The present invention relates to the general field of strands and cables, such as structural stays and tensioning cables, composed of several wires twisted together.

The invention more particularly concerns the protection against corrosion of strands used in the construction of prestressing cables, guy wires, stay cables for the construction of engineering structures, cables used in building suspension and stay cable bridges or for the reconstruction of engineering structures, building structures and floors. These strands often have poorly protected areas and are consequently sensitive to the action of oxidising agents. The lifespan of these strands can thus be adversely affected.

Known strands are generally made up of metallic wires, for example steel wires. These wires are generally twisted together and may be wrapped in a protective sheath of polymeric material, which may be extruded around the bundle of twisted-together wires.

In addition, the metallic wires of known strands may be directly protected against corrosion by a thin layer of metal having a high degree of resistance to oxidation, such as zinc, applied by galvanization. In some countries, the wires in structural steel cables are required to be galvanised. In other countries, however, galvanising is not used, on the grounds that the galvanisation process is highly polluting, and that it can substantially impair the mechanical properties of the steel. The galvanising process can introduce hydrogen into high tensile steel wires, which may impair the durability of some prestressing steels. Galvanising not only has environmental and mechanical disadvantages, but is also expensive.

It is also known in the prior art to further improve protection of the wires inside the sheath, by filling the spaces (interstices) between the wires with a viscous material, such as a grease or a wax, which forms a

supplementary obstacle to oxidizing agents such as air, humidity and water. However, experience shows that, in spite of these precautions, oxidising agents may still subsist in the strands, for example in the filling material, and are sometimes the cause of degradation of the metal wires. The latter cannot then fully perform their function, or no longer have the required mechanical properties.

The object of the present invention is therefore to provide a method of producing a strand which does not have the aforementioned inconveniences and which has an improved protection against corrosion. In particular, the invention is aimed at providing a strand which benefits from improved protection against corrosion, without the need for the wires to be galvanised.

It is another object of the present invention to provide a strand whose protection against corrosion does not adversely affect its possibilities of use, in particular its size or its mechanical properties.

It is another object of the present invention to provide a new method of producing a strand, in which the steps relating to protection against corrosion are implemented in a simple and inexpensive way.

According to the invention, a method is provided for producing a strand comprising a plurality of wires twisted together. The method of the invention comprises a first temporary opening-up step, in which the wires of the strand are separated from each other along at least a first opened-up length portion of the wires, and at least one coating step, in which a protective coating substance is applied to each of the plurality of the wires over the opened-up length portion so as to form a discrete individual protective coating covering the entire surface of said opened-up length portion of each wire, the protective coating substance containing a volatile migratory corrosion inhibiting agent.

According to a variant of this method, the coating step includes curing the discrete individual coating on each of the wires.

The method may further comprise a closing-up step, performed after the coating step, in which the coated wires are closed up together to re-form the strand. It may also comprise an extrusion step in which a protective sheath is formed around the plurality of wires coated in the coating step, and the material of the protective sheath may contain a volatile migratory corrosion inhibiting agent. In the latter case, the protective sheath may advantageously be formed by coextruding an inner sheath layer containing volatile migratory corrosion inhibiting agent, and an outer sheath layer not containing a volatile migratory corrosion inhibiting agent.

According to one embodiment of the invention, the method compries a filling step, in which interstices between and around the plurality of

individually coated wires are filled with a common protective filler substance. The common protective filler substance may advantageously comprise a volatile migratory corrosion inhibiting agent.

In a variant of this embodiment, a second temporary opening-up step may be performed before the filling step such that the wires are separated from each other along at least a second opened-up length portion of the wires so as to permit filling of the interstices between the wires with the common protective filler substance.

The protective coating substance can be applied as a sprayable liquid, and in which the coating step includes spraying the protective coating substance on to the surface of each of the wires.

According to another embodiment of the invention, a strand is provided comprising a plurality of wires twisted together, in which the surface of each of the wires is individually protected against corrosion by a discrete individual protective coating comprising a volatile migratory corrosion inhibiting agent.

