PROCESS FOR ELECTROPLATING STEEL WIRES AND COATED WIRES THUS PRODUCED

Abstract: A process for the production of steel wires coated with a thin layer of an alloy containing copper, zinc, nickel and lead, said process comprising the following series of operations: a) simultaneously electrodepositing copper and lead from an alloy bath on a steel wire, which has been previously subjected to the usual preliminary treatments, b) electrodepositing zinc from a zinc plating bath, c) electrodepositing nickel from a nickel plating bath, d) solid state diffusion by Joule effect of the layers obtained by electroplating in steps a), b) and c); and the products so obtained, which can be employed in the production of reinforcing structures for elastomeric material articles, in particular for tires.

Description: PROCESS FOR ELECTROPLATING STEEL WIRES AND COATED WIRES THUS PRODUCED ¥* The present invention relates to a process for electroplating ' steel wires as well as to the products thus obtained, which products can be employed for the manufacture of reinforcing structures for ela¬ stomeric material articles, and more particularly for tires. More particularly, this invention relates to a process for coating steel wires with a thin layer of an alloy, containing copper, zinc, nicke and lead, by electroplating and subsequent diffusion by Joule effect. 10 In the production of reinforced elastomeric materials, in parti¬ cular of radial tires, wire cords have been employed up to the present time, as is well known, which cords are made up of steel wires coated by electroplating with brass, preferably of a composition between Cu/ Zn = 60/40 and Cu/Zn = 70/30 {% by weight) at the amounts, between 2 15 and 8 g/Kg of steel and of 1-2 μ thickness for a wire diameter be¬ tween 0.75 and 1.40 mm. Such wires are then wet-drawn according to a chipless procedure and employed for the production of said cords. It is also well known that such cords, in addition to a high ultimate tensile stress and to a high flexibility, also show, during 20 the curing process with a standard mix, an adhesion to rubber (or oth¬ er elastomeric material) which is much larger than the value of ad¬ hesion that can be ascribed to the mere friction effect. At the pre¬ sent time such property is considered to stem substantially from the o formation and growth of films of some hundred A thickness of copper 25 and zinc sulfides at the rubber/brass interface, as a consequence of the chemical reactions occurring during vulcanization. On the other hand, it.has been observed that the adhesion be¬ tween steel and the coating is also of the same importance. Thus it is clearly evident that any physical or chemical phenomenon, such as for instance corrosion, which alters the state of surfaces, af¬ fects the cord/elastomer adhesion negatively. Such drawback manifests .itself much frequently, because rubber mixes commercially employed in the production of tires are hardly ever anhydrous, and give rise during vulcanization to the formation of a ZnO film, in addition to sulfides films, and next, owing to material aging, to the formation of hydroxides with the consequent definitive separation of the cord from the rubber. This was shown by a prolonged treatment with overheated steam (120°C or more), in the presence of chlorides. In that case the examination of the state of surfaces put into evidence the presence of rust at the interface between steel and coating. On the other hand, it is well known that car and truck tires are subjected during employment to severe mechanical stresses,, espe¬ cially at the highest speeds or in off-road runs. As the safety of passengers and of load depends on tire re¬ liability, it is necessary that the cord/elastomer adhesion be particularly strong, not only at the very beginning, but also during the full tire life. Thus it is a main object of the present invention to realize a kind of coating for steel wires that allows an improved starting adhesion cord/elastomer to be obtained and that warrants its sta¬ bility in time, as a result of the increase in the corrosion re- sistance. The coating is also to warrant the possibility of mechan¬ ically working the wire .so treated, and to allow a good behaviour to drawing, which must be chipless and with no remarkable changes in the