CA1185152A - Selective chemical removal of hard surface coatings from superalloy substrates - Google Patents

Abstract

SELECTIVE CHEMICAL REMOVAL OF HARD SURFACE
COATINGS FROM SUPPERALLOY SUBSTRATES
ABSTRACT

This invention consists of a chemical stripping composi-tion comprising an aqueous solution of H2SO4, a nitro-substituted aromatic compound, a fluoride ion producing species, and a surfac-tant, together with an accompanying method wherein the composition can selectively remove hard surface layers deposited through flame, plasma spray, and detonation gun techniques on superalloy substr-ates by contacting the above stripping solution in a heated bath wherein the bath is continuously agitated by an ultrasonic agitation means.
BACKGROUND OF THE INVENTION
This invention is related to the removal of ceramic, cermet and metallic coatings from high strength, high temperature re-sistant metal substrates and more particularly, to the removal of these hard surface coatings from superalloys, particularly nickel-base substrates by use of an aqueous acidic stripper bath equipped with a high energy density agitation means.
One of the most difficult requirements in the chemical stripping art has been to develop a composition which will chemi-cally remove the "hard surface" coatings, that is, molten particle coatings which are applied by detonation gun, plasma, and flame spray methods onto metal surfaces, such as the nickel-base jet engine combustion chambers. Such coatings, which are usually applied in thin layers to provide protection against high temper-ature oxidative gas streams, include heat resistant ceramics, cermets, and nickel-base mixtures.

Description

S~2 SELECTIVE CHEMICAL REMOVAL OF HAR~ SURFACE
COATINGS FROM SUPERALLOY SUBSTRATES
ABSTRACT

l'his invention consists of a chemical stripplng composi-tion comprising an aqueous solution o:E H2SO4, a nitro-substituted aromatic compound, a fluoride ion producing species, and a surfac-tant, together with an accompanying method wherein the composition can selecti.vely remove hard surface layers deposited through Elame, plasma spray, and detonation gun techniques on superalloy substr-ates by contacting the above stripping solution in a heated bath wherein the bath is continuously agitated by an ultrasonic agitation means.
BACKGROUND OF T~E INVENTION
This invention is related to the removal of ceramic, cermet and metallic coatings from high strength, high temperature re-sistant metal substrates and more particularly, to the removal of these hard surface coatings from superalloys, particularly nickel-base substrates by use of an aqueous acidlc stripper bath equi.ppedwith a high energy density agitation means.
One of the most difficult requi:rements in th~ chemical.
stripping art has been to develop a composition w:hich will chemi-cally remove the "hard surface" coatings, that is, molten parti.cle coatings which are appl.ted by detonation cJUn, plasma, and :Elame spray methods onto metal surfaces, such as the nickel-base jet engine combustion chambers. Such coatings, which are usually applied in thin layers to provide protection against high temper-ature oxidative gas streams, include heat resistant ceramics, cermets, and nickel-base mixtures.

,~
.

~he Prior Art has lacked an efficient ch~mical stripper which can quickly and selectively remove such coatings without damaging the underlying substrateO Currently used techniques feature mechanical methods such as machining, grinding and S abrasive blasting the coating, and non-mechanical methods such as chemical imrnersion in solutions which only soften a ceramic coating, e.g. hydrochloric acid base solutions, and the use o:E
molten salt baths. ~Iowever, mechanical me-thods are very -time consuming, and adclitionally can easily damage a very expensive piece of equipment, due to the ease in creatiny extensive dimen-sional damage on the metal substrate, which is frequently used in a "high techno:Logy" application. Although current chemica:L
methods will soften certain ceramic coatings, subsequent mech-anical removal of the ceramic, along with the underlying inner bond coating is still re~uired. Molten salt baths are ha7.ardous to operate, and are effective on only a limited number of coating substrate combina-tions. Also these techniques are all deficient in their speed of removal, coas-t of operation, ancl effectiveness of result~ U.S. Patent 2,698,781 discl.oses sul.:Eu.r:i.c acid solutions containing mitroaromatic additives wh:ich are ~E:Eect.ive in removing oxide scale and electroplated nickel f:roTn copper substrates. However, there is no teaching in~err:ing -that ceramic, cermet or hard surface rnetallic coa-tings might be chemically removable from a nickel base subs-trate.
OBJECTS OF THE INVENTION
It is an object of this invention to create an effective composition and accompanying method for the removal of hard surface coatings from high strength, high temperature resistant alloy substrates.

