DE102010038270A1 - Process for cleaning substrates - Google Patents

Abstract

A method for removing sand particles from a substrate is described. The method includes the step of treating the substrate with an acid solution comprising H _x AF ₆ , wherein A is selected from the group consisting of Si, Ge, Ti, Zr, Al and Ga, and wherein H _x AF ₆ in a concentration in the Range from about 5% to about 40% by weight.

Description

BACKGROUND

The invention relates to the cleaning of substrate surfaces and, more particularly, to the removal of sand particles from turbine engine surfaces, such as blades.

Sand contamination has recently emerged as a key factor that significantly degrades performance in turbine components. For example, aircraft engines that fly inland routes often experience significant sand contamination due to heavy sand intake during idle, takeoff, and landing. It has been found that the primary pollution mechanism is the increased roughness of compressor blades due to the suction of sand. Specifically, this increased roughness results from the formation of micro-pits during impact of particles. Thereafter, sand particles of sizes less than 10 microns accumulate in these pits to form the soil layers. High temperatures in downstream stages of the compressor lead to sintering of the sand particles, which increases the blade-sand adhesion. Consequently, washing water treatments, which are often used to clean the turbine components, are often unsuccessful at removing the accumulated sand particles.

There remains therefore a need in the art for a method of removing sand particles and sand pollutant layers from, e.g. Turbine engine components, while having minimal or no effect on the blade surface below or any protective coatings on such surface.

SHORT DESCRIPTION

A primary embodiment of this invention is directed to a method of removing sand particles from a substrate. The method includes the step of treating the substrate with an acid solution comprising H _x AF ₆ , wherein A is selected from the group consisting of Si, Ge, Ti, Zr, Al and Ga, and wherein H _x AF ₆ in a Concentration of about 5 wt .-% to about 40 wt .-% is present.

DRAWING

These and other features, aspects, and advantages of the present invention will become better understood upon reading the following detailed description with reference to the accompanying drawings in which like reference numerals represent like parts, wherein: FIG.

1 FIG. 3 is a cross-sectional view of a portion of a turbine engine component; and FIG

2 a method of removing sand particles from a substrate is illustrated.

DETAILED DESCRIPTION

Embodiments of the present invention relate to a method for removing sand particles from a substrate. The method includes the step of treating the substrate with an H _x AF ₆ acid solution having a concentration in the range of about 5% to about 40% by weight. As will be described in detail below, typically, milder processing conditions, such as lower concentrations of H _x AF ₆ acid in the aqueous solution, lower temperatures, and shorter treatment _{times are} preferred to remove the sand particles from the substrate without significant damage to the underlying To cause plating.

The term "sand" as used herein may be broadly defined as a naturally occurring granular material composed of finely divided rock and mineral particles. The most common component of sand is often silica (SiO ₂ ), typically in the form of quartz. However, the composition of sand can vary widely. In some embodiments, the term "sand" specifically refers to "CMAS" materials that are materials based on a mixture of calcia, magnesia, alumina, and silica. The particle size of the sand particles is usually submicron. In one embodiment, the mean particle size of the sand particles ranges from about 0.5 μm to about 10 μm.

It has been found that sand pollution often occurs in aircraft engines flying in geographic areas that have deserts. Blades in the compressor sections of such aircraft engines show significant accumulations of fine sand. Surfaces of turbine components, such as blades, can accumulate sand, resulting in reduced airflow and a significant increase in specific fuel consumption (SFC) The accumulation of sand exists in both particulate and glassy layers.

Sand particles with sizes of less than 10 μm accumulate on the blades of the compressor. High temperatures in the downstream stages of the compressor lead to a Sintering the sand particles, which reinforces the blade-sand adhesion. The sintering of the sand particles forms a cementitious layer of the sand particles on the blade. Consequently, washing with water, which is often effective in cleaning the turbine components, is often unsuccessful in removing the accumulated sand particles.

The method includes treating the substrate with a mild aqueous solution of H _x AF ₆ acid at a selected temperature for a selected period of time and then purging the turbine engine component in an aqueous medium. The term "mild aqueous solution of H _x AF ₆ acid" as used herein refers to an aqueous solution of H _x AF ₆ acid that is substantially free of strong acids such as phosphoric acid. The process of this invention utilizes fluorosilicic acid or its derivatives having the formula H _x AF ₆ . In this formula, A is selected from the group consisting of Si, Ge, Ti, Zr, Al and Ga. "X" has a value of 1 to 6, and more typically 1 to 3. Materials of this type are commercially available or can be used without undue reliance Effort to be made. The preferred acid is H ₂ SiF ₆ . It is referred to by various names, such as "hydrofluorosilicic acid", "fluorosilicic acid", "fluosilicic acid", "hexafluorosilicic acid" and "HFS". Materials of this kind are in the U.S. Patent 6,599,416 (Kool et al.), Which is incorporated herein by reference.

