Power cable having an aluminum corrosion inhibitor

Abstract

A power cable having a metallic electrical conductor surrounded by one or more semiconductive layer and more or more insulating layer, wherein the cable has at least one metallic element made of aluminum, having a corrosion inhibitor provided in direct contact with the at least one metallic element made of aluminum.

Claims

1. Power cable comprising a metallic element made of aluminum, wherein a corrosion inhibitor is provided in direct contact with the metallic element, the corrosion inhibitor having the following general formula (I):
R1-ArR2(I) wherein R1 is a 1-hydroxy-tetrazol-5-yl group a 2-hydroxy-tetrazol-5-yl group, 1-acryloxy-tetrazol-5-yl group or 1-(2-carboxyethenyl)-tetrazol-5-yl group; Ar is a monocyclic or bicyclic aromatic moiety; and R2 is a hydrogen atom (H) or a 1-hydroxy-tetrazol-5-yl group, a 2-hydroxy-tetrazol-5-yl group, a hydroxyl group (OH), a vinyl group, an allyl group or a OCOR4 group, where R4 is an alkenyl group having from 2 to 6 carbon atoms.

2. Power cable according to claim 1, wherein, when Ar is a monocyclic aromatic moiety and R2 is different from hydrogen atom, R1 and R2 are in ortho, meta, or para position with respect to each other.

3. Power cable according to claim 1, wherein, when Ar is a bicyclic aromatic moiety and R2 is different from hydrogen atom, R1 and R2 are in peri position with respect to each other, or are substituents of the same cycle.

4. Power cable according to claim 1, wherein the corrosion inhibitor has the following general formula (Ia) ##STR00003## wherein R1 and R2 have the same meanings as defined in formula (I) of claim 1.

5. Power cable according to claim 1, wherein R1 is a 1-hydroxy-tetrazol-5-yl group and R2 is a hydrogen atom.

6. Power cable according to claim 1, wherein R2 is a hydrogen atom, a 1-hydroxy-tetrazol-5-yl group, preferably in ortho position with respect to R1, or a hydroxyl group (OH).

7. Power cable according to claim 1, wherein the corrosion inhibitor of formula (I) is associated with a supporting material to form a corrosion inhibiting element, where the corrosion inhibitor is in direct contact with the metallic element made of aluminum.

8. Power cable according to claim 7, wherein the corrosion inhibitor is absorbed in or adsorbed on the supporting material.

9. Power cable according to claim 1 wherein the corrosion inhibitor of formula (I) in direct contact with the metallic element made of aluminum is in an average amount of from 110.sup.3 g/cm.sup.2 to 10010.sup.3 g/cm.sup.2 with respect to the surface of the metallic element.

10. Process for producing a power cable comprising a metallic element made of aluminum and a corrosion inhibiting element comprising a supporting material associated to a corrosion inhibitor of formula (I),
R1-ArR2(I) wherein R1 is a 1-hydroxy-tetrazol-5-yl group or a 2-hydroxy-tetrazol-5-yl group; Ar is a monocyclic or bicyclic aromatic moiety; and R2 is a hydrogen atom (H) or a 1-hydroxy-tetrazol-5-yl group, a 2-hydroxy-tetrazol-5-yl group, a hydroxyl group (OH), a vinyl group, an allyl group or a OCOR4 group, where R4 is an alkenyl group having from 2 to 6 carbon atoms; the process comprising the steps of sprinkling the supporting material with said corrosion inhibitor of formula (I) in dry form or dissolved in a polar solvent to be subsequently evaporated, to provide said corrosion inhibiting element; and positioning the corrosion inhibiting element in direct contact with the metallic element made of aluminum.

11. Process according to claim 10 wherein the polar solvent is selected from water, acetone and hydroxyl-containing solvents.

12. Process according to claim 10 wherein the corrosion inhibitor is dissolved in the polar solvent at a concentration of up to 250-300 ppmw (parts per million weight).

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The present invention will be better understood by reading the following detailed description, given by way of example and not of limitation, to be read with the accompanying drawings, wherein:

(2) FIG. 1 shows a perspective view of a power cable according to an embodiment of the present invention;

(3) FIG. 2 shows a cross section of a power cable according to an embodiment of the present invention;

(4) FIG. 3 shows a graph plotting the linear polarization resistance (LPR) variation over time with and without corrosion inhibitor, as described in Example 2; and

(5) FIG. 4 shows a graph plotting the corrosion rate variation over time with and without corrosion inhibitor, as described in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 1 shows a perspective view of a power cable 11 according to an embodiment of the present invention.

