Hexavalent Chromium-Free Stripping Bath and Process for Aluminum Anodizing Removal

Abstract

A method for removing aluminum oxide coating from a substrate includes exposing the coating to a solution of: 3.0 wt. % to 10.0 wt.% oxalic acid; 5.0 to 25.0 combined nitric acid in wt. % and LiNO3 in mM. The solution may include: 5 mM to 75.0 mM combined Zr4+, Mo6+, Nb5+, and Cr3+; and / or at least one of 8-hydroxyquinoline, 2-quinolinecarboxylic acid, salicylaldoxime, and benzotriazole.

Description

HEXAVALENT CHROMIUM-FREE STRIPPING BATH AND PROCESS FOR ALUMINUM ANODIZING REMOVALCROSS-REFERENCE TO RELATED APPLICATION

[0001] Benefit is claimed of U.S. Patent Application No. 63 / 670,037, filed July 11, 2024, and entitled “Hexavalent Chromium-Free Stripping Bath and Process for Aluminum Anodizing Removal”, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length.BACKGROUND

[0002] The disclosure relates to gas turbine engines. More particularly, the disclosure relates to removal of aluminum anodic layers.

[0003] Anodization of aluminum (Al) is widely used across various industries to provide desired surface properties, such as enhanced corrosion resistance, durability, wear and chemical resistance, or electrical insulation. Industries may also need to strip aluminum anodized layers and re-anodize aluminum components for many reasons including rework and repair, which is common in aerospace, automotive, and electronics industries where precision and appearance are critical. Stripping and re-anodizing ensures that parts meet stringent quality and performance standards, it also extends the lifespan of the component.

[0004] Stripping of aluminum anodized coatings is traditionally done with a chromate-based phosphoric acid solution. The chromic acid / phosphoric acid solution used per ASTM -137 Standard ’lest Method for Measurement of Coating Mass Per Unit Area on Anodically Coated .Aluminum will remove the Al anodized layer without affecting the underlying substrate. Many countries have stringent regulations regarding the use and disposal of hexavalent chromium. Regulations such as the European Union's REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals) and the U.S. Environmental Protection Agency (EPA) guidelines are pushing industries to find alternatives. Development of chromate-free alternatives can reduce these costs and potentially offer more efficient and cost-effective processes in the long term.

[0005] Gas turbine engines (used in propulsion and power applications and broadly inclusive of turbojets, turboprops, turbofans, turboshafts, industrial gas turbines, and the like) have a variety of anodized components.

[0006] In a variety of circumstances, it is desirable to remove anodic layers from components themselves (e.g., locally from an area where anodizing is not desired) or from anodizinghardware such as racks. Traditional chemical removal has involved use of hexavalent chromium solutions.

[0007] US4244792 discloses a stripping solution of oxalic acid and nitric acid as an alternative to chromic acid.SUMMARY

[0008] One aspect of the disclosure involves a method for removing aluminum oxide coating from a substrate. The method comprises exposing the coating to a solution of: 3.0 wt. % to 10.0 wt.% oxalic acid; 5.0 to 25.0 combined nitric acid in wt. % and LiNOa in mM; and 7.5 mM to 75.0 mM combined Zr4+, Mo6+, Nb5+, and Cr3+.

[0009] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the solution has: at least one of 8-hydroxyquinoline, 2-quinolinecarboxylic acid, salicylaldoxime, and benzo triazole.

[0010] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively: the sum in mM of a hydroxy quinoline content, a 2-quinolinecarboxylic acid content, a salicylaldoxime content, and 10.0% of a benzotriazole content is 0.10 to 15.0; or the sum in mM of a hydroxyquinoline content, a 2-quinolinecarboxylic acid content, a salicylaldoxime content, 10.0% of a citric acid content, and 10.0% of a benzotriazole content is 0.10 to 15.0.

[0011] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, said sum is 0.50 to 5.0.

[0012] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, one to all of: said 2-quinolinecarboxylic acid content is 0.50 mM to 2.0 mM; said 8-hydroxyquinoline content is 0.25 mM to 1.0 mM; and said salicylaldoxime content is 0.50 mM to 2.0 mM.

[0013] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, a 2-quinolinecarboxylic acid content of the solution is at least 0.50 mM.

