Non-magnetic stainless steel wire as an armouring wire for power cables

Abstract

A non-magnetic stainless steel wire with an adherent corrosion resistant coating is disclosed. The surface of the non-magnetic stainless steel is pre-treated so as to be sufficiently free from oxides and form a good adhesion with the above corrosion resistant coating. The non-magnetic stainless steel wire is used as a armoring wire for a power cable for transmitting electrical power.

Claims

1. A power cable for transmitting electrical power, comprising: an armouring layer comprising non-magnetic stainless steel wire and magnetic low-carbon steel wire, said non-magnetic stainless steel wire comprising a corrosion resistant coating on the surface of the non-magnetic stainless steel, wherein said surface is pre-treated, the pre-treated surface being sufficiently free from oxides and adhering to said corrosion resistant coating, said corrosion resistant coating is zinc and/or zinc alloy, and said corrosion resistant coating is present in an amount of 20 g/m.sup.2 to 600 g/m.sup.2, and said magnetic low-carbon steel wire comprising a corrosion resistant coating.

2. A power cable for transmitting electrical power as in claim 1, wherein said corrosion resistant coating on the surface of the non-magnetic stainless steel is a hot dipped zinc and/or zinc alloy coating.

3. A power cable for transmitting electrical power as in claim 1, wherein an intermediate layer of electroplated nickel is present between the non-magnetic stainless steel wire and said corrosion resistant coating on the surface of the non-magnetic stainless steel.

4. A power cable for transmitting electrical power as in claim 1, wherein said surface of the non-magnetic stainless steel wire is obtainable by a pre-treatment of electroplating with zinc and/or zinc alloy.

5. A power cable for transmitting electrical power as in claim 1, wherein said surface of the non-magnetic stainless steel is obtainable by a pre-treatment of being held in inert and/or reduction atmosphere before the corrosion resistant coating is formed thereon.

6. A power cable for transmitting electrical power as in claim 1, wherein said non-magnetic stainless steel wire has a round diameter ranging between 1.0 mm to 10.0 mm.

7. A power cable for transmitting electrical power according to claim 1, wherein the power cable is a tri-phase submarine power cable.

8. A power cable for transmitting electrical power according to claim 1, wherein said power cable is a high voltage cable of more than 110 kV.

9. A power cable for transmitting electrical power according to claim 1, wherein said non-magnetic stainless steel wire is wound around at least part of said power cable.

10. A power cable for transmitting electrical power according to claim 1, wherein said armouring layer is an annular armouring layer.

11. A power cable for transmitting electrical power according to claim 1, wherein said non-magnetic stainless steel wire and said magnetic low-carbon steel wire are arranged parallel to each other in the armouring layer.

12. A power cable for transmitting electrical power according to claim 1, wherein said non-magnetic stainless steel wire and said magnetic low-carbon steel wire alternate in the armouring layer.

13. A power cable for transmitting electrical power according to claim 1, wherein said non-magnetic stainless steel wire and said magnetic low-carbon steel wire are interspersed in the armouring layer.

14. A power cable for transmitting electrical power according to claim 1, wherein said magnetic low-carbon steel wire renders the location of the power cable detectable.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

(1) The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

(2) FIG. 1 is a high voltage power cable according to prior art.

(3) FIG. 2 is a cross-section of a non-magnetic stainless steel wire according to the first aspect of the invention.

(4) FIG. 3 is a cross-section of a tri-phase power cable having armouring wires.

MODE(S) FOR CARRYING OUT THE INVENTION

(5) FIG. 2 is a cross-section of a coated non-magnetic stainless steel wire 20. Non-magnetic stainless steel wire 22 is covered by a pre-coated adherent layer 24 and a corrosion resistant coating 26.

Example 1

(6) A steel wire, ref. AISI 202, of a diameter of 1.9 mm is treated according to a first embodiment of the process.

(7) The composition (in percentage by weight) of the wire rod is as follows: C less than 0.08; Si less than 0.75; Mn ranging from 6.6 to 8; P less than 0.045; S less than 0.015; N less than 0.15; Cr ranging from 15 to 17; Ni ranging from 3.5 to 5; Cu less than 2; and the balance is Fe.

(8) The steel wire is processed continuously on one or more lines depending on the capabilities of the production site.

(9) This steel wire is first degreased in an degreasing bath (containing phosphoric acid) at 30 C. to 80 C. for a few seconds. An ultrasonic generator is provided in the bath to assist the degreasing.

