Heat exchanger coating

Abstract

A heat exchanger for an air-conditioning circuit of a vehicle. The heat exchanger enables heat exchange between fluids and has a surface in contact with one of the fluids. The surface is formed from aluminum and/or from aluminum alloy. The surface is coated with an alumina layer and a reinforcing layer. The reinforcing layer includes an organic substance and a mineral substance. The organic substance includes at least one polymer and the mineral substance is capable of reacting with the aluminum in order to form an anticorrosion material.

Claims

1. A heat exchanger for an air-conditioning circuit of a vehicle, said heat exchanger enabling heat exchange between a first fluid and a second fluid and comprising: a surface in contact with at least one of said first fluid and said second fluid, said surface being formed from at least one material selected from the group consisting of aluminum and aluminum alloy, wherein said surface is coated with an alumina layer and a reinforcing layer, wherein said reinforcing layer comprises: an organic substance including at least one polymer; and a mineral substance capable of reacting with the aluminum to form an anticorrosion material and wherein said mineral substance capable of reacting with the aluminum allows for regeneration of the alumina layer coated on said surface.

2. The heat exchanger as claimed in claim 1, wherein said mineral substance capable of reacting with the aluminum allows for filling in surface irregularities of at least one selected from the group consisting of the surface and the alumina layer.

3. The heat exchanger as claimed in claim 1, wherein said mineral substance capable of reacting with the aluminum comprises from at least one material selected from the group consisting of a chromium-based substance and a fluoride-based substance.

4. The heat exchanger as claimed in claim 1, wherein from at least one material selected from the group consisting of said organic substance and said mineral substance provide said reinforcing layer with from at least one property selected from the group consisting of hydrophilic properties and antimicrobial properties.

5. The exchanger as claimed in claim 4, wherein said organic substance occupies between 50% and 80% of said reinforcing layer.

6. The exchanger as claimed in claim 5, wherein said mineral substance occupies between 20% and 50% of said reinforcing layer.

7. The heat exchanger as claimed in claim 1, wherein said reinforcing layer additionally comprises at least one corrosion inhibitor.

8. The heat exchanger as claimed in claim 1, wherein said reinforcing layer additionally comprises at least one antimicrobial substance.

9. The heat exchanger as claimed in claim 1, wherein said reinforcing layer weighs between 0.5 and 1.5 g per 1 m.sup.2 of said surface.

10. The heat exchanger as claimed in claim 1, wherein said heat exchanger comprises one face having a first surface and a second surface opposite to the first surface, a first portion of said reinforcing layer covering the first surface having a thickness greater than the thickness of a second portion of said reinforcing layer covering the second surface.

11. The heat exchanger as claimed in claim 1, wherein the heat exchanger is an evaporator.

12. A heat exchanger for an air-conditioning circuit of a vehicle, said heat exchanger enabling heat exchange between a first fluid and a second fluid and comprising: a first surface in contact with said first fluid and being formed from at least one material selected from the group consisting of aluminum and-aluminum alloy; and a second surface in contact with said second fluid and being formed from at least one material selected from the group consisting of aluminum and-aluminum alloy, wherein said first surface and said second surface are opposite surfaces of a face in the heat exchanger and are coated with an alumina layer and a reinforcing layer, wherein said reinforcing layer comprises a mineral substance capable of regenerating said alumina layer, and wherein a first portion of said reinforcing layer covering the first surface has a thickness greater than the thickness of a second portion of said reinforcing layer covering the second surface.

13. A heat exchanger for an air-conditioning circuit of a vehicle, said heat exchanger enabling heat exchange between a refrigerant and an external air flow and comprising: a first surface in contact with said external air flow and being formed from at least one material selected from the group consisting of aluminum and-aluminum alloy; and a second surface in contact with said refrigerant and being formed from at least one material selected from the group consisting of aluminum and-aluminum alloy, wherein said first surface and said second surface are coated with an alumina layer and a reinforcing layer, wherein said reinforcing layer comprises a mineral substance capable of regenerating said alumina layer, and wherein a first portion of said reinforcing layer covering the first surface in contact with said external air flow has a thickness greater than the thickness of a second portion of said reinforcing layer covering the second surface in contact with said refrigerant.

Description

(1) FIG. 1 represents, in a general manner, a portion of a heat exchanger enabling a heat exchange between a first fluid and a second fluid. It may be, for example, an evaporator for a vehicle air-conditioning circuit. In this case, one of the two fluids is a flow of air to be cooled and/or the other fluid a refrigerant. In particular, it is in this example a brazed evaporator made of aluminum or of aluminum alloy.

(2) Said exchanger is formed here of a bundle 10 comprising ducts 12 for circulation of the refrigerant. Said ducts 12 are formed, for example, of tubes, connected to one or some collectors, and/or of pairs of stacked plates, communicating with one another. Between the tubes and/or pairs of plates, it is possible to provide disrupters 14, especially separators of serpentine shape. These separators 14 disrupt the circulation of the air flow and increase the exchange area. The heat exchange between the air flow and the refrigerant is thus improved. These separators 14 are in contact with the tubes and/or the pairs of plates, especially via the apices of their bends.

(3) Said exchanger has surfaces intended to come into contact with one of said fluids, especially the flow of air to be cooled, in particular the walls of said tubes, pairs of plates and/or separators.

(4) According to the invention, said surfaces are formed from aluminum and/or from aluminum alloy. They are coated with an alumina layer and with a layer referred to as a reinforcing layer that reinforces the natural protection of the aluminum. This reinforcing layer comprises a portion of organic substances comprising at least one polymer and a portion of mineral substances comprising at least one substance capable of reacting with the aluminum in order to form an anticorrosion material. Said reinforcing layer advantageously constitutes a coating of said alumina layer.

