JPS6261670B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the removal of ceramic, celmet, and metal coatings from high strength, high temperature metal substrates, and more particularly, to the removal of ceramic, celmet, and metal coatings from high strength, high temperature metal substrates using an aqueous acidic stripping bath equipped with high energy density agitation means. The present invention relates to the removal of these hard surface coatings from superalloys, particularly nickel-based substrates.

One of the most difficult requirements for chemical stripping techniques is the application of "hard surface" coatings to nickel-based metal surfaces, such as jet engine combustion chambers, by detonation guns, plasma, and flame spray methods. The objective was to develop a composition that chemically removes fused particle coatings. Such coatings, usually applied as thin layers to provide protection against hot oxidizing gas flows, include heat-resistant ceramics, Celmet and nickel-based mixtures.

The prior art lacks an efficient chemical stripper that can quickly and selectively remove such coatings without damaging the underlying substrate. The techniques used so far are characterized by mechanical methods such as machining, polishing, and blasting of the coating, as well as chemical immersion in solutions that only soften the ceramic coating, such as hydrochloric acid, and molten salt baths. There are non-mechanical methods such as use. However, mechanical methods are very time consuming and, moreover, are prone to large dimensional deviations in the metal substrates frequently used in "high technology" applications, damaging very expensive parts of the equipment. easy. Although conventional chemical methods soften some ceramic coatings, it is still necessary to subsequently mechanically remove the ceramic together with the underlying inner adhesive coating. Molten salt baths are cumbersome to operate and are only effective for a limited number of coating and substrate combinations. Also, these techniques are all deficient in removal speed, operating cost, and effectiveness of results. U.S. Pat. No. 2,698,781 discloses a sulfuric acid solution containing nitroaromatic additives effective in removing oxide scale and electrodeposited nickel from copper substrates. However, ceramic, Celmet or metal hard surface coatings are not effective for chemical removal from nickel-based substrates.

It is an object of the present invention to create an effective composition and associated method for removing hard surface coatings from high strength, high temperature resistant alloy substrates.

Another object of the present invention is to provide compositions and compositions effective for selectively and non-corrosively removing deposited hard high temperature ceramic, Celmet and nickel based coatings from nickel based substrates. The goal is to create a method to accompany this.

Another object of the present invention is to devise a method for selectively stripping hard surface coatings from the metal surfaces of jet engine components without causing dimensional distortion of the metal surfaces undergoing the stripping process.

These and other objects of the present invention are to provide a new and novel method for selectively removing hard surface coatings, particularly high temperature ceramic, Celmet and nickel based coatings from high strength high temperature resistant metal substrates, particularly nickel based substrates. was completed by discovering a unique composition and associated method. The stripping composition comprises H ₂ SO ₄ , preferably about 90-450 g/concentrated H ₂ SO ₄ , e.g.
゜Be′δ specific gravity 1.84 (66゜Be′＠1.84Spec.grav.),
However, preferably about 250-300g/; about 20g/
or saturated water-soluble nitro-substituted aromatic compound, and preferably about 100-120 g/
about 0-70 g of a water-soluble fluorine-containing species, and most preferably about 10-20 g of fluoroboric acid; about 0
- 1.0 g/g of surfactant, and preferably about 0-0.5 g/diphenyl ether sulfonate; the remainder being about 2-90% by weight of the solution.
consists of an amount of water.

The stripping method of the present invention involves removing hard surface coatings on metal substrates, typically heat-resistant ceramic, Celmet or nickel-based coatings, using an acidic stripping solution as described above, preferably sulfuric acid, soluble fluorine-containing species, soluble nitro-substituted aromatic compounds. , a solution consisting of a surfactant and water, and this contact is continued until the hard surface coating is selectively removed from the metal surface. The contacting is carried out by placing the coated substrate in a solution bath maintained at a temperature of about 48-83°C (120-180°), preferably about 54-66°C (130-150°), while the bath is heated by ultrasonic agitation means. Preferably, a fully powered ultrasound generating transducer produces at least about 0.62 watts/cm ² (about 4 watts/cm 2 ).
in ² ), preferably about 1.08 to 1.25 watts/cm ² (about 7
This is done by immersion while being actively stirred at a power density of -8 watts/in ² ). After the coating has been completely removed, the substrate is removed from the solution and rinsed to make it suitable for further processing.