[0014] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, a 8-hydroxyquinoline content of the solution is at least 0.25 mM.

[0015] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, a salicylaldoxime content of the solution is at least 0.50 mM.

[0016] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the solution has: at most 0.50 wt. % nitric acid, if any.

[0017] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the solution has: at most 0.50 mM LiNOa, if any.

[0018] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the solution has: at least 5.0 wt. % nitric acid and at least 5.0 mM LiNCh.

[0019] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the combined Zr4+, Mo6+, Nb5+, and Cr3+comprises at least 5.0 mM Zr4+.

[0020] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the combined Zr4+, Mo6+, Nb5+, and Cr3+comprises at least 5.0 mM each of Nb5+and Cr3+.

[0021] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the combined Zr4+, Mo6+, Nb5+, and Cr3+comprises at least 5.0 mM each of Nb5+and Cr3+.

[0022] A further aspect of the disclosure involves an anodization method including a method above and further comprising: after the exposing, anodizing.

[0023] A further embodiment of any of the foregoing embodiments may additionally and / or alternatively include: before the exposing abrasive blasting; and after the exposing, but before the anodizing, rinsing.

[0024] A further aspect of the disclosure involves a method for removing aluminum oxide coating from a substrate, the method comprising: exposing the coating to an acidic solution containing at least one of 8-hydroxyquinoline, 2-quinolinecarboxylic acid, salicylaldoxime, benzotriazole, and citric acid.

[0025] A further embodiment of any of the foregoing embodiments may additionally and / or alternatively include forming the solution by forming a blend of: 3.0 wt. % to 10.0 wt.% oxalic acid; 5.0 to 25.0 combined nitric acid in wt. % and LiNOa in mM; and said at least one of 8-hydroxyquinoline, 2-quinolinecarboxylic acid, salicylaldoxime, and benzo triazole.

[0026] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the sum in mM of a hydroxyquinoline content, a 2-quinolinecarboxylic acid content, a salicylaldoxime content, 10.0% of a benzotriazole content, and 10.0% of a citric acid content is 0.10 to 15.0.

[0027] A further aspect of the disclosure involves a solution of: 3.0 wt. % to 10.0 wt.% oxalic acid; 5.0 to 25.0 combined nitric acid in wt. % and LiNOa in mM; and at least one of 8- hydroxyquinoline, 2-quinolinecarboxylic acid, salicylaldoxime, and benzo triazole.

[0028] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the sum in mM of a hydroxyquinoline content, a 2-quinolinecarboxylic acid content, a salicylaldoxime content, and 10.0% of a benzotriazole content is 0.10 to 15.0.

[0029] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the solution has a pH of 0.50 to 1.0 and / or the solution has not more than 5.0 ppm Cr6+, if any.

[0030] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, at least one of: a 2-quinolinecarboxylic acid content of the solution is at least 0.50 mM; a 8 -hydroxy quinoline content of the solution is at least 0.25 mM; and a salicylaldoxime content of the solution is at least 0.50 mM.

[0031] The details of one or more embodiments are set forth in the description below. Other features, objects, and advantages will be apparent from the description and from the claims.DETAILED DESCRIPTION

[0032] A stripping solution comprises: oxalic acid; nitric acid or an alternative; a transition metal cation additive; and optionally an organic additive. The cation additive and organic additive reduce attack on substrate relative to attack on oxide and can compensate for the loss of the favorable performance of chromic acid in this regard. Absent such compensation, in the case of uneven anodization / oxidation the exposure needed to remove the oxide from thicker oxide areas will cause excessive pitting of initially non-anodized areas or more thinly anodized areas. Similar issues may attend situations where there is uneven stripping of a more uniform anodize. The example solution is Cr6+-free or essentially free. For example, the solution may have not more than 5.0 ppm Cr6+, if any, more broadly not more than 10.0. Example pH is 0.50 to 1.0.

[0033] The nitric acid alternatives function to provide nitrate in order to help passivate / prevent pitting of the aluminum. Relative to nitric acid itself, the alternative may have an advantage of allowing a slightly less acidic solution. Examples are lithium nitrate (LiNCh) or sodium nitrate (NaNOa). LiNOs may also act by providing lithium while also providing nitrogen (which acts as to form nitric acid).

