CA2071944C

CA2071944C - Stripping solution and process for stripping compounds of titanium from base metals

Info

Publication number: CA2071944C
Application number: CA 2071944
Authority: CA
Inventors: Jiinjen Albert Sue
Original assignee: Praxair ST Technology Inc
Current assignee: Praxair ST Technology Inc
Priority date: 1990-10-19
Filing date: 1991-10-18
Publication date: 1998-02-03
Anticipated expiration: 2011-10-18
Also published as: DE69113020T2; WO1992007110A1; JP2989265B2; CA2071944A1; DE69113020D1; EP0506928B1; JPH05503320A; EP0506928A1

Abstract

An aqueous stripping solution and method for selectively removing a titanium compound from a base metal. The aqueous solution contains a source of hydrogen peroxide, an alkali source of hydroxyl ions and an acid with the components in a concentration such that the pH of the solution is above 8.

Description

STRI PP ING SOI,UTIO~ ANU PROCESS FOR STRIPPINC;
!;~OI~lP0~7NDS OF TITANIIJM FROM BASE ~IFTAI,S
Field of the Invention This invention is a continuation-in-part of U.S. Patent Application Serial No. 599,833 filed October l9, 1990 and relates to an aqueous stripping solution for selectively removing a titanium compound, such as TiN or Ti~2, 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 I~vention 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 charac~eristics and to provide erosion protection. The engine parts are cast or otherwise molded or machined ~rom superalloys, stainless steels or alloy steels and represent ~ery e~pensive precisio~ 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 comp~nents. It is essential to strip the protective coating from the base metal wi~hout suffering any detrimental attack to the underlying base metal.
To selecti~ely 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 D-16302-l ~ ' - 2 - ~a7I9~ ~

a high corrosion resistance characteristic.
Stripping is e~en 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 pero~ide are known there is no known aqueous based stripping solution using hydrogen peroside 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 compri~ing hydrogen Rero~ide 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 ~ontaining a source of hydrogen pero~ide, an alkali source of hydro~yl ions, and an ~cid, maintaining the solution temperature between 25~C and 85~C and ad~usting the molar ratio of the components to cause the pH of the a~ueous solution to be above a pH of 8. ,:

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The stripping composition of the present invention comprises an aqueous solution including an alkali source of hydroxyl ions, a source of hydrogen pero~ide and an acid with the constituents of the solution in a concentration such that the pH o~ the solution is above 8.
~rie~ Descriptlon of the ~rawin~s: :
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 conkent of NH40H 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 ~s a function of the content of hydrogen pero~ide in mole per liter in the stripping solution of the present invention;
Figure 4 is a plot o~ the solution ~tripping rate ~or stripping TiN ~rom an Inconel 718 coupon as a function o~ the solution operating temperature; and Figure 5 is a plot o~ the solution active li~e of a preferred solution composition for removing TiN from Inconel 718 base metal substrates and the stripping efficiency as a ~unction o~ temperature.
~etailed ~escriptio~ of the Invention ~ ssentially any coating composition of a titanium compound can be removed from any b~se metal substrate by the process of the present invention without detrimentally attacking the base metal. The _ 4 _ -2~ ~ 7 9 ~4 invention is particularly suited to the removal of TiN or Ti~2 from a base metal composed of stainless steels, superalloys or alloy steels.
The stripping solution of th~ present invention comprises the following three components:
a source of hydrogen pero~ide, an alkaline source of hydro~yl 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 pero~ide 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 o~ between .29 to about 4.71 mole per liter (mole/L). Any source of hydrogen pero~ide such as a perborate, as is well known to those skilled in the art, may be used. Othes compounds which readily dissociate into hydrogen peroxide upon contact with water are also suitable.
The alkali source of hydro~yl ions (OH) is then added to the solution. The hydro~yl ion is preferably added in combination with ammonium ions through the addition of ammonium hydro~ide (NH4OH). The source of hydroxyl ions should be present in the stripping ~olution in a concentration of at least 0.29 mole/L
and prefeIably b~tween .29 mole/L and 3.23 mole~
An acid must also be present in the solution at a ' concentration of 0.026 mole/L a~d 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 car~oxyl or carbo~yl-hyarogyl group acid such as lactic acid, o~alic acid, tartaric acid, formic -.

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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 an~ 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 bet~7een 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 ~ach 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 concentratio~ and/or to adjust the pH of the solution.
The ef~ectiveness of the stripping solution of the present invention is det:ermined by the ef~iciency in which the titanium compound coating is removed from the substrate without suf~ering any deleterious effect on the ~ase metal. A minimum stripping efficiency of 1 ~ 10~2g~cm2/L and preferably above 2 ~ 10~2g/~m2/~ 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.

