DETAILED DESCRIPTION What is claimed is: 1.
A metallic article comprising: a substrate of a superalloy based on an element selected from the group consisting of Fe, Co and Ni, and a graded coating diffused with said substrate to increase the hot corrosion resistance of the substrate, the graded coating consisting essentially of: (i) an inner coating portion adjacent to and diffused with the substrate, and consisting essentially of, by weight, 10-50% Cr, up to 30% Al, up to 10% Hf, up to 30% of elements selected from the group consisting of Pt, Rh and Pd, up to 3% Y, with the balance at least one element selected from the group consisting of Fe, Co and Ni, and elements diffused from the substrate and the outer coating portion, and (ii) an outer coating portion adjacent to and diffused with the inner coating portion, and consisting essentially of, by weight, 10-50% Al and 1-40% of elements selected from the group consisting of Hf, Pt, Rh and Pd, with the balance elements diffused from the inner coating portion.
2.
The article of claim 1 in which the outer coating portion includes 10-30% Al and 2-5% Hf.
3.
The article of claim 1 in which the outer coating portion includes 10-30% Al and 5-40% Pt.
4.
The article of claim 3 in which the Pt is 20-40%.
5.
The article of claim 1 in which the outer coating portion includes 10-30% Al, 2-5% Hf and 20-40% Pt.
6.
The article of claim 1 in which the inner coating portion consists essentially of, by weight, 8-20% Al, 10-40% Cr, up to 5% Hf, up to 20% Pt, up to 2% Y, with the balance at least one element selected from the group consisting of Fe, Co and Ni.
7.
The article of claim 6 in which the Al is 8-16% and the Cr is 15-30%.
Description:
CROSS REFERENCE TO RELATED INVENTIONS This invention is related to patent application Ser.
No.
508,747, filed Sept.
24, 1974 now U.
S.
Pat.
No.
4,080,486, issued Mar.
21, 1978 and to concurrently filed application Ser.
No.
835,543, each relating to a coated metallic article of improved environmental resistance, and assigned to the assignee of the present invention
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