surface composition and in the continuity of the coating itself. Aiming at obtaining such double object the present invention suggests a coating made up of copper, zinc, nickel and lead .electro- deposited as successive layers'on steel wires and then transformed into an alloy through solid state diffusion by Joule effect. It is worthwile observing that the proposal of the present invention is not limited to the selection of the components men- . tioned above and of their percentage amounts in the alloy, but, in the formation of said coating, the present invention also sets forth the order and the manner in which the electroplating process onto the steel support is to be carried out. Namely, copper and lead are electrodeposited simultaneously in a first step, and next the electro- deposit of zinc is performed, followed by the electrodeposit of nickel. In such a way a four-component coating is obtained which, in addition to be unprecedented in the technical literature, also shows a- very high corrosion resistance due to the presence of nickel, as well as a very high mechanical workability, which is ensured by the presence of lead. The adhesion between said coating and the rubber, which adhesion in the case of the known brass-coated wires stems from the formation of copper and zinc sulfides films, is increased in the coating accord¬ ing to the present invention by the further formation of a nickel sulfide film. The production of steel wires coated with a thin layer of the alloy according to the present invention can be carried out in any plant designed for continuous traditional type working processes, both preliminary and successive to the electroplating operation. More particularly, steel wires of diameters between 0.75 and 1.4 mm, obtained by dry-drawing of a steel rod of 5.5 mm diameter are , subjected before the electroplating process to a heat treatment car¬ ried out at about 1,000°C in a furnace, and to a patenting process at a temperature between 500 and 600°C. After such treatment, the wire surface is drastically cleaned by passing it through a series of tanks containing 2M H ? S0. at 35°C, in which said wire takes on in turn positive and negative polarities with current densities between 2 700 and 800 A/dm . After such treatment and a suitable washing of the wire surface, such surface is brilliant and free from impurities and oxides, as can be proved by a check under a metallographic icro- scope. As regards the final treatment, the wire coated according to the process of the present invention is cleaned by removing the surface oxides by means of a washing operation with 10% H3PO4 at 40°C and then with water. Hot air drying is then performed, followed by winding of the wire and the transfer of the same to the drawing and stranding departments. The basic feature of the present invention ' lies thus in the way of carrying out the electrodeposit operations of the above- mentioned alloy components on the steel wire support and of perform- ing the diffusion process by Joule effect. More specifically, the present invention concerns a process for the production of electroplated steel wires, intended for the manufac¬ ture of reinforcing structures for elastomeric material articles, especially for tires, said process being characterized by the follow- ing succession of operations: a) simultaneously electrodepositing copper and lead from an alloy bath on a steel wire, which has been previously subjected to the above-mentioned preliminary treatment, b) electrodepositing zinc from a zinc plating bath, c) electrodepositing nickel from a nickel plating bath, d) solid state diffusion by Joule effect of the electroplated layers obtained in the steps a), b) and c), washing steps being provided after each of the electroplating oper¬ ations performed in the steps a), b) and c). Preferably the electroplating step a) of copper and lead is carried out employing a pyrophosphate alloy bath of a new composi¬ tion within the limits set forth in the following table 1, wherein the pertinent operative parameters are also reported. TABLE 1 Composition of the alloy bath for the simultaneous electrodeposit of copper and lead and the pertinent operative conditions Composition of the bath ionic species concentration (moles/liter) maximum minimum 5- 10 " ■ 3 0,1 α,2 0,1 2 7 NH. α,ι α NO. 0,1 0 CU(HP 2 O 7 ) 2 ~ C P 2 Q 2 7 " trace amounts Uperative conditions t = 50 + 2°C; pH = 8.8; Cu/P ? O y ratio = 1.7 by weight; current density 2 10-12 A/dm ; countercurrent stirring. A typical composition example according to the present inven¬ tion, which composition has been used in many experimental tests, is the following one: ^P-O " 3H-0, 360 g/1; Cu 2 2 0 7 .5H 2 0, 90 g/1; q.s. to pH=8.8 (at 25°C); NH 3 , 3 g/1; The amount of lead contained in the coating depends: a) on the amount of the Pb(P ? 