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E~periments were conducted using numerous aqueous compositions all containing various proportions of hydrogen pero~ide, an acid and an alkali source of hydro~yl ions. The following tables I, II, III and IV identify the different solution compositions all of which had no deleterious e~fect on the base metal. All of the tests shown in the Tables 1, II, III and lV were carried out by i~mersing a TiN coated Inconel 71B* coupon (1.5 ~ 25 ~ 50 mm) into the test stripping solution at between 60 and 85~ C~
Tabl e I
Effect of Citr;c Acid Content (H3C6H507~ on Stripping Efficiency Stri ppi r~g --Composi ti on Mol etE-_ Ef f i ci ency Solution H20 Hzo2HH40H H3C6HS~7 pH ~10 2 g/c~2/L) , 1 bal. 1.321.09 0 10 0.38 .
: - -2 ba1. 1.321.09 O.û5 10 3.1 .

3 bal. 1.321.09 0.10 10 3.4 4 ~al. 1.321.0g 0.16 10 3.8 tal. 1.32 l.û9 0.21 lû 4.0 ~ 6 bal. 1.32 l.û9 0.~6 10 4.1 - 7 bal. 1.321.09 û.42 9 5.7 8 bal. 1.321.09 0.59 9 4.4 9 bal. 1.321.09 û.73 ~.5 2.0 ~ Inconel 718 is a registered trademark of the Internatonal Nickel Corporation. .

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_ 7 ~ 9 ~ 4 Table I ~hould 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 ef~iciency 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 cit~ic acid provided the pH level is above 8.5.
The concentration of hydrogen pero~ide and the alkali componen~ were held constant. It was determined from e~perimentation that the presence of a minimum concentration of acid was necessary to stabilize the solution and to permit the stripping efficiency to e~ceed the minimum level. The concentration ~f citric acid should be above at least about 0.026 mole/L and pre~erably above O.O!j2 mole~L. The m~imllm concentration of citric acid is appro~imately 0.76 mole/L, Upon exceeding thle maximum concentration the pH of the solution drops to below 8.5 which reduces the stripping efficiency below the effective minimum level.

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Table II
, Effect oE NH4~H Content on Stripping Efficiency : .
. StrippingComposition ~ole~L E~ficienc~
Solut~on ~2~ ~2~2 NH40H ~3C6~5~7 p~ (10 2 g/cm /L) .. -bal. 1.3Z 0 0.16 2 0.39 11 b~l. 1.32 0.37 0.16 10 300 ' 4 bal. 1.32 1.09 0.16 10 3.8 . 12 bal. 1.32 1.46 O.lS 10 4.2 :~ 13 bal. 1.32 1.80 0.16 lO 4.0 ,. . .
.; 14 bal. 1.32 2.51 0.16 ll 5.3 , ~ . .
~ 15 bal. 1.32 3.23 0.16 11 5.1 ~ ,. .
Table II should be read in conjunction with Figure 2 which is basea on the data of Table II ~and -shows the effect of varying the concentration of ammonium hydroxide (NH40H) in the stripping solution. Ammonium hydroxide i.s the preferred alkali source. The concentration level of citric acid and pero~ide were held constant wh:;le adjusting the concentration of NH40H. From Table II and Figure 2 it is apparent that the stripping solution does not function effectively until the concentration of NH40H
is raised to a minimum level of about 0.29 molefL at a pH of 8 or higher. The 1atte~ was confirmed by the data show~ in Table IV as will be discussed in greater detail later in the specification.

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Effect of H20z Content on Stripping Efficiency Stripping Composition Mole/LEfficienc~
Solution 1120 ~2~2 NH40~ ~3C6~507 P}~ ~10 2 glcm IL) 16 bal. 0.44 1.0~ 0.16 9 1.9 17 bal. 0.88 1.09 0.16 9 3.6 4 bal. 1.32 1.09 0.16 10 3.8 18 bal. 2.65 1.09 0.16 10 6.3 19 bal. 4.41 1.09 0.16 lO 6.9 bal. 2.65 2.17 0.16 11 6.2 Table III should be read in conjunction with Figure 3 from which it is apparent that the stripping ef~iciency directly increases with increasing concentrations of hydrogen pero~ide up to about 2.94 mole/L at which concentration the stripping ef~ici~ncy of the solution levels off. Accordingly, although the hydrogen pero~ide concentration may be further increased the ma~imum level should be about 9.71 mole/~ above which, ~or practical considerations, ~here is a negati~e incentive to further raise the hydrogen peroxide concentration.
The m;nimum concentration of hydrogen peroxide is about ~.29 mole/~ and preferably abo~e 0.59 mole/~.
Typically the temperature of the solution has an influence on the stripping rate and efficiency. The reactivity o~ the solution increases with increasing operating temperature and the solution life decreases with increasing operation ':

