Method of continuous coating of electrically conductive substrates

The invention relates to a method of continuously coating electrically conductive substrates using high-speed electrolysis in which the substrate is immersed successively in an electrolytic activating bath and an electrolytic coating bath. The two baths are of the same composition and the substrate is constantly maintained in one bath. The method is applicable especially to the nickel plating of fine aluminum wires intended for the production of flexible cable for aeronautical applications. These wires may be treated in layers and at high speed.

The present invention relates to a method of and an apparatus for the 
continuous coating of electrically conductive substrates by the use of 
high-speed electrolysis. 
In this instance, the term substrate is understood as being any product 
which takes the form of a round, bar, tube, flat or strip of considerable 
length and in particular wire; the said substrates may consist of a 
material which is conductive of electricity, such as graphite, metals and 
more particularly aluminium and its alloys. 
With regard to the coating, this may be any coating which covers the 
substrate in a continuous manner even to a minimal thickness and which 
offers suitable adhesion to withstand handling and stresses such as 
friction or clamping. 
But of more particular interest here is the nickel plating of relatively 
fine aluminium wires intended to be used as electrical conductors in the 
production of flexible cables. This coating is obtained by electrolysis in 
which deposition takes place at high speed. 
Among the documents representing the prior art in this field, it is 
possible to mention U.S. Pat. No. 4,097,342 which describes a method of 
producing aluminium members such as wire or strip which comprises passing 
the member continuously through a bath which has a high capacity for 
dissolving alumina, such as that formed by a solution of sulphuric acid 
and concentrated phosphoric acid and then through an electrolytic coating 
bath, the first bath being provided with a cathode in order to render the 
member anodic and the coating bath which is fitted with an anode. 
This method may be used for coating aluminium with brass, zinc, lead, 
nickel or copper, coating baths of a suitable composition being employed. 
Thus, for example, in the case of tin, the bath consists of 300 g/l Sn 
(BF.sub.4).sub.2, 200 g/l HBF.sub.4, 25 g/l H.sub.3 BO.sub.3, 30 g/l 
gelatin and 1 g/l .beta.-naphthol. If the first process is applied to an 
aluminium wire with a diameter of 3.2 mm in a bath at 35.degree. C. in 
which the wire is maintained for 5 seconds under a current density of 100 
to 120 A/sq.dm, a thickness of tin of 5 .mu.m is obtained, with a rate of 
travel of 36 m/min through baths 3 m in length. 
In the same field, it is likewise possible to cite two other American 
patents which belong to the Applicants: 
U.S. Pat. No. 4,492,615 which teaches the coating of a very long metal 
member with nickel and in which the member passes successively through a 
stripping die and then through a bath referred to as a "liquid connector" 
in which a cathode is immersed and finally through an electrolytic nickel 
plating bath, the passage from one bath to another possibly being via a 
phase in which the members are rinsed. In this method, the liquid 
connection may, for example, consist of a bath containing 125 g/l 
NiCl.sub.2, 6 H.sub.2 O; 12.5 g/l H.sub.3 BO.sub.3 and 6 cc/l of HF, while 
the coating bath consists of 300 g/l Ni (NH.sub.2 SO.sub.3).sub.2 
(sulphamate), 30 g/l NiCl.sub.2, 6 H.sub.2 O and 30 g/l H.sub.3 BO.sub.3. 
These baths are used at respective temperatures of 40.degree. and 
65.degree. C. in order to achieve equivalent resistivity levels. 
Under these conditions, aluminium wire of 1.78 mm diameter has been 
successfully coated with a thickness of 1 .mu.m of nickel using current 
densities of 175 A/sq.dm with rates of travel which may be as much as 300 
m/min. 
U.S. Pat. No. 4,741,811 relates to a nickel plating process based on the 
same principle as that in the above-mentioned patent, comprising an 
activating bath and a coating bath having the same compositions as 
hereinabove but in which the current density is modulated so that it can 
at the same time be applied to several wires of relatively small diameter, 
of around 0.51 mm to 0.15 mm. Thus it is possible to exceed a thickness of 
1.5 .mu.m of nickel with speeds of passage which are between 25 and 50 
m/min. 
