EP0506928A1 - Stripping solution and process for stripping compounds of titanium from base metals. - Google Patents

Abstract

Aqueous pickling bath and method for selectively removing a titanium compound from a base metal. The aqueous solution contains a source of hydrogen peroxide, an alkaline source of hydroxyl ions and an acid, the concentration of the constituents being such that the pH of the solution is greater than 8.

Description

STRIPPING SOLUTION AND PROCESS FOR STRIPPING COMPOUNDS OF TITANIUM FROM BASE METALS

Field of the Invention

This invention is a continuation-in-part of U.S. Patent Application Serial No. 599,833 filed October 19, 1990 and relates to an aqueous stripping solution for selectively removing a titanium

compound, such as TiN or TiB₂, from a solid base metal without chemically attacking the solid base metal and to an accompanying process for stripping compounds of titanium from base metals.

Background of Invention

High performance components in aircraft engine turbomachines such as compressor blades, bearings and gears are typically coated with a titanium metal compound such as TiN to improve their wear characteristics and to provide erosion

protection. The engine parts are cast or otherwise molded or machined from superalloys, stainless steels or alloy steels and represent very expensive precision components. Removal of the coating from the underlying base metal is necessary if a defect is discovered in the coating and/or for restoring worn components. It is essential to strip the protective coating from the base metal without suffering any detrimental attack to the underlying base metal.

To selectively strip a titanium compound such as TiN from a solid base metal composed of a superalloy, stainless steel or alloy steel without chemically attacking the base metal is particularly difficult when both the base metal and coating have a high corrosion resistance characteristic.

Stripping is even more difficult when the corrosion resistance of the coating is equal to or greater than the corrosion resistance of the substrate.

Although, stripping solutions containing hydrogen peroxide are known there is no known aqueous based stripping solution using hydrogen peroxide which will permit the removal of a coating of a titanium compound from a solid base metal composed of a superalloy, stainless steel or alloy steel without causing detrimental attack to the underlying base metal. A chemical stripping solution comprising hydrogen peroxide is described in U.S. Patent Nos. 4,554,049, 4,410,396 and

4,545,918 respectively. The stripping solutions disclosed in these patents are either unable to strip compounds of titanium from base metals composed of superalloys stainless steels and alloy steels or will actively attack both the titanium compound coating and the base metal.

Summary of the Invention

The process of the present invention for stripping a coating of a titanium compound from an underlying base metal without suffering chemical attack to the base metal comprises the steps of:

immersing the base metal and coating into an aqueous solution containing a source of hydrogen peroxide, an alkali source of hydroxyl ions, and an acid, maintaining the solution temperature between 25°C and 85°C and adjusting the molar ratio of the components to cause the pH of the aqueous solution to be above a pH of 8.

The stripping composition of the present invention comprises an aqueous solution including an alkali source of hydroxyl ions, a source of hydrogen peroxide and an acid with the constituents of the solution in a concentration such that the pH of the solution is above 8.

Brief Description of the Drawings;

Figure 1 is a plot of stripping efficiency versus the content of the preferred acid in mole per liter for removing a TiN coating from an Inconel 718 base metal;

Figure 2 is a plot similar to that of Figure 1 showing stripping efficiency as a function of the content of NH₄OH in mole per liter in the stripping solution of the present invention;

Figure 3 is another plot similar to that of Figure 1 of stripping efficiency as a function of the content of hydrogen peroxide in mole per liter in the stripping solution of the present invention;

Figure 4 is a plot of the solution stripping rate for stripping TiN from an Inconel 718 coupon as a function of the solution operating temperature; and

Figure 5 is a plot of the solution active life of a preferred solution composition for removing TiN from Inconel 718 base metal substrates and the stripping efficiency as a function of temperature. Detailed Description of the Invention

Essentially any coating composition of a titanium compound can be removed from any base metal substrate by the process of the present invention without detrimentally attacking the base metal. The invention is particularly suited to the removal of TiN or TiB₂ from a base metal composed of stainless steels, superalloys or alloy steels.

