EP0197981B1

EP0197981B1 - Catalytic polymer electrode for cathodic protection and cathodic protection system comprising same

Info

Publication number: EP0197981B1
Application number: EP19850904921
Authority: EP
Inventors: John E. Bennett; Donald S. Novak; Thomas A. Mitchell
Original assignee: Eltech Systems Corp
Current assignee: Eltech Systems Corp
Priority date: 1984-10-01
Filing date: 1985-09-19
Publication date: 1990-02-28
Also published as: WO1986002106A1; DE3576174D1; AU4960285A; BR8506959A; EP0197981A1; CA1278775C

Abstract

A catalytic polymer electrode comprises a current conducting polymer body forming an electrode base which is provided with catalytic valve metal particles fixed to its surface. A current conducting body of carbon filled thermoplastic polymer is heated to soften its outer surface, and the catalytic valve metal particles are pressed onto its softened surface and thereby attached to the surface of the polymer body. Such a catalytic polymer electrode used as an anode in an impressed-current cathodic protection system comprises a catalyst to provide a reduced oxygen potential. Such catalytic polymer anodes may be applied in systems for impressed current cathodic protection of reinforced concrete structures, such as bridge decks, support members, parking garages, or of buried or submerged steel structures such as gas and oil pipelines, offshore production platforms, fuel storage tanks, well casings.

Description

Technical field

The present invention relates to anodes of impressed-current cathodic protection systems comprising a body of current-conducting polymer in the surface of which are fixed electrochemically active elements, and a method of making such catalytic polymer anodes.

