Caustic process for replacing thermal barrier coatings

Abstract: An aqueous caustic process can be used for replacing a thermal barrier coating on a metal surface. In the process, the metal surface is contacted with a caustic solution to remove the thermal barrier coating. The processing conditions for contacting the caustic solution are selected to avoid the formation of a peel layer during a subsequent application of a replacement thermal barrier coating and heat treatment. A replacement thermal barrier coating is then reapplied to the metal surface.

Description: This invention is related to removing thermal barrier coatings from surfaces. More particularly, the invention is related to removing thermal barrier coatings without attacking underlying coatings, such as nickel aluminide or platinum nickel aluminide. Higher operating temperatures increase the efficiency of gas turbine engines. Advanced propulsion technology now involves the operation of engines at temperatures of 1000°C and above. Such high operating temperatures challenge the durability of engine components. The use of nickel and cobalt-base superalloys has provided some advance in high temperature durability. However in certain sections of a gas turbine engine, such as in the turbine, combustor and augmentor, the metallic components must be coated with a thermal insulator to provide further high temperature operation durability. Thermal barrier coatings (TBC) are typically used on metallic components to provide the further high temperature operation durability required in many sections of the modern turbine engines. Thermal barrier coatings typically include a ceramic coating deposited on a metallic bond layer that is deposited on the metallic component surface. Typically the bond layer is platinum nickel aluminide, nickel aluminide, aluminided MCrAlY (where M equals nickel, cobalt, nickel-cobalt and iron; Cr is chromium; Al is aluminum; Y is yttrium) or mixtures thereof. The ceramic coating can be some form of stabilized zirconia, such as zirconia stabilized with calcia, hafnia, magnesia, yttria or any of the rare earth oxides, which form a refractory oxide thermal barrier coating. The thermal barrier coating has a very low thermal conductivity, low density and high melting point. During service life, coated metallic components of engines for commercial aircraft, some military aircraft and for power generation, crack, spall or undergo chemical and physical attack. Hence, periodically these engines must be overhauled. During overhaul, turbine blades and vanes that have not exceeded creep limits and are not otherwise severely eroded or damaged can be refurbished for reuse. Coatings, such as the thermal barrier coating and bond coat, are stripped from the metallic component and the component is reworked and cleaned, recoated and returned to service. The thermal barrier coating must be completely removed before recoating the metallic surface with fresh bond layer and thermal barrier coating. Ault, U.S. Pat. 4,439,241 discloses cleaning a turbine part, repairing the part, e.g., by welding or straightening, and reapplying the external coating. In the process, a part, such as a turbine blade, is disposed in a holder in an autoclave and is subjected to the flow of an alkaline solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) and water at a temperature between 150°C to 235°C, at a pressure between 1034 kPa to 2413 kPa (150psi to 350psi). The potassium hydroxide solution is 20-45 weight percent. First stage turbine blades experience a build up of silicon-calcium (Si-Ca) rich dirt and debris during engine service that inhibits the flow of cooling air and creates hot spots, which lead to accelerated internal corrosion that may cause through-wall material failure. Previous autoclave processes have been ineffective at removing large amounts of this internal debris. Irvine et al., New Method for Turbine Blade Internal Cleaning, a paper presented at the Internal Gas Turbine and Aeroengine Congress and Exposition, Houston, Texas, June 5-8, 1995, proposes pumping heated caustic at pressures approaching 2758 kPa (400psi) through the internal cavities of turbine blades. The increased caustic flow from this process, compared to previous autoclave processes, improves cleaning effectiveness. The process reduces repair costs by improving part yield during repair. The caustic process can be used to clean engine parts such as air foils and can also be used to remove ceramic cores from airfoil castings. The caustic reacts with oxides and hydroxides to form salts of the alkali metals which are soluble in water. This chemical reaction makes it possible to clean airfoils and to remove ceramic cores. Additionally, the chemical reaction of the caustic process has been proposed to remove thermal barrier coatings by dissolving the alumina interface between the thermal barrier coating and the underlying metallic component. Unfortunately under the conditions taught in the art, the caustic process also attacks the nickel aluminide or platinum nickel aluminide of the underlying metallic component. When the metallic component is subsequently cleaned and recoated with thermal barrier coating and heat treated, a black layer believed to be mostly nickel oxide, peels from the airfoil internal and external surface. Thus, there is a need to provide a caustic process to remove thermal barrier coatings from an underlying metallic component without forming the undesired black peel (flakes). The present invention is based on the discovery that the peel layer (flakes) may be avoided by conducting the caustic process under selected processing conditions. US-A-5643474 