AU2012203900A1 - Electric Cable with Limited Corrosion and Enhanced Fire Resistance - Google Patents

Abstract

The present invention relates to an electric cable (1) comprising an elongate element (4) surrounded by a first layer (2) comprising an assembly of at least two metallic strands (2a), characterized in that the entire circumference of said at least two metallic strands comprises a layer of hydrated alumina (9). Figure to be published: Fig.1.

Description

1 Electric cable with limited corrosion and enhanced fire resistance [001] The present invention relates to the field of electric cables. It applies typically, but not exclusively, to the high-voltage electric transmission cables or overhead energy transport cables, well known as "overhead lines" (OHLs). [002] The OHL cables conventionally consist of bare electrically conductive elements, held taut on an appropriate assembly of pylons. These lines are conventionally intended to transport electrical energy at a high alternating voltage (225 to 800 kV). [003] The present invention relates to an electric cable that has a high resistance to corrosion, so as to withstand rough atmospheric conditions such as the salty atmosphere close to coastlines or the sulfurous atmosphere of industrialized urban environments. [004] The OHL cables are generally manufactured based on aluminum. This is because this material has a fairly low weight compared to other conductive materials. However, the latter has a fairly low corrosion resistance. It has in fact been observed that, after 2-3 years in a highly corrosive atmosphere (salty or sulfurous atmosphere), a conductor made of aluminum or of aluminum alloy exhibited cracks that might result, in the long term, in the overhead line dropping (breaking of the strands forming the cable). [005] This is why it is known practice to protect the aluminum or aluminum alloy cables by applying a layer of grease to their outer surface. However, this solution is not satisfactory given that the layer of grease has a time-limited action. Furthermore, the layer of grease creates a corona effect which in turn causes a sound nuisance which is disagreeable to the population living in the vicinity of the line. [006] The patent FR 676 889 describes a high-voltage electric cable comprising a central conductive element formed by round metallic wires made of aluminum and covered by an outer layer formed by Z-shaped metallic wires also made of aluminum. However, such a type of electric cable does not provide sufficient resistance over time to atmospheres charged with salt or with sulfur.

2 [007] The present invention also relates to an electric cable which can withstand heat generated, for example, by a fire. [008] The aluminum or aluminum alloy cables, because of their low resistance to heat (the melting point of aluminum being, in fact, 658*C), are not used in electrical applications where the temperature may be high, for example where a fire resistance is required (e.g.: emergency exit lamp). [009] When there is such a requirement, it is known practice in the prior art to use copper-based electric cables. The melting point of copper is in fact much higher than that of aluminum and is of the order of 1083 0 C. [010] The aim of the present invention is to propose a novel electric cable which avoids all or some of the abovementioned drawbacks. In particular, the aim of the electric cable according to the invention is to withstand severe atmospheric conditions and thus avoid the corrosion of the overhead lines. Its aim is also to withstand high temperatures, such as fire temperatures which may be of the order of 600 to 1200*C, while allowing for a continuity of the electrical signal. [011] To this end, the subject of the invention is an electric cable comprising an elongate element, surrounded by a first layer comprising an assembly of at least two metallic strands (or metallic wires), characterized in that at least a portion of the circumference of said at least two metallic strands, and preferably the entire circumference of said at least two metallic strands, comprises a layer of hydrated alumina. In other words, said at least two metallic strands are each surrounded, at least partly, even totally, by a layer of hydrated alumina. [012] The Applicant has discovered, surprisingly, that the first layer of the invention, formed with metallic strands in which the edge or periphery of said metallic strands is made of hydrated alumina, exhibits an extremely high corrosion resistance. [013] Furthermore, said first layer of the invention exhibits an enhanced temperature resistance, while allowing for a continuity of the electrical signal. The electric cable of the invention is thus capable of withstanding fires, and does so notably despite the low melting point of the aluminum or of the aluminum alloys 3 that are likely to form the cable. In practice, by considering that the metallic strands forming the first layer are of aluminum or of aluminum alloy, the layer of hydrated alumina makes it possible to jacket the aluminum or aluminum alloy, even when the latter is melting. Furthermore, the layer of hydrated alumina will directly follow the expansion of the aluminum or of the aluminum alloy when melting, thus increasing the malleability and deformability of the strands forming the cable during thermal shocks. This is why, because of this expansion, the continuity of the electrical signal is still maintained (the metallic strands forming the cable are not broken under the effect of the heat). [014] In a particular embodiment, each of the metallic strands forming the assembly of the first layer comprises a layer of alumina over their entire circumference. [015] Because of this, all of the outer surface of the first layer is covered by a layer of alumina. In other words, the outer surface of the first layer comprises said layer of alumina, this layer extending notably along the longitudinal axis of the electric cable. [016] The expression "outer surface" should be understood to mean the surface which is most distant from the elongate element. [017] Preferably, the metallic strands forming the first layer are capable of conferring on said first layer a substantially regular surface, each of the strands forming the first layer being able notably to have a transversal section with a form complementing the strand(s) adjacent to it. [018] According to the invention, the expression "metallic strands capable of conferring on said first layer a substantially regular surface, each of the strands forming the first layer being able notably to have a transversal section with a form complementing the strand(s) adjacent to it" should be understood to mean that: the juxtaposition or the fitting together of all of the strands forming the first layer forms a continuous (irregularity-free) jacket, for example of circular or oval, or even square, section.

