EP0297178A1

EP0297178A1 - Electrodeposition of metals

Info

Publication number: EP0297178A1
Application number: EP87201265A
Authority: EP
Inventors: Michel Neirynck
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 1987-07-03
Filing date: 1987-07-03
Publication date: 1989-01-04

Abstract

A device for electroplating a plurality of steel wires (l) comprises an electrolytic bath, means for guiding the steel wires (l) in parallel in a substantially horizontal plane through said electrolytic bath, a substantially horizontal and inert anode support (3) and a soluble anode (2) lying on said inert anode support (3). The soluble anode (2) consists of a multitude of pieces having a volume in the range of O.l cm³ to l5 cm³.

Description

The invention relates to a device and a method for electroplating a plurality of steel wires.
Devices for electroplating a plurality of steel wires have been employed for many years and are well-known in the art. Such devices may comprise an electrolytic bath, means for guiding the steel wires horizontally through said electrolytic bath, an anode support, and a dissolvable anode lying on said anode support. The dissolvable anode is in the form of a plate or of a block. A device according to the prior art is disclosed in U.S. patent 4.O6l.553.
However, the devices according to the prior art present many drawbacks. The dispersion, over the different parallel wires, of the coating thickness, increases according as the consumption of the anodes progresses, and becomes prohibitive when nearing complete consumption. The necessary standstills for replacement of the anodes constitute another drawback.
It is an object of the invention to avoid the drawbacks of the prior art. More particularly, it is an object of the invention to provide a device which shows a uniformity over the time of deposition on the different wires. It is a further object of the invention to avoid standstills for replacement of the anodes.
According to the invention there is provided a device for electroplating a plurality of steel wires, said device comprising an electrolytic bath, means for guiding the steel wires in parallel in a substantially horizontal plane through said electrolytic bath, a substantially horizontal and inert anode support and a soluble anode lying on said inert anode support, characterized in that said soluble anode consists of a multitude of pieces having a volume in the range of O.l to l5 cm³, preferably in the range of O.2 to 2 cm³.
Anode pieces are known as such, where they are contained in baskets, submerged in the bath, but up to now no effort has been made to use such pieces in such a way as to solve the above problems, a.o. of uniformity of deposition, by disposing them in the form of a horizontal bed, facing a parallel horizontal plane of travelling wires.
A device for electroplating a plurality of steel wires means that two, three, or more, e.g. 2O or 4O steel wires are electroplated at the same time in the same electrolytic bath.
The type of electrolytic bath used depends on the metal to be electroplated on the steel wires. If copper is to be plated one may use copper-citrate, copper-amine or copper-tartrate, copper-sulfate or copper fluoroborate, and more preferably copper-cyanide or copper-pyrophosphate.
By substantially horizontal is meant that the plane of the wires or the plane of the anode support, or both, may have an inclination in the direction of the wires. The angle of inclination is, however, smaller than three degrees (3°).
The inert anode support may be a grid or preferably a full plate. By inert anode support is meant that the anode support is made of a noble metal such as platinum or is coated with a noble metal, or is made of a passivated metal, e.g. passivated stainless steel or titanium. In an aggressive electrolytic bath such as copper-sulfate the anode support may be made of titanium or zirconium, in less aggressive mediums such as copper-pyrophosphate the anode support may be made of a passivated stainless steel.
The anode pieces may be made of copper, zinc, brass or bronze depending on the metal to be electroplated on the steel wires. The anode pieces may have different forms such as small spheres or beams. The anode pieces may be wire rods cut to short ends such that they present a cylindrical form. Preferably, the length of such a cylinder is equal to the diameter in order to form easily a bed of equal height over the surface of the anode support. The anode pieces may also be slugs having different and irregular forms.
As consumption proceeds the anode pieces will become smaller and smaller. If a grid is used for the anode support the distance between the bars of the grid should be kept as small as possible in order to prevent the pieces to fall through the grid. This is the reason why the anode support is preferably a full plate. However, it is also possible to have an anode support which consists partly of a grid and partly of a full plate. The almost consumed anode pieces are then raked on the full plate whilst new anode pieces are dumped and distributed over the other part.
Usually there are two, three or more layers of anode pieces on the bed surface. As the anode pieces of the surface layer become smaller they are "replaced" by the anode pieces of the following layers.
The greatest dimensions of the new anode pieces should be preferably smaller than the smallest distance between the steel wires. This prevents standstills while adding new anode pieces.
The invention also relates to a method for electroplating more than one steel wire characterized in that a device according to the invention is used.
The invention will now be explained more in detail with reference to the accompanying drawing, wherein

figure l represents a cross-section of a device according to the invention, perpendicular to the direction of movement of the steel wires.

