EP0771888A1

EP0771888A1 - Process for the protection against external corrosion in copper-based heat exchangers

Info

Publication number: EP0771888A1
Application number: EP96915036A
Authority: EP
Inventors: José Manuel ARAGUES BERNAD
Original assignee: Valeo Termico SA
Current assignee: Valeo Termico SA
Priority date: 1995-05-16
Filing date: 1996-05-14
Publication date: 1997-05-07
Also published as: ES2129282B1; ES2129282A1; MX9700433A; WO1996036749A1

Abstract

The process comprises the coating and heat diffusion of an Sn-based alloy on a Cu-based core so as to obtain a composite material comprised of a Cu-based core, an external surface formed of a Sn-based alloy and an intermediary layer comprised of Cu-Sn alloys of variable compositions. Said composite material thus formed is appropriate for the fabrication of gills for heat exchangers, particularly radiators for motor vehicles, and whose honeycombs can be welded in an oven with oxidizing or non oxidizing atmosphere.

Description

SCOPE OF THE INVENTION

The invention refers to a procedure for protecting copper-based heat exchangers against external corrosion which consists of the coating and thermal diffusion of a tin-based alloy on a copper-based nucleus, by which a composite material which offers excellent performance against corrosion is formed. The invention also refers to the said composite material, to fins for heat exchangers made of the said material and to heat exchangers which incorporate such fins, and also to their manufacturing process.

BACKGROUND TO THE INVENTION

Heat exchangers, especially radiators intended for the cooling of motors in automobiles and agricultural and industrial machinery (heat engines), which consist of a central nucleus, known as the core, consisting of a set of tubes for the circulation of the liquid coolant, plus several fins in contact with the tubes in order to produce the exchanging of heat. The radiator is completed by tanks and header plates which close the circuit of the liquid coolant. In addition some lateral steel supports are included in order to increase its rigidity.
In the case of copper radiators, the core tubes are of brass, the fins are of copper (Cu) and the tube-fin joint is made with tin-based solders (Sn). An Sn-based solder is provided by the brass tube, which has been previously coated with solder, mainly by immersion in a bath of molten solder.
Soldering of the core can be carried out in continuous or static furnaces with an oxidizing atmosphere. Usually the soldering of the core is carried out in two stages, the first in a furnace, in which the tubes are soldered to the fins, and the second in which the ends of the tubes are soldered to the header plates by systems such as capillary attraction (immersion in baths of molten Sn), the projection of Sn (splashing or spraying), etc.
In order to obtain correct soldering between the tubes and the fins a solder "flux" (or a scourer of the metals to be joined and a protector of the actual soldering during the operation) should be applied, either by immersion of the core or by the projection of the flux onto the core.
The fluxes normally used are those of a mineral type which contain inorganic halides, such as zinc or ammonium chlorides, hydrochloric acid, etc., although other fluxes of a mixed type, with organic and inorganic components, such as amino hydrochlorates and hydrobromates, hydrobromic acid, etc. are also used.
In general, these products are aggressive to the radiator materials and their residues cause appreciable corrosion, so that it is necessary to carry out washing and drying operations on the radiators, which implies considerable energy consumption and the production of residual acid fluids and significant concentrations of metallic ions, making it necessary to carry out adequate treatment on them prior to their disposal or re-use. Furthermore, vapours are given off which contain, among other compounds, hydrochloric acid, hydrobromic acid, ammonia and amine compounds, which are also aggressive to the environment, so that additional operations of cleaning the vapours generated have to be carried out, which makes the process more expensive and complicated.
A further additional problem caused by current radiator manufacturing processes relates to the application of an anti-corrosion protection, since at present this operation is carried out once the manufacture of the radiator is completed, by means of the application of an anti-corrosion paint by projection, which only covers the external areas of the radiator with paint, but does not cover the core nucleus. Normally the paint penetrates only about 2 mm into each face of the core.
In general, copper radiators provide excellent service in respect of thermal transfer, mechanical strength and internal corrosion. However, resistance to external corrosion is problematic.
In recent years, the demands for protection against corrosion have increased notably, not only in the automotive sector due, among other reasons, to the increasing use of halogenated salts, such as NaCl and MgCl₂, for de-icing roads, but also in other sectors where it is necessary to guarantee effective protection against other aggressive environments, fundamentally marine or industrial ones. These environments cause significant corrosion on tubes, fins and on soldering, with the consequent loss of mechanical strength and of the thermal characteristics of the radiators.
Therefore, there continues to exist a series of problems associated not only with current copper radiator manufacturing procedures, such as the need to use a soldering flux which requires washing and drying operations on the radiators, which in turn, generates an environmental problem due to the effluents and vapours produced, as well as a significant consumption of energy, which complicates and puts up the cost of current radiator manufacturing procedures, but also with the actual copper radiators manufactured with existing technology, since these continue to have little resistance to external corrosion.
The present invention provides a solution to the problems raised.