4 [019] Thus, the strands with the transversal section in the form of a Z or in trapezoid form are suitable for the present invention, whereas strands of circular section (whose assembly does not make it possible to obtain a regular jacket) do not fall within the above definition. In particular, strands with a Z-shaped transversal section are preferred. [020] Even more preferably, the first layer has a transversal section in the form of a ring. [021] According to a first variant embodiment, the first layer is an outer layer. According to the invention, the term "outer layer" of the electric cable should be understood to mean the final layer of the electric cable (i.e., the outermost layer of the electric cable), in particular, the one which is intended to be in contact with the environment external to the cable, in other words generally with the atmosphere. Because of this, the electric cable of the invention does not comprise other layers surrounding the first layer. Thus, when all the metallic strands forming the first layer are surrounded by said layer of alumina and the first layer is the outer layer, the outer surface of the electric cable of the invention comprises said layer of alumina along its longitudinal axis. [022] According to a second variant embodiment, the first layer is covered with an electrically insulating layer or an insulating sheath. [023] In the invention, the layer of hydrated alumina is a layer of aluminum oxide hydroxide or, in other words, a layer of alumina hydroxide. [024] According to a first variant, the layer of hydrated alumina is a monohydrated layer. [025] Examples of monohydrated alumina that can be cited include boehmite, which is the gamma polymorph of AIO(OH) or A1203.H20; or diaspore, which is the alpha polymorph of AIO(OH) or Al2O3.H20. [026] According to a second variant, the layer of hydrated alumina is a polyhydrated layer, and preferably a trihydrated layer.

