Electrodes for use in electrochemical processes

Abstract: Electrodes for electrochemical processes, especially anodes for the cathodic protection of metal substrates, e.g. reinforcing bars in concrete, comprise a conductive core (11) which acts as a current-distributing member, an outer member (13) which provides an electrochemically active outer surface, and an intermediate member (12) composed of a material which is of higher resistivity, and/or which is less electrochemically active, than the material of the outer member. The higher the resistivity of the intermediate member, the more regular the current distribution along the length of the electrode. When the intermediate member is less electrochemically active, this protects the core from corrosion if the outer member is damaged by physical means or through electrochemical erosion. Preferably at least one of the intermediate member and the outer member is composed of a conductive polymer, especially one comprising carbon black or graphite as conductive filler.

Description: It is well known to carry out electrochemical reactions by maintaining a potential difference between two electrodes which are exposed to and electrically connected by at least one electrolyte. A particularly important electrochemical reaction is the prevention of corrosion of a substrate by maintaining a potential difference between the substrate and an electrode so that current passes between the electrode and the substrate. In such methods, the substrate is usually the cathode. Suitable anodes include discrete anodes (for example anodes comprising a metallic core surrounded by graphite, a mixture of graphite and carbon, or a dispersion of graphite or carbon black in a thermoset resin) and distributed anodes (for example conductive paints, and platinum or platinum-coated wires). For further details of anodes which have been used, or proposed for use, reference may be made for example to U.S. Patents Nos. 4,502,929 (Stewart et al), 4,473,450 (Nayak et al), 4,319,854 (Marzocchi), 4,267,029 (Massarsky), 4,255,241 (Kroon et al), 4,196,064 (Harms et al), 3,868,313, (Gay), 3,798,142 (Evans), 3,391,072 (Pearson), 3,354,063 (Shutt), 3,151,050 (Wilburn), 3,022,242 (Pearson) and 2,053,214 (Brown), European Patent Publication No. 0147977, UK Patents Nos. 1,394,292 and 2,046,789A and Japanese Patent Publications Nos. 34293 (1973) and 48948 (1978). In recent years, increasing attention has been directed to distributed anodes having an electrochemically active surface which comprises a conductive polymer, this term being used to denote a composition which comprises a polymer component and, dispersed in the polymer component, a particulate conductive filler which has good resistance to corrosion, especially carbon black or graphite. Thus U.S. Patent No. 4,502,929 (Stewart et al) describes distributed anodes whose electrochemically active surface is provided at least in part by an element which is is composed of a conductive polymer and which is preferably at least 500 microns thick. Preferred electrodes are flexible and comprise a metal core and an element which surrounds the core and is composed of a conductive polymer which has a resistivity of 0.1 to 1000 ohm.cm and an elongation of at least 10%. U.S. Patent No. 4,473,450 (Nayak et al), the disclosure of which is incorporated herein by reference, notes that failure of the anodes described in Patent No. 4,502,929 takes place when degradation of the conductive polymer permits ingress of the electrolyte to the metal core, and discloses that the rate of ingress can be reduced by means of second elements which are partially embedded in and project from the conductive polymer element and which are composed of a material such that the electrochemical reaction takes place preferentially on the projecting surfaces of the second elements. In Patent No. 4,473,450, it is theorized that the improved properties of such anodes result at least in part from the ability of damaging electrochemical reaction products to escape more easily if they are generated on the protruding portions of the second elements than they can if they are generated within the mass of conductive polymer. European Patent Publication No. EP 0147977 discloses an anode which is particularly suitable for use in the cathodic protection of reinforcing bars in concrete, and which comprises a plurality of elongate strands which are joined together to form a flexible open mesh, at least some of the strands being electrically conductive and comprising carbonaceous material. We have discovered that in electrodes comprising (i) a conductive core which is composed of a first conductive material having a first resistivity at 23°C and which acts as a current-distributing member and (ii) an outer element which provides an electrochemically active surface, improved current distribution is obtained if the conductive core is electrically surrounded by an intermediate element which is composed of a second conductive material having a second resistivity at 23°C which is higher than the first resistivity, the intermediate element preferably having a transverse resistance which is at least 1 ohm.meter. The higher the transverse resistance of the intermediate element, the more uniform the current distribution. We have further discovered that in electrodes comprising (i) a conductive core which acts as a current-carrying member and (ii) an outer element which provides an electrochemically active surface, the useful life of the electrodes is substantially increased by the presence of an intermediate element which electrically surrounds the core and which is composed of a material which is less electrochemically active than the outer element. The advantages of the latter discovery are particularly apparent when the current density on the anode varies substantially along its length, thus causing erosion to be concentrated at small sections of the anode. The present invention provides an article which is suitable for use as an electrode in an electrochemical process and which comprises (a) a