Tartrate-containing alloy bath for electroplating brass on steel wires and procedure for employing the same

Tartrate-containing alloy bath for electroplating brass on steel wires which are particularly designed for use in the production of radial tires, and the employment of said alloy bath in a continuous manufacturing process in which brass is plated directly on said steel wires in the desired amounts and compositions.

The present invention relates to a tartrate-containing alloy bath for 
electroplating brass on steel wires as well as to the procedure for 
employing said bath. More particularly, the present invention relates to 
the definition of parameters concerning the composition and the operative 
conditions of the bath, which is specifically designed for electroplating 
brass on steel in a continuous way and at high current densities. The 
resulting brass-coated wires are widely employed in the production of 
radial tires and pipes for high pressure service. 
With reference to the fundamental application of the bath according to the 
present invention, it is to be kept in mind that brass-coated steel wires, 
usually having diameters between 0.60 and 2.00 mm, must satisfy a number 
of requirements, and more particularly the following ones: 
brass coatings must be substantially of the .alpha. (cubic) structure, 
which is necessary to obtain a satisfying behaviour to drawing as well as 
a satisfying adhesion to the steel surface; the surface further, 
composition of the coating must be such as to warrant an optimal adhesion 
to rubber as a function of the rubber batch employed; 
homogenity of the coating composition from the steel/brass interface to the 
brass/rubber interface is required; 
a coating thickness between 0.1 and 3.5 .mu.m is required, which thickness 
warrants the possibility of drawing the wire and, after drawing and 
stranding, assures a continuous coating of the steel surface so as to 
avoid the presence on that surface of unprotected areas and therefore 
corrosion risks. 
As is well known, brass-coated wires of the type mentioned above are 
obtained at the present time through a manufacturing procedure comprising 
the following operations as successive steps: 
(a) heat treatment at about 1000.degree. C. of a steel rod (of a suitable 
composition--C about 0.7%; Mn about 0.6%--and of perlitic structure) and 
drawing of said rod to the diameter required; 
(b) heat treatment within a furnace at 1000.degree. C. of the steel wire 
(which is kept moving at a speed between 10 and 50 m/min), of the same in 
a molten lead bath at 500.degree.-600.degree. C. and electrochemically 
pickling said wire in 2M H.sub.2 SO.sub.4 at 35.degree. C.; 
(c) electroplating the following on the polished wire: (1) copper from 
H.sub.2 SO.sub.4 acid bath; (2) copper from pyrophosphate alkaline bath; 
(3) zinc from H.sub.2 SO.sub.4 acid bath; 
(d) diffusion of copper and zinc layers by heating to about 700.degree. C. 
through a heating step and a soaking and tempering step to form the alloy; 
(e) drying the wire so coated and freeing the same from the surface oxides 
by acid treatment. 
However, it is to be observed that such production process is not free from 
drawbacks, and more particularly the following ones: 
complexity of the galvanic operation plant for realizing three separate 
electroplating processes from galvanic baths which are alternately 
acid/alkaline/acid, which imposes the need for intermediate washing (hot 
washing and cold washing) to avoid pollution, due to entrainment, of the 
successive electrolytic baths; high cost and difficulties of operation of 
three separate galvanic baths, with particular reference to the alkaline 
copper plating bath whose sensitivity to the polluting agents (such as 
chloride, sulfate, ferric, plumbous ions, etc.) is very high; 
zinc losses through evaporation and oxidation during the diffusion process; 
interruptions in the continuity of the coating due to sparking originating 
from unsteady contacts during said process; 
compositional inhomogeneities of the coating through its thickness. 
The last mentioned drawback can be evidenced through AES spectrometry 
(Auger electrons spectrometry), as can be seen in FIG. 1 of the enclosed 
drawings, which illustrates results of an AES analysis of a steel wire 
coated with brass through diffusion by Joule effect. The so-called 
etching-time is reported as the abscissas, which corresponds to the depth 
from the surface, while the percentage concentration of the two alloy 
elements is reported as the ordinates. 
It is to be observed that, employing the traditional procedure disclosed 
above, the homogeneity of the coating composition depends on the duration 
and temperature of the diffusion step, as well as on the average size of 
the copper microcrystals forming the stationary phase, so that such 
homogeneity cannot be easily warranted, mainly because the copper crystal 
growth during electroplating of the metal depends on such a large amount 
of parameters (pH, stirring, temperature and composition of the bath, 
current density etc.) as to be very hard to control. 
