Process for forming surface diffusion alloy layers on refractory metallic articles

In a thermochemical process for the formation of surface diffusion alloy layers on refractory metallic articles having perforations therein whose transverse cross-sectional dimension is less than 0.4 mm, said process comprising submerging the treated articles in a reagent containing a mixture of a first powder of metal or alloy for supplying at least one metal to the treated articles, and a second powder of an inert diluent, said first and second powders having a particle size ranging between 1 and 50 microns, and thereafter subjecting the reagent and the article to a thermal treatment in a halogen-hydrogen atmosphere at a temperature between 750.degree. and 1200.degree. C, the improvement wherein prior to submerging the metallic article to be treated in the said reagent, the said article is individually covered, on at least the part of the external surface thereof having said perforations therein, with a metallic felt coated with a film of stable refractory oxide, said metallic felt being previously cut into a thin sheet of a thickness of a few tenths of a millimeter to a few millimeters, and preshaped so that one of the faces thereof adapts itself closely to the external surface of the article to be treated, the other face of the said metallic felt sheet being in intimate contact with the reagent when the article and the metallic felt sheet associated therewith is submerged in the said reagent, the said metallic felt sheet coated with a film of stable refractory oxide being obtained from a sheet of a refractory metallic felt, having a thickness of 0.3 to 1 mm., containing chromium in proportion by weight between 10% to 60% and having been subjected to selective oxidation to produce chromium oxide, said film of stable refractory oxide being produced by action of a reducer of the chromium oxide, and said selective oxidation being effected so that the fibers constituting the metallic felt sheet are covered with a layer of chromium oxide that represents at least 5% of the weight of the metallic felt sheet.