0,) ~ ion contained in the bath; b) on the current density, as can be observed from the results of a series of exper¬ imental tests reported in Figure 1 of the enclosed drawings. In said Figure, the ordinates show the concentrations of metal ion in the coating as a function of the concentrations of the ion in the galvanic bath as the abscissas (g/Kg), for some different values of the current density (A/dm ), and more exactly: 8.6 (1); 13.2 (2); 16.1 (3); 19.3 (4). Thus it can be clearly seen that the concentration of metallic lead in the coating is proportional to the concentration of the ion in the. galvanic bath and it decreases on increasing the current density. Accordingly, by previously stating the value of the operative current density, it is quite easy to obtain, by interpolation of the curves plotted in Figure 1, the required value of the Pb(P ? 0 7 ) concentration that results in a coating having a definite percentage of such metal. As regards the anodic process, the anodes employed are made up of electrolytic copper. Since in the case of the simultaneous electro- deposit (co-electrodeposit) of copper and lead the anodic current yield could be higher than the cathodic current yield referred to copper, an undesired increase in the concentration of the cupric ion can be avoided employing a parallel-connected electrolysis tank where¬ in the discharge of the excess copper is performed between inert metal electrodes. The concentration of the lead ion is to be monitored and suitably restored through the addition of the most suitable salts; in all experimental tests the nitrate salt was employed (the nitrate . ion, which performs the function of a depolarizing agent, undergoes a constant consumption due to cathodic reduction; lead nitrate ad¬ ditions make up fully or partially for such consumption). As regards the zinc electrodeposit step b), an acid bath containing sulfuric acid is preferably employed, which bath has the composition reported in Table 2, wherein the pertinent operative parameters are also put into evidence. TABLE 2 Zinc plating bath Ionic species Concentration (moles/liter) maximum minimum Zn 2+ 2.0 0.7 SO 2' 2.5 1.0 Operative conditions tt == 2200--4400°°CC;; ppHH == 3.0 + 0.5; current density 10-20 A/dm m ; counter- current stirring. Anodic and cathodic current yields are close to 100%. Anodes are made up of 99.9% zinc. The electrodeposit of nickel (step c)) is carried out preferably from an acid bath having the composition shown in Table 3, wherein the pertinent operative conditions are also put into evidence. TABLE 3 Zinc plating bath ./. Chemical species concentration (moles/liter) maximum minimum Ni 2+ _ _ 1.5 0.7 SO 2" 1.3 0.5 Cl " 0.5 0.1 H 3 B0 3 0.6 0.3 Operative conditions t = 30-60°C; pH = 3.5 +_ 0.5; current density 2-10 A/dm ' coun- ] Q tercurrent stirring Anodes are made up in each case of 99.9% pure nickel foil. A specific example of the nickel plating bath according to the invention is as follows: NiS0..7H 2 0, 330 g/1; NiC .δH D, 45 g/1; H BO,, 38 g/1; pH = 3.5; t = 50 + 1°C; (specific conductivity: 70 S 15 at 50°C). As already mentioned'above, the various layers obtained in the galvanic electroplating 'operations disclosed previously are subjected, • after a final washing with cold water, to a reciprocal diffusion by Joule effect (d), by applying to the wire a suitable voltage value, 20 for instance by means of three stainless steel rolls; thus a first heating step is realized and afterwards a second soaking step is performed; the ratio between the length of the first section and that of the second one is of about 1:2. The values of tension and current density required for a correct 25 diffusion process depend: 1) on the wire diameter; 2) on the thickness of the coating; 3) on