core which (i) is composed of a conductive material having a first resistivity at 23°C e.g. metal, and (ii) does not provide any of the electrochemically active surface of the electrode; (b) a continuous intermediate element which (i) is secured to, and electrically surrounds the core, and (ii) is composed of a conductive polymer material which has a second resistivity at 23°C, which is higher than the first resistivity; and (c) an outer element which (i) is secured to and is in electrical contact with the core and the intermediate element so that all electrical paths between the core and the outer element pass through the intermediate element, and (ii) is composed of a conductive material having a third resistivity at 23°C, characterised in that the outer element surrounds and covers substantially the entire outer surface of the intermediate element, and (a) either (i) the intermediate element provides part of the electrochemically active surface of the electrode, or (ii) the outer element comprises a conductive material other than a conductive polymer, or (b) both Preferred articles of the invention comprise an intermediate element composed of a material which has a high resistivity and which is less electrochemically active than the material of the outer element. In another aspect, the invention provides an electrochemical process in which an electrode of the invention is surrounded by an electrolyte, and current passes between the anode and the electrolyte, particularly a cathodic protection method wherein an electrode of the invention is used as an anode. BRIEF DESCRIPTION OF THE DRAWING The invention is illustrated in the accompanying drawings, in which Figure 1 is a cross-sectional view of an electrode of the invention, and Figure 2 is a cross-sectional view of another electrode of the invention. DETAILED DESCRIPTION OF THE INVENTION The core of the electrodes of the present invention acts as a current distributor and is composed of a material of relatively low resistivity, generally less than 10 -2 ohm.cm. When the electrode is relatively long, e.g. 30m (100ft) or more, it is preferred that the core be composed of a material of still lower resitivity e.g. less than 5 x 10 -4 ohm.cm, particularly less than 3 x 10 -5 ohm.cm, e.g. copper or another metal. The resistivities given herein are measured at 23°C. For shorter lengths, e.g. of less than 18m (60 feet), a carbon fiber or graphite fiber core may be of sufficiently low resistance. The core is usually of constant cross-section along its length. When the electrode is a long one, e.g. of 30m (100 feet) or more, or is in the form of an open mesh which is powered from a limited number of contact point, the dimensions of the core are selected so that it has a suitably low resistance, preferably an average resistance of less than 10 -2 ohm/0.3m (foot), particularly less than 10 -3 ohm/0.3m (foot), especially less than 10 -4 ohm/0.3m (foot). The core can be for example a short rod, e.g. of metal, graphite or carbon, 76mm to 1219mm (3 to 48 inches) long, a long metal wire, solid or stranded, a metal plate, or a mesh structure, e.g. of expanded metal or a net formed by joining metal, graphite or carbon fiber strands together. The intermediate element electrically surrounds the core, the term "electrically surrounds" being used to mean that when the electrode is immersed in an electrolyte and is in use, all electric current passing between the core and the electrolyte passes through the intermediate element, so that the electrolyte cannot contact and corrode the core. The intermediate element is usually in the form of a coating which is of constant cross-section and which completely surrounds and is in direct physical contact with the core, e.g. a coating of annular cross-section around a core of round cross-section. However, other arrangements are possible. For example, the core can have some sections coated with an insulating polymer and others coated with a conductive polymer. The intermediate element can provide part or none, but not all, of the surface of the electrode (ie. if the electrode is immersed in a liquid, the outer element is contacted by the liquid, and the intermediate element may or may not be contacted by the liquid). The intermediate element has at least one of the following characteristics: (1) it has dimensions, and is composed of a material, such that it has a transverse resistance which is sufficiently high to produce a useful improvement in the uniformity of the current distribution, preferably a transverse resistance of at least 1 ohm.meter; and (2) it is composed of a material which is less electrochemically active than the material of the outer member. In order to determine whether one material is less electrochemically active than another material, the following test should be carried out. A test cell is constructed in which the cathode is graphite or carbon rod, the reference electrode is a a silver/silver chloride electrode, the anode is the material to be tested, and the electrolyte is a 3% by weight solution of sodium chloride in water. The anode is polarized + 2.0 volts with reference to the silver/silver chloride electrode, and the current density on the anode is measured after the current has reached a steady state. The anode material which has the lower current density is the less electrochemically active. The current density of the second material is preferably less than 0.2 times, particularly less than 0.1 times, especially less than 0.01 times, the current density or the third material. The intermediate element preferably has both characteristic (1) and characteristic (2). This can be achieved through the use of a conductive polymer of sufficiently high resistivity as the material of the intermediate element. When the outer element is of low resistivity, eg. 0.1 to 50 ohm.cm, useful improvements can be obtained by using as the second conductive material (for the intermediate element) a conductive polymer whose resistivity is a few times greater, eg. at least 2 times greater. However, when long