(5) The presence of the mineral substances capable of reacting with the aluminum makes it possible to compensate for attacks on the alumina layer, or even to regenerate said alumina layer. A protective layer is thus reformed and the consequences of corrosion attacks of the surfaces of the exchanger are limited. By reacting with the aluminum, said substances could also make it possible to fill in surface irregularities of said surfaces and/or of the alumina layer thereof.

(6) The organic substances and/or the mineral substances in the coating could additionally provide the surfaces of the exchanger with hydrophilic and/or antimicrobial properties. Thus, the drops of water flatten out in the form of a film which is easier to drain and avoids the spraying of water caused by the removal of drops of water.

(7) Said substances are advantageously substances known for fixing oxygen molecules. Said substances are preferably chromium-based substances, in particular trivalent chromium salts.

(8) This mechanism may be improved by being in an acid medium, especially at a pH of between 1.5 and 5, preferably between 2 and 3.5. This pH may be obtained by the addition of acids, in particular organic acids that could furthermore be binders capable of reacting with the polymer(s) of the portion of organic substances. Said polymer comprises, for example, a hydroxylated polymer of vinyl alcohol type. The mechanism is also improved if the coating comprises a small portion of fluoride, of less than 1000 ppm.

(9) The anticorrosion properties provided by the substances capable of reacting with the aluminum may be reinforced by the addition of one or more corrosion inhibitors to said reinforcing layer. These may be titanium-based substances, especially titanium salts. These corrosion inhibitors may be substances that fix oxygen.

(10) Likewise, for improving the antimicrobial properties, said reinforcing layer may comprise one or more antimicrobial substances. These substances may be selected from bronopol, carbendazim, isothiazolinone and zinc-based substances, such as zinc pyrithione. In the case of a coating having a pH between 1.5 and 5, the antimicrobial substance(s) must withstand the acid media.

(11) Such a heat exchanger may be obtained, for example, by the process of the invention.

(12) In a first step, said exchanger is formed with surfaces that are intended to come into contact with one of the fluids to be cooled and said surfaces are formed from aluminum and/or from aluminum alloy and coated with an alumina layer. Preferably, the exchanger is obtained by a brazing step.

(13) In a second step, said alumina layer is covered with a coating that reinforces the natural protection of the aluminum. Said coating comprises a portion of organic substances comprising at least one polymer and a portion of mineral substances comprising at least one substance capable of reacting with the aluminum in order to form an anticorrosion material. According to the invention, the coating is applied directly after the brazing step without an additional step, such as a surface treatment of activation or pickling type, or the preparation for a conventional immersion processclosing off the internal circuit, inserting into equipment for immersion in a bath.

(14) This step makes it possible to form a layer that regenerates and/or that reinforces the natural protection of the aluminum, advantageously by regenerating the alumina layer.

(15) As described above, the pH of the coating is preferably between 1.5 and 5 and in an optimized manner between 2 and 3.5 in order to obtain a good regeneration of the alumina layer and a good protection of the surfaces of the exchanger by the reinforcing layer.

(16) The coating is generally applied to the exchanger in a uniform manner, for example by spraying and/or by blowing. Preferably, the amount applied is between 15 and 20 ml/m.sup.2.

(17) The covering step may be followed by a step in which the exchanger is blown. This step makes it possible to adjust the thickness of coating present on two of the opposite faces of the exchanger. Generally, the thickness of the coating on the face opposite the spraying and blowing is slightly greater. Preferably, this face is the face exposed to the air flow since this overthickness leads to the corrosion resistance of the exchanger being improved. Advantageously, this step takes place before the drying step.

(18) These steps are followed by a step in which the coating is dried. This drying step especially enables the formation of the reinforcing layer. Indeed, this step enables the polymerization of the polymer(s) present in the portion of organic substances of the coating. The polymerization may be reinforced by the crosslinking of the polymers, in particular if crosslinking agents are added to the coating. The crosslinking agents may be of organic acid, epoxy or acrylic type, selected to react with the free hydroxyl functions of the polymer.

(19) The drying and the polymerization may also be reinforced in an advantageous manner if said coating is dried during a step of heating between 130 C. and 180 C., preferably at 150 C. Said heating step lasts from 1 to 10 min; preferably it lasts 5 min.

(20) An example of a reinforcing layer that has made it possible to obtain an exchanger having satisfactory hydrophilic, antibacterial and anticorrosion qualities comprises 60% of organic substances and 40% of mineral substances, including 30% of chromium-based substances. The polymer included in the portion of organic substances is especially a vinyl alcohol polymer. This layer has a surface density of between 0.8 and 1.2 g/m.sup.2 and an overthickness of 20% on the face exposed to the air flow obtained for example by the blowing step described above.

(21) Comparative tests were carried out between such an exchanger and an exchanger from the prior art.

(22) It was thus shown that such an exchanger had equivalent results regarding its hydrophilic, antimicrobial and anti-odor qualities, relative to an exchanger from the prior art known for these same qualities.

(23) In addition, the corrosion resistance is considerably improved owing to the presence of the reinforcing layer. Corrosion tests made it possible to demonstrate that: the signs of corrosion attack of the aluminum surface were retarded by more than 60 days on an exchanger according to the invention; the depth of the corrosion pitting on the surface of an exchanger according to the invention was stable after 90 days in contact with a corrosive solution in a corrosion chamber, whereas the depth of the corrosion pitting on the surface of an exchanger from the prior art was increased; the failure of the exchanger according to the invention is obtained around 50 days after that of the exchanger from the prior art.

(24) It should be noted that embodiment variants are of course possible and that the present invention is not limited to an evaporator for vehicle air conditioning. In particular, it is possible to extend the present invention to other types of heat exchanger, and to other fields.