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A method for selectively removing one or more coatings from the surface of a substrate is described. The coating is treated with an aqueous composition which includes an acid of the formula H x AF 6 , or precursors to such an acid. In that formula, A is Si, Ge, Ti, Zr, Al, and Ga; and x is 1-6. Th

A method for selectively removing one or more coatings from the surface of a substrate is described. The coating is treated with an aqueous composition which includes an acid of the formula H x AF 6 , or precursors to such an acid. In that formula, A is Si, Ge, Ti, Zr, Al, and Ga; and x is 1-6. The acid is often H 2 SiF 6 . The composition may sometimes include at least one additional acid, such as phosphoric acid. The coating being removed is often an aluminide coating or an MCrAl(X)-type material. The substrate is usually a polymer or a metal, such as a superalloy.

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1. A method for selectively removing at lease one coating from the surface of a substrate, comprising the step of contacting the coating with an aqueous composition which comprises at least one of an acid having the formula H x AF 6 , and precursors to said acid, wherein A is selected from the gro

1. A method for selectively removing at lease one coating from the surface of a substrate, comprising the step of contacting the coating with an aqueous composition which comprises at least one of an acid having the formula H x AF 6 , and precursors to said acid, wherein A is selected from the group consisting of Si, Ge, Ti, Zr, Al, and Ga; wherein x is 1-6; and wherein contacting said coating further comprises contacting a coating comprising at least one ofa. an aluminide material, andb. MCrAl(X), where M is an element selected from the group consisting of Ni, Co, Fe, and combinations thereof; and X is an element selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof;said coating comprising at least one of a diffusion coating and an overlay coating. 2. The method of claim 1, wherein x is 1-3. 3. The method of claim 1, wherein the acid is present at a level in the range of about 0.05 M to about 5 M. 4. The method of claim 3, wherein the acid is present at a level in the range of about 0.2 M to about 3.5 M. 5. The method of claim 1, wherein the precursor is a salt of the acid. 6. The method of claim 1, wherein the aqueous composition comprises the compound H2SiF6 or H2ZrF6. 7. The method of claim 6, wherein the H2SiF6 compound is formed in situ within the aqueous composition, by the dissociation of a corresponding salt of the compound; or by the reaction of a silicon-containing compound with a fluorine-containing compound. 8. The method of claim 7, wherein the silicon-containing compound is SiO2, and the fluorine-containing compound is HF. 9. The method of claim 1, wherein the aqueous composition further comprises at least one additional acid or precursor thereof. 10. The method of claim 9, wherein the additional acid has pH of less than about 7 in pure water. 11. The method of claim 10, wherein the additional acid has a pH of less than about 3.5 in pure water. 12. The method of claim 9, wherein the additional acid is a mineral acid. 13. The method of claim 9, wherein the additional acid is selected from the group consisting of phosphoric acid, nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid, hydrobromic acid, hyrdriodic acid, acetic acid, perchloric acid, phosphorous acid, phosphinic acid, alkyl sulfonic acids, and mixtures of any of the foregoing. 14. The method of claim 9, wherein the additional acid is present in the composition at a level in the range of about 0.1 M to about 20 M. 15. The method of claim 14, wherein the additional acid is phosphoric acid. 16. The method of claim 15, wherein the phosphoric acid is present at a level in the range of about 0.5 M to about 5 M. 17. The method of claim 1, wherein the substrate is immersed in a bath of the aqueous composition. 18. The method of claim 17, wherein the bath is maintained at a temperature in the range of about room temperature to about 100° C., while the substrate is immersed therein. 19. The method of claim 18, wherein the temperature is the range of about 45° C. to about 90° C. 20. The method of claim 18, wherein the immersion time is in the range of about 10 minutes to about 72 hours. 21. The method of claim 20, wherein the immersion time is in the range of about 60 minutes to about 20 hours. 22. The method of claim 17, wherein the bath further comprises at least one additive selected from the group consisting of inhibitors, dispersants, surfacants, chelating agents, wetting agents, deflocculants, stabilizers, anti-settling agents, and anti-foam agents. 23. The method of claim 1, wherein the aluminide material is selected from the group consisting of aluminide, noble metal-aluminide, nickel-aluminide, noble metal-nickel-aluminide, and mixtures thereof. 24. The method of claim 1, wherein the substrate is selected from the group consisting of a metallic material and a polymeric material. 25. The method of claim 24, wherein the polymeric material is selected from the group consisting of polyolefi ns, polytetrafluroethylenes, epoxy resins, polystyrenes, polyphenylene ethers; mixtures comprising one of the foregoing; and copolymers comprising one of the foregoing. 26. The method of claim 24, wherein the metallic material comprises at least one element selected from the group consisting of iron, cobalt, nickel, aluminum, chromium, titanium, and mixture which include any of the foregoing. 27. The method of claim 26, wherein the metallic material comprises a superalloy. 28. The method of claim 27, wherein the superalloy is nickel-based or cobalt-based. 29. The method of claim 27, wherein the superalloy is a component of a turbine engine. 30. The method of claim 29, wherein the component comprises an airfoil. 31. A method for selectively removing at least one coating material from the surface of a metallic substrate, comprising the step of contacting the coating with an aqueous composition which comprises at least one of an acid having the formula H x AF 6 , and precursors to said acid, wherein A is selected from the group consisting of Si, Ti, and Zr; wherein x is 1-3; and wherein contacting said coating further comprises contacting a coating comprising materials selected from the group consisting of aluminides and MCrAlY materials, wherein M is an element selected from the group consisting of Ni, Co, Fe, and combinations thereof. 32. The method of claim 31, wherein the acid is present at a level in the range of about 0.05 M to about 5 M. 33. The method of claim 31, wherein the aqueous composition further comprises at least one additional acid or precursor thereof, selected from the group consisting of phosphoric acid, nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid, and mixtures thereof. 34. The method of claim 33, wherein the additional acid is present in the composition at a level in the range of about 0.1 M to about 20 M. 35. The method of claim 31, wherein the coating material comprises an MCrAlY layer which is diffusion-aluminided. 36. The method of claim 31, wherein the metallic substrate comprises a nickel-base or cobalt-base superalloy. 37. The method of claim 36, wherein the metallic substrate is a turbine engine airfoil.