Electrolytic process for coating a metal element with a layer of brass

Abstract: A process for electrolytic codeposition of copper and zinc on a metal element wherein the electrolytic bath consists of an aqueous solution of at least a bivalent copper salt, at least a zinc salt and an alkali pyrophosphate, and an amino acid compound of formula wherein Y, R 1 , and R 2 have the meanings shown in the description, or an acid-addition salt of said amino acid compound, at a temperature of from 40°C to 55°C.

Description: The present invention relates to an electrolytic process for coating a metal element with a layer of brass, and the use of some metal elements thus coated in reinforcing structures of articles made of vulcanized elastomeric material. More in particular, the present invention relates to the electrolytic codeposition of a layer of copper and zinc on a metal element such as, for example, a steel wire. It is known that some articles of manufacture made of vulcanized elastomeric material are reinforced by embedding suitable metal structures in an elastomeric matrix. Generally, said metal structure is made of steel wires and/or cords. However, steel, that is the preferred material due to its mechanical properties, has the disadvantage of not adhering sufficiently to the vulcanized elastomeric material. In order to obtain a good adhesion to the elastomeric material the steel is therefore usually coated with a layer of a suitable material. The preferred coating material is brass. In this case adhesion is enhanced by the formation, during the vulcanizing process, of disulfide bridges (-S-S-) between the elastomeric matrix and the copper that coats, as a component of brass, the reinforcing metal element. The brass layer on the metal element must be as compact and homogeneous as possible both to ensure a uniform formation of disulfide bridges, and thus a uniform adhesion of the elastomeric matrix to the reinforcing metal structure, and to prevent the triggering of corrosive reactions in the possible areas of the reinforcing metal structure that are not sufficiently coated. In fact, when adhesion is not sufficiently uniform, it occurs, right from the start of use, some detachements of the elastomeric matrix from the reinforcing metal structure that propagate rapidly along the reinforcing metal structure and quickly render the article unreliable. As regards corrosion, the problem is particularly felt in tires. In fact, it frequently occurs, especially in tires of heavy vehicles used on rough and stony ground, that the tread is cut even deeply thus exposing the reinforcing metal structure to contact with water and dampness. This promotes the start of reactions that corrode the metal structure all the more quickly the less compact and uniform is the brass coating and they propagate throughout the reinforcing metal structure rendering the tire unsuitable for use. The known processes for coating a metal element with a layer of brass can be divided into two families: codeposition and diffusion. The codeposition processes are characterized by the electrolytic codeposition of copper and zinc. The conventional process of codeposition that gives the best results, in terms of compactness, uniformity and homogeneousness of the deposited layer of brass, is that which involves the use of alkali cyanide baths containing a complex cyanide of bivalent copper, a complex cyanide of zinc, free cyanide, and other compounds acting as brighteners, such as carbonates, ammonia, etc.. This process is known as "cyanide process". It is, however, well known that cyanides imply serious problems of toxicity for the operators and their disposal involves costly and not altogether satisfactory techniques. In order to overcome these drawbacks, many electrolytic baths have been evaluated having various compositions. For examples, sulfates, tartrates, oxalates, pyrophosphates and ethylenediamine, or even mixed systems such as chlorides/oxalates. However, the results achievable with the cyanide process have not yet been attained. It has also been proposed to add histidine to a pyrophosphate bath. This system has given satisfactory results when carried out with a low density of cathode current and at 30°C. However, when the density of cathode current exceeds 3 A/dm 2 , the deposit has a powdery appearance and a burnt colour (Y. Fujiwara and H. Enomoto "Composition, Structure and Morphology of Cu-Zn Alloy Deposits from Pyrophosphate Baths", Plate and Surface Finishing, January 1993, pages 52-56). Substantially the same process is disclosed also by JP-A-63 203 790 (Shimizu KK). A second way by which attempts have been made to replace the cyanide process are that of the diffusion processes. These methods comprise the electrodeposition of one or two layers of copper on steel, followed by the electrodeposition of a layer of zinc and by a heat treatment whose purpose is to diffuse zinc in the layers of copper, thus forming a layer of brass. However, even the processes of this family have several drawbacks. A first drawback is that each