ARTICLE EXHIBITING IMPROVED RESISTANCE TO FORMICARY CORROSION

Abstract: An article resistant to formicary corrosion in accordance with the principles of the present invention includes a copper member having a surface and coating on the surface for preventing formicary corrosion of the copper member. In an embodiment, the article is a copper tube in a plate-fin coil of an air conditioning unit and the coating formed from one of tin and a tin alloy. Preferably, the coating has a thickness in the range of 0.10 to 1.00 mil and is uniformly applied to the surface. Most preferably, the thickness of the coating is 0.50 mil.

Description: ARTICLE EXHIBITING IMPROVED RESISTANCE TO FORMICARY CORROSION Technical Field This invention relates generally to protecting metals from a corrosive environment, and more specifically, to protecting copper from formicary corrosion when used in a corrosive environment. Background Art Corrosion is a process that involves two simultaneous reactions which are called half- cells. One half-cell reaction is the oxidation or corrosion ofthe metal. This process involves the loss of electrons, e.g., 2M →2M + +2e Electrons from the oxidation process are in turn used by an associated reduction half- cell reaction, which is often the reduction of oxygen or hydrogen, e.g., O 2 +4H + +4e → 2H 2 O 2H + +2e " → H 2 The oxidation reaction (corrosion process) can only proceed at a rate governed by the reduction reaction that uses the electrons from the oxidation process. This is because charge neutrality must be maintained. A unique type of localized corrosion called formicary corrosion, also referred to as ant nest corrosion, occurs in copper when subjected to organic acids in the presence of moisture and air. The result is a rapid tunneling through the copper surface in a random manner, which ultimately leads to through-wall penetration. Prevention of this form of corrosion has typically been accomplished by elimination at least one of the three necessary constituents for its occurrence, i.e., air, moisture, or organic acid. However, when at least one of these constituents cannot be removed, as would be the case in an indoor air conditioning application in which an evaporator coil is wet by condensate by virtue of its dehumidification function, alternate protection methods are needed. There exists a need, therefore, for an improved method of protecting copper in environments which promote formicary corrosion. Disclosure of Invention The primary object of this invention is to provide an article, such as the copper tubes of an evaporator coil, having improved formicary corrosion resistant properties when used in a corrosive environment. Another object of this invention is to provide a copper article having a coating for inhibiting corrosive activities due to formicary corrosion. Yet another object of this invention is to provide an article formed from copper and having a tin or tin alloy coating for inhibiting formicary corrosion. The foregoing objects and following advantages are achieved by the article of the present invention including a copper member having a surface and a coating on the surface for preventing formicary corrosion ofthe copper member. In accordance with the principles of this invention, the article is a copper tube and the coating formed from one of tin and a tin alloy. Preferably, the coating has a thickness in the range of 0.10 to 1.00 mil and is uniformly applied to the surface. Most preferably, the thickness ofthe coating is 0.50 mil. Brief Description of Drawings FIG. 1 is a perspective view of an evaporator coil incorporating copper tubing treated in accordance with the principles ofthe present invention; FIG. 2 is a graph illustrating the performance improvements achieved in accordance with the principles ofthe present invention. Best Mode for Carrying Out the Invention As will be described in detail below, the present invention provides for formicary corrosion protection of copper, such as copper tubing used in plate-fin coils. However, it should be evident 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 copper members are used in a corrosive environment. FIG. 1 illustrates a plate-fin coil 1 ofthe type typically used in air conditioning units. The coil includes one or more flow circuits for carrying refrigerant therethrough. For the purposes of explanation, the coil 1 contains a single flow circuit tube 2 consisting of an inlet line 3 and an outlet line 4, and a plurality of fins 5 extending radially therefrom. As noted above, evaporator coils of this type are commonly used in corrosive environments. In a typical arrangement, coils of this type are fabricated utilizing copper tubing for the circuit flow tubes. Copper is utilized in tube construction because of its good heat transfer properties, general resistance to corrosion, and ease of fabrication and repair. In accordance with the principles ofthe present invention, the exposed surface ofthe copper tube 2 is coated or enriched with a material for preventing formicary corrosion. Tin and tin alloys are the best candidates for this material. Thus, substantial improvements in the resistance to formicary corrosion is achieved by coating or impregnating the copper tubing's surface with a layer of material such as tin or tin alloy. The improvement achieved using the principles ofthe present invention is illustrated graphically in FIG. 2. Testing has shown a greater than twenty fold (20x) improvement in the failure rate of copper when, for example, copper tubing is coated with 0.5 mil of electroplated tin. Other methods of prevention such as using formicary corrosion inhibitors (not including removing one ofthe three factors causing formicary corrosion which is rarely a feasible option), typically show only a three fold (3x) improvement. Accordingly, by coating the formicary-corrosion- susceptible copper tubing with a metal that is resistant to such corrosion, the corrosion resistance ofthe copper tubing is significantly improved. While pure tin is the preferred material, tin alloys containing such metals as zinc, magnesium, copper, gallium, cadmium and lead will also result in improved resistance to formicary corrosion ofthe copper. Other metals also increasing the resistance to formicary corrosion may be used as well. The coating or surface enrichment ofthe copper tubes 2, in the embodiment illustrated, with tin or other discussed material is accomplished prior to the assembly ofthe heat exchanger 10. There are several methods for applying the protective coating systems to copper tubing. Potential methods include: (1) hot dipping, (2) electroplating (3) vapor deposition, and (4) ion implantation. This invention includes all coatings that work to prevent formicary corrosion of copper. The proposed approach is very cost effective and maintains the thermal conduction ofthe copper tubing in heat exchanger applications. High thermal conduction is accomplished by using a coating with a relatively high thermal conductivity or by applying very thin coatings. An important aspect ofthe present invention is the production of a uniform coating of formicary corrosion reaction resistant material over the entire exterior surface ofthe flow circuit tubes 2. Regardless ofthe process used, the variables of tube surface preparation, tube preheat temperature, coating composition, and coating thickness need to be carefully controlled to achieve the proper results ofthe present invention. The preparation ofthe exposed surfaces ofthe tube is designed to remove the surface oxide layer from the copper to ensure that the coating material will adhere well to the tube. A number of surface preparation processes are known in industry and include the use of acid reducing gases, fluxes and mechanical abrasion such as shot blasting It is preferred that the coating have high ductility to allow for the subsequent assembly ofthe heat exchanger without damaging the coating. The ductility ofthe coating is determined in part by the coating composition and the thickness ofthe coating. The coating must be thick enough to prevent the penetration ofthe electrolyte, and thin enough to have good formability and cost benefits. The optimal range of thickness is 0.10 mils to 1.0 mils. The primary advantage of this invention is that an article is provided, such as a plate- fin heat exchanger coil, having improved formicary corrosion resistant properties when used in a corrosive environment. Another advantage of this invention is that a copper tube evaporator is provided having a coating for inhibiting corrosion activities due to formicary corrosion. Yet another advantage of this invention is that a plate-fin coil is provided formed from copper tubing with a tin or tin alloy coating for inhibiting formicary corrosion. Although the invention has been shown and described with respect to the best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made without departing from the spirit and scope ofthe invention.