SUMMARY OF THE INVENTION

By means of laboratory tests to analyze the problems of external corrosion on copper radiators, the principal mechanisms which produce the corrosion of tubes and fins have been identified, which has permitted the creation of a new material, to be precise, a composite material, suitable for the manufacture of fins or insertions, which offers an excellent performance against corrosion, retaining its thermal and mechanical properties, which also allows the traditional thicknesses of these parts to be reduced. At the same time, the composite material provides a very effective protection against the perforating corrosion which is produced in radiator core tubes as a consequence of the mechanisms of corrosion which are produced in copper radiators manufactured with current technology. All of this ensures the retention during long-term service of the functional characteristics of a radiator manufactured in accordance with the precepts of the present invention, as has been demonstrated by an extensive programme of accelerated corrosion tests.
The new material provided by this invention consists of a Cu-based nucleus, an intermediate layer consisting of Cu-Sn alloys of variable composition and an external surface formed, basically, by an Sn-based alloy. This material can be obtained by means of thermal diffusion, under controlled conditions, of a coating of Sn-based alloys deposited on a Cu-based nucleus.
The use of this new material involves a series of changes in radiator manufacturing procedure, one of which lies in the possibility of carrying out the soldering of the core in ovens with a controlled atmosphere. By operating under these conditions and due to the favourable soldering characteristics provided by the new material, the soldering can be carried out without the use of any kind of flux, which is impossible to achieve with current technologies. This method of carrying out soldering of the core provides enormous environmental improvements since gaseous effluents and aqueous contaminants are not produced and, at the same time, significant reductions in energy consumption are achieved.
Alternatively, the cores constructed with this new material can also be soldered in an oven with an oxidizing atmosphere, although in this case an organic non-corrosive flux which does not require cleaning may be used, so that notable advantages are achieved in comparison with current radiator manufacturing technology.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a photograph which shows perforating corrosion in a core tube, of arsenical brass [Cu-Zn 70/30, As 0.03%], after a test of 92 hours in a saline mist at a magnification of 20/1. In the photograph the leakage area is indicated by an arrow.
Figure 2 is a photograph showing perforating corrosion in a core tube, of arsenical brass (Cu-Zn 64/36, As 0.03%], after 120 hours of testing in a saline mist, at a magnification of 20/1. In the photograph, the leakage area is indicated by an arrow.
Figure 3 is a photograph showing intercrystalline corrosion on and removal of zinc from a core tube, of arsenical brass [Cu-Zn 67/33 As 0.03%], as well as corrosion in the Sn-Pb solder, after 144 hours of testing in a saline mist, at a magnification of 390/1 (Figure 3A) and at a magnification 325/1 (Figure 3B).
Figure 4 is a photograph showing corrosion by the removal of zinc from a core tube, of phosphorized brass [Cu-Zn 66/34 P], after 244 hours of testing in a saline mist, at a magnification of 260/1 (Figure 4A), and perforating corrosion (intercrystalline and zinc removal) in the said core tube after 244 hours of testing in a saline mist, at a magnification of 260/1 (Figure 4B).
Figure 5 is a photograph showing corrosion on a fin after 144 hours of testing in a saline mist, at a magnification of 260/1 (Figure 5A), and also at a magnification of 360/1 (Figure 5B). In the photograph, the joint of the core tube and the fin can be seen. The core tube is made of arsenical brass [Cu-Zn 67/33 As 0.03%] and the fin of Cu-Sn (0.01% Sn).
Figure 6 is a photograph showing the appearance of a radiator core which incorporates fins manufactured with a composite material provided by this invention, after 1,170 hours of testing in a saline mist (0.5 enlargement [0.5X]).
Figure 7 is a photograph showing an enlarged detail of Figure 6 (2X).
Figure 8 is a photograph showing the appearance of the tube after 1,008 hours of testing in a saline mist (5X). As can be seen, there are no noticeable attacks due to corrosion on the brass and the Sn-Pb film on the base metal is maintained.
Figure 9 is a photograph showing the appearance of a cross-section of the tube after 1,008 hours of testing in a saline mist (1,000X). The Cu-Sn alloy film on the surface of the brass can be seen, but there are no noticeable points of corrosion.
Figure 10 is a photograph showing the appearance of a cross-section of a fin, manufactured with a material provided by this invention, after 1,008 hours of testing in a saline mist (1,000X). By examination through an optical microscope the Cu nucleus and the Cu-Sn alloy film of approximately 1 to 2 µm over the whole surface of the fin can be seen.
Figure 11 is a photograph showing the appearance of the tube-fin joint after 1,008 hours of testing in a saline mist (100X). It can be seen that there is a slight attack on the Sn-Pb solder, but a considerable meniscus is maintained.
Figure 12 is a photograph showing an enlarged detail of Figure 16 (200X).
Figure 13 is a photograph showing the appearance of a core which contains fins manufactured with a material provided by this invention and soldered in a static oven, in a vacuum, under N₂ pressure and without the use of any kind of flux.

