CN1234079A - Advanced electrochemical corrosion protection methods - Google Patents

Abstract

A method for reducing the galvanic corrosion of dissimilar metals in contact with each other is characterized in that the exposed surface of the noble metal is coated with a more electrochemically compatible metal that is less noble.

Description

Advanced electrochemical corrosion protection methods

The present invention relates generally to the protection of metals in corrosive environments, and in particular to the protection of copper-aluminum heat exchangers for air conditioners.

Galvanic corrosion occurs when two dissimilar metals come into contact with each other in the presence of an electrolyte to form a pair of electrodes. More expensive (higher potential series) metals provide surface area for reduction reactions, while less expensive (lower potential series) metals corrode during oxidation. Oxidation occurs most at the interface of two metals, but can also occur at some distance from the actual interface. In coastal areas, the most common electrolyte is brine from the air. A fine mist of salt water can be blown inland as far as fifty miles from the coast. Sulfur dioxide from industrial pollution also forms an electrolyte when it combines with moisture in the air.

The usual method of preventing galvanic corrosion is to coat exposed metal surfaces with various paints. The effectiveness of these protective coatings is limited for a number of reasons. The main problem with coatings is that their effectiveness in preventing corrosion is reduced by exposure to environments such as UV light and acid rain. Another common problem is that coating materials often do not adhere well to the substrate and actually flake off or erode away to expose the metal substrate. Furthermore, this protective coating is porous and allows the electrolyte to pass through the surface of the substrate to connect the electrode pairs. Additionally, applying a protective coating to the surface of some items can have an adverse effect on their performance.

Attempts have been made with varying degrees of success to extend the useful life of the unit by coating conventional copper-aluminum heat exchangers with various materials. These coating materials often reduce the heat transfer capability of the device, adhere poorly, and do not penetrate all areas that may be exposed to corrosive environments.

The present invention generally provides an advanced method of electrochemical corrosion protection. According to the invention, in an article made of two metals, one of which is more noble than the other, the outer surface of the more noble metal is electrically bonded to the less noble metal. Chemically compatible metals are treated to form a protective layer between the two dissimilar metals, which prevents reduction reactions of the electrode pair. The present invention greatly reduces the redox reaction process that occurs when two dissimilar metals come into contact with each other in the presence of an electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a heat exchanger with heat exchanger tubes treated according to the method of the present invention.

The present invention will be specifically described below in conjunction with electrochemical corrosion protection for copper-aluminum heat exchangers. It will be apparent, however, that those skilled in the art are not limited to this particular example, but can use many configurations for bringing dissimilar metals into contact with each other in the presence of an electrolyte.

Figure 1 shows a fin-and-tube heat exchanger 10 typically used in air conditioning units. The heat exchanger includes one or more flow lines for carrying refrigerant through the heat exchanger assembly. For ease of illustration, the heat exchanger 10 includes a single flow line tube 2 consisting of an input line 3 and an output line 4 connected at one end of the heat exchanger 10 by a 90° elbow 5 . However, it is obvious that more lines can be added to the device according to the requirements of the system. The device also includes a series of cooling fins 6 consisting of radially arranged plate-like elements spaced along the length of the flow path. The cooling fins 6 are supported in the assembly between a pair of end plates 7 and 8 to define a gas flow passage through which gas passes over the conduit 2 and between the spaced apart cooling fins 6 .

As noted above, heat exchangers of this type are typically exposed to corrosive environments. In a typical configuration, this type of heat exchanger is made with copper tubes for the line flow tubes and aluminum for the fins. The fins are configured to contact the line tubes and remove heat from the line tubes by conductive heat transfer and then convective heat transfer to a gas (usually air) flowing around the line tubes. Copper is used in line pipe construction because of its good heat transfer characteristics, corrosion resistance, and ease of repair. Aluminum is used for the heat sink because of its good heat transfer characteristics, ease of manufacture, and low cost. In some applications, heat exchangers are all copper and all aluminum to avoid electrochemical corrosion problems, but the cost is significantly increased.

Aluminum is very low in potential series, ie it is less expensive than copper. Therefore, aluminum oxidizes or corrodes when it comes into contact with copper in the presence of an electrolyte. In the structure shown in Figure 1, the interface between the line pipe and the heat sink is where the electrode pair is generated and the aluminum heat sink is corroded. Once the fins corrode at the intersections, the fins are no longer in contact with the conduits, and thus the heat exchange efficiency is greatly reduced as the fins lose their ability to conduct heat away from the conduits.