[0034] Example transition metal cations are Zr4+(e.g., zirconyl (IV) nitrate hydrate as zirconium precursor), Mo6+(e.g., (ammonium paramolybdatetetrahydrate ((NH4)6Mo?O24- 4H2O)) as precursor), Nb5+((e.g., ammonium niobium oxalate (NH4[NbO(C2O4)2(H2O)2]) as precursor); Cr3+(e.g., Cr2(SO4)3 as precursor). These function as corrosion inhibitors. Specifically, they reduce attack on substrate relative to attack on oxide.

[0035] Example organic additives include one or more of 8 -hydroxy quinoline (C9H7NO, quinolin-8-ol), 2-quinolinecarboxylic acid (C10H7NO2, quinaldinic acid), salicylaldoxime (C7H7NO2, saldox), and benzotriazole (C6H5N3, BTA). These may function as surfactants as well as corrosion inhibitors.

[0036] Example processes involve rework during original equipment manufacture (OEM) or remanufacture (e.g., in aircraft maintenance / repair / overhaul (MRO)). For example, an aluminum fan case is typically treated with a sulfuric acid anodize in OEM or MRO. The anodized layer is typically located on the inner diameter (ID) and outer diameter (OD) surfaces of such case. The anodized layer may need to be stripped during OEM production if there are significant defects (areas of missing anodize or variability in anodize layer thickness). The anodized layer may also need to be stripped from fan cases during MRO.

[0037] A first step in the fan case MRO example is disassembly of case to detach all attached vanes and other components. A further step is removal of organic coating such as via abrasive blasting and / or chemical removal. A further step is cleaning of the surface to remove blasting and paint residues such as via solvent wiping.

[0038] A further step is immersing the (cleaned) coated substrate in a bath of the aforementioned stripping solution. An example solution is at a temperature of 50°C (more broadly 20°C to 60°C or 35°C to 60°C or 35°C to 50°C) and has a pH of about 0.7 to 1.0 (more broadly 0.50 to 4.0 or 0.50 to 2.0 or 0.50 to 1.0 or 0.70 to 1.5). pH may be adjusted by addition of ammonia solution (NH4OH).

[0039] Example duration is about 30 minutes (more broadly 10 to 60 minutes). The duration may be determined such as by visual inspection of the coated substrate during immersion. During immersion, the fluid may be circulated and / or agitated. Use of the additives may reduce pitting sensitivity and allow more flexibility in duration of immersion.

[0040] At the end of the immersion, the substrate may be removed from the bath and neutralized (e.g., via rinsing with deionized water).

[0041] Then, the anodize layer may be reapplied. Example anodize is sulfuric acid anodize (SAA). This may include multiple substeps including masking of areas not to be anodized and final rinsing. Optionally, there may be repair steps including welding and / or machining.

[0042] Table I below provides parameters of several examples of a solution with transition metal cation additive(s):Table I Example Bath Compositions* Actually tester t Comparative

[0043] Citric acid appears in Table I due to testing. It is listed as an acid for convenience. As a practical matter, the lower concentration of citric acid relative to oxalic acid, nitric acid, and / or a nitric acid substitute may more properly cause citric acid to be considered an organic additive discussed below relative to Table III. Regarding the tested examples, PAI yielded in two 50°C thirty minute tests a 979 and 987 mg / ft2oxide removal. Ex. 2 had 1107 and 1047. Ex. 12 had 976 and 948. Ex. 15 had 1002 yet 925 in a shorter twelve minute test. The thirty minute Ex. 15 exhibited a pit probability distribution very close to that of PAI with a slightly lower probability of deep pits. The twelve minute Ex 15 was smaller yet and approached that of a thirty minute chromium phosphate immersion that had a 914 mg / ft2oxide removal.

[0044] Table II below provides parameters of several example ranges of a solution with transition metal cation additive(s):

[0045] Table III below provides parameters of several example ranges of organic additives for the solutions:Table IIIExample Bath Organic Additive Ranges

[0046] More specific variations on the transition metal cation additive ranges may be created by adding minimum contents of one or more individually and / or combined contents of two to all. For example, a combined content for one group of solutions having one to all may be 7.5 mM to 75.0 mM or 10 mM to50 mM.