According to a variant of this embodiment, interstices between and around the wires, are filled with a common protective filler substance

comprising a volatile or migratory corrosion inhibiting agent. According to another variant of this embodiment, the wires are enclosed in a protective sheath. The protective sheath may be formed from a solid material containing a volatile migratory corrosion inhibiting agent.

According to a further variant of the invention, the protective sheath comprises an inner sheath layer containing volatile migratory corrosion inhibiting agent, and an outer sheath layer not containing a volatile migratory corrosion inhibiting agent.

The corrosion inhibiting agent used in the coating for the wires, the common protective filler substance and/or the protective sheath, can be initially in powder or granular form and made from a mixture of volatile and non-volatile corrosion inhibiting substances. Such a powder can be mixed or melted with the polymeric material for extrusion to form the protective sheath, or it can be mixed with the common protective filler substance without any further treatment, or it can be mixed or dissolved in the coating substance used to create the individual wire coatings.

The fact that zinc galvanization is no longer needed in the production of a strand according to the invention is environmentally beneficial, and can reduce the risk of impairing mechanical characteristics of the wires. Avoiding galvanisation means avoiding exposing the wires to acid and heat, both of which can affect the structural behaviour of each individual wire or the strand as a whole. Hydrogen embrittlement is also less likely to occur in ungalvanised wires. By avoiding galvanisation, the cost of producing such a strand is also significantly reduced.

Note that, although the method of the invention enables the production of corrosion-resistant strands without galvanisation, the method can also be used to provide extra protection for wires which are galvanised.

The invention and its advantages will appear in more detail in the following description, with illustrative example embodiments and

implementations given with reference to the accompanying drawings. Note that the drawings are included as an aid to understanding the invention, and should not be taken as limiting the scope of the invention, which is defined in the attached claims.:

Fig. 1 illustrates an example embodiment of a structural cable according to the invention, made up of a group of strands according to the invention,

Fig. 2 illustrates a cross-section of an example embodiment of a strand according to the invention, and

Fig. 3 illustrates an example embodiment of a strand according to the invention, at various stages of the method of producing, according to the invention.

Fig. 4 illustrates various steps which may be included in the method of the invention.

The various embodiments of the invention will now be described in more detail, with reference to the drawings. Note that identical functional and structural elements which appear the different drawings are assigned the same reference numerals.

Fig. 1 shows an example implementation of a structural cable (1 ) made up of strands (2), each of which is made of steel wires (3) enclosed in a protective sheath (4). The steel wires (3) are normally twisted together.

Fig. 2 illustrates a cross-section of a strand (2) according to an embodiment of the invention. Each strand (2) is made up of several metallic wires (3) which are twisted together and enclosed in a protective sheath (4) of polymeric material, such as high density polyethylene (HDPE), which encloses and protects the wires (3). The protective sheath may be loose fitting around the strands, as suggested in figure 2, or it may be made so as to fit elastically or tightly around the strands. Stay cables may typically require a tightly-fitting protective sheath to reduce or avoid relative movement between the sheath and the strand. The wires (3) of each strand (2) are each directly and individually protected against corrosion by a coating (5), which acts to oppose the action of oxidizing and other corrosive agents (not represented). Each coating (5) is designed to prevent contact between the material (eg steel) of one individual wire and such corrosive and oxidizing substances. In the context of this application, the term corrosion is used to mean any process, for example chemical or electrolytic, which can have a deleterious effect on the chemical integrity, and hence the mechanical properties, of the wires (3). The individual coatings are applied such that each coating forms an individual protective coating over the entire surface of one wire.