Background art

It is known that cathodic protection is effective to prevent corrosion of reinforcing steel in concrete bridge decks, support structures and parking garages, which are subject to extensive damage by corrosion of the steel reinforcement due to the presence of salt and moisture in the normally alkaline concrete environment. Such damage of reinforced concrete by corrosion results more particularly from the practice of spreading large amounts of salt on roads in winter, while coastal structures are attacked by seawater and salt spray.
Existing cathodic protection techniques for reinforcing steel are nevertheless limited by the anodes presently available for this purpose, which must have a long life and a chemically resistant anode structure that is readily adaptable from case to case according to the size and configuration of the reinforced concrete structure to. be cathodically protected from corrosion.
A known type of polymer anode commercially available for use in impressed-current cathodic protection systems consists of a carbon loaded, current conducting polymer body with a copper core and operates at a current density limited to a maximum of about 0.02 A/m² to avoid causing damage to the polymer anode surface.
Another type of anode which is used for cathodic protection of reinforcing steel consists of carbon fibers which are placed in a groove in the concrete, the groove then being filled with a grout of electronically conductive carbon-loaded backfill. Here again, the use of carbon presents serious limitations, since this material is subject to high operating voltages and therefore a limited lifetime as an anode. This is a serious limitation since replacement of anodes embedded in concrete is very difficult. This type of anode also has a high electronic resistivity, so that current can be carried longitudinally only over very short distances through the carbon fibers.
Other anodes which are traditionally used for impressed current cathodic protection are constructed of platinized titanium or platinized tantalum with a more electronically conductive copper core. Such electrodes are often used for cathodic protection of underground pipelines, well casings, ship hulls, jetties, drilling rigs, and oil platforms. These electrodes are expensive and must therefore be used at a higher current density, up to 1000 A/m² in some cases. The expense of such platinized titanium or tantalum electrodes entails special design problems since a very low current density must be applied to the structure being cathodically protected. This results in a mismatch of current density between anode and cathode. Various system designs attempt to accommodate this mismatch, usually by installation of small anodes at certain locations which are intended to protect large structures over great distances. Unfortunately, this often leads to unforeseen current density disparities and inadequate protection of more distant parts of the structure.
It is understood that continuous, wire-like flexible anodes for example are more suitable for cathodic protection of many structures such as underground pipelines in particular, but until now such anodes were not capable of functioning over extended periods of time.
Numerous composite electrodes have moreover been proposed which comprise a polymer material combined with a dispersed conductive filler, or an electrocatalyst, or both. The state of the art relating to such composite electrodes may be illustrated for example by U.S. Patents No. 3,629,007; 3,751,301; 4,118,294; 4,473,450 and European Patent Application No. 0 067 679.
European Patent Application No. 0 122 785 for instance proposed and claimed in article which is suitable for use as an anode in a method for protecting an electrically conductive substrate from corrosion and which comprises: a first element which provides part of the electrochemically active surface of the article, and is composed of a conductive polymer; and a plurality of second elements which provide part of the electrochemically active surface of the article, are partially embedded in, and project from the surface of, the first element, and are composed of a material such that, when the article is used as an anode in a method for protecting an electrically conductive substrate from corrosion, the electrochemical reactions at the anode take place preferentially on the second elements rather than the first element. According to the disclosure, these second elements were carbon fibres or graphite fibres, typically in the form of multifilament yarn.
In the unrelated field of anodes for oxygen evolution in an acid electrolyte such as is used e.g. in metal electrowinning, European Patent Application No. 0 046 727 described and claimed an anode for oxygen evolution in an acid electrolyte, comprising a base of lead or lead alloy, having catalytic particles comprising at least one catalyst for oxygen evolution fixed to a support particle consisting of a valve metal. These particles were partly embedded in the lead or lead alloy base and uniformly distributed at the surface of the base, so that the particles were firmly anchored and electrically connected to the base while a substantial non-embedded part of the particles remained projecting from the surface of the base for contact with the acid electrolyte. Oxygen could thereby be evolved on the surface of the particles at a reduced potential at which the underlying lead or lead alloy base essentially served as a current conducting support for the catalytic particles. Such anodes were useful as replacement of the conventional lead or lead alloy anodes without catalytic particles.
An object of the invention is to provide a catalytic polymeric anode with a long service life which is particularly suitable for the cathodic protection of reinforced concrete structures, such as bridge decks, parking garages, and coastal structures exposed to seawater and salt spray, as well as the cathodic protection of underground pipelines, well casings, ship hulls, jetties, drilling rigs, oil platforms, and the like.
According to the invention, there is provided an anode of an impressed-current cathodic protection system, the anode comprising a body of current conducting polymer in the surface of which are fixed electrochemically active elements, wherein these active elements are catalytic particles of valve metal surface-coated with an electrocatalyst.
The anode according to the invention may comprise any suitable current conducting polymer body. Carbon loaded polymers are advantageous. The current conducting body may be made from thermoplastic polymer compounds. The preferred thermoplastic resins include: polyolefins such as polymers of ethylene and/or propylene; halocarbon polymers such as polyvinyl chloride, polyvinylidene fluoride and halogen substituted olefinic polymers; styrenic polymers such as polystyrene, and copolymers of styrene with acrylonitrile, etc; polyamides such as polycaprolactam; thermoplastic polyesters; and acrylic resins such as polyacrylates and polymethacrylates. However, higher strength thermoplastics including polyimides; polyarylene resins, such as polycarbonates, polysulfones and polyphenylene. oxides and sulfides; and various heterocyclic resins may also be used.