[0047] One particularly relevant transition metal cation additive is Zr4+. An example minimum content of that may be 5.0 mM.

[0048] A particularly relevant combination is Nb5+and Cr3+. Thus example minimum contents of each may be 5.0 mM.

[0049] More specific variations on the organic additive ranges may be created by adding minimum contents of one of more individually and / or combined contents of two to all. For example, a combined content for one group of solutions having one to all may be such that the sum in mM of a hydroxyquinoline content, a 2-quinolinecarboxylic acid content, a salicylaldoxime content, 10.0% of a benzotriazole content, and 10.0% of a citric acid content is 0.10 to 15.0 or 0.50 to 15.0 with alternative lower limits of 0.25, 1.0, and 2.0 and alternative upper limits of 2.0, 5.0, and 10.0 in any combination. Example individual such acid contents may be lower such as lower ends of 0.1, 0.25, or 0.50 and upper ends of 1.0 and 2.0 in any combination.

[0050] One particularly relevant organic additive is 2-quinolinecarboxylic acid

[0051] A particularly relevant combination is hydroxyquinoline and salicylaldoxime.

[0052] The resistance to attack on substrate may be measured by observation of pitting upon achieving a threshold of oxide removal. The nature of tests may depend on whether the additives are believed to substantially affect the oxide removal. If not, then a baseline solution and exposure time may be used with differences only in the additives. After the stripping process, the (post-stripped) samples may be cross-sectioned, mounted and polished for pit depth measurements. The pit depths may be plotted to provide a graphical representation of substrate attack.

[0053] If additives would substantially affect oxide removal, it may be desirable to first precalibrate an amount of oxide removal (e.g., determine by observation the required time for an experimental solution to achieve the same degree of removal that the baseline does). Then the pitting results of such parameters may be compared to the baseline.

[0054] Solution blending techniques and apparatus may be otherwise conventional as may be the stripping bath apparatus and any other steps in the OEM or MRO process.

[0055] The use of “first”, “second”, and the like in the following claims is for differentiation within the claim only and does not necessarily indicate relative or absolute importance or temporal order. Similarly, the identification in a claim of one element as “first” (or the like) does not preclude such “first” element from identifying an element that is referred to as “second” (or the like) in another claim or in the description.

[0056] One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied to an existing baselineconfiguration, details of such baseline may influence details of particular implementations.Accordingly, other embodiments are within the scope of the following claims.

Claims

CLAIMSWhat is claimed is:

1. A method for removing aluminum oxide coating from a substrate, the method comprising: exposing the coating to a solution of:3.0 wt. % to 10.0 wt.% oxalic acid;5.0 to 25.0 combined nitric acid in wt. % and LiNOa in mM; and7.5 mM to 75.0 mM combined Zr4+, Mo6+, Nb5+, and Cr3+.

2. The method of claim 1 wherein the solution has: at least one of 8-hydroxyquinoline, 2-quinolinecarboxylic acid, salicylaldoxime, and benzo triazole.

3. The method of claim 2 wherein: the sum in mM of a hydroxyquinoline content, if any, a 2-quinolinecarboxylic acid content, if any, a salicylaldoxime content, if any, and 10.0% of a benzotriazole content, if any, is 0.10 to 15.0.

4. The method of claim 3 wherein: said sum is 0.50 to 5.0.

5. The method of claim 3 wherein one to all of: said 2-quinolinecarboxylic acid content is 0.50 mM to 2.0 mM; said 8-hydroxyquinoline content is 0.25 mM to 1.0 mM; and said salicylaldoxime content is 0.50 mM to 2.0 mM.

6. The method of claim 3 wherein: said 2-quinolinecarboxylic acid content is 0.50 mM to 2.0 mM.

7. The method of claim 3 wherein: said 8-hydroxyquinoline content is 0.25 mM to 1.0 mM.

8. The method of claim 3 wherein: said salicylaldoxime content is 0.50 mM to 2.0 mM.

9. The method of claim 2 wherein: the sum in mM of a hydroxyquinoline content, a 2-quinolinecarboxylic acid content, a salicylaldoxime content, 10.0% of a citric acid content, and 10.0% of a benzotriazole content is 0.10 to 15.0.

