Patent Application
20200055615
The present invention is directed to processes and apparatus for removing coatings from gas turbine engine components.
A typical gas turbine engine includes a compressor section with shaft mounted blades for compressing air that is then directed into a combustor where fuel is mixed with the air and ignited, the heated gases then expanding through a high-pressure turbine (HPT) which includes stationary vanes and rotating turbine blades mounted on the same shaft driving the compressor, and then through a low pressure turbine (LPT) with blades mounted on a second shaft which drives a fan to provide thrust in the case of an aircraft jet engine, or drives an electrical generator in the case of a power generating industrial gas turbine engine (IGT). The HPT and LPT blades (“buckets” in IGT) are circumscribed by shrouds (“tiles” in IGT) to form the flowpath for the working gas. The efficiency of any gas turbine engine is enhanced by reaching higher temperatures. Components of gas turbine engines thus exposed to high temperature environments are conventionally manufactured from nickel-, cobalt-, or iron-based superalloy materials which exhibit improved mechanical properties at high operating temperatures. The operating environment leads to three types of degradation limiting the component's useful life; hot corrosion, stress corrosion cracking (also generally referred to as Type I and Type II corrosion, respectively, and as sulfidation), and high temperature oxidation. The temperature ranges at which Type I and Type II corrosion, and High Temperature Oxidation, degrade the superalloy depends on the superalloy composition.
The ability to achieve even higher engine operating temperatures has been enabled through the use of coatings on superalloy components. Coatings can be used either alone as an environmental coating (to protect the component, also referred to hereinafter as a part, directly from corrosion or oxidation) or as a bond coat for a subsequently applied thermal barrier coating (TBC), such as Yttria stabilized zirconia (YSZ) applied to surfaces exposed to hot gases, particularly flowpath surfaces. Exemplary superalloy coatings include MCrAlY (where M represent one or more of Fe, Co, and Ni), platinum aluminides, and nickel aluminides, each of which provide a source of aluminum to form and replenish a thermally grown oxide (TGO) layer of alumina (Al2O3) on their surface when exposed to oxygen at high temperatures, the alumina providing an effective protection against high temperature oxidation. Other coatings, such as wear coatings or abradable coatings, may also be applied on components, such as Chromium Carbide—Nickel Chromium (CrC—NiCr). Over time, however, high temperature oxidation and hot corrosion, may form corrosive deposits which attack and degrade the protective oxide scale. In particular, conventional IGTs operate for long periods of time at a constant high temperature resulting in type I and type II corrosion. Whatever the source, coatings and base material of gas turbine engine components experience degradation which may be repairable in order to return these high-value parts to service.
As shown in
In order to repair such engine components, they are removed from the engine for a repair process which includes cleansing all contaminants from the base material and restoring the components to an operable condition. In order to do this, as depicted schematically in
Airfoils are involved in repair operations, including blades (or buckets) and vanes (or nozzles), which may be cast using equiax, directional solidification, or single crystal methods depending on the superalloy. Items like transition ducts, liners, combustors (including combustor liners and fuel nozzles) and end caps have also been involved in repair operations. One component subject to repair is a tile (functionally the same as a shroud in aero engines). In the past the repair method included immersion in acid to dissolve the coating, followed by FIC to remove the deposits that developed in the cracks during engine operation. Original manufacture of these components typically involved casting using equiax methods.
Two exemplary parts, shrouds (or tiles), and vanes (or nozzles) are both subject to cyclic fatigue cracks in the corners and filet radii, or just in general on a tile or shroud. Repair requires coatings be stripped from the parts which are then subjected to FIC to remove the deposits. Repair alloys are then applied to the parts to heal the cracks, such as GE's Activated Diffusion Healing (ADH) alloy and Partitioned Alloy Component Healing (PACH) alloy. See e.g., U.S. Pat. No. 4,830,934.
Tiles or shrouds have been attacked by the acids used in the chemical stripping such that it has become a standard trade practice to only grit blast the coatings off, followed by FIC.
FIC also has some peculiar shortcomings, especially when it comes to chromium rich either substrates or coatings, or spinels. Chromium has been observed on occasion to react during FIC becoming like a coating and depositing on the FIC chamber such that parts were coated rather than cleaned. It has been reported that cobalt based substrate IGT components with CoCrAlY's as a coating must be run either twice or three times through the FIC to fully remove the spinels. Occasionally the FIC's themselves must be cleaned out by running empty heat cycles.
Based on the issues discussed above, there is a need for improving the process to economically clean coatings from components used in gas turbine engines, particularly cobalt based substrates and CoCrAlY coatings and nickel based substrates with MCrAlY coatings.
A method for removing a coating from a gas turbine engine component is provided, including the steps of providing a bath with a conformal cathode and electrolyte solution, placing the component in the bath and connecting a power source to the component such that it will act as an anode, and agitating the solution while passing an electric current between the components and the cathode to electrolytically remove the coating and spinels that may have formed in stress corrosion cracks in the substrate. The components can then be removed from the bath, rinsed, and optionally undergo a high temperature, hot vacuum cycle in a heat treat furnace and have a braze alloy applied to the cracks without requiring an intervening fluoride ion cleaning cycle. The electrolyte solution has a pH that is basic and can have a chemical composition including one or more of sodium citrate, sodium percarbonate, sodium bicarbonate, and sodium phosphate. The electric current can be supplied by a rectifier 42 capable of providing up to 4000 A at no more than 5 V, the amperage actually supplied calculated based on the surface area to be cleaned. The conformal cathode is made from an electrically conductive material, having a high incipient melting point and which is inert when immersed in a solution having a pH in a range of about 9 to 11. The electrolyte solution is heated to maintain a temperature between about 48° C. to about 75° C. The components has a cobalt base substrate and/or a cobalt containing coating such as CoCrAlY or a CrC—NiCr coating.