(10) Alternatively, the steel wire may be first degreased in an alkaline degreasing bath (containing NaOH) at 30 C. to 80 C. for a few seconds. Electrical assistance is applied in the bath to assist the degreasing.

(11) This is followed by a pickling step, wherein the steel wire is dipped in a pickling bath (containing 100-500 g/l sulphuric acid) at 20 C. to 30 C. to remove the instantaneously formed chromium oxide. This is followed by another successive pickling carried out by dipping the steel wire in a pickling bath (containing 100-500 g/l sulphuric acid) at 20 C. to 30 C. for a short time to further remove the chromium oxide on the surface of the steel wire. All pickling steps may be assisted by electric current to achieve sufficient activation.

(12) After this second pickling step, the steel wire is immediately immersed in a electrolysis bath (containing 10-100 g/l zinc sulphate) at 20 C. to 40 C. for tens to hundreds of seconds. The steel wire is pre-electroplated with zinc and/or zinc alloy. To electrogalvanise, an electrical charge is applied on the steel wire, which attracts the zinc ions to bond to the surface. In current example, the electrogalvanized layer has a coat weight of 10-50 g/m.sup.2. During this step the wire is running at a speed in the range of 20 to 100 m/min, preferably approximately at a speed of 30 m/min. Then the steel wire is rinsed in water and the excess of water is removed.

(13) The electro-plated steel wire is further treated in a fluxing bath. The temperature of fluxing bath is maintained between 50 C. and 90 C., preferably at 70 C. Afterward, the excess of flux is removed. The steel wire is subsequently dipped in a galvanizing bath maintained at temperature of 400 C. to 500 C.

(14) In present application, a coating formed on the surface of the stainless steel wire by galvanizing process is zinc and/or zinc alloy. The thickness of the galvanized coating is ranging from 20 g/m.sup.2 to 600 g/m.sup.2, e.g. ranging from 50 g/m.sup.2 to 300 g/m.sup.2. A zinc aluminum coating has a better overall corrosion resistance than zinc. In contrast with zinc, the zinc aluminum coating is more temperature resistant. Still in contrast with zinc, there is no flaking with the zinc aluminum alloy when exposed to high temperatures. A zinc aluminium coating may have an aluminium content ranging from 2 wt % to 23 wt %, e.g. ranging from 2 wt % to 12 wt %, or e.g. ranging from 5 wt % to 10 wt %. A preferable composition lies around the eutectoid position: aluminium about 5 wt %. The zinc alloy coating may further have a wetting agent such as lanthanum or cerium in an amount less than 0.1 wt % of the zinc alloy. The remainder of the coating is zinc and unavoidable impurities. Another preferable composition contains about 10 wt % aluminium. This increased amount of aluminium provides a better corrosion protection than the eutectoid composition with about 5 wt % of aluminium. Other elements such as silicon and magnesium may be added to the zinc aluminium coating. More preferably, with a view to optimizing the corrosion resistance, a particular good alloy comprises 2 wt % to 10 wt % aluminium and 0.2 wt % to 3.0 wt % magnesium, the remainder being zinc.

(15) After hot-dip galvanising tie- or jet-wiping can be used to control the coating thickness. Then the wire is cooled down in air or preferably by the assistance of water. A continuous, uniform, void-free coating is formed. Several hot-dip galvanizing trials after a pre-electrogalvanizing and with different final coating thickness are summarized in table 1.

(16) TABLE-US-00001 TABLE 1 Hot-dip galvanizing trials after a pre-electrogalvanizing. Sample Speed [m/min] Coat weight [g/m.sup.2] 1 80 21 2 120 265 3 80 228 4 40 217

Example 2

(17) A steel wire, ref. AISI 202, of a diameter of 1.9 mm is treated according to a second embodiment of the process.

(18) This steel wire is first degreased in an acid degreasing bath with the assistance of an ultrasonic generator or degreased in an alkaline degreasing bath with electrical assistance. The steel wire is continued with a pickling step, wherein the steel wire is dipped in a pickling bath (containing 100-500 g/l sulphuric acid) at 20 C. to 30 C. for a few seconds to remove the instantaneously formed chromium oxide. This is followed by another successive pickling carried out by dipping the steel wire in a pickling bath (containing 100-500 g/l sulphuric acid) at 20 C. to 30 C. for a very short time to further and sufficiently remove the chromium oxide on the surface of the steel wire.