The need for a simplification of the traditional electroplating process, 
mainly as regards the employment of a number of galvanic baths, has been 
felt for a long time, and quite precise information is available as 
regards the possibility of simultaneously electrodepositing metals having 
very different discharge potentials such as copper and zinc, for instance, 
through the use of complexing agents, which are capable of lowering such 
differences. 
With reference to that problem, the possibility has been known form some 
time of employing such agents in the so-called "alloy baths", such as for 
instance cyanide baths, baths containing variously polymerized phosphoric 
ions, or ethylenediamine or polyethylenpolyamine of various natures, for 
electroplating brass directly with no successive diffusion process, thus 
eliminating or reducing the drawbacks mentioned above. 
However, such baths were shown to be unsatisfactory both owing to the 
toxicity of the reactants and to the difficulties involved in using the 
same for a continous, high production rate operation, because said baths 
do not result in stability of the coating composition with time 
(reproducibility) and the obtainment of the desired Cu/Zn ratio in the 
coating through the simple control of the value of that ratio in the bath. 
Thus, it is clearly evident that the employment of one only alloy bath such 
as that proposed in the present invention is very important, said alloy 
bath allowing the electroplating of brass to be performed directly at the 
desired amounts and compositions in a continous operation process. 
It is interesting to note that the prior art studies as regards the 
complexing agents put into evidence, among the other things, the use of 
tartrate (S. S. Abd El Rehim and M. E. El Ayashy, Journal of Applied 
Electrochemistry, 8, (1978) 33-39; S. S. Abd El Rehim, ibidem, 8 (1978) 
569-572), which, in an alkaline environment, allows the obtainment of 
copper and zinc discharge potentials so close to one aother that the 
electroplating of such metals occurs substantially simultaneously. 
Indeed, it has been observed that the predominant ionic species in the 
solution, in the presence of tartrate ion (C.sub.4 H.sub.4 O.sub.6.sup.2-) 
and at alkaline pH's, are those which originate from the complexing 
equilibriums balanced by the following constants: 
______________________________________ 
for copper Cu(OH).sub.2 C.sub.4 H.sub.4 O.sub.6.sup.2- 
Ki = 7.3 .times. 10.sup.-20 
pH .gtoreq. 10 
for zinc Zn(OH)C.sub.4 H.sub.4 O.sub.6.sup.- 
Ki = 2.4 .times. 10.sup.-8 
pH 5.5 .div. 11 
11 Zn(OH).sub.3.sup.- 
Ki = 3 .times. 10.sup.-16 
14 Zn(OH).sub.4.sup.2- 
Ki = 2 .times. 10.sup.-13 
______________________________________ 
However, such information relates to laboratory experiments in which no 
suggestion can be seen of an operative procedure suitable for industrial 
application; in particular, no concrete teaching exists as regards the 
actual definition of a continuous operating process, and no indication 
exists that the notion has been conceived that it is necessary to set 
forth and control the value of the Cu/Zn ratio in the solution so as to 
warrant the obtainment of a given composition of the plated coating. 
Finally, no transportation seems to have been performed of the values 
obtained in the laboratory (for instance, at current density of about 0.2 
A/dm.sup.2) to the situation actually occurring in an industrial plant 
(current densities up to 40 A/dm.sup.2). 
In order to realize a continous process on an industrial scale for 
electroplating brass of the desired composition on steel wires, the 
present invention suggests a tartrate-containing alloy bath containing: 
tartrate ion: 0.8-1.5M 
copper ion: 0.3-0.6M 
zinc ion: 0.1-0.3M 
alkaline hydroxide: 1.5-3M 
with a Cu/Zn ratio of 1.5-3.5 and a density of 1.10-1.3 g/cm.sup.3 at 
20.degree. C. 
According to a preferred embodiment of the present invention, such bath 
also contains: 
ammonium chloride: 0.05-0.1M 
ammonium nitrate: 0.05-0.1M 
Preferably said alkaline hydroxide is caustic soda (NaOH) or potash ((KOH). 
The application of the bath is to be performed according to the present 
invention respecting the following conditions: 
Temperature: 25.degree.-50.degree. C. 
Current density: 5-40 A/dm.sup.2 
For the preparation of the bath (in order to avoid the presence of foreign 
ions) basic copper carbonate and basic zinc carbonate are employed, with 
removal of carbon dioxide by reaction with tartaric acid. When the 
shifting reaction has been completed, the necessary amount of potassium 
sodium tartrate (Seignette salt) is added to bring the total concentration 
of the tartrate ion to the required value. Next, sodium hydroxide as well 
as the other components are added. 