The individual coating applied to the surface of each wire may be a substance, such as an aromatic urethane or an epoxy-based material, which can be applied as a liquid or a powder, for example, but which subsequently polymerises, hardens, crystallises or solidifies after application to form a contiguous protective coating over the entire surface of the portion of the length of wire being treated. The coating process may also involve a curing step, during which the applied coating is allowed to, or encouraged to cure before subsequent steps are performed. Curing may be a single, passive step, or it may involve several steps and various chemical or thermal processes. The curing may involve one or more processes of submitting the coating to humidity, or heating, or cooling, or ultraviolet light, or dry air, or any other appropriate curing process. The protective coating achieved by the method of the invention is sufficiently mechanically robust that, once cured, it can remain intact during a subsequently twisting together of the wires, and if the finished strand is later physically manipulated or stressed. The coating used in the method and strand of the present invention is preferably a polymeric coating such as an aromatic urethane, which bonds tightly to the surface of the metal of the wires and forms a highly corrosion-resistant (contact inhibiting) barrier. Coating thickness can be regulated to suit the materials and conditions of the particular strand and its application, but might typically be in the region of 50 to 100 microns, and may be applied in a single layer or in multiple layers.

The individual corrosion protection coating of each wire (3) contains at least one volatile migratory corrosion inhibiting (VMCI) agent. Such corrosion inhibiting agents are known variously by the acronyms "VPI" (Vapour Phase Inhibitor) or "VPCI" (Vapour Phase Corrosion Inhibitor) or VCI (Volatile Corrosion Inhibitor) or MCI (Migratory Corrosion Inhibitor). In this application, the term Volatile Migratory Corrosion Inhibitor (VMCI) is used to describe the family of such agents.

Note that the volatile VMCI component described here is distinct from the volatile components (solvents) which act as a vehicle for applying (by spraying, for example) barrier films on to the surfaces to be protected. Such solvents are high-volatility compounds designed to evaporate and disperse in as short a time as possible (preferably a few minutes), and might have a vapour-pressure of 10 mm Hg or more. VMCI components, on the other hand, have a much lower volatility (having a vapour pressure in the region 10 ^"4 to 10 ^"6 mm Hg, for example) and are designed to evaporate or otherwise migrate out of a coating or substrate slowly, over months or years.

The VMCI agent in the individual coatings helps to inhibit any undercutting of the barrier film by corrosive agents at locations where a coating is incomplete, damaged or otherwise compromised, and the VMCI agent thereby provides a further protection against corrosion, in addition to the protection provided by the coating's barrier film itself. The coating is thus self-healing, as it tends to repair itself.

For example, the coating substance may be a product such as that produced by the company Cortec under the trade name VpCI, and in particular the type VpCI 396. Such products release molecules of corrosion inhibiting agent which then travel, as a vapour through gaseous media or by migration through solids, for example, and which, when they come into contact with a metal such as steel, are adsorbed by the steel to form a permanent

hydrophobic barrier at the surface of the metal. Suitable VMCI agents can comprise amino-carboxylate compounds, for example, on a calcium-sulfonate basis. The amino part of each molecule bonds strongly to steel, while the carboxylate part repels water, thus setting up a powerful hydrophobic barrier which effectively inhibits corrosion which would otherwise occur due to contact with water, humidity, air, or electrolytes in the vicinity of the surface of the wires (3). The longer the exposure time of the metal to the vapours, or to VMCI molecules migrating through surrounding media, the more effective the corrosion protection.

In a structural strand comprising multiple high-tensile steel wires twisted together, the coating is preferably applied directly to the surface of the tensile steel. In this way, the coating acts as a replacement for the galvanising traditionally used in the prior art, with the consequent advantages described above - less complex and costly production, and reduced environmental impact. In an alternative embodiment, however, the individual protective coatings may be applied to the wires over existing coatings, such as a galvanisation layer. The self-healing advantage of the VMCI agent in the coatings can still be effective even when the coatings are applied over a galvanisation layer.

In an embodiment of the invention, the protection of the wires (3) against corrosion may be improved still further, by means of a common protective filler substance (7) used to fill the spaces (6) between the wires (3), and between the wires (3) and the sheath (4), thereby forming a further obstacle to oxidizing and corrosive agents. The term "common" is used in this application to denote a substance which is applied to all the wires (3) collectively, and thereby to distinguish the common protective filler substance (7) from the individual protective coating (5) which ensures that each wire (3) is individually protected against corrosion.