discloses a wet chemical process for removing physical vapour deposited or air plasma sprayed thermal barrier coatings from coated parts without damaging or affecting the bond coat or the base metal substrate The process entails using an autoclave with an organic caustic solution to fully remove the thermal barrier coating. A comparative example using 50 weight % NaOH in water for 2 hours at 285°C and 13790 KPa (2000 psi) results in damage to the bond coating as well as removing the thermal barrier coating. FR-A-2483962 discloses a method for removing scale, particularly oxide scale, from the surface of a high temperature alloy which has been exposed to high temperature conditions. An aqueous alkaline hydroxide solution comprising 20-75 weight % of NaOH or KOH is contacted with the scale in an autoclave at a temperature of from 200-340°C in a non-oxidizing atmosphere. The invention is an aqueous caustic process for replacing a thermal barrier coating on a metal surface comprising: (A) contacting the metal surface with an aqueous caustic solution to remove the thermal barrier coating from said surface under processing conditions selected to avoid the formation of a peel layer during a subsequent application of a replacement thermal barrier coating and heat treatment, the selected conditions including a temperature in a range from 230°C to 250°C, a pressure of from 2758kPa to 5516kPa (400psi to 800psi), an oxygen level of less than fifty parts per million, the caustic solution comprising 40-45 weight percent KOH or 40 to 50 weight percent NaOH; and (B) applying said replacement thermal barrier coating. In particular, this invention is related to the chemical removal of yttria-stabilized zirconia thermal barrier coatings deposited by physical vapor deposition or air plasma spray on superalloy substrates having a bond coat. In the process, the surface of the coated part is contacted with a caustic solution to remove the thermal barrier coating. The processing conditions for contacting the caustic solution are selected to avoid the formation of a peel layer during a subsequent application of a replacement or repair thermal barrier coating and heat treatment. A replacement or repair thermal barrier coating is then reapplied to the metal surface. The peel layer is an oxidation product of nickel aluminide or platinum nickel aluminide. In another aspect, the invention is an aqueous caustic process for treating a nickel aluminide-containing metal surface, platinum nickel aluminide-containing metal surface, MCrAlY aluminide-containing metal surface and mixtures thereof to remove a thermal barrier coating or other caustic soluble ceramic coating, comprising: (A) contacting the metal surface with an aqueous caustic solution under processing conditions selected to avoid the reaction of said caustic with said nickel aluminide, platinum nickel aluminide, or mixtures thereof to form a peel layer during a subsequent application of a thermal barrier coating and heat treatment, the selected conditions including a temperature in a range from 230°C to 250°C, a pressure of from 2758kPa to 5516kPa (400psi to 800psi), an oxygen level of less than fifty parts per million, the caustic solution comprising 40-45 weight percent KOH or 40 to 50 weight percent NaOH; and (B) applying said thermal barrier coating. The invention provides a method for removing thermal barrier coatings from flat and contoured surfaces by treating the thermal barrier coated surface in an autoclave with a caustic solution as defined in claim 1. The autoclave chamber is purged with an inert gas, such as argon, helium, and nitrogen, so that the oxygen level is less than fifty parts per million, and alternatively, the oxygen level is less than ten parts per million. The thermal barrier coating (TBC) is generally a chemically stabilized zirconia, such as yttria stabilized zirconia, calcia, stabilized zirconia or magnesia stabilized zirconia. Other oxide or ceramic coatings that act as a thermal barrier may also be referred to as the thermal barrier coating for the purposes of this invention. Also, the invention includes the removal of other caustic soluble ceramic oxide coatings that might not be used as thermal barrier coatings. Herein, bond coats are usually meant to be metallic compositions, including platinum-nickel aluminum, aluminum, nickel aluminum, nickel-chromium-aluminum-yttrium, iron-chromium-aluminum-yttria, cobalt-chromium-aluminum-yttrium and nickel-cobalt-chromium-aluminum-yttrium. Substrate materials often used in turbines for aircraft engines and power generation equipment may be nickel, cobalt or iron based superalloys. Examples of such substrates are GTD-111, GTD-222, Rene-80, Rene 41, Rene 142, N-5, Rene 125, Rene 77, Rene 95, Inconel 706, Inconel 718, Inconel 625, cobalt-based HS188, cobalt-based L-605 and stainless steels. The process is especially suited for thermal barrier coated parts and hardware used in turbines or on airfoils. An example of a turbine part is a turbine blade or vane. The term airfoil refers also to turbine ;parts, such as blades, vanes, buckets, nozzles and the like. Additional substrate materials that can accommodate a thermal barrier coating for applications other than turbine parts, may be used in this invention. For instance, it is also contemplated that this invention may be utilized for removal of thermal barrier coatings in electronic applications and power generation such as gas, steam and nuclear to mention a few. Use of additives, such as surfactants and chelates, to further reduce the surface tension of the caustic solution can be beneficial. The caustic compound and the water may be