All three of the documents quoted employ a method comprising an anodic 
pickling stage followed by a cathodic coating stage. The baths used for 
carrying out these stages always differ from one to another, which 
generally makes it necessary to carry out an intermediate rinsing stage in 
order to avoid the baths mixing. Furthermore, the point at which the wires 
pass through the facing walls of the tanks containing the baths are not 
absolutely fluid-tight and screening means will generally be provided to 
recover any entrained bath material, prior to rinsing, in order that it 
can be recycled. Such screening means are generally empty chambers in 
which the product is in contact with ambient air. As the densities of 
current which pass through the product between the two tanks are those of 
the main current and since they may attain very high levels of around 700 
A/sq.mm cross-section, the consequence of the JOULE'S effect is a quite 
considerable release of heat. 
It is obvious that this heat which is naturally dissipated through the 
liquid mass in each of the baths may not be so dissipated in the screening 
means and the result is excessive heating which may give rise to 
deformation or even breakage when the product consists of fine wires. 
To avoid this phenomenon, it is necessary to reduce the intensity which 
passes through the wire while maintaining high passage speeds in order to 
minimise the time of exposure to the free air, which restricts the 
thickness of the coating. 
Well, it is known that for a constant speed of travel, the relationship is: 
##EQU1## 
EQU and I max=K.sub.2 .phi..sup.2 
in which 
e max=maximum thickness of the coating 
I max=strength acceptable by the wires at a given speed 
.phi.=diameter of the wires. 
Therefore e max=K'.sub.1 .multidot.K.sub.2 .multidot..phi.. 
Consequently, the smaller the diameter of the wires, the more reduced are 
the maximum thicknesses of coating which can be deposited on them. 
Thus, when it is desired to deposit a sufficient thickness of coating on a 
fine wire, it will be necessary to provide for a relatively slow speed of 
passage which results in a relatively long period of exposure to the air 
and hence an increased risk of breakage. 
It is to avoid this drawback due to the products having to pass outside a 
liquid bath for cooling that the Applicants have perfected a method for 
the continuous coating of at least one electrically conductive substrate 
by high-speed electrolysis in which the substrate passes successively 
through an activating bath into which at least one cathode is plunged and 
then through a coating bath into which at least one anode is plunged, 
characterised in that the said baths are identical in composition. 
It is obvious that under these conditions it is possible to dispense with 
intermediate rinsing and hence recovery of bath and consequently the 
substrate can be kept constantly submerged in a liquid so that any heating 
is avoided. Consequently, it becomes possible to increase the current 
densities and to carry out coating of fine wires. 
Preferably, these baths circulate in relation to the electrodes in the same 
direction or in the opposite direction to that of the substrate in order 
to ensure a better heat exchange. Thus, one has a common bath which is 
shared into two portions, each being capable of being heated or cooled at 
will and propelled to the points of use and then returned to a common 
collecting point where they can be likewise subjected to heat or cleansing 
treatments. 
Furthermore, in order to optimise the method so that, if necessary, 
activating and coating baths can be made to operate at different 
temperatures, the substrate may be passed between the two baths through a 
buffer bath of the same composition as the other two. 
In this case, it is likewise preferable to ensure circulation of the said 
bath which may be carried out in the form of a third portion taken from 
the common bath and possibly subjected to heat treatments and to specific 
conditions of movement. 
Another function of this buffer bath is to avoid having the substrates pass 
suddenly from anodic polarity to cathodic polarity, so enjoying a 
progressive variation from the activating bath current to the coating bath 
current so that optimum electrical conditions for coating are provided 
immediately the substrate enters the bath. 
