The invention relates to a colored wire and to a method of manufacturing a colored wire.

Background of the invention.

The application of a zinc or zinc alloy coating to protect steel against corrosion is widely known.

Zinc or zinc alloy coatings are applied on steel wires to provide a cathodic protection of the steel. The thicker the coating, the longer it takes till the steel wire starts to corrode, ie. till red rust appears.

Currently, there is a general demand to increase the corrosion resistance without applying thicker coatings. Therefore, a subsequent protection or passivation of the coating is required.

Another known problem associated with zinc and most zinc alloy coatings, especially under aggressive environments as for instance environments with a high humidity, is the occurrence of white rust.

For that reason, it is necessary that the zinc or zinc alloy coating is protected or passivated to maintain its decorative appearance of gloss.

At present, there is a high demand in the market for colored wire. Such type of wire has to be characterised by a uniform color, it has to keep its beautiful appearance over a long period of time and at the same time it has to be characterised by a good corrosion resistance.

A number of attempts have already been made to obtain a colored wire with a good corrosion resistance.

One of these attempts consists in applying a duplex coating on a steel wire by first galvanizing a steel wire and then applying a lacquer on the galvanized steel wire.

Such a solvent based method has the drawback that a separate installation is required to apply the lacquer, which has a considerable influence on the costs.

Another known method is the coloring of wires by applying a polymer based coating comprising a coloring agent onto a zinc coated steel wire.

These polymer based coatings have a rather high thickness, which is limiting the flexibility of the wire.

Furthermore, these methods have the drawback that they require intensive curing.

Summary of the invention.

It is an object of the present invention to provide steel wires in a variety of colors.

It is another object to provide colored wires having a long lasting shining aspect and a good corrosion resistance.

It is a further object to provide a method of manufacturing colored wire in an environment friendly way.

According to a first aspect of the present invention, a colored wire is provided. This colored wire comprises : - a steel core; - a first coating layer comprising zinc or a zinc alloy, applied on the steel core; - a second coating layer comprising a metal oxide and at least one coloring agent, applied on the first coating layer.

According to the present invention the use of a polymer based coating to color the wire is not required.

Possibly, the colored metal wire comprises additional coating layers such as: - a coating layer comprising at least one coloring agent; and/or - a coating layer comprising a polymer such as PVC, a polyurethane a polyacrylate, a polyester, an epoxy resin,...

The invention is applicable to steel wires as core. The steel wires may be chosen within a high diameter range. Either steel wires with a low or a high carbon content can be considered.

The steel wires can have any cross-section such as round, square, rectangular, oval or half oval cross-sections.

The first coating layer comprises a zinc or a zinc alloy layer.

This zinc or zinc alloy coating has a thickness ranging preferably between 1 and 100 um and more preferably between 1 and 50 um, for example 5 or 20 um.

As zinc alloy coating one can consider for example Zn-Fe, Zn-Ni and Zn-AI alloys. A preferred zinc alloy coating is a Zn-AI alloy coating comprising between 2 and 15 % Al.

Possibly, between 0.1 and 0.4 % of a rare earth element such as Ce and/or La can be added in order to improve properties like fluidity and wettability of the melt.

The zinc or zinc alloy coating used in the present invention is substantially free of Mn, Ti and Cu. With"substantially free"is meant that no Mn, Ti or Cu is added to the zinc or zinc alloy. Possibly, some inevitable traces of Mn, Ti or Cu can be present.

The second coating layer, applied on the first coating layer, comprises a metal oxide and at least one coloring agent.

Typical metal oxide coatings result from a conversion process, such as a chromating process.

The metal oxide may also be obtained by oxidation of the base metal.

The metal oxide layer may for example comprise a zinc oxide coating layer obtained by oxidation of the zinc or zinc alloy.