The definition of the above-mentioned critical parameters of the bath 
according to the present invention has been obtained through a set of 
experimental tests aiming at checking the possibility of using a tartrate 
bath on an industrial scale and at studying the behaviour of the bath as a 
function of such parameters. 
Said tests have been carried out with a rotating brass electrode (satured 
calomel electrode (SCE) as a reference electrode; counterelectrode: Pt) by 
means of potentiodynamic polarizaion measurements, and they gave the 
results reported in FIGS. 2-6, in which the current densities (A/dm.sup.2) 
are reported as the ordinates, and the discharge potential, expressed as 
Volts and referred to the saturated calomel electrode (SCE), is reported 
as the abscissas.

FIG. 2 of the enclosed drawings illustrates the electroplating curves of 
copper, zinc and of the alloy, from tartrate baths containing one only 
metal ion or both metal ions. As it can be clearly observed, the curve 
relating to the discharge of the alloy is intermediate between the 
discharge curves of the two single metals, so that the effective 
electroplating of brass from a tartrate galvanic bath is confirmed. 
FIG. 3 shows that, for the same ionic ratio, the discharge potential is 
shifted towards less negative values on increasing the concentration of 
the alkaline compound (NaOH). 
FIG. 4 puts into evidence that, at the same value of the Cu/Zn ratio and at 
constant alkalinity, the discharge potential is shifted, on the contrary, 
towards more negative values on increasing the concentration of the 
tartrate ion. It can be observed that the addition of an alkaline compound 
such as caustic soda, which is indispensable for the establishment of the 
alkaline conditions necessary to keep the complexing equilibriums 
mentioned above, performs the function of compensating for the shift 
towards more negative dischage potentials which results from the addition 
of the tartrate ion. 
FIG. 5 puts into evidence that the addition to the bath of increasing 
amounts of NH.sub.4 Cl, which is necessary to give the plated coating a 
brilliant yellow color, occurs with no significative changes in the 
discharge potential, whereas in the case of addition of KNO.sub.3 as a 
depolarizing agent (FIG. 6) the influence of the addition is not 
negligible. 
The analysis of the coatings obtained in all said experimental tests showed 
the possibility of electroplating brass essentially of the .alpha. phase 
and with an excellent adhesion to steel. 
The possibility of industrial application of the bath according to the 
present invention has been confirmed, among the other things, by 
experimental observations as regards the possibility of obtaining in the 
anodic process a "simultaneous" type dissolution employing brass anodes of 
the desired composition. 
Aiming at that object the selective coefficient was determined, as defined 
by the relationship: 
##EQU1## 
as a function of time, working with a tartrate alkaline solution at 
25.degree. C. and at current densities as follows: anodic current density, 
0.45 A/dm.sup.2 ; cathodic current density, 15 A/dm.sup.2. 
Results obtained are reported in the following Table 1 and they show that, 
in a bath which at 0 time is free from Cu.sup.++ and Zn.sup.++ ions, the 
dissolution is at first of a preferential type but after about 1 hour it 
becomes of a simultaneous type. 
TABLE 1 
______________________________________ 
Values of the selectivity coefficient Z at 25.degree. C. with brass 
anodes (Cu 67.5%, Zn 32.5%) at various concentrations of the 
nitrate ion. 
Z VALUES 
time 3 g/l 6 g/l 9 g/l 
(min) NO.sub.3 NO.sub.3 
NO.sub.3 
______________________________________ 
0 3.56 5.4 2.71 
4 1.56 1.87 1.51 
8 1.40 1.40 1.30 
10 1.27 1.29 1.30 
20 1.24 1.31 1.32 
30 1.16 1.17 1.17 
40 1.06 1.14 1.25 
50 1.09 1.10 1.06 
60 1.06 1.10 1.02 
75 1.03 1.11 1.05 
90 1.03 1.03 1.01 
120 0.99 1.00 1.01 
180 0.96 1.00 0.97 
______________________________________ 
Analyses were carried out by atomic absorption spectrophotometry (AAS). 
FIG. 7 shows the behaviour of the selectivity coefficient Z (as the 
ordinates) as a function of time, for the dissolution of brass anodes 
(68/32) in a tartrate alkaline solution (the curve being averaged over 65 
determined values--KNO.sub.3 from 3 to 9 g/l--25.degree. C.--cathodic c.d. 
