FR2930023A1 - Surface treatment method for motor vehicle's charge air cooler, involves carrying out hydrothermal treatment on components and brazing points to cover components and points with boehmite film and protect components and points from corrosion - Google Patents

Abstract

The method involves carrying out a hydrothermal treatment e.g. boehmite formation, on aluminum components of a heat exchanger and brazing points to cover the components and the points with a boehmite film (306) and to protect the components and the points against corrosion. The treatment is carried out by forming an interface between a deionized water solution and an assembly of the components/points, and forming pseudo-boehmite (304) and bayerite on the assembly, where the solution comprises an additive e.g. EDTA solution. Each point connects each component (300) with the other components. An independent claim is also included for a heat exchanger comprising aluminum components that are assembled with each other by brazing points.

Description

Surface treatment method of a heat exchanger and heat exchanger resistant to corrosion.

The invention relates to the field of brazed heat exchangers, in particular for motor vehicles and to a surface treatment process for such heat exchangers.

Many heat exchangers are made by assembling different pieces of metal material including aluminum. The parts are generally assembled together by brazing to form a heat exchanger of more or less complex geometry.

Metal heat exchangers are sensitive to their sometimes hostile environment and are often affected by corrosive reactions. The chemical corrosion reaction causes progressive degradation of said heat exchangers.

Corrosion is often due to the condensation of exhaust gases during start-up, on the heat exchanger in the engine of a vehicle. Also, the fouling of a pollution heat exchanger promotes corrosion. The condensation of the exhaust gases is increased during start-up due to the fact that all the components of an engine are cold. The degradation of the heat exchangers results in the lowering of the performance and the life of an engine. There is therefore a need to treat the heat exchangers of a vehicle to increase their corrosion resistance.

Corrosion can affect heat exchangers, both on their outside (so-called external corrosion) and on their inside (called internal corrosion). External corrosion is mainly related to the surrounding environment, namely mainly the exhaust / pollution gas assembly. In particular, in urban areas the surrounding environment is particularly aggressive.

Until recently, internal corrosion was generally less widespread. This was because the interior of an exchanger was less exposed to the surrounding environment. New technologies such as charge air coolers (RAS) have turned things around. Indeed, modern vehicle engines are frequently supplied with charge air. This air contains in many cases recycled exhaust gases. The interior of RAS is therefore exposed to an aggressive environment and requires special treatment.

In particular, surface treatments using hydrophobic coatings are known. WO2006 / 034876A1 discloses a heat exchanger for charge air cooling, which comprises a hydrophobic surface for the purpose of removing condensed water in the form of droplets.

The prior art coating surface treatments are often complex and do not provide uniform coating. In addition, the coatings are generally made by means of a complex and expensive step of soaking and immersion in a hydrophobic lacquer type chemical composition.

Prior art soaking steps do not provide uniform access to heat exchangers of complex geometry, such as charge air exchangers and / or recycled exhaust gas exchangers. In addition, hydrophobic coatings do not provide effective protection at low pH, which is necessary for corrosion resistance.

The invention improves the situation.

The present invention consists of a surface treatment method of a heat exchanger having aluminum components joined to each other by soldering points, characterized in that it comprises the application of a hydrothermal treatment of boehmitage type on at least one part; said components and brazing points, to cover them at least partially with a boehmite film and protect them against corrosion.

For this purpose, the invention introduces a method of surface treatment of a heat exchanger having aluminum components joined together by soldering points. The method comprises the application of a hydro-thermal boehmitage type treatment on at least part of said components and soldering points, to cover them at least partially with a boehmite film. The method of the invention notably allows effective protection against corrosion. In addition, the hydro-thermal application can be carried out in a regular and uniform manner on all of said components and said soldering points.

3 The hydro-thermal treatment comprises the following steps: a. forming an interface between a deionized water solution and at least a portion of said aluminum components and soldering points, b. applying a succession of temperature variations to said interface.