~Q~S~

It is another object of this invention to crea-te an e:E:Eective composition and accompanying method for the selective, non-corrosive removal of hard, high temperature resistant ceramics, cermets and nic~cel-base bond coats from nickel-base substrates.
It is still another object of -this invention to clevise a method of selectively stri.pping only the hard surEace coating :Erom je-t engine component metal surfaces without dimensional clamacJe to the stripped me-tal surfaces.
.~0 SU~M RY O~ THE INVENTION

These an other objec-ts o:E -the invention have been accomplished by -the discovery of a novel composition a:ncl accompanying method for the selec-tive removal o:E harcl sur:Eace eoatings particularly hea-t resi.stant ceramic, cerme-ts and nickel.-base eoats, :Erom high strength, high tempe:ra-ture resis-tant metal subst:l^ates, in partieular nic]~el-base substra-tes. The stripping eompos.ition eomprises ~12SO~/ preferably abou-t 90-~50 grams/liter (g/l) oE coneentrated ll2SO~, e.g., 66Be'@ 1.8~ spee~ grav., most pr~:Eerably about 250 300 g/l; abou-t 20 g/l to saturatio.rl o.E a ~() watc~:r so:lu~le, n.it:ro-substituted aroma-tic compound, and most pre:Eerably about ].00-120 g/l o:E meta-ni-trobenzene socliurrl sul.:Eo--r~ate; about 0-70 g/l o:E a wate.r solub:l.e, :E:Luorine-containinc3 speeies, most preferably abou-t l.G-20 g/l oE fluoroboric acid;
about 0-1.0 g/l of a surfactant, most preferably about 0-.5 g/l of one of the group of diphenylether sulfonates; the rema:inder being water in amount of about 2-90 wt.-6 of the solution. The stripping method of the invention involves contac-ting -the hard surface eoatings, usually a heat resis-tant ceramic, cerme-t, or nickel-base coating on -the me-tal subs-trate with -the acidic stripping solu-tion desc.ribed above, pre:Eerably comprising sulfuric acid, a soluble fluorine- containing species, a soluble nitro-substituted aromatic compound, a sur:Eactan-t and water, and continuing the contacting un-til the hard surEace coating has been selectively removed from -the metal. surface.
The eontaeting is earried ou-t by immersing -the deposi-ted substrate in the solution bath which is ]cep-t at about 120-1~0 L0 .rl'~, most preferably about 130-150:~., as -the ba-th is ac-tively ayitated by an ultrasonic ac3ita-tion means, mos-t~preferably al~
adequately powered ultrasonic generator--transducer, to a power density of at least about 4wa-tts/in~, and pre:Eerably about 7--~
watts/in . Upon eomplete coating removal, the substrate is removed from the so]ution, :rinsed, and is in condition :Eor :Eurther proeesslng~
DESCRIPTION OF` THE INVENTION
. . _ ~ well known -technique, particularly :in the "h:i.gh tecllno:l.ocJy"
lnclustr:ies involvinc~ the construct:ion o:E jet encJ:ines~ arl(l gas

2~ tu.rb:ine encJines w:hich employ superalloys -that are use:Eul, :Eor e~amp:le, in aeronautica:L applicat:i.ons, is to coat the metaL
su.rEaees wh:ich wil:L be exposed to severe hi~h temperature oxi-dation environments with a thin, i.e., a few mils, coa-tings of a protective metallic or nonmetallie layer. These eoatings, commonly referred to as "hard surface" coatings, can be any one of a number of high s-trength heat and corrosion resis-tant substanees, and usually include a "nickel-base", hereinaf-ter defined as "a metallic mixture rich in nickel which can func-tion as both a bonding coat for a protective ceramic layer, or, the protective layer i-tself"; a ceramic substance, or a cerme-t, as ~5~2 well as certain mixtures of these substances. One particularly hard coating which is Erequently used on aircraft combustion chambers as a protective coating is a three layer system, which has the following composi-tion:

Bond Coat: 3-5 mils, Ni 80~, Cr 15%

Intermediate Coat: 4-6 mils, 35-65~ mixture of Ni 8n%-Cr 20% and MgO-ZrO2 Outer Coat: 6-10 mils Mg0-Zr02.