electrodes are to be used, eg. 30m (100 feet) or more, it is preferable for the second conductive material to have a resistivity of at least 1,200 ohm.cm, particularly at least 3,000 ohm.cm, especially at least 8,000 ohm.cm. Such compositions contain lower concentrations of conductive filler than those which have previously been recommended for use in electrodes. The term "conductive polymer" is used herein to denote a composition which contains a polymer component and, dispersed in the polymer component, a particulate conductive filler which has good resistance to corrosion, especially carbon black or graphite or both. The conductive polymer is preferably prepared by melt-shaping, eg. by pressure extrusion around the core. However, improved results can be obtained when the intermediate element has only one of characteristics (1) and (2). Thus characteristic (1) above can be achieved through the use of a material for the intermediate element which has high resistivity but which is more electrochemically active than the material of the outer element. In that case, the intermediate element will provide improved current distribution, but will be eroded more rapidly than the outer element if contacted by electrolyte; accordingly, when using such an intermediate element, it preferably does not provide any of the surface of the electrode (ie. if the electrode is immersed in a liquid, the intermediate element is not contacted by the liquid). Similarly, characteristic (2) above can be achieved through the use of a material for the intermediate element which is highly conductive but which has high resistance to corrosion, eg. titanium, niobium or platinum. In that case, however, the electrode must be used under circumstances in which less uniform current distribution can be tolerated. Characteristic (1) above results in an electrode having improved current distribution. The term "transverse resistance" is used to denote the resistance between the inner surface and the outer surface of the intermediate element. The higher the transverse resistance, the better the current distribution, but this must be balanced against other factors such as ease of manufacture, the desired dimensions of the electrode, the desired current off the anode, the available power supplies and the power consumption. In addition, the extent of the improvement in current distribution depends also on the resistance of the electrolyte between the electrode and the substrate to be protected. I have found that the intermediate layer preferably has a resistance of at least 1 ohm.meter, particularly at least 1.5 ohm.meter, especially at least 4 ohm.meter. When using a distributed anode, the use of a high resistance intermediate layer increases the length of the anode which can be employed while keeping the substrate potential within permissible limits. When using a discrete anode comprising a metal core surrounded by an electrochemically active material such as graphite, or a mixture of graphite and carbon, or a dispersion of carbon black or graphite or both in a polymer, eg. a thermoset resin, the use of a high resistance intermediate layer lengthens the life of the anode by reducing the current density at the point of critical weakness, which is the junction of the metal core and the electrochemically active material. Characteristic (2) above results in an electrode in which the core is protected from corrosion if the outer member comprises a plurality of spaced-apart portions and/or if the outer member is damaged by physical means or through electrochemical erosion. As indicated above, when the intermediate element is composed of a conductive polymer, there are concentrations of conductive filler which will provide characteristic (1) as well as characteristic (2). Such concentrations also produce compositions which, by comparison with the conductive polymers containing greater amounts of the filler previously recommended for use in electrodes, have improved physical properties, eg. tensile strength, elongation and impact resistance, making such compositions all the more satisfactory as a protective layer over the core. The physical properties can be yet further improved by crosslinking, eg. with the aid of radiation, preferably to a dosage of at least 5 Mrads. The intermediate element provides protection for the core when the outer element is damaged, either by purely physical means or by electrochemical erosion. The latter type of damage is particularly Serious when the electrode is used in a situation in which the current density on the surface of the outer element varies substantially over its length, with, in consequence, a similar variation in the rate of ingress. When the damage has reached a point at which electrolyte contacts the intermediate element, through the outer element, the smaller electrochemical activity of the intermediate element causes the electrochemical activity to be transferred to another location. The outer element of the electrodes of the invention provides at least part, of the electrochemically active surface of the electrode. In some cases, the outer element will provide the whole of the exposed surface of the electrode (i.e. if the electrode is immersed in a liquid, the liquid does not contact the intermediate layer at all). In such cases, the outer element may be in the form of a coating which is of constant cross-section and which completely surrounds a single intermediate element and is in direct physical contact with the intermediate element, eg. a coating of annular cross-section around a single intermediate element, or in the form of a tape with two or more parallel intermediate elements embedded therein. Such an outer element is preferably prepared by melt-shaping, eg. by pressure extrusion of a conductive polymer around the intermediate element or elements. In other cases, the outer element provides only part of the exposed surface of the electrode. In preferred embodiments of the present invention , at least one of the second and third conductive materials (for the intermediate and outer