Method for removing aluminide coating from metal substrate and turbine engine part so treated

1. A method for selectively removing an aluminide coating from at least one surface of a metal-based substrate, comprising the following steps in sequence:

(a) contacting the surface of the substrate with an aqueous solution comprising from about 10% to about 50%, by weight of the solution, of caustic at a temperature of from about 60° C. to about 100° C. for from about 20 minutes to about 4 hours;

(b) contacting the surface of the substrate with at least one stripping composition comprising from about 20% to about 40% nitric acid, by weight of the composition, at a temperature less than about 20° C. for at least about 2 hours to degrade the coating without damaging the substrate; and

(c) removing the degraded coating without damaging the substrate.

2. The method of claim 1 , wherein the stripping composition comprises from about 25% to about 35% nitric acid, by weight of the composition.

3. The method of claim 2 , wherein the stripping composition comprises from about 28% to about 32% nitric acid, by weight of the composition.

4. The method of claim 1 , wherein the stripping composition has a temperature of from about 0° C. to about 15° C.

5. The method of claim 4 , wherein the stripping composition has a temperature of from about 4° C. to about 12° C.

6. The method of claim 1 , wherein the stripping composition contacts the substrate for a period of from about 4 hours to about 10 hours.

7. The method of claim 6 , wherein the stripping composition contacts the substrate for a period of from about 6 hours to about 8 hours.

8. The method of claim 6 , wherein the stripping composition comprises from about 25% to about 35% nitric acid, by weight of the composition, and has a temperature of from about 4° C. to about 12° C.

9. The method of claim 8 , wherein the stripping composition further comprises from about 01. % to about 5%, by weight of the composition, of a wetting agent selected from the group consisting of polyalkylene glycols, glycerol, fatty acids, soaps, emulsifiers, and surfactants.

10. The method of claim 9 , wherein the substrate is immersed in a bath of the stripping composition in step (b) for a period of from about 6 hours to about 8 hours.

11. The method of claim 1 , wherein the stripping composition further comprises from about 0.1% to about 5%, by weight of the composition, of a wetting agent.

12. The method of claim 1 , wherein the substrate is immersed in a bath of the stripping composition in step (b).

13. The method of claim 12 , wherein the bath is maintained at a temperature of from about 0° C. to about 15° C. while the substrate is immersed therein for a period of from about 4 hours to about 10 hours.

14. A method for selectively removing an aluminide coating from at least one internal surface of a metal-based substrate, comprising the following steps in sequence:

(a) contacting the surface of the substrate with an aqueous solution comprising from about 10% to about 50%, by weight of the solution, of caustic at a temperature of from about 60° C. to about 100° C. for from about 1 to about 4 hours;

(b) contacting the surface of the substrate with at least one stripping composition comprising from about 20% to about 40% nitric acid, by weight of the composition, at a temperature less than about 20° C. for at least about 2 hours to degrade the coating without damaging the substrate;

(c) contacting the surface of the substrate with an aqueous solution comprising from about 10% to about 50%, by weight of the solution, of caustic at a temperature of from about 60° C. to about 100° C. for from about 20 minutes to about 2 hours;

(d) contacting the surface of the substrate with at least one stripping composition comprising from about 20% to about 40% nitric acid, by weight of the composition, at a temperature less than about 20° C. for at least about 2 hours to degrade the coating without damaging the substrate; and

(e) contacting the surface of the substrate with an aqueous solution comprising from about 10% to about 50%, by weight of the solution, of caustic at a temperature of from about 60° C. to about 100° C. for from about 1 to about 4 hours to remove the degraded coating without damaging the substrate.

15. The method of claim 14 , wherein the substrate is immersed in a bath of the stripping composition maintained at a temperature of from about 0° C. to about 15° C. in each of steps (b) and (d).

16. The method of claim 15 , wherein the stripping composition comprises from about 25% to about 35%, by weight, of nitric acid.

17. The method of claim 16 , wherein the substrate is immersed in the bath of the stripping composition having a temperature of from about 4° C. to about 12° C. for from about 3 to about 4 hours in each of steps (b) and (d).

18. The method of claim 17 , wherein the substrate is immersed in a bath of the caustic solution having a temperature of from about 70° C. to about 85° C. in each of steps (a), (c) and (e).

19. The method of claim 18 , wherein the caustic solution comprises from about 15% to about 30%, by weight, of potassium hydroxide.

20. The method of claim 19 , wherein the caustic solution is ultrasonically agitated while in contact with the substrate.

21. The method of claim 20 , wherein the aluminide coating is selectively removed from the internal surfaces of a high-pressure turbine blade.

22. A method for selectively removing an aluminide coating from at least one internal surface of a superalloy substrate, comprising the following steps in sequence:

(a) contacting the surface of the substrate with an aqueous solution comprising from about 10% to about 50%, by weight of the solution, of caustic at a temperature of from about 60° C. to about 100° C. for from about 20 minutes to about 4 hours,

(b) contacting the surface of the substrate with at least one stripping composition comprising from about 25% to about 35% nitric acid, by weight of the composition, at a temperature of from about 0° C. to about 15° C. for from about 4 hours to about 10 hours to degrade the coating without damaging the substrate; and

(c) removing the degraded coating without damaging the substrate.

23. The method of claim 22 , wherein the stripping composition comprises from about 28% to about 32% nitric acid, and the substrate is immersed in a bath of the stripping composition having a temperature of from about 4° C. to about 12° C. for from about 6 hours to about 8 hours.

24. A method for replacing a worn or damaged aluminide coating on at least one surface of a metal-based substrate, comprising the following steps in sequence:

(i) selectively removing the aluminide coating from the surface of the substrate by (a) contacting the surface of the substrate with an aqueous solution comprising from about 10% to about 50%, by weight of the solution, of caustic at a temperature of from about 60° C. to about 100° C. for from about 20 minutes to about 4 hours; (b) contacting the surface of the substrate with at least one stripping composition comprising from about 20% to about 40% nitric acid, by weight of the composition, at a temperature of less than about 20° C. for at least about 2 hours to degrade the coating without damaging the substrate; and (c) removing the degraded coating without damaging the substrate; and

(ii) applying a new aluminide coating to the surface of the substrate.

25. The method of claim 24 , wherein the stripping composition comprises from about 28% to about 32% nitric acid, and the substrate is immersed in a bath of the stripping composition having a temperature of from about 4° C. to about 12° C. for from about 6 hours to about 8 hours.