The stripping solution of the present invention comprises the following three components: a source of hydrogen peroxide, an alkaline source of hydroxyl ions and a suitable acid in various

proportions to cause the pH of the solution to be above 8 without corroding the substrate. The

stripping solution is prepared by first combining the source of hydrogen peroxide with water. The source of hydrogen peroxide should be present in a minimum concentration of .29 mole per liter and in a

preferred concentration range of between .29 to about 4.71 mole per liter (mole/L). Any source of hydrogen peroxide such as a perborate, as is well known to those skilled in the art, may be used. Other

compounds which readily dissociate into hydrogen peroxide upon contact with water are also suitable. The alkali source of hydroxyl ions (OH) is then added to the solution. The hydroxyl ion is preferably added in combination with ammonium ions through the addition of ammonium hydroxide (NH₄OH). The source of hydroxyl ions should be present in the stripping solution in a concentration of at least 0.29 mole/L and preferably between .29 mole/L and 3.23 mole/L. An acid must also be present in the solution at a concentration of 0.026 mole/L and preferably between 0.026 mole/L and 0.76 mole/L. Any acid which will not corrode the base metal may be used, preferably an organic carboxyl or carboxyl-hydroxyl group acid such as lactic acid, oxalic acid, tartaric acid, formic acid, propionic acid or citric acid. Alternatively, a diluted inorganic acid such as, for example, acetic acid, nitric acid, hydrochloric acid and sulfuric acid may also be used provided it will not chemically attack the base metal and is low enough in

concentration to maintain the solution pH above 8.

The pH of the stripping solution is critical to the present invention and must be above pH 8 to be effective. The preferred pH range is between pH 9-14 with a pH range of 10-12 being optimum. The pH of the solution may be controlled by adjusting the concentration of alkali, peroxide and organic acid relative to one another provided each is held to a concentration within the preferred range.

Additionally, other alkali ions such as sodium or potassium ions may be added to the stripping solution by the addition of NaOH and/or KOH to establish the desired mole concentration and/or to adjust the pH of the solution.

The effectiveness of the stripping solution of the present invention is determined by the

efficiency in which the titanium compound coating is removed from the substrate without suffering any deleterious effect on the base metal. A minimum stripping efficiency of 1 x 10^-2g/cm²/L and preferably above 2 x 10^-2g/cm²/L is necessary for the stripping solution to be acceptable for commercial practice. The stripping efficiency is determined based on total weight loss of the coating per unit coating surface area for a given volume of stripping solution over a time period until the solution is considered inactive.

Experiments were conducted using numerous aqueous compositions all containing various

proportions of hydrogen peroxide, an acid and an alkali source of hydroxyl ions. The following tables I, II, III and IV identify the different solution compositions all of which had no deleterious effect on the base metal. All of the tests shown in the Tables I, II, III and IV were carried out by

immersing a TiN coated Inconel 718* coupon (1.5 x 25 x 50 mm) into the test stripping solution at between 60 and 85° C.

Table I

Effect of Citric Acid Content (H₃C₆H₅O₇) on Stripping Efficiency

* Inconel 718 is a registered trademark of the

Internatonal Nickel Corporation.

Table I should be read in conjunction with

Figure 1, which is based on the data of Table I, showing the effect of citric acid on the stripping efficiency of the solution. Citric acid is the preferred acid component although any of the other acids, as heretofore described, may be substituted for citric acid at equivalent concentration or equivalent pH levels to produce substantially

equivalent results. The stripping efficiency

increases monotonically with increasing concentration of citric acid provided the pH level is above 8.5. The concentration of hydrogen peroxide and the alkali component were held constant. It was determined from experimentation that the presence of a minimum concentration of acid was necessary to stabilize the solution and to permit the stripping efficiency to exceed the minimum level. The concentration of citric acid should be above at least about 0.026 mole/L and preferably above 0.052 mole/L. The maximum concentration of citric acid is approximately 0.76 mole/L. Upon exceeding the maximum

concentration the pH of the solution drops to below 8.5 which reduces the stripping efficiency below the effective minimum level.

Table II

Effect of NH₄OH Content on Stripping Efficiency

Table II should be read in conjunction with Figure 2 which is based on the data of Table II and shows the effect of varying the concentration of ammonium hydroxide (NH₄OH) in the stripping

solution. Ammonium hydroxide is the preferred alkali source. The concentration level of citric acid and peroxide were held constant while adjusting the concentration of NH₄OH. From Table II and Figure 2 it is apparent that the stripping solution does not function effectively until the concentration of NH₄OH is raised to a minimum level of about 0.29 mole/L at a pH of 8 or higher. The latter was confirmed by the data shown in Table IV as will be discussed in greater detail later in the specification.