Well-known technology in the "advanced technology" industry, particularly related to the construction of gas turbine engines using superalloys useful in jet engines and aeronautical applications, involves the use of metal surfaces that are exposed to harsh, high-temperature, oxidizing environments. Coating with a thin (several mil) metallic or non-metallic protective layer is necessary. These coatings are commonly referred to as "hard surface" coatings and may be any of a number of high strength, heat and corrosion resistant materials and are typically described below as either an adhesive coat to the protective ceramic layer or as the protective layer itself. ``Nickel-based'', defined as ``a nickel-rich metal mixture that can also work'';
"Ceramic materials" or "Celmets" as well as some mixtures of these materials can be used. A particularly hard coating frequently used as a protective layer in aircraft combustion chambers is a three-layer system and has the following composition:

Adhesive coat: approx. 0.07-0.13m/m (3-5mil),
Ni80%, Cr15% Intermediate coat: approx. 0.10-0.16m/m (4-6mil),
Ni80%-Cr20% and MgO- _ZrO2
35-65% mixture outer coat: approx. 0.15-0.26m/m (6-10mil),
The hard surface coatings that have begun to be applied to the combustion chambers of MgO-ZrO ₂ jet engines are notable as follows.

A: Co23, Cr18, Al12, Yo5, Ni balance B: Y stabilized zirconia C: Co Cr Al Y D: Ni + Cr94, Al6, Y1 These hard surface coatings can be applied to a variety of metal substrates (in the broadest embodiment of the invention). can be any metal that is resistant to acid or alkali corrosion, and more particularly can be exfoliated from metal superalloys that have both high temperature resistance and high strength at high temperatures.
In particular, alloys of metals in the group of the periodic table, namely iron, nickel and cobalt, especially nickel-based alloys such as the well-known "Hastelloy Then it will peel off. The coating can be applied using various well-known techniques such as detonation gun,
It is applied to the substrate by plasma atomization and flame-blown particle application. All of these well-known application techniques essentially involve grinding a composition into a fine powder, heating this powder until it is in a molten state, and then turning the molten material, accelerated in a gas plasma, into a fine, very dilute spraying on the substrate to be coated in the form of a metallic mist;
It consists of the metal mist curing to form an extremely tough protective coating.

The present invention is further directed to improved compositions and associated methods for removing these coatings applied as described above.
In order for powerful chemical strippers to work effectively, several problems must first be overcome. The stripping composition must be capable of stripping at a satisfactory rate, e.g., on the order of about 0.012-0.051 mm/hour (1/2-2 mil/hour), and must be free of toxic gases produced by practicing the process. Must be carefully ventilated to remove. This ventilation is always a major consideration in both laboratory and factory environments and applications. Such a solution was found by creating an aqueous acidic bath with the following composition: Thus, sulfuric acid H ₂ SO ₄ (any commercial grade is sufficient, but preferably concentrated, eg 66Be'.) is an essential component of the solution.
The concentration of H ₂ SO ₄ is about 90-450 g/, preferably about
It is in the range of 250-300g/. Too little acid will not provide the bath with the acid strength necessary to remove the coating at a reasonable rate, while using more than 450 g/l will not provide additional good stripping results and will not be economical. can't be justified either. However, in theory H ₂ SO ₄ may make up up to 90% of the solution.

The second essential component that functions as an oxidizing agent in the stripping solution is, in the broadest embodiment of the invention, a water-soluble nitro-substituted aromatic compound, more particularly such as the class of compounds disclosed in U.S. Pat. No. 2,698,781. It is a water-soluble nitro-substituted benzene compound.
Most preferably, sodium metanitrobenzenesulfonic acid or -sulfonic acid, for example, a suitable alkali metal or alkaline earth metal salt of nitrobenzenesulfonic acid (containing an ammonium group as a functional equivalent) are useful. It is believed that this class of drugs functions as reaction accelerators in solution, but is not bound by this theory.

