MXPA99003645A - Advanced galvanic corrosion protection - Google Patents

Abstract

A method for reducing the galvanic corrosion of dissimilar metals in contact with one another wherein the exposed surface of the more noble metal is coated with a metal more galvanically compatible with the less noble metal.

Description

PROTECTION AGAINST GALVANIC CORROSION ADVANCED DESCRIPTION OF THE INVENTION This invention relates generally to the protection of metals from a corrosive medium, and more specifically to the protection of copper-aluminum heat exchangers, for use in air conditioners. Galvanic corrosion occurs when two different metals make contact with one another in the presence of an electrolyte, which forms a galvanic couple. The more noble the metal (higher in the galvanic series) provides the surface area for the reduction reaction and the less noble the metal (lower in the galvanic series) it corrodes in an oxidation process. Oxidation occurs largely at the interface of two metals, but it can also occur at a distance from the current interface. In coastal regions, the most common electrolyte is salt water in the air. A fine spray of salt water can be blown inland up to fifty miles from the coast. Sulfur dioxide from industrial pollution also creates electrolytes when combined with the humidity of the air. A common method to prevent galvanic corrosion has been to coat the exposed surfaces of metals with various types of paint. These protective coatings have satisfied only with limited success for a number of reasons. The main problem with coatings is that their objectivity to prevent corrosion is degraded by exposure to the environment such as acid rain and aviolet light. Another common problem is that the coating materials often do not adhere well to the metal substrates and eventually peel or fall by erosion exposing the metal substrates. On the other hand, such protective coatings are somehow porous and allow the electrolyte to penetrate the surface of the substrates and connect the galvanic couple. In addition, the application of protective coatings to the surfaces of certain articles can negatively affect their performance. Attempts have been made, with varying degrees of success, to coat conventional copper-aluminum heat exchangers with various materials in an effort to extend the life of the unit. These coating materials very often reduce the heat transfer capacity of the unit, exhibit poor adhesion properties and fail penetration within all fa that may be exposed to the hostile environment. Japanese patent publication JP 53 132 449 A discloses a heat exchanger made of iron tube and aluminum fins where the complete assembly is coated with an aluminum welding plating. The British patent publication GB 2 284 882 A discloses a tube and fin heat exchanger made of tube, preferably steel, which is a metallic coating, in particular, a soft malleable metallic coating, to provide a corrosion barrier in the tube to prevent corrosion of the tube itself and to improve the bond between the tube and the fins. The French patent publication FR 2 179 317 describes a heat exchanger made of copper tubes and aluminum fins, where the tube is coated with a solder tin applied by means of a hot bath process until a thickness of 20 to 50 micrometers. Japanese patent publication JP 63 034 495A discloses a heat exchanger having aluminum fins and aluminum alloy tubing, which has a flame coating with an aluminum alloy solder, which may contain zinc. In general, the present invention provides an advanced galvanic protection method against corrosion. According to the present invention, in an article made of two metals, one being more noble than the other, the outer surface of the most noble metal is treated with a metal which is more galvanically compatible with the less noble metal than the metal more noble that is galvanically compatible with the less noble metal before assembly to form a protective layer on the surface of the most noble metal and between the least noble metal and the noblest metal where the galvanic corrosion, the less noble metal is reduced. Through the optimal selection of metal treatment applied to the most noble metal, the invention largely reduces the process of oxidation reduction which causes corrosion of the less noble metal, for example, an aluminum wing mounted on a copper tube of a heat exchanger, in the presence of an electrolyte. Figure 1 is a perspective view of a heat exchanger incorporating heat exchanger tubes treated in accordance with the present invention. As will be described in detail below, the present invention will be described to provide galvanic protection against corrosion of a copper-aluminum heat exchanger. However, it should be apparent to one skilled in the art that the present invention is not limited to this specific example and could be used in connection with a number of arrangements, where different metals are in contact with one another in the presence of an electrolyte Figure 1 illustrates a fin / tube heat exchanger 10 of the type normally used in air conditioning units. The heat exchanger includes one or more flow circuits for transporting the refrigerant through the heat exchanger unit. For the purposes of the explanation, the heat exchanger 10 contains a single flow circuit tube 2 consisting of an inlet line 3 and an outlet line 4 which are connected at one end of the heat exchanger 10 by means of a tube bent at 90 ° 5. It should be evident, however, that more circuits can be added to the unit depending on the demands of the system. The unit further includes a series of fins 6 comprising radially spaced-apart plate-like elements along the length of the flow circuit. The fins 6 are supported in the assembly between a pair of plate ends 7 and 8 to define a gas flow passage through which the gas passes over the extension of the tube 2 and between the spaced fins 6. As noted in FIG. the foregoing, heat exchangers of this type are commonly exposed in use to corrosive environments. In a typical arrangement of heat exchangers of this type, they are manufactured using copper tubes for the circuit flow tubes and aluminum for the fins. The fins are in contact with the tubes and extract the heat from the tubes through a conductive heat transfer and then dissipate the heat through convective transfer of heat to the gas (commonly air) flowing over the tubes. Copper is used in the construction of pipes due to its good heat transfer properties, its general resistance to corrosion, and its ease of repair. The fins are made of aluminum for its good heat transfer properties, easy fabrication and low cost. The heat exchangers made entirely of copper, like all aluminum, are used in certain applications to avoid the problems of galvanic corrosion, but at an exchange cost previously characterized. Aluminum is significantly lower in the galvanic series, that is, less noble, than copper. It is for this reason that aluminum oxidizes or corrodes when in contact with copper in the presence of an electrolyte. In the arrangement shown in Figure 1, the interface of the tube and the fin, is where the galvanic couple is made and where corrosion of the aluminum fin occurs. Once the fin has corroded, at the intersection, the fin is no longer in contact with the tube and thus the efficiency of the heat exchanger is greatly reduced because the fin loses its ability to conduct heat away from the tube . As will be explained with. Further detail below, according to the present invention, the exposed surfaces of the tubes 2 are coated or enriched with aluminum or a metal more galvanically compatible with the aluminum. Aluminum is the best candidate as material, since a galvanic couple will not form between the aluminum cladding and the aluminum fins 6. However, such active metals as zinc, magnesium tinplate, gallium, cadmium and lead also reduce the extension of the torque galvanic and in this way the degree of oxidation of the fin material. Coating or surface enrichment of copper 12 tubes with aluminum is done before the heat exchanger 10 is assembled. Copper aluminization is a well-known practice and can be performed to a degree of precision to virtually eliminate the problems cited above, with conventional coatings for corrosion protection. Various aluminization processes of copper tubes are known in the industry and are contemplated by the present invention. The coating processes include the hot bath, the electroplating, paints with aluminum loading and mixing, and thermal spraying. Surface improvement processes include the deposition of ion vapor, the deposition of chemical vapor and the deposition of physical vapor. The critical aspect of the present invention is the production of a uniform coating of aluminum over the entire surface of the flow circuit tubes 2. Without taking into account the processes contemplated, the variables of the pipe surface preparation, the temperature of Tube preheating, the composition of the coating and the thickness of the coating should be carefully controlled to achieve adequate results of the present invention. The preferred preparation of the exposed surface of the tube is to remove the oxide layer from the surface of the copper to ensure that the material to be coated will adhere well to the tube. A number of surface preparation processes are known in the industry and include the use of reducing gases, flows and shot blasting. The preheating temperatures of the tube should be controlled between 24 ° C and 600 ° C to avoid copper dissolution and to limit intermetallic growth during the coating process. It is preferred that the coating have high ductility to allow subsequent assembly of the heat exchanger without damaging the coating. The ductility of the coating is determined in part by the composition of the coating and the thickness of the coating. As mentioned above, any metal composition more galvanically compatible with the fin material than the tube material, would stop the degree of oxidation of the fins 6, while the ideal coating material would exactly match the fin material. Certain aluminum alloys are considered to be used in the ". present invention and comprise combinations of aluminum with silicon and aluminum combined with zinc. The coating should advantageously be thick enough to prevent the penetration of electrolyte. However, since any coating has some negative effect of some kind on the heat transfer of the unit, the excessively thick protective layer should be avoided. The optical range of thickness contemplated by the present invention is .1 mils to 2 mils (2.5 to 51 microns).