According to a variant of this embodiment of the invention, the common protective filler substance (7) used to fill the spaces (6) within the strand (2), contains a corrosion inhibiting agent which may include a volatile corrosion inhibitor. The protective substance (7), used to fill the spaces (6) within each strand (2) may be a wax, grease or gel based substance, to which the corrosion inhibitor has been added in liquid or powder form. Suitable VMCI products which can be added in this way include the product produced by the company Cortec under the trade name M-369, which may be supplied in liquid or powder form, is compatible with oils, grease, waxes, gels or solvents, and can be added to such substances to give contact, vapour phase and/or migratory protection, thanks to its self-healing properties. Mobility of the VMCI agent molecules of this particular product through solids or semi-solids such as grease or wax is by migration (crawling).

According to another embodiment of the invention, the individually coated wires (3) of the strand (2), as well as the common protective filler substance if present, can be enclosed within a protective sheath (4). Such a sheath may be formed, for example, by extrusion of a thermoplastic polymeric material, such as high-density polyethylene (HDPE), around the coated wires (3) of the strand (2). This extrusion process can either be performed directly after the coating of the wires, or it can be performed in a separate operation at a later time.

In a variant of this embodiment, the protective sheath (4) enclosing the wires (3) is made from a material containing a VMCI product. In this case, the protective function of the sheath (4) against corrosion is thus made more efficient. The agent in this case may be, for example, a volatile or migratory corrosion inhibitor, VMCI, and may be added to the polymer material in powder or granulate form, during or before the polymeric sheath material is extruded around the coated wires (3). Once the sheath is formed, molecules of the volatile or migratory corrosion inhibitor agent can travel into and through the materials of the protective filler substance (gel, wax, grease etc) and, if they are able to reach the surface of the wires (3), they become adsorbed on to the steel of the wire and create a hydrophobic barrier at the surface, thus further improving the corrosion resistance of the wires. This VMCI provision in the sheath may be in addition to or instead of the VMCI in the common protective filler substance. It may be a mixture of VMCI and contact inhibitors, which migrates out of the HDPE sheathing, sublimates and adheres to the metal surface. It forms a tight nitrogen bond to steel which provides anodic-cathodic corrosion protection even in the presence of moisture and chlorides.

In a closed environment such as the closed volume encompassed by the sheath (4) which surrounds the wires (3) of each strand (2), the corrosion inhibiting agents released from the sheath (4) react in the presence of water, such as the water suspended in the air (humidity), and then form ionic molecules which form an ionic film on any exposed surface (3a) of each metallic wire (3). The ionic film acts as a hydrophobic barrier and inhibits any electric corrosion phenomena at the surface. The coating (5) thus provides a cathodic and anodic protection of the metallic wires (3). In the event that the coating of the wires becomes damaged, the volatile migratory corrosion inhibitor forms a hydrophobic barrier at the surface of the wire where the coating is damaged.

Note that the coating process of the method of the invention is designed to provide a contiguous and complete individual protective coating on each of the wires. However, in the event that the individual protective coating on one of the wires is damaged or incomplete, then the molecules of the migratory or volatile corrosion inhibitor emitted from the polymeric sheath will be adsorbed on to any exposed metal surface of the wire and form a protective hydrophobic barrier in the manner described above.

Figure 3 shows in general fashion how the wires in a strand may be opened up to improve or facilitate the application of the individual coating substance (5). In phase A1 , the wires are still in their twisted, closed state, but are then opened up in in the transition to phase A2, so that application (8) of the coating (5) may occur evenly and completely over each of the wires (3) individually. Once the coating is cured, the coated wires (3) are then twisted together again in the transition to region A3, after which the sheath (4) is formed around the twisted wires (3).