Apparatus for use in performing the method are described, including a bath holding electrolyte solution, a conformal cathode in the shape to enable electrolytic stripping of the component, a heater for controlling the temperature of the solution, a rectifier for providing a direct current between the component as an anode and the conformal cathode, and an agitator for agitating the solution. The rectifier 42 is capable of providing up to 4000 A while controlling the voltage between 3.75 volts to 4.25 volts. The pH of the solution is alkaline or basic enabled by a chemical composition such as sodium citrate, sodium percarbonate, sodium bicarbonate, or sodium phosphate. The conformal cathode is made from an electrically conductive material that is inert when immersed in a solution having a pH in a range of about nine to about 11 and with the thickness capable of conducting 4000 A at about 4 V without loss of material. The agitator can be a direct in the tank agitation pump or an external pump flowing the solution into the tank, such as through a sparger flowing only liquid.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the detailed description given below, serve to explain various aspects of the invention.
As previously discussed,
Tile/shroud samples from components that had been run in an engine and had stress corrosion cracks with spinels were tested using the method and apparatus described above. The substrate was a cobalt based superalloy, GE FSX414 having an environmental coating. Coatings were removed from a tile/shroud sample using a method including the steps of providing the bath with a conformal cathode and an electrolyte solution, placing the tile/shroud sample in the bath and electrically connecting tile/shroud sample to a rectifier power source which enabled the tile/shroud sample to act as an anode while passing an electric current through the solution between the tile/shroud sample and the conformal cathode. Surprisingly coatings were stripped so effectively that it was determined that it was not necessary to perform the conventional FIC cleaning prior to application of the braze alloy. The tiles came out of the stripping process clean enough that they could be brazed directly, although it may be advantageous to run the parts in a high hot vacuum cycle, like 1975° F. for two hours at a range of 10−4 or 10−5 Torr. Coming out of this cycle the braze can be applied directly to the part. The recipe for the chemical solution was ⅓rd molar sodium citrate and ⅓rd molar sodium percarbonate. The pH of the solution was around 9-10. Voltage was applied through the rectifier between the parts (as an anode) and a conformal cathode. This voltage was controlled to be around 4 volts and restricted so as to never permit it to exceed, five volts.
The conformal cathodes were made from SS 304. The thickness of these cathodes must be adequate to carrying the current load. In one bath a rectifier 42 capable of 4000 amperes was used, so the cathodes were large and thick to carry this current. While ability to carry such a high current would suggest copper being better than SS 304, copper is a metal which would deleteriously affect the substrate of the desired part or component by incipient melting and thus should not be used. Copper, silver or other such metals should be avoided for use as cathodes, and metals like titanium are not conductive enough and would need to be very thick to be effective.
The baths can be run from 48° C. as a standard but can be up to 75° C. Higher temperatures of the bath begins to drive off the percarbonate portion and the cost of the operation increases substantially.
It has been determined that the electrolyte solution must be agitated. In one embodiment, direct in the tank agitation can be accomplished by a pump such a Flo-King Pump. In another embodiment the solution can exit the tank for filtering the solution and reentering the tank via a sparger. Ventilation is not necessary.
When percarbonate is used, a portion of the molecule, that portion which is a peroxide ion is eventually consumed and driven off. The remainder of the molecule is a carbonate, really a carbon and a number of oxygens. When this occurs the chemical reaction will continue to work just at a slower rate.
There is some deep chemistry which is behind the excellent performance of this bath. First, Aluminum, yttrium and chromium can all be stripped in a solution wherein the pH is 9-10 regardless of the chemical method whereby it was contrived. Sodium hydroxide might therefore be an alternative except that it is very high pH. A typical 1 molar solution would be around pH 12. When such a solution was attempted in the same manner in a laboratory large holes developed in the substrate much more dramatically than typical Hydrochloric acid pitting. Therefore, gentle dilute basic solution like those proposed are the likely best candidate for this operation, with a preferred solution being generally basic and one embodiment having an electrolyte solution with a pH of between about 9 and about 12, and another embodiment with a pH of between about 9 and about 10. It has been determined acidic electrolytic solutions such as using oxalic acid would cause significant loss of substrate material when a current is applied. Alternative alkaline compositions such as sodium phosphate could be used. The electrolyte solution can be optimized based on the component materials and coatings involved, the current and the conformal cathode.
While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in some detail, it is not the intention of the inventor to restrict or in any way limit the scope of the appended claims to such detail. Thus, additional advantages and modifications will readily appear to those of ordinary skill in the art. The various features of the invention may be used alone or in any combination depending on the needs and preferences of the user.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2018/025771 | 4/2/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62453006 | Feb 2017 | US |
