BACKGROUND

[0001] 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.

[0002] 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.

[0003] 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.

[0004] Electrochemical stripping method is one known method for stripping metallic coatings, such as aluminide coatings, from turbine blades. Referring to FIG. 1, a conventional electrochemical stripping system 1 is shown. The system 1 may include an electrolyte bath receptacle 2 containing an electrolyte 3, a cathode 4, and a direct current (DC) power supply 5. When an article 6, such as a turbine blade, needs to be stripped, the cathode 4 is submerged in the electrolyte 3 and electrically coupled to the negative terminal of the DC power supply 5, and the article 6 as an anode is submerged in the electrolyte 3 and electrically coupled to the positive terminal of the DC power supply 5. After activating the DC power supply 5, the metallic coatings on the article 6 are corroded away by the electrolyte 3 during a one-step electrochemical reaction known in the art.

[0005] 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.

[0006] For these and other reasons, there is a need for embodiments of the invention.

BRIEF DESCRIPTION

[0007] 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.

DRAWINGS

[0008] 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:

[0009] FIG. 1 is a schematic view of a conventional electrochemical stripping system.

[0010] FIG. 2 is a schematic view of an electrochemical stripping system according to one embodiment.

[0011] FIG. 3 is a schematic view of an electrochemical stripping system according to another embodiment.

[0012] FIG. 4 is a block diagram of a control circuit of the electrochemical stripping system of FIG. 2.

[0013] FIG. 5 is a flowchart of a multi-step electrochemical stripping method according to one embodiment. [0014] FIG. 6 is a diagram of a current density for the metallic coatings and the substrate of an article, as a function of an electrode potential between the article and a reference electrode, for stripping system treated by the present invention.

[0015] FIG. 7 is a schematic view of a turbine blade after electrochemical stripping by using a conventional one-step electrochemical stripping method and by using the multi-step electrochemical stripping method of FIG. 6, respectively.

DETAILED DESCRIPTION

[0016] 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.

[0017] 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.

[0018] Referring to FIG. 2, an electrochemical stripping system 10 according to one embodiment includes an electrolyte bath receptacle 20 (hereinafter "receptacle") containing an electrolyte 30, an electrode 40, a control circuit 50, a coated article 60 to be stripped, and a reference electrode 70. In one embodiment, the electrode 40 may act as a cathode, and the coated article 60 may act as an anode corresponding to the electrode 40. In an alternative embodiment, the electrode 40 may act as an anode, and the coated article 60 may act as a cathode. The reference electrode 70 is used to provide a reference voltage, for example received from an external power supply (not shown).

[0019] 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.

[0020] This illustrated embodiment of FIG. 2 shows only one electrode 40, however this is merely exemplary and not intended to limit the invention in any manner. In other embodiments, there may be two or more electrode 40 's, to further improve efficiency. The electrode 40 is formed with an appropriate geometry that is configured to direct electrical fields to the surfaces of the article 60. Appropriate geometric configurations for the electrode 40 falling within the scope of the invention include, but are not limited to, planar geometries, cylindrical geometries, and combinations thereof. Alternatively, the electrode 40 can include a complex geometrical configuration, such as a geometrical configuration that is approximately complementary to the geometry of the article 60 that is to be stripped. The electrode 40 is generally non-consumable and remains intact throughout the electrochemical stripping process.

[0021] 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.

[0022] 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 FIG. 3). In the embodiment of FIG. 3, an electrolyte pool 21 is introduced, which contain sufficient electrolyte 31. The electrolyte 31 is the same as the electrolyte 30, and is used to refresh the electrolyte 30 by using the pumping device 90. For example, the pumping device 90 pumps the electrolyte 31 through a pipe 55 and transports the electrolyte 31 into the receptacle 20 through a pipe 56. Meanwhile, the electrolyte 30 is transported to the electrolyte pool 21 through a pipe 57 by some control devices (not shown), such as control valves, sensors, etc., according to conventional control methods. Therefore, an electrolyte circle is established to refresh the electrolyte 30 during the stripping process. In one embodiment, the circulating rate of the electrolyte 30 in the electrolyte circle may be 100ml/min-800ml/min. [0023] 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 Rene 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.