As described herein, the use of the H _x AF ₆ compound provides significant benefits to the sand removal process. In many cases, such a method is very suitable, for. Where there is a need for high removal rates. Surprisingly, it has been found that by employing the aqueous solution of fluosilicic acid and / or the derivatives of fluosilicic acid, the sand particles can be removed from the surface of the blades without damaging the underlying metal alloy surface or any intermediate coating as described below.

Precursors for the H _x AF ₆ acid can also be used. A "precursor" as used herein refers to any compound or group of compounds that can be combined to form the acid or its dianion AF ₆ ^-2 or that can be converted to the acid or its dianion under reaction conditions, e.g. By the action of heat, stirring, catalysts and the like. The acid can thus in situ, for. B. in a reaction vessel can be formed.

As an illustration, the precursor may sometimes be a metal salt, inorganic salt or an organic salt in which the dianion is ionically bonded. Non-limiting examples include salts of Ag, Na, Ni and K as well as organic salts such as a quaternary ammonium salt. Dissociation of the salts in an aqueous solution often yields the acid. In the case of H ₂ SiF ₆ , a suitable salt that can be used is Na ₂ SiF ₆ .

The person skilled in the art is familiar with the use of compounds which cause the formation of H _x AF ₆ within an aqueous composition. So can, for. B., H ₂ SiF _{6 are formed} in situ by the reaction of a silicon-containing compound with a fluorine-containing compound. An exemplary silicon-containing compound is SiO ₂ while an exemplary fluorine-containing compound is hydrofluoric acid (ie, aqueous hydrogen fluoride).

Now up 1 Referring to FIG. 1, there is depicted a cross-sectional view of a portion of an exemplary turbine engine component generally indicated by reference numeral 10 referred to as. The turbine engine component in this case is a bucket. The component 10 closes a concave surface 12 and a convex surface 14 one. The sand particles can be removed from any surface or from passageways, indentations or various voids in such surfaces.

The nature of the turbine engine component is not intended to be limited and may vary between a shroud ring, a bucket or blade, a nozzle or vane, a diaphragm component, a seal component, a valve stem, a nozzle box, a nozzle plate, or the like. The terms "blade" and "bucket" can be used interchangeably. In general, a blade is a rotating blade of an aircraft turbine engine and a bucket is a rotating blade of a turbine engine for power generation on land. Also, the term "nozzle", which is generally related to a stationary blade in a steam or gas turbine, can be used interchangeably with the term "vane."

The turbine engine component, e.g. B. substrate 12 , may generally comprise a variety of metallic alloys, e.g. Steel or alloys thereof or titanium alloys (eg Ti-6A1-4V). In some preferred embodiments, the turbine engine component is formed from superalloys that can be used at high temperatures, often above about 0.7 of the absolute melting temperature. The superalloys are based on nickel, cobalt, iron or some combination thereof. Many are disclosed in the above-referenced PS by Kool et al. described.

As those skilled in the art know, turbine engine components are usually provided with one or more protective coatings. The present invention therefore provides for the removal of sand from the surfaces of these coatings as well as the removal of sand from an uncoated substrate. Many different types of protective coatings are typically used, e.g. As diffusion coatings or topcoats, as described in Kool-PS. In addition, overcoats such as the "MCrAl (X)" materials (which are considered to be metallic coatings) may be covered by thermal barrier coatings (TBCs). Non-limiting examples of the latter include ceramic coatings such as yttria-stabilized zirconia. Some of the TBS's are in the U.S. Patent 6,921,586 (Zhao et al.), Which is incorporated herein by reference. The thickness of the protective coating (s) depends on various factors such as the type of article coated, the composition of the substrate and the environmental conditions to which the article is exposed.

2 Figure 3 is an illustrative flow sheet for an exemplary, non-limiting, method of removing sand particles from a substrate. At block 20 For example, the substrate is treated with an H _x AF ₆ acid solution having a concentration in a range of about 5% to about 40% by weight. The substrate may, for. Example, be treated by at least partial immersion of the substrate in a bath of H _x AF ₆ acid solution. The bath can be stirred mechanically during the treatment.