More particularly in the case of nickel plating, the single bath employed 
is a mixture having the following composition: 
nickel sulphamate which makes it possible to obtain deposits exhibiting the 
qualities required in a very high density current, 
nickel chloride which acts as an electrode depassivator, 
nickel fluoroborate which, at the activation stage, makes it possible to 
obtain a very fine attack on the substrate so creating a vast number of 
nickel germination sites and therefore a much finer deposit than that 
created on the germination sites obtained with the nickel chloride bath 
described in the Applicants' above-mentioned patents, 
fluoroboric acid which makes it possible to regulate the pH of the bath at 
between 1.5 and 3, 
possibly orthoboric acid, the function of which is to buffer the solution 
on the surfaces which are subject to electrochemical reactions. 
In the case of substrates in the form of fine wires, by avoiding exposure 
to the air, the method according to the invention has the advantage that 
it is possible to multiply the maximum admissible current strengths by a 
factor of 4. 
Thus, for example, an aluminium wire of type 1310-50 to the Standards of 
the Aluminium Association and having a diameter of 12/100 would melt under 
a current strength of 8 A according to the prior art techniques, which 
made it necessary to restrict the speed of passage to 32 m/min in order to 
obtain a 1 .mu.m nickel coating under a current strength of 6 A. 
With the method according to the invention, the same wire is capable of 
withstanding a 24 A current without breaking, which makes it possible to 
nickel plate at speeds of around 130 m/min in an installation, the 
electrolytic part of which does not exceed 2.5 m in length. 
Furthermore, the diameter of the nickel nodules obtained is far smaller 
than in the prior art, which results in a better rate of coverage of the 
wire. Similarly, the contact resistance measured by the crossed wires 
method yields values below 500 g of loading of between 0.2 and 0.7 
m.OMEGA. whereas in U.S. Pat. No. 4,741,811 these levels were between 1.5 
and 2 m.OMEGA.. 
Finally, thanks to the greater fineness of the deposit, the nickel lends 
itself well to subjacent deformation of the substrate. Indeed, the 
adhesion test which consists of rolling up the nickel plated wire on its 
own diameter shows that the nickel film is able perfectly well to follow 
the deformation without becoming detached. 
The invention likewise relates to an apparatus which applies the method 
according to the invention, which is characterised in that it is formed by 
a tank separated into two compartments by a partition of electrically 
insulating material containing the baths of identical composition into 
which are plunged at least one electrode connected to the positive pole of 
a current source at least in the continuous part, while into the other is 
plunged an electrode connected to the negative pole of the same source, 
the said compartments and the partition each being provided with inlet and 
outlet pipes each connected by a tank for a pumped supply and a heat 
exchanger and being traversed on either side while the partition is 
traversed by at least one fluid-tight aperture through which passes the 
substrate which is to be coated. 
Thus, the apparatus differs from that of the prior art represented by U.S. 
Pat. No. 4,492,615 by the absence of a space between the "liquid 
connection" compartment or activating compartment and the coating 
compartment, in order to avoid any contact between the substrate and the 
air and in order to ensure continuous cooling via the bath. 
This tank is of preferably parallelepiped form having a vertically disposed 
partition which divides it into two compartments each of which is about 1 
m long. The walls of the tank which are parallel with this partition and 
the said partition are provided with a plurality of apertures which are so 
disposed that it is possible to pass a sheet of wires, for example at 
suitable intervals, for them to enjoy the very best electrical processing 
conditions. The dimensions of the apertures are such as are needed to 
obtain satisfactory sealing-tightness in respect of the tank so that any 
loss of bath material is avoided. 
In order to overcome the considerable variation in polarity resulting from 
passage of the wire from one compartment to the other, the Applicants 
likewise propose an alternative form of apparatus characterised in that it 
consists of a tank separated into three compartments by two partitions of 
electrically insulating material and containing baths of identical 
composition, one of the end compartments being equipped with at least one 
electrode which plunges into the bath and which is connected to the 
positive pole of a current source at least in the continuous part while 
the other is equipped with at least one electrode plunging into the bath 
and connected to the negative pole of the same current source, the said 
compartments being each provided with a bath inlet and outlet pipe each 
connected to a supply tank via a pump and a heat exchanger and being 
traversed on either side by at least one fluid-tight aperture through 
which passes the substrate which is to be coated. 