Claims (9)

1. CLAIMS 1. A method for reducing the galvanic corrosion of a less noble first metal member in a mounting with a second nobler metal member, when the assembly is exposed to an electrolyte, the. The first, less noble metallic member has a surface in contact with an exposed surface of a second nobler metal member, characterized in that it comprises the step of treating the exposed surface of the second nobler metal member with a metal more galvanically compatible with the first metal member less noble than the second nobler metal member which is galvanically compatible with the first less noble metal member before mounting the first less noble metal member to the second nobler metal member so that a protective layer is formed on the exposed surface of the second metal member more noble and between the first less noble metal member and the second nobler metal member. The method according to claim 1, characterized in that the second nobler metal member consists essentially of copper, the first less noble metal member consists essentially of aluminum and the treatment metal comprises a metal selected from the group consisting of aluminum, an aluminum alloy containing silicon, and an aluminum alloy containing zinc, zinc, tin, magnesium, gallium, cadmium, lead and combinations thereof. The method according to claim 2, characterized in that the treatment step comprises coating the exposed surface of the second nobler metal member with the treatment metal to form a layer of the treatment metal on the exposed surface of the second member. . The method according to claim 3, characterized in that the coating process is a hot bath process, an electroplating process, a painting process or a diffusion coating process. 5. A method for reducing the galvanic corrosion of a fin in a heat exchanger, which is used exposed to an electrolyte, includes at least one metal heat transfer tube, which has an external surface and at least one metal fin mounted to the outer surface of the heat transfer tube, at least one fin is formed of a less noble metal than the metal of the heat transfer tube, characterized in that it comprises the step of treating the outer surface of the heat transfer tube with a metal more galvanically compatible with the less noble metal of the fin than the nobler metal tube that is galvanically compatible with the less noble metal of the fin before mounting at least one fin to the outer surface of the heat transfer tube . 6. The method of compliance with the claim 5, characterized in that the metal-heat transfer tube is copper, the metal wing is made of aluminum, and the treatment metal comprises a metal selected from the group consisting of aluminum, an aluminum alloy containing silicon, an alloy of aluminum containing zinc, zinc, tin, magnesium, gallium, cadmium, lead and combinations thereof. 7. The method of compliance with the claim 6, characterized in that the treatment step comprises coating the outer surface of the heat transfer tube with the treatment metal to form a treatment metal layer on the external surface of the heat transfer tube. 8. The method of compliance with the claim 7, characterized in that the coating process is a hot bath process, an electroplating process, a painting process or a diffusion coating process. The method according to claims 3, 4, 7 and 8, characterized in that the layer of the treatment metal has a thickness of between 0.0001 inches and 0.002 inches (2.5 to 51 microns).