15 A/dm.sup.2, anodic c.d. 0.45 A/dm.sup.2). The concentration of the 
depolarizing agent (the NO.sub.3.sup.- ion), which allows the copper to be 
dissolved at the anode, is not a determining factor at least in the range 
from 3 to 9 g/l. 
Another factor which according to the present invention was shown to be of 
fundamental importance for setting forth the effective industrial 
potentiality of the process is the dependence of the coating composition 
on the temperature as well as on the Cu/Zn ratio in the solution. 
With reference to such result, FIG. 8, in which the copper percentage 
concentration in the coating (Cu %) is reported as the ordinates and the 
cathodic current density (A/dm.sup.2) is reported as the abscissas, shows 
the curves relating to coatings obtained from various galvanic baths at 
different values of current density, operating at a given temperature 
value (baths H,L) or at different temperature values (bath E). Baths H, E 
and L are selected among those which are typical of the present invention 
and which are reported in the following in Table 2. 
Such curves show that the composition of the coating is not fundamentally 
affected by the values of the current density but, for a certain value of 
the Cu/Zn ratio, it depends quite remarkably on temperature. 
FIG. 9 shows, as the ordinates, the composition of the coating (on the 
left) given as the copper percentage concentration in the alloy, and (on 
the right) the cathodic current yield (.eta..sub.cat) at various Cu/Zn 
ratios at 30.degree. C. and at 16 A/dm.sup.2, as functions of the Cu/Zn 
ratio in the bath. 
The cathodic current yield is always quite lower than 1; operating at a 
current density of 16 A/dm.sup.2 and at 30.degree. C., the value of such 
parameter is close to 0.5. However, such parameter is to be previously 
determined for each composition as well as for each set of operative 
conditions. 
Finally, it was established that the coating so obtained has a composition 
which is stable during the time of employment of the bath, as is clearly 
shown in FIG. 10, wherein the copper percentage concentration in the 
coating is reported as the ordinates and the ratio between the amount of 
the alloy coated and the initial amount of copper and zinc in the bath (R) 
is reported as the abscissas. The behaviour of the coating composition on 
increasing the total amount of the current passed through the galvanic 
cell is represented by a horizontal straight line. 
The bath according to the present invention can be operated quite easily as 
it substantially requires just the continuous control of the following 
parameters: 
Cu/Zn ratio in the solution 
anode composition 
nitrates and ammonia 
free and total alkalinity 
operating temperature 
galvanic bath density. 
Thus it is necessary to employ thermoregulated plants and an analytical 
service station equipped with atomic absorption spectrophotometer and with 
potentiometers having electrodes specific for the nitrate ions and for 
ammonia. Such analyses can be performed directly in the galvanic bath at 
its own service concentration introducing any suitable corrections in a 
continuous way by means of metering pumps, so that all parameters pointed 
out above are kept constant with time. 
Within the class of tartrate-containing alloy baths for electroplating 
brass on steel wires, the galvanic baths which were shown to be 
particularly preferable according to the present invention are those 
having the compositions shown in the following Table 2. 
TABLE 2 
__________________________________________________________________________ 
Compositions of some typical baths 
Abbreviations of the bath 
H E L M N 
Reactant g/l M g/l M g/l M g/l M g/l M 
__________________________________________________________________________ 
tartaric acid 90 111 83 90 97.5 
Seignette salt 
112 203 127 112 99 
total tartrate ion 
1 1.46 1 1 1 
bas.CuCO.sub.3 (titr. 56% Cu) 
45.4 55.6 45.4 45.4 45.4 
corresp. to Cu ion 
0.4 0.49 0.4 0.4 0.4 
bas.ZnCO.sub.3 (titr. 57.4% Zn) 
22.6 28 17 22.6 28.3 
corresp. to Zn ion 
0.2 0.25 0.15 0.2 0.25 
NaOH 80 2 120 3 100 2.5 
100 2.5 
120 3 
NH.sub.4 Cl 2.7 0.05 
-- -- 2.7 0.05 
2.7 0.05 
2.7 0.05 
NH.sub.4 NO.sub.3 
4.0 0.05 
-- -- 4.0 0.05 
4.0 0.05 
4.0 0.05 
Cu/Zn ratio 2 1.96 2.67 2 1.6 
density (g/cm.sup.3) 
1.175 1.259 1.212 1.204 1.222 
__________________________________________________________________________ 
As already pointed out above, the tartrate-containing alloy baths according 
to the present invention can be specifically employed in the continuous 
production of steel wires coated with brass, which are designed for the 
production of radial tires. Such baths allow said brass coating operation 
to be carried out by means of one only electroplating process, performed 
on steel wires pre-treated according to the commonly employed techniques, 
the electroplating operation being followed by water washing at room 
temperature. 
