Patent Application
20140311918
Embodiments of the invention relate generally to electrochemical stripping methods for stripping metallic coatings of coated articles, such as aluminide coatings, from surfaces of metallic or non-metallic articles.
Stripping of metallic coatings is an important step in a number of manufacturing processes, such as turbine blade repair, for example. Metallic coatings are provided on articles to provide protection, for example environmental protection, to the articles. Removal of a metallic coating permits at least one new coating to be applied to such an article to restore its protective properties for subsequent use. For example, the composition of diffusion or overlay metallic coatings on turbine blades typically includes, but is not limited to, platinum aluminide (PtAl). The composition of the substrate (also referred to as a “base alloy” or a “parent alloy”) of the turbine blades typically includes, but is not limited to, Rene N5® brand superalloy.
A stripping process should be sufficiently selective, meaning that the stripping process removes only intended materials, while preserving an article's desired structures. For example, stripping processes should remove metallic coatings from the turbine blade without consuming or otherwise modifying the underlying substrate. Thus, the turbine blade's structural integrity will be maintained after the stripping process.
Electrochemical stripping method is one known method for stripping metallic coatings, such as aluminide coatings, from turbine blades. Referring to
However, the above one-step electrochemical reaction is sometimes “non-selective,” meaning that the stripping process cannot sufficiently distinguish between the metallic coatings and the substrate of the article 6, leading to degradation of the article's performance and reliability. In a worst-case scenario, the article 6 may be rendered unusable and scrapped.
For these and other reasons, there is a need for embodiments of the invention.
In accordance with an embodiment disclosed herein, a multi-step electrochemical stripping method for stripping metallic coatings of a coated article is provided. The multi-step electrochemical stripping method includes:
(a): providing a determined electrode potential between a reference electrode and the coated article submerged in an electrolyte;
(b): recording a current peak value of a current signal flowing through the coated article;
(c): removing the voltage provided to the coated article when the current signal falls to a determined first current value after passing the current peak value;
(d): refreshing the electrolyte;
(e): providing the determined electrode potential between the reference electrode and the coated article for a determined time and determining whether the current signal is less than a determined second current value during the determined time;
(f): repeating steps (d) and (e) if the current signal is not less than the determined second current value; and
(g): removing the voltage provided to the coated article if the current signal is less than the determined second current value, wherein the determined first current is greater than the determined second current value.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Embodiments of the invention relate to a multi-step electrochemical stripping method for stripping metallic coatings of a coated article. The multi-step electrochemical stripping method includes providing a determined electrode potential between a reference electrode and the article submerged in an electrolyte; recording a current peak value of a current signal flowing through the article; removing the voltage provided to the article when the current signal falls to a determined first current value after passing the current peak value; refreshing the electrolyte; providing the determined electrode potential again for a determined time and determining whether the current signal is less than a determined second current value during the determined time. If not, the electrolyte is refreshed again and the process continues. If the current signal is less than a determined second current value during the determined time, the process ends.
Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items, and terms such as “front”, “back”, “bottom”, and/or “top”, unless otherwise noted, are merely used for convenience of description, and are not limited to any one position or spatial orientation. Moreover, the terms “coupled” and “connected” are not intended to distinguish between a direct or indirect coupling/connection between two components. Rather, such components may be directly or indirectly coupled/connected unless otherwise indicated.
Referring to
When the article 6 needs to be stripped, the electrode 40, the article 60, and the reference electrode 70 are submerged in the electrolyte 30 and electrically coupled to the control circuit 50. The control circuit 50 controls an electrode potential between the article 60 and the reference electrode 70 within a determined voltage range by providing a cell voltage between the electrode 40 and the article 60. Therefore, a desired electric field can be established between the electrode 40 and the selected coated surfaces of the article 60, which can corrode away the metallic coatings of the article 60.
This illustrated embodiment of
The receptacle 20 can be any appropriate non-reactive receptacle. The shape and capacity of the receptacle 20 may vary according to the application, as long as the receptacle 20 is sized sufficiently to receive the electrolyte 30, the electrode 40, the article 60, and the reference electrode 70. The material of the receptacle 20 may also vary as long as it is non-reactive and does not interfere with the electrochemical stripping process, such as glass material.