According to one embodiment, step b. includes at least the following substeps: b1. apply a selected temperature of between 20 ° C and 80 ° C, preferably 80 ° C for 10 to 20 minutes, preferably 20 minutes, b2. apply a selected temperature of between 80 ° C and 100 ° C, preferably 100 ° C for 1 to 15 hours, preferably 1 hour, (1 hour preferably to obtain the minimum layer thickness for effective protection against corrosion), b3. alternatively to obtain a thicker boehmite layer, apply a selected vapor phase temperature of between 100 ° C and 374 ° C, preferably 120 ° C, for 1 to 15 hours (depending on the desired thickness).

Often before the application of sub-step b1, the soldering points are coated at least partially with silicon particles. According to one embodiment, after the application of the sub-step b3, the soldering points are entirely covered with a boehmite film.

According to one embodiment, the heat exchanger is of the charge air cooler type. As mentioned above, exchangers of this type are particularly susceptible to corrosion. The method of the invention makes it possible to protect them effectively.

In step a, the formation of the interface can be carried out either by dipping at least a portion of said components and brazing points in said deionized water solution, or by internal circulation in the heat exchanger or by spraying said deionized water on at least part of said components and soldering points. By dipping is generally meant partial or complete immersion of the components and / or soldering points.

It is noted that the realization by internal circulation of the deionized water is particularly adapted to confer effective anti-corrosion resistance to the RAS. The deionized water solution can be in either liquid or gaseous form, or both.

According to one embodiment, the deionized water solution may comprise a catalyst-type reaction accelerator additive. A solution of ethylenediaminetetraacetic (EDTA) is particularly suitable.

The invention also introduces a heat exchanger with aluminum components joined together by soldering points. In this exchanger, the aluminum components and soldering points are at least partially covered with a boehmite film, which allows a high corrosion resistance.

As mentioned above, the heat exchanger may be of the charge air cooler type for heat exchange between a first fluid and a second fluid. having a heat exchange bundle, an envelope housing the bundle, an inlet manifold with an inlet manifold and an outlet manifold with an outlet manifold for circulating said first fluid.

Such a heat exchanger can be made by a method as described above.

Other advantages and characteristics of the invention will appear on reading the detailed description below and on the accompanying drawings, in which: FIG. 1 shows a perspective view of a charge air heat exchanger for motor vehicles,

FIG. 2 represents a flowchart of a hydrothermal process according to one embodiment of the invention,

FIG. 3 schematically shows the formation of a double film of pseudoboehmite and bayerite on a surface of heat exchanger, and

Figure 4 shows schematically the formation of pseudo-boehmite and boehmite film on the surface of a heat exchanger.

The drawings and the description below contain, for the most part, elements of a certain character. They are an integral part of the description, and thus may serve not only to make the present invention easier to understand, but also to contribute to its definition, if necessary. Now, briefly, we will turn to motor vehicle heat exchangers.

A heat exchanger generally allows the exchange of heat between fluids 5 (liquid or gaseous) and comprises a heat exchange bundle, a logean envelope: this heat exchange bundle, an inlet manifold and a outlet manifold for a fluid, arranged on either side of the bundle.

As described above, the engines of modern vehicles are frequently supplied with compressed air (charge air comprising recycled exhaust gas) by a compressor driven by exhaust gas. This compression has the effect of: heating the air so that it is necessary to cool it to lower its temperature before its introduction into the engine cylinders. For this purpose, the vehicle is equipped with a charge air cooler. As mentioned above, these heat exchangers comprise metal parts assembled together by brazing.

Brazing means an assembly of materials by means of a base metal, co-laminated to the base material, having a lower melting temperature than the materials to be assembled. Generally, in the automotive industry, brazing is carried out in an oven. It is a heterogeneous assembly which leaves on the final product brazing points marking the merger of different parts that contained the material of supply. It will be understood that soldering points may be more or less extensive soldering areas or areas.

