JP2008190771A - Plate fins with excellent corrosion resistance - Google Patents

Abstract

A heat exchanger plate fin made of aluminum or an aluminum alloy and excellent in corrosion resistance is provided.
Plate fins 1 and 2 for heat exchangers made of aluminum or aluminum alloy and having an electroless nickel plating layer formed on the surface thereof. The thickness of the nickel plating layer is desirably 10 μm or more. If these plate fins 1 and 2 are used as components of a plate fin type cooler for vehicles, they exhibit excellent corrosion resistance against scattering of a highly corrosive cleaning agent during vehicle washing.
[Selection] Figure 1

Description

  The present invention relates to plate fins for plate fin type heat exchangers used in industrial machinery, railway vehicles, electric power facilities, and a wide range of other industrial fields, and is particularly excellent for scattering of acidic or alkaline cleaning agents. The present invention relates to a plate fin made of aluminum or aluminum alloy having high corrosion resistance.

Plate fin type heat exchangers are small and light heat exchangers that excel in heat exchange efficiency, and are used in industrial machinery, railway vehicles, electric power facilities, and a wide range of other industrial fields. For example, in rail vehicles and the like that require high-speed travel, plate fin heat exchangers made of aluminum or aluminum alloy that are lightweight and excellent in formability are frequently used. Also, in chemical plants that often handle highly corrosive fluids, stainless steel plate fin type heat exchangers with excellent corrosion resistance are often used, but in air or near neutral water, Since a highly protective oxide film (Al ₂ O ₃ ) is formed on the surface, a plate fin type heat exchanger made of aluminum or aluminum alloy is also used.

  1A and 1B are diagrams for explaining the configuration of a core portion in a plate fin heat exchanger. FIG. 1A is an exploded perspective view of the core portion, and FIG. 1B is an assembled perspective view of the core portion. The core portion 4 which is the heart of the heat exchanger is formed by alternately laminating plate-like tube plates 1 that partition the low-temperature fluid passage and the high-temperature fluid passage, and corrugated types for promoting heat transfer and fins 2 having various shapes. In addition, a spacer bar 3 for keeping the space between the tube plates 1 and sealing the flow path is constituted by a basic structure arranged on both side surfaces.

  When assembling the core part, a sheet-like or powdery brazing material is arranged between the tube plate 1 and the fins 2 or between the tube plate 1 and the spacer bar 3, and the laminated and assembled core part 4 has an atmosphere. It is inserted into the adjusted heating furnace, heated to the melting point of the brazing material while being pressurized, and brazing is performed to form an integral core part.

  The flow methods in the core portion of the low temperature fluid and the high temperature fluid are basically classified into a counter flow type and a direct flow type, and various flow methods are configured by combining them. An arrow shown in FIG. 1A indicates a direct flow type flow in which a low-temperature fluid and a high-temperature fluid cross flow, and an arrow shown in FIG. 1B indicates a flow in which both fluids counter-flow.

  In heat exchangers configured using plate fins made of aluminum or aluminum alloy, liquid materials such as hot oil and high-temperature water, and gaseous substances such as air are often used as the heat medium. Is circulated in the cold fluid passage or the hot fluid passage.

Aluminum or an aluminum alloy is an active metal, but exhibits excellent corrosion resistance in an environment where the above-described oxide film (Al ₂ O ₃ ) is stably present. However, when chlorine ions or the like that locally destroy this oxide film are present, corrosion tends to proceed in the form of pitting corrosion. Moreover, when it is in contact with copper or a copper alloy, contact corrosion occurs. Dissolved copper ions also promote corrosion.

  In addition, heat exchangers having plate fins made of aluminum or aluminum alloy are normally not used in such an environment because the oxide film dissolves in non-oxidizing acids and alkalis. Depending on the apparatus in which the heat exchanger is disposed, the mode of use of the facility, or the maintenance management, a situation occurs in which the plates and fins constituting the core portion of the heat exchanger are severely corroded in a relatively short time.

  For example, in railway vehicles, particularly for Shinkansen vehicles, plate fin type coolers made of aluminum or aluminum alloy are used as main converter coolers and main transformer coolers for weight reduction. By the way, in general, in the vehicle body cleaning work in the regular maintenance of the vehicle, since the blower fan that sends air to the cooler is often kept in operation, a cleaning agent, particularly a strong acid-based cleaning of about pH 1 having a large cleaning effect. When an agent (the type of acid is mainly oxalic acid) is used, it may be scattered and remain inside the cooler air passage, resulting in the occurrence of corrosion, and depending on conditions, leakage may occur in a short period of time.