DETAILED DESCRIPTION OF THE INVENTION

Identification of the mechanisms of external corrosion

In order to identify possible mechanisms of external corrosion which affect copper radiators, perforating corrosion in core tubes, corrosion of solders and corrosion of fins have been studied, as well as the electro-chemical aspects involved.
To study perforating corrosion in core tubes, tests were made on copper radiators which had suffered leaks in core tubes after carrying out tests of accelerated corrosion, which has enabled the mechanisms of the development of this type of corrosion to be established.
At the tube-solder-fin joint the aggressive action of the chemicals used in the accelerated corrosion tests (basically sodium chloride in the continuous saline mist test in accordance with standard NFX 41002), there develops a process of galvanic corrosion sustained by the various electro-chemical potentials of the metals which are joined by means of Sn solders, which produce corrosion pits in the brass tubes and end up affecting the whole thickness of the tube (Figures 1, 2, 3A, 3B, 4A and 4B). This perforating corrosion is produced after 100 hours of testing in a saline mist, which is clearly insufficient to satisfy the requirements of external protection imposed by automobile manufacturers and necessitates the technical development of plans to solve this important problem presented by copper radiators.
Solders also suffer galvanic corrosion after the accelerated external corrosion tests (Figure 3A). At the tube-solder-fin joint, the Sn solder and the brass tube are attacked preferentially because of their anodic character as opposed to the copper fins.
Finally, outside the tube-fin joint area, corrosion is also produced in the copper fins, because of the action of the sodium chloride deposited in a saline mist test. (Figures 5A and 5B).
The situation with regard to the redox potentials of the core components, can be summarised as follows:

Anodic end:
- Sn-Pb (residual solder coating)
- Cu-Zn (core tubes)
- Cu-Sn (diffusion coating on the tube)
Cathodic end:
- Cu (fins)

The differences in potential between the Sn-Pb solders and the Cu (fins) reach values of 300mV so that high-intensity galvanic batteries are formed which cause the corrosion detected in the tests.

Description of the Invention

The procedure for protection against external corrosion in heat exchangers based on copper, especially in radiators for cooling heat engines, motors, and more particularly, automobile radiators, consists of the coating and thermal diffusion of a tin-based alloy on a copper-based nucleus, such that a composite material is formed which has an excellent performance against corrosion.
The procedure of this invention eliminates the corrosion mechanism which produces perforated points in core tubes (brass) manufactured in accordance with state of the art procedures and corrosion in the solder and on the fins is significantly reduced. All these corrosion processes involve a great reduction in the operating characteristics of heat exchangers and especially in automobile radiators since, whilst perforating corrosion in the core tubes makes the radiator useless, corrosion on the fins and in the solder causes significant losses in the thermal and mechanical characteristics of the radiator.
In order to resolve the problem raised, the invention provides a procedure for the protection of heat exchangers which consists of the formation of a new composite material, suitable for the manufacture of fins for heat exchangers, consisting of a Cu-based nucleus, and external surface which consists of an Sn-based alloy and an intermediate layer consisting of Cu-Sn alloys of variable composition, obtained by the coating and thermal diffusion, under controlled conditions, of an Sn-based alloy deposited on a Cu-based nucleus.
Consequently, this invention provides a procedure for protection against external corrosion in heat exchangers based on copper characterized by

a) a coating being applied with an Sn-based alloy onto a Cu-based nucleus; and
b) the Cu-based nucleus coated with an Sn-based alloy being subjected to suitable heat treatment, under controlled conditions, for the purpose of inducing thermal diffusion of the said alloy into the said Cu-based nucleus in the event that the said thermal diffusion has not taken place simultaneously with the application of the coating, so that a composite material is formed consisting of a Cu-based nucleus, an external surface consisting of an Sn-based alloy and an intermediate layer consisting of Cu-Sn alloys, of variable composition, which ensure the total adhesion and continuity of the various layers of the composite material.

The term "Cu-based nucleus" refers to a material constituted principally and basically of Cu which, optionally, may be weakly alloyed with one or more metals, selected from the group consisting of, for example, Te, Mg, Zn, Sn, Cd, Cr, Ag, Pb, In, Be, Zr, Fe, P, Al and Ni, which in their entirety may be present in a concentration less than 0.2% by weight. These elements added to the Cu have the purpose of increasing the thermal resistance of the Cu so that its mechanical properties are maintained after the thermal cycles of soldering of the core. At the same time, these elements enable the highest possible values of thermal conductivity to be assured in order to obtain the most appropriate radiator performance. A Cu-based nucleus, capable of being coated with an Sn-based alloy, may be in the form of Cu strips, Cu fins for heat exchangers and cores for heat exchangers consisting of brass tubes and Cu fins.
The term "Sn based alloys" includes pure Sn and any alloy of Sn with other metals. In particular, the preferred Sn-based alloys used to prepare the composite material of this invention, consist of:

a) Sn-Pb binary alloys, in any proportion, preferably in a proportion of between 1% and 99% of Sn and from 99% to 1% of Pb;
b) Sn-Pb alloys with the addition of other elements, such as Sb, Ag, Cu, Zn, Bi, Cd, In, Ni, Pb, in the following proportions:
- Sn: from 0.5 to 99% Sb: from 0.01 to 7%
- Ag: from 0.01 to 5% Cu: from 0.01 to 2%
- Zn: from 0.01 to 1% Bi: from 0.01 to 2%
- Cd: from 0.01 to 5% In: from 0.01 to 5%
- Ni: from 0.01 to 1% Pb: from 0.5 to 99%
c) Sn-Sb alloys, in proportions of between 93% and 99.5% of Sn and 7% and 0.5% of Sb;
d) Sn-Ag alloys, in proportions of between 95% and 99% of Sn and 5% and 1% of Ag;
e) Sn-Zn alloys, in proportions of between 97% and 99% of Sn and 3% and 1% of Zn; and
f) Pure Sn, with a 99% minimum percentage of Sn.