:l.0 Other hard surface coatings which are finding application in -jet engine combustion chambers and are oE particular note are:
A: Co 23, Cr 18, Al 12, Yo 5, Ni balance;
B: Y stabilized zirconia;
C: Co Cr Al Y;
D: Ni -~ Cr 94, Al 6, Y 1.
These hard surface coatinys can be stripped from a varie-ty of Metal substrates which, in the broadest embodiment of the invention, can be of any metal which is resistant to acid or al]caline corrosion, and more specifically, metal superal:loys ~hich have both high temperature resistance and high streng-th at these elevated temperatures. In par-ticular, alloys locatecl in the VII[ A Group on the Periodic Chart, i.e., iron, nicl~el and cobalt, and particularly nickel-base alloys, such as -the well known "Hastelloy X" are stripped of -their coatings by -the composition of the invention. The coatings are applied to -the substrates through a variety of well known -techniques in the art, e.g., detonation gun, plasma spray, and flame spray particle applications, which all essentially consist of grinding the compositions into a fine powder, heating the powder up into its molten :Eorm, and spraying the mol-ten material, which has been accelerated in a gas plasma to high speeds, onto -the substrate to be coated in -the :Eorm of a Eine, very -thin metal mist, which hardens to form an ex-tremely tough, pro-tec-tive coa-ting.
This invention is further directed to an improved composition and accompanyi.ng method of removal of these coa-tings which have been appl.ied in the above-described manner. In order for a strong chemical stripper to function effec-tively seve.ral problems must :Eirs-t be overcome. The stripp:Lng eomposi-tion must be able to strip a-t a satisfactory rate, e.g., of the o.rder of 1/2-2 mi.ls per/hour, and must be carefully ventilated so as to remove toxi.c vapors resulting from the process, which is always a major consideration when dealing with chemicals in thi.s art, bo-th in the lab in their industrial environments and applications. Such a solutlon has been :Eound by creating an aqueous acidic bath llav:ing the following composition: Sulfuric acid, H2SO4, preferably in :its concentrated ~orm, e.g., 66 Be', al-though any commerc:La:Lly available brand is satis:cactory~ is an essentia] co:rlsti.tl.ler-lt ol the solu-tion. The concentrat:ion of ll2SO4 shoulcl:range l~el:weell abo~lt .~)~-450 g/:L, ancl pre:Eerably be-tween about 250-300 g/l.. rrOO
.litt:l.e aeicl will not qive the ba-th -the necessary acLd st:rencJtll ko .remove the coatings a-t a suitable rate, wh:Lle amounts :Ln e~cess oE 450 g/:L do not provide additional superior s-tripping results and eannot be economically ~ustified; however, in the -theory, H2SO4 eould eonsist of as mueh as 90% of the solution.
A seeond, essenti.al element in the stripping solution, functioning as an oxidizing agent, is, in the broadest embodiment of the invention, a water soluble nitro-substituted aroma-tic compound, or, more preferably, a water soluble ni-tro-substituted benzene eompound such as -the class o:E compounds disclosed in U.S. Patent 2,698,781. Mos-t preferably, meta-nitrobenzene sodium sulfonate or sulfoni.c acid, or -the like, e.g., any suitable alkali or alkaline earth me-tal ni-trobezene sulfonate, including the ammonium radieal as a :cunetional equivalent thereof, will fune-tion effeetively. ~'his class oF
eheml.eals is believed to funetion in solution as a kine-tic aeeele:rator, al-thoucJh applieants' do no-t wish to be bound by :L0 Iheory.
~ he eoneentration of the nitro subsituted aroma-t:ie eonsti-tuen-t ean range from about 20 g/l to satura-tion, ancl should range between about 20-120 g/1, with the higher levels, e.g., 60-120 g/l, being the mos-t preferred.
Another eonstituent of the bath whieh although not essentia:L
is hic~hly pre:Ee.rred, :is the addition of a soluble :Eluorine-eontain~ cJ speeies, most preferably fluoroborie aeid, H:BF~, but other fluorine-eontaining speeies, e.g., HF, NH~HF2, Na3~:L.~i'6 et al that ean disassoeiate in ac~ueous solu-tion to gene.r.ate .Lc-~w ;E:I.uor:i.de ion eoneent:rations, are pa:rtieu:lar:ly suitclbLe. 'l'he eoneellt:rat.l.on o:E the :Eluor:ine-eontaining spee:ies is cle-termi.ned bY l10W mueh of i-t :is required to produee a desired co~centxclt:ion o.E :Eluor:ide ions :in solution; and gellerally wl.ll range between about 0-70 g/l, and preferably, about 10-20 g/l for substances whieh highly disassoeiated in aqueous solution. As -the coneen-t-ration of fluoride ions increase, -the chance of harmful corrosion to the substrate ri.ses, creating an upper limit barrier on the amount which ean be added to the stripping solution.
It is preferred, although not essential, to inelude a small amount, e.g., 0-3.0 g/l and most preferably about 0-1.0 g/l of ,~, ~ i ..