In addition, even if there is no abnormality in the normal use state, the plate fin type heat exchanger is arranged and used as a part of other devices and equipment, so that the surface oxide film (Al ₂ O ₃ ) may not be able to exist stably and may require anticorrosion measures for the plate fins.

As a countermeasure for improving the corrosion resistance of aluminum or an aluminum alloy, an anodizing treatment (alumite method) has been widely performed. In this method, a highly protective oxide film (Al ₂ O ₃ ) is formed by anodic oxidation in sulfuric acid, and a thick film can be formed. Furthermore, when the film is formed by boiling with hot water or subjecting to hot water vapor, the film partially swells (thus closing the pores) and the resistance to pitting corrosion increases. However, the oxide film is soluble in non-oxidizing acids such as hydrochloric acid and sulfuric acid, and alkalis.

  Chemical conversion film treatment is also performed for the purpose of coating ground treatment (pretreatment for improving coating adhesion and rust prevention), corrosion prevention, lubrication, and the like. For example, a chromate method of immersing in an aqueous solution containing chromate or fluoride for a predetermined time, a dihydrogen phosphate (such as acidic zinc phosphate), a phosphate method of immersing in an aqueous solution containing phosphoric acid, or the like is used. A slightly soluble film is formed. However, since the coatings are thin and have low hardness, these coatings alone cannot provide sufficient corrosion resistance, and usually, treatment such as painting is often performed.

  Patent Document 1 describes a heat exchanger having aluminum alloy fins and plates each having a fluoride layer formed on a surface layer portion. This fluoride layer has excellent corrosion resistance in 150 cycles of dry and wet repeated tests using a mixed acid such as hydrochloric acid and sulfuric acid (pH = 1.3), and is used as an evaporator, condenser, radiator, and oil cooler. It is applied to an exchanger, and is particularly suitable for a heat exchanger used in a fuel cell.

International Publication Number WO2003 / 036216

  This invention is made | formed in view of such a condition, The objective is to provide the plate fin for heat exchangers made from aluminum or aluminum alloy excellent in corrosion resistance. Specifically, it has excellent corrosion resistance even when temporarily exposed to an acidic or alkaline environment, and in particular, a plate fin type attached to the main converter or main transformer of the aforementioned Shinkansen vehicle. It is an object of the present invention to provide a plate fin having excellent corrosion resistance against scattering of a cleaning agent such as a strong acid during vehicle cleaning.

  The present inventors have repeatedly studied to achieve the above object, and have tried to use a plate fin in which the surface of aluminum or an aluminum alloy, which is a material of the plate fin, is subjected to nickel plating.

  Nickel has corrosion resistance against non-oxidizing acids such as hydrochloric acid and has high corrosion resistance against alkali. Nitric acid has corrosion resistance if it is a dilute acid of 0.5% or less. Nickel is also excellent in weather resistance, has corrosion resistance to seawater, and is said to withstand high-temperature pure water well. Therefore, nickel plating can be an effective means for achieving the object of the present invention (that is, provision of plate fins having excellent corrosion resistance, particularly excellent corrosion resistance against scattering of acids and the like).

  However, when a nickel plating process is applied to the plate fin, a commonly used electroplating method cannot be applied. This is because the core portion of the plate fin type heat exchanger has a complicated shape as shown in FIG. 1, has a large heat transfer area, and uniform plating is difficult.

  Accordingly, electroless nickel plating is applied to the core portion produced by performing brazing using plate fins made of aluminum or aluminum alloy.

  In electroless nickel plating, for example, a material to be plated is immersed in a plating bath mainly composed of nickel sulfate and sodium hypophosphite (reducing agent), and electrons are generated in the bath by an oxidation reaction of sodium hypophosphite. In this method, the nickel ions are reduced and deposited as a metal film on the surface of the material to be plated. Since nickel is deposited on the surface of the material to be plated by a chemical reaction, the deposition rate (film formation rate) is slower than electroplating, but a dense plating layer is easily obtained.

  In the field of electronics industry, this electroless nickel plating is used for various purposes in order to provide excellent solderability, bonding properties, or to ensure the electrical conductivity of contacts (under-treatment of gold plating). Widely done. Also, in the aircraft industry and further in the automobile industry, aluminum or aluminum alloy is partially used for weight reduction, and electroless nickel plating is performed for the purpose of improving wear resistance and corrosion resistance.