These Sn-based alloys present an anodic electro-chemical potential with respect to the Cu-based nucleus, which permits effective protection of the Cu against attack caused by natural or artificial environments which contain, among other compounds, inorganic chlorides, nitrogenous compounds and sulphur oxides, thereby significantly increasing the life of the radiator fin and by achieving satisfactory preservation of the mechanical and thermal properties of the fins it allows the radiator thicknesses and weights to be reduced.
In order to obtain effective protection against external corrosion, the composite material formed should have a minimum thickness of 1 micron (µm). In this description, this thickness is considered to be determined by the sum of the thickness of the intermediate layer, consisting of the Cu-Sn alloys of variable compositions, and the thickness of the external surface, consisting of the Sn-based alloy. In view of the fact that the protection against corrosion increases with the thickness of the layer, it can be adjusted to that thickness which is most suitable in order to obtain a determined level of protection. In general, the thickness of the intermediate layer and the outermost surface should be between 1 µm and 1/5 of the total thickness of the composite material, including the thickness of the Cu-based nucleus. In practice, it has been noted that a total thickness of between 2 and 4 µm gives good results.
The manufacture of the composite material consists of the coating of the Cu-based nucleus with an Sn-based alloy and the thermal diffusion of the said alloy deposited on the Cu-based nucleus.
The depositing or application of the said Sn-based alloy on the Cu-based nucleus may be carried out by various procedures. In the first alternative, the coating of the nucleus may be carried out by immersing the said Cu-based nucleus continuously in a bath of molten Sn-based alloy, the coating layer being controlled by a stream of air, inert gas, water or lamination. In the latter case, the thermal diffusion of the alloy is produced simultaneously with the coating of the nucleus.
In another alternative, the said coating may be carried out by projecting the molten Sn-based alloy, by wave or by cascade, onto the Cu-based nucleus. In this case, the thermal diffusion of the alloy is also produced simultaneously with the coating of the nucleus with the said alloy.
In another alternative procedure, the coating may be carried out by depositing a metallic powder which consists of pore Sn or an Sn-based alloy, or of pastes which contain a metallic powder consisting of pure Sn or an Sn-based alloy, on the Cu-based nucleus, followed by thermal treatment at a temperature equal to or greater than 300°C so that thermal diffusion of the alloy on the Cu-based nucleus takes place.
In addition, the coating of the Cu-based nucleus with an Sn-based alloy may be carried out by electrodeposition of pore Sn or Sn-based alloys onto the Cu-based nucleus, followed by thermal diffusion at a temperature equal to or greater than 300°C.
The coating with the Sn-based alloy can be applied onto the Cu-based nucleus in the form of a strip with a thickness suitable for the manufacture of fins for heat exchangers, or alternatively, it can have a thickness greater than that necessary for the manufacture of such fins, in which case, a stage of lamination of the composite material can be carried out, once it has been formed, until a thickness suitable for the manufacture of fins is obtained.
In addition, the Sn-based alloy can be applied onto the fins constructed from strips of uncoated Cu and also onto a conventional core constructed of brass tubes and Cu fins.
The coating of the Cu-based nucleus with the Sn-based alloy may be total or partial. In the latter case, the partial coating may preferably be carried out by electrodeposition of pure Sn or of the Sn-based alloy, or by projecting the molten Sn-based alloy, or by deposition either of a metallic powder which contains pure Sn or an Sn-based alloy, or of pastes which contain a metallic powder consisting of pure Sn or the Sn-based alloy, onto the area of the Cu-based nucleus to be coated, followed by thermal diffusion at a temperature equal to or greater than 300°C.
When the Sn-based alloy is applied onto the core, the coating may be applied either onto the whole core or onto its external surfaces. In view of the fact that corrosion on the external surfaces of the core is more important, the coating may be applied in such a way that it only affects the external areas of the core and not its central nucleus. In an application peculiar to this invention, the composite material may not be present over the whole width of the fins but may cover only the front and rear surfaces of the core to a depth of up to 1/3 of the width of the core. In this case, the partial coating may preferably be applied to the surfaces to be coated by electrodeposition of pure Sn or of an Sn-based alloy, by projecting either molten Sn or molten Sn-based alloy, or by depositing either a metallic powder containing pure Sn or an Sn-based alloy, or of pastes which contain a metallic powder consisting of pure Sn or an Sn-based alloy, onto the core surface to be coated, followed by thermal diffusion at a temperature equal to or greater than 300°C.
Besides protection against corrosion of the actual fins, a large increase in resistance to corrosion of the whole core assembly is obtained. With the new material, a balance is obtained in the electro-chemical potentials of the metals which make up the core, thus avoiding the galvanic mechanism which produces points of perforating corrosion in the tubes and a rapid deterioration of the soldering meniscuses.
Using the procedure provided by this invention avoids the mechanism which affects the operation of the radiator in the short term, and instead produces a slow attack on the Sn alloys which are characteristically anodic as opposed to the materials making up the tubes and fins. The results obtained in carrying out comparative tests of accelerated corrosion [Continuous Saline Mist, Standard NFX 41002] on conventional Cu radiators and those to which the protective treatment proposed by this invention has been applied or which incorporate fins manufactured from the said composite material, reveal that perforating corrosion is produced after a time period some 10 times greater in Cu radiators treated with the procedure of the invention than in conventional Cu radiators, i.e. uncoated ones (see Examples 1 and 2).
At the same time, it is very interesting to observe how the mechanical strength and the thermal efficiency of radiators treated with the protection procedure proposed by this invention, is maintained after the accelerated corrosion tests, which indicates the limited attack produced on the tubes, fins and soldering meniscuses. Tests carried out have revealed that, after 1,000 hours of a saline mist test, the thermal efficiency of the treated radiators is reduced by only 10%.
All of this ensures the operation of radiators, treated with this protective procedure, in service in aggressive environmental conditions, over long periods.
With regard to the manufacture of heat exchangers which incorporate parts manufactured from the composite material provided by this invention, and specifically, in the soldering of the core operation, the said composite material allows the use of both an oven with an oxidizing atmosphere and an oven with a controlled non-oxidizing atmosphere.
When the said soldering is carried out in an oven with an oxidizing atmosphere, conditions for soldering the core, provided by the composite fin material, are much more favourable, since the said composite material, thanks to its Cu-Sn alloy based intermediate layer and its Sn-based external surface, presents a much improved solderability compared to the normally used Cu-based material. This allows the use of soldering flux materials consisting of organic acids, amines and resins, with no inorganic components. Conditions for the soldering process are therefore much milder and furthermore the operations of washing and drying of the cores are eliminated.
In addition, soldering of the core can be carried out in ovens with a controlled non-oxidizing atmosphere, so that the soldering operation is carried out without any kind of flux, which enormously simplifies the heat exchanger manufacturing process since the installations for fluxing, washing and drying are eliminated. By the use of this technology significant energy savings are obtained and gaseous and aqueous effluents are eliminated, so it presents a very favourable impact on the environment.
At the same time, this technology also takes into account the possibility of carrying out the soldering of the ends of the tubes to the header plates in the same operation as that of soldering the tubes to the fins. To do this, it is only necessary to apply locally an Sn-based solder, without flux or including a very weak, non-corrosive organic flux, which produces no environmental problem. This is possible due to the favourable conditions of the controlled non-oxidizing atmosphere of the oven. Among the non-corrosive organic fluxes which could be mentioned are alcoholic rosin solutions, de-activated or activated with organic acids or amines, for example rosin:isopropanol (10:90), rosin: glutamic acid:isopropanol (10:2:88) or rosin:dibutylamine hydrochlorate:dimethylamine hydrochlorate:isopropanol (10:2:4:84).
With all the above, in comparison with current heat exchanger manufacturing procedures which require the core to be soldered in two stages, great simplification in manufacture and a reduction of energy costs is achieved, as well as a very significant improvement in productivity.
In consequence, an additional aim of this invention, consists of a procedure for the manufacture of heat exchangers based on copper, especially radiators for cooling heat engines, and more especially automobile radiators, in which the fins are composed of, or are totally or partially coated with, the composite material provided by this invention, which includes a stage of soldering the tubes to the said fins, which can be carried out:

a) in a continuous or static oven with an oxidizing atmosphere, by using an organic non-corrosive soldering flux which does not require washing, consisting of organic acids, amines and resins, with no inorganic components; or alternatively,
b) in an oven with a controlled non-oxidizing atmosphere, without the incorporation of any type of flux. In this case, such ovens may be continuous or static vacuum ovens, or continuous or static inert atmosphere ovens, in the presence of inert gases such as N₂, CO₂,and other inert gases and the absence of O₂ and H₂O.

Another additional aim of this invention consists of fins for heat exchangers, especially suitable for use in the manufacture of radiators for cooling heat engines, such as radiators for automobiles, essentially consisting of the composite material provided by this invention. Such fins can be manufactured by means of coating and thermal diffusion of an Sn-based alloy onto a Cu-based nucleus in the form of a strip with a thickness appropriate for the manufacture of the fins, or alternatively, it can have a thickness greater than that necessary for the manufacture of such fins, in which case, a stage for the lamination of the composite material can be carried out, once it has been formed, until a thickness appropriate for the manufacture of fins is obtained.
In a particular application, such fins are manufactured starting with a Cu-based nucleus, in the form of a fin and with the appropriate thickness, onto which is applied a coating of an Sn-based alloy over the whole surface of the fin or onto the external edges of such a fin, by means of the application of preformed sheets, threads or cords of Sn alloys, or by means of the electrodeposition of pure Sn or an Sn-based alloy, or by means of the projection of a molten Sn-based alloy, or by the deposition of either a metallic powder which contains pure Sn or an Sn-base alloy, or pastes which contain metallic powder which includes either pure Sn or an Sn-based alloy, followed by thermal diffusion at a temperature equal to or greater than 300°C.
Alternatively, the coating of the said fin, consisting of a Cu-based nucleus, may be carried out by immersion of the Cu-based nucleus or the surface strip to be coated in a bath of a molten Sn-based alloy, by projecting pure molten Sn or an Sn-based alloy, by wave or by cascade, with thermal diffusion simultaneous with the coating.
A further additional aim of this invention consists of a beat exchanger based on copper, such as a radiator intended for cooling heat engines, more especially a radiator for automobiles, which contains some fins totally or partially manufactured from the composite material obtained by the procedure of this invention.
Finally, the invention also provides a procedure for depositing Sn-based alloys onto the core of a heat exchanger based on copper, manufactured with state of the art technology, i.e. made of brass tubes and copper fins, which is characterized by the application of the said alloy onto the core in such a way that it covers principally the outer edges of the fins, by electrodeposition, applied to both faces of the core, of either pure Sn or an Sn-based alloy, or of pastes which contain a metallic powder consisting of pure Sn or an Sn-based alloy, followed by thermal diffusion at a temperature equal to or greater than 300°C.
The following examples are used to illustrate the invention, and should not be considered as limiting its scope.