5 [027] Examples of trihydrated alumina that can be cited include gibbsite or hydrargillite, which is the gamma polymorph of AI(OH)3; bayerite, which is the alpha polymorph of AI(OH)3; or nordstrandite, which is the beta polymorph of AI(OH)3. [028] The layer of alumina of the invention (i.e. the layer of hydrated alumina) is a layer whose thickness is controlled. In other words, it is obtained by a manufacturing method that makes it possible to obtain a substantially constant and uniform thickness over the entire circumference of the metallic strand or strands. As an example, this layer of hydrated alumina can be obtained by anodization (see controlled oxidation). [029] In a first variant embodiment, said layer of hydrated alumina is not present on one or more portions of the electric cable intended for electrical connection, in order to facilitate its installation. [030] In a second variant embodiment, the layer of hydrated alumina is capable of being broken at a connection area (e.g. electrical junction or electrical anchorage), so as to avoid, in an operational cable configuration, any overheating thereof at said connection. [031] Conventionally, the connections at an electrical junction (cable-cable connection) or at an electrical anchorage (post-cable) are made via a sleeve made of conductive material, such as steel or aluminum. For example, at a junction, the end of two cables (with a length of approximately 80 cm) is inserted into the sleeve which is then compressed by a crimping means. In the connection area, the ends of the cable are thus protected from corrosion by the sleeve. [032] The electric cables of the prior art do not comprise any layer of hydrated alumina on their outer surface; the current flowing in the cable is dispelled from the material of the outer layer to the conductive material of the sleeve. [033] In the electric cable according to the invention, the layer of hydrated alumina, which preferably covers the outer circumference of the first layer of the electric cable, is an electrical insulator (1 pm of alumina can electrically insulate a 6 voltage of 40 V). It might then be thought that it causes an overheating in the first layer by not allowing the current flowing in the electric cable to be evacuated to the sleeve. This would be all the more prejudicial since the standard IEC 61284 specifies to this end that the temperature of a conductor should not exceed 105 0 C at the risk of causing a creep of the conductor (beyond this temperature a tempering treatment is in fact observed which modifies the mechanical characteristics of the cable, particularly when the latter is aluminum alloy based) and of resulting in sagging of the overhead lines which could then be in contact with the roofs of homes or in contact with trees. [034] However, the Applicant has discovered that the presence of the layer of hydrated alumina, particularly at said connection area, was not restrictive and did not result in overheating given that the latter breaks upon the installation of the electric cable. In fact, the compression exerted (according to the current standards) on the sleeve via the crimping means is sufficient to break the layer of alumina and thus cause the electric current to pass between the first layer and the sleeve, particularly when the first layer is an outer layer. [035] Preferably, the thickness of this layer of alumina (see strands of the first layer) is at most 20 pm, and preferably at least 5 pm. Particularly preferably, the thickness of the layer of alumina may range from 6 to 15 pm, and even more preferably from 8 to 12 pm (terminals included). [036] The elongate element of the electric cable of the invention can preferably be positioned at the center of the cable (i.e. central position). It may be an electrically conductive element and/or a mechanical reinforcing element. [037] According to a characteristic of the invention, between the elongate element and the outer layer, a second layer is arranged. This may more particularly be referred to as a second layer called inner layer. [038] According to a first variant embodiment, the inner layer comprises an assembly of metallic strands, each of the strands forming the inner layer having a transversal section with a form complementing the strand(s) adjacent to it. Preferably, the strands of the inner layer, once assembled, thus form an outer 7 jacket having a regular section, for example circular, oval or square. Even more preferably, the strands of the inner layer, once assembled, have a transversal section in the form of a ring. As an example, the strands of the inner layer may have a Z- or trapezoid-form transversal section, the Z form being preferred. [039] In a variant embodiment, the strands of the inner layer may have a round-form transversal section. [040] According to one embodiment, at least a portion of the circumference of the metallic strands, and preferably the entire circumference of the metallic strands of the inner layer, is also formed by a layer of alumina, and preferably a layer of monohydrated alumina. [041] The thickness of this layer of alumina (see strands of the second layer) also varies from 5 to 20 pm, preferably from 6 to 15 pm, and even more preferably from 8 to 12 pm (terminals included). [042] In particular, the elongate element, the first layer (or more particularly the metallic strands forming the first layer) and/or the second layer (or more particularly the metallic strands forming the second layer) are preferably made of aluminum or of aluminum alloy. [043] The term "aluminum alloy" should be understood as the alloys of aluminum defined in the Washington DC Aluminum Association Directive 2086 or the alloys that meet European standard EN573. These standards define a number of classes of aluminum alloy that have references ranging from 1000 to 8000. [044] Preferably, the electric cable of the invention is a high-voltage electric transmission cable (OHL). [045] Another subject of the invention relates to an electric cable comprising at least one metallic strand (or metallic wire), notably made of aluminum or of aluminum alloy, characterized in that said metallic strand comprises, over its entire periphery, a layer of hydrated alumina, said metallic strand and the layer of hydrated alumina being as defined in the present description. This metallic strand surrounded by its layer of hydrated alumina may 8 notably be obtained by the step a) of the manufacturing method described below, and more particularly by controlled oxidation. [046] Thus, the metallic strand or strands whose edge or periphery is totally surrounded by hydrated alumina exhibits/exhibit, on the one hand, an extremely high corrosion resistance, and on the other hand an enhanced temperature resistance, while allowing for a continuity of the electrical signal. [047] This metallic strand may conventionally be surrounded by an electrically insulating layer or an insulating sheath. [048] In the present invention, whatever the object of the invention taken into consideration, the metallic strand or strands preferably do not comprise a layer of ceramic alumina, or more generally do not comprise a layer if ceramic, surrounding the layer of hydrated alumina. Thus, the fire resistance can be optimized by the non-presence of a layer of ceramic alumina, or the non-presence of a layer of ceramic, around the layer of hydrated alumina. [049] This is because, in a fire, a layer of ceramic alumina surrounding the layer of hydrated alumina could significantly damage the metallic strand. The layer of ceramic alumina would thus limit, in a fire, the continuity of the electrical signal of the electric cable concerned, that is to say, when the metallic strand or strands is/are melting. [050] The electric cable defined thus in this other object of the invention can be used notably in the field of aeronautics, in the railway sector or in the buildings sector, for example to power a lamp of an emergency exit panel. [051] Another subject of the present invention is a method for manufacturing an electric cable as described above, characterized in that it comprises the following steps: a) producing a controlled oxidation on the surface of at least one metallic strand, so as to form a layer of hydrated alumina on at least a portion of the circumference of said metallic strand, and preferably over the entire circumference of said metallic strand, and 9 b) assembling a number of strands obtained according to the step a) in order to form the first layer, and optionally the second layer, around the elongate element. [052] The controlled oxidation makes it possible to obtain a layer of hydrated alumina whose thickness is substantially constant and uniform over the circumference of the metallic strand, unlike what could be obtained with a so called "open air" oxidation. [053] As an example, the controlled oxidation can be produced by anodization. Anodization is more particularly a controlled and electrochemical oxidation of the surface of a material, such as a material made of aluminum or of aluminum alloy. [054] Preferably, the metallic strand obtained in the step a) may undergo a filling of the layer of hydrated alumina, in order to improve its compactness. [055] This filling may, for example, be done by performing a hot hydration of the metallic strand obtained in the step a), by dipping said strand in boiling water. This filling step is performed prior to the step b). [056] Advantageously, the strand obtained in the step a) or the strand obtained after filling is rinsed in osmosed water. [057] In a preferred embodiment, in the first layer, and optionally in the second layer, each strand has a transversal section with a form complementing the strand(s) adjacent to it, and being able to confer on the layer concerned a substantially regular surface. [058] The invention will be better understood, and other aims, details, features and advantages thereof will become more clearly apparent from the following description of particular embodiments of the invention, given solely as nonlimiting illustrations, with reference to the appended drawings. [059] In these drawings: [060] Figure 1 is a schematic cross-sectional view of an electric cable according to an embodiment of the present invention; 10 [061] Figure 2 is an enlarged view of the outer layer of the electric cable according to figure 1; [062] Figure 3 is a schematic cross-sectional view of an electric cable according to another embodiment of the present invention; [063] Figure 4 is an enlarged view of the outer layer of the electric cable according to figure 3; [064] Figure 5 is a photograph showing the layer of hydrated alumina, formed according to the method of the invention; [065] Figure 6 is a schematic drawing of an accelerated corrosion test conducted by the present applicant; [066] Figure 7 is a photograph showing the surface of an electric cable according to the prior art ("standard OHL with internal grease") after the latter has undergone the corrosion test of figure 6; [067] Figure 8 is a photograph showing the surface of an electric cable according to the invention after said electric cable has undergone the corrosion test of figure 6; [068] Figure 9 is a graph showing the trend of the corrosion (average depth of breaks formed by the corrosion as a function of time) for three electric cables: a first electric cable according to the prior art comprising an outer layer comprising strands of Z-form transversal section ("standard OHL without internal grease"), a second electric cable according to the prior art comprising an outer layer comprising strands of Z-form transversal section with an internal packing of grease ("standard OHL"), and another electric cable according to the invention ("solution OHL"); [069] Figure 10 is a macroscopic photo of a raw aluminum alloy wire having undergone a heat test (440 watts heat power); and [070] Figure 11 is a macroscopic photo of an anodized aluminum alloy wire according to the invention having undergone the same heat test as the wire of figure 10 (440 watts heat power).