In figure l steel wires l are guided horizontally and in parallel through an electrolytic bath. The anode pieces 2 are lying on a full plate anode support 3. The anode support 3 is connected to the positive terminal of the direct current source. The steel wires l are connected to the negative terminal of the direct current source by means of guiding rollers (not shown). If the smallest distance between the steel wires is e.g. 2O mm then the greatest dimension of the anode pieces will be smaller than this value. e.g. l3 mm. If the anode pieces have a cylindrical form with diameter equal to the length and equal to l3 mm. then they have a volume of l.73 cm³.
As a matter of example, for the electrodeposition of copper on steel wires having a diameter of O.5 to 2.5 mm following parameters may be used :
- a copper-pyrophosphate bath
- a cathode current density of 2 - l5 A/dm²
- velocity of the steel wires : 2O - lOO m/min.
Application of the device and the method according to the invention is possible in plating lines of alloys (binary or ternary or with multiple alloying elements) in which two or more layers of different metals are firstly electroplated on the steel wires in subsequent plating baths and then continuously in the same line, are caused to diffuse into each other by electrical resistance heating or by inductive heating of the wire. For brass, for instance, a first copper layer can be laid (using a copper-pyrophosphate bath and a copper sulphate bath, the one after the other, cathode current density in a range between e.g. 2 and 6O Amps/dm²), and then a zinc layer using a zinc-sulfate bath. For ternary alloys, a cobalt, or nickel or iron layer may e.g. also be electroplated between or on top of the layers of copper or zinc. Such brass coated wire (6O to 75 % of copper in the brass) can, after twisting into cord if so desired, be used for rubber reinforcement.
The following test has been carried out.
In a method A (prior art) 2O steel wires are electroplated with copper in a copper-pyrophosphate bath. The anode consists of copper plates. The dispersion of electrodeposition of copper over the wires is measured a first time immediately after replacement of the anodes and a second time one week after the replacement of the anodes.
In a method B (invention) 2O steel wires are electroplated with copper in a copper-pyrophosphate bath. The anode consists of a multitude of cylindrical copper pieces. Here again the dispersion of electrodeposition of copper over the wires is measured immediately after adding the anode pieces and one week after adding the anode pieces.
Table l summarizes the results of this test :
Table l shows that the uniformity of electrodeposition over the steel wires decreases in time for a method according to the prior art whereas for a method according to the invention the uniformity of electrodeposition over the steel wires is relatively stable, even if no advantage is taken of the possibility to add new pieces in the meantime without stopping the process.
Other advantages of the device and method according to the invention have also been recognized.
The surface of a multitude of anode pieces is much larger as compared to a same volume in plate form. This allows to use much lower surface current densities at the anode, so that the risk of polarization of the anodes is much lower. Moreover, due to the less smooth surface of the anode pieces in comparison with an anode plate, the turbulence of the electrolytic bath is higher in the neighbourhood of anode pieces than in the neighbourhood of an anode plate. Still another advantage is that with a multitude of anode pieces the electrical contact between the anode and the anode support is better than between an anode plate and an anode support.
Dissolving small pieces of metal for electroplating may also occur in the plating bath itself. It can also occur in a separate tank where an aqueous liquid, rich in ligands is prepared for use as a liquid in a plating bath with inert anodes.
The invention is not limited to the electrodeposition of metals to steel wires having a round cross-section. The steel wires may also have a flat, a polygonal or another cross-section.
The invention neither is limited to the use of a soluble anode consisting of a multitude of pieces of the same material. It is e.g. possible to use a bed consisting of zinc and iron pieces.

Claims

1. A device For electroplating a plurality of steel wires (l), said device comprising an electrolytic bath,
means for guiding the steel wires (l) in parallel in a substantially horizontal plane through said electrolytic bath,
a substantially horizontal and inert anode support (3)
and a soluble anode (2) lying on said inert anode support (3)
characterized in that said soluble anode (2) consists of a multitude of pieces having a volume in the range of O.l cm³ to l5 cm³.

2. A device according to claim l, characterized in that said pieces are made of a copper alloy.

3. A device according to claim l, characterized in that said pieces are made of copper.

4. A device according to anyone of claims 2 to 3, characterized in that said electrolytic bath contains copper ions.

5. A device according to claim 4, characterized in that said electrolytic bath is a copper-pyrophosphate bath.

6. A device according to anyone of claims l to 5, characterized in that the greatest dimension of said anode pieces is smaller than the smallest distance between two steel wires.

7. A device according to anyone of claims l to 6. characterized in that said anode pieces have a substantially cylindrical form.

8. A device according to anyone of claims l to 7, characterized in that said anode support (3) is a full plate.

9. A device according to anyone of claims l to 7, characterized in that said anode support (3) is made of passivated stainless steel.

lO. A method for electroplating more than one steel wire (l), characterized in that a device according to anyone of claims l to 9 is used.