the running speed of the wire; 4) on the absolute and relative values of the lenghts of the heating and soaking sections. Such values, for the coatings according to the present invention disclosed in the 30 following, are not much different from those usually employed for the coating of steel wires with Cu and Zn. However, they are to be determined empirically by checking, at a steady-state condition, the obtainment of the temperature value that ensures a complete homogenization of the coating, through the observation of the coating colour which becomes bright yellow in the absence of nickel, whereas at increasing nickel concentrations it takes on lighter tones (till the silver white tone). For indicative purposes, the following values are reported which were obtained in pilot plant experimental tests and which showed optimal: A = 27.5 + 0.5; V = 2.1 _ 0.1 at running speeds of 2 m/min.; wire diameters 1.4 mm; coating of 4.2 g/Kg, containing 3% Ni. Coatings so obtained have an average composition between the following minimum and maximum values: Cu 59-70% Zn 30-33% Ni 0.75-8% Pb ' - -0.2-2% and, in the case-of application to steel wires under the operative conditions mentioned above, they give rise to final products having the characteristic features of the present invention. The following Table 4 shows for exemplification purposes the results in a number of experimental tests performed on steel wires of 1.3 mm diameter coated with a minimum amount of 3.7 g/Kg and a maximum amount of 4.4 g/Kg of the alloy of the average composition mentioned above. Such coating shows rubber adhesion values always comparable to those of a 69/31 brass coating (such coating being considered as a reference and being obtained under the same operat¬ ive conditions); at nickel concentrations between 0.75 and 3.0% the values of the adhesion strength to rubber show a sharp improvement. (Table 4 on the next page) TABLE 'I Analytical results obtained with a l mm diameter wire Test % composition of the alloy alloy Adhesion No. amount CR CRS ΔCRS Cu Zn Ni Pb g kg Kg Kg/mm Kg 0 68.8 31.2 - - 3.76 167 111.6 -10.8 56.5 9.5 1 65.8 32.6 0.75 0.85 3.91 181 120 - 2.4 64.6 16.2 2 65.0 32.0 2.1 0.90 3.87 185 125.5 + 3.1 7.1.6 7.7 3 64.1 32.0 3.0 0.90 4.05 188 123.7 + 1.3 62.6 10.0 10 4 64.7 30.8 3.6 0.90 4.04 189 124 + 1.6 55.6 7.2 5 62.6 32.0 4.5 0.90 4^16 189 124 + 1.6 53.2 11.2 6 61.2 31.5 6.4 0.90 4.23 189 123.8 + 1.4 59.4 11.7 7 60.0 32.1 7.0 0.90 4.19. 189 124.3 + 1.9 49,2 9.8 8 59.0 32.6 7.6 0.90 4.40 189 122.9 + 0.5 51.5 10.2 15 Copper and nickel determined by AA, 2 analytical determinations for each one; g/kg 2 deter ¬ minations; CR: ultimate tensile stress; CRS: specific ultimate tensile stress; CRS referenc 122.4; adhesion 15 detj≡rminations, / 2 inch immersion (about 1.25 cm), time 35', temperature 155 + f°C. Rubber mix:' the standard mix; (T:standard deviation. The analysis of the surface of the sample No. 3 carried out by XPS spectrometry shows the lead signal at 144 eV (E. .). Such signal is also present after removal of a 400 A layer by bombard¬ ment with 5 keV Ar , as can be observed in Figure 2 of the enclos- ed drawings, wherein the abscissas show the bond energies (E. .) in eV and the ordinates show the number of electrons (N,-. . A scanning Auger spectrometry analysis of samples 3 and 7 gave the following results: Sample 3 - Sample 3 consists of an alloy (4.05 g/kg) contain- ing 3 % nickel and 0.90 % lead. Carbon and oxygen are present on the surface due to atmospheric pollution, and trace amounts of chlorine and sulfur are also present. The surface of the sample is well coated. The composition profile in the first 0.2 μn thick¬ ness layer puts into evidence a remarkable homogeneity with respect to Cu and Zn (0 and C only are present on the surface, and their » signals disappear after about one hundred A); iron is present just ' in trace amounts at the detectability limit. Sample 7 - The chemical analysis of the coating gives the fol¬ lowing average values of nickel and lead concentrations: Ni, 7 % of the total amount and Pb 0.91 % of the total amount. The investigations of the state of the surface of sample 7 car¬ ried out by Secondary Electron Micrography (SEM) at 500 X and 2000 X shows the presence of stains which are irregular both as regards their