Other examples of oxide coatings comprise zirconium oxide, aluminium oxide, chromium oxide and mixtures of these oxides. Preferred metal oxide layers comprise chromium oxide (Cr203) or a mixture of zinc oxide and chromium oxide.

The metal oxide layer forms preferably a chemically inert layer that protects the zinc or zinc alloy coating from corrosion.

The thickness of the second coating layer is preferably between 10 and 150 nm, and more preferably between 20 and 80 nm, for example 40 nm.

The coloring agents are for example absorbed by or penetrated into the porous metal oxide layer.

It can be preferred that the metal oxide layer has a porous structure so that the coloring agents can easily be absorbed.

Preferably, the functional groups of the coloring agents react with the oxides or hydroxides of the coating layer (s). These functional groups are preferably N-and/or S-based groups. Such functional groups may react with metal or metal oxides for instance with Zn or ZnO.

The coloring agent comprises at least one organic or inorganic dye or a combination of organic and inorganic dyes. Preferred dyes comprise a chromophore such as an azo group.

Preferably, the coloring agentis soluble in a solvent, such as water, an alcohol, for example methanol or ethanol, aceton,...

The colored wires which are obtained according to the present invention pertain to the whole visible spectrum, from violet till red.

Possibly, an additional layer comprising at least one coloring agents can be applied on top of the metal oxide layer.

For some applications, it can be desired that an additional coating layer comprising a polymer such as PVC, a polyurethane, a polyacrylate, a polyester, an epoxy resin,... is applied on top of the second coating layer or on top of the layer comprising one or more coloring agents.

This polymer based layer further intensifies the color and further increases the corrosion resistance of the coated steel wire.

Possibly, the polymer based coating also comprises one or more coloring agents.

The colored wire according to the present invention can be used for any kind of application whereby a colored wire and/or an improved corrosion resistant wire is desired. The invention is applicable to steel wires either with a low or a high carbon content.

The colored wires can for example be used for decoration, anti-corrosion and identification application.

Examples where a decorative aspect is desired are for example wires for handles of buckets or pails, wires for the closing of bottles or jars, coat hanger wire.

Colored wires according to the present invention can be used for fences since they are giving a nice decorative aspect and at the same time an improved corrosion resistance. They can for example be used as barbed wire, for knotted fences, for welded fences,... Structures comprising a number of colored wires according to the present invention are described below.

A further application of the colored wire according to the present invention is for the manufacturing of springs.

Furthermore, the colored wires are suitable for al kind of identification purposes, for example for filament identification in cables, for identification of springs or for applications where the visibility of the wire is important, such as vineyard wire.

According to a second aspect of the invention a structure comprising a number of colored wires as described above is provided.

The structure may for example comprise a welded, woven or braided structure.

Wires according to the present invention can be used in wire beads for reinforcing internal or external corners of a wall or for reinforcing window or door openings. Such wire beads are sold by the applicant under the trademark WIDRA@.

According to a third aspect of the invention a method of manufacturing a colored wire is provided.

This method comprises the steps of - providing a steel core; - applying a first coating layer comprising zinc or a zinc alloy on said steel core; - applying a second coating layer comprising a metal oxide and at least one coloring agent on top of said first coating layer.

The colored wire according to the present invention is manufactered in a continuous, in line process. By using this continuous process, the manufacturing costs of the colored wire are considerably reduced. This is a great advantage over the processes known in the art.

The method may further comprise one or more drying steps and/or the application of an additional coating layer as described above.

Also these steps can be performed in a continuous, in line process.

The zinc or zinc alloy coating can be applied by any conventional technique, for example by hot dip, electrolysis or cladding.

Also the metal oxide layer comprising one or more coloring agents can be applied by a number of different techniques.

The metal oxide and the coloring agent (s) can be applied in one step by applying a metal oxide layer comprising the coloring agent (s).

Alternatively, a metal oxide layer can be applied in a first step and the coloring agent (s) can be applied in a subsequent step, for example by passing the wire through a coloration bath or by subjecting the wire to a spraying of a coloration solution.