11 [071] For reasons of clarity, only the elements that are essential to the understanding of the invention have been represented schematically in these figures, and without observing scale. [072] [073] The electric cable 1, illustrated in figures 1 and 2, corresponds to a high-voltage electric transmission electric cable of OHL type. [074] This electric cable 1 comprises: an elongate central electrically conductive element 4 and, in succession and coaxially around this central conductive element 4, an inner layer 3, and an outer layer 2. The inner 3 and outer 2 layers are also electrically conductive. In particular, the central element 4 is in contact with the inner layer 3, which in turn is in contact with the outer layer 2. [075] The conductive element 4 is formed from round cylindrical strands 4a of aluminum or of aluminum alloy, seven of them, each strand 4a being covered with greases 5. This grease 5 thus fills both the interstices present between the cylindrical strands 4a and between the strands 4a and the inner layer 3. [076] The inner layer 3 and the outer layer 2 consist of an assembly of strands (3a and 2a) also of aluminum or of aluminum alloy whose transversal section is in Z form (or in "S" form depending on the orientation of the Z). The geometry of the strands in "Z" form thus makes it possible to obtain a surface that has almost no interstices that can generate accumulations of moisture and therefore centers of corrosion. As represented in figure 1, the inner layer 3 comprises 13 strands 3a and the outer layer 18 strands 2a. The inner layer 3 differs from the outer layer 2 in that the outer layer consists of strands 2a of which the circumference (of each strand) is formed by a layer of alumina 9, preferably monohydrated. This layer of alumina 9 is generally formed by anodization. The particular geometry of the strands 2a (Z-form transversal section) and their protection by the layer of alumina 9 thus form a barrier against corrosion and do so even if the electrical conductor 1 is in severe marine and industrial exposure conditions (presence in the air of elements such as sodium, chlorine, sulfur, etc.). This will elsewhere be demonstrated in test 1 below.

