EP0320482A1 - Device for electrolytic metal deposition and operation method thereof - Google Patents

Abstract

The device comprises a wall consisting of a plurality of electrically conductive members (15) separated by electrically insulating members (16). A proportion of the slots (5) for feeding electrolyte are situated in the electrically conductive members, while a proportion of the slots (6) for removing the electrolyte are situated in the electrically insulating members. At least a proportion of the electrically conductive members are connected to a terminal of a source of direct current (14), the other terminal of the source of direct current being connected to the substrate (2). The height of the electrolysis gap (11) may vary periodically. <IMAGE>

Description

The present invention relates to a device for the electrolytic deposition of a metal on a mobile substrate. The invention also relates to the method of using such a device, as well as to the product obtained by the implementation of this method.

To fix the ideas, the description which follows will be essentially devoted to a device intended for the manufacture of an extra-thin sheet, hereafter called foil, preferably in iron, by continuous electrolytic deposition on a mobile substrate and separation foil and substrate. The invention is however not limited to this single application, and it extends in an equally advantageous manner to the formation of a permanent electrolytic deposit, in particular to the electrolytic coating of a steel strip.

The device of the invention belongs to the type described in patent BE-A-08700561, which reveals an electrode comprising a body, one wall of which is profiled so as to match the shape of the substrate. In this walls are provided with narrow and parallel slots, serving alternately to introduce the electrolyte into the electrolysis interval and to take it back from this interval; this arrangement ensures turbulent circulation and a short trajectory of the electrolyte between the electrodes as well as a uniform flow over the entire width of the substrate.

However, it appeared that this known device did not make it possible to produce extremely thin and puncture-free foils, for example foils having a thickness of less than 9 μm and having an almost total absence of porosity.

Furthermore, the reduction of installation and operating costs is always an important factor that should be taken into account when designing an industrial electroplating installation.

In this regard, an increase in the current density in the electrolysis cells makes it possible, for a given deposit thickness, to reduce the number of electrolysis cells and / or to increase the speed of the substrate. This current density is nevertheless limited by the phenomenon of concentration polarization, corresponding to a local depletion of the electrolyte near the electrodes, which results in the formation of fragile, even powdery deposits, called "burnt deposits".

There are several methods that can alleviate the disadvantages just mentioned. Mention may be made in this regard of the introduction of various additives into the electrolyte, the adjustment of the pH and the temperature of the electrolyte or else the stirring thereof. These methods make it possible to raise the admissible current density somewhat, but do not completely and satisfactorily resolve the above-mentioned difficulties.

It is also possible to practice electrolytic deposition using pulsed currents. This technique clearly improves the situation with regard to stings in the depot. However, the application of this technique is not economically possible for electrolytic deposition because of the extremely high price of pulse generators.

The object of the present invention is to provide a device for the electrolytic deposition of a metal on a mobile substrate, which makes it possible to use the pulsed current technique under advantageous conditions not only from the economic point of view, but also from the flexibility of the deposition process and the quality of the products obtained.

According to the present invention, a device for the electrolytic deposition of a metal on a mobile substrate, which comprises an electrode having at least one wall, the outer surface of which is located opposite said substrate with which it delimits an electrolysis interval and a plurality of parallel slots formed in said wall and connected alternately to electrolyte supply means and to electrolyte discharge means, is characterized in that said wall consists of a plurality of electrically elements conductors separated by electrically insulating elements, in that at least part of the electrolyte supply slots are located in said electrically conductive elements, while part of the electrolyte discharge slots are located in said elements electrically insulating, and in that at least a portion of said electrically conductive elements are connected to a terminal of a source direct current, the other terminal of the direct current source being connected to said substrate.

This arrangement makes it possible to create, in the electrolysis interval, a maximum electric field under the electrically conductive elements and a minimum electric field under the electrically insulating elements. The substrate circulating in said electrolysis interval is thus alternately subjected to a maximum current density corresponding to the maximum electric field and to a minimum current density corresponding to the minimum electric field.

According to a particular variant of the device of the invention, said electrically conductive elements separated by electrically insulating elements are only provided in an initial portion of said wall, considered in the direction of progression of said substrate.

This arrangement makes it possible to apply the technique of pulsed currents in the period when they are most useful, that is to say at the start of the electrolytic deposition, which corresponds to the birth of the bites.

According to another variant of the device of the invention, the height of the electrolysis interval varies periodically and continuously between minima situated in line with the electrolyte supply slots and maxima situated in line with the discharge slots of the electrolyte, at least in the portion of the wall where said electrically conductive elements are separated by electrically insulating elements.

In this variant, the low height of the electrolysis interval makes it possible on the one hand to reduce the ohmic losses in the zones corresponding to the high current densities and on the other hand to reduce the pressure losses and the energy consumption resulting in areas with low current density.

