WO2015169572A1 - Aquaculture net with coated steel wires - Google Patents

Aquaculture net with coated steel wires Download PDF

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Publication number
WO2015169572A1
WO2015169572A1 PCT/EP2015/058479 EP2015058479W WO2015169572A1 WO 2015169572 A1 WO2015169572 A1 WO 2015169572A1 EP 2015058479 W EP2015058479 W EP 2015058479W WO 2015169572 A1 WO2015169572 A1 WO 2015169572A1
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WO
WIPO (PCT)
Prior art keywords
stainless steel
aquaculture net
net according
weight
ferritic
Prior art date
Application number
PCT/EP2015/058479
Other languages
French (fr)
Inventor
Stefaan Debondt
Bart ALLAERT
Eline DE ROOSE
Jan Mestdagh
Filiep VANHEE
Florence LACANTE
Original Assignee
Nv Bekaert Sa
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Application filed by Nv Bekaert Sa filed Critical Nv Bekaert Sa
Publication of WO2015169572A1 publication Critical patent/WO2015169572A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • A—HUMAN NECESSITIES
    • A01—AGRICULTURE; FORESTRY; ANIMAL HUSBANDRY; HUNTING; TRAPPING; FISHING
    • A01K—ANIMAL HUSBANDRY; AVICULTURE; APICULTURE; PISCICULTURE; FISHING; REARING OR BREEDING ANIMALS, NOT OTHERWISE PROVIDED FOR; NEW BREEDS OF ANIMALS
    • A01K61/00—Culture of aquatic animals
    • A01K61/60—Floating cultivation devices, e.g. rafts or floating fish-farms
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • B32B15/015—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium the said other metal being copper or nickel or an alloy thereof
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C19/00—Alloys based on nickel or cobalt
    • C22C19/03—Alloys based on nickel or cobalt based on nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C19/00—Alloys based on nickel or cobalt
    • C22C19/03—Alloys based on nickel or cobalt based on nickel
    • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
    • C22C19/058—Alloys based on nickel or cobalt based on nickel with chromium without Mo and W
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C9/00—Alloys based on copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C9/00—Alloys based on copper
    • C22C9/04—Alloys based on copper with zinc as the next major constituent
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C9/00—Alloys based on copper
    • C22C9/06—Alloys based on copper with nickel or cobalt as the next major constituent
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A40/00—Adaptation technologies in agriculture, forestry, livestock or agroalimentary production
    • Y02A40/80—Adaptation technologies in agriculture, forestry, livestock or agroalimentary production in fisheries management
    • Y02A40/81—Aquaculture, e.g. of fish

Definitions

  • the invention relates to an aquaculture net with metal wires.
  • Aquaculture nets or fish-farnning nets are used to raise aquatic life such as fish.
  • the aquaculture net keeps the aquatic life controlled and contained and protects the aquatic life inside the net against predators such as sharks and sea wolfs.
  • the aquaculture nets are usually of the chain-link fence type. This is a fence of wires woven into diamond pattern openings.
  • the mesh openings have a dimension that is smaller than the dimension of the fish contained in the nets.
  • Each wire is preformed with bends so that it exhibits a wavy pattern with maxima and minima.
  • the maxima of a wire interlock with the minima of a neighbouring wire to form the patterns of a series of diamonds.
  • aquaculture nets have an acceptable resistance against bio- fouling, i.e. against fouling material that may grow on the mesh structure.
  • fouling material refer to fouling organisms such as barnacles, algae or molluscs, which may attach and grow to the wire material of the mesh structure.
  • this fouling mechanism may be so persistent that entire openings in the meshes may be filled blocking any introduction of fresh water or nutrition into the volume inside the mesh structure.
  • cladded wire Due to suboptimal annealing and high difference in strain hardening of core wire and thick coating, cladded wire can be cold deformed only to a limited extent.
  • the invention provides the combination of ferrite for interface roughness and austenite for strength of the cladded wire.
  • Duplex type of stainless steel offers this combination: it consists of a mixture of ferrite and austenite, suitable for creating interface roughness during wire drawing (both of half product before annealing and of end product) and suitable for sufficient strength to carry the cage weight without fracture.
  • Duplex does not need to be rough as core wire in the cladding process, which solves the dirt and gas problem associated with cladding rough austenitic steel core wire.
  • the invention provides Cu-alloy cladded coating on duplex core wire.
  • the invention provides an aquaculture net with Cu-alloy cladded duplex wire, which combines excellent anti-fouling and fracture free properties.
