Method for checking in a moving mode the continuity of a metal covering on a metal wire of different nature

A method is disclosed for checking and obtaining in a moving mode the continuity of a metal covering on a metal wire of different nature belonging to the group of a wire being treated by a current strength and issuing from a covering bath and wire, the treatment of which is achieved and coming from a storage wheel. The method comprises continuously bringing the coated wire in contact with a volume of electrolyte solution chemically and physically inert with respect to the wire, and in which the sensitive part of a reference electrode is immersed, sliding the wire issuing from the volume against an electrical contact, measuring the electrical potential variations between the electrode and the contact, and when the potential current oscillates, noting the poorly covered wire and increasing the current strength on the wire being treated.

The invention concerns an apparatus for checking in a moving mode the 
continuity of a metal covering on a metal wire of different nature and 
electrochemical potential. 
U.S. Pat. No. 4,492,615 claims a process for covering a great length of 
metal with a metal layer adhering thereto, by electrolysis in a continuous 
mode and at a high speed of movement. 
That process is applied in particular to nickel plating wires of aluminium 
or aluminium alloys which are intended to be used in particular as 
electrical conductors. Indeed, such a covering gives the wire a low level 
of contact resistance which remains stable with the passage of time and 
permits copper advantageously to be replaced by aluminium for producing 
most industrial or domestic single-wire or multi-wire conductors. 
However, in order for the level of stability of the contact resistance to 
be maintained with the passage of time, it is necessary for the covering 
to be in as continuous a condition as possible. 
As the thickness of nickel is generally very small and of the order of a 
micron, it only needs a fleeting mishap on the production line or poor 
conditioning or packaging of the wire produced, for the covering to suffer 
from heterogeneities or breaks therein, so that the continuity of the 
covering is interrupted. It is therefore a matter of interest to provide 
an apparatus which makes it possible to check the continuity of the 
covering and which can serve both as an acceptance test for the user and 
as a quality control means for the manufacturer. 
It will be apparent that, by virtue of the form of the product and the way 
in which it is obtained, the apparatus must be such that it can be applied 
in a moving mode and can thus give an instantaneous response. 
Now, the conventional methods used for checking the continuity of the 
coverings on a metal wire generally involve using chemical reactants and 
are therefore essentially discontinuous. Moreover, they are only applied 
to copper wires which are tin-plated, nickel-plated or silver-plated, and 
not to nickel-plated aluminium wires. 
It is for that reason that the applicants sought and developed an apparatus 
for checking, in a moving mode, the continuity of a metal covering on a 
metal wire, characterised in that it comprises: 
a cell of electrically insulating material, the lateral walls of which are 
provided with two openings so disposed that the wire to be checked which 
passes between same is brought into contact with a solution of electrolyte 
which at least partially fills said cell and in which the sensitive part 
of a reference electrode is immersed, 
an electrical contact which slides on the wire issuing from the cell, and 
a millivoltmeter connected on the one hand separately to the contact and to 
the electrolyte and on the other hand to a recording means. 
Thus, the apparatus according to the invention firstly comprises a cell, 
that is to say, a container, which may or may not be closed by a cover and 
which is made of a material such as glass or a plastics material or a 
metal which is covered internally with an insulating covering. The 
container is provided on the side walls thereof with two openings provided 
with sealing means and within which the wire to be checked passes. It 
contains a solution of electrolyte in a sufficient amount for the wire to 
be completely immersed. The solution which is to be chemically and 
physically inert with respect to the wire and the material constituting 
the cell is preferably an aqueous solution of boric acid in a level of 
concentration of between 1 g/l and saturation at the the temperature which 
can be withstood by the reference electrode. 
The sensitive end of a reference electrode which may be for example a 
saturated calomel electrode dips into that solution. A sliding electrical 
contact is placed on the wire which issues from the cell. The electrode 
and the contact are connected to a millivoltmeter in such a way as to be 
capable of measuring the potential difference between the wire and the 
reference electrode, which measurement is continuously transmitted to a 
recording means. 
The millivoltmeter must have a high input impedance, preferably of higher 
than 10.sup.9 .OMEGA., so as to avoid a slow degradation in the reference 
electrode and in consequence drift in the measurement results. The 
configuration of the curve obtained at the recording means indicates 
whether the covering is continuous or not. 
Indeed, it has been observed that, whether the curve results from a 
measurement carried out on a wire when it issues from the tank in which 
the covering is produced and which therefore carries voltage, or on a 
covered wire which carries no voltage, coming from a storage reel, the 
curve had certain characteristics which were related to the quality of the 
covering. 
In particular, in the former case, as long as the covering is 
heterogeneous, the potential difference is unstable, which is revealed by 
oscillations of greater or lesser magnitude but of substantial frequency, 
whereas once the covering becomes homogeneous, the potential difference 
stabilises. Thus, in production of the covering, it is possible to find 
operating conditions which make it possible to attain that stability and 
consequently a suitable quality of wire. 
In the second case, it is noted that there are also variations in potential 
on a poorly covered wire whereas that potential is constant on a 
high-quality wire.