The coating shows a constant composition throughout its full thickness as 
is proved by the AES spectrometry. With reference to that, FIG. 11 shows 
an AES spectrum of the surface of a steel wire of 1.4 mm size, coated with 
brass of 2 .mu.m thickness and of composition 64.2% Cu and 35.8% Zn. Such 
coating was obtained at 30.degree. C. with a current density of 16 
A/dm.sup.2. The derivative (dN/dE) of the number of electrons with respect 
to the kinetic energy of the same is reported as the ordinates, and the 
values of the kinetic energy E.sub.kin are reported as the abscissas 
expressed as eV. The complete composition of the coating at its surface 
can be read off such diagram. 
FIG. 12, in which the ordinates show the values of a "peak-to-peak" 
parameter proportional to the concentration and the abscissas show the 
so-called etching time in seconds, illustrates the results of an AES 
analysis of the same wire, from which the stability of the alloy 
composition throughout its full thickness can be clearly appreciated. The 
initial variations in the composition should not be taken into account as 
they depend on the concentration drop of C and O, which are present at the 
surface as polluting agents. 
The coating shows a very high degree of continuity as well as a very high 
corrosion resistance. The corrosion rate was shown to be of 0.073 mm/year 
at 20.degree. C. in a 0.1M HCl solution added with 1 g/l NaCl and 0.3 g/l 
CaCl.sub.2. 
Under the same conditions the average corrosion rate of the steel wire is 
of 0.63+0.1 mm/year. 
Adhesion to rubber was shown satisfying and substantially corresponding to 
the common standard. 
The present invention will be disclosed in the following by an example 
referring to a preferred procedure for the production of a steel wire 
coated with brass by means of a tartrate-containing alloy bath. 
EXAMPLE 
The brass coating experiments were carried out on a small-scale pilot plant 
(scale about 1:15) with speeds between 1 and 5 m/minute. Such pilot plant 
consists of a series of galvanic bath tanks (PVC, titanium bolts and nuts 
and titanium contacts) with weirs and of suitable lenghts, each bath tank 
being fed independently by its own thermostat (.+-.0.5.degree. C.); the 
plant also comprises an unwinding device and a winding device, both of 
them being operated with controlled linear velocity (.+-.1 cm/minute). The 
electrical circuits are fed by AMEL galvanometer/potentiometer controllers 
(.+-.1 mA). 
A typical example of the procedure followed in the experimental tests is as 
follows: 
steel wire samples (perlitic structure, 0.7% C, 0.6% Mn, 1.4 mm diameter) 
coming from previous mechanical and heat treatments, were coated with 
brass in the pilot plant mentioned above according to the following 
operations: 
washing with water at room temperature 
brass coating 
washing with water at room temperature 
drying with compressed air 
Employing the composition bath L and Cu/Zn anodes (67/33% by weight) under 
the following operative conditions: 
temperature: 35.degree..+-.0.5.degree. C. 
current density: 20 A/dm.sup.2 
wire speed: 1.+-.0.01 m/minute 
the following coating was obtained: 
amount: 5.09 g/kg 
composition: Cu, 65.6%; Zn, 34.5% 
adhesion: 80.5.+-.15 kg 
standard: 80.9.+-.12 kg 
(procedure for the ASTM adhesion test: STANDARD rubber batch, 155.degree. 
C., 35 minutes 1/2 inch immersion, 1/2 inch pull hole). 
FIG. 13, in which the peak-to-peak parameter is reported as the ordinates 
and the etching-time expressed in minutes is reported as the abscissas, 
shows the results of an AES analysis of the coating obtained according to 
the present example. More particularly, the analysis can be appreciated of 
the coating at the point corresponding to the brass/steel interface, for 
an etching-time of about 27 minutes, that is at 2 .mu.m depth. 
The present invention has been disclosed with particular reference to some 
of its preferred embodiments but it is to be understood that modifications 
and changes can be introduced by those who are skilled in the art without 
departing from its true spirit and scope.