In one embodiment, the electrolyte 30 may include a charge-carrying component in a solvent, such as but not limited to a halide salt solution. The electrolyte 30 can be delivered into the receptacle 20 by any appropriate means. For example, and in no way limiting of the invention, the electrolyte 30 may be poured into the receptacle 20. Alternatively, the electrolyte 20 can be delivered into the receptacle 20 by a pumping device 90 (shown in
In non-limiting embodiments, the composition of diffusion or overlay metallic coatings of the article 60 may include, but is not limited to, platinum aluminide (PtAl). The composition of the substrate of the article 60 may include, but is not limited to, a nickel-based superalloy (such as René N5® brand superalloy). In other embodiments, the article 60 and its composition also can be changed according to requirements. For example, the metallic coatings of the article 60 also may be aluminide, nickel-aluminide, platinum-nickel-aluminide, and mixtures thereof. The substrate of the article 60 also may be a cobalt-based superalloy or an iron-based superalloy, etc.
Referring to
Referring now to
In one embodiment, when the article 60 may be a turbine blade, the composition of diffusion or overlay metallic coatings of the turbine blade includes platinum aluminide (PtAl), and the composition of the substrate of the turbine blade includes René N5® brand superalloy, the charge-carrying components of the electrolyte 30 can be selected according to the following table, and the solvent can be selected from distilled water or tap water.
The charge-carrying components of the electrolyte 30 includes three component groups A, B, and C, and each group includes at least one component selected from the corresponding components listed in the above table. In one embodiment, the group A includes components which are used to implement a stripping function to the article 60. The group B includes components which are used to improve ion transfer within the electrolyte 30 and therefore enhance the uniformity of ion concentration at the coating surface when the electrochemical stripping occurs. The group C includes components which are used to provide an acid environment (Namely PH<7, and in certain embodiments, PH<1.5) to achieve a better selectivity (the term “selectivity” means that the stripping process removes only intended materials, while preserving article's desired structures).
In a particular embodiment, the charge-carrying components of the electrolyte 30 may include about 3 wt % NaCl (from A), 5 wt % NH4Cl (from B), and 3 wt % H3PO4 (from C). The solvent of the electrolyte 30 may include distilled water. The PH value of the fresh solution of the electrolyte 30 is about 0.8. The temperature of the electrolyte 30 is controlled at 30±3 degrees C. The circulating rate of the electrolyte 30 is about 400 ml/min by using the pump device 90.
In this embodiment of
At step S2 of
At step S3, the controller 51 outputs a control command to the first voltage regulator 52 and the second voltage regulator 53 to remove the voltage provided to the electrode 40 and the article 60 when the current signal falls to a determined first current value after passing the current peak value Ipeak, that is suspending the stripping process. In one embodiment, the determined first current value can be selected in a current range, such as 20%-90% of the current peak value Ipeak. In a particular embodiment, the determined first current value may be 40%±5% of the current peak value Ipeak.
At step S4, the electrolyte 30 is refreshed, namely the ion concentration at the surface of the article 60 is refreshed to the initial state. For example, the electrolyte 30 can be refreshed by itself, which results in the stripping process being suspended for a determined time, such as 30 minutes. In an alternative embodiment, the electrolyte 30 also can be refreshed by using the pump device 90 (shown in
At step S5, the controller 51 sends control commands to the first voltage regulator 52 and the second voltage regulator 53 to control the electrode potential between the reference electrode 70 and the article 60 to reach the determined voltage again (restart stripping process) for a determined time, such as 10 minutes, and then to remove the voltage provided to the electrode 40 and the article 60 (suspend the stripping process again). Meanwhile, the controller 51 determines whether the current signal is less than a determined second current value during the determined time. The determined second current value is less than the determined first current value. If yes, the entire stripping process ends. If not, the stripping process goes back to the step S4. In a particular embodiment, the determined second current value may be about 10% of the current peak value Ipeak. In an alternative embodiment, the determined second current value may be a stable current value, such as 0.002 A.
Referring to
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
It is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201110337199.3 | Oct 2011 | CN | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2012/059677 | 10/11/2012 | WO | 00 | 4/29/2014 |