Referring to FIG. 1, shown therein is a covering or coating bath 1 through 
which the wire 3 passes in the direction indicated by the arrow 2, and the 
apparatus according to the invention comprising a cell 4 provided with 
sealing means 5 and containing a solution 6 into which dips a reference 
electrode 7, an electrical contact 8 which slides against the wire, a 
millivoltmeter 9 which is connected separately to the reference electrode 
and to the contact and a recording means 10 for displaying the result of 
the checking operation. The apparatus also comprises a rinsing tank 11 for 
removing the solution from the wire and a wire drying system 12. 
Referring to FIG. 2, shown therein is the same apparatus as described 
above, which is no longer supplied with the wire issuing from the covering 
bath 1, but a covered wire coming from a reel 13, the wire then being 
wound on the reel 14 after the checking operation. 
FIG. 3 which relates to production control shows the values in respect of 
potential in volts with respect to the saturated calomel electrode, which 
are given by the recording means in dependence on time and the different 
current strengths which are applied in steps to form a covering of nickel 
on a wire of aluminium or aluminium alloy, with a diameter of 0.51 mm, 
moving at a speed of 100 meters per minute, in accordance with the process 
claimed in U.S. Pat. No. 4,492,615 namely successively passing the wire 
into a liquid current supply means containing nickel chloride, boric acid 
and hydrofluoric acid and then through a covering bath based on nickel 
sulphamate also containing boric acid and nickel chloride. 
Three ranges as indicated at 1, 2, 3 may be distinguished, according to 
increasing values in respect of current strength: 
range 1: at low strength, the potential oscillates. On examination, the 
nickel obtained has a low level of covering capacity; 
range 2: as from 30 amperes, the potential rises rapidly and the recorded 
signal oscillates greatly. That corresponds to an unfavourable 
modification in the chemical nature of the deposit; and 
range 3: for strengths of close to 80 amperes, the recorded signal becomes 
very stable and no longer varies when the strength rises, indicating that 
the degree of covering is at a maximum. 
On the basis of those recordings, for each particular situation involving a 
covering, irrespective of the nature of the substrate, the diameter 
thereof and the speed of its movement, it is possible to determine the 
optimum conditions in respect of current density to provide a homogeneous 
covering which satisfies the required function. 
FIG. 4 which relates to a nickel-plated wire which is unwound from a 
storage reel gives the curve A in respect of potential in mV in dependence 
on current strength in amperes to which the wire was subjected upon 
treatment thereof, while curve B gives the degree of 
covering.theta.=nickel surface area/ substrate surface area, in dependence 
on the same current strengths. Those curves show the correlation which 
exists between measured potential and the degree of covering. It will be 
seen in particular that, when the covering attains 100%, the potential no 
longer varies. Thus, by measuring the level of potential and verifying the 
degree of stability thereof, it is possible to deduce therefrom that the 
wire complies with the requirement for continuity of the covering, which 
is imposed by the user. 
FIG. 5 is a micrograph with an enlargement of 3000 of a wire on which the 
degree of covering is close to 100%. 
The invention finds application in checking, while on the move, of any 
metal substrate covered with another metal, in particular nickelplated 
aluminium wires, in respect of which there is a wish to be sure of the 
continuity and quality of the covering and more particularly when dealing 
with electrical conductors.