 to 0.60 wt. % La, from 0.0 to 0.65 wt. % Ce, from 0.06 to 1.8 wt. % Nb, and up to 2.5 wt. % of one or more trace elements, carbon, and silicon.