The concentration of nitro-substituted aromatic compound may range from about 20 g/ to saturation, with about 20-120 g/, with higher concentrations such as 60-120 g/ being preferred.

Add as a third essential bath component a soluble fluorine-containing species, most preferably fluoroboric acid _HBF4 , but other fluorine-containing species that can dissociate in aqueous solution to generate low concentrations of fluorine ions. , for example HF, NH ₄ HF ₂ , Na ₃ AlF ₆ and the like are particularly suitable. The concentration of the fluorine-containing species is determined by the amount required to produce the desired concentration of fluoride ions in solution, but is generally less than about 70 g/g, preferably for substances that are highly dissociated in aqueous solution. Approximately 10−
It may be in the range of 20g/. As the fluorine ion concentration increases, the chance of harmful corrosion to the substrate increases, and there is an upper limit to the amount of fluorine ion that can be added to the stripping solution.

Although not essential, a small amount, e.g. 0-3.0g/
Preferably, about 0-1.0 g/g of surfactant is included to act as a wetting agent to the surface to be dissolved. Particularly preferred compounds for this purpose are diphenyl ether sulfonates, such as the general formula (In the formula, R represents an alkyl group or a hydrogen atom, and X represents an alkali metal such as sodium.) Such a product is sold under the trade name "Dowfax 3B2" by the Dow Chemical Company of Midland, Michigan. Most diphenyl ether sulfonic acid alkali metal salts are suitable equivalents, and in the broadest embodiment of this invention, any surfactant that is hydrolytically and oxidatively stable to the stripping solution may be substituted. It can be used as an object. The stripping composition is completed by adding 2-90% water by weight to bring the solution to the desired strength. Water reduces the viscosity of the stripping solution and increases the mobility of ionic species in the solution.

Adding water to sulfuric acid has the desirable effect of lowering the cavitation threshold of the solution, ie, the minimum force required to bring the local pressure acting on the solution to a value less than the vapor pressure of the solution. This threshold occurs due to an increase in vapor pressure and a decrease in viscosity and dilution density. The cavitation threshold at 20 KHz for room temperature water is approximately 0.38 W/cm ² (2.45 W/in ² ).

The method of the present invention comprises contacting the hard surface coating with an acidic stripping solution and continuing the contact until the hard surface coating is substantially removed from the metal substrate without dimensional change of the underlying surface. In order for exfoliation to occur at an acceptable rate, it is essential that the solution bath be continuously stirred by suitable stirring means, preferably an ultrasound-generating transducer, either magnetostrictive (preferred) or a piezoelectric transducer. It is. The converter is approximately 0.62W/cm ² (approximately 4W/
in ² ), preferably about 1.08 to 1.25 W/cm ² (about 7-
Must be engineered to yield a minimum power density of 8W/in ² . Merely conventional agitation does not provide the necessary solution agitation in the stripping bath to produce effective velocities. In combination with an ultrasonic stirrer that inherently supplies energy to the solution, the solution was heated at approximately
~83℃ (about 110-180〓), but preferably about
It is necessary to maintain the temperature at 54-66°C (approximately 130-150°C).

The invention will be explained in further detail by the following examples.

Example 1 The aircraft combustor used in the jet engine was Hastelloy X (22 Cr, 18.5 Fe, 9.0Mo,
1.5 Co, 0.6 W, balance Ni) and adhesive coat (approximately 0.07-0.13 mm (3-5 mil) thick, Ni/Cr 95%)
and intermediate coat (approximately 0.10-0.16mm (4-6mil)
thickness, 35-36% mixture of Ni80%-Cr20% and MgO- _ZrO2 ) and outer coat (approximately 0.15-0.26 mm (6-
10mil) thick, with a triple layer of MgO−ZrO ₂ ) approximately
Flame spray was applied over an area of 0.34 m ² (520 in ² ). Two of these cylindrical combustion chambers have a temperature of approximately 48-77°C (120°C).
The coating was stripped for 10-16 hours using ultrasonic stirring in a bath maintained at a temperature in the range -170〓). Approximately 114 (30 gallon) stainless steel hot water tanks with two 1200 watt side-mounted submersible ultrasonic transducers are used during the stripping operation, and the stripping solution (275 g. / H ₂ SO ₄ 66゜Be′, 14g / HBF ₄
(48%), 120g/sodium m-nitrobenzenesulfonate, 0.1g/dowfax 3B2)
22.7 (6 gallons) was used. A metallographic examination revealed no damage to the base of the cylindrical combustion chamber.