Table III

Effect of H₂O₂ Content on Stripping Efficiency

Table III should be read in conjunction with Figure 3 from which it is apparent that the stripping efficiency directly increases with increasing concentrations of hydrogen peroxide up to about 2.94 mole/L at which concentration the stripping

efficiency of the solution levels off. Accordingly, although the hydrogen peroxide concentration may be further increased the maximum level should be about 4.71 mole/L above which, for practical

considerations, there is a negative incentive to further raise the hydrogen peroxide concentration. The minimum concentration of hydrogen peroxide is about 0.29 mole/L and preferably above 0.59 mole/L.

Typically the temperature of the solution has an influence on the stripping rate and

efficiency. The reactivity of the solution increases with increasing operating temperature and the

solution life decreases with increasing operation temperature. Accordingly, to determine the optimum solution temperature two test solutions were prepared using a different peroxide to alkali molar ratio at a constant acid concentration. The stripping rate was evaluated as a function of the operating temperature as shown in Figure 4. The composition of the two test solutions were as follows:

Solution 12. 1.32 mole/L H₂O₂ + 1.46 mole/L NH₄OH + 0.16 mole/L H₃C₆ H₅O₇ balance water (marked "O" in Figure 4).

Solution 4. 1.32 mole/L H₂O₂ + 1.09 mole/L NH₄OH + 0.16 mole/L H₃C₆ H₅O₇ balance water (marked "Δ" in Figure 4).

The stripping rate is expressed in terms of the total weight loss (in grams) of the coating per unit area (in cm²) per unit volume (in liters) per unit time (in minutes). As shown in Figure 4 the optimum stripping rate is realized at a solution temperature exceeding 50°C and preferably between 60°C and 85ºC.

Although the optimum solution temperature is above 50°C the solution may be operated at a

temperature within a wide range extending from about 25°C to about 95°C as is evident from Figure 5 which is a plot of the solution active life in minutes as well as stripping efficiency against temperature. A preferred solution of H₂O + 1.32 mole/L H₂O₂ + 1.09 mole/L NH₄OH + 0.16 mole/L citric acid was used to develop the plot. The solution active life was found to decrease exponentially with increasing temperature from about 1000 minutes at 25°C to about 24 minutes at about 95°C. The stripping efficiency also

decreases rapidly with increasing temperature. At higher operating temperatures of above about 85°C the solution active life is simply too short for any practical commercial use. Figure 5 should be

evaluated in conjunction with Figure 4 which

substantiates that the stripping rate is highest above 50°C. Accordingly from both Figure 4 and 5 a wide operating solution temperature of between 25°C to 85°C is practical although the highest stripping rate occurs above between 50°C and 85°C with 60°C - 80°C being the preferred range for optimum stripping with a reasonable solution active life.

The following Table IV is a compilation of the data obtained using various alkali ammonium compounds and NaOH at different pH levels for

comparison with the results of Table II on the effect of stripping efficiency for the various test solutions.

Table IV

Effects of Composition and pH Value on Stripping Efficiency

* Ammonium Bicarbonate

** Ammonium Sulfate

*** Ammonium Tartrate

From the above Table IV it is apparent that a pH above 8 is necessary for the solution to provide an effective stripping efficiency and that ammonium compounds other than NH₄OH do not produce effective stripping efficiencies unless combined with NH₄OH or another source of hydroxyl ions such as NaOH.

However, it is clear from all of the test data that NH₄OH is the preferred alkali source. The effective concentration for the three critical components, viz., a source of hydrogen peroxide, an alkali source of hydroxyl ions and acid is 0.29 mole/L to 4.71 mole/L, 0.29 mole/L to 3.23 mole/L and 0.026 mole/L to 0.76 mole/L, respectively. For the preferred components H₂O₂; NH₄OH and citric acid the preferred concentration is 0.59 mole/L to 4.71 mole/L, 0.37 mole/L to 3.23 mole and 0.05 mole/L to 0.66 mole/L, respectively.

Although the base metal in the test coupons were all of Inconel 718 other coupons including TiN coated stainless steels such as AISI44OC and AISI 17-4 PH and alloy steels such as M50, M50NIL and Pyrowear 53 were tested using the preferred stripping solution. All demonstrated similar behavior to the TiN coated Inconel 718 coupons with no deleterious effect on the base metal.

The hydrogen peroxide component in the stripping solution of the present invention may be generated in situ from any source of peroxide which dissociates in water to form hydrogen peroxide such as a perborate, e.g. sodium perborate tetrahydrate (NaBO₃·4H₂O) or any other know peroxide compound which will readily dissociate into hydrogen peroxide in the presence of water at atmospheric pressure and within the operating temperatures of the present invention. Ammonium peroxydisulfate ((NH₄)₂S₂O₈) is not a suitable source of hydrogen peroxide for the present invention as is evident from the following Table V despite the fact that ammonium

peroxydisulfate is used to commercially produce hydrogen peroxide by hydrolysis at reduced pressure and elevated temperature.