12 [077] [078] The electric cable 1, illustrated in figures 3 and 4, corresponds to a high-voltage electric transmission electric cable of OHL type, but with a structure slightly different to that of the electric cable described in figures 1 and 2. [079] This electric cable 1 comprises: an elongate central electrically conductive element 4 and, in succession and coaxially around this central conductive element 4, an inner layer 3, and an outer layer 2. The inner 3 and outer 2 layers are also electrically conductive. In particular, the central element 4 is in contact with the inner layer 3, which is in turn in contact with the outer layer 2. [080] The elongate element 4 is formed from round cylindrical strands 4a of aluminum or of aluminum alloy, 19 of them, each strand 4a being able to be covered with greases. [081] The inner layer 3 and the outer layer 2 consist of an assembly of strands (3a and 2a) also of aluminum or of aluminum alloy with a trapezoidal transversal section. The geometry of the trapezoid-form strands offers the advantage of obtaining a surface that has almost no interstices that can generate accumulations of moisture and therefore centers of corrosion. As represented in figure 3, the inner layer 3 comprises 18 strands 3a and the outer layer 24 strands 2a. The inner layer 3 consists of strands 2a in which the circumference (of each strand) is formed by a layer of hydrated alumina 9, preferably a boehmite (see figure 4 or 5). This layer of alumina 9 is generally formed by anodization. The layer of alumina 9 thus forms a jacket that is able to contain the aluminum or the aluminum alloy when the latter is melting because of high temperatures. This effect will elsewhere be demonstrated in test 3 below. [082] [083] In variant embodiments represented in figures 1 to 4, it is possible to modify the number of strands 3a, 2a of the inner and outer layers, their form, the number of inner layers or even the number of round wires, as well as the nature of the aluminum. [084] 13 [085] A method for manufacturing the electric cable according to the invention will now be described. [086] This method comprises a number of steps: a step of degreasing and pickling strands, a first rinsing step, a neutralization step, a second rinsing step, a step of anodizing under current in an electrolyte based on sulfuric acid, a third rinsing step, a step of filling the pores with hot water and a fourth rinsing step. [087] The starting material is, for example, a strand or wire of Z-form transversal section made of AGS-type aluminum alloy (aluminum, magnesium, silica, bearing the reference 6201 of European standard EN573), the height of the Z is 2.9 mm or an equivalent diameter of 3.2 mm. The wire is wound on a spool. These wires are marketed with a film of grease linked to the drawing process. This is why, for the manufacturing method, it is generally necessary to proceed with a degreasing step. [088] The degreasing and the pickling of the wires are performed mostly by a chemical method or assisted by an electrolytic method. The aim of the degreasing operations is to eliminate the various bodies and particles contained in the greases while the pickling operation serves to eliminate the oxides present on the metal. There are a number of pickling methods: chemical, electrolytic or mechanical. These methods are known to those skilled in the art. Chemical pickling consists in eliminating the oxides by dissolving, even bursting the layer, without attacking the underlying metal. For the degreasing/pickling, it is possible, for example, to use a 45 ml/L industrial solution of GARDOCLEAN@ (Company CHEMETALL). The solution mainly consists of soda (approximately 30 g/L to 45 ml/L) and of surface active agents. [089] The step for neutralizing the wires makes it possible not to pollute the bath used for the anodization. Furthermore, this step makes it possible to eliminate certain traces of oxides that may damage the anodization. This step is carried out in a bath identical to the anodization bath. A solution of 200 g/L sulfuric acid H2SO4 at ambient temperature will make it possible to eliminate any residues 14 of soda from the degreasing. The neutralization makes it possible to set the surface of the aluminum to the same pH value as the anodic bath. [090] Then, the strands are anodized. The anodization is based on the principle of water electrolysis. In a tank filled with treatment for the process, that is to say, in an acid medium such as sulfuric acid, the part is placed at the anode of a direct current generator. The cathode of the system is generally made of lead (inert in the medium). It may also be made of aluminum or stainless steel, in some installations. During the electrolysis, the layer of oxide is created from the surface towards the core of the metal, unlike an electrolytic deposition. For aluminum, a layer of alumina is formed which has an electrical insulating capability. Thus, the current no longer arrives at the substrate, and it is then protected. [091] The reactions are as follows: - at the cathode: 2H+ + 2e- -+ H 2 - at the anode: Al-+ 3e~ + A13*, then: 2 A13+ + 3 H 2 0 -- A1 2 0 3 + 6 H* * balance equation: 2 Al + 3 H 2 0 -* A120 3 + 3 H 2 [092] These reactions therefore cause a layer of aluminum oxide 9, the alumina which is an insulator, to be formed. The current therefore no longer arrives at the layer. This is why an electrolyte which dissolves the layer must be used, such as sulfuric acid, phosphoric acid, chromic acid or even oxalic acid. Equipotential spheres are then obtained which progress by producing porous hexagonal structures. The anodization process depends on the speed of dissolution. In practice: . if Vdissolution > Voxidation, pickling occurs e if Vdissolution Voxidation, electrolytic polishing occurs e if Vdissolution < Voxidation, anodization occurs. [093] The layer of hydrated alumina 9 in sulfuric anodization is formed from outside to inside. The coloration is made by impregnating colorant through absorption in the pores. [094] The electrolytic parameters are imposed by a current density and a conductivity of the bath. For the desired thickness on the prototype