EXAMPLE 1

Strips of Cu-Sn (0.1% Sn) 0.042 mm thick were coated continuously with an alloy of Sn-Pb (15% Sn + 85% Pb), in a molten bath with a coating thickness of 2 µm/face. Fins for automobile radiators were manufactured with the material obtained and radiators were assembled including such fins. The soldering of the core was carried out in a continuous oven with an oxidizing atmosphere, using an organic flux. Thermal diffusion was achieved by coating the strip with a molten Sn alloy.
A continuous saline mist test was carried out, in accordance with Standard NFX 41002, with 5% NaCl at 35°C. After 1,170 hours, the radiator presented no perforating corrosion of any kind, either on the tubes or on the fins, only a slight superficial attack being noted on the coating of the fins (Figures 6 and 7), while on the copper radiators manufactured with current technology, i.e. without the protective treatment proposed by this invention, on carrying out the same saline mist test, perforating corrosion were noted on tubes and fins between 100 and 200 hours of the test period.

EXAMPLE 2

Strips of Cu-Cd (0.2% Cd) 0.04 mm thick were coated continuously, with an Sn-Pb molten alloy (25% Sn + 75% Pb), by immersion in a molten bath, and with a coating of 4 µm/face. Fins for automobile radiators were manufactured with the material obtained and radiators were assembled including these fins. The soldering of the core was carried out in a continuous oven, with an oxidizing atmosphere, using an organic flux. Thermal diffusion was achieved by coating the strip in the molten Sn alloy.
A continuous saline mist test was carried out [Standard NFX 41002], with 5% NaCl at 35°C, for 1,008 hours. After this time only a slight attack on the coating of the fins was observed. The radiator maintained its mechanical strength perfectly and no point of perforating corrosion was produced either on the fin or on the tubes. Only the meniscuses of the Sn-based solder were weakly attacked (Figures 8, 9, 10, 11 and 12).
Figures 8 to 12, in comparison with Figures 1 to 5 which correspond to radiators constructed according to techniques appertaining to the previous state of the art, submitted to a saline mist test in short duration tests (less than 244 hours) and on which very marked corrosion was produced, demonstrate the greater resistance to external corrosion of copper radiators treated with the protective procedure of the present invention.

EXAMPLE 3

Fins were manufactured from a strip of Cu coated by an Sn-Pb alloy (60/40), by electrolysis in an aqueous phase. After electrodeposition, thermic diffusion treatment was carried out at 300°C for 30 seconds. Subsequently, radiators were constructed which included the fins described above. The core was soldered in a static vacuum oven. The radiators were submitted to a continuous acetic saline mist test, containing CuCl₂ - CASS TEST - in accordance with Standard ASTM B 368. When the radiator was examined, no significant corrosion was observed on the tubes, the fins or the solder meniscuses.

EXAMPLE 4

A radiator core was manufactured containing fins manufactured using a composite material provided by this invention having a film of Sn-Cu alloys and Sn with a total thickness of 3 µm. The soldering was carried out in a static oven, in a vacuum and under a nitrogen pressure of 40 mbars. No kind of flux was used.
When the soldering of the core was examined, it was observed that the said core was perfectly soldered and had a clean, bright appearance and was totally free of oxides (Figure 13).

Claims

A procedure for protection against external corrosion of copper-based heat exchangers which consists of coating and thermal diffusion of an Sn-based alloy onto a Cu-based nucleus, such that a composite material is formed which consists of a Cu-based nucleus, an external surface consisting of an Sn-based alloy and an intermediate coating consisting of Cu-Sn alloys of variable compositions, characterized by such Sn-based alloys including:
a) binary Sn-Pb alloys, in any proportion, preferably in a proportion of between 1% and 99% of Sn and 99% and 1% of Pb;

b) Sn-Pb alloys with the addition of other elements, such as Sb, Ag, Cu, Zn, Bi, Cd, In, Ni, Pb in the following proportions:
Sn: from 0.5 to 99% Sb: from 0.01 to 7%

Ag: from 0.01 to 5% Cu: from 0.01 to 2%

Zn: from 0.01 to 1% Bi: from 0.01 to 2%

Cd: from 0.01 to 5% In: from 0.01 to 5%

Ni: from 0.01 to 1% Pb: from 0.5 to 99%

c) Sn-Sb alloys in proportions of between 93% and 99.5% of Sn and 7% and 0.5% of Sb;

d) Sn-Ag alloys in proportions of between 95% and 99% of Sn and 5% and 1% of Ag;

e) Sn-Zn alloys in proportions of between 97% and 99% of Sn and 3% and 1% of Zn;

