The invention relates generally to aluminum alloys and, more particularly, to corrosion resistance of aluminum alloys.
Due to its wide availability and excellent thermal conductivity properties, many heat exchangers are made from aluminum. Extruded aluminum headers and or tubes are used because of their low cost and ease of fabrication. Heat exchangers can be manufactured from several grades of aluminum, and extrudable and rolled aluminum products are most common. Aluminum alloy material typically used in constructing extruded tubes for use in heat exchangers is known as low alloy aluminum base (such as 3000 series aluminum). The aluminum materials typically contain tramp elements such as iron, silicon, magnesium, and the like as impurities introduced in the smelting process, especially when scrap material is used. These minor elements usually form intermetallic particles within the aluminum matrix that have unique electrochemical and chemical properties. These intermetallic particles may act as local anodes and cathodes that initiate the corrosion process and thereby impair the corrosion resistance of the base material.
Specifically, when exposed to a corrosive, aqueous environment, the aluminum is susceptible to localized corrosion modes such as pitting, intergranular, stress cracking (SCC) and general corrosion. Due to the presence of surface particles, or other abnormal surface features like pits, accelerated oxidation or corrosion is initiated in these areas and eventually degrades the entire surface. Pitting corrosion is known to significantly reduce fatigue strength and life. Typically, fatigue endurance limits are reduced by pitting to nominally half or less than the limit of uncorroded alloys. Many methods exist for increasing the corrosion resistance of aluminum alloys such as painting, electroplating, anodizing and chromating the surfaces of the metal. Many of these processes are expensive, many are environmentally unfriendly, and none offer long term low maintenance protection. For example, anodizing involves a complex and expensive multi-step procedure. Chromate passivation is less complex, but does not provide sufficient pitting corrosion protection to allow aluminum based materials to be used in a tropical environment for a long design life.
An all-aluminum heat exchanger, particularly one intended for use as a condenser or evaporator is continually exposed to a moisture containing environment and can be highly susceptible to corrosion. Localized corrosion, the usual process for aluminum alloys, results in pitting of the tube surface. Eventually as corrosion continues to eat away at the tubes, holes will form allowing one of the heat exchanger fluids to leak. Pitting corrosion rates are often fast enough to cause perforation of the tubing holding the refrigerant within a few years of service if the material is not properly prepared. Gradual loss of refrigerant results in lower efficiency operation of a cooling system, along with eventual system shut down. The release of refrigerants may also have adverse environmental impact.
According to one embodiment of the invention, a heat exchanger is provided including at least one metal tube. The surface of the tube has a modified microstructure. At least one inhomogeneity has been removed or refined from the surface of the tube.
According to another embodiment of the invention, a method of manufacturing a metal tube having enhanced corrosion resistance, for use in a heat exchanger, is provided including extruding a metal into a tube. The microstructure of the surface of the tube is modified by removing or refining inhomogeneities as the tube is extruded or immediately afterwards.
According to yet another embodiment of the invention, a method for manufacturing a metal tube having an enhanced corrosion resistance, for use in a heat exchanged, is provided including forming a sheet of metal into a tube having a desired shape. The edges of the metal are bonded together to form a seal of the tube. The microstructure of the surface of the tube is modified by removing or refining inhomogeneities of the metal before or after the tube is formed.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.
The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
Referring now to
With reference now to
The corrosion resistance of an aluminum alloy, such as aluminum alloy 100 for example, may be improved by modifying the microstructure at the surface 102 of the aluminum alloy 100. Because the inhomogeneities at the surface 102 of the alloy 100 reduce the stability of the passive layer 110, removal or refinement of these inclusions will result in an alloy 100 having a greater corrosion resistance.
Various processes may be used to alter the surface composition of a component made from aluminum alloy 100. These processes may be applied to various parts of a heating ventilation and air conditioning system made of aluminum, such as a heat exchanger tube or a fin. In one embodiment, the inhomogeneities of the surface 102 may be refined or dissolved away by plastically deforming the surface 102 of the alloy. Methods for plastically deforming the surface 102 include shot peening, laser shock peening, ultrasonic peening, low plasticity burnishing, and other similar methods known to persons having ordinary skill in the art. Each of these methods creates a layer of compressive residual stress having a certain magnitude and depth. Such plastic deformation results in a generally homogeneous deformation of the surface 102 of the alloy 100. By creating a layer of compressive residual stress of a sufficient magnitude and depth, it may be possible to prevent or inhibit the growth of inclusions or pits that lead to pitting corrosion, and the growth of cracks such as those required for stress cracking corrosion.
In another embodiment, the inhomogeneities adjacent the passive layer 110 may be removed or refined by applying a heat source to the surface 102 of the alloy 100. Exemplary thermal processes include, but are not limited to, laser surface melting, laser surface alloying, laser cladding, thermal arc spray, plasma processes, and other similar processes known to persons having ordinary skill in the art. For those thermal processes using a laser, generally a high intensity laser is applied to the surface 102 of the alloy 100 for a relatively short duration as the tube is moved at a relatively high speed and collected on a spool. In a thermal process, the heat source causes the near surface region of the alloy to liquefy. The majority of the alloy 100 is unaffected by the heat, and therefore, acts as a heat sink to rapidly cool the melted surface, creating a new microstructure. These thermal processes produce enhanced corrosion resistance as a result of altering the surface composition and redistributing the impurities and second phase particles 120. The resultant aluminum alloy has a more uniform structure with superior homogeneity compared to conventional surfaces. For example, a laser surface melting process may completely melt or dissolve inclusions on the surface 102 of the alloy 100 or the surface alloy created using a laser surface alloying process will have compositional uniformity.
In another embodiment, the inhomogeneities of the passive layer 110 may be removed or refined by applying a chemical or electrochemical process to the alloy 100. For example, the aluminum alloy 100 may be placed in a chemical etching bath, which will attack the structural inhomogeneities by selectively leaching the second phase particles 120 from the surface 102 of the alloy 100 without damaging the alloy 100. The chemicals combined to form the bath will vary depending on the composition of the intermetallic particles being removed. Suitable bath solutions for selectively removing second phase particles from the surface of an aluminum alloy may include sodium hydroxide generally in the range of between 5 and 20 percent, hydrochloric nitric acid, hydrofluoric acid and combinations thereof. A chemically etched surface of an aluminum alloy may additionally be smoothed after being treated with a chemical solution to eliminate surface roughness and pits that may induce local corrosion. Exemplary processes for smoothing the surface of the alloy include, but are not limited to, rolling, burnishing, grinding, fine wire burnishing, and thermal processes.
Referring now to
A method 300 of forming a tube, having an enhanced corrosion resistance, for use in a heat exchanger is illustrated in
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/616,542 filed Mar. 28, 2012, the contents of which are incorporated herein by reference thereto.
Filing Document | Filing Date | Country | Kind |
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PCT/US2013/034246 | 3/28/2013 | WO | 00 |
Number | Date | Country | |
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61616542 | Mar 2012 | US |