electrodeposition step requires its own tank and this involves a costly and bulky plant. A second drawback is that this process requires a careful selection and control of several parameters in a plurality of tanks. A third drawback is the cost of the energy required in the heat treatment that causes the diffusion. A fourth drawback is that the heat treatment causes some worsenings in the mechanical properties of steel; in particular the ultimate tensile strength shows a reduction of some 5 percent. A further drawback of these processes is that the heat treatment causes the formation of some quantities of zinc oxide. In order to overcome these drawbacks, it is an object of the present invention to provide a process of electrolytic codeposition of copper and zinc on a metal element wherein said process, while not using any cyanide, gives results at least equal to those of the cyanide process. This and other objects have been achieved with the process of the present invention that is based on the finding that an electrolytic bath similar to that described by Y. Fujiwara and H. Enomoto gives altogether unexpected results when carried out at temperatures of from 40°C to 55°C. In fact, when working at these temperatures, excellent brass coatings are obtained even with a density of cathode current equal to or greater than 25 A/dm 2 and this means a productivity equal to at least 8 times that of the process of Y. Fujiwara and H. Enomoto. Unless otherwise specified , in the present description and in the claims, the expression "metal element" means any metal article that can be coated with a layer of brass by an electrolytic method such as, for example, articles for bathroom, sanitary and electrical use, door and window frames and articles for personal use. Specific examples of such articles are: taps, handles, small metal items, chandeliers, spectacle frames and the like. Preferably, the metal element consists of iron and alloys thereof, aluminium and zinc. Even more preferably, the metal element consists of a steel wire having a carbon content of from 0.6 to 0.95%. Typically, said steel wire is suitable for being corded into cords and forming internal reinforcing structures for articles made of vulcanized elastomeric material such as tires, conveyor belts, transmission belts and flexible pipes of natural or synthetic rubber or mixtures thereof. The term "brass" means a metal composition, as homogeneous as possible, consisting of from 10% to 50% by weight of zinc and from 90% to 50% by weight of copper, preferably from 20% to 40% by weight of zinc and from 80% to 60% by weight of copper and, even more preferably, from 30% to 40% by weight of zinc and from 70% to 60% by weight of copper. The term "cord" means a cord obtained by cording, according to conventional techniques, of steel wires coated with a layer of brass that, before drawing, is from 1 to 3 microns (µm) thick while, after drawing, it is from 0.1 to 0.4 microns thick. Generally, the diameter of said wires is about 1-3 mm before drawing and 0.1-0.5 mm after drawing. Typically, a cord commonly used in reinforcing structures of giant radial tires consists of 7 strands, each of 4 wires with a diameter of about 0.175 mm, round which a wire is wound, known as wrapper, having a diameter of 0.15 mm. The expression "cathodic efficiency" means the percent ratio between the amount of metal actually deposited and the theoretical amount of metal that should have been deposited according to the Faraday's rule. The present invention relates to a process of electrolytic codeposition of copper and zinc on a metal element according to claim 1. A preferred meaning of Y is NR 3 . Preferred meanings of R 1 , R 2 and R 3 , are hydrogen, methyl and ethyl. Preferably, the amino acid compound of formula (I) is histidine hydrochloride. As opposed to brass deposits obtained by means of diffusion, the deposit of brass obtained with the process of the present invention is characterized by a substantially constant content of copper and of zinc throughout the depth of the deposit. Fig. 1 is a diagram showing the amount of copper deposited depending on the density of the cathode current applied when using a galvanic bath of the present invention that had the following characteristics: Cu ++ = 0.2M, Zn ++ = 0.12M, histidine = 0.01 M, T = 45°C. Fig. 2 is a diagram showing the cathode efficiency depending on the density of the cathode current applied when using a galvanic bath of the present invention that had the following characteristics: Cu ++ = 0.2M, Zn ++ = 0.12M, histidine = 0.01 M, T = 45°C. Fig. 3 is a diagram showing the amount of copper deposited depending on the Cu ++ /Zn ++ mole ratio present in a galvanic bath of the present invention run under the following conditions: pH = 9, T = 45°C, i = 24 A/dm 2 . Fig. 4 shows a cord made of metal wires of the present invention embedded in a vulcanized elastomeric matrix. Fig. 5 shows a cross-sectional view of a tire reinforced with cords made of metal wires of the present invention. Fig. 6 shows a cross-sectional perspective