CS222661B2 - Chlorine-resistant metal core electrode - Google Patents

Abstract

The invention relates to electrodes with a chlorine-resistant metal core made of titanium or tantalum. The core is provided with a semiconductor coating consisting of at least one platinum group metal, at least one oxide from the group consisting of titanium dioxide and tantalum oxide, and at least one dopant oxide from the group consisting of oxides of silver, gold, tin, chromium, tantalum, lanthanum, aluminum, cobalt, antimony, molybdenum, nickel, iron, tungsten, vanadium, phosphorus, boron, beryllium, sodium, calcium, strontium, lead, copper and bismuth and mixtures thereof. The weight fraction of the dopant oxide or oxides is in the range from 0.10 to 50% by weight of the titanium and/or tantalum. thallium in titanium dioxide or tantalum, and the ratio of the mass of platinum group metals to the mass of other metals in the oxide coating is from 20:100 to 85:100. The electrode according to the invention can be used in any liquid or gaseous electrolyte, in particular in aqueous salt solutions or in molten salts.

Description

The electrode according to the invention can be used in any liquid or gaseous electrolyte, in particular in aqueous salt solutions or in molten salts.

The invention relates to an electrode with a core made of a chlorine-resistant metal.

Titanium and tantalum are known to have the ability to conduct current in the anode direction and to reduce the resistance to the passage of current in the cathode direction and are also sufficiently resistant to the electrolyte and to the conditions inside the electrolyzer used, for example, for the production of chlorine and caustic soda, so that they can be used as electrodes in electrolytic processes. However, in the anode direction, their resistance to the passage of current increases very rapidly due to the formation of an oxide layer on these electrodes, so that it is not possible to conduct current to a significant extent into the electrolyte without significantly increasing the voltage, which leads to