According to yet another variant of the device of the invention, at least one of said electrically conductive elements is connected to a terminal of a direct current source, the polarity of which is opposite to that of the terminal to which the other electrically connected elements are connected conductive, the electrical potential of the substrate being intermediate between the respective potentials of said electrically conductive elements.

It has been found to be particularly advantageous that said electrically conductive elements having said opposite polarity are periodically inserted between the other electrically conductive elements.

Such an arrangement makes it possible to intensify the effects of pulsed current electrolysis by interposing, preferably periodically, a current pulse of opposite direction in a succession of pulses of electrolysis current of direct direction.

It goes without saying that, in general, the substrate is connected to the negative terminal of a source of direct electric current and that it thus constitutes the cathode of the electrolytic deposition device, while the anode consists in particular by the above electrically conductive elements.

It should however be emphasized that, in the last variant mentioned above, said electrically conductive elements having said opposite polarity in fact constitute cathodes with respect to the substrate which is thus locally anodic.

In the device of the invention, said substrate can be either a temporary support such as a metal belt, for example made of titanium, on which a detachable film is deposited, or a product such as a steel strip on which one deposits a permanent coating.

• Fig. 1 illustre schématiquement un dispositif de dépôt électrolytique appartenant à la technique antérieure; la
• Fig. 2 présente le principe du dispositif de la présente invention, permettant la création de courants pulsés; la
• Fig. 3 montre une première variante du dispositif de la Fig. 2, pré­sentant un intervalle d'électrolyse de hauteur variable; et la
• Fig. 4 illustre un montage destiné à produire des impulsions de courant de sens inverse intercalées dans une séquence d'impulsions de courant de sens direct.

Other characteristics and advantages of the present invention will appear on reading the description which follows, of various embodiments of the device of the invention. This description, which is given for the simple purpose of illustrating the present invention, refers to the appended drawings, in which the:

• Fig. 1 schematically illustrates an electrolytic deposition device belonging to the prior art; the
• Fig. 2 shows the principle of the device of the present invention, allowing the creation of pulsed currents; the
• Fig. 3 shows a first variant of the device of FIG. 2, having an electrolysis interval of variable height; and the
• Fig. 4 illustrates an assembly intended to produce pulses of current of opposite direction interspersed in a sequence of pulses of current of direct direction.

In all the figures, identical or analogous elements are designated by the same reference numerals. The directions of circulation of the electrolyte and of the electric currents are indicated by appropriate arrows. Finally, the elements which are not directly necessary for understanding the invention have not been shown, so as not to unnecessarily overload the drawings.

In particular, the fragmentary views of Figs. 2 to 4 show portions of the wall of the electrode which faces the substrate and the general arrangement of which is visible in FIG. 1.

Fig. 1 schematically illustrates in partial section an electrolytic deposition device belonging to the prior art, in which an anode 1 and a mobile substrate 2 are connected respectively to the positive terminal and to the negative terminal of a direct current source. The anode 1 has a flat wall 10 which faces the substrate 2 and which defines therewith an electrolysis interval 11 of constant height. The wall 10 is provided with narrow parallel slots 5, 6 transverse to the substrate, which open into the electrolysis interval; these slots respectively provide the supply (5) of electrolyte and the evacuation (6) of the electrolyte from the electrolysis interval 11. The electrolyte circuit is represented by the arrows a, b, c, d , e, f. In this known embodiment, the electrolysis interval 11 is the seat of a constant electric field and the current density is normally uniform there. The substrate 2 moves in the direction of the arrow g.

The wall fragment, according to the invention, illustrated in FIGS. 2 to 4 corresponds to a portion comprising two supply slots 5 and a discharge slot 6, such as the portion framed by a dashed line in FIG. 1. The substrate 2 is covered with an electrolytic deposit 12; it is connected by means of brushes 13 to the negative terminal of a source of direct electric current 14.

Fig. 2 shows that, in accordance with the present invention, the wall 10 consists of electrically conductive elements 15, separated by electrically insulating elements 16. The supply slots 5 are located in the conductive elements 15, while the slots discharge 6 are located in the insulating elements 16. The conducting elements 15 are connected to the positive terminal of the source of direct electric current 14.

In the direction of progression of the substrate 2, the conductive elements 15 have a length A and the insulating elements 16 have a length B.

This device works as follows.

When the indicated electrical connections are made, there is established in the electrolysis interval 11 an electric field varying periodically according to the direction of movement of the substrate 2. On the other hand, this electric field must be perfectly uniform in the transverse direction .