Brazing points are particularly sensitive to clogging and condensation of exhaust gases. It is therefore important to protect places with soldering points to ensure corrosion resistance. Anticorrosion resistance is understood to mean resistance to corrosion.

Condensation of the exhaust gases usually takes place when a vehicle is started when the engine is cold. The exhaust gases contain chemical species of pH 52.5 such as sulphates, nitrates or organic acids. In addition, the pollution of the atmosphere, in particular with particles of pH <2.5, such as chlorides, mixes with the exhaust gas and fouls the heat exchanger.

The organic acids mentioned above are especially of the acetic acid, propanoic acid, glycolic acid, formic acid, etc. type.

Added to this is the fact that pollution standards are becoming increasingly severe and require increasing activity of the charge air heat exchangers, resulting in increased condensation and fouling of said heat exchangers. and has the effect of accelerated corrosion.

The Applicant has discovered, not surprisingly, that an application of a hydrothermal treatment of boehmitage type on a heat exchanger having. Aluminum components joined together by soldering points provides effective corrosion resistance. In particular, according to one embodiment, the inventior provides protection against chemical agents and attacks of pH 2.5.

In addition, the invention allows simple operation and uniform protection over the entire heat exchanger when it is desired.

FIG. 1 shows a charge air cooler type heat exchanger 100. According to the embodiment described herein, the heat exchanger 100 undergoes a boehmite hydrothermal treatment by total or partial immersion in a solution of immersion at regulated temperatures.

The immersion solution is preferably deionized water. The use of deionized water has the particular advantage of not chemically attacking the heat exchanger 100, which allows a total soaking of the exchanger 100 in order to access any location of a complex geometry exchanger such as the charge air cooler.

According to one embodiment of the invention, deionized water is used in which one or more additives such as a solution of ethylenediaminetetracetic acid (EDTA) is added or not. EDTA makes it possible in particular to increase the reaction rate for the formation of boehmite, as detailed below. However, the hydrothermal treatment according to the invention does not require the addition of chemical agents in the immersion solution. Figure 2 shows a flow chart of the hydrothermal treatment. This is a boehmitage treatment in which deionized water and 3% of a solution of ethylenediaminetetraacetic acid are used. A first operation 200 involves contacting aluminum components joined together by soldering points of a heat exchanger with deionized water.

In the embodiment described here, it is a heat exchanger of the charge air cooler type as shown in Figure 1. The contacting of water and said components can be performed by dipping or by internal circulation of the deionized water in the vapor phase in the heat exchanger. The purpose of the dipping operation 200 is to form an interface between the deionized solution and at least a portion of said aluminum components and soldering points. It will be understood in the present description that the term interface is not understood in the strict sense, but corresponds substantially to the set of deionized water / aluminum surface.

The operations 202, 204 and 206 correspond to a succession of temperature variations of said interface. The action of the heat on the set deionized water / aluminum surface has the effect of forming a hydrated aluminum oxide of high corrosion resistance.

Operation 202 comprises forming a double film comprising pseudo-boehmite (γ-A / OOH) and bayerite [Al (OH) 3]. Thermal operation 202 involves applying a temperature of between about 20 ° C and 80 ° C for 10 to 20 minutes.

A subsequent operation 204 comprises the formation of boehmite film. Operation 204 comprises applying a temperature of between 80 ° C and 100 ° C for 1 to 25 hours.

An alternative for forming a thicker boehmite film, thermal operation 206, comprises applying a temperature of between 100 ° C and 374 ° C in the water vapor phase, for 1 to 15 hours depending on the temperature. desired layer thickness. The aluminum components and brazing points of the heat exchanger described herein are therefore substantially coated with a boehmite film by means of operations 200 to 206. The boehmite is of a mineral nature and is a mono aluminum oxide. -hydrate [Al 2 O 3 H 2 O]. 30 35

A last operation 208 includes returning to room temperature, emersion and drying of the aluminum components of the heat exchanger.