(7) The power cable 11 of FIG. 1 is a single core cable and comprises a conductor 12, an inner semiconductive layer 13, an insulating layer 14 and an outer semiconductive layer 15, which constitute the cable core. The cable core is surrounded by, a metal screen 16 and an outer sheath 17.

(8) The conductor 12 generally comprises metal wires, which are preferably made of copper or aluminium, and which are braided together by using conventional technique.

(9) The cross sectional area of the conductor 12 is determined in relationship with the power to be transported at the selected voltage.

(10) Preferred cross sectional areas for power cables according to the present invention range from 16 mm.sup.2 to 1,600 mm.sup.2.

(11) Inner semiconductive layer 13, insulating layer 14 and outer semiconductive layer 15 are made of polymeric materials.

(12) Polymeric materials suitable for layers 13, 14 and 15 can be selected from the group comprising: polyolefins, copolymers of different olefins, copolymers of an olefin with an ethylenically unsaturated ester, polyesters and mixtures thereof.

(13) Examples of suitable polymers are: polyethylene (PE), in particular low density PE (LDPE), medium density PE (MDPE), high density PE (HDPE), linear low density PE (LLDPE), ultra-low density polyethylene (ULDPE); polypropylene (PP) and copolymers thereof; elastomeric ethylene/propylene copolymers (EPR) or ethylene/propylene/diene terpolymers (EPDM); ethylene/vinyl ester copolymers, for example ethylene/vinyl acetate (EVA); ethylene/acrylate copolymers, in particular ethylene/methyl acrylate (EMA), ethylene/ethyl acrylate (EEA) and ethylene/butyl acrylate (EBA); ethylene/-olefin thermoplastic copolymers or mechanical mixtures thereof.

(14) In the case of inner semiconductive layer 13 and outer semiconductive layer 15, the above listed polymeric materials are added with an electro-conductive carbon black, for example electro-conductive furnace black or acetylene black, so as to confer semiconductive properties to the polymer material.

(15) The insulating layer 14, inner semiconductive layer 13 and outer semiconductive layer 15 can be made of polymeric a thermoplastic material, preferably comprising a thermoplastic polymer material including a predetermined amount of a dielectric liquid. Example of thermoplastic insulating layers are disclosed in WO 02/03398, WO 02/27731, WO 04/066318, WO 07/048422 e WO 08/058572.

(16) The metal screen 16 is made of a metal braid, made for example of aluminium, wrapped around the outer semiconductive layer 15.

(17) The outer sheath 17 is preferably made of polymer material, such as polyvinyl chloride (PVC) or polyethylene (PE).

(18) In the embodiment of FIG. 1, a corrosion inhibiting element 18, in form of a tape of supporting material bearing, on the surface facing the metal screen 16, a corrosion inhibitor of formula (I), is provided around and in contact with the metal screen 16.

(19) Similarly, but not shown in FIG. 1, a corrosion inhibiting element can be provided, preferably in form of yarns, within the metal wires of the conductor 12 and/or between the inner semiconductive layer 14 and the conductor 12 and/or between the metal screen 16 and outer semiconductive layer 15.

(20) FIG. 2 shows another embodiment of the invention. FIG. 2 illustrates a cable 21 comprising three cable cores. Each cable core comprises a conductor 22, an inner semiconductive layer 23, an insulating layer 24 and an outer semiconductive layer 25. Each cable core is surrounded by a metal screen 26. An outer sheath 27 surrounds all of the three screened cable cores. Conductors 22 are each made of a solid aluminium rod.

(21) The three screened cable cores are stranded and embedded into filler (or bedding) 29 which, in turn, is surrounded by an outer sheath 27. Outer sheath 27 can be made of the same material already disclosed in connection with outer sheath 17 of FIG. 1.

(22) The materials of inner semiconductive layer 23, insulating layer 24, and outer semiconductive layer 25 can be as those already mentioned in connection with cable 11 of FIG. 1 for analogous cable portions.