wire of 8- 10 15 pm, the current density will be set at 55-65 Aldm 2 and the voltage will be set at 20 21 V with a current intensity of 280-350 A. The strand or wires 2a are thus obtained. [095] Filling is the technique used to block or close the porosities that exist in each cell of the layer of oxide. This blocking is obtained by transformation of the hydrated alumina forming the anodic layer, resulting in an expansion and therefore a progressive closing of the pores. This operation is performed by immersing the anodized parts in boiling water (osmosed water that has a temperature above 80"C) to promote the reaction kinetics. The filling thus promotes a good resistance to corrosion. [0961 The various rinses are defined by 3 steps: rough rinsing, clean rinsing, drying in compressed air. The rinsing is done using osmosed water. [097] Finally, the strands 2a with a Z-form transversal section are assembled in a standard manner so as to obtain an electric cable with a cross sectional area of 455 mm 2 . The latter consists of a central conductive element formed by 19 round wires made of AGS 6201, on which is arranged an inner layer consisting of 18 strands/wires with a Z-form transversal section of aluminum alloy AGS 6201 and on which is arranged an outer layer comprising 24 wires, also of Z form section, obtained according to the method described above. [098] [099] The electric cable according to the invention makes it possible to obtain anticorrosion characteristics superior to the standard conductor as will be demonstrated below. Test 1: anticorrosion test [0100] An anticorrosion test was conducted in order to compare the mechanical resistance of the electric cable according to the invention with standard cables of the prior art. [0101] For this, the electric cable according to the invention that is tested, "solution OHL", is the electric cable obtained according to the above method and 16 that has, to recap, the following characteristics: a central electrically conductive element of AGS 6201 consisting of 19 round wires, on which is arranged an inner layer formed by 18 strands of Z-form section of AGS 6201 and on which is arranged an outer layer comprising 24 strands of Z-form section of AGS 6201, the edge of which is formed by a layer of monohydrated alumina 8 to 10 pm thick (hereinafter called conductor AEROZ 1). [0102] The electric cable "standard OHL without internal grease" is an electric cable comprising a central electrically conductive element formed by 19 round wires of AGS 6201, surrounded by a first layer formed by 18 strands of Z form section of AGS 6201 on which is arranged a second layer of 24 strands of Z form section of AGS 6201. The conductor has a cross-sectional area of 455 mm2. For this cable, the grease has been removed. [0103] The electric cable "standard OHL" is the same electric cable as the one described previously except that the internal grease has been left. [0104] The accelerated corrosion test combines two standardized tests: the salt mist test and the Kersternich test. The salt mist reveals a moist corrosion with the presence of sodium chloride (NaCl) allowing for an increase in the conductivity of the moisture through a greater exchange of ions accelerating the corrosion effect. The Kersternich test makes it possible to reveal an observable corrosion in an industrial or urban environment by the injection of sulfurous products in a moist environment. [0105] The test put in place combines the two tests as is illustrated in figure 6. A 5% solution of NaCl is placed in the bottom of a sealed chamber and is heated to 50-60 0 C in order to reproduce the salt mist while the addition of sulfurous products in gaseous form is created by dissolving copper in sulfuric acid and spraying it into the chamber. The samples 6 are placed in the chamber in an ordered manner, allowing for a uniform circulation of the polluted environment. [0106] The tracking parameters used to obtain a reproducible test are: the temperature of the NaCl solution, the concentration of the NaCl solution, the flowrate of air injected into the sulfuric acid for return into the chamber, the 17 quantity of copper dissolved and the concentration of the sulfuric acid used to dissolve the copper. [0107] The results given in figures 7 to 9 are obtained. [0108] As the graph of figure 9 shows, the electric cable according to the invention exhibits no corrosion mark/break which is not the case with the electric cables according to the prior art. After 120 days in a salty and sulfurous atmosphere, the electric cable of the prior art without grease exhibits more than 350 microns of observed corrosion pitting depths, more than 150 for the electric cable with grease and none or almost none for the electric cable according to the invention. This graph also shows the important role of the grease which, through its drop point, will flow toward the outside of the electric cable in order to protect it from corrosion. [0109] The photographs of figures 7 and 8 show the outer surfaces of the electric cable according to the invention (figure 8) and of the electric cable according to the prior art with grease (figure 7) after they have been exposed for more than 200 days to the hostile atmosphere of the experimentation chamber. It can be seen that the surface of the electric cable according to the invention is not damaged unlike that of the electric cable according to the prior art. Numerous erosions, breaks can in fact be seen in figure 7. Consequently, the electric cable according to the invention effectively withstands a highly corrosive atmosphere. Test 2: validity test according to standard IEC61284 [0110] Tests were conducted by an independent laboratory, at the company DERVAUX, in order to measure the temperature of the electric cable according to the invention. [0111] For these tests, the conductor AEROZ 1 was tested, together with a conductor AEROZ 2 (conductor identical to AEROZ 1 except that the Z-form section strands of the inner layer also have an 8 to 10 pm thickness of alumina). [0112] In order to measure the temperature, the company DERVAUX followed the protocol set out in the standard IEC61284.