Figure 3 schematically shows the formation of pseudo-boehmite 304 and bayerite 302 on an aluminum component 300 of a heat exchanger. The formation of pseudo-boehmite 304 and bayerite 302 progresses to a uniform film of boehmite 306 as shown in FIG. 4. The boehmite film covers not only the aluminum components of the heat exchanger, but particularly the brazing formed by the assembly of said components.

It is noted that during brazing, so-called silicon needles are created at the brazing points. Silica needles are often affected by an agglomeration of impurities around them. Therefore, said needles and their vicinity are particularly affected by corrosion.

According to one embodiment, the boehmite film 306 substantially covers all the silica needles found at the brazing points. This allows for proper corrosion resistance.

In addition, the boehmite film 306 is covalently bonded to the aluminum components and is resistant to temperatures of -40 ° C to 280 ° C without loss of any adhesion.

The duration of the hydrothermal treatment of the boehmitage type comprising the operations 200 to 206 is generally from 1 to 15 hours. Preferably, the treatment is carried out by steam (spray) to prevent contamination by chemical impurities or inhibitors such as calcium, copper, iron, fluorides, chlorides, silicon oxide and / or phosphate. The invention also relates to a heat exchanger with aluminum components 30 joined together by brazing points having undergone a process treatment of the invention. A heat exchanger according to the invention is therefore a heat exchanger whose assembled aluminum components and brazing points are at least partially covered with a boehrite film.

According to one embodiment, the heat exchanger is of the supercharging air cooler type for exchanging heat between a first fluid and a second fluid, comprising a heat exchange bundle, an envelope housing this bundle, an inlet manifold with an inlet manifold and an outlet manifold with an outlet manifold for circulating said first fluid. It is a heat exchanger 100 like that shown in FIG.

Examples

Example: Heat Exchanger Table 1 shows the corrosion resistance of heat exchangers of the invention on exposure to hostile environment conditions. The conditions of the example are defined on the one hand by alternately immersing heat exchangers in two media: medium A and medium B, whose respective pH is 2.5 and 1.5; and secondly by simulation cycles.

Medium A: - 300 ppm Sulfates, (as sodium sulphate) - 50 ppm Nitrates, (as sodium nitrate) - 3000 ppm Formic Acid, - 500 ppm Acetic Acid, and - 400 ppm Acid propanoic.

The resulting pH of this mixture is 2.5. Medium B: - 1300 ppm Sulfates, (as sulfuric acid 96%) 25 - 50 ppm Nitrates, (as sodium nitrate) 3000 ppm Formic Acid, 500 ppm Acetic Acid, and - 400 ppm Acid propanoic.

The resulting pH of this mixture is 1.5.

The simulation cycles comprise on the one hand a driving cycle of 68h / week, immersion for 15s in a selected medium (medium A or medium B) and drying for 15min at 170 ° C .; and other bet: a stop cycle of 100h / week, immersion for 15s in a selected medium (medium A or medium B) and drying for 15min at room temperature.

PH media Anticorrosion resistance of heat exchangers Medium A 2.5 No leakage after 4000 simulation cycles Medium B 1.5 No leakage after 1630 simulation cycles Table 1 Example: Aluminum components joined to each other by soldering points Table 2 shows a test Comparative between elements having undergone treatment: hydrothermal according to the invention (TH element) and elements that have not undergone hydrothermal treatment (element 0). The comparison is based on the loss of material. caused by corrosion, aluminum components joined together by soldering points. Corrosion is favored by hostile environmental conditions defined on the one hand by the alternating immersion of said components in four media: medium C, medium D, medium E and medium F, with a variable pH ranging from 1.5 to 2, Depending on the amount of sulfuric acid present in each medium; and on the other hand corrosive cycles. Each medium (C, D, E and F) comprises: - 1000 ppm Sulfates, - 60 ppm Nitrates, and - and a variable amount of Chloride (see Table 2).

