GB2175609A

GB2175609A - Electrode

Info

Publication number: GB2175609A
Application number: GB08608459A
Authority: GB
Inventors: Peter Charles Steele Hayfield
Original assignee: Marston Palmer Ltd
Current assignee: Marston Palmer Ltd
Priority date: 1985-04-12
Filing date: 1986-04-07
Publication date: 1986-12-03
Also published as: GB8509384D0; GB8608459D0; GB2175609B; CA1314518C

Abstract

An extended area electrode comprises one or more wires of a valve metal in the form of an open mesh, the wires having on their surface an anodically active layer. The electrode is particularly suitable for use in low current density applications e.g. electrosmotic dewatering or cathodic protection of reinforcement bars 2 in reinforced concrete 1. A graphite slab 3 acts as an anode and the extended area electrode in the form of a platinum clad niobium wire mesh 4 provides an electrical connection to the slab 3. By using a wire mesh, an extensive area of contact can be produced which enables the anode to operate for long periods of time. <IMAGE>

Description

SPECIFICATION Electrode This invention relates to electrodes and has particular reference to extended area electrodes.

Electrodes are used in many applications and the invention is particularly concerned with the provision of anodes and anode connectors.

It is well known to use anodes in impressed current cathodic protection systems particularly those involving the protection of a steel structure immersed in water. Essentially in such an arrangement the structure is made cathodic relative to an anode to prevent oxidation of the steel structure i.e. to prevent rusting. The chemical reaction taking place at the anode is, in the case of an impressed current system, the evolution of oxygen.

It is well known that certain materials when connected as anodes will rapidly dissolve. Such materials may be used as consumable electrodes but cannot be used as non-consumable electrodes for impressed current systems. Thus it is not possible to use copper in a commercial impressed current system as an anode because it would dissolve.

Other materials, such as metals of the platinum group, are substantially unaffected when connected as an anode. Such materials evolve oxygen and only very slowly dissolve - the wear rate frequently quoted for platinum in an aqueous sodium cloride solution is one microgram per ampere hour. This means that one microgram of platinum dissolves for each ampere hour of current area passed. Such low wear rates gives rise to platinum anodes being considered as non-consumable.

However such terms as non-consumable are really relative and when used herein the terms non-consumable and substantially non-consumable are used to mean a material which is consumed at such a low rate as to be economically useful as an anodically active material in transferring electrons to an electrolyte.

The metals and compounds of the platinum group are classically used in cathodic protection systems as non-consumable surfaces. The platinum group metals are frequently used as a thin layer on a valve metal because of the cost of the platinum group metal itself.

Other materials such as lead dioxide may be used as non-consumable anode surfaces since they are relatively inexpensive and the higher wear rates associated with their use can be compensated by the use of thicker coatings.

When used in sea water it is possible to operate an anode at a high current density because the throwing power of the anode, i.e. the volume which could be electronically influenced by the anode, is high as the electrical conductivity of sea water is high.

There has recently developed a need to produce an anode capable of operating economically at low current densities. It is fairly clear that any anode which can be operated at a high current density could in fact be operated at a low current density.

However the cost of platinum coated anodes is relatively high and the economic viability of their use has in the past depended on their being operated at high current densities.

In the case ofcathodic protection of reinforcement bars (rebars) in reinforced concrete the concrete itself must carry the electrical current to the rebars which are cathodically protected relative to an externally applied anode. The need in such a system is for an anode having an extensive surface area in contact with a large area of concrete because the throwing power of the anode through the concrete is very low. There is, therefore, a need for an anode in such a situation which can operate at a low current density and will be economically provided at a cost which will enable it to be used commercially.

There are other applications for the use of anodes operating at low current densities and having extended areas. The range of such applications is wide and includes electro-osmosis to remove water from water-logged slag heaps, railway sidings, land drainage and other civil engineering applications and electro damp-proofing to dampproof buildings.