The preferred level of H _x AF ₆ acid used depends on several factors. They include the type and amount of sand removed, the location of the sand on a substrate, the type of substrate, the thermal history of the substrate and sand material (eg how tight the sand adheres to the substrate), the time and temperature for the treatment and the stability of the acid in the treatment solution.

As a general rule, the H _x AF ₆ acid is present in a treating composition at a level of from about 5% to about 40% by weight. Usually, the level is in the range of about 10% to about 30% by weight. In the case of H ₂ SiF ₆ , a preferred concentration (here in molar units) is from about 0.2 M to about 2.2 M. Adjusting the amount of H _x AF ₆ acid and other components described below. can be readily accomplished by considering stoichiometric parameters and by observing the effect of particular compositions on the removal of sand from the substrate.

The temperature of the H _x AF ₆ acid solution can usually be maintained at up to about 100 ° C. In some specific embodiments, the temperature is kept below about 50 ° C. In some particularly preferred embodiments, the temperature is in the range of about 50 ° C to about 80 ° C.

Typically, ambient pressure is suitable during treatment of the substrate with the H _x AF ₆ acid.

The time required to remove the sand particles (ie, the time of immersion in the aqueous composition) can vary considerably. Factors that influence the selection of an appropriate time include the specific composition of the removed, sand-based material as well as its density and thickness. The time may also vary with the temperature of the H _x AF ₆ acid solution. For a higher temperature of the fluorosilicic acid solution, the treatment time, for. B., be shorter and vice versa. Typically, the substrate is at least partially immersed in a wetting agent bath for treatment for a period of time in the range of about 1 minute to about 36 hours, and preferably from about 5 minutes to about 8 hours. In some cases, a particularly preferred dipping time is in the range of about 10 minutes to about 2 hours. The time of immersion of the substrate in the bath usually depends on the temperature of the bath and the nature of the protective coating on the substrate. Longer immersion times can, for. For example, it may be acceptable for substrates that have protective coatings such as MCrAlY and TBC because the nature of such protective coatings is inert to H _x AF ₆ acid solution. In the case of such protective coatings, the underlying substrate usually remains undamaged - even after prolonged immersion of the substrate in the H _x AF ₆ acid solution.

The aqueous composition used for the present invention may include various other additives serving a variety of functions. Nonlimiting examples of these additives are inhibitors, dispersants, surfactants, chelating agents, wetting agents, flocculating agents, stabilizers, anti-settling agents and antifoaming agents. Those skilled in the art are familiar with specific types of such additives and with effective levels for their use. An example of an inhibitor of the composition is a relatively weak acid, such as acetic acid, mentioned above. Such a material tends to reduce the activity of the primary acid in the composition. This is desirable in some cases, e.g. For example, the possibility of any damage to the underlying one (s) Coating (coatings) or the substrate surface to reduce.

The aqueous solution of the H _x AF ₆ acid of the present invention is not only effective in removing the deposit containing the sand particles on the substrate but is also usually mild for the underlying metal of the blade and for any of the protective coatings. In other words, with proper monitoring of the treatment process, the underlying surfaces are not affected.

Optionally, as in block 22 illustrates treating the substrate with a wetting agent such as a surfactant prior to treatment with the H _x AF ₆ acid solution. The treatment with the wetting agent may facilitate a better interaction between the sand particles and the H _x AF ₆ acid to facilitate the removal of the coating.

Non-limiting examples of suitable surfactants include detergents, acid stable surfactants, non-acid stable surfactants, and combinations thereof. An example of an acid-stable surfactant may include polyethylene oxide commercially available as Triton x-100 ^™ (manufactured by Research Chemicals Limited [CAS No. 9002-93-1]). An example of a non-acid-stable surfactant includes trisodium phosphate, which is commercially available as Alconox TSP ^TM.

Optionally, the substrate becomes as in block 24 illustrated, rinsed with water before treatment with the wetting agent. Rinsing the substrate with water eliminates any debris or loose sand particles from the surface of the substrate.

The substrate can, as in block 26 optionally rinsed with water or other liquid after treatment with the H _x AF ₆ acid solution to remove any debris or unwanted chemicals from the surface of the substrate. In one embodiment, rinsing may be accomplished by applying a jet of water.