shapes and as regards their depths, as well as of longitudinal striations. A point-to-point analysis of the first kind of stains (which are on the other hand very rare) showed that the same consisted of sur¬ face scales of carbon materials, as the carbon (C 1s) and oxygen (0 1s) signals only were shown to be present in the same. On the coπtary, a point-to-point analysis of the nearby zones showed that the coating is made up of Cu, Zn, Ni and Pb. Figure 3 shows the compositional map of an area comprising one of the observed longitudinal striations. Such analysis was performed for: a): copper (peak at 919 eV), b): nickel (peak at 713 ev), c): zinc (peak at 992 eV); it is evident that copper is absent from said striations whereas only nickel is present therein. On the contrary zinc and lead are spread almost homogeneously everywhere both inside and outside said longitudinal striations. Accordingly, it can be concluded-that the surface coating of sample No. 7 does not consist of a quaternary alloy but that a full separation of nickel occurs, which nickel has formed an al- loy with zinc but not with copper. . The decrease in the adhesion already remarked in Table 4 for sample 7 can be presumably ascribed to such fact, which occurs for a nickel percentage of 7% or higher. This is confirmed by the adhesion result relating to sample 8 (Ni: 7.6 %), which also is lower than the value of the standard sample. Moreover, a compositional profile of sample No. 7 carried out by Auger spectrometry, starting from the coating surface and going towards the coating steel- interface showed the presence of signi- ficative amounts of oxygen down to about one thousand A from the surface. The compositional profile of oxygen follows very closely the compositonal profile of zinc so that the presence can be suspected of oxide traces also within the mass of the coating itself. It is to be observed that the steel wire coated with an alloy amount between 2 and 8 g/kg in the presence of 0.75-7 % Ni and 0.2-2 % Pb can be very easHy drawn.. The wire, examined under the electron microscope or by scanning Auger spectrometry after drawing shows a surface homogeneously coat¬ ed on which quite a negligible number of defects are present. Analytical tests and adhesion tests carried out on a standard cord made up of four wires of 0,25 mm diameter (average thickness of the coating 0.3 jim) gave results which are reported in Table 5. • TABLE 5 Wire cords of steel coated with Cu/Zn/Ni/Pb Analyses and adhesion strengths Chemical analysis Tesf 1.4 mm diam. wire Cord 2 + 2X0.25 STD 4.05 62,5 37,5 - - 3.32 69.2 30.8 — m 9 4.34 01. Q 34.2 3.9 0.9 3.92 65.5 31.4 2.1 1.0 10 4.55 62.3 32.0 4.9 0.8 3.46 63.7 32.4 2.8 1.1 11 4.49 64.1 33.4 1.6 0.9 3.67 63.3 32.4 3.2 1.1 12 4.37 64.1 32.5 2.4 0.9 3.48 64.3 32.4 2.2 1.1 13 4.50 62.4 33.8 2.9 0.9 3.64 64.4 33.2 1.3 1.1 TABLE 5 continued Test specific ultimate tensile adhesion No. stress as obtained (1) after aging(2) CRS ΔC S . Kg & Δ K* G STD 122 ,4 - S#6 3βS " 2 ,1 3#5 9 120.1 -2,3 * 8 4 '° +2«4 2 4 2 +0.9 1) Average over 18 determinations of two different vulcanizations: reference: cords 2 +2X0.25 with a 69/35 brass coating, 4.2 g/kg (rubber mix: the standard mix, 1/2 inch immersion (about 1.25 cm), vulcanization 155 - 1°C- for 35 minutes). 2) treated with steam for 12 hours on 10% NaCl solution at 120°C. Samples examined had a percentage amount of lead close to 1% and a percentage amount of nickel between 1.3 and 4.0%. All cord samples show after vulcanization (the standard mix was employed according to the ASTM standard procedure for such tests) a value of the adhesion strengths that is sharply higher than the referen- ce sample value; such improvement in the adhesion strength in some instances is higher than 10%. Moreover, after steam treatment under pressure of a 10% NaCl -solution at 120°C for 4 hours, the adhesion loss is always much lower with respect to the standard sample with a gain in the case of sample No. 11, containing 3.2% Ni and 1% lead, of about 20%. The present invention has been dislcosed with particular re¬ ference to some of its specific embodiments but it is to be un¬ derstood that modifications and changes can be introduced in the same by those who are skilled in the art without departing from its true spirit.and scope.