  • an aquaculture net comprising chain-linked metal wires, said metal wires having a ferritic- austenitic duplex stainless steel core cladded with a copper alloy coating to give an anti-corrosion and anti-fouling property to said metal wires, wherein said ferritic-austenitic duplex stainless steel core has a ferrite content in the range of 10 to 90% by volume and has an average surface roughness Ra in the range of 1 to 5 micro meters at the interface between the ferritic-austenitic duplex stainless steel core and the copper alloy coating.
  • Copper alloy coating has substantially face centered cubic (fee) structure.
  • substantially means said copper alloy coating holds more than 90 % in volume having face centered cubic structure.
  • the copper alloy coating may contain a small portion (less than 10 % in volume, e.g. related to impurities) having different crystal structure.
  • the invention aquaculture net has a prolonged life time as a consequence of several advantages of the solution.
  • the aquaculture net has anti- corrosion and anti-fouling property due to the metal wires cladded with the specific Cu-alloy coating.
  • crevices can be avoided by having the combination of the core wire holding ferrite (bec) on one hand and the copper alloy coating of fee type on the other hand.
  • bee type core wire is favorable for creating interface roughness and for reaching adhesion between core wire and cladded Cu-alloy coating.
  • a reasonable adhesion between the interface of the duplex core wire and the copper alloy coating is built as the interface roughness increases during wire drawing. Adhesion is important for prolonged life because interface gaps - once exposed to sea water - can trigger crevice corrosion of the core wire.
  • Most ferritic stainless steels (with bec crystal structure) are not suitable for exposure to sea water.
  • the interface roughness is homogeneous; the level of roughness can be meticulously modified by tuning the content of ferrite and by changing wire drawing and heat treatment parameters.
  • the more standard way of getting a rough interface i.e. by roughening the core wire, e.g. by sand blasting
  • rough core wire is difficult to clean and drags dirt and gas into the cladding tube; dirt and gas reduce adhesion.
  • the stainless steel core wire has ferrite content in the range of 10 to 90% by volume.
  • the stainless steel core wire could be the ferritic-austenitic duplex stainless steel.
  • the tensile strength of duplex stainless steel is generally higher than the commonly used austenitic stainless steel.
  • the aquaculture net according to the present invention would be lighter or allow thicker copper alloy coating compared with a net having austenitic stainless steel as core wire in order to reach the same strength. Lower weight is beneficial for cost; thicker copper alloy coating is beneficial for corrosion resistance.
  • the yield strength of duplex stainless steel is substantially (almost factor 2) higher than the commonly used austenitic stainless steel. This is relevant to failure caused by fretting and fatigue: low yield strength allows the cage weight to deform the normally rounded bend point in chain-linked wire to more sharply bended shape, which leads to higher stress concentration at contact points. If the core wires are made of material having high yield strength, the original chain link shape would be kept and the failure of the net due to fretting and fatigue would be limited.
  • the core wire is a ferritic-austenitic duplex stainless steel wire.
  • the stainless steel core wire has ferrite content in the range of 10 to 90% by volume, and preferably has ferrite content in the range of 40-70% by volume.
  • the stainless steel can be duplex stainless steel having a mixed microstructure of austenite and ferrite.
  • the ferritic-austenitic duplex stainless steel wire has a ferrite content of about 50% by volume.
  • Duplex stainless steels are characterized by high chromium (19-35% by weight) and molybdenum (up to 5% by weight) and lower nickel contents than austenitic stainless steels.
  • the duplex stainless steel contains nickel preferably in the range of 1.5 to 8% by weight. Nickel helps to obtain the desired phase balance and provides toughness. The desired toughness can be adjusted depending on the requirement of chain-linking process.
  • the exposed stainless steel surfaces are prepared to best possible finish.
  • copper alloy coating is applied on top of the ferritic-austenitic duplex steel wire.
  • the roughness of ferritic- austenitic duplex steel core at the interface is controlled by the composition and the process of the coated metal wire.
  • the roughness of ferritic-austenitic duplex steel core at the interface can be measured with image analysis of cross sections by subtraction of wire shape from the interface profile. Wire shape is calculated by applying a filter on the interface profile; the filter uses a cut-off length of 0.25 mm. Roughness values Ra >1 ⁇ are preferred for good adhesion.
  • An average surface roughness Ra and its standard deviation are calculated for test specimens. Average or mean number and standard deviation are commonly applied in statics.