In addition to (or instead of) the rinse step 26 For example, the treatment of the sand-soiled surface may also include at least one subsequent step to remove any remaining residual sand or other. As described in the aforementioned Kool-PS, a post-cleaning or "soil removal" operation may be performed. This may be in the form of a grinding step that minimizes damage to the substrate or protective coatings. As an example, sand blasting may be performed by directing a stream of compressed air containing alumina particles over the surface. The air pressure is usually less than 100 psi. Sandblasting is performed for a sufficient amount of time to remove any residual material. Other known techniques for abrading the surface can be used instead of sandblasting. Many of these techniques are in the U.S. Patent 5,976,265 described herein by reference. So the surface, z. B., manually scrubbed with a fiber pillow, z. As a pillow, with polymeric, metallic or ceramic fibers. Alternatively, the surface may be polished with a flexible disk or tape in which alumina or silicon carbide particles have been embedded. Liquid abrasive materials may alternatively be used on the disks or tapes. These alternative techniques are controlled in a manner that maintains contact force against the surface that is not greater than the force used in the sandblasting techniques discussed above.

Other techniques (or combinations of techniques) could be used instead of abrading to remove any residual material. Examples include spinning the article (eg, water spin) or laser ablation of its surface. Alternatively, the remaining material could be scraped off the surface. As yet another alternative, sound waves (eg, ultrasound) could be directed against the surface, generating vibrations that would dislodge the destroyed material. For each of these alternative techniques, those skilled in the art would be familiar with operating adjustments to be made to control the relevant force exerted on the surface of the article (as in the case of the abrading technique) to minimize damage to the substrate or overlying protective coatings. The object is sometimes rinsed after this stage, z. B., using water or a combination of water and a wetting agent.

The bath can be stirred during this stage to remove the remainder. For example, the bath may be mechanically agitated (as described above) while the substrate is being treated in the H _x AF ₆ acid solution. In one embodiment, the bath may be mechanically agitated while the substrate is being treated with a wetting agent.

As previously mentioned, the present invention can also be applied to effectively remove sand from any internal region or cavity in an article, e.g. B. Depressions, hollow regions, passages or holes. In the case of a turbine blade, the internal region is often in the form of radial cooling holes or serpentine passages. The process of the present invention produces a synergistic effect and results in an extremely effective cleaning process for blades and other parts of turbine engines having strong adherent sand particles.

While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is therefore to be understood that the appended claims encompass all such modifications and changes that fall within the scope of the invention.

A method for removing sand particles from a substrate is described. The method includes the step of treating the substrate with an acid solution comprising H _x AF ₆ , wherein A is selected from the group consisting of Si, Ge, Ti, Zr, Al and Ga, and wherein H _x AF ₆ in a concentration in the Range from about 5% to about 40% by weight.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 6599416 [0012]
• US 6921586 [0020]
• US 5976265 [0033]

Claims (10)

A method for removing sand particles from a substrate, comprising the step of treating the substrate with an acid solution comprising H _x AF ₆ , wherein A is selected from the group consisting of Si, Ge, Ti, Zr, Al and Ga, and wherein H _x AF _{6 is present} in a concentration ranging from about 5% to about 40% by weight. The method of claim 1, wherein the H _x AF ₆ acid solution is an aqueous solution. The process of claim 1 wherein the concentration of H _x AF ₆ acid is in a range of from about 10% to about 30% by weight. The method of claim 2, wherein the H _x AF ₆ acid solution is maintained at a temperature of not more than about 100 ° C during the treatment. The method of claim 1, wherein the substrate is treated with a wetting agent prior to treatment with the fluorosilicic acid solution. The method of claim 5, wherein the wetting agent comprises a surfactant. The method of claim 5, further comprising rinsing the substrate with water prior to treatment with the wetting agent. The method of claim 1, further comprising rinsing the substrate with water after treatment with the H _x AF ₆ acid solution. The method of claim 1 wherein at least one protective coating is disposed over the substrate and the removed sand particles are those adhered to a surface of the protective coating. A method for removing sand particles from a surface of at least one protective coating disposed on a superalloy substrate, comprising the step of treating the coated substrate with an acid solution comprising H _x AF ₆ , wherein A is selected from the group consisting of Si, Ge, Ti , Zr, Al and Ga, and wherein H _x AF _{6 is present} in a concentration ranging from about 5% to about 40% by weight.

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