elements respectively) is a conductive polymer, preferably a melt-extruded conductive polymer having an elongation of at least 10%, particularly at least 25%. The outer layer is preferably at least 500 micrometres thick, particularly at least 1,000 micrometres thick. When the intermediate layer is not contacted by electrolyte (unless and until physical damage to or electrochemical erosion of the outer element exposes the intermediate layer), it is preferably at least 200 micrometres thick, particularly at least 350 micrometres thick, e.g. 350 to 1,500 micrometres thick. When the intermediate layer is contacted by electrolyte when the electrode is first used, similar thicknesses can be used, but somewhat greater thicknesses are preferred, e.g. at least 500 micrometres, particularly at least 1,000 micrometres. When the third conductive material is a conductive polymer, it preferably has a third resistivity of 0.01 to 300 ohm.cm, particularly 0.1 to 50 ohm.cm. The second conductive material preferably has a second resistivity which is at least 2 times, particularly at least 10 times, especially at least 100 times, the third resistivity, and/or which is at lest 500 ohm.cm above, particularly at least 1,200 ohm.cm above, especially at least 5,000 ohm.cm above, the third resistivity. When one or both of the second and third conductive materials is a conductive polymer, the conductive filler is preferably carbon black and/or graphite. When both are conductive polymers, the fillers can be the same or different, and useful advantages may result from the use of different fillers which are selected with a view to the different functions of the intermediate and outer elements. For good properties in the intermediate layer, carbon blacks having high structure (e.g. a DBP value of 80 or more) have the advantage that they can impart satisfactory conductivity at relatively low loading. Tests have shown that the electrochemical activity of these carbon blacks falls rapidly in use, which is a positive advantage in the intermediate layer. The interface between the intermediate and outer elements is preferably free from portions which are re-entrant into the intermediate element, particularly a smooth regular surface such as is obtained for example by melt-extruding or molding the outer element(s) around a melt-extruded or molded intermediate element. The electrodes of the present invention can be composite articles which comprise two (or more) cores, each electrically surrounded by an intermediate element, and a single outer element in which the intermediate elements are fully embedded. In use of such composite articles, both (or all) of the cores can be connected to the power supply and used as an electrode, or only one (or some) of the cores can be used as an electrode, with the other(s) being left for future use when the initially used electrode(s) has (or have) become inoperable, The electrodes of the invention can also comprise one or more insulated conductors for use as part of a monitoring or fault-finding system, or to feed power to other electrodes or to the far end of the core or cores of the same electrode. Referring now to the drawing, Figure 1 is cross-sectional view of a distributed electrode of the invention which comprises a metal core 11; a continuous intermediate element 12 which surrounds the core 11 and is composed of a conductive polymer having a relatively high resistivity, eg. about 500 ohm.cm or more; and an outer element 13 which is composed of a conductive polymer having a relatively low resistivity, eg. less than 300 ohm.cm, particularly less than 50 ohm.cm. The distributed electrode has a constant cross-section along its length. Figure 2 is a cross-sectional view of a discrete electrode of the invention which comprises a metal core 11; an intermediate element 12 which surrounds the core 11 and is composed of a conductive polymer having a relatively high resistivity; and an outer element 13 which is composed of a mixture of a graphite and carbon having a relatively low resistivity.

Claims: An article which is suitable for use as an electrode in an electrochemical process, and which comprises: (a) a core which (i) is composed of a conductive material having a first resistivity at 23°C e.g. metal, and (ii) does not provide any of the electrochemically active surface of the electrode; (b) a continuous intermediate element which (i) is secured to, and electrically surrounds the core, and (ii) is composed of a conductive polymer material which has a second resistivity at 23°C, which is higher than the first resistivity; and (c) an outer element which (i) is secured to and is in electrical contact with the core and the intermediate element so that all electrical paths between the core and the outer element pass through the intermediate element, and (ii) is composed of a conductive material having a third resistivity at 23°C, characterised in that the outer element surrounds covers substantially the entire outer surface of the intermediate element, and (a) either (i) the intermediate element provides part of the electrochemically active surface of the electrode, or (ii) the outer element comprises a conductive material other than a conductive polymer; or (b) both An article according to Claim 1 wherein the conductive polymeric material of the intermediate element is a melt-extruded conductive polymer having a resistivity at 23°C of a least 1,200 ohm.cm. An article according to Claim 1 wherein the first material is a metal, the second material is a conductive polymer, and the third material is graphite, a mixture of graphite and carbon, or a dispersion of a carbonaceous material in a thermoset resin. An article according to Claim 1 wherein the second resistivity is at least 3,000/log e (A 2 /A 1 ), where A 1 is the interior area of the intermediate layer and A 2 is the exterior area of the intermediate layer.

Inventors: STEWART RAY F, THOMPSON JAMES C, STEWART, RAY F., THOMPSON, JAMES C.
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1988
Main classification: C23F13
IPC codes: C23F13