A single coating step is illustrated in figure 3, but it should be understood that the coating may alternatively be applied in more than one layer. In this case, the strand is either passed through the same coating equipment two or more times, or the coating equipment may consist of more than one coating station, in which case the coating layers can be applied successively in one operation. A combination of multiple passes and multiple stations is also possible, of course. Curing may be provided between the applications of the layers, as required. The composition of the successive layers may also be altered to improve the mechanical and/or protective characteristics of the coating. For example, the first layer(s) to be applied may have a higher concentration of VMCI component, while the outer layer(s) may have a lower (or zero) concentration of VMCI component. Similarly, the outer layer or layers may comprise components which reduce the layers' permeability to the VMCI component. In this way, the VMCI component in the inner layer(s) may be contained within the coating, and as close as possible to the surface of the metal to be protected.

The steps of forming the protective sheath and filling the spaces may be performed at the same time, as part of a combined extrusion-plus-filling step. The filling of the interstices is advantageously performed while the wires of the strand are opened, as shown in figures 3 and 4. The opening-up for the filling process may be the same opening-up step as performed for the coating step, or it may be a separate process. In the latter case, the wires are first opened up and coated, then re-twisted together, and then subsequently reopened up later for the filling operation. The subsequent opening-up, filling, extruding and closing-up sequence may be performed at a time later than the coating of the individual wires.

Thus, to produce a strand (2) according to an example method of the invention, a coating containing at least one volatile and/or migratory corrosion inhibiting agent is applied to each of the metallic wires (3) individually. Note that the application (by spraying, for example) may be performed on one, some or all of the wires at once, but in such a way as to form a separate, discrete, individual coating on each wire. Subsequently, the sheath (4) may be formed on the outside of the strand (2). The filling of the spaces (6) with the common protective substance (7), to which a corrosion inhibiting agent may have been added, can be carried out just before the forming of the sheath or almost simultaneously. The polymeric material used to produce the protective sheath (4) may be pre-mixed with a corrosion inhibiting agent (VMCI), or the corrosion inhibiting agent may be added during the extrusion process which forms the sheath.

In implementing the coating (5), according to one embodiment of the invention, at least one corrosion inhibiting agent is sprayed (8) on to each of the metallic wires (3) intended to make up said strand (2), before the wires are combined to form the strand (2). The process of re-forming the strand (2) can include the twisting of the metallic wires together (3), and the outer sheath may then be formed around the re-twisted-together wires. Spraying is just one possible choice of application method which may be used, but has the advantage of achieving relatively even coatings in a rapid application.

The method steps are repeated along the strand (2) so as to form a continuous coating (5) over the entire surface (3a) of each of the metallic wires (3), along the desired length.

According to the example implementation, the production method of the invention consists of spraying (8) the VMCI product, on its own or as a product containing the corrosion inhibiting agent, on to the part (A2) of the length where the metallic wires (3) have been opened up, so as to enable an evenly distributed deposition of the corrosion inhibiting agent, or the product containing the corrosion inhibiting agent, on each individual metallic wire (3).

Figure 4 shows in more detail, but still in schematic form, how the individual protective coating, the common protective filler substance and the sheath can be applied to the wires (3) in a strand (2) in a method according to the various embodiments of the invention. Note that in figure 4, the strand is moved through the opening, coating, curing, twisting, filling and sheathing processes, in that order, in a continuous process, or in a series of sub- processes.

The twisted wires (3) of the strand are first opened up, for example using a nylon strand-splitter (10). The separated strands are then coated with the individual protective coating substance containing a volatile or migratory corrosion inhibitor. In figure 4, this coating step is illustrated by spray nozzles (1 1 ), but any suitable method could be used for ensuring that the coating substance is applied evenly and completely over the wires (3) - vapour deposition in an enclosed chamber. Alternatively, the coating may be supplied in particulate form, such as a powder, which can be adhered to the wires while being thermally or otherwise converted into a contiguous coating, or by dipping in a bath of liquid. The coating substance is chosen such that it hardens or solidifies or otherwise changes into a contiguous and mechanically robust coating. In a preferred embodiment of the invention, a polymeric material such as a curable urethane coating is used. Alternatively, an epoxy-based material may be used.