By the present invention there is provided an extended area electrode comprising a plurality of wires of valve metal in the form of an open mesh, the wires having on their surface a material having anodically active properties, the material being substantially non-consumable in operation.

The material on the surface may be in the form of a coating.

The anodically active coating may be a platinum group metal or platinum group metal compound or may comprise a doped layer of an oxide of a valve metal. Alternatively the anodically active layer may comprise lead or lead oxide or a sub-stoichiometric tin oxide or antimony oxide.

The mesh structure may be formed by weaving or knitting or may comprise a welded structure in the form of a network of individual strands welded together where they cross.

The valve metals wires may be provided with a core of a metal of higher electrically conductivity than the valve metal, preferably a copper or aluminium core.

The present invention also envisages the use of such an electrode as a connector to a further electrode body such as coke, electrically conductive bitumen, electrically conductive plastics material or electrically conductive cement. The electrically conductive plastics material may be in the form of a sheet with the mesh embedded therein. The mesh may have a plurality of holes in its surface.

The present invention further provides a cathodic protection system for the cathodic protection of reforcement bars in reinforced concrete comprising an anode in electrical connection with the concrete where in the anode comprises a mesh of wire of a valve metal chosen from the group titanium, zirconium, niobium, hafnium and tantalum or alloys based thereon having comparable anodic properties, the wire having on at least part of its surface an anodically active coating of substantially nonconsumable material.

The mesh may be in contact with a further anodically active anode material such as electrically conducting concrete, electrically conducting plastics material, electrically conductive paint, graphite or electrically conducting coke. The electrically conducting plastics material may have the mesh embedded therein.

The mesh may be knitted or woven or may be a welded structure. The thickness of the wires may be different in different portions of the net. The wires may have a core of a metal of higher electrical conductivity than the valve metal, preferably a copper or aluminium core.

By way of example embodiments of the present invention will now be described with reference to the accompanying drawings of which Figure 1 is a schematic sectional view of a cathodically protected reinforced concrete beam, Figure 2 is an enlargement of the portion of Figure 1 within the circle II of Figure 1, Figure 3 is a perspective view of a wire mesh connected directly to a concrete slab, Figure 4 is a schematic view of a knitted wire mesh Figure 5 is a plan view of a form of embedded mesh, and Figure 6 is an enlarged section of part of Figure 5 in situ.

Referring to Figure 1, this shows a concrete beam 1 reinforced by a series of steel reinforcement bars or rebars 2. Although theoretically the rebars when surrounded by an alkaline cement should not rust or corrode, it has been found that corrosion of rebars can take place. It is thought that the most common causes of rebar corrosion are either that the concrete contained chloride ions when it was manufactured or that the application of chloride ions to the outside of the concrete has resulted in penetration of chloride ions to the re bars and their subsequent corrosion. This is a particular problem with bridge decks and their concrete supports where the bridges have been salted for safety reasons throughout the ice and snow season of the year.

Because the electrical conductivity of the concrete of the beam 1 is low, any attempt to cathodically protect the rebars must use an extended surface area anode which is capable of throwing its current across the portion of the concrete beam between the anode and the rebar 2. A single anode at one end of the concrete beam would have insufficient throwing power to protect the rebars throughout the entire concrete beam.

As shown in Figure 2, therefore, a graphite slab 3 is bonded to the concrete beam 1 to act as an anode. It is, however, necessary to provide an electrical connection with the graphite layer 3, which can itself be quite difficult to accomplish. As shown in Figure 1 this electrical connection is by means of a platinum clad niobium wire mesh 4 which is bonded to the graphite slab 3 by means of an adhesive layer 5. By using a wire mesh, an extensive area of contact can be produced which enables the anode to operate satisfactorily for long periods of time.

The best form of wire from which to manufacture the mesh comprises a platinum clad niobium wire having a copper core. Such a wire is preferably formed by co-extrusion in accordance with the method set-out in British Patent No 1 457 511 the details of which are incorporated herein by way of reference.