10. The method of claim 2 wherein: a 2-quinolinecarboxylic acid content of the solution is at least 0.50 mM.

11. The method of claim 2 wherein: a 8 -hydroxy quinoline content of the solution is at least 0.25 mM.

12. The method of claim 2 wherein: a salicylaldoxime content of the solution is at least 0.50 mM.

13. The method of claim 1 wherein the solution has: at most 0.50 wt. % nitric acid, if any.

14. The method of claim 1 wherein the solution has: at most 0.50 mM LiNCh, if any.

15. The method of claim 1 wherein the solution has: at least 5.0 wt. % nitric acid and at least 5.0 mM LiNOa.

16. The method of claim 1 wherein the combined Zr4+, Mo6+, Nb5+, and Cr3+comprises at least 5.0 mM Zr4+.

17. The method of claim 16 wherein the combined Zr4+, Mo6+, Nb5+, and Cr3+comprises at least 5.0 mM each of Nb5+and Cr3+.

18. The method of claim 1 wherein the combined Zr4+, Mo6+, Nb5+, and Cr3+comprises at least 5.0 mM each of Nb5+and Cr3+.

19. An anodization method including the method of any previous claim and further comprising: after the exposing, anodizing.

20. The anodization method of claim 19 further comprising: before the exposing abrasive blasting; and after the exposing, but before the anodizing, rinsing.

21. A method for removing aluminum oxide coating from a substrate, the method comprising: exposing the coating to an acidic solution containing at least one of 8-hydroxyquinoline, 2-quinolinecarboxylic acid, salicylaldoxime, benzotriazole, and citric acid.

22. The method of claim 21 further comprising forming the solution by forming a blend of:3.0 wt. % to 10.0 wt.% oxalic acid; 5.0 to 25.0 combined nitric acid in wt. % and LiNOa in mM; and said at least one of 8-hydroxyquinoline, 2-quinolinecarboxylic acid, salicylaldoxime, benzotriazole, and citric acid.

23. The method of claim 21 wherein: the sum in mM of a hydroxyquinoline content, a 2-quinolinecarboxylic acid content, a salicylaldoxime content, 10.0% of a benzotriazole content, and 10.0% of a citric acid content is 0.10 to 15.0.

24. The method of claim 21 wherein the acidic solution contains said 8-hydroxyquinoline.

25. The method of claim 21 wherein the acidic solution contains said salicylaldoxime.

26. The method of claim 21 wherein the acidic solution contains said benzo triazole.

27. The method of claim 21 wherein the acidic solution contains said citric acid.

28. An anodization method including the method of any of claim 21 to claim 27 and further comprising: after the exposing, anodizing.

29. The anodization method of claim 28 further comprising: before the exposing abrasive blasting; and after the exposing, but before the anodizing, rinsing.

30. A solution of:3.0 wt. % to 10.0 wt.% oxalic acid;5.0 to 25.0 combined nitric acid in wt. % and LiNOa in mM; and at least one of 8-hydroxyquinoline, 2-quinolinecarboxylic acid, salicylaldoxime, and benzo triazole.

31. The solution of claim 30 wherein: the sum in mM of a hydroxyquinoline content, a 2-quinolinecarboxylic acid content, a salicylaldoxime content, and 10.0% of a benzotriazole content is 0.10 to 15.0.

32. The solution of claim 31 wherein: the solution has a pH of 0.50 to 1.0.

33. The solution of claim 32 wherein: the solution has not more than 5.0 ppm Cr6+, if any.

34. The solution of claim 31 wherein: the solution has not more than 5.0 ppm Cr6+, if any.

35. The solution of any of claim 30 to claim 34 wherein: a 2-quinolinecarboxylic acid content of the solution is at least 0.5 mM.

36. The solution of any of claim 30 to claim 34 wherein: a 8-hydroxyquinoline content of the solution is at least 0.25 mM.

37. The solution of any of claim 30 to claim 34 wherein:a salicylaldoxime content of the solution is at least 0.5 mM.

38. A method for using the solution of any of claim 30 to claim 34 to remove aluminum oxide coating from a substrate, the method comprising: exposing the coating to the solution.

39. An anodization method including the method of claim 38 and further comprising: after the exposing, anodizing.

40. The anodization method of claim 39 further comprising: before the exposing abrasive blasting; and after the exposing, but before the anodizing, rinsing.

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