a sur~actant to func-tion as a wetting agent on the surEace to be dissolved. Compounds which are particularly pre~erred for -this task are -the diphenylether sulfonates, e.g., compounds having the generic ~ormula ~ ~ - O ~ -R

where l~ is an alk~l group or hydrogen, and X is an alkali metal, such as sodium. Such a product is curren-tly sold under -the trade name "Dowfax 3B2", which is a trademark owned by the Dow Chemical Company, and whicll is marketed by -the Dow Chemical Co., Midland, Michigan. Most alkali me-tal diphenylether sulEona-tes are suitable equivalents, and, in the broadest embodiment of the invention/ any surEactant which possesses hydrolytic and oxidative s-tability toward the stripping solution is an accept-able substitute. The stripping composition is comp:Leted byadding water, in amounts ranging Erom 2-90wt.o, to br-ing the solution to the desired strength. Water lowers the v:isco5:i ty o~ the strlpping solution and increases the mobil:ity oE ion:ic spec.ies in solution.
The add:ition of water to sulfuric acid has the des:ireab:Le eLEect of lowering the solution cavitation threshold, i.e., -the minimum power required to reduce the local pressure on a liquid to a value less than its vapor pressure. This arises through an increase in vapor pressure and decrease in viscosity and density upon dilution. For room tempera-ture wa-ter at 20 KHz, the caviation threshold is 2.45 ~7/in .

The method of this invention involves contacting the hard surface coating with the acidic stxipping solution, and continuing the contacting until the hard coating has been substantially removed from the metal substra~e without any dimensional change occurring to ~he underlying surface? In order for the strippillg to occur at an accep~able ra~e, it is essential that the solution bath be continually agitated by a suitable agitation means, and preferably by an ultrasonic gener-.
ator-transducer, either a magnetostrict:ive (preferred) or a piezoelectric transducer. The transducer should operate .in such a manner so as to produce a minimal power density of about 4 watt.s/in2, and preferably about 7-8 watts/in2. 5t.irring by conventional techniques simply will not produce the required solution agitation in the stripper hath needed to produce an effective rate. Coupled with the ultrasonic agitator, which inherently w.ill sllpply energy to the solution it is necessary to keep the solution at a temperature range of about llO-180F.
in order to produce an acceptable rate of st.ripping, and most prefer~bly about 130-150F. during operat.ion.
The .~olling examples serve to furthex illustrate the illvention:

An aircxaEt combustor used in jet engilles is made of Hastelloy-X (22 Cr, 18.5 Fe, 9.0 Mo, 1.5 Co, 0.5 W, balanc* Ni) and plasma flame sprayed with a triple layer coating over 520 in2 3 comprised of a bond coat, 3-5 mils thickness~ Ni/Cr 95% inter-mediate coat, 4-6 mils, 35 36% mixture of Ni 80% -Cr 20% and MgO-ZrO2; outer coat, 6-10 mils, MgO-ZrO2. Two of ~hese combustion can chambers where stripped of their coatings i.n 10 and 16 hours, using ultrasonic agitation in a bath kep-t at a temperature range of 120-170F. A 30 gallon s-tainless steel hot water tank containing two 1200 watt side mounted immersible ultrasonic transducers was employed during the stripping operation and 6 gallons of the stripping solu-tion (275 g/l H2SO4 66 Be', 14 g/l HBF4 (48%), 120 g/l M-nitrobenzene sodium sulfonate, 0.1 g/l Dowfax 3B2) contained in a square polypropylene container was used. Metallagraphic examina-tion failed to show any substrate degradation of the combustion can chambers.