However, nickel plating, especially electroless nickel plating, is not applied to the surface of aluminum or aluminum alloys used as the main constituent material of relatively large machinery and equipment for the purpose of improving corrosion resistance. . As described above, aluminum has a high affinity with oxygen, and in air or near neutral water, a highly protective oxide film (Al ₂ O ₃ ) is formed on the surface. This is presumably because this oxide film has been widely used as a lightweight material in the atmosphere in which the oxide film is stably present or in the vicinity of neutral water or aqueous solution.

  In the heat exchanger having aluminum alloy fins and plates with a fluoride layer formed on the surface layer portion described in the above-mentioned Patent Document 1, it is described that an electroless nickel plating layer is formed. It is formed as an intermediate layer between the ground and the fluoride layer and is not intended to improve the corrosion resistance by the plating layer itself.

  The inventors of the present invention actually investigated and investigated the corrosion resistance of the electroless nickel plating layer. As a result, when an aluminum test piece subjected to electroless nickel plating was immersed in an aqueous oxalic acid solution (pH = 1), the change in the mass of the test piece and the observation of the corrosion status were observed. It was confirmed that the layer functions as a barrier layer against corrosion of aluminum.

  The present invention has been made based on such examination results, and the gist thereof is the following plate fin.

  That is, it is a plate fin for heat exchangers made of aluminum or an aluminum alloy, which has an electroless nickel plating layer formed on its surface and is excellent in corrosion resistance.

  Here, “aluminum or aluminum alloy” means Al (industrial pure aluminum such as JIS alloy numbers 1050 and 1100), Al—Mn (such as 3003), Al—Mg—Si (such as 6063). ), And other aluminum alloys used as a constituent material of plate fin type coolers.

  In this plate fin, if the thickness of the electroless nickel plating layer is 10 μm or more, good corrosion resistance can be obtained.

  If the surface of the plate fin is the surface after the zinc diffusion treatment is performed, it is effective for improving the adhesion of the nickel plating layer.

  In addition, if the plate fin is used as a component member of a plate fin type cooler for a railway vehicle, it has good corrosion resistance against scattering of the cleaning agent on the plate fin when washing the vehicle. Demonstrated.

  In the plate fin of the present invention, since the base aluminum or aluminum alloy is separated from the external environment by the nickel plating layer, even when temporarily placed in an acidic or alkaline environment different from the normal use state, Exhibits excellent corrosion resistance.

  In particular, in the case where the plate fin of the present invention is used as a plate fin for a cooler attached to the main converter or main transformer of the above-mentioned Shinkansen vehicle, a cleaning agent such as oxalic acid during vehicle washing is used. It can be expected to show excellent corrosion resistance against scattering and the like.

  The plate fin of the present invention is a plate fin for a heat exchanger made of aluminum or an aluminum alloy, and is an excellent plate resistance having an electroless nickel plating layer formed on the surface thereof.

The electroless nickel plating layer is formed on the surface of the plate fin even if it is placed in an acidic or alkaline environment where the surface oxide film (Al ₂ O ₃ ) cannot exist stably. This is to provide excellent corrosion resistance.

  The thickness of the nickel plating layer is not particularly limited. The effect of improving the corrosion resistance by forming a nickel plating layer on the surface of the plate fin is due to blocking the contact between aluminum or an aluminum alloy and the environment (for example, the above-mentioned highly acidic cleaning agent). It is desirable that the thickness of the film is thicker because complete blocking is possible over a long period of time.

  From the examples described later, the thickness of the plating layer is preferably 10 μm or more. If the thickness of the plating layer is within this range, good corrosion resistance can be obtained.

  The plating process may be performed according to a conventionally performed method. After performing the necessary pretreatment, for example, nickel sulfate, a hypophosphite as a reducing agent as a main component, a buffer that maintains the pH of the plating solution within a predetermined range, a nickel soluble complex is formed to form nickel Immerse in a plating solution containing a complexing agent for preventing precipitation of ions for a predetermined time.