  • An average surface roughness is the sum of a collection of numbers of surface roughness divided by the number of numbers in the collection. For instance, for one test specimen twelve images are analyzed and thus twelve roughness numbers are averaged.
  • the standard deviation of a surface roughness is the square root of the variance, wherein the variance is the average of the squared differences from the average.
  • the ferritic-austenitic duplex steel wire preferably has an average surface roughness Ra in the range of 1 to 5 micro meters, more preferably in the range of 1.2 to 3 micro meters, and most preferably in the range of 1.2 to 2 micro meters.
  • the standard deviation of the measured surface roughness may be in the range from 0.1 to 1.5 micro meters, e.g. from 0.1 to 0.5 micro meters.
  • the range of average surface roughness Ra is optimized to resist crevice corrosion and to create a good adhesion at the interface between the duplex stainless steel core and its top copper alloy coating.
  • duplex stainless steel with pitting and crevice corrosion resistance (best expressed by the pitting resistance equivalent number) is applied as the core material of the wire in the present invention due to its excellent corrosion resistance.
  • Pitting resistance equivalent number (PREN) is a measurement of the corrosion resistance of stainless steel.
  • the most relevant testing procedures are specified in the ASTM G48 and ASTM G150 standards. In general: the higher the PREN-value, the more corrosion resistant the steel.
  • the ferritic-austenitic duplex stainless steel contains the following elements (in % by weight):
  • the PREN-value of the duplex stainless steel exceeds 25.
  • the PREN-value is at least 20 in both the ferritic and austenitic phases. In order to provide an even better corrosion protection, the PREN-value preferably exceeds 34.
  • the copper alloy coating can be in the form of a strip fixed around said duplex steel core.
  • said strip has been drawn on said ferritic-austenitic duplex stainless steel core.
  • a strip of a suitable metal of controlled composition and predetermined and desired thickness can be formed into a tube form. The width of this strip is somewhat greater or equal to the circumference of the steel core to be covered.
  • the strip is shaped into a tube and seam welded on or around the steel core.
  • two strips can be used to cover the steel core. Instead of seam welding, these two strips are drawn onto the steel core.
  • said copper alloy coating is a copper nickel alloy. Copper nickel coatings have proven to provide a good resistance against corrosion because of the nickel, and good resistance against fouling because of the effect of copper.
  • said copper alloy coating is CuNixFey whereby x is 9, 10 or 1 1 and y is 1.
  • said copper alloy coating is CuNixSny whereby x is 8, 9, 10 or 1 1 and y is 1 , 2 or 3.
  • x and y are weight percentages.
  • said copper nickel alloy comprises at least 80 per cent by weight copper and between 5 per cent by weight and 15 per cent by weight nickel.
  • a composition of 90 wt% Cu and 10 wt% Ni has proven to be an acceptable composition.
  • the ferritic-austenitic duplex stainless steel core presents a gradient of Ni close to the interface between said ferritic-austenitic duplex stainless steel core and said copper alloy coating. This is in particular observed by annealing at an appropriate temperature. Tests have been performed at different temperatures, e.g. annealing a duplex steel wire coated with CuNi for 2 hours at 1070 °C. A significant enrichment in Ni is observed towards the outer surface of a duplex stainless steel core. In other words, a gradient in Ni close to the interface between said steel core and said copper alloy coating is noticed. This further improves adhesion between the stainless steel core and the copper alloy coating.
  • the coated ferritic-austenitic duplex stainless steel wire is further drawn after the application of copper alloy coating.
  • the further drawing of coated wire can be done by using Turks heads at high temperature. Turks heads can be applied just after welding to press the coating onto the core wire. All cross sections showed perfect adhesion. After combining with drawing in one die pass, no gaps were seen at the interface, even at the welding zone. Characteristic voids or gaps at the welding zone were not observed during the further drawing.
  • Figure 1 shows a cross section of a wire according to the invention.
  • Figure 2 shows the process of welding a copper alloy coating to a steel core.
  • Figure 1 shows a cross section of a steel wire 12 according to the invention.
  • a copper alloy coating 16 is welded to or around a ferritic- austenitic duplex stainless steel core 14.
  • Figure 2 shows a process of welding a copper alloy coating 16 to a steel core 14.
  • a strip of a suitable metal and predetermined thickness can be formed into a tube form. The width of this strip is somewhat greater or equal to the circumference of the steel core 14 to be covered. The strip is closed in a tube and welded around the steel core 14. After welding, Turks heads 60 presses the copper alloy coating 16 to the steel core 14.