f) Pure Sn, with a 99% minimum percentage of Sn.
A procedure according to claim 1, characterized by the composite material formed having a minimum thickness, considered as the sum of the thickness of the intermediate layer and the thickness of the external surface, of 1 micron (µm).
A procedure according to claim 2, characterized by the thickness of the composite material formed being between 1 µm and 1/5 of the total thickness of the composite material, including the thickness of the Cu-based nucleus.
A procedure according to claim 2, characterized by the thickness of the composite material formed being between 2 and 4 µm.
A procedure according to claim 1, characterized by the depositing or application of the said Sn-based alloy on the Cu-based nucleus being carried out by the immersion of the said nucleus in a bath of a molten Sn alloy.
A procedure according to claim 5, characterized by the coating layer of the said Sn-based alloy being controlled by means of a jet of air, an inert gas, water or lamination.
A procedure according to claim 5, characterized by the thermal diffusion of the Sn-based alloy being produced simultaneously with the coating of the Cu-based nucleus with the said alloy.
A procedure according to claim 5, characterized by the coating of the Cu-based nucleus with the Sn-based alloy being carried out by projecting a molten Sn-based alloy, by a wave or cascade, onto the Cu-based nucleus.
A procedure according to claim 3, characterized by the thermal diffusion of the Sn-based alloy being produced simultaneously with the coating of the Cu-based nucleus with the Sn-based alloy.
A procedure according to claim 5, characterized by the coating of the Cu-based nucleus with the Sn-based alloy being achieved by the depositing of either a metallic powder which contains pure Sn or an Sn-based alloy, or of pastes containing a metallic powder which consists of pure Sn or an Sn-based alloy, onto a Cu-based nucleus.
A procedure according to claim 10, characterized by the thermal diffusion of the Sn-based alloy on the Cu-based nucleus being achieved by heating to a temperature equal to or higher than 300 °C.
A procedure according to claim 5, characterized by the coating of the Cu-based nucleus with the Sn-based alloy being achieved by the electrodeposition of either pure Sn or Sn-based alloys onto the Cu-based nucleus.
A procedure according to claim 12, characterized by the thermal diffusion of the Sn-based alloy on the Cu-based nucleus being achieved by heating to a temperature equal to or higher than 300°C.
A procedure according to claim 1, characterized by the Cu-based nucleus to be coated being a copper fin for a heat exchanger.
A procedure according to claim 14, characterized by the coating with the Sn-based alloy applied to the said copper fin being partial.
A procedure according to claim 15, characterized by the partial coating and thermal diffusion of the Sn-based alloy being achieved by the electrodeposition of pore Sn or the Sn-based alloy, or by the projection of the molten Sn-based alloy, or by the depositing of either a metallic powder containing pore Sn or the Sn- based alloy, or of pastes containing a metallic powder which contains pure Sn or the Sn-based alloy, onto the area of the Cu-based nucleus to be coated, followed by thermal diffusion at a temperature equal to or higher than 300°C
A procedure according to claim 1, characterized by the Cu-based nucleus to be coated being a core constructed with brass tubes and copper fins, suitable for heat exchangers.
A procedure according to claim 17, characterized by the coating with the Sn-based alloy applied to the said core being partial and being applied to the front and rear surfaces of the core, to a depth of up to 1/3 of the width of the core.
A procedure according to claim 18, characterized by the partial coating of the core with Sn-based alloy and its thermal diffusion being effected locally on the surfaces of the core to be coated, by means of an electrodeposition either of pore Sn or the Sn-based alloy, or by means of projecting either pure Sn or the molten Sn-alloy or by the depositing of either a metallic powder which contains pure Sn or the Sn-based alloy, or of pastes containing a metallic powder which consists of pure Sn or the Sn-based alloy, followed by thermal diffusion at a temperature equal to or higher than 300°C.
A procedure for the manufacture of copper-based heat exchangers, which includes a stage of soldering the tubes to the fins, characterized by the fins being composed, totally or partially, of a composite material consisting of a Cu-based nucleus, an outer surface consisting of an Sn-based alloy and an intermediate layer consisting of Cu-Sn alloys of variable composition, and the said soldering being carried out in an oven with an oxidizing atmosphere or a controlled non-oxidizing atmosphere.
A procedure according to claim 20, characterized by the soldering being carried out in a continuous or static oven with an oxidizing atmosphere.
A procedure according to claim 21, characterized by the soldering being carried out incorporating an organic non-corrosive soldering flux which does not require washing.
A procedure according to claim 22, characterized by the said soldering flux being composed of organic acids, amines and resins, without inorganic components.
A procedure according to claim 20, characterized by the soldering being carried out in an oven with a controlled non-oxidizing atmosphere without incorporating flux of any kind.
A procedure according to claim 24, characterized by the soldering being carried out in a continuous or static vacuum oven.
A procedure according to claim 24, characterized by the soldering being carried out in a continuous or static oven with an inert atmosphere in the presence of inert gases and in the absence of O₂ and H₂O.
A procedure according to any of the claims 20 to 26, characterized by the said manufactured heat exchanger being a radiator intended for cooling heat engines, in particular radiators for automobiles.
Composite material characterized by the said Sn-based alloys consisting of:
a) binary Sn-Pb alloys in any proportion, preferably in a proportion of between 1% and 99% of Sn and 99% and 1% of Pb;

b) Sn-Pb alloys with the addition of other elements, such as Sb, Ag, Cu, Zn, Bi, Cd, In, Ni, Pb in the following proportions:
Sn: from 0.5 to 99% Sb: from 0.01 to 7%