  In performing the plating treatment, it is desirable to perform a zinc diffusion treatment on the surface of the material to be plated in advance. Aluminum is a remarkably base metal, and when it is immersed in a plating solution as it is, a substitution reaction occurs in which aluminum is dissolved and nickel ions are deposited, which tends to cause problems in the adhesion between the material to be plated and the plating layer. Therefore, if the surface of the material to be plated is subjected to zinc diffusion treatment in advance, a substitution reaction occurs relatively slowly between the surface zinc and nickel ions in the plating solution, thereby improving the adhesion of the nickel plating layer. . Furthermore, the sacrificial anticorrosive action of zinc on aluminum or aluminum alloy can also be expected.

  If the plate fin is used as a component of a plate fin type cooler for a railway vehicle (including a Shinkansen vehicle), a corrosive cleaning agent is added to the plate of the cooler when the vehicle is washed. Even when splashing on the fins and adhering to them, or when they are temporarily placed in an acidic or alkaline environment that is different from normal use, the plate fin base is corroded by the nickel plating layer on the surface. Can prevent the progress of corrosion.

  In particular, if the plate fin is used as a constituent member of a plate fin type cooler for Shinkansen vehicles and has corrosion resistance against oxalic acid, the effect of the nickel plating layer is remarkably recognized. In a Shinkansen vehicle, a plate fin type cooler made of aluminum or aluminum alloy is attached as a cooler for a main converter or a cooler for a main transformer. Although oxalic acid is mainly used, if the plate fin of the present invention has an electroless nickel plating layer formed on the surface, droplets of highly corrosive cleaning agent adhere to the plate and fin during vehicle cleaning. Even so, the progress of corrosion can be sufficiently prevented.

  As described above, the plate fin of the present invention is a plate fin on which an electroless nickel plating layer is formed, and the surface of the plate fin is separated from the external environment by the nickel plating layer. Even when it is temporarily placed in an acidic or alkaline environment different from the normal use state, it has excellent corrosion resistance.

  A 1.8% oxalic acid aqueous solution (pH = 1) containing 20 mg of copper pieces was placed in a beaker, the test pieces were immersed in a state heated to 70 ° C., and the change in mass of the test pieces and the observation of the corrosion state were performed.

  The specimen was brazed with a fin of A1100 material and a brazing sheet of BAS231P material in order to equalize the air side passage structure of the actual cooler. (B) brazed, zinc diffused, (c) brazed, electroless nickel plated, and (d) brazed There are four types in total: those after zinc diffusion treatment and after electroless nickel plating treatment. In addition, it brazes about any test piece in order to perform the test according to the actual usage pattern in consideration of the adhesiveness of the plating layer in the brazed part.

  FIG. 2 shows the change over time in the amount of corrosion in each test piece with a plating layer thickness of 25 μm. The amount of corrosion is expressed as the weight loss per unit area obtained by dividing the mass difference between the test piece before and after the test by the surface area of the test piece. Also, in the explanation of symbols such as ○ mark and ● mark used in the figure, for example, “brazing + Ni” is a test piece subjected to electroless nickel plating after the brazing of (c). In addition, “brazing + Zn + Ni” represents a test piece subjected to electroless nickel plating after zinc diffusion treatment after brazing (d).

  As is clear from the results shown in FIG. 2, the amount of corrosion of the nickel-plated test pieces (△ and ▲ in the figure) is the same as that of the non-plated test pieces (○ in the figure). , ● marked) is significantly smaller. Moreover, as a result of analyzing the component contained in the aqueous solution used for the test, the elution of aluminum or zinc was not observed, and the decrease in the mass of the test piece was due to the elution of the nickel plating layer. From this, it was confirmed that the nickel plating layer functions as a barrier layer against corrosion of aluminum (in the case of zinc diffusion treatment, zinc).

  In this test, there was no difference between the case where the zinc diffusion treatment was performed and the case where the zinc diffusion treatment was not performed, and the influence on the corrosion resistance such as the improvement of the adhesion of the nickel plating layer by the zinc diffusion treatment (reduction of the corrosion amount) I could not confirm.

  FIG. 3 is a diagram showing the relationship between the thickness of the nickel plating layer and the amount of corrosion. As described above, a 1.8% aqueous solution of oxalic acid (pH = 1) containing 20 mg of copper pieces was placed in a beaker and heated to 70 ° C., and then subjected to electroless nickel plating after brazing in (c). It is the result obtained by immersing the test piece for 24 hours and measuring the mass change of the test piece.

  When the thickness of the plating layer is 5 μm, a considerable amount of the nickel plating layer remained after immersion of the test piece, but the plating layer was locally consumed and peeled off due to corrosion. Was significantly eroded.