The coloring agents are then absorbed by or penetrated into the metal oxide layer.

Preferably, the functional groups of the coloring agents react chemically with the metal oxides.

In principle, any technique that allows to obtain a metal oxide layer on the zinc or zinc alloy coated steel wire can be used. The metal oxide coating can for example be applied chemically by simple immersing or by an electrochemically assisted process.

A preferred method to obtain a metal oxide layer is by chromating the zinc or zinc alloy coated wire. Such a process is for example based upon an aqueous solution of hexavalent and more recently upon a solution of trivalent chromium.

To further intensify the color of the wire, an additional layer can be applied on top of the second coating layer by immersing the coated wire in a solution comprising at least one coloring agent.

The method may further comprise an additional step of applying a polymer based coating. The polymer based layer can for example be applied by extrusion. It may be applied on top of the second coating layer or on top of the additional coloring layer.

By optimising the process parameters, such as the application temperature, the concentration of the chromating bath, the concentration

of accelerative additives such as H202, the pH of the coloration bath, the temperature of the coloration bath, the way of drying; the total time of the process can be reduced.

Since the applied coating has a very thin thickness, the flexibility and deformability of the wire are remained after the coating is applied.

A further advantage of the very thin coating of the colored wires according to the present invention is that the wires can easily be welded.

Brief description of the drawings.

The invention will now be described into more detail with reference to the accompanying drawings wherein - FIGURE 1 shows the cross-section of a colored wire according to the present invention.

Description of the preferred embodiments of the invention.

FIGURE 1 represents a transversal cross-section of a colored wire 10 according to the invention. The steel wire 10 comprises a steel core 12, a zinc-aluminium coating 14 applied on the steel core and a metal oxide layer 16 applied on top of the zinc or zinc alloy coating. This metal oxide layer comprises substantially Cr203 and a coloring agent.

A steel wire according to the invention can be manufactured as follows : Starting material is a low carbon wire rod with a diameter of about 5.5 mm. This wire rod is drawn either to an intermediate diameter or to the final diameter. Subsequently, the drawn wire is subjected to a heat treatment and is hot dip galvanized at this intermediate or final diameter.

The zinc coated wire can be further drawn to its final diameter in case of an intermediate drawing step.

The zinc coating has for example a thickness of 20 um.

Subsequently, the zinc coated wires are dipped in a solution comprising trivalent chromium ions. The concentration of the chromium ions is between 6 en 100 %, preferably higher than 12 %.

Before the zinc coated wires are dipped in the chromium solution, it is preferred that the wire is degreased. Degreasing can be done in many different ways such as by steam degreasing, chemical degreasing, electrochemical degreasing or by a combination thereof. A preferred way of degreasing is a chemical alkaline degreasing.

The chromium passivated wires are then immersed in a solution comprising a water soluble coloring agent. The coloring agents are absorbed by the formed oxide layer, which mainly comprises chromium oxide. The longer the wire is immersed into the solution comprising the coloring agent, the more intense the color of the wire will be.

The chromium oxide layer comprising the coloring agent has a thickness of for example 20 um.

In an alternative method, the zinc coated wires are dipped in a solution comprising chromium ions and at least one coloring agent.

The above described colored wire is characterised by a long lasting shiny aspect.

To evaluate the corrosion resistance, the wire is subjected to a corrosion test. The above described wire, a hot dip galvanised steel wire having a zinc coating of 20 um on which a chromium oxide layer comprising a coloring agent is applied is subjected to a neutral salt spray test (ISO 9227).

5 % dark brown rust (DBR) is observed after 300-600 hours.

When a steel wire having only a zinc coating of 20 um is subjected to the same spraying test, 5 % DBR is already observed after 100-200 hours.

This means that by using a colored wire according to the present invention, the formation of 5 % DBR is retarded 3 times compared with a zinc coated steel wire.