view of a conveyor belt reinforced with cords made of metal wires of the present invention. Fig. 7 shows a cross-sectional perspective view of a transmission belt reinforced with cords made of metal wires of the present invention. Fig. 8 shows a cross-sectional perspective view of a flexible pipe reinforced with cords made of metal wires of the present invention. Before being dipped in the galvanic bath to be coated with a layer of brass according to the process of electrolytic codeposition of the present invention, the metal element is pickled according to conventional techniques, to remove any possible surface layer of oxides. Preferably, the galvanic bath of the present invention is characterized by: A) a content of Cu ++ of from 0.1 to 0.35 moles/litre; B) a Cu ++ /Zn ++ mole ratio of from 0.9 to 9; C) a (Cu ++ )+(Zn ++ )/alkali pyrophosphate mole ratio of from 0.13 to 0.45; D) a (Cu ++ )+(Zn ++ )/amino acid compound mole ratio of from 13 to 45. Examples of suitable copper salts are hydrate or anhydrous cupric sulfate. Examples of suitable zinc salts are hydrate or anhydrous zinc sulfate. Typical examples of an alkali pyrophosphate are sodium and potassium pyrophosphate. A typical example of an amino acid compound is histidine, preferably in the form of an acid addition salt such as, for example, hydrochloride. The density of cathodic current is from 10 to 40 A/dm 2 , more preferably it is of from 20 to 30 A/dm 2 . According to a preferred embodiment of the present invention, a galvanic bath for depositing a brass layer comprising from 60% to 70% of copper and from 40% to 30% of zinc has the following composition: mols/litre Cu ++ 0.18 - 0.26 Zn ++ 0.087 - 0.14 alkali pyrophosphate 0.9 - 1.09 amino acid compound 0.09 - 0.011 Suitable materials to prepare the abovementioned bath are: grams/litre cupric sulfate pentahydrate 45 - 65 zinc sulfate heptahydrate 25 - 40 potassium pyrophosphate 300 - 360 histidine monohydrochloride 1.8 - 2.2 Bath temperature : 40 - 55°C; preferably 45°C. Bath pH : 8.5 - 9.7; preferably 8.8 - 9. Density of cathode current : 10 - 40 A/dm 2 . Both soluble anodes and insoluble anodes may be used. In the first case brass anodes are used, having the same copper percent as that desired in the deposit. Example of a suitable material for insoluble anodes is titanium, possibly coated with oxides of suitable materials such as iridium and tantalum. The anodes may be placed parallel to the wire or on the bottom of the cups. After electrodeposition of a brass layer, the metal element is preferably treated on its surface with a phosphating bath consisting of a 30 - 50 g/l solution of phosphoric acid at a temperature of 30°C - 50°C, for an immersion time of from 3 to 5 seconds. When the metal element is a steel wire coated with a layer of brass (1 - 2 microns), it is preferably drawn using a conventional emulsion lubricant. The wire can then be used to produce cord of the type shown in Fig. 4. As shown in the diagram of Fig. 1, a further advantage of the present invention is that the density of the cathode current does not have a significant effect on the percent of deposited copper. The distribution of the current itself (depending on different cell forms, different bath stirring, and the like) also has no significant effect on the percent of deposited copper. As shown in Fig. 2, the cathode efficiency of the process of the present invention is about 70% while, in the case of the cyanide bath it is, for the same cathode current, substantially less. As shown in Fig. 3, the amount of copper deposited with the process of the present invention is a linear function of the copper/zinc ratio. This further advantage of the present invention allows the composition of the bath to be preselected very easily in accordance with the brass composition that it is desired to deposit. Fig. 5 shows a tire produced in a known way but using cords made of steel wires coated with brass deposited according to the present invention. The tire is fitted on a rim 13 and consists of a bead 10, a bead ring 12, a carcass ply 14, belt strips 15, tread 16 and sidewalls 17. The strips 15 are formed by cords of steel wire coated with brass with the process of the present invention. Figs. 6-8 show a conveyor belt 20, a transmission belt 30 and a flexible pipe 40 made of vulcanized natural or synthetic rubber, respectively, each reinforced with cords made of steel wire coated with brass according to the process of the present invention. The following examples are intended to further illustrate the present invention without, however, limiting it in any way. EXAMPLE 1 A galvanic bath capable of depositing a layer of brass consisting of about 67% of copper and about 33% of zinc on a steel wire has the following composition: mols/litre Cu ++ 0.15 Zn ++ 0.06 alkali pyrophosphate 1 histidine 0.01 The materials used to prepare the abovementioned bath were: grams/litre cupric sulfate pentahydrate 60 zinc sulfate heptahydrate 38 potassium pyrophosphate 330 histidine monohydrochloride 2 Working conditions: Bath temperature 45°C