The invention provides a particularly simple method of manufacturing such a catalytic polymer anode with an extended service life. According to the method the polymer anode is made by heating a cable of thermoplastic polymer so as to produce a softened external layer of the thermoplastic polymer and pressing catalytic valve metal particles onto said softened external layer of the polymer cable. The catalytic valve metal particles may advantageously be heated before pressing, the pressing being carried out so that upon cooling a uniform outer layer of catalytic valve metal particles is anchored to the surface of the electrode base.
It has been found that when a carbon loaded polymer forming the anode body is admixed with up to 50% by weight of conductive fibers such as carbon or metallic fibers the resulting anode obtained upon extrusion of such material around a metallic core, heating of the current conducting body and pressing the catalytic valve metal particles has advantageous properties. Such anodes are found to sustain high electrical currents while avoiding an excessive voltage drop within the electrode.
Although the amount of fibers may be as high as 50 weight %, the best results are obtained when the amount of fibers in the carbon loaded polymer is kept in the range of 5 to 30% by weight of the polymer. This is based on the finding that the polymer body made with more than 50% of fibers loses its mechanical properties and when the amount of fibers is kept below 5% the conductivity of the polymer body is not adequate. Various conductive fibers may be employed e.g. carbon, glassy carbon, nickel, copper, aluminium or stainless steel fibers.
Use of a body of thermoplastic polymer is particularly advantageous in that it allows a catalytic polymer anode to be produced according to the invention by this extremely simple and reproducible method, while ensuring an excellent fixation and electrical connection of said catalytic particles to the surface of the polymer body.
The anode according to the invention may be produced in a highly simplified manner in the form of a continuous electrode of any suitable cross-section, for example in the form of a wire, rod, strip, or sheet.
This possibility of manufacturing continuous electrodes of any desired cross-section and length in a simple and reproducible manner is particularly important for the industrial manufacture of continuous anodes for cathodic protection systems, which generally entail very high installation costs, and more particularly require anodes which are readily adaptable to the particular configuration of the structure to be protected from case to case.
The body formed of current conducting, carbon-loaded polymer will advantageously comprise an internal metallic reinforcing core, preferably a copper core, which is embedded in the carbon loaded polymer body, in order to allow the catalytic polymer electrode to conduct a sufficiently high electrical current while avoiding an excessive voltage drop within the electrode.
The catalytic particles used in the invention advantageously consist of one of the valve metals titanium, niobium, tantalum, zirconium, or an alloy thereof which exhibits substantially the same anodic film-forming properties as these valve metals.
Catalytic particles of titanium sponge which have an irregular size and shape and are readily deformable may be advantageously pressed into a coherent layer adhering to the surface of a current conducting polymer body.
These catalytic valve metal particles are advantageously activated with an electrocatalyst which provides a reduced oxygen potential and which may comprise at least one precious metal selected from ruthenium, palladium, iridium, platinum, and rhodium in the metallic state or, preferably, as an oxide.
Good results were obtained with a catalytic polymer anode according to the invention having particles of titanium sponge coated with a ruthenium based catalyst.
A very small amount of precious metal may be applied to the valve metal particles and the proportion of precious metal applied may advantageously be at most in the order of 1 % by weight of the valve metal particles, and advantageously considerably below 1%. This proportion of precious metal applied may preferably lie in the range from 0.1% to about 1.0% but may if necessary amount up to about 5%.
The particle loading of the catalysed valve metal particles may advantageously be in the range 10 to 100 grams per square meter of the electrode base surface to which they are applied, but this loading may amount to up to 500 grams per square meter or more in some cases.
The catalytic particles employed according to the invention may be prepared in any suitable manner, for example by a process as described in U.S. Patent No. 4,454,169, or in U.S. Patent No. 4,425,217.
The catalytic particles may be simply applied, fixed, and electrically connected by pressing them onto the surface of a heated thermoplastic polymer body forming the anode base. These particles may thus be applied by means of rollers, or by drawing the polymer body through a die. An electronically conductive glue or adhesive may likewise be used for their electrical connection.
The impressed-current anodes for cathodic protection systems according to the invention are used at current densities that do not exceed 500 Alm² and are preferably between 10 and 350 A/ m². The catalytic polymer anode of the invention is particularly effective in protecting buried or submerged steel structures such as gas and oil pipelines.
Test results have shown that the catalytic anode can operate at a much higher current density and a much lower potential than the conventional polymer anode. Moreover, it has been established that catalysed valve metal particles such as are applied according to the invention retain their catalytic activity under extremely harsh anodic corrosion conditions during operation at a many times higher anode current density.
The catalytic anode according to the invention may thus be expected to exhibit a long service life in cathodic protection systems due to the fact that it can operate at a much lower potential and can thereby protect the current conducting polymer body from damage by oxidation during operation at a relatively high anode current density.
The activated polymer anode of the invention may be in the form of cable, sheet, wire, perforated plate or any other convenient form. However, it has been established that, for the anode of the invention in which the active catalytic material (e.g. Ru0₂) is carried on a conductive valve metal carrier and the carrier with the catalyst thereon is supported on a carbon loaded thermoplastic polymer, the preferred form is the cable.
The invention may further be illustrated by the following examples:

Example I

A catalytic polymer anode was made by applying catalytic titanium particles to an anode base consisting of a conventional current conducting polymer anode of carbon loaded polyolefin with a copper core, which is a conventional, wire-shaped polymer anode (diameter 1 cm) commercially available for impressed-current cathodic protection.
For this purpose, the polymer anode body was heated to 120°C for 10 minutes and catalytic titanium particles were then pressed with a roller onto the softened anode surface, which provided good adherence of the catalytic particles to the anode surface.
The catalytic particles thus applied consisted of activated titanium sponge with a particle size in the range from 300 to 840 micrometers. These sponge particles were activated by impregnation with an activating solution comprising 2.38 g RuCI₃- aq. (40 wt% Ru), and 3.36 g tetra-ortho- butyl titanate dissolved in 3.2 ml concentrated HCI and 80 ml butylalcohol, then drying at 100°C in air for 120 minutes, and heat treating the dried particles in air at 300°C for 30 minutes, at 425°C for 30 minutes, and finally at 500°C for 10 minutes. This activating treatment qas carried out 2 times and the catalytic particles thus obtained contained 1% Ru by weight of Ti.
The total loading of the catalytic particles applied as described corresponded to 100 g/m² of the polymer anode surface.
The catalytic anode thus obtained was tested in a concrete block containing a steel reinforcement bar, and compared with a conventional polymer anode (without catalyst) as described above. For this purpose, the catalytic anode and the conventional polymer anode were symmetrically positioned in the same vertical plane and on opposite sides of a vertical steel reinforcement bar (at 5 cm from the steel bar), and each anode was provided with a reference electrode (Ag/AgCI) for measuring its single electrode potential (S.E.P).
A block of concrete (9x13x30 cm) containing 8.8 kg/m³ of NaCI was then cast around the anodes and the steel bar so that they were embedded while their top ends projected from the concrete block for connection to a D.C. supply source.
An impressed current corresponding to an anode current density of 0.11 A/m² was passed through the concrete block between the steel bar connected to the negative terminal of the D.C. source and the anodes connected to the positive terminal, while the anode potentials were measured over a test period of 1 month.
The described catalytic anode operated for 30 days at a constant potential of 0.390 V vs. CSE (Copper Sulfate Electrode). On the other hand, the conventional polymer anode exhibited a potential which rose from about 1 V to about 2 V vs. CSE in the first 8 days, decreased slightly to 1.8 V after 20 days, then increased slowly once more to 2.0 V after 30 days.
The catalytic anode thus operated at a constant potential up to about 1.6 V lower than the non- catalytic anode, while it may be noted that the anode current density applied in this test is several times higher than that at which the described conventional polymer anode can be operated with a satisfactory service life.
The catalytic anode according to the invention may thus be expected to exhibit a long service life in cathodic protection systems, due to the fact that it can operate at a much lower potential, and can thereby protect the current conducting polymer body from damage by oxidation during operation at a relatively high anode current density. The catalytic anode according to the invention may be expected to be functional up to about 500 _A/m2.

Example II

A catalytic polymer anode 150 meters long was made by applying catalytic particles to an anode base consisting of a carbon loaded polyolefin with a copper core. The anode base was first prepared by extruding a conductive thermoplastic compound around a conductive copper core of 1.50 mm diameter wire.
Catalyst was prepared by activating 500 grams of titanium sponge with a particle size in the range of 300 to 840 micrometers. After rinsing in acetone and drying at 120°C, the titanium particles were activated by mixing with an activating solution comprising 17.31 grams RuC1₃ aq. (43 wt.% Ru) in 250 ml acetone, drying for 2 hours, prebaking at 340°C for 30 minutes, and postbak- ing at 400°C for 40 minutes. This activating treatment was carried out two times and the catalytic particles thus obtained contained 3% Ru by weight of titanium.
Catalyst was then continuously applied in a separate step by passing the anode base through a tube furnace at about 385°C to rapidly heat the polymer surface, passing through a fixed bed of heated catalyst particles, and passing through a series of rollers to press the catalyst particles onto the surface of the anode base. The anode thus prepared was catalyzed with 330 grams of said particles per square meter of anode surface.
110 meters of this anode was then placed in a porous duct for the purpose of protecting lead- sheathed utility cables. 0.93 amps was applied to the anode resulting in a current density at the anode surface of 0.35 amps per square meter. Cable to soil potentials were acceptable for cathodic protection of the cable, ranging from 0.685 to 0.90 volts vs. a C_US0₄ reference electrode, and contact resistance was measured to be equivalent to, or slightly lower than, cast iron anodes. Ground anode potentials measured vs. a C_US0₄ reference electrode along the length of the anode at even intervals remained constant, and were reported as follows:

Example III

A catalytic polymer anode 100 meters long was made by applying catalytic particles to an anode base consisting of carbon loaded polyolefin with a copper core. The anode base was first prepared by extruding a conductive thermoplatic compound around a conductive copper core of 3.73 mm diameter wire.
Catalyst particles were prepared and were applied to the anode base as described in Example II. An anode thus prepared was catalyzed with 300 grams of said particles per square meter of anode surface.
A representative sample of this anode 10 centimeters long was operated in 1.0 M H₂S0₄ at 0.327 A, equivalent to a surface current density of 100 amps per square meter. This anode has operated for over 1,150 hours at a cell voltage of 2.6 to 2.7 volts without any sign of failure. An anode base of carbon loaded thermoplastic without catalyst particles failed after only 20 hours of operation, at which time its cell voltage exceeded 10.0 volts.

