The present invention relates to a method of applying a protective coating to an interior surface. More specifically, the present invention relates to a method of electrodepositing a thin coating uniformly to all interior surfaces of a device.
A coating may commonly be applied to metal surfaces to form a protective layer, such as for corrosion resistance. In many applications it may be important that the coating be thin, yet uniformly applied to the surface. For example, if the coating is for an interior or an exterior of a heat exchanger, it may be important to minimize a thickness of the coating in order to minimize heat transfer losses.
Electrodeposition may commonly be used to apply a coating to a metal surface. However, it may be difficult to uniformly apply a thin coating to interior surfaces of a device, particularly devices having complex shapes and/or small passageways.
The present invention relates to a method of applying a coating to an internal surface of a device. The method comprises applying an electric current through an interior space of the device to electrodeposit resin particles onto a first portion of the internal surface and curing the resin particles to form a coating on the first portion of the internal surface. The method further comprises repeating an application of the electric current through the interior space of the device to electrodeposit resin particles onto a second portion of the internal surface and curing the resin particles to form a coating on the second portion of the internal surface. The application of the electric current through the interior space and the curing of the resin particles may be repeated until a coating is formed on all of the internal surface.
A method is described herein for electrodepositing a thin coating on internal surfaces of a device. The method is well-suited for complex shaped devices that may include areas that are commonly hard to reach and present a challenge to uniformly coating all interior areas of the device.
Electrodeposition or electroplating may be used to coat a metal surface of a device with a resin using electric current. A flow of current from an anode causes resin particles to be deposited onto the surface of the grounded metal device. The deposited resin may then be cured to form a protective coating, which may be used, for example, for corrosion resistance.
The electrodeposition process may be used for applying a coating to internal surfaces of a device. However, if a single application of current is applied to the anode, it may be difficult to deposit resin particles on the surface of recessed areas of the device. This may be due in part to an inability to place the anode inside the device or in proximity to all interior spaces of the device. In that case, the resin particles may deposit on a portion of the internal surface located closest to the anode.
Once the deposited resin particles are cured on the metal surface to form a hardened coating, the coating may insulate the metal surface from further deposition of resin particles. Thus, as described in further detail below, the insulative properties of the coating may be used, in a subsequent application of current and additional resin, to drive the flow of current from the anode further into the recesses of the device. This method makes it feasible to uniformly apply a thin protective coating to all interior surfaces of a complex shaped device, such as a heat exchanger or a radiator.
First reservoir 24 of device 14 is configured as an entrance reservoir and includes inlet port 30, and second reservoir 26 is configured as an exit reservoir and includes outlet port 32. As such, resin may be delivered from pump 22 into device 14 through inlet port 30 and out of device 14 through outlet port 32. (Inlet and outlet ports 30 and 32 may similarly be used for pumping or circulating fluid through device 14 during operation of device 14 for heat exchange.)
Tubes 28 may be long and narrow, making it difficult to deposit resin into a center portion of each of tubes 28. In some embodiments, tubes 28 may have a flattened shape, as opposed to having a circular diameter. Using system 10, it is possible to apply a uniform coating to all interior surfaces of device 14, including all interior surfaces of tubes 28.
In system 10, DC power supply 16 has a positive terminal (designated as + in
The electric field between the positively charged anodes 18 and 20 and the negatively charged cathode (i.e. device 14) causes resin particles (not shown) being pumped through an interior of device 14 to be attracted to and deposit onto the negatively charged metal surfaces of device 14. System 10 uses a cathodic electrocoating process, meaning that the resin particles deposit onto a negatively charged surface (device 14), which is the cathode. In alternative embodiments, an anodic electrocoating process may be used; in that case, the terminals are reversed, such that device 14 is positively charged (i.e. an anode) and an anodic resin may be deposited onto the positively charged metal surface of device 14.
By using pump 22 (see
The resin solution may be any type of solution suitable for forming a coating on a metal surface, including, but not limited to an organic coating, such as an epoxy. In some cases, a particular resin may be designed for only a cathodic electrocoating process or only an anodic electrocoating process. For example, since system 10 uses a cathodic electrocoating process, the resin solution that includes resin particles 38 is a cathodic resin that is configured to deposit onto the negatively charged surface of device 14. If system 10 alternatively used an anodic electrocoating process, an anodic resin may be used.
As described above in reference to
As current flows as a result of voltage V, resin particles 38 are attracted to the negative charge on the bare metal surfaces of device 14, including interior surfaces 40 of first and second reservoirs 24 and 26, and interior surfaces 42 of tubes 28a, 28b and 28c. The attractive forces between the resin and the metal cause particles 38 to deposit onto interior surfaces 40 and 42. A thickness of a coating formed by resin particles 38 on interior surfaces 40 and 42 is a function in part of voltage V. Thus, voltage V may be controlled in order to control the thickness of the coating, as explained in further detail below.
After resin particles 39 are deposited onto interior surfaces 40, a next step is to cure resin particles 39 such that the resin particles harden and form coating 44 on interior surfaces 40. Prior to a curing process, a rinse solution may be pumped through the interior of device 14. In addition, deionized water may be flushed through the interior. At that point, anodes 18 and 20 may be removed from device 14. The curing of particles 39 to form coating 44 may be performed by exposing device 14 to a high temperature.
The steps described above are then repeated in order to deposit resin particles onto interior surfaces 42 of tubes 28a, 28b and 28c. Thus, anodes 18 and 20 are inserted back into first and second reservoirs 24 and 26. Resin particles 38 are again pumped through interior surfaces of device 14, and voltage V is redelivered from power supply 16 to anodes 18 and 20.
As described above in reference to
Once voltage V has been applied for the designated time, power supply 16 may be turned off and anodes 18 and 20 may be removed from device 14, and the interior of device 14 may be flushed out as described above. The same curing process may then be used to cure resin particles 39 formed on first portions 50 of interior surfaces 42 to form coating 44 (see
Finally, in
In an exemplary embodiment of system 10, the electrocoating process, as shown in
As stated above, a thickness of coating 44 may be controlled as a function of how much voltage is applied to anodes 18 and 20 and for how long. In order to determine a value or a range of values for voltage V for coating interior surfaces 42 of tubes 28, experiments may be done on individual tubes having similar dimensions to tubes 28. After each deposition of resin particles 39 and a curing process, the tube may be cut open or otherwise examined to determine a thickness of the coating and how far the coating penetrated into an interior of the tube. If these experiments are performed over a range of voltages for a given time and a given tube size, it may be possible to determine a thickness of the coating formed as a function of the voltage. Moreover, the experiments may be used to determine how many times the process must be repeated to coat all of the interior of the tube.
In the exemplary embodiment of FIGS. 1 and 2A-2E, device 14 is a heat exchanger that may be used for an aircraft. However, it is recognized that the method described herein may be used for coating an interior of any type of device, including, but not limited to, other types of heat exchangers and any type of radiator.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
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Number | Date | Country | |
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20080156647 A1 | Jul 2008 | US |