[0024] Referring to FIG. 4, an exemplary embodiment of a control circuit 50 of the electrochemical stripping system 10 is shown. The control circuit 50 may include a controller 51, a first voltage regulator 52 electrically coupled to the electrode 40, a second voltage regulator 53 electrically coupled to the article 60, and a current sensor 54 electrically coupled between the second voltage regulator 53 and the article 60. The first voltage regulator 52 is used to regulate voltage to the electrode 40 according to control commands from the controller 51. The second voltage regulator 53 is used to regulate voltage to the article 60 according to control commands from the controller 51. The current sensor 54 is used to sense current flowing through the article 60 and the sensed current is received by the controller 51 during the entire stripping process. The controller 51 also receives the reference voltage of the reference electrode 70. In various embodiments, the controller 51 may be any appropriate programmable device which can receive sensed signals and output control signals according to determined programs therein. For example, the controller 51 may be a micro control unit or a processor. The voltage regulators 52 and 53 may be any appropriate circuit configurations, such as voltage transformers, which can regulate different voltage values to elements. The voltage regulators 52 and 53 also can be integrated together as one element. In other embodiments, the electrode 40 also can receive a stable voltage, which means the first voltage regulator 52 can be omitted accordingly. [0025] Referring now to FIG. 5, a flowchart of a multi-step electrochemical stripping method according to one embodiment is shown. The electrochemical stripping system 10 applies the multi-step electrochemical stripping method to strip the article 60, which can achieve a better performance than the conventional one-step electrochemical stripping method. In the multi-step electrochemical stripping process, as embodied by the invention, electrochemical stripping process parameters (hereinafter "stripping parameters") also define the stripping characteristics. These stripping parameters influence the rate of material removal and thus the efficiency of the stripping process. The stripping parameters include, but are not limited to, electrode geometry, electrode potential between reference electrode and article, reference electrode material, electrolyte composition and concentrations, distance between electrode and article, electrolyte temperature, and so on. The stripping parameters may vary over operational ranges. For example, the electrode potential may vary from a trace voltage (the term "trace" means a small but measurable value) to at least about 30V. The distance between the article 60 and the electrode 40 may vary in a range from about 0.1 inches to about 10 inches. The temperature of the electrolyte 30 may vary from 20 decrees C to 60 degrees C. The stripping time depends on the metallic coating composition, microstructure, density, and thickness. The electrochemical stripping time may increase with thicker coatings. For example, the stripping time of an electrochemical stripping process may vary in a range from about 0.1 minutes to about 4 hours.

[0026] 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 Rene 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.

Potassium

KC1 <10 3

chloride

Sodium

NaBr <10 3

bromide

Potassium

KBr <10 3

bromide

Sodium nitrate NaN03 3-15 9

Ammonium

NH4C1 <10 5

chloride

B

Ammonium

NH4N03 <10 5

nitrate

Hydrochloric

HC1 <10 5

acid

Sulfuric acid H2S04 <10 5

C

Nitric acid HN03 <10 5

Phosphoric

H3P04 <10 3

acid

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).

[0027] In a particular embodiment, the charge-carrying components of the electrolyte 30 may include about 3wt% NaCl (from A), 5wt% NH4C1 (from B), and 3wt% H3P04 (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 400ml/min by using the pump device 90. [0028] FIG. 5 is a flowchart of a multi-step electrochemical stripping method according to one embodiment. During the entire stripping process, the controller 51 receives the current signal sensed by the current sensor 54. In one embodiment, the multi-step electrochemical stripping method may begin at step SI . At step SI, the controller 51 sends control commands to the first voltage regulator 52 and the second voltage regulator 53 to provide a cell voltage between the electrode 40 and the article 60, which can control an electrode potential between the article 60 and the reference electrode 70 to reach a determined voltage value. The electrode potential between the reference electrode 70 and the article 60 is also a stripping parameter, which can be determined by experimental data. For example, FIG. 6 depicts a diagram of a current density for the metallic coatings and the substrate of the article 60, as a function of the electrode potential between the article 60 and the reference electrode 70, for stripping system treated by the present invention.

[0029] In this embodiment of FIG. 6, the reference electrode 70 is an Ag/AgCl reference electrode. In other embodiments, the reference electrode 70 also can be other kinds of reference electrodes, such as a standard hydrogen electrode (SHE), a saturated calomel electrode (SCE), etc. In FIG. 6, a selective window can be selected, in which the current density difference between the metallic coatings (curve Al) and the substrate (curve A2) is biggest. Thus, a determined voltage range could be selected according to the selective window, such as 0.05V-0.3V. Further, a particular determined voltage value also can be selected in the determined voltage range, such as about 0.2V. Therefore, the stripping process is triggered by providing the determined voltage value between the reference electrode 70 and the article 60.

[0030] At step S2 of Fig. 5, the controller 51 records a current peak value of the current signal sensed by the current sensor 54. In particular, once the stripping process starts, the current signal will increase gradually to achieve a current peak value Ipeak and decrease gradually. Thus, the controller 51 can record the peak value Ipeak according to above current trend feature. [0031] 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 I _pea k, 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 I _pea k.

[0032] 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 FIG. 3) to transport fresh electrolyte 31 into the receptacle 20. In a particular embodiment, the electrolyte 30 is refreshed by the pump device 90 with a circulating rate being about 400ml/min for about 5 minutes.

[0033] 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 0.002A. [0034] Referring to FIG. 7, a schematic view of a turbine blade after electrochemical stripping by using a conventional one-step electrochemical stripping method and after using the multi-step electrochemical stripping method disclosed herein, is shown and labeled as 100 and 200, respectively. Comparing the stripped turbine blade 200 with the stripped turbine blade 100, it can be seen that the residual coating 150 resulting from the conventional one-step electrochemical stripping method performed on turbine blade 100 is not present in turbine blade 200. Thus, the multi-step electrochemical stripping method described herein and illustrated by turbine blade 200 clearly improves the quality of the stripped surface of the turbine blade over that of the prior art methods depicted by turbine blade 100. That is, the multi-step electrochemical stripping method disclosed herein sufficiently distinguishes between the metallic coatings and the parent alloy, leading to a better stripping effect.

[0035] 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.

[0036] 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.