During its movement in the electrolysis interval 11, each point of the substrate 2 is subjected to the above-mentioned electric field varying in space; the current density prevailing at this point therefore varies between a maximum value corresponding to the passage through a maximum of electric field and a minimum value corresponding to the passage through a minimum of field. This minimum value can moreover be positive, zero or even negative. The maximum value of the electric field, and consequently that of the current density, is located under the conductive elements 15, and more precisely at the level of the slots. feed 5; similarly, the minimum value of the electric field, and consequently that of the current density, is located under the insulating elements 16, and more precisely at the level of the evacuation slots 6. The electric field and the current density vary continuously between these extreme values.

The electrolytic deposition 12 on the substrate 2 is therefore carried out under pulsed current conditions, although the electric field does not vary over time and the electric supply voltage therefore remains constant.

In an interesting particular embodiment, which is not specially illustrated but which is easily understood, the arrangement of FIG. 2 is only produced in the initial portion of the wall 10, relative to the direction of progression of the substrate 2. This portion is for example that which is included in the box in phantom in FIG. 1. The rest of the wall 10 is then formed in accordance with the prior art, that is to say by a single conductive wall in which are formed the supply slots 5 and the discharge slots 6. This arrangement allows reduce the energy consumption due to ohmic losses while maintaining the advantageous use of pulsed currents in the most interesting portion of the electrolysis interval. It is indeed in this initial portion that the pits responsible for the porosity of the electrolytic deposit arise.

In the variant of the device of the invention illustrated in FIG. 3, the height of the electrolysis interval 11 varies periodically, and continuously, in the direction of movement of the substrate 2. This height is minimum in line with the feed slots 5, that is to say in areas with a maximum electric field and maximum current density. This low height is justified in these places, because it reduces ohmic losses and therefore the consumption of electrical energy. On the other hand, this height is maximum at the level of the evacuation slots 6, that is to say in areas where a minimum electric field and a current density prevail. minimum. In this zone, the ohmic losses are low and little influenced by the height of the electrolysis interval. On the other hand, a greater height of this electrolysis interval results in an increase in the cross-section of the electrolyte and a reduction in the pressure drops. This results in a reduction in energy consumption to ensure the circulation of the electrolyte.

Finally, FIG. 4 shows that an electrically conductive element such as 15a can be connected to the negative terminal of a direct current source 14a, while the other electrically conductive elements 15 are connected to the positive terminal of another source of direct electric current 14. In this arrangement, the substrate 2 is, by means of the brushes 13, connected between the two sources 14 and 14a placed in series. The conductive element 15 is therefore anodic with respect to the substrate 2, while the element 15a is cathodic with respect to this substrate 2.

It goes without saying that cathode elements 15a can be made up of any number of these conductive elements, and in any order relative to the anode elements 15. In this way, the substrate 2 traversing the electrolysis interval 11 can be subjected to any desired sequence of anodic and cathodic pulses, caused by electric fields considered here as positive and negative respectively. This results in an accentuation of the effects of electrolysis, in particular an improvement in the regularity of the deposition of the metal.

The length of the electroplating device according to the invention depends in particular on the required production capacity and on the average current density.

As shown in Figs. 2 to 4, the wall 10 of this device consists of conductive elements 15 of length A and insulating elements 16 of length B, the lengths A and B being considered in the direction of movement of the substrate 2. In total, the device includes a certain number n of identical groups (conductive element 15 - insulating element 16) and its total length is n (A + B). The slots supply 5 and discharge 6 are preferably centered respectively in the conductive 15 and insulating 16 elements.

We will now describe, by way of example, a preferred embodiment of the device of the invention, in which reference will again be made to FIG. 2.

The different quantities used are defined as follows:
- active length : length corresponding to the high intensity electric field; given the small height of the electrolysis interval, it can be considered that the active length is substantially equal to the length "A" of an electrically conductive element;
- length inactive : length corresponding to an electric field of almost zero average intensity; for the same reason as above, it can be considered that the inactive length corresponds substantially to the length "B" of an electrically insulating element;
- active ratio : ratio w = A / (A + B);
- active period : duration required at a point on the substrate to travel, at speed V, the active length A;
- inactive period : time required at a point on the substrate to travel, at speed V, the inactive length B;
- peak current density (Dc) : average value of the current density during an active period;
- current density (Dm) : average value of the current density during a period comprising an active period and an inactive period; neglecting the very weak current corresponding to an inactive period, we can write
Dm = w. Dc
with sufficient precision in industrial operation.

Furthermore, it appeared that the optimal use of the device of the invention with a view to eliminating punctures even in the thinnest foils, implied compliance with the following operating conditions:
- the electrolyte must be based on chlorides and it must contain 150 to 180 g / l Fe⁺⁺, 0.8 to 2 g / l Fe⁺⁺⁺, 20 to 30 g / l Ca⁺⁺, 0.01 at 0.001 g / l H⁺ and 2 to 3 ml / l of wetting agent;
- the electroforming temperature must be between 90 ° C and 100 ° C;
- the average current density Dm must be between 50 and 100 A / dm²;
- the active period must be between 1 and 10 ms;
- the active ratio w must be between 0.2 and 0.8.