  • a process is provided wherein a copper alloy coating of predefined composition and thickness is cladded onto a steel core wire by providing the coating in the form of one or more strips, deforming the strip or strips to make it surround the core wire and fixing it onto the core wire.
  • the strip is fixed onto the core wire by seam welding.
  • the process step of welding may be followed by a step of pressing the coating against the steel core by means of Turks heads at a minimum temperature of 200 °C.
  • the process step of enclosing the steel core with a strip or foil of metal may be followed by a step of annealing the wire at a temperature above 950 °C for a time period of at least one hour.
  • Said copper alloy coating is a copper nickel alloy.
  • the steel core 14 is made from a ferritic-austenitic duplex stainless steel which provides good corrosion resistance and strength.
  • the steel core 14 has an average surface roughness Ra in the range of 1 to 5 micro meters at the interface between the ferritic-austenitic duplex stainless steel core and the copper alloy coating.
  • Table 1 Chemical composition for ferritic-austenitic duplex stainless steels used according to the present invention (% in weight). type C Cr Ni Mo N Others PREN
  • the corrosion resistance of the duplex stainless steel wire according to the present invention is tested in an environment with high chloride concentration, wherein rod and drawn or processed steel wire of austenitic stainless steel type 304L are taken as references.
  • Stainless alloys normally do not corrode because they are in a passive state. However, once passivity is broken they become active and can corrode very fast. Factors influencing the breakdown of passivity are chloride concentration (attacking the passive layer), temperature (higher is worse), oxidants, acidity and potential. ASTM G48 and ASTM G150 describe the practices for evaluating pitting and crevice corrosion resistance of stainless steels. ASTM G48 applies Fe(lll)Cl3 solutions which create an environment with high chloride, acidity and Fe(lll).
  • 304L reference material (rod and processed wire) is not resistant to ASTM G48 practice A and B.
  • Ferritic-austenitic duplex stainless steels perform much better according to the practices of ASTM G48, with increased performance of the duplex steels being related with higher PREN values. This difference is even more pronounced for Cu- alloy coated wire because austenitic core wire has lower adhesion to the coating (i.e. more crevices) than ferritic-austenitic duplex stainless steel core wire.

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  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
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  • Metallurgy (AREA)
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Abstract

An aquaculture net comprising chain-linked metal wires, said metal wires having a ferritic-austenitic duplex stainless steel core (14) cladded with a copper alloy coating (16) to give an anti-corrosion and anti-fouling property to said metal wires, wherein said ferritic-austenitic duplex stainless steel core has a ferrite content in the range of 10 to 90% by volume and has an average surface roughness Ra in the range of 1 to 5 micro meters at the interface between the ferritic-austenitic duplex stainless steel core and the copper alloy coating.

Description

AQUACULTURE NET WITH COATED STEEL WIRES
Description Technical Field
[0001] The invention relates to an aquaculture net with metal wires. Background Art
[0002] Aquaculture nets or fish-farnning nets are used to raise aquatic life such as fish. The aquaculture net keeps the aquatic life controlled and contained and protects the aquatic life inside the net against predators such as sharks and sea wolfs.
[0003] The aquaculture nets are usually of the chain-link fence type. This is a fence of wires woven into diamond pattern openings. The mesh openings have a dimension that is smaller than the dimension of the fish contained in the nets. Each wire is preformed with bends so that it exhibits a wavy pattern with maxima and minima. The maxima of a wire interlock with the minima of a neighbouring wire to form the patterns of a series of diamonds.
[0004] Preferably, aquaculture nets have an acceptable resistance against bio- fouling, i.e. against fouling material that may grow on the mesh structure. Within the context of the present invention, the terms fouling material refer to fouling organisms such as barnacles, algae or molluscs, which may attach and grow to the wire material of the mesh structure. However, this fouling mechanism may be so persistent that entire openings in the meshes may be filled blocking any introduction of fresh water or nutrition into the volume inside the mesh structure.
[0005] It is known that Cu-alloy cladded wire for aquaculture cages exists (WO 2009095135). One of the advantages of cladded wire is in the high thickness of the coating, which should be favorable for corrosion life time. Some Cu-alloys are known to have excellent anti-fouling properties. [0006] It is known that chain-link fence type metal wire based aquaculture cages suffer from fretting at contact points between wires (WO 2009095135). As fretting exposes the core wire of coated wire to sea water, the core wire is preferred to be stainless steel.