In accordance with the following Table V TiN coated Inconel 718 coupons (1.5x25x50 mm) were immersed into separate peroxide containing solutions with a specified pH of above 8 and at temperatures of between 60°C and 65°C to evaluate the stripping effectiveness of the solutions with the different sources of peroxide.

As is evident from the above table no stripping action was observed in the solutions 34 and 35 containing ammonium peroxydisulfate and no weight loss was found on the test coupons. The solutions 32 and 33 with sodium perborate tetrahydrate were capable of stripping the TiN coating from an Inconel 718 substrate but at a reduced stripping efficiency. This is in sharp contrast to the effect of an

otherwise identical stripping solution composition containing hydrogen peroxide.

Tables V and VI show the results of corrosion on the base metal when the acid component in the stripping solution contains the Cl- ion. In solution No. 34 and 36, NH₄Cl and CH₃OH were used instead of an organic acid and in solutions No. 37-40 HCl was used. Both TiN coated Inconel 718 and 410 stainless steel coupons (1.5x25x50 mm in size) were immersed into the solution No. 36 and only 410 stainless steel exhibited corrosion attack due to the presence of the Cl- ion from the NH₄Cl solution. In the tests in the following Table VI HCl was used as the acid component to strip TiN from different substrate materials at different concentration levels. Accordingly, the chloride concentration levels that cause pitting vary with the substrate material composition. If an acid containing the chloride ion is used in the stripping solution, the concentration of acid should be determined according to the substrate material used.

Claims

What I claim is :

1. A process for stripping a coating of a titanium compound from a base metal of a superalloy, stainless steel or alloy steel without suffering chemical attack to the base metal comprising the steps of:

immersing the base metal and coating into an aqueous solution containing hydrogen peroxide or a compound which dissociates into hydrogen peroxide in water, an alkali source of hydroxyl ions, and an acid, maintaining the solution temperature between about 25°C and 85°C and maintaining the pH of the aqueous solution at a pH of above at least 8.

2. A process as defined in claim 1 wherein the minimum concentration of hydrogen peroxide, said source of hydroxyl ions and said acid is .29 mole/L, 0.29 mole/L and 0.026 mole/L respectively.

3. A process as defined in claim 2 wherein said alkali source further comprises ammonium ions.

4. A process as defined in claim 3 wherein said acid is an organic acid selected from the carboxyl group or carboxyl-hydroxyl group.

5. A process as defined in claim 4 wherein the concentration of hydrogen peroxide, said alkali source and said organic acid is 0.29 mole/L to 4.71 mole/L, 0.37 mole/L to 3.23 mole/L and 0.026 mole/L to 0.76 mole/L respectively.

6. A process as defined in claim 5 wherein said alkali source comprises ammonium hydroxide.

7. A process as defined in claim 6 wherein said organic acid is citric acid.

8. A process as defined in claim 7 where said hydrogen peroxide is formed in situ from a perborate.

9. A process as defined in claim 8 wherein the concentration of hydrogen peroxide, said alkali source and said organic acid is .59 mole/L to 4.71 mole/L, 0.37 mole/L to 3.23 mole/L and 0.05 mole/L to 0.66 mole/L respectively.

10. A metal stripping composition for stripping a titanium compound from a base metal of a superalloy, stainless steel or alloy steel comprising an aqueous solution of an alkali source of hydroxyl ions; a source of hydrogen peroxide and an acid with the components in a concentration such that the pH of the solution is above 8.

11. A metal stripping composition as defined in claim 10 wherein said titanium compound is selected from the group consisting of TiN and TiB₂.

12. A metal stripping composition as defined in claim 11 wherein said acid is an organic acid selected from the carboxyl group or

carboxyl-hydroxyl group.

13. A metal stripping composition as defined in claim 12 wherein the minimum concentration of said source of peroxide, said source of hydroxyl ions and said acid is .29 mole/L, 0.29 mole/L and 0.026 mole/L respectively.

14. A metal stripping composition as defined in claim 13 wherein said alkali source comprises ammonium hydroxide.

15. A metal stripping composition as defined in claim 14 wherein said source of hydrogen peroxide is selected from the group consisting of hydrogen peroxide and a perborate.

16. A metal stripping composition as defined in claim 15 wherein said organic acid is citric acid.