18 [0113] This independent company found that, for the two types of conductor according to the invention, the temperature did not exceed 105*C and therefore conformed to the standard IEC61284. [0114] Although the invention has been described with regard to a particular embodiment, it is obvious that it is in no way limited thereto and that it embraces all the technical equivalents of the means described and their combinations if they fall within the context of the invention. [0115] [0116] The electric cable according to the invention also makes it possible to obtain anti-fire characteristics that are better than the standard conductor. [0117] Test 3: heat-resistance test [0118] To carry out the heat-resistance test, raw aluminum wires were compared to wires according to the invention, in particular to wires of aluminum alloy AGS 6201 covered with a layer of boehmite. The thickness of the layer of hydrated alumina varied from 7 to 10 pm along the wire. The wires tested all have a diameter of 8 mm. [0119] The principle of the test that was performed on the wires (samples) is based on induction. Using a coil, a magnetic field is created around the samples. Through a principle of physics, the electrons of the substance (the aluminum) will be excited. This excitation will generate heat until, at a given point (in the middle of the coil), the melting of the substrate occurs. The melting point of aluminum (658 0 C) is then reached. [0120] The sample heating parameters will depend on the power emitted by the inductor. [0121] For this test, this power is varied and the time it takes for the sample to reach melting point and possibly break is recorded. [0122] During the test, a camera is used to measure the exact time at which the samples will, possibly, break. [0123] The various results obtained are given in Table I below.