(23) In the embodiment of FIG. 2, a corrosion inhibiting element 28, in form of a tape supporting material bearing a corrosion inhibitor of formula (I) on the surface facing the conductor 22, is provided at the interface between conductor 22 and the inner semiconductive layer 23 of each cable core.

(24) The corrosion inhibiting element 28 can be, alternatively or additionally, a yarn or tape wound positioned as already said in connection with cable 11 of FIG. 1.

(25) Similarly, but not shown in FIG. 2, a corrosion inhibiting element can be provided in direct contact with metal screen 26, either between the metal screen 26 and outer semiconductive layer 25 or between the metal screen 26 and outer sheath 27.

(26) The cable according to the present invention can be manufactured as disclosed above. The corrosion inhibiting element can be supplied using common process apparatus at a suitable step of the manufacturing process. For example, when the corrosion inhibiting element is to be positioned within the wires of an electric conductor, the inhibiting element in form of yarn(s) is stranded together with the wires. For example, when the corrosion inhibiting element is to be positioned between the electric conductor and the protecting layer (insulating layer or inner semiconducting layer), the corrosion inhibiting element in form of yarn(s) or tape is wound around the conductor before extruding said layer.

(27) The following examples are intended to further illustrate the present invention, without however restricting it in any way.

Example 1

Synthesis of 5-phenyl-1-hydroxy-(1H)-tetrazole

Step 1: Preparation of N-Hydroxybenzimidoyl Chloride

(28) Ethanol (30 mL) was poured in a 250 mL three-necked round-bottomed flask equipped with internal temperature probe, reflux condenser and nitrogen inlet. Benzoyl chloride (19.71 mL, 23.87 g, 0.17 moles) was added via syringe and the solution was stirred during the addition. Hydroxylamine hydrochloride (21.20 mL, 35.41 g, 0.51 moles) was added in one portion, followed by sodium hydroxide 97% (27.2 g, 0.68 moles). The reaction flask was placed in an oil bath and heated at 60 C. with stirring for 1 hour. The reaction flask was removed from the oil bath and was left to cool to ambient temperature.

(29) The mixture was transferred to a single-necked round-bottom flask and concentrated by rotary evaporation at temperature of 45 C. and vacuum of 40 mbar.

(30) The solid residue was transferred to a separatory funnel and was extracted three times with ethyl acetate. The combined organic layers were dried over Na.sub.2SO.sub.4. The drying agent was removed by filtration and then the organic layer was concentrated by rotary evaporation at temperature of 45 C. and vacuum of 40 mbar.

(31) The solid residue was recrystallized from 60 mL of hexane at 5 C. for 2 hours to give crystals of N-hydroxybenzimidoyl chloride (20.21 g, 0.13 moles).

(32) The resulting yield of reaction was of: 76.9%

Step 2: Preparation of N-Hydroxybenzimidoyl Azide

(33) Sodium azide (9.75 g, 0.15 moles) dissolved in 10 mL of water was charged in a 250 mL three-necked round-bottomed flask equipped with internal temperature probe and reflux condenser. A solution of N-hydroxybenzimidoyl chloride (18.65 g, 0.12 moles) in 20 mL of methanol was added dropwise. The solution was placed in an oil bath and heated at 45 C. with stirring for 2.5 hours.

(34) At the end of the reaction, the reaction flask was removed from the oil bath and was left to cool to ambient temperature.

(35) The mixture was transferred to a single-necked round-bottom flask and the solvent of the solution was distilled off by rotary evaporation at temperature of 25 C. and vacuum of 55 mbar.

(36) The residue was transferred to a separatory funnel and was extracted three times with diethyl ether (330 mL). The aqueous phase was further extracted three times with diethyl ether (330 mL).

(37) The organic layers were dried with sodium sulfate filtered and evaporated to give N-hydroxybenzimidoyl azide (16.2 g, 0.10 moles).

(38) The resulting yield of reaction was of: 89.9%

Step 3: Preparation of N-Acetoxybenzimidoyl Azide

(39) N-Hydroxybenzimidoyl azide (16.2 g, 0.10 moles), dissolved in 10 mL of dichloromethane and pyridine (11.85 g, 12.12 mL, 0.15 moles) was charged in a 100 mL three-necked round-bottomed flask, equipped with internal temperature probe.