The corrosive cycles comprise five successive dipping cycles. The soaking cycles are carried out over a period of 7 days and include immersion in a medium (C, D, E or F) for 8 hours at 60 ° C and 16 hours of exposure to air over a period of 5 days. successive days, followed by immersion in a medium (C, D, E or F) for 2 successive days.

Media pH Quantity Loss of Loss of Loss of Chlorides matter - material - matter - material - (ppm) element o element element element (g / cm2) TH ra (%) TH (° / o) (g / cm2) Medium C 2.5 60 0.06372 0.05244 6.4 5.2 Medium D 2 80 0.08778 0.05943 8.78 5.94 Medium E 1.5 60 0.1231 0.1119 12.24 11.66 Medium F 1.5 100 0.12986 0.1927 13.0 11.93 Table 2

Claims (16)

Revendications1. Process for the surface treatment of a heat exchanger having aluminum components joined to each other by soldering points, characterized in that it comprises the application of a hydrothermal treatment of boehmitage type on at least part of said components and brazing points, to cover them at least partially with a boehmite film and to protect them against corrosion. 2. Method according to claim 1, characterized in that the hydrothermal treatment comprises the following steps: forming an interface between a deionized water solution and at least a portion of said aluminum components and soldering points, b. applying a succession of temperature variations to said interface. 15 3. Method according to claim 2, characterized in that step b. includes at least the following substeps: b1. apply a selected temperature of between 20 ° C and 80 ° C, preferably 80 ° C, for 10 to 20 minutes, preferably 20 minutes, b2. apply a selected temperature of between 80 ° C and 100 ° C, preferably 100 ° C, for 1 to 15 hours, preferably 1 hour,, b3. alternatively to obtain a thicker boehmite layer, apply a selected vapor phase temperature of between 100 ° C and 374 ° C, preferably 120 ° C, for 1 to 15 hours. 25 4. Method according to claim 3, characterized in that: - step b1. comprises forming pseudo-boehmite and bayerite on at least part of said aluminum components and soldering points, step b2. comprises forming pseudo-boehmite and boehmite on at least a portion of said aluminum components and soldering points; step b3. comprises forming boehmite on at least a portion of said aluminum components and soldering points. 5. Method according to claim 4, characterized in that before application of the sub-step b1, the soldering points are coated at least partially by 12 silicon particles, and in that after application of the substep b3, said Brazing points are fully covered with a boehmite film. 6. Method according to one of the preceding claims, characterized in that the heat exchanger 5 is of the charge air cooler type. 7. Method according to one of claims 2 to 6, characterized in that in step a. forming said interface is performed by dipping at least a portion of said components and soldering points in said deionized water solution. 8. Method according to one of claims 2 to 6, characterized in that in step a. forming said interface is performed by internal circulation in the heat exchanger of said deionized water on at least part of said components and soldering points. 15 9. Method according to one of claims 2 to 6, characterized in that in step a. forming said interface is performed by spraying said deionized water on at least part of said components and soldering points. 10. Method according to one of claims 2 to 9, characterized in that the deionized water solution is at least partially in liquid form. 11. Method according to one of claims 2 to 10, characterized in that the deionized water solution is at least partially in gaseous form. 25 12. Method according to one of claims 2 to 11, characterized in that the deionized water solution further comprises an additive. 13. The method of claim 12, characterized in that the additive is a solution of ethylenediaminetetraacetic acid (EDTA). 14. Heat exchanger with aluminum components joined together by soldering points, characterized in that said aluminum components and soldering points are at least partially covered with a boehmite film. 35 15. Heat exchanger according to claim 14, characterized in that the heat exchanger is supercharge air cooler type for heat exchange between a first fluid and a second fluid, comprising a heat exchange beam. heat, a housing housing this bundle, an inlet manifold with an inlet manifold and an outlet manifold with an outlet manifold for circulating said first fluid. 16. Heat exchanger according to one of claims 14 or 15, characterized in that it is carried out by a method according to one of claims 1 to 13.