Claims: CLAIMS : 1. A process for the production of electroplated steel wires, characterized by the following series of operations: a) simultaneously electrodepositing copper and lead from an alloy bath onto a steel wire which has been previously subjected to the usual preliminary treatments, b) electrodepositing zinc from a zinc plating bath, c) electrodepositing nickel from a nickel plating bath, d) solid state diffusion by Joule effect of the layers ob- 0 tained by the electroplating steps a), b), and c), a washing step being provided after each one of the electroplat¬ ing operations a), b) and c). 2. A process for the production of electroplated steel wires according to claim 1, wherein said simultaneous electrodeposit of _. 5 " copper and lead of step a) is carried out employing a.pyrophosphate alloy bath of the following composition: Cu( 2 0 7 ) 2 " 0.3 - 0.7 moles/1 Pb(P 2 0 7 ) 2 5" 0.25 x 10 3 -10x10 "3 moles/1 - P 2 0 7 0.1 - 0.2 moles/1 0 HP 2 0 7 3~ 0.1 - 0.2 moles/1 NH 3 0-0.1 moles/1 NO " 0-0.1 moles/1 Cu(HP 2 0 7 ) 2 " , CuP 2 0 7 " , H 2 P 2 0 7 " , b( 2 0 ? ) 2" trace amounts; Cu/P 2 0 7 ratio = 1/7 by weight; pH = 8.8; the bath being kept under the following operative conditions: temperature = 50 - 2°C current density = 10-20 A/dm- 3. A process for the production of electroplated steel wires according to claims 1 or 2, wherein said zinc electrodeposit - step b) is carried out employing an acid bath of the following compositon: Zn 2+ 0.7-2.0 moles/ 1 S0 4 2" 1.0-2.5 moles/1 said bath being kept under the following operative contions: temperature = 20 - 40°C 2 current density = 10-20 A/dm countercurrent stirring. 4. . A process for the production of electroplated steel wir- es according to claims 1, 2 or 3, wherein said nickel electrode¬ posit of step c) is carried out from an acidbath of the following compositon: Ni 2+ 0.7-1.5 moles/1 S0 4 2" 0.5-1.3 moles/1 CI " 0.1-0.5 moles/1 H BO. 0.3-0.6 moles/1 pH 3.5 - 0.5 said bath being kept under the following operative conditons: temperature = 30-60°C 2 current density = 2-10 A/dm countercurrent stirring. 5. A pyrosphosphate alloy bath for electrodepositing copper and lead on steel, said bath being characterized in that it has the following composition. Cu( 2 0 7 ) 2 " 0.3-0.7 moles/1 Pb(P 2 0 7 ) - 0.25x10 '3 -10x10 '3 moles/l Pb 2 0 ? 0.1-02 moles/1 HP 2 0 7 3" 0.1-0.2 .oles/1 NH 0-0.1 moles/1 NO ~ 0-0.1 moles/1 Cu(HP 2 0 ? ) " CuP 2 0 7 2" , H P-0- 2" , Pb( 2 0 y ) 2 ~ trace amounts; Cu/ PJ)- ratio = 1/7 by weight; pH = 8.8 6. Coated steel wires obtained by the process claimed in any one of claims 1-4, said wires being characterized by an al¬ loy coating containing copper, zinc, nickel and lead, of the following average composition: Cu 59-70 % Zn 30-33 % Ni 0.75-8 % Pb 0.2-2 % .

Inventors: AMBROSIO LEO, DE FILIPPO DOMENICO, AMBROSIO, LEO, DE FILIPPO, DOMENICO;
Applicants: CONSIGLIO NAZIONALE RICERCHE, GENCORD SPA, CONSIGLIO NAZIONALE DELLE RICERCHE, GENCORD S.P.A.
Application year: 1986
Main classification: C25D5
IPC codes: C25D3, C25D5