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temperature. Accordingly, to determine the optimum solution temperature two test solutions were pxepared using a different pero~ide 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 ~he two test solutions were as follows:
Solution 12. 1.32 mole/L H202 + 1.46 mole/L
NH40H ~ 0.16 mole~L H3C6 H~07 balance water (marked "0~ in Figure 4).

Solution 4. 1.32 mole/L H202 ~ 1.09 mole/L
NH~OH + 0.16 mole/L H3C6 H507 balance water (marked ~" in Figure 4).

The stripping rate is e~pressed in terms of the total weight loss ~in grams) of the coating per unit area (in cm2) per unit ~olume (in liters) per unit time (in minutes). As shown in Figure 4 the optimum stripping rate is realized at a solution temperature e~ceeding 50~C and prefera~ly between 60~C and 85~C.
Although the optimum solution temperature is above SO~C the solution may be. operated at a temperature within a wide range e~tenaing from about 2S~C to about 9S~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 H20 ~ 1.32 mole/L H~O~ ~ 1.09 mole/L NH40H + 0.16 mole/L citric acid was used to develop the plot. The solution active life was found to decrease e~ponentially with increasing temperature from about 1000 minutes at 25~C to about 2~ minutes ,, , ,. . . .. . . .. .. .
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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 S 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 6D~C -80OC 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.

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- }3 -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 NH40H do not produce effective stripping efficiencies unless combined with NH4OH or another source of hydro~yl ions such as-NaOH.
However, it is clear from all of the test data that NH40H is the preferred alkali 60urce~ The ef~ective concentration for the three critical components, - viz., a source of hydrogen pero~ide, an alkali source of hydro~yl ions and acid is 0.29 mole~L to ~.71 mole~L, 0.29 mole~L to 3.23 mole/L and 0.026 mole~L
to 0.76 moleJL, respectively. For the preferred components H2O2; NH40H and citric acid the preferred concentration is 0.59 mole/L to 4.71 mole/L, 0.~7 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 71~ other coupons including TiN
coated st~inless steels such as AISI440C and AISI
17-4 PH and alloy steels such as M50, M50NIL and Pyrowear 53 were tested using the preferred ~tripping solution. All demonstrated similar behavior to the ~ TiN coated Inconel 718 coupon~ with no deleterious ; . effect on the base metal.
~ he hydrogen pero~ide component in the stripping solution of the present invention may be generated in situ from any source of pero~ide which : dissociates in water to form hydrogen pero~ide such as a perborate, e.g. sodium perborate tetrahydrate ~NaBO3-9H2O) or any other know pero~ide compound :: which will readily dissociate into hydrogen peroside , D-16302-1 ~
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in the presence of water at atmospheric pressure and within the operating temperatures of the present invention. Ammonium pero~ydisulfate ((NH4)2S208) is not a suitable source of hydxogen pero~ide for the :
present invention as is evident from the following Table V despite the fact that ammonium pero~ydisulfate is used to commercially produce hydrogen pero~ide by hydrolysis at r~duced pressure and ~levated temperature.
In accordance with the following Table Y TiN
coated Inconel.718 coupons ~1.5x25x50 mm) were immersed into separate pero~ide containing solutions with a specified pH of a~ove 8 and at temperatures of between 60~C and 6S~C to evaluate the stripping effectiveness of the solutions with the different sources of pero~ide.

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As is evident from the above table no ~-stripping action was obser~ed 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 o~ 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 pero~ide.
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 CH30H 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 (1.5~25s50 mm in size) were immersed into the solution No. 36 and only 410 stainless steel e~hibited 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 con~entration of acid should be determined according to the substrate material used.

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_ Composition (Hole/L)- Substrate Solution HzO H2~2 HH40H tlC1 Mater;al Comments 37 Bal. 1.32 l.û9 O.lZ ~50 Steel P;tt;ng corrosiDn attack 38 Bal. 1.32 l.û9 0.35 410 SS P;tt;ng corrosion attack :~ 39 Bal. 1.3Z 1.09 û.35 Inconel 718 No corrDsion attack Oal. 1.32 1.09 1.16 Inconel 718 Pitt;ng corrosion ~ttae.

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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 ~tripping 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 de~ined 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.