19 Aluminum wire Wire according to the invention Heat power Time to Heat power Time to (watts) breaking of the breaking of the wire (min) wire 467 1 min 26 sec 440 8 min 10 sec 436 1 min 55 sec 436 >10 min 440 1 min 23 sec 440 >15 min 440 1 min 32 sec 440 >15 min 720 30 sec 720 >2 min 720 35 sec 720 1 min 34 sec 720 36 sec 720 1 min 45 sec Table I: Comparative results, at the same power, of the resistance to melting between a wire of aluminum alloy and a wire of anodized aluminum alloy. [0124] As the above test shows, the strands according to the invention withstand high temperatures (figure 11) and are not cut, unlike the strands of pure aluminum (figure 10). [0125] Although the invention has been described in relation to particular embodiments, it is obvious that it is in no way limited thereto and that it embraces all the technical equivalents of the means described and their combinations, provided the latter fall within the context of the invention. [0126] Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the priority date of this application. [0127] Where ever it is used, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of'. A corresponding meaning 20 is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear. [0128] It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text. All of these different combinations constitute various alternative aspects of the invention. [0129] While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be obvious to those skilled in the art are therefore intended to be embraced therein.

Claims (21)

1. Electric cable comprising an elongate element surrounded by a first layer comprising an assembly of at least two metallic strands , characterized in that the entire circumference of said at least two metallic strands comprises a layer of hydrated alumina .

2. Electric cable according to claim 1, characterized in that the outer surface of the first layer comprises said layer of alumina.

3. Electric cable according to claim 1 or 2, characterized in that the assembly of the metallic strands is capable of conferring on said first layer a substantially regular surface.

4. Electric cable according to one of the preceding claims, characterized in that each of the metallic strands forming the first layer has a transversal section with a form complementing the strand(s) adjacent to it.

5. Electric cable according to one of the preceding claims, characterized in that the first layer is an outer layer.

6. Electric cable according to one of the preceding claims, in which the layer of alumina is a layer of monohydrated alumina.

7. Electric cable according to one of the preceding claims, in which the layer of alumina is a layer of boehmite.

8. Electric cable according to one of claims 1 to 5, in which the layer of alumina is a layer of polyhydrated alumina.

9. Electric cable according to one of the preceding claims, in which the transversal section of the metallic strands is of Z or trapezoid form.

10. Electric cable according to one of the preceding claims, in which the layer of alumina is capable of being broken at a connection area.

11. Electric cable according to one of the preceding claims, in which the layer of alumina has a thickness of at most 20 pm.

12. Electric cable according to one of the preceding claims, in which the layer of alumina has a thickness of at least 5 pm. 22

13. Electric cable according to one of the preceding claims, in which, between the elongate element and the first layer , a second layer, called inner layer , is arranged.

14. Electric cable according to claim 13, in which the inner layer comprises an assembly of metallic strands , each of the strands forming the inner layer having a transversal section with a form complementing the strand(s) adjacent to it.

15. Electric cable according to claim 14, in which at least a portion of the circumference of the metallic strands , and preferably the entire circumference of the metallic strands of the inner layer , is formed by a layer of hydrated alumina.

16. Electric cable according to one of the preceding claims, in which the elongate element , the first layer and/or the second layer are made of aluminum or of aluminum alloy.

17. Electric cable according to one of the preceding claims, characterized in that it is a high-voltage electric transmission cable (OHL).

18. Electric cable comprising at least one metallic strand made of aluminum or of aluminum alloy, characterized in that said metallic strand comprises, over its entire periphery, a layer of hydrated alumina, said metallic strand and the layer of hydrated alumina being defined in any one of the preceding claims.

19. Electric cable according to claim 18, characterized in that the metallic strand does not comprise a layer of ceramic surrounding the layer of hydrated alumina.

20. Method for manufacturing an electric cable according to one of claims 1 to 17, characterized in that it comprises the following steps: a) producing a controlled oxidation on the surface of at least one metallic strand , so as to form a layer of hydrated alumina over the entire circumference of said metallic strand; and 23 a) assembling a number of strands obtained according to the step a) in order to form the first layer, and optionally the second layer, around the elongate element .

21. Method for manufacturing an electric cable according to claim 20, characterized in that the controlled oxidation step is an anodization step.