[0007] It is known that high tensile wire for aquaculture cages exists (US 201 1 0265729); such high tensile wire can be stainless steel wire. High tensile wire is expected to allow for reduced weight of cages, which should be favorable for cost.
[0008] Reaching high tensile strength (e.g. UTS>1000 MPa) with stainless steel requires cold deformation (wire drawing strain).
[0009] Due to suboptimal annealing and high difference in strain hardening of core wire and thick coating, cladded wire can be cold deformed only to a limited extent.
[0010] Given the diameter typically used for wire in aquaculture (i.e. >1.5 mm), it is not obvious to reach strength of e.g. UTS 2000 MPa; such strength level is reached with limited wire drawing strain only with spring wire type of austenitic stainless steel.
Disclosure of Invention
[001 1] It is the object of the invention to avoid the drawbacks of the prior art in chain-linked fence type aquaculture nets with cladded wire.
[0012] The invention provides the combination of ferrite for interface roughness and austenite for strength of the cladded wire. Duplex type of stainless steel offers this combination: it consists of a mixture of ferrite and austenite, suitable for creating interface roughness during wire drawing (both of half product before annealing and of end product) and suitable for sufficient strength to carry the cage weight without fracture. Duplex does not need to be rough as core wire in the cladding process, which solves the dirt and gas problem associated with cladding rough austenitic steel core wire.
[0013] The invention provides Cu-alloy cladded coating on duplex core wire.
[0014] The invention provides an aquaculture net with Cu-alloy cladded duplex wire, which combines excellent anti-fouling and fracture free properties. [0015] According to the present invention, it is provided an aquaculture net comprising chain-linked metal wires, said metal wires having a ferritic- austenitic duplex stainless steel core cladded with a copper alloy coating to give an anti-corrosion and anti-fouling property to said metal wires, wherein said ferritic-austenitic duplex stainless steel core has a ferrite content in the range of 10 to 90% by volume and has an average surface roughness Ra in the range of 1 to 5 micro meters at the interface between the ferritic-austenitic duplex stainless steel core and the copper alloy coating.
[0016] Copper alloy coating has substantially face centered cubic (fee) structure.
Herein, "substantially" means said copper alloy coating holds more than 90 % in volume having face centered cubic structure. The copper alloy coating may contain a small portion (less than 10 % in volume, e.g. related to impurities) having different crystal structure.
[0017] The invention aquaculture net has a prolonged life time as a consequence of several advantages of the solution. First, the aquaculture net has anti- corrosion and anti-fouling property due to the metal wires cladded with the specific Cu-alloy coating.
[0018] Secondly, crevices can be avoided by having the combination of the core wire holding ferrite (bec) on one hand and the copper alloy coating of fee type on the other hand. Indeed, it is known that the use of bee type core wire is favorable for creating interface roughness and for reaching adhesion between core wire and cladded Cu-alloy coating. A reasonable adhesion between the interface of the duplex core wire and the copper alloy coating is built as the interface roughness increases during wire drawing. Adhesion is important for prolonged life because interface gaps - once exposed to sea water - can trigger crevice corrosion of the core wire. Most ferritic stainless steels (with bec crystal structure) are not suitable for exposure to sea water. Due to the fine microstructure of duplex, the interface roughness is homogeneous; the level of roughness can be meticulously modified by tuning the content of ferrite and by changing wire drawing and heat treatment parameters. [0019] The more standard way of getting a rough interface (i.e. by roughening the core wire, e.g. by sand blasting) is not advantageous for thick cladded coatings, because rough core wire is difficult to clean and drags dirt and gas into the cladding tube; dirt and gas reduce adhesion.
[0020] According to the present invention, the stainless steel core wire has ferrite content in the range of 10 to 90% by volume. The stainless steel core wire could be the ferritic-austenitic duplex stainless steel. In annealed condition, the tensile strength of duplex stainless steel is generally higher than the commonly used austenitic stainless steel. In the other words, the aquaculture net according to the present invention would be lighter or allow thicker copper alloy coating compared with a net having austenitic stainless steel as core wire in order to reach the same strength. Lower weight is beneficial for cost; thicker copper alloy coating is beneficial for corrosion resistance.
