EP1192295B1

EP1192295B1 - Article exhibiting improved resistance to formicary corrosion

Info

Publication number: EP1192295B1
Application number: EP00930472A
Authority: EP
Inventors: Sandra J. Downey; Thomas J. Garosshen; Daniel P. Gaffaney
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 1999-06-01
Filing date: 2000-05-08
Publication date: 2005-11-16
Anticipated expiration: 2020-05-08
Also published as: WO2000073537A1; DE60024087T2; DE60024087D1; ES2249268T3; EP1192295A1

Description

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.
Corrosion is a process that involves two simultaneous reactions which are called half-cells. One half-cell reaction is the oxidation or corrosion of the metal. This process involves the loss of electrons, e.g., 2M →2M⁺ +2e^-
Electrons from the oxidation process are in tum used by an associated reduction half-cell reaction, which is often the reduction of oxygen or hydrogen, e.g., O₂ +4H⁺ +4e^- → 2H₂O 2H⁺ +2e^- → H₂
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 occurence, 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.
JP 08/78585 dicloses a process in which the inner surface of a copper tube is plated with tin. JP 59229200 discloses plating tin, nickel, lead or aluminium ores components of a heat exchanger.
The present invention provides an article as claimed in claim 1.
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.
In accordance with a preferred embodiment of this invention, the protective layer comprises a 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 (0.0025 to 0.025 mm) and is uniformly applied to the surface. Most preferably, the thickness of the coating is 0.50 mil (0.063 mm)
FIG. 1 is a perspective view of an evaporator coil incorporating copper tubing treated in accordance with the principles of the present invention;
FIG. 2 is a graph illustrating the performance improvements achieved in accordance with the principles of the present invention.
As will be described in detail below, the present invention provides for formicary corrosion protection of copper tubing used in plate-fin coils.
FIG.1 illustrates a plate-fin coil 1 of the 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 of the present invention, the exposed surface of the 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 of the 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 (0.063 mm) of electroplated tin. Other methods of prevention such as using formicary corrosion inhibitors (not including removing one of the 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 of the 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 of the copper. Other metals also increasing the resistance to formicary corrosion may be used as well.
The coating or surface enrichment of the copper tubes 2, in the embodiment illustrated, with tin or other discussed material is accomplished prior to the assembly of the 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 of the 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 of the present invention is the production of a uniform coating of formicary corrosion reaction resistant material over the entire exterior surface of the flow circuit tubes 2. Regardless of the 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 of the present invention. The preparation of the exposed surfaces of the 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 of the heat exchanger without damaging the coating. The ductility of the coating is determined in part by the coating composition and the thickness of the coating. The coating must be thick enough to prevent the penetration of the electrolyte, and thin enough to have good formability and cost benefits. The optimal range of thickness is 0.10 mils to 1.0 mils (0.0025 to 0.025 mm)
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.

Claims

An article resistant to formicary corrosion comprising a flow circuit tube (2) made from copper, said tube having a refrigerant inlet (3) and a refrigerant outlet (4), and further comprising a plurality of fins (5) connected to, and extending radially from, the tube (2), the external surface of said tube (2) having a layer provided on its surface for preventing formicary corrosion of the tube, characterised in that said layer extends under the fins (5) between the fins (5) and the tube (2).
An article according to claim 1 wherein the layer comprises a coating on the surface, said coating being of a metal that has a greater resistance to formicary corrosion than does copper.
An article according to claim 1 or 2 wherein said coating is applied uniformly to said surface.
An article according to claim 3 wherein the coating has a thickness in the range 0.10 to 1.00 mil (0.0025 to 0.025 mm).
An article according to claim 1 wherein said layer is a coating comprising one of tin and a tin alloy.
An article according to claim 5 wherein the coating has a thickness in the range 0.10 to 1.00 mil (0.0025 to 0.025 mm) .
An article according to claim 1 wherein said layer is a tin coating having a uniform thickness of about 0.5 mil (0.0063 mm) .
A method of protecting from formicary corrosion an article which comprises a flow circuit tube (2) made from copper, said tube having a refrigerant inlet (3) and a refrigerant outlet (4), and a plurality of fins (5) connected to and extending radially from the tube (2) comprising the steps of:

providing on the external surface of said tube, prior to the attachment of the fins thereto, a layer which is resistant to formicary corrosion.
A method according to claim 8 wherein the layer comprises a coating on the surface, said coating being of a metal that has a greater resistance to formicary corrosion than does copper.
A method according to claim 8 or 9 wherein said coating is applied uniformly to said surface.
A method according to claim 10 wherein the coating has a thickness in the range 0.10 to 1.00 mil (0.0025 to 0.025 mm) .
A method according to claim 8 wherein said layer is a coating comprising one of tin and a tin alloy.
A method according to claim 12 wherein the coating has a thickness in the range 0.10 to 1.00 mil (0.0025 to 0.025 mm).
A method according to claim 8 wherein said layer is a tin coating having a uniform thickness of about 0.5 mil (0.0063 mm) .