A military jet engine combustion can chamber was immersed in a stripping solution containing 275 grams/liter concentrated sulfuric acid H2SO4 (66Be'), 145 grams/liter fluoroboric acid, HBF4 (48~), and 116 grams/liter meta-nitrobenzene sodium sulfonate. After 20 hours at 130-170F., the three layer coating had been removed from the burner can. A 5 gallon stainless s-teel ~an]c, fitted with a 1000 watt bottom-mounted ultrasonic transducer, containing 2 gallons of the stripping solution, in a square polypropylene container was used. No substrate degraclation was observed.

A Hastelloy-X coupon with Metco 443 (Ni/Cr 93, 6 Al), a product of Metco Inc., Westbury, N.Y., was immersed into a stripp:ing solution containing 90 grams/liter concentrated sulfuric acid H2SO4 (66Be'), 72 grams/liter meta-nitrobenzene sodium sulEonate, 46 grams/liter sodium sulfate Na2SO4, and 1.2 grams/liter ammonium thiocyanate NH4SCN.
After 45 minutes with ultrasonic agitation a-t 145-15~ F.
the several mil thic]c coating had been removed from the coupon.

.~

This lnvention is capable of removing no-t only ceramics, cerme-ts and nickel-base substances from metals, particularl,y nic]cel-base supera]loy subs-trates, wi-thout causing unacceptable dimensional changes or s-tructural changes -to -the base metal, but also of removing a wide variety of hard surface coa-tings from these substrates without damage to the underlying material.
Obvi,ously, numerous modifications and variations of -the present invention are possible in light of the above teachings.

:[t is therefore to be understood that within the scope of the ,1.0 appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A composition for the selective removal of hard surface metal coatings, selected from the group consisting of ceramic, cermet and nickel-base mixtures, from high strength, high temperature resistant substrates, comprising:
about 90-450 g/l of H2SO4;
about 60-240 g/l of a water soluble nitro-substituted aromatic compound;
the remaining being water.

2. A composition as claimed in claim 1 wherein the solution further includes about 0-70 g/l of a water soluble fluorine-containing species which disassociates to provide a source of fluoride ions in solution.

3. A composition as claimed in claim 2 wherein the solution further includes about 0-3.0 g/l of a surfactant selected from the class of diphenylether sulfonates.

4. A composition for the selective removal of hard surface metal coatings, selected from the group consisting of ceramic cermet and nickel-base mixtures, from high strength, high temperature resistant substrates, wherein the composition comprises:
about 250-300 g/l of H2SO4;
about 60-120 g/l of a water soluble nitro-substituted benzene compound;
about 10-20 g/l of a water soluble fluorine-containing species which disassociates to provide a source of fluoride ions in solution;
about 0-1.0 g/l of a surfactant selected from the class of diphenylether sulfonates;
the remainder being water.

5. A composition as claimed in claim 4 wherein the composition comprises about 270-280 g/l of H2SO4.

6. A composition as claimed in claim 4 wherein the water soluble nitro-substituted benzene compound is about 100-120 g/l of meta-nitrobenzene sodium sulfonate.

7. A composition as claimed in claim A wherein the water soluble fluorine-containing species is about 10-20 g/l of fluoro-boric acid.

8. A composition as claimed in claim 4 wherein the diphenylether sulfonate is about 0-.5 g/l of sodium dipheny-lether sulfonate.

9. A method for the selective removal of hard surface metal coatings, selected from the group consisting of ceramic, cermet and nickel-base mixtures from high strength, high temperature resistant substrates comprising contacting the hard surface coating with an acidic stripping solution as claimed in claim 1, in a suitable container means which is kept at a solution temperature of about 120-180°F. and is continually agitated by an ultrasonic agitation means;
continuing the contacting until the hard surface coating has been substantially removed from the metal substrate without damaging the underlying substrate;
removing the substrate from the solution bath.

10. A method as claimed in claim 9 wherein the temperature of the solution is kept at about 130-150°F. during operation.

11. A method as claimed in claim 9 wherein the ultrasonic agitation means is an ultrasonic generator-transducer which continually supplies a power density to the solution of at least 4 watts/in2 during operation.

12. A method as claimed in claim 11 wherein the transducer is kept at a power density of about 7-8 watts/in2.

13. A method as claimed in claim 9 wherein the composition of the solution bath is that claimed in claim 4.

14. A composition as claimed in claim 1 wherein the aromatic compound is a meta-nitrobenzene sodium sulfonate.