[0021] In annealed condition, the yield strength of duplex stainless steel is substantially (almost factor 2) higher than the commonly used austenitic stainless steel. This is relevant to failure caused by fretting and fatigue: low yield strength allows the cage weight to deform the normally rounded bend point in chain-linked wire to more sharply bended shape, which leads to higher stress concentration at contact points. If the core wires are made of material having high yield strength, the original chain link shape would be kept and the failure of the net due to fretting and fatigue would be limited.
[0022] In a preferred embodiment, the core wire is a ferritic-austenitic duplex stainless steel wire. According to the present invention, the stainless steel core wire has ferrite content in the range of 10 to 90% by volume, and preferably has ferrite content in the range of 40-70% by volume. The stainless steel can be duplex stainless steel having a mixed microstructure of austenite and ferrite. Preferably, the ferritic-austenitic duplex stainless steel wire has a ferrite content of about 50% by volume. Duplex stainless steels are characterized by high chromium (19-35% by weight) and molybdenum (up to 5% by weight) and lower nickel contents than austenitic stainless steels. The duplex stainless steel contains nickel preferably in the range of 1.5 to 8% by weight. Nickel helps to obtain the desired phase balance and provides toughness. The desired toughness can be adjusted depending on the requirement of chain-linking process.
[0023] In aggressive marine environment, normally the exposed stainless steel surfaces are prepared to best possible finish. Mirror-finish resists pitting best. According to the present invention, copper alloy coating is applied on top of the ferritic-austenitic duplex steel wire. The roughness of ferritic- austenitic duplex steel core at the interface is controlled by the composition and the process of the coated metal wire. The roughness of ferritic-austenitic duplex steel core at the interface can be measured with image analysis of cross sections by subtraction of wire shape from the interface profile. Wire shape is calculated by applying a filter on the interface profile; the filter uses a cut-off length of 0.25 mm. Roughness values Ra >1 μηη are preferred for good adhesion. An average surface roughness Ra and its standard deviation are calculated for test specimens. Average or mean number and standard deviation are commonly applied in statics. An average surface roughness is the sum of a collection of numbers of surface roughness divided by the number of numbers in the collection. For instance, for one test specimen twelve images are analyzed and thus twelve roughness numbers are averaged. The standard deviation of a surface roughness is the square root of the variance, wherein the variance is the average of the squared differences from the average.
[0024] At the interface between the ferritic-austenitic duplex stainless steel core and the coating, the ferritic-austenitic duplex steel wire preferably has an average surface roughness Ra in the range of 1 to 5 micro meters, more preferably in the range of 1.2 to 3 micro meters, and most preferably in the range of 1.2 to 2 micro meters. The standard deviation of the measured surface roughness may be in the range from 0.1 to 1.5 micro meters, e.g. from 0.1 to 0.5 micro meters. The range of average surface roughness Ra is optimized to resist crevice corrosion and to create a good adhesion at the interface between the duplex stainless steel core and its top copper alloy coating. [0025] In a preferred embodiment, duplex stainless steel with pitting and crevice corrosion resistance (best expressed by the pitting resistance equivalent number) is applied as the core material of the wire in the present invention due to its excellent corrosion resistance. Pitting resistance equivalent number (PREN) is a measurement of the corrosion resistance of stainless steel. The PREN-value is calculated using the following formula: PREN = 1 x %Cr + 3.3 x %Mo + 16 x %N (% in weight). The most relevant testing procedures are specified in the ASTM G48 and ASTM G150 standards. In general: the higher the PREN-value, the more corrosion resistant the steel. According to the present invention, the ferritic-austenitic duplex stainless steel contains the following elements (in % by weight):
Cr in the range of 22.0-35.0% by weight
Mo in the range of 0.3-5.0% by weight
N in the range of 0.1 -0.6% by weight
and the PREN-value of the duplex stainless steel exceeds 25. Preferably, the PREN-value is at least 20 in both the ferritic and austenitic phases. In order to provide an even better corrosion protection, the PREN-value preferably exceeds 34.
[0026] Preferably, the copper alloy coating can be in the form of a strip fixed around said duplex steel core. In a preferred example, said strip has been drawn on said ferritic-austenitic duplex stainless steel core. A strip of a suitable metal of controlled composition and predetermined and desired thickness can be formed into a tube form. The width of this strip is somewhat greater or equal to the circumference of the steel core to be covered. The strip is shaped into a tube and seam welded on or around the steel core. Alternatively, two strips can be used to cover the steel core. Instead of seam welding, these two strips are drawn onto the steel core.
[0027] In a preferred embodiment said copper alloy coating is a copper nickel alloy. Copper nickel coatings have proven to provide a good resistance against corrosion because of the nickel, and good resistance against fouling because of the effect of copper.