(40) The solution was placed in an ice/ethanol bath and kept at 0 C. while stirring, subsequently acetyl chloride (10.19 g, 9.23 mL, 0.13 moles) was added dropwise. After the addition was completed, the mixture was stirred at room temperature for 4 hours. After reaction terminated, water (20 mL) was added to the mixture and distilled off to remove methylene chloride.

(41) The obtained solid was filtered through a folded filter and left to dry overnight. The residue was extracted three times with toluene (310 mL) in a separatory funnel, to remove water and then the organic layer was dried with sodium sulfate and filtered. The solution was cooled to 0 C. for 2 hours to give crystals that were filtered and dried to 60 C. for 7 hours, to give crystals of N-acetoxybenzimidoyl azide (18.36 g, 0.09 moles).

(42) The resulting yield of reaction was of: 92.3%

Step 4: Preparation of 5-phenyl-1-hydroxy-(1H)-tetrazole

(43) N-acetoxybenzimidoyl azide (18.36 g, 0.09 mol) dissolved in 30 mL of diethyl ether, and zinc chloride were charged in a 100 mL single-necked round-bottomed flask. The solution was cooled to 20 C. with stirring for 2 hour to give de-acetylation and intramolecular cyclization. After reaction terminated, the solvent was removed by rotary evaporation at temperature of 30 C. and vacuum of 400 mbar, to obtain crystals of 5-phenyl-1-hydroxy-(1H)-tetrazole (11.34 g, 0.07 moles).

(44) The resulting yield of reaction was of: 77.7%

Example 2

Evaluation of Corrosion Inhibiting Property

(45) The 5-phenyl-1-hydroxy-(1H)-tetrazole corrosion inhibitor prepared in Example 1 was used to evaluate the corrosion inhibition of the aluminium shield 16 in the cable 11 of FIG. 1.

(46) The three-electrode linear polarization resistance (LPR) method was used for evaluating the corrosion rate in presence and absence of inhibitor, as shown, for example, in http://www.gamry.com/application-notes/corrosion-coatings/corrosion-techniques-polarization-resistance/.

(47) The three-electrode system was realized by using (i) a Standard Calomel Reference Electrode as a reference electrode, (ii) an aluminium screen, insulated with a polyester tape, as a working electrode (WE), and (iii) an aluminium wire of 1.6 mm diameter and 45 cm long, insulated with a polyester tape, inserted between the aluminium screen and a polyethylene outer sheath, as a counter electrode (CE).

(48) For comparison purpose, two systems were realized, with and without corrosion inhibitor.

(49) In the system with the corrosion inhibitor, the aluminium wire employed as counter electrode was previously immersed into tap water solution containing 0.7 mmoles/L of 5-phenyl-1-hydroxy-(1H)-tetrazole as corrosion inhibitor (I). In the system without the corrosion inhibitor, the aluminium wire employed as counter electrode was previously immersed into tap water.

(50) Using the linear polarization resistance set up the material was polarized, typically on the order of 10 mV, relative to the open circuit potential. As the potential of the working electrode was changed, a current was induced to flow between the working and counter electrodes, and the material's resistance to polarization was found by taking the slope of the potential versus current curve. This resistance was then used to find the corrosion rate of the material using the Stern-Geary equation.

(51) The results are illustrated in FIG. 3, showing a graph plotting the polarization resistance variation (ohm.Math.cm.sup.2) in ordinate over time (hours) in abscissa with (straight line and filled marker) and without (dashed line and empty marker) corrosion inhibitor, and in FIG. 4, showing a graph plotting the corrosion rate variation (as current density, Amp/cm.sup.2) in ordinate over time (hours) in abscissa with (dashed line and rhombic markers) and without (straight line and round markers) corrosion inhibitor.

(52) The values of polarization resistance obtained with the sample with corrosion inhibitor were substantially higher than those obtained with the sample without corrosion inhibitor, as from FIG. 3. The results illustrated in FIG. 4 show that the calculated corrosion rate based on LPR method in the presence of inhibitor was about half the calculated corrosion rate in the absence of inhibitor.

(53) These results confirmed that a corrosion inhibitor of formula (I) can effectively inhibit the corrosion of aluminium elements of power cables.