EP0276190B1 - Process and apparatus for the electrolytic deposition of a continuous nickel film on wire - Google Patents

Abstract

The invention relates to a process and an apparatus for electrolytically depositing, in a moving mode, a continuous film of nickel on metal wire for electrical use. The process comprises using an activation bath and a nickel-plating bath in which the current density is reduced in the upstream portion of the nickel plating bath and/or the downstream portion of the activation bath and the acidity of the nickel-plating bath is so regulated as to develop the nickel in the form of strongly adhering globules of small diameter, which completely cover the wire. The reduction in the above mentioned current density and control in respect of the current density profile along the bath may be achieved by acting on the position of the electrodes in the bath and/or by interposing screens between the electrodes and the wire.

Description

The invention relates to a method and a device for electrolytically depositing, on parade, a continuous film of nickel in the form of globules of adjustable size on metallic wire for electrical use.

French Patent No. 2,526,052, belonging to the applicant, teaches a process and a device for coating a long length of metal with a metal layer. It applies in particular to direct nickel plating, that is to say without the application of intermediate layers, electrical conductors of aluminum or one of its alloys, with a diameter between 1.5 and 3 mm and used for either industrial or domestic purposes.

The process consists in passing the wire, previously freed from lubrication residues, firstly through a liquid current socket which will be called hereinafter activation bath where, by passing a continuous or pulsed electric current, it charges positively and acquires, under the action of acidic and / or saline compounds contained in the bath, a so-called active surface which is perfectly suitable for subsequent coating; then, through a nickel-plating bath where, by passing the same current, it becomes negatively charged and gradually becomes covered with nickel until it forms a continuous film. This process made it possible to obtain, on wires traveling at nearly 300 m / minute, a nickel film with a thickness of a few microns having in particular good adhesion and a weak and non-evolving contact resistance, properties which are essential for producing conductors. reliable electrical. This adhesion was such that the wire could then be drawn to a diameter of 0.78 mm without there being any detachment or tearing of the nickel coating.

The aforementioned patent also teaches a compact device in which the activation and nickel-plating tanks each have a length close to 5 meters and are each equipped with a planar electrode extending parallel to the wire over its entire path in the bath.

The applicant now aiming to manufacture electrical cables from strands made of a number of nickel-plated aluminum wires with a diameter of less than 1 mm had considered using the above method by supplementing it with a additional range of drawing operations intended to bring the wires to the desired diameter. However, it then encountered certain difficulties with regard to the behavior of the nickel coating which, for significant reductions in section, degraded and led to an unfavorable change in the contact resistance of the wire obtained.

This is why she tried to apply her process to the coating of fine wires in order to be able to bend it directly and thus to avoid any drawing operation. However, new drawbacks have appeared, such as the formation of powdery deposits or discontinuous films.

It is with the aim of proposing a solution to this problem, a solution which can also be transposed to wires of all sizes, that the Applicant has developed according to the invention a method for electrolytically depositing a continuous film of nickel during the process. in the form of globules of adjustable size on metallic wire for electrical use in which, after degreasing, the wire is subjected to a current density which charges it positively by passing it through a energizing bath under tension then, after rinsing, to a current density which charges it by passing it through an acid nickel bath under tension and finally to a rinsing and a drying and which is characterized in that in order to reduce the size of the globules is modulated the current density along the path of the wire, reduces the current density in the upstream part of the nickel plating bath and / or downstream of the activation bath and adjusts the acidity of the nickel plating bath to a pH value between 1 and 5.

Thus do we find in this process the means used in the aforementioned patent but to which are added particular means which allow in the nickel-plating bath, on the one hand to develop a deposit of nickel in a particular form of globules adhering perfectly wire and thus avoid any powdery deposit and, on the other hand to control the size and distribution of these globules so as to ensure a continuous coating of the wire. As for the activation bath, it can be seen that these particular means have a beneficial effect on the preparation of the surface of the wire, in particular by developing attachment centers for nickel.

These means consist, on the one hand, of a reduction in the current density in the upstream part of the nickel-plating bath or downstream of the activation bath. Indeed, it has been found that in the prior art where there is a single electrode or even several electrodes distributed regularly along the bath and parallel to the wire, the current density was very high in the upstream part of the nickel-plating bath and this, all the more so as the current intensity admitted was great; which could affect the quality of the deposit.

The Applicant has found that it is necessary to reduce the current density in the upstream part of the nickel-plating bath in order to see a layer of nickel appear in the form of globules which adhere and cover the substrate better and which confer low contact resistance.