[0028] In one embodiment said copper alloy coating is CuNixFey whereby x is 9, 10 or 1 1 and y is 1. In another embodiment said copper alloy coating is CuNixSny whereby x is 8, 9, 10 or 1 1 and y is 1 , 2 or 3. Herein x and y are weight percentages. These particular alloys have the advantage that they harden during an annealing process resulting in an increased abrasion resistance. Other alloys are also possible, such as CuZnSn alloys and CuZnNi alloys.
[0029] In another embodiment said copper nickel alloy comprises at least 80 per cent by weight copper and between 5 per cent by weight and 15 per cent by weight nickel. A composition of 90 wt% Cu and 10 wt% Ni has proven to be an acceptable composition.
[0030] In an example according to the present invention, the ferritic-austenitic duplex stainless steel core presents a gradient of Ni close to the interface between said ferritic-austenitic duplex stainless steel core and said copper alloy coating. This is in particular observed by annealing at an appropriate temperature. Tests have been performed at different temperatures, e.g. annealing a duplex steel wire coated with CuNi for 2 hours at 1070 °C. A significant enrichment in Ni is observed towards the outer surface of a duplex stainless steel core. In other words, a gradient in Ni close to the interface between said steel core and said copper alloy coating is noticed. This further improves adhesion between the stainless steel core and the copper alloy coating.
[0031] In a preferred embodiment, the coated ferritic-austenitic duplex stainless steel wire is further drawn after the application of copper alloy coating. The further drawing of coated wire can be done by using Turks heads at high temperature. Turks heads can be applied just after welding to press the coating onto the core wire. All cross sections showed perfect adhesion. After combining with drawing in one die pass, no gaps were seen at the interface, even at the welding zone. Characteristic voids or gaps at the welding zone were not observed during the further drawing.
Brief Description of Figures in the Drawings
[0032] Figure 1 shows a cross section of a wire according to the invention.
[0033] Figure 2 shows the process of welding a copper alloy coating to a steel core. Mode(s) for Carrying Out the Invention
[0034] Figure 1 shows a cross section of a steel wire 12 according to the invention. A copper alloy coating 16 is welded to or around a ferritic- austenitic duplex stainless steel core 14.
[0035] Figure 2 shows a process of welding a copper alloy coating 16 to a steel core 14. A strip of a suitable metal and predetermined thickness can be formed into a tube form. The width of this strip is somewhat greater or equal to the circumference of the steel core 14 to be covered. The strip is closed in a tube and welded around the steel core 14. After welding, Turks heads 60 presses the copper alloy coating 16 to the steel core 14.
[0036] More generally, a process is provided wherein a copper alloy coating of predefined composition and thickness is cladded onto a steel core wire by providing the coating in the form of one or more strips, deforming the strip or strips to make it surround the core wire and fixing it onto the core wire.
[0037] Preferably the strip is fixed onto the core wire by seam welding.
[0038] Preferably the process step of welding may be followed by a step of pressing the coating against the steel core by means of Turks heads at a minimum temperature of 200 °C.
[0039] Alternatively or additionally, the process step of enclosing the steel core with a strip or foil of metal may be followed by a step of annealing the wire at a temperature above 950 °C for a time period of at least one hour.
[0040] Said copper alloy coating is a copper nickel alloy.
[0041] According to the present invention, the steel core 14 is made from a ferritic-austenitic duplex stainless steel which provides good corrosion resistance and strength. The steel core 14 has an average surface roughness Ra in the range of 1 to 5 micro meters at the interface between the ferritic-austenitic duplex stainless steel core and the copper alloy coating. As examples, the compositions of ferritic-austenitic duplex stainless steel used as the core of the wire according to the present invention are shown in table 1. [0042] Table 1 - Chemical composition for ferritic-austenitic duplex stainless steels used according to the present invention (% in weight). type C Cr Ni Mo N Others PREN
Figure imgf000010_0001
(LDX2404)
C 0.02 5.7 ™5 » i 0.17
(2205)
""D " 0.02 "25 7 4 " " " o' '27 43
(2507)
[0043] The corrosion resistance of the duplex stainless steel wire according to the present invention is tested in an environment with high chloride concentration, wherein rod and drawn or processed steel wire of austenitic stainless steel type 304L are taken as references.
[0044] It has been observed that when the crevice corrosion occurs, the stainless steel core wire is no longer protected but corrodes under the CuNi cladding. The galvanic couple has changed from the desired situation because in the crevice the environment is different than outside the crevice.
