EP0222724A1 - Process and apparatus for producing an extra-thin metal foil by electroplating - Google Patents

Abstract

An anode arranged very close to a cathode in an electrolytic circuit is constituted by a plate formed with first and second sets of orifices, both distributed over the surface of the plate. The first set is connected to electrolyte supply means and the second set to electrolyte discharge means with the aim of producing high turbulence in the narrow space between the anode and cathode.

Description

The present invention relates to the electrolytic deposition of a metallic material on a substrate.

The technique of electroplating has long been used to form both adherent coatings and thin non-adherent coatings, which can then be separated from their substrate in the form of an extra thin sheet.

In this technique, it is known that the deposition rate of the metallic material depends, among other parameters, on the current density used, and that the practical realization of this current density is itself linked to the "turbulence" of the electrolyte.

On the other hand, it is known that the cost of an electrolysis operation depends in particular on the potential difference existing between the electrodes, and that this can be all the less the smaller the distance between the electrodes .

The economic implementation of such an electrolysis operation therefore implies that the electrolyte is circulated at high speed between two electrodes as close together as possible.

This problem has already received various solutions, which essentially consist in sending the electrolyte either tangentially or perpendicular to the surfaces of the electrodes present. These solutions are however only applicable for electrodes of small extent. Indeed, when the surfaces present are large, as is the case for example during the coating of a sheet or a steel strip of large width or during the manufacture by electroforming of thin sheets of large width , the pressure drops are enormous due to the small flow section and the large distance traveled by the electrolyte; it is then necessary to use very powerful pumps which apply very high pressures to ensure the circulation of the electrolyte. These pressures in turn develop significant forces on the electrodes which may be deformed, which causes an uncontrollable variation in their spacing and thwarts the regularity of the electrolysis.

The present invention provides a device for ensuring, economically, a high turbulence of an electrolyte between two very close electrodes, without using excessive driving pressures.

The device for electrolytic deposition on a substrate, which is the subject of the present invention, intended to ensure a high turbulence of the electrolyte between an anode and a very close cathode, the cathode generally being constituted by the substrate, is essentially characterized in that it comprises a plate pierced with a pre first series of orifices distributed over the surface of the plate and connected to electrolyte supply means, as well as a second series of orifices also distributed over the surface of the plate and adjacent to the orifices of said first series, the orifices of said second series being connected to means for discharging the electrolyte, and in that said plate constitutes the anode of the electrolysis circuit.

• (a) au moins un orifice, dit orifice d'alimentation, traversant une paroi du caisson et donnant accès audit volume intérieur;
• (b) des tubes raccordés aux orifices de ladite seconde série, tra­versant ledit volume intérieur sans communiquer avec lui et dé­bouchant à l'extérieur dudit caisson.

According to a particular embodiment of this device, said plate forms, with a plurality of other walls, a box which delimits a closed interior volume and which comprises:

• (A) at least one orifice, said supply orifice, passing through a wall of the box and giving access to said interior volume;
• (b) tubes connected to the orifices of said second series, passing through said interior volume without communicating with it and opening out to the exterior of said box.

According to another embodiment of the device of the invention, said tubes connected to the orifices of the second series are, at their other end, connected to suction means, for example to a pump via a collector .

The operating principle of this device is as follows: the box, connected to the positive pole of a direct current source, is arranged in such a way that its wall provided with the two series of orifices is located near the surface of the substrate, which is connected to the negative pole of the same DC source. The electrolyte is introduced, under moderate pressure, into the interior volume of the box through the supply orifice. Under the effect of the supply pressure, the electrolyte leaves the box through the orifices of the first series, and it circulates in the narrow space existing between the wall of the box and the substrate, therefore between the anode and the cathode. After a short trip in this space, the electrolyte is taken up by the orifices of the second series, for example by suction, and leads, by the tubes connected to these orifices, to a dis sufficient substrate strength. It can then be introduced again into the box, possibly after regeneration, and again run through the circuit which has just been described.

• la figure 1 représente schématiquement un caisson destiné au revête­ment d'une surface plane, telle qu'une face d'une bande;
• la figure 2 montre un caisson adapté à la formation d'une feuille ex­tra mince par dépôt non adhérent sur un cylindre rotatif;
• la figure 3 présente la variation du débit spécifique de l'électrolyte en fonction de sa pression entre les électrodes, pour di­verses valeurs de la distance anode-cathode;
• la figure 4 exprime l'influence de la distance anode-cathode sur la pression de l'électrolyte entre les électrodes, pour un débit spécifique constant de l'électrolyte.