Example 2 The cylindrical combustion chamber of a military jet engine is 275
g/concentrated sulfuric acid H ₂ SO ₄ (66°Be′), 145 g/fluoroboric acid HBF ₄ (48%), and 116 g/
of sodium metanitrobenzene sulfonate. Approximately 54~77℃ (130−
170〓) for 20 hours, the three layers of coating were removed from the burner barrel. A 5 gallon stainless steel tank fitted with a 1000 watt bottom mounted ultrasonic transducer and containing 2 gallons of stripping solution in a square polypropylene container was used. No destruction of the substrate was observed.

According to the invention, materials based on ceramics, celmets and nickel can be combined with metals, in particular
Not only can a wide range of hard surface coatings be removed from nickel-based superalloys without causing unacceptable dimensional or structural changes to metal substrates, but also a wide range of hard surface coatings can be removed from these substrates to the underlying materials. can be removed without causing damage.

Obviously, various modifications may be made to the invention in light of the above description. Therefore, within the scope of the appended claims, the invention may be practiced otherwise than as described above.

Claims (1)

[Scope of Claims] 1. A composition for selectively removing ceramic, celmet, and nickel-based hard surface coatings from high-strength, high-temperature-resistant metal substrates, the composition comprising: be done. _H2SO4 in an amount of about 90-450 g/, _a water-soluble nitro-substituted aromatic compound in an amount of about 60-240 g/, an effective amount of fluoroboric acid ( _HBF4 ), and the balance water. 2. The composition according to claim 1, characterized in that fluoroboric acid is added in an amount of 70 g/or less. 3. The composition of claim 1, wherein the solution further contains a surfactant selected from the class of diphenyl ether sulfonates in an amount of up to about 3 g/l. 4. The composition according to claim 2, characterized in that the composition is composed of the following components: H ₂ SO ₄ in an amount of about 250-300 g/ and about 60-120
a water-soluble nitro-substituted benzene compound in an amount of about 10 to 20 g/a, fluoroboric acid in an amount of about 10 to 20 g/a;
A surfactant selected from the class of diphenyl ether sulfonates in an amount of up to 5. The composition according to claim 1, wherein the water-soluble nitro-substituted benzene compound is sodium meta-nitrobenzene sulfonate. 6. A method for selectively removing ceramic, celmet, and nickel-based hard surface coatings from high-strength, high-temperature-resistant metal substrates, the method comprising the following steps. Contacting the hard surface coating with the acidic stripping solution of claim 1, the solution temperature of which is maintained at about 43-83°C (110-180°) and constantly agitated by ultrasonic stirring means, forms the base. The hard surface coating is removed from the metal substrate without damaging the substrate, and the metal substrate is then removed from the bath of the solution. 7 The temperature of the solution is approximately 54-66℃ (130-66℃) during the work.
150〓). 8 The ultrasonic stirring means shall be used at least during operation.
7. A method according to claim 6, characterized in that it is an ultrasonic generating transducer that continuously supplies the solution with a power density of 0.6 W/cm ² (4 W/in ² ). 9 Converter is approximately 1.08~1.25W/cm ² (7~8W/
9. A method according to claim 8, characterized in that the power density is maintained at a power density of in ² ). 10 Approximately 70g/fluoroboric acid is added to the acidic stripping solution.
7. A method according to claim 6, characterized in that the following amounts are added: