Coating for internal surfaces of an airfoil and method of manufacture thereof

1. A method of coating, comprising:

providing an article having an internal passage therein to be coated;

electrolytically applying a first layer that comprises chromium or a chromium alloy onto a surface of the internal passage;

electrolytically applying a second layer comprising aluminum or an aluminum alloy onto the first layer; and

heat treating the article to promote interdiffusion between the first layer and the second layer; where the electrolytically applying of the first layer and the second layer is conducted by using a conforming electrode that traverses internal convoluted passages of the article without contacting a surface of the article; where the internal convoluted passages are insulated from the conforming electrode by a porous sheath that is disposed on the conforming electrode; and wherein the porous sheath comprises an electrically insulating material that comprises a polyolefin, a polyvinyl acetate, a polyvinyl chloride, a polyimide, a polysiloxane, a polystyrene, a polymethylmethacrylate, or a combination thereof.

2. The method of claim 1 , wherein the electrolytically applying of the first layer and/or the electrolytically applying of the second layer is conducted via electrodeposition.

3. The method of claim 1 , where the conforming electrode is an anode and where the article is a cathode.

4. The method of claim 1 , where the electrolytically applying of the first layer comprises using an electrolyte that comprises a suspension of chromium or nickel-chromium.

5. The method of claim 1 , where the electrolytically applying of the second layer comprises using an electrolyte that comprises a suspension of aluminum or an aluminum alloy.

6. The method of claim 1 , where the first layer has a thickness of 50 to 100 micrometers.

7. The method of claim 1 , where the second layer has a thickness of 25 to 75 micrometers.

8. The method of claim 1 , where the heat treating is conducted at a temperature of 800 to 1600° C.

9. The method of claim 1 , where the heat treating results in the formation of a layer of thermally grown oxides that comprise alumina and chromium oxide that is disposed on a layer of Ni—Cr/aluminide or a layer of Cr/aluminide.

10. The method of claim 9 , where the Ni—Cr/aluminide layer or the Cr/aluminide layer contains aluminum and chromium that vary in amount inversely with one another with distance from a surface of the article or from a surface of the thermally grown oxide layer.

11. The method of claim 1 , where the article is an airfoil.