In order to clearly understand the principle of the device of the invention as well as its operation, we will now give a detailed description with reference to the accompanying drawings, in which:

• FIG. 1 schematically represents a box intended for coating a flat surface, such as one face of a strip;
• FIG. 2 shows a box suitable for the formation of an extra thin sheet by non-adherent deposition on a rotary cylinder;
• FIG. 3 shows the variation of the specific flow rate of the electrolyte as a function of its pressure between the electrodes, for various values of the anode-cathode distance;
• FIG. 4 expresses the influence of the anode-cathode distance on the pressure of the electrolyte between the electrodes, for a constant specific flow rate of the electrolyte.

In these figures, the same elements are each designated by the same reference numerals.

Referring first to Figure 1, the device of the invention comprises a box 1, one of the walls of which is arranged in parallel and at a very short distance from the surface of a metal strip 2 in movement. In a side wall, the box 1 is provided with an orifice 3 to which is connected a pipe 4 for supplying electrolyte from a source not shown.

In the wall of the box 1 facing the strip 2 is provided a first series of orifices 5 making the interior volume of the box 1 communicate with the narrow space existing between the box 1 and the strip 2. In this same wall is provided a second series of orifices 6 to which tubes 7 are connected which pass through the interior volume of the box 1 and which exit therefrom passing in leaktight manner through another wall of the box 1. In the embodiment illustrated by the In FIG. 1, these tubes open out into a manifold 8 which can be connected to a pump 9.

To form an electrolytic deposit on the strip 2, the latter is connected to the negative pole of a direct current source, or possibly to the ground, while the box 1 is connected to the positive pole of this same direct current source. The box therefore constitutes the anode and the strip constitutes the cathode of an electrolysis circuit.

In Figure 1, the electrical connections are shown schematically, because the technology of these connections is well known and is not part of the present invention.

The electrolyte enters the interior volume of the box 1 through the supply orifice 3 connected to the pipe 4. Under the effect of the supply pressure, the electrolyte fills the interior volume of the box, then it flows through the orifices 5 to occupy the narrow space existing between the box 1 and the strip 2. An electric current can thus flow between the anode and the cathode and ensure the desired electrolytic deposition on the strip 2. Thanks to the short distance separating the orifices 5 from the orifices 6, the electrolyte is very quickly taken up, by suction, through the orifices 6 and the tubes 7, to the collector 8 and to the pump 9. After having possibly been regenerated and having received a quantity backup by known means not shown, the electrolyte is then returned, by the action of the pump 9, in the supply line 4 and starts the circuit again.

In a preferred variant of the invention, the suction devices, namely the manifold 8 and the pump 9, are eliminated completely. The box 1 is completely submerged in the tank containing the electrolyte and the tubes 7 open directly into this tank. The electrolyte then flows through the tubes 7 under the effect of the pressure established between the anode and the cathode.

Due to the shortness of the path taken by the electrolyte in the narrow space existing between the box and the substrate, between an orifice 5 and a neighboring orifice 6, the pressure drop opposite to the circulation of the electrolyte is very reduced. . The pressure required to ensure this circulation is therefore lower than in the previously known solutions. In addition, the almost immediate recovery of the electrolyte through the orifices 6 prevents, or at least strongly limits the lateral flow of the latter.

The above example refers more particularly to the coating of flat products, such as strips. It goes without saying, however, that the invention is not limited to this type of product, and that its use also extends to the coating of products of any cross-section, by the use of plates, in particular of box walls matching the contour of the substrate.

The device of the invention also makes it possible to deposit non-adherent coatings, which can be detached from their substrate to obtain very thin sheets.

Figure 2 illustrates this application.

FIG. 2 represents a box 1, one wall of which comprises a semi-cylindrical cavity where the orifices 5 and 6 defined above are formed. The orifices 6 are connected to a manifold 8 by tubes 7. In the semi-cylindrical cavity is disposed a cylinder 10, coaxial with said cavity in which it can rotate. The outside diameter of the cylinder is slightly smaller than that of the cavity, so as to leave a narrow annular gap between them. The box 1 and the cylinder 10 are respectively connected to the positive pole and to the negative pole of a direct current source. The electrolyte is introduced through the supply line 4, arrives through the orifices 5 in the annular slot where it undergoes electrolysis, then is taken up through the orifices 6 and the tubes 7 towards the collector 8. The metal sheet 11 formed on the cylinder is then detached in a manner known per se.

The tests carried out by the Applicant have shown that the installations in FIGS. 1 and 2 have several advantages over the known devices.

The following description is devoted more particularly to the production of extra-thin sheets by the installation of FIG. 2. The effects and the advantages described are however also true in the case of coating by the installation of FIG. 1.

From a hydraulic point of view, tests have confirmed that shortening the hydraulic path of the electrolyte is an excellent way to reduce the pressure between the electrodes.