From the results shown in FIG. 3, when the thickness of the plating layer was 5 μm, the mass loss of the entire test piece was 0.03 mg / mm ² . On the other hand, when the thickness of the plating layer is 25 μm, the mass reduction amount is 0.008 mg / mm ² , and as shown in FIG. 2, the mass reduction of the test piece is due to elution of the nickel plating layer ( (Displayed as a white bar graph in FIG. 3).

Even when the thickness of the plating layer is 5 μm, the same amount (that is, 0.008 mg / mm ² ) of the nickel plating layer is considered to be eluted, so that the mass loss of the entire test piece (0.03 mg / Mm ² ) subtracting the decrease due to elution of the nickel plating layer (outlined portion in FIG. 3), the corrosion amount of aluminum when the thickness of the plating layer is 5 μm (that is, the consumption of the plating layer, the peeling portion) (Corrosion amount of the base aluminum) is estimated to be 0.022 mg / mm ² (shaded portion in FIG. 3).

Further, from the result shown in FIG. 2, the immersion time for the corrosion amount of aluminum to be 0.022 mg / mm ² is about 4 hours (see the curve of-○-or-●-in Fig. 2), and the entire surface of the test piece. Considering that it takes a considerable amount of time even when the metal is corroded and the above-mentioned corrosion pattern (local significant erosion), if the plating layer thickness is 5 μm, the plating is performed for some reason. It is inferred that the base aluminum was corroded in a very short time after a part of the layer was locally consumed.

  Although the thickness of the nickel plating layer depends on its purpose, it is generally about several μm from the viewpoint of protecting the surface of the member including corrosion. However, as a result of the above test, it became clear that a sufficient anticorrosive effect cannot be expected with a film thickness of about 5 μm under the conditions (corrosion environment) used here.

  By the way, nickel plating includes a plating method by energization in addition to the electroless method used in this study. In the case of plating by energization, an appropriate plating thickness is said to be about 15 μm, particularly when corrosion resistance is important. On the other hand, in the case of the electroless method, it is known that the denseness of the plating layer to be formed is much better than that in the case of the energization method. Considering the results of this experiment, at least 10 μm in electroless plating. If the plating layer thickness is sufficient, it can be determined that the film has sufficient corrosion resistance.

  The plate fin of the present invention is a heat exchanger plate fin made of aluminum or aluminum alloy having an electroless nickel plating layer formed on the surface thereof, and the base aluminum or aluminum alloy is separated from the external environment by the plating layer. Therefore, it has excellent corrosion resistance even when it is temporarily placed in an acidic or alkaline environment.

  In particular, when the plate fin of the present invention is used as a plate fin of a cooler attached to the main converter or main transformer of the above-described Shinkansen vehicle, Excellent corrosion resistance against scattering.

  Therefore, the plate fin of the present invention can greatly contribute to the improvement of durability of the heat exchanger made of aluminum or aluminum alloy and the expansion of the application range in the field of manufacturing the plate fin type heat exchanger.

It is a figure explaining the structure of the core part in a plate fin type heat exchanger, The same (a) is an exploded perspective view of a core part, The same (b) is an assembly perspective view of a core part. It is a figure which shows the result of the Example of this invention, and is a figure which shows the time-dependent change of the corrosion amount in the oxalic acid aqueous solution of the test piece which has a nickel plating layer (film thickness of 25 micrometers) on the surface of an aluminum base. It is a figure which shows the result of the Example of this invention, and is a figure which shows the relationship between the film thickness of a nickel plating layer, and the amount of corrosion in the oxalic acid aqueous solution.

Explanation of symbols

1: Tube plate 2: Fin 3: Spacer bar 4: Core part

Claims (4)

Plate fin for heat exchanger made of aluminum or aluminum alloy,
A plate fin excellent in corrosion resistance, characterized in that an electroless nickel plating layer is formed on the surface thereof.   The plate fin excellent in corrosion resistance according to claim 1, wherein the electroless nickel plating layer has a thickness of 10 μm or more.   The plate fin with excellent corrosion resistance according to claim 1 or 2, wherein the surface of the plate fin is a surface after being subjected to zinc diffusion treatment.   The said plate fin is used as a structural member of the plate fin type cooler for rail vehicles, The plate fin excellent in corrosion resistance in any one of Claims 1-3 characterized by the above-mentioned.

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