This improvement has been accentuated by replacing the profile of the current density inherent in the prior process, namely a decreasing curve from the entry to the exit of the bath, a profile in which a regular growth succeeds a slow decay and, in particular , by locating the maximum density at a point in the bath located between a third of the length from the inlet and the middle while reducing as much as possible the difference between this maximum density and the minimum density.

Under these conditions, a reduction in the size of the globules is observed, which leads to a greater rate of covering of the wire and consequently to a significantly better contact resistance.

The means of the invention consist, on the other hand, in adjusting the acidity of the nickel-plating bath to a pH value of between 1 and 5 because within this range, the Applicant has found that the size of the nickel globules with the advantages mentioned above and this especially as the acidity increases. These results are particularly clear for pH values between 2.5 and 3.5.

This acidity can be increased, for example, by increasing the amount of sulfamic acid in the nickel plating bath which, as described in the aforementioned patent further contains nickel chloride and orthoboric acid while in the plating bath. 'activation, we find the same constituents described in said patent.

The method of the invention can be applied to any metallic wire such as copper wire for example. However, it finds a particular interest in the nickel plating of aluminum wires or in one of its alloys for electrical use because it allows, because of its relatively low specific mass and the lightening which results therefrom, a substantial saving of energy. when it is substituted for copper for making cables intended to equip, for example, land or air transport vehicles.

This process is particularly suitable for nickel plating of small section strands (less than 1 mm) because it gives a very adherent coating which makes it suitable for making strands and cables which can be obtained by simultaneously nickel plating several strands placed in vertical tablecloth in the same bath.

The invention also relates to a device for applying the method described above.

This device comprises, as in the aforementioned patent, in the direction of travel of the wire, a first tank containing the activation bath, a rinsing compartment, a second tank containing the nickel-plating bath, the two tanks each being equipped with at least two pairs of planar electrodes, each pair being formed of electrodes placed on either side of the wire or wires and at least partially immersed in their respective bath, the pairs of the activation bath being connected to a source of negative current and those of the nickel bath being connected to a positive current source. However, it is characterized in that the electrodes of at least one of said pairs are removable, placed at an adjustable distance from at least one neighboring pair and from the wire and which is interposed between each of said electrodes and the wire at least one removable screen made of electrically insulating material.

Thus, unlike the prior art, instead of having one or more electrodes distributed regularly along the tanks and at equal distance from the wire, the device according to the invention consists firstly of pairs of electrodes that can be moved either along the length of the tank to bring them closer or away from each other or leave free spaces in particular at one of the ends of the tanks, or according to the other dimension of the tank to bring them closer or less of the wire (s) to be coated. In this way, one can modulate the current density profile along the wire knowing that the absence of electrodes decreases this density and that the approximation of the electrodes of the wire increases it.

In particular, the profile defined above can be obtained either by leaving a free space in the upstream part of the nickel-plating tank and / or downstream of the activation tank, or by bringing the electrodes of the wire in the part opposite to the aforementioned part. As for the specific means for moving the electrodes, they can be produced from the knowledge of a person skilled in the art.

The device of the invention also consists of the presence of at least one removable screen between each of the electrodes of at least one pair and the wire. These screens are made of an electrically insulating material and preferably have good resistance to the activation or nickel-plating bath. These screens are placed more or less far from the wire and at least partially mask the electrodes so that they interrupt or deflect the lines of current flowing in the baths and therefore make it possible to reduce the current density at precise locations in the bath.

In order to achieve the profile described above, these screens are placed in the upstream part of the tank and / or downstream of the activation tank. However, we act even better on this profile by using screens provided with holes of variable diameter. Preferably, the number of holes is varied as a function of the position of the screen in the tank and, in particular, their number is increased in the direction of travel of the wire. Thus, we can combine full screens and pierced screens.

The device thus designed is suitable for the treatment of one or more wires by providing in the walls of the tanks located at the end of suitable openings located next to each other and equipped with seals. Preferably, to promote exchanges between the wire and the baths, it is possible to ensure a circulation of the latter by means of pumps. The device is also fortunately completed by a rinsing compartment intended to eliminate by means of demineralized water the bath which may have been entrained out of the nickel-plating tank, the water which wets the wire then being evaporated in a drying compartment.

All the rinsing tanks and compartments are designed as modular elements, of length and section adaptable to the coating problem posed and easily associable with each other.