Depending on the type of coating substance used, one or more curing or other treatment stages may be required to convert the coating substance into a form suitable for protecting the wires in the strand. For example, the curing of Cortec products such as VpCI 396 may be activated and/or accelerated by increasing the humidity and temperature in the vicinity of the newly-coated wires. Water or water-vapour (21 ) nozzles 12 are shown in figure 4 for this purpose, as are the means (13) for providing a supply of warm or hot air. Curing proceeds as the coated wires are moved through the curing region A2.

Once the coatings are sufficiently cured, the coated strand wires (3) may pass into the next phase, A3, of the process, in which the spaces between the wires are filled with protective filler substance (7), and the wires are twisted together again and then enclosed in the protective sheath (4). The filling and extruding processes can be arranged one after the other, as indicated in figure 4, or they can be performed at the same time.

The filling and sheathing steps may be performed directly after the coating steps if they are to be performed at all. Alternatively, as discussed earlier in this application, the filling and sheathing steps may be performed separately from the coating steps. In this case, the coated and cured wires are re-twisted together and then, if and when the strand requires filling and sheathing, the wires are opened up again using a similar opening-up process, so that the filler (grease, wax gel etc) can be applied to all surfaces of the wires. The wires are then re-twisted together again and the sheath is applied to the outside of the strand and filler.

The volatility of the corrosion inhibitor can be different in the different parts of the strand covering. In the sheath, for example, it is preferable to use a large amount of relatively low volatility corrosion inhibitor. This provides a long- lasting but relatively low-level release of corrosion inhibiting molecules into the interior of the sheath (4) which may provide back-up corrosion protection for the wires over years or even decades. Using a low-volatility (low vapour- pressure) corrosion inhibitor also means that less of the corrosion inhibiting agent is released outside the sheath (4). A second, outer sheath layer may also be extruded around the first sheath. The purpose of this outer sheath is to prevent migration of the volatile migratory corrosion inhibitor substance outside the strand, and thereby to maximise its protective effect within the strand. The second sheath can advantageously be co-extruded with the inner sheath, the material of the inner sheath containing the volatile migratory corrosion inhibitor (VMCI), and the material of the outer sheath not containing any VMCI.

The common filler substance (7), on the other hand, can be mixed with a lower concentration of more volatile corrosion inhibitor. Because it is enclosed within the protective sheath (4), and because its volatility is high, the VMCI in the common filler substance creates a relatively high vapour-pressure of the corrosion inhibitor inside the sheath, providing a long-lasting protective environment in the vicinity of the wires and a self-healing of the protective coatings. As time passes, the corrosion inhibitor is contained inside sheath, loss through the sheath is minimised, and the protective effect is therefore maximised over time.

According to the embodiments of the method and the strand of the invention, the individual coating on each wire provides an individual barrier against moisture, and thereby a primary protection against corrosion. The individual protection coating also contains VMCI agent(s) which provide subsidiary protection in case the protective function of the coating itself is compromised. The VMCI present in the coating provides a self-healing property for the coating, such that it can compensate for damaged or incomplete parts of the coating, by ensuring that the VMCI molecules are adsorbed on to any exposed steel, thereby giving the exposed areas a protective hydrophobic barrier. The combination of the physical barrier provided by the bonding of the coating to the metal surface of the wire, with the hydrophobic barrier provided by the VMCI agent, results in a greatly superior resistance to undercutting of the coating by aggressive, corrosive agents. According to the invention, therefore, an individual protective coating is provided separately on each wire. These protective coatings provide a first level of corrosion protection for the metal of the wire. A second level of self- healing protection is provided by the inclusion of VMCI (volatile and/or migratory) corrosion inhibiting agents within the coating substance. In other embodiments, the volatile and/or migratory VMCI corrosion inhibiting agents may be provided in the sheath and/or in the common filler substance. In these variants, each wire in the strand is protected not only by its individual protective coating and by the VMCI in its individual protective coating, but also by at least one other common, separate source of VMCI corrosion inhibition.