The device according to the invention made it possible to achieve a specific electrolyte flow rate of 20 l / m².s under a pressure of 1 kg / cm² with an anode-cathode distance equal to 0.1 mm. Such a specific flow ensures high turbulence, which in turn promotes the electrical behavior of the installation.

FIG. 3 illustrates the variation of the specific flow rate of the electrolyte as a function of the pressure, for different values of the anode-cathode distance. It clearly shows that the device of the invention makes it possible to very greatly reduce this distance while ensuring appreciable specific flow rates and without requiring excessive pressures.

This characteristic is illustrated by the diagram in FIG. 4, which shows the influence of the anode-cathode distance on the pressure of the electrolyte ensuring a predetermined specific flow rate.

For a constant specific flow q equal to 46 l / m².s, the device of the invention made it possible to reduce the anode-cathode distance to 0.2 mm, while the pressure only went from 0.4 to 0 , 6 kg / cm².

With regard to the electrical aspect of the formation of an extra-thin sheet, these tests also stressed the importance of the den sity of current (D) and turbulence of the electrolyte during the deposition operation. All the other conditions being constant, these two parameters largely condition the cohesion and the surface quality of the extra-thin sheet obtained. As already indicated in the introduction to the present application, the practical realization of the current density also depends on the speed of circulation of the electrolyte, that is to say ultimately on its specific flow rate. By an appropriate increase in the specific flow rate for a constant anode-cathode distance, the device of the invention has made it possible to produce perfectly healthy extra-thin sheets at current densities well above 100 A / dm².

The device of the invention also proves to be advantageous in this respect, because an increase in the current density leads to the possibility of increasing the speed and therefore the productivity of the deposition lines, for a given thickness of extra-thin sheet at produce.

An additional advantage of the installation according to the invention is that it makes it possible to achieve high levels of turbulence and current density while using only low pressures. As a result, the anode and the substrate are not subjected to significant stresses and therefore do not deform significantly. In addition, the energy consumption of the pump remains low.

The increase in turbulence which the device of the invention allows, has further led to a decrease in the apparent resistivity of the electrolysis cell. Thus, for the same anode-cathode distance of 1 mm, an increase in the electrolyte pressure from 0.5 kg / cm² to 1 kg / cm² caused an increase in the specific flow rate of 53.9 l / m².s at 80 l / m².s and a drop in the apparent resistivity of the electrolysis cell from 2.21 ohm-cm to 1.43 ohm-dm. This resulted in a further reduction in energy consumption during deposition.

In the above description, we have systematically referred to an electrolyte supply provided by the orifices of the pre first series, while the recovery and return of the electrolyte were carried out by the orifices of the second series and by the tubes associated with these orifices. In particular, it has been proposed to use a box to supply this power. It would not, however, depart from the scope of the present invention to supply the orifices of any one of the two series by directly connecting these orifices to individual supply pipes, in the absence of any box; these supply pipes could then themselves be connected, individually or in any groups, to an electrolyte source. The orifices of the other series would then advantageously be provided with return tubes of the electrolyte.

The device of the present invention also makes it possible to vary the width of the area coated with the substrate or the width of the extra-thin sheet, by adapting the length - in the transverse direction of the product - of the box 1 or of the cylinder 10, respectively .This adaptation can be carried out for example by modifying the number of boxes juxtaposed according to the width of the product or by partitioning a long box into several compartments with clean supply and evacuation, or by closing off part of the orifices for passage of the electrolyte.

Finally, it is possible to combine various devices in accordance with the invention to simultaneously coat either the two faces of the same flat product, in particular a strip, possibly with different coatings, or a single face of two flat products, or again to produce several sheets simultaneously from a single electrolytic solution.

In the case of the production of an extra-thin sheet, the operation of forming the sheet can advantageously be followed, in line, by a heat treatment intended to fix the properties. Given the very wide use of this type of product in the packaging industry, one of its essential properties is its ability to fold, which implies a low elastic limit and an absence of spring effect after folding.

To this end, the process which is the subject of the present invention may include a heat treatment operation comprising a first step of reheating the extra-thin sheet beyond its recrystallization temperature, followed by a cooling step. fast to a temperature close to room temperature. By "close to room temperature" is meant a temperature at which the extra-thin sheet no longer undergoes any metallurgical transformation.

In practice, the reheating temperature is higher than approximately 650 ° C., in order to cause the recrystallization of the metal, and thus to improve its ductility by reducing its elastic limit and its breaking load compared to the levels observed immediately after l '' electroforming. The reheating step is preferably carried out by direct resistance heating.

Rapid cooling can be carried out, for example, by immersing the extra-thin sheet in an aqueous quenching bath which may be at a temperature above ambient temperature. Such rapid cooling exerts on the extra-thin sheet a softening effect which promotes the appearance of the ability to fold. The degree of softening thus reached depends of course on the purity of the metal constituting said sheet, and in particular on its contents of carbon and free nitrogen.