The invention will be better understood with the aid of the attached FIG. 1 which represents a perspective view of a nickel-plating tank cut lengthwise along a vertical plane placed a little in front of the plant of median symmetry .

We distinguish the tank 1 of parallelepiped shape whose small faces 2 are each pierced with three holes 3 through which pass three metal strands 4 which pass in the direction of the arrow 5 in the nickel plating bath 6. In this tank, we see four of the eight electrodes 7 placed vertically on either side of the sheet and approaching the latter in the direction of travel of the strands. These electrodes are connected to a positive current source, not shown, while the strands 4 are negatively charged.

We also see between the electrodes and the sheet four of the eight screens 8 placed parallel to the sheet and at equal distance between them, the first two, in the direction of travel, are full, the the third has six holes 9 and the last has twelve holes.

The electrodes and the screens are suspended in the bath by means not shown which allow them to be moved longitudinally and transversely in the tank. The bath is driven in a circulation movement from bottom to top by means of a pump, not shown, supplied by the flow of overflows 10 and which discharges said bath into the distribution ramp 11.

Such a representation is also valid for the activation tank.

The invention can be illustrated using the following application examples:

Example 1

• - un premier compartiment de rinçage de dimensions intérieures 1000 x 120 x 120 mm contenant 9 litres de solution à 70_°C pompée à partir d'un bac de réserve de 80 litres
• - un compartiment de rinçage
• - une cuve d'activation de dimensions intérieures 1000 x 120 x 120 mm garnie d'électrodes de dimensions 100 x 80 x 80 mm reliées à une source de courant négatif pouvant débiter 2000 A sous 40 Volts et d'écrans de dimensions 120 x 40 x 5 mm en polypropylène disposés de manière à obtenir une répartition de la densité de courant judicieusement choisie,ladite cuve renfermant une solution à 45_°C contenant 125 g/1 de chlorure de nickel à 6 H₂0, 12,5 g/I d'acide orthoborique, et 6 cm³/1 d'acide fluorhydrique et circulant de bas en haut à raison de 6 m³/h
• - un deuxième compartiment de rinçage
• - une cuve de nickelage de dimensions intérieures 1000 x 120 x 120 mm garnie d'électrodes reliées au pô- le positif de la même source de courant qui alimente la cuve d'activation et d'écrans de mêmes dimensions que ceux de la cuve d'activation, l'ensemble étant disposé suivant la figure 1; cette cuve de nickelage renfermant une solution à 65°C contenant 300 g/I de sulfamate de nickel, 30 g/I de chlorure de nickel, 30 g/I d'acide orthoborique ayant un pH de 3,2 et circulant de bas en haut à raison de 6 m3/h.
• - un troisième compartiment de rinçage
• - un four de séchage.

In a device comprising successively:

• - a first rinsing compartment with internal dimensions 1000 x 120 x 120 mm containing 9 liters of 70 _° C solution pumped from an 80-liter reserve tank
• - a rinsing compartment
• - an activation tank with internal dimensions 1000 x 120 x 120 mm fitted with electrodes with dimensions 100 x 80 x 80 mm connected to a negative current source capable of delivering 2000 A at 40 Volts and screens with dimensions 120 x 40 x 5 mm in polypropylene arranged so as to obtain a distribution of the judiciously chosen current density, said tank containing a solution at 45 _° C. containing 125 g / 1 of nickel chloride at 6 H ₂ 0, 12.5 g / I orthoboric acid, and 6 cm ^3/1 of hydrofluoric acid and circulating from bottom to top at a rate of 6 m ³ / h
• - a second rinsing compartment
• - a nickel-plating tank with internal dimensions 1000 x 120 x 120 mm fitted with electrodes connected to the positive pole of the same current source which supplies the activation tank and screens of the same dimensions as those of the tank d 'activation, the assembly being arranged according to Figure 1; this nickel-plating tank containing a solution at 65 ° C. containing 300 g / I of nickel sulfamate, 30 g / I of nickel chloride, 30 g / I of orthoboric acid having a pH of 3.2 and circulating from bottom to high at the rate of 6 m3 / h.
• - a third rinsing compartment
• - a drying oven.

5 aluminum strands of the 1310.50 type were passed simultaneously according to the standards of the Aluminum Association with a diameter of 0.51 mm traveling at a speed of 50 m / min.

The strands obtained were each covered with an average thickness of nickel of 1.5 μm in the form of globules with a diameter of 1.0 μm, a representation of which at 3000 magnification is shown in FIG. 2 and which can be compared in FIG. 3 corresponding to the prior art and which gave much larger globules (3 wm) and not forming a continuous layer.