Such a co-extruded wire can be formed with a well bonded platinum layer which platinum layer is pore free, is acid resistant and protects the underlying valve metal. The wire is also smooth and easy and safe to handle.

In the event of failure of the anode layer 3 the wire netting will itself function as an anode to continue to protect the rebars 2. Thus the electrical connector nature of the wire mesh 4 serves a double function of forming the electrical connection to the main anode material and being a back-up in the event of failure of the main anode 3.

As shown in Figure 3 the wire mesh 6 may comprise a series of parallel strands 7 crossed by transverse strands 8. The transverse strands need not be formed of platinised metal but may comprise a valve metal without the platinum layer.

In place of the platinum metal other anodically active coatings such as lead, lead dioxide or substoichiometric or doped oxides of titanium or tantalum can be used. Because of the low current densities involved, such doped oxides - the doping may comprise partial removal of oxygen atoms could be acceptable as anodically active layers.

In addition to the provision of a copper core within the wires, a steel core could be provided for enhanced strength.

The platinum layers may be in the form of platinum or iridium or iridum containing layers may be used. Electroplating may be used to provide the platinum group metal layer or fired coatings may be used formed by the application of a paint containing a platinum group compound to the wires and subsequently fired. A titanium palladium alloy such as Ti and 0.15% Pd may be used to form the wires as such a material is both anodically active and electrically conducting and may be used in place of anodically coated titanium alloys.

In addition to the protection of rebars in the concrete the extended area electrode may be used for the electro-osmosis of water logged slag heaps, railway sidings, land drainage, desalting and other civil engineering applications. For large areas of mesh knitting provides a suitable method of manufacturing an open mesh. As shown in Figure 4 an open mesh may be provided from a knitted wire structure 9.

The extended area electrode may be used for electro damp-proofing by being bonded to brickwork or stone work sited a short distance away from the structure to be damp-proofed and set di rectly into the ground. Under the influence of an applied electric field, water is electro-osmotically attracted towards the cathode and dries out the brick-work.

Claims

1. An extended area electrode comprising a plurality of wires of valve metal in the form of an open mesh, the wires having on their surface a material having anodically active properties, the material being substantially non-consumable in operation.

2. An electrode as claimed in Claim 1 in which the material on the surface is in the form of a coating.

3. An electrode as claimed in Claim 1 or 2 in which the anodically active coating is a platinum group metal or platinum group metal compound or comprises a doped layer of an oxide of a valve metal.

4. An electrode as claimed in Claim 2 in which the anodically active layer comprises lead or lead oxide or a sub-stoichiometric oxide of tin or titanium or antimony oxide.

5. An electrode as claimed in any one of Claims 1 to 4 in which the mesh structure is formed by weaving or knitting or comprises a welded structure in the form of a network of individual strands welded together where they cross.

6. An electrode as claimed in any one of Claims 1 to 5 in which the valve metals wires are provided with a core of metal of higher electrically conductivity than the valve metal, preferably a copper or aluminium core.

7. The use of an electrode as claimed in any one of Claims 1 to 6 as a connector to a further electrode body such as coke, electrically conductive bitumen, electrically conductive plastics material or an electrically conductive cement.

8. The use of an electrode as a connector as claimed in Claim 7 in which the electrically conductive plastics material is in the form of a sheet with the mesh embedded therein.

9. The use of an electrode as a connector as claimed in Claim 8 in which the mesh has a plurality of holes in its surface.

10. A cathodic protection system for the cathodic protection of reinforcement bars in reinforced concrete comprising an anode in electrical connection with the concrete where in the anode comprises an electrode as claimed in any one of Claims 1 to 6.

11. A system for de-watering a structure of earth or other material in which there is provided an anode and a cathode, in which the anode is an electrode as claimed in any one of Claims 1 to 6.