These conditions were applied in an iron foil manufacturing device, with a view to ensuring a production of 736 kg / h of foil by electrolytic deposition on a cathode moving at a speed V = 250 m / min.

The electrolyte had the following composition: Fe⁺⁺: 165 g / l Ca⁺⁺: 21 g / l Fe⁺⁺⁺: 1 g / l H⁺: 0.005 g / l wetting agent: 2.5 ml / l and a temperature of 98 ° C.

The device included 1622 identical A-B groups; all the conductive elements 15 were anodic. The active length A was 12.5 mm, the inactive length B was 37.5 mm, and the resulting active ratio was thus 0.25. The active period was 3 ms and the inactive period 9 ms. The electrolysis interval was 1 mm high.

The electric generator supplied a current of 810.8 kA at a DC voltage of 10.25 V, which corresponded to an installed power of 8476 kW. The peak current density Dc reached 320 A / dm² and the average current density Dm was 80 A / dm².

Under these conditions, an iron foil with a width of 1250 mm and a thickness of 5 μm was produced, free of pitting. The specific energy consumption here amounted to 11.52 kWh / kg of iron foil.

Alternatively, a device was used in which only the first 300 elements were made in accordance with the present invention. All other parameters being unchanged, iron foil identical to the previous one was manufactured with a specific consumption of 7.44 kWh / kg of iron foil.

It goes without saying that, for the electrolytic deposition of a permanent coating on a substrate such as a steel strip, the operating conditions, in particular the composition of the electrolyte, must be adapted to each case.

Claims (10)

1. Device for the electrolytic deposition of a metal on a mobile substrate (2), which comprises an electrode having at least one wall (10) whose external surface is located opposite said substrate with which it defines an electrolysis interval (11), and a plurality of parallel slots (5, 6) formed in said wall and connected alternately to means for supplying electrolyte and to means for discharging the electrolyte, characterized in that said wall is consisting of a plurality of electrically conductive elements (15) separated by electrically insulating elements (16), in that at least a portion of the electrolyte supply slots (5) are located in said electrically conductive elements, while that part of the electrolyte discharge slots (6) are located in said electrically insulating elements, and in that at least a part of said electrically conducting elements are connected to a terminal of a source of direct current (14) the other terminal of the direct current source being connected to said substrate. 2. Device according to claim 1, characterized in that said electrically conductive elements (15) separated by electrically insulating elements (16) are only provided in an initial portion of said wall (10), considered in the direction of progression of said substrate. 3. Device according to either of Claims 1 and 2, characterized in that the height of the electrolysis interval (11) varies periodically and continuously between minima situated in line with the slots (5) supply of electrolyte and maxima located in line with the slots (6) for discharging the electrolyte, at least in the portion of the wall (10) where said electrically conductive elements are separated by electrically insulating elements. 4. Device according to either of claims 1 to 3, characterized in that at least one of said electrically conductive elements (15a) is connected to a terminal of a direct current source, the polarity of which is opposite to that of the terminal to which the other electrically conductive elements (15) are connected, the electrical potential of the substrate (2) being intermediate between the respective potentials of said electrically conductive elements (15, 15a). 5. Device according to claim 4, characterized in that the electrically conductive elements (15a) having said opposite polarity are periodically interposed between the other electrically conductive elements (15). 6. A method for the electrolytic deposition of a metal on a mobile substrate, in which said substrate moves along an electrode wall with which it delimits an electrolysis interval in which an electrolyte circulates, characterized in that l 'is created in at least part of said electrolysis interval, a stationary electric field whose intensity is variable in the direction of movement of said substrate and substantially uniform in the direction perpendicular to this movement, this variable electric field giving rise, at within the electrolyte, at a variable electric current density in the direction of movement of said substrate and substantially uniform in the direction perpendicular to this movement, and in that the substrate is passed through in said variable electric field so as to subjecting said substrate to a plurality of successive electric current pulses. 7. Method according to claim 6, characterized in that the intensity of the electric field is varied periodically, and therefore the density of electric current in said electrolysis interval. 8. Method according to either of Claims 6 and 7, characterized in that the duration of an electric current pulse is between 1 ms and 10 ms. 9. Process according to either of Claims 6 to 8, characterized in that the ratio (w) between the duration of an electrical current pulse and the duration separating the start of two successive pulses is between 0 , 2 and 0.8. 10. Method according to either of claims 6 to 9, characterized in that the average current density (Dm) during the period separating the start of two successive pulses is respectively between 50 A / dm² and 100 A / dm² for the manufacture of a foil, or between 200 A / dm² and 300 A / dm² for the formation of a permanent coating.

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