Therefore, this alternative embodiment resides in incorporating between the 
two compartments of the above-mentioned apparatus as buffer compartment 
containing the same bath as the other two. 
All these compartments are connected to a common supply tank through two 
pipes in order to form circuits which may be partly independent and into 
which the bath may be conveyed, passing through heat exchange and 
purification appliances. 
With regard to the intermediate compartment, it must be sufficiently long 
to ensure a progressive change in polarity. Preferably, this length is 
between 50 and 200 mm.

FIG. 1 shows a tank 1 consisting of an activating compartment 2 and a 
coating compartment 3 containing a bath 4 and into which are respectively 
immersed a cathode 5 and an anode 6. This tank is traversed by a layer of 
five wires 7 passing through apertures 8 in the direction of travel 9. 
Each of the compartments is connected to a supply tank, not shown, by 
respective pipes 10-11 and 12-13. 
The same elements as in FIG. 1 can also be found in FIG. 2 and to them is 
added the compartment 14 which is fitted with pipes 15 and 16. 
The invention may be illustrated by means of the following examples of 
application: 
EXAMPLE 1 
An aluminium wire of type 1310.50 according to the Standards of the 
Aluminium Association, with a diameter of 0.12 mm, was activated and then 
nickel plated in a bath having the following chemical composition: 
______________________________________ 
50% nickel sulphamate 330 cc/l 
nickel fluoborate 55 cc/l 
50% fluoboric acid 5 cc/l 
nickel chloride 21 g/l 
orthoboric acid 16 g/l 
______________________________________ 
ph of the bath--1.6 
temperature--60.degree. for the two compartments 
graphite plates in the activating compartment 
nickel electrodes in the nickel plating compartment. 
The two compartments were separated by a single partition. 
______________________________________ 
Results: 
Speed Current Voltage Thickness 
Potential 
m/min strength A V Ni (um) mV* 
______________________________________ 
32 6 10.5 1.00 400 
45 9 17 1.07 460 
62 12 22 1.03 480 
80 15 28 1.00 540 
100 19 35.5 1.01 600 
133 24 46** 0.96 700 
______________________________________ 
*measured according to U.S. Pat. No. 4741811 
**maximum voltage of the rectifier 
No breakage was found which could be attributed to overheating of the wire. 
These results should be compared with those obtained when using different 
activating and nickel plating baths with intermediate rinsing and passage 
through compressed air screening devices, conditions in which the 0.12 mm 
diameter wire melts at 8 A. 
Characteristics of the wires obtained: 
excellent adhesion of the nickel 
contact resistance (m.OMEGA.) under a load of 500 g: 0.19-0.15-0.28-0.24, 
microscopic scanning inspection: FIG. 3 shows an excellent level of 
deposited nickel coating with nodules which are less pronounced than those 
obtained by the prior art (FIG. 4) but which are clearly smaller than 1 
.mu.m. 
EXAMPLE 2 
Four wires with a diameter of 0.15 mm were treated at the same time in a 
vertical layer in the bath described hereinabove in Example 1. The 
activating compartment was at a temperature of 45.degree. C. while the 
nickel plating compartment was at a temperature of 60.degree. C. 
Circulating in the central compartment was bath liquid which had been 
drawn off and discharged into the storage tank from the activating tank. 
______________________________________ 
Results: 
Speed Current Voltage Thickness 
Potential 
m/min strength A V Ni (um) mV* 
______________________________________ 
30 37 16 1.32 430 
60 74 33 1.32 450 
90 100 46** 1.07 540 
______________________________________ 
*measured according to U.S. Pat. No. 4741811 
**maximum voltage of the rectifier 
characteristics of the wires: 
excellent adhesion of the nickel 
contact resistance is comprised between 0.20 and 0.33 m.OMEGA. under a 500 
g loading. 
The invention can be applied particularly to the nickel plating of fine 
aluminium wires intended for the production of flexible cables for 
aeronautical applications. These fils can be treated in layers and at high 
speed.