These strands could have been stranded and gave during contact resistance tests under 500 g values between 1.5 and 2 m Q whereas in the prior art for such strands, values greater than 2 m Q.

Example 2

On the same device, plies of 5 wires of diameter smaller than that of Example 1 were treated, that is to say wires of diameter 0.32 - 0.30 - 0.25 - 0.20 and 0.15 mm at running speeds between 25 and 50 m / min and obtained a nickel deposit of 1.0 μm of average thickness, formed of globules of diameter less than one micron, having contact resistances less than m Q.

These nickel-plated wires were stranded and wired, then insulated with materials approved by the aeronautics industry.

The invention finds its application in the nickel plating of metallic wires in particular of aluminum, of any diameter and, in particular, less than 1 mm and allows obtaining by stranding and wiring of light and reliable electrical conductors which are particularly advantageous for equipping spacecraft. air or land transport where energy saving by reducing equipment is much appreciated.

Claims (18)

1. A process for electrically depositing, in a moving mode, a continuous film of nickel in the form of globules of controllable size, on metal wire for electrical use, wherein, after degreasing, the wire is subjected to a current density which charges it positively by passing it through an activation bath under voltage and then, after rinsing, a current density which charges it negatively by passing it through an acid nickel-plating bath under voltage and finally a rinsing operation and a drying operation, characterised in that, in order to reduce the size of the globules, the current density along the path of movement of the wire is modulated, the current density is reduced in the upstream portion of the nickelplating bath and/or the downstream portion of the activation bath, and the acidity of the nickel-plating bath is regulated to a pH-value of between 1 and 5.

2. A process according to claim 1 characterised in that the current density is increased and then decreased slowly in the direction of movement of the wire.

3. A process according to claim 2 characterised in that the density is increased so as to have a maximum between the first third and the middle of the bath.

4. A process according to claim 3 characterised in that the difference between the maximum density and the minimum density is reduced, to reduce the size of the globules.

5. A process according to claim 1 characterised in that the acidity of the nickel-plating bath is increased to reduce the size of the globules deposited.

6. A process according to claim 1 characterised in that the acidity is of a value of between 2.5 and 3.5 pH-units.

7. A process according to claim 1 characterised by nickel plating wire of aluminium or one of the alloys thereof, for electrical use.

8. A process according to claim 7 characterised by nickel plating wire of a diameter of less than 1 mm.

9. A process according to claim 8 characterised by nickel plating wire in the form of an aligned vertical array of at least two separated strands which are then straded together.

10. Apparatus for carrying out the process according to claim 1 comprising, in the direction (5 of movement of the wire (4), a first tank containing the activation bath, a rinsing compartment, a second tank (1) containing the nickel-plating bath, the two tanks each being provided with at least two pairs of flat electrodes (7), each pair being formed by electrodes disposed on respective sides of the wire or wires and at least partially immersed in their respective baths, the pairs of the activation bath being connected to a negative current source and those of the nickel-plating bath being connected to a positive current source, characterised in that the electrodes of at least one of said pairs are movable and placed at an adjustable distance with respect to at least one adjacent pair and with respect to the wire and that interposed between each of said electrodes and the wire is at least one movable screen (8) of electrically insulating material.

11. Apparatus according to claim 10 characterised in that the distance with respect to an adjacent pair is so regulated as to leave a free space in the upstream portion of the nickel-plating tank and/or the downstream portion of the activation tank.

12. Apparatus according to claim 10 characterised in that the distance with respect to the wire is so regulated that it is larger in the upstream portion of the nickel-plating tank and/or the downsteam portion of the activation tank.

13. Apparatus according to claim 10 characterised in that the screens are positioned in the upstream portion of the nickel-plating tank and/or the downstream portion of the activation tank.

14. Apparatus according to claim 10 characterised in that at least one of the screens is provided with holes (9).

15. Apparatus according to claim 14 characterised in that the number of holes is variable depending on the position of the screen in the tank.

16. Apparatus according to claim 15 characterised in that the number of holes increases in the direction of movement of the wire in the nickel-plating tank and in the opposite direction in the activation tank.

17. Apparatus according to claim 10 characterised in that the nickel-plating tank is followed by a rinsing compartment and a drying compartment.

18. Apparatus according to claim 10 characterised in that the tanks and compartments are in the form of elements of modular type.

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