Example IV

A catalytic polymer anode was prepared by applying catalytic particles to an anode base consisting of a modified carbon loaded polyolefin with a copper core. The anode base was prepared by extruding a mixture of a conductive thermoplastic compound with 15% by weight of added carbon fibers around a conductive copper core of 3.73 mm diameter wire. Addition of the carbon fibers had the effect of lowering the volumetric resistivity of the polymeric phase from 20 ohm-centimeters to 0.20 ohm-centimeters making possible operation at higher current density.
Catalytic particles containing 3% Ru by weight of titanium were prepared as described in Example II, and were then applied to the surface of the anode base by heating the anode base to 120°C for 10 minutes and rolling the catalytic titanium particles onto the softened anode surface. The anode thus prepared contained 380 grams of said particles per square meter of anode surface.
A representative sample of this anode 2 centimeters long was operated in 1.0 M H_ZS0₄ at 0.655 A, equivalent to a surface current density of 1000 amps per square meter. This anode operated for over 268 hours at a cell voltage of 3.1 to 3.2 volts without sign of failure.
This is a dramatic improvement over the conventional commercially available wire-shaped polymer anode which cannot be operated at such high current density at all, and which has a recommended operating current density maximum of only 0.5 amps per square meter.

Technical applicability

Catalytic polymer anodes of impressed-current cathodic protection systems according to the invention are especially suitable to prevent corrosion damage of reinforced concrete structures such as bridge decks, support members and parking garages, or buried or submerged steel structures such as gas and oil pipelines, offshore production platforms, fuel storage tanks and well casings.

Claims

1. An anode of an impressed-current cathodic protection system, the anode comprising a body of current conducting polymer in the surface of which are fixed electrochemically active elements, characterized in that said active elements are catalytic particles of valve metal surface-coated with an electrocatalyst.

2. The anode of claim 1, wherein said body is formed of a carbon loaded polymer.

3. The anode of claim 1 or 2, wherein said current conducting polymer body contains dispersed therethrough up to 50% by weight of carbon or metal fibers.

4. The anode of claim 1, 2 or 3 wherein said polymer is a thermoplastic polymer.

5. The anode of claim 4, wherein said thermoplastic polymer is selected from polyolefins, halocarbon polymers, styrenic polymers, polyamides, thermoplastic polyesters, acrylic resins and blends of same.

6. The anode of claim 1, 2 or 3 wherein a metallic core is embedded within said current conducting polymer body.

7. The anode of claim 1 or 2, wherein said valve metal is selected from titanium, tantalum, niobium, and zirconium.

8. The anode of claim 1 or 7, wherein said electrocatalyst comprises at least one precious metal selected from ruthenium, palladium, rhodium, iridium and platinum, in either the metallic state or as an oxide.

9. The anode of claim 8, wherein the catalytic particles comprise said precious metal in an amount from 0.1% to 5.0% by weight of valve metal.

10. The anode of claim 1, wherein said catalytic valve metal particles are applied to the surface of said current conducting polymer body in an amount corresponding to a loading from about 10 to about 500 grams of said catalytic particles per square meter of said surface.

11. The anode of any of claims 1 to 10, wherein the valve metal is titanium and the electrocatalyst comprises ruthenium oxide.

12. The anode of any of claims 1 to 11, wherein the polymer body is in the form of a cable.

13. A method of manufacturing an anode according to claim 12, comprising the steps of:

(a) heating said cable of a thermoplastic polymer so as to produce a softened external layer of the thermoplastic polymer; and

(b) pressing said catalytic valve metal particles onto said softened external layer of thermoplastic polymer, so as to obtain, on cooling, a uniform outer layer of said catalytic valve metal particles anchored to the surface of said cable of thermoplastic polymer.

14. The method of claim 13, wherein said catalytic valve metal particles are heated prior to pressing them onto said softened external layer of thermoplastic polymer.

15. Use of the anode according to any of claims 1 to 12 in an impressed-current anode of a cathodic protection system for the prevention of corrosion of reinforcing steel in concrete or buried or submerged steel structures, at an anode current density up to 500 Alm².