As will be explained in more detail below, according to the invention, the exposed surface of the conduit 2 is coated or enriched with aluminum or a metal which is galvanically compatible with aluminum. Since no electrode pair is formed between the aluminum coating and the aluminum heat sink 6, aluminum is the best candidate metal. However, active metals such as zinc, tin, magnesium, gallium, cadmium, and lead will also degrade the electrode pair, thereby reducing the oxidation rate of the heat sink material.

Coating or surface enrichment of copper conduit 12 with aluminum is performed prior to assembly of heat exchanger 10 . Aluminization of copper is a well known practice and can be carried out to a degree of precision that virtually eliminates the above-mentioned problems of conventional coatings for corrosion protection. Several processes for aluminizing copper tubing are available in the industry and are used in the present invention. The coating process includes hot dipping, electroplating, aluminum-filled paints and coatings, and thermal spraying. Surface concentration processes include ionic vapor deposition, chemical vapor deposition, and physical vapor deposition.

The main aspect of the invention is the production of a uniform coating of aluminum over the entire surface of the flow line tube 2 . Regardless of the process under consideration, variables such as pipe surface preparation, pipe preheat temperature, coating composition, and coating thickness must be carefully controlled to achieve the results characteristic of the present invention. The preparation of the exposed surface of the conduit is best done by removing the surface oxide from the copper to ensure good adhesion of the coating material to the conduit. A number of surface preparation processes exist in industry, including the use of reducing gases, fluxes, and sandblasting. The line pipe preheating temperature should be controlled between 24°C and 600°C to prevent copper dissolution and limit the growth of intermetallic compounds during the coating process.

It is preferred that the coating has high ductility so that subsequent assembly of the heat exchanger will not damage the coating. The ductility of the coating depends in part on the composition of the coating and the thickness of the coating. As noted above, any metallic composition that is electrochemically more compatible with the heat sink material than the line tubing material will reduce the oxidation rate of the heat sink 6, while ideally the coating material will be perfectly compatible with the heat sink material. Certain aluminum alloys are contemplated for use in the present invention, including silicon-incorporated aluminum and zinc-incorporated aluminum. The coating must be thick enough to prevent electrolyte penetration. However, since any coating has a detrimental effect on the heat transfer of the device, excessively thick protective layers should be avoided. The optimum thickness range contemplated by the present invention is 0.1 mil to 2 mil.

Claims (10)

1. A method of preventing galvanic corrosion of the surface of a first member disposed in contact with a second member, said first member being formed from a less noble metal than the second member, said first member having a suitable a surface in contact with a surface of said second member, comprising the steps of: Treat the exposed surface of the second piece with some less expensive metal. 2. The method of claim 1, wherein said treating step includes coating the exposed surface of the second member with a less expensive metal to form a more expensive metal on the exposed surface of the second member. Fewer metal layers. 3. 2. The method of claim 2 wherein said less expensive metal layer has a thickness of between about 0.0001 and 0.002 inches. 4. The method according to claim 3, wherein the coating process is a hot dipping process, an electroplating process, a painting process or a diffusion coating process. 5. 2. The method of claim 1, wherein said treating step is a surface enrichment process. 6. The method of claim 5, wherein said treating step is an ion vapor deposition process, a chemical vapor deposition process or a physical vapor deposition process. 7. The method of claim 1 wherein said second piece consists essentially of copper, said first piece consists essentially of aluminum, and said less expensive metal consists essentially of aluminum or includes silicon or zinc. Aluminum alloy composition. 8. The method of claim 1 wherein said second piece consists essentially of copper, said first piece consists essentially of aluminum, and said less expensive metal comprises a group consisting of zinc, tin, magnesium , gallium, cadmium, lead, and combinations thereof selected from the group of elements less expensive than aluminum. 9. A heat exchanger produced by the method of claim 1. 10. The heat exchanger according to claim 9, wherein said first piece is a heat sink made of aluminum, said second piece is a wiring tube made of copper, said relatively expensive The lesser metal is selected from the group consisting of aluminum, silicon aluminum alloy, zinc aluminum alloy, zinc, tin, magnesium, gallium, cadmium, lead, and combinations thereof.

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