Claims: CLAIMS What is claimed is: 1. An article resistant to formicary corrosion comprising a copper member having a surface, and characterized by means on said surface for preventing formicary corrosion of said copper member. 2. The plate-fin coil according to claim 1, further characterized in that said copper member is a copper tube. 3. An article according to claim 1 further characterized in that said copper member comprises a flow circuit tube formed from copper and having a refrigerant inlet and a refrigerant outlet; and by a plurality of fins connected with said flow circuit tube and extending radially therefrom. 4. An article according to claim 1, 2 or 3 further characterized in that said means for preventing formicary corrosion comprises a coating on said surface, said coating being of a metal that has a greater resistance to formicary corrosion than does copper. 5. The plate-fin coil according to claim 4, further characterized in that said coating is uniformly applied to said surface. 6. The plate-fin coil according to claim 5, further characterized in that said coating has a thickness in the range of 0.10 to 1.00 mil. 7. The plate- fin coil according to claim 1, 2, 3 or 4, further characterized in that said means for preventing is a coating comprising one of tin and a tin alloy. 8. The plate- fin coil according to claim 7, further characterized in that said coating has a thickness in the range of 0.10 to 1.00 mil. 9. The plate-fin coil according to claim 2, further characterized in that said means for preventing comprises a tin coating on said copper tube, said coating having a relatively uniform thickness of about 0.50 mil.

Inventors: DOWNEY SANDRA J, GAFFANEY DANIEL P, GAROSSHEN THOMAS J, DOWNEY, SANDRA, J., GAFFANEY, DANIEL, P., GAROSSHEN, THOMAS, J.
Applicants: CARRIER CORP, CARRIER CORPORATION
Application year: 2000
Main classification: C23C30
IPC codes: C23C30