For example, an extra-thin sheet (10 µm) of iron containing less than 0.002% carbon and less than 0.0007% nitrogen, was recrystallized by heating and maintaining between 650 ° C and 850 ° C , then cooled to around 5600 ° C / s by immersion in boiling water. It thus acquired an excellent folding ability without spring effect without losing anything of its flatness or its surface appearance.

An extra-thin sheet of iron, produced and heat treated according to the process of the invention, has a folding ability at least equivalent to that of the aluminum sheets currently used.

The device and the method according to the present invention make it possible to manufacture coated products and in particular to produce extra-thin sheets of high quality, with high productivity of the installation and with limited energy consumption.

These excellent results have been obtained thanks to the combination of the two fundamental characteristics of the invention, namely a high turbulence of the electrolyte and a very short hydraulic path between the electrodes. The high turbulence is ensured by the fact that the electrolyte exhibiting a high momentum arrives perpendicular to the surface of the substrate, that is to say the cathode, and that it spreads between the anode and the cathode before being quickly taken up by the evacuation orifices. Under these conditions, practically no laminar flow is established parallel to the electrodes. Furthermore, it is generally considered that the turbulence of the electrolyte depends on its speed of circulation between the electrodes. The arrangement which is the subject of the present invention however makes it possible to achieve high turbulence without requiring significant electrolyte flow rates; moreover, the required pressure being low, the pumping power and consequently the energy consumption are limited. The hydraulic path is very short due to the short distance separating the supply ports from the discharge ports. The arrangement of the invention makes it possible to discharge practically all of the electrolyte through the discharge orifices; there is therefore no significant lateral flow and the device of the present invention has the advantage, important in practice, of not requiring side seals.

Claims (10)

1. Device for electrolytic deposition on a substrate, intended to ensure high turbulence of the electrolyte between an anode and a very close cathode, the cathode generally being constituted by the substrate, characterized in that it comprises a plate pierced with a first series of orifices distributed over the surface of the plate and connected to electrolyte supply means, as well as a second series of orifices, also distributed over the surface of the plate and adjacent to the orifices of said first series, the orifices of said second series being connected to means for discharging the electrolyte, and in that said plate constitutes the anode of the electrolysis circuit. 2. Device according to claim 1, characterized in that said plate forms, with a plurality of other walls, a box which delimits a closed interior volume and which comprises: (A) at least one orifice, said supply orifice, passing through a wall of the box and giving access to said interior volume; (b) tubes connected to the orifices of said second series, passing through said interior volume without communicating with it and opening out to the exterior of said box. 3. Device according to claim 2, characterized in that at least a portion of said tubes connected to the orifices of the second series are, at their other end, connected to suction means. 4. Device according to claim 3, characterized in that said suction means comprise a manifold and a pump. 5. Device according to claim 1, characterized in that the orifices of any one of the two series of orifices are connected to individual supply pipes, in that these individual supply pipes are connected, individually or in groups any source of electrolyte, and in that the orifices of the other series are connected to return tubes of the electrolyte. 6. Device according to either of claims 1 to 5, characterized in that it further comprises means for separating the substrate and an extra-thin sheet formed by the electrolytic coating deposited on said substrate, means heating to heat said extra-thin sheet to a temperature above its recrystallization temperature, as well as means for rapidly cooling the extra-thin sheet to a temperature close to room temperature. 7. A method of electrolytic deposition on a substrate using a device according to either of the preceding claims, characterized in that said plate is placed, respectively the wall of said box pierced with two series orifices, in parallel and at a very short distance from the surface of the substrate, and in that one connects on the one hand said plate, respectively said wall, to the positive pole of a direct current source and on the other hand said substrate at the negative pole of said source, in that the electrolyte is supplied to the orifices of one of said series of orifices, in order to introduce this electrolyte into the narrow space between the plate and the wall, and the substrate, in that the electrolyte is circulated in said narrow space and in that it is taken up, at least partially, through at least part of the orifices of the other series of orifices. 8. Method according to claim 7, characterized in that at least part of said electrolyte is taken up by applying a vacuum to at least part of the orifices of this other series of orifices. 9. A method according to either of claims 7 and 8, characterized in that said plate is at least partially immersed, respectively said wall, as well as the substrate, in the electrolyte. 10. Method according to one of claims 7 to 9, characterized in that one separates the substrate and an extra-thin sheet formed by the electrolytic coating deposited on said substrate, in that one warms said extra sheet thin up to a temperature above its recrystallization temperature and in that the extra-thin sheet is then rapidly cooled to a temperature close to room temperature.

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