[0045] Stainless alloys normally do not corrode because they are in a passive state. However, once passivity is broken they become active and can corrode very fast. Factors influencing the breakdown of passivity are chloride concentration (attacking the passive layer), temperature (higher is worse), oxidants, acidity and potential. ASTM G48 and ASTM G150 describe the practices for evaluating pitting and crevice corrosion resistance of stainless steels. ASTM G48 applies Fe(lll)Cl3 solutions which create an environment with high chloride, acidity and Fe(lll).
[0046] It is known that 304L reference material (rod and processed wire) is not resistant to ASTM G48 practice A and B. Ferritic-austenitic duplex stainless steels perform much better according to the practices of ASTM G48, with increased performance of the duplex steels being related with higher PREN values. This difference is even more pronounced for Cu- alloy coated wire because austenitic core wire has lower adhesion to the coating (i.e. more crevices) than ferritic-austenitic duplex stainless steel core wire.

Claims

Claims
1. An aquaculture net comprising chain-linked metal wires, said metal wires having a ferritic-austenitic duplex stainless steel core cladded with a copper alloy coating to give an anti-corrosion and anti-fouling property to said metal wires, wherein said ferritic-austenitic duplex stainless steel core has a ferrite content in the range of 10 to 90% by volume and has an average surface roughness Ra in the range of 1 to 5 micro meters at the interface between the ferritic-austenitic duplex stainless steel core and the copper alloy coating.
2. An aquaculture net according to claim 1 , wherein the ferrite content is in the range of 40 to 70% by volume.
3. An aquaculture net according to claim 1 or 2, wherein the stainless steel contains the following elements (in % by weight):
Cr in the range of 22.0-35.0% by weight
Mo in the range of 0.3-5.0% by weight
N in the range of 0.1 -0.6% by weight
and for which the relation PREN=%Cr+3.3%Mo+16%N exceeds 25.
4. An aquaculture net according to claim 3, wherein the stainless steel has PREN value exceeding 34.
5. An aquaculture net according to claim 3 or 4, wherein the PREN-value is at least 20 in both the ferritic and austenitic phases of the stainless steel.
6. An aquaculture net according to any one of the preceding claims, wherein the stainless steel contains Ni in the range of 1.5 to 8% by weight.
7. An aquaculture net according to any one of the preceding claims, wherein said copper alloy coating takes more than 15% by volume of the metal wire.
8. An aquaculture net according to any one of the preceding claims, wherein said copper alloy coating takes more than 30% by volume of the metal wire.
9. An aquaculture net according to any one of the preceding claims, wherein said copper alloy coating is a copper nickel alloy.
10. An aquaculture net according to claim 9, wherein said copper nickel alloy comprises at least 80 per cent by weight copper and between 5 per cent by weight and 15 per cent by weight nickel.
1 1. An aquaculture net according to any one of the preceding claims, wherein said stainless steel core comprises nickel, said nickel presenting a gradient close to the interface between said stainless steel core and said copper alloy coating.
12. An aquaculture net according to any one of the preceding claims, wherein said metal wire is end drawn.
13. An aquaculture net according to any one of the preceding claims, wherein said metal wire is end annealed.
PCT/EP2015/058479 2014-05-06 2015-04-20 Aquaculture net with coated steel wires WO2015169572A1 (en)

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US6451133B1 (en) * 1998-10-23 2002-09-17 Sandvik Ab Stainless steel for use in seawater applications
US20070098588A1 (en) * 2005-11-03 2007-05-03 Daido Steel Co., Ltd. High-nitrogen austenitic stainless steel
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WO2009095135A1 (en) * 2008-01-30 2009-08-06 Nv Bekaert Sa Aquaculture net with steel wires coated with metal strip

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US6451133B1 (en) * 1998-10-23 2002-09-17 Sandvik Ab Stainless steel for use in seawater applications
US20070098588A1 (en) * 2005-11-03 2007-05-03 Daido Steel Co., Ltd. High-nitrogen austenitic stainless steel
US20080240970A1 (en) * 2007-03-31 2008-10-02 Daido Tokushuko Kabushiki Kaisha Austenitic free-cutting stainless steel
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* Cited by examiner, † Cited by third party
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CN110088305A (en) * 2016-12-21 2019-08-02 山特维克知识产权股份有限公司 The purposes of two-phase stainless steel part

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