present in about a two to three ratio. The concentrations of the bases range from forty to forty-five weight percent for potassium hydroxide and forty to fifty weight percent of sodium hydroxide. The process of the invention is conducted in an autoclave reactor. The autoclave reactor is a pressure vessel and is built to withstand high pressures at high temperatures. Pressure in the system is elevated by heating the contents (reaction mixture) in the autoclave or by using an external source of compressed gases to over pressurize the vessel. The autoclave may be operated in batch fashion; that is, the ingredients of the caustic organic solution are charged, the unit is closed, and the charge is brought to the desired conditions of temperature and pressure. Continuous or semicontinuous operation can be undertaken if one or more of the reactants are continuously fed and products withdrawn. The treatment may involve combinations of ultrasonic agitation, mechanical mixing, and boiling with an autoclave treatment. The autoclave treatment can be conducted under several conditions. The pressure ranges from 2758 kPa (400 pounds per square inch) to 5516 kPa (800 pounds per square inch) or higher, and the temperature ranges from 230°C to 250°C. Also, pressurization can be achieved at room temperature using compressed gases. Still yet, the process can start with zero pressure and by increasing the temperature of the reaction mixture, the autoclave pressure automatically rises resulting from the increase in the vapor pressure of the reaction mixture. The time to remove the thermal barrier coating depends on the amount of the coating to be removed and the temperature and pressure conditions that are applied. Usually, the time is between 5 minutes to 4.0 hours preferably 30 minutes to 4 hours. Also, it should be noted that using a mixer, such as a mechanical stirrer, a magnetic stirrer, or an ultrasonicator, at high pressures may enhance the ability of the caustic solution to remove the thermal barrier coating in torcherous locations and within a shorter duration of time. EXAMPLES: A number of caustic solutions containing potassium hydroxide (KOH) and sodium hydroxide (NaOH) were tested at different processing conditions. A list of selected conditions is provided in TABLE I. Wt% of Caustic in Water Temperature (°C) Pressure w/Nnitrogen (psi) kPa Process Time (Hours) TBC Removed Attack on NiAl Coating 40 KOH 230 (800) 5516 4 Yes No 45 KOH 250 (800) 5516 4 Yes No 40 KOH 250 (600) 4137 1 Yes No 45 KOH 230 (600) 4137 1 Yes No 40 NaOH 230 (800) 5516 1 Yes No 50 NaOH 250 (800) 5516 1 Yes No 40 NaOH 250 (600) 4137 1 Yes No 50 NaOH 230 (600) 4137 1 Yes No 30 KOH 200 (160) 1103 5 Min Partially No 45 KOH 200 (200) 1379 5 Min Yes No 30 KOH 230 (425) 2930 4 Yes Yes 45 KOH 230 (400) 2758 4 Yes No The results indicate that lower concentrations (30 weight percent) of caustic attack the nickel aluminide layer and cause the formation of a peel layer (flakes) during subsequent reapplication of the thermal barrier coating.

Claims: An aqueous caustic process for replacing a thermal barrier coating on a metal surface, comprising: (A) contacting the metal surface with an aqueous caustic solution to remove the thermal barrier coating from said surface under processing conditions selected to avoid the formation of a peel layer during a subsequent application of a replacement thermal barrier coating and heat treatment, the selected conditions including a temperature in a range from 230°C to 250°C, a pressure of from 2758 kPa to 5516 kPa (400 psi to 800 psi), an oxygen level of less than fifty parts per million, the caustic solution comprising 40-45 weight percent KOH or 40 to 50 weight percent NaOH; and (B) applying said replacement thermal barrier coating. An aqueous caustic process for treating a nickel aluminide-containing metal surface, platinum nickel aluminide-containing metal surface, MCrAlY aluminide-containing metal surface and mixtures thereof to remove a thermal barrier coating or other caustic soluble ceramic coating, comprising: (A) contacting the metal surface with an aqueous caustic solution under processing conditions selected to avoid the reaction of said caustic with said nickel aluminide, platinum nickel aluminide, or mixtures thereof to form a peel layer during a subsequent application of a thermal barrier coating and heat treatment, the selected conditions including a temperature in a range from 230°C to 250°C, a pressure of from 2758 kPa to 5516 kPa (400 psi to 800 psi), an oxygen level of less than fifty parts per million, the caustic solution comprising 40-45 weight percent KOH or 40 to 50 weight percent NaOH; and (B) applying said thermal barrier coating. The process of claim 1 or claim 2, wherein said selected conditions comprise a processing time of between 5 minutes to 4 hours. The process of any preceding claim, wherein said metal surface is contacted with a caustic solution under a nitrogen atmosphere or under an argon atmosphere. The process of any preceding claim, wherein said metal surface includes a nickel aluminide coating or a platinum nickel aluminide coating. The process of any preceding claim, wherein said metal surface is in the form of an airfoil. The process of claim 1 where the process takes place in an autoclave chamber. The process of claim 7 where the autoclave atmosphere contains less than ten parts per million oxygen.

Inventors: FARR HOWARD JOHN, SANGEETA D, WUSTMAN ROGER DALE, FARR, HOWARD JOHN, SANGEETA, D., WUSTMAN, ROGER DALE
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1999
Main classification: C23G1
IPC codes: C23C4, C23G3, C23G1, F01D5, C23F1, C23C10, B08B3, C23C16