Ag: from 0.01 to 5% Cu: from 0.01 to 2%

Zn: from 0.01 to 1% Bi: from 0.01 to 2%

Cd: from 0.01 to 5% In: from 0.01 to 5%

Ni: from 0.01 to 1% Pb: from 0.5 to 99%

c) Sn-Sb alloys in proportions of between 93% and 99.5% for Sn and 7% and 0.5% for Sb;

d) Sn-Ag alloys in proportions of between 95% and 99% for Sn and 5% and 1% for Ag;

e) Sn-Zn alloys in proportions of between 97% and 99% for Sn and 3% and 1% for Zn;

f) Pure Sn, with a 99% minimum percentage of Sn.
Material according to claim 28 characterized by having a minimum thickness, considered as the sum of the thickness of the intermediate layer and the thickness of the external surface, of 1 micron (µm).
Material according to claim 29, characterized by having a thickness between 1 µm and 1/5 of the total thickness of the composite material, including the thickness of the Cu-based nucleus.
Material according to claim 29, characterized by having a thickness between 2 and 4 µm.
Material according to claim 28, characterized by the depositing or application of the said Sn-based alloy onto the Cu-based nucleus being carried out by the immersion of the said nucleus in a bath of a molten Sn-based alloy, the coated layer being controlled by means of a jet of air, an inert gas, water or lamination, and the thermal diffusion of an Sn-based alloy being produced simultaneously with the coating of the Cu-based nucleus with the said alloy.
Material according to claim 28, characterized by the depositing or application of the said Sn-based alloy onto the Cu-based nucleus being achieved by projecting the molten Sn-based by waves or by cascade, onto the Cu-based nucleus and the thermal diffusion of the Sn-based alloy being produced simultaneously with the coating of the Cu-based nucleus with the said Sn-based alloy.
Material according to claim 28, characterized by the depositing or application of the said Sn-based alloy onto the Cu-based nucleus being achieved by depositing of either a metallic powder which contains pure Sn or an Sn-based alloy or pastes which contain a metallic powder which consists of pore Sn or an Sn-based alloy onto a Cu-based nucleus and the thermal diffusion of the Sn-based alloy on the Cu-based nucleus being achieved by heating to a temperature equal to or higher than 300°C.
Material according to claim 28, characterized by the depositing or application of the said Sn-based alloy onto the Cu-based nucleus being achieved by the electrodeposition of either pure Sn or Sn-based alloys onto a Cu-based nucleus and the thermal diffusion of the Sn-based alloy on the Cu-based nucleus being achieved by heating to a temperature equal to or higher than 300°C.
Material according to claim 28, characterized by the Cu-based nucleus being in the form of a strip, with a thickness equal to or greater than that required for the manufacture of fins for heat exchangers.
Material according to claim 28, characterized by the coating of the Cu-based nucleus with the Sn-based alloy being total.
Material according to claim 28, characterized by the coating of the Cu-based nucleus with the Sn-based alloy being partial.
Material according to claim 38, characterized by the partial coating of the Cu-based nucleus with the Sn-based alloy and its thermal diffusion being achieved locally by the electrodeposition of either pore Sn or an Sn alloy, or by the projecting of the molten Sn-based alloy or the depositing either of a metallic powder which contains the Sn-based alloy, or of pastes which contain a metallic powder which consists of Sn-based alloy, on the area of the Cu-based nucleus to be coated, followed by thermal diffusion at a temperature equal to or higher than 300°C.
Material according to claim 28, characterized by it being suitable for the manufacture of fins for heat exchangers, especially for radiators intended for cooling heat engines, in particular radiators for automobiles.
A fin for a heat exchanger based on copper, characterized by being consisting of a composite material according to one of claims 28 to 40.
A fin according to claim 41, characterized by being obtainable by coating a Cu-based nucleus by means of the application of laminas, threads or preformed cords of Sn alloys, or by means of the electrodeposition of pore Sn or the alloy of Sn, or by the projection of a molten alloy based on Sn, or the depositing of a metallic powder containing pore Sn or an Sn-based alloy, or pastes containing a metallic powder which contains pore Sn or an Sn-based alloy, onto the Cu-based nucleus or the superficial strip to be coated, followed by thermal diffusion at a temperature equal to or higher than 300°C.
A fin according to claim 41, characterized by the coating of the Cu-based nucleus being carried out by means of the immersion of the Cu-based nucleus or the surface border to be coated in a bath of molten Sn-based alloy, wave or cascade, with thermal diffusion simultaneous with the coating.
A fin according to either claim 42 or claim 43, characterized by the Cu-based nucleus being in the shape of a strip, with a thickness equal to or greater than that required for the manufacture of fins.
A fin according to claim 41, characterized by being suitable for use in the manufacture of radiators intended for cooling heat engines, in particular radiators for automobiles.
A procedure for depositing Sn-based alloys onto the core of a heat exchanger based on copper, consisting of brass tubes and copper fins, characterized by the said alloy being applied on the core so that it coats mainly the external borders of the fins, by the electrodeposition applied to both faces of the core, of either pure Sn or Sn-based alloys, or by projecting either a metallic powder which contains pore Sn or an Sn-based alloy, or pastes which contain a metallic powder which consists of pure Sn or an Sn-based alloy, followed by thermal diffusion at a temperature equal to or higher than 300°C.