Bath pH 9 Density of cathode current 25 A/dm 2 . EXAMPLE 2 A galvanic bath capable of depositing a layer of brass consisting of about 64% of copper and about 36% of zinc on a steel wire has the following composition: mols/litre Cu ++ 0.2 Zn ++ 0.118 alkali pyrophosphate 1 histidine 0.01 The materials used to prepare the abovementioned bath were: grams/litre cupric sulfate pentahydrate 50 zinc sulfate heptahydrate 34 potassium pyrophosphate 330 histidine monohydrochloride 2 Working conditions are: Bath temperature 45°C Bath pH 9 Density of cathode current 2 A/dm 2 . EXAMPLE 3 A galvanic bath capable of depositing a layer of brass consisting of about 60% of copper and about 40% of zinc on a steel wire has the following composition: mols/litre Cu ++ 0.1 Zn ++ 0.05 alkali pyrophosphate 1 histidine 0.01 The materials used to prepare the abovementioned bath were: grams/litre cupric sulfate pentahydrate 25 zinc sulfate heptahydrate 14 potassium pyrophosphate 330 histidine monohydrochloride 2 Working conditions: Bath temperature 45°C Bath pH 9 Density of cathode current 5 A/dm 2 . EXAMPLE 4 A galvanic bath capable of depositing a layer of brass consisting of about 67% of copper and about 33% of zinc on a steel wire has the following composition: mols/litre Cu ++ 0.1 Zn ++ 0.1 alkali pyrophosphate 1 histidine 0.01 The materials used to prepare the abovementioned bath were: grams/litre cupric sulfate pentahydrate 25 zinc sulfate heptahydrate 29 potassium pyrophosphate 330 histidine monohydrochloride 2 Working conditions: Bath temperature 45°C Bath pH 9 Density of cathode current 16 A/dm 2 . Similar results have been obtained by working under conditions similar to those described in the preceding Examples from 1 to 6 both at 40°C and at 50°C but the appearance of the layer of brass deposited was slightly less uniform than that deposited at 45°C. TESTS The morphological and structural properties of brass deposits obtained according to Examples from 1 to 6 have been evaluated both under an electron microscope with a magnification of 2,500 and Auger spectroscopy. Examination under the electron microscope has shown that the deposits richest in zinc (i.e. copper< 60%) exhibit a substantially smooth surface while the poorest ones (i.e. copper ≥ 60%) exhibit a compact surface with a semispheroidal morphology having a size of from about 0.5 to 1 microns. This morphological feature sharply distinguishes the brass deposits of to the present invention over those obtained by diffusion which, before drawing, show a highly irregular surface with holes of a size of from 0.1 to 0.5 microns. Further differences have been detected by means of the Auger spectroscopy. In fact the brass deposits according to the present invention are characterized by a substantially constant content of copper and zinc throughout the depth of the deposit while those obtained by diffusion show, before the drawing step, deviations of some 10%, as gradient of diffusion (diffusion % of Cu between the outside and the inside). Moreover, the surfaces of the brass deposits obtained by diffusion show, before the drawing step, some amounts of oxides, particularly of zinc, evaluated by detecting the content of oxygen, [estimated as the ratio O/(Cu+Zn)], which is of from about 5% to about 10%. In contrast, the surface of deposits obtained according to the present invention have a content of oxygen equal to or less than about 3%.

Claims: A process for electrolytic codeposition of copper and zinc on a metal element wherein the electrolytic bath consists of an aqueous solution of at least a salt of bivalent copper, at least a salt of zinc and an alkali pyrophosphate, characterized in that said bath also comprises an amino acid compound of formula where Y is O, S or NR 3 , R 1 , R 2 and R 3 , the same, or different , are hydrogen or lower alkyl, or an acid-addition salt thereof, and that the bath temperature is of from 40°C to 55°C, and the density of the cathode current is of from 10 A/dm 2 to 40 A/dm 2 . A process according to claim 1, characterized in that Y is NR 3 . A process according to claim 1 or 2, characterized in that R 1 is hydrogen, methyl or ethyl. A process according to any claim from 1 to 3, characterized in that R 2 is hydrogen, methyl or ethyl. A process according to any one of the claim from 1 to 4, characterized in that R 3 is hydrogen, methyl or ethyl. A process according to any claim from 1 to 5, characterized in that the amino acid compound of formula (I) is histidine hydrochloride. A process according to any claim from 1 to 6, characterized in that the density of the cathode current is of from 20 A/dm 2 to 30 A/dm 2 . A process according to any claim from 1 to 7, wherein said metal element is a steel wire for use in reinforcing structures of articles made of vulcanized elastomeric material such as a tire for a vehicle, a transmission belt, a conveyor belt or a pipe.

Inventors: PAVAN FEDERICO, PAVAN, FEDERICO
Applicants: PIRELLI, PIRELLI PNEUMATICI SOCIETA' PER AZIONI
Application year: 1996
Main classification: C25D3
IPC codes: C25D3