(19) After the second pickling step, the steel wire immediately flash coated by nickel sulfamate solution (containing 50-100 g/l) at 20 C. to 60 C. Then the steel wire is dipped in electrolysis bath (containing 50-100 g/l nickel sulfamate) at 20 C. to 60 C. for several minutes. To electroplate nickel, an electrical charge is applied on the steel wire, which attracts the nickel ions to bond to the surface. In this example, the electroplated nickel layer has a coat weight of 20-60 g/m.sup.2. During this step the wire is running at a speed in the range of 20 to 100 m/min, preferably approximately at a speed of 30 m/min. Afterwards, the steel wire is rinsed in water and the excess of water is removed.

(20) The steel wire with a pre-electroplated nickel coating on the surface is further treated in for example a zinc and ammonium chloride fluxing bath and dipped in a galvanizing bath, similar to example 1. After tie- or jet-wiping and cooling, a continuous, uniform, void-free coating was formed on the surface of the steel wire. Several hot-dip galvanizing trials after a pre-electroplated nickel coating and with different final coating thickness are summarized in table 2.

(21) TABLE-US-00002 TABLE 2 Hot-dip galvanizing trials after a pre-electroplated nickel coating. Sample Speed [m/min] Coat weight [g/m.sup.2] 1 80 42 2 40 151 3 80 217

Example 3

(22) A steel wire, ref. AISI 202, of a diameter of 1.9 mm, 6 mm, 7 mm and 8 mm is respectively treated according to a third embodiment of the process.

(23) The steel wire is first degreased and then followed by pickling in acid solution. These processes are similar as in examples 1 and 2.

(24) After the pickling process, the steel wire is rinsed in a flowing water rinsing bath.

(25) In this example, after the excess of water is removed, the wires are further transferred under the protection of the tube filled with a heated reduction gas or gas mixture of argon, nitrogen and/or hydrogen to the galvanizing bath. Preferably, the wires are heated to 400 C. to 900 C. in the tube before the galvanizing bath.

(26) The post steps in this example are similar to the steps illustrated in the above examples 1 and 2.

(27) As a comparison, galvanizing trials are also performed through a conventional process, i.e. the steel wires are not pre-electroplated or there is no inert atmosphere protection during galvanizing process. Wrapping tests are performed on the final products to test the adhesion of coatings with steel wires. Steel wires coated with a pre-treatment step as in above illustrated examples show a very good surface quality: there is no micro-cracks and no delamination. While steel wires, which are not pre-electroplated or there is no inert atmosphere protection during galvanizing process, present a bad surface quality and some coatings are delaminated or peel off.

(28) As a precaution, although steel wires, ref. AISI 202, of a diameter of 1.9, 6, 7 and 8 mm are used herewith as a half-product in the examples, other grade steel wire or steel wire with larger/smaller diameter can also be applied in the invention. It should be noted that a further wire drawing after galvanizing may be applied depending on the application if improvement of the tensile strength of the coated steel wires is desired.

(29) FIG. 3 represents a cross-section of a tri-phase submarine power cable armoured with the non-magnetic stainless steel wires of present invention.

(30) The tri-phase submarine power cable 30 is shown in the illustration. It includes a compact stranded, bare copper conductor 31, followed by a semi-conducting conductor shield 32. An insulation shield 33 is applied to ensure that the conductor do not contact with each other. The insulated conductors are cabled together with fillers 34 by a binder tape, followed by a lead-alloy sheath 35. Due to the severe environmental demands placed on submarine cables, the lead-alloy sheath 35 is often needed because of its compressibility, flexibility and resistance to moisture and corrosion. The sheath 35 is usually covered by an outer layer 37 comprising a polyethylene (PE) or polyvinyl chloride (PVC) jacket. This construction is armoured by steel wire armouring layer 38. The steel wires used herein are according to the invention, i.e. they are non-magnetic stainless steel wires with an adherent galvanized layer for strong corrosion protection. An outer sheath 39, such as made of PVC or cross-linked polyethylene (XLPE) or a combination of PVC and XLPE layers, is preferably applied outside the armouring layer 38.

LIST OF REFERENCE NUMBERS

(31) 10 steel wire armoured cable 12 conductor 14 insulation 16 bedding 18 armour 19 sheath 20 coated non-magnetic stainless steel wire 22 non-magnetic stainless steel wire 24 pre-coated adherent layer 26 corrosion resistant coating 30 power cable 31 copper conductor 32 semi-conducting conductor shield 33 insulation shield 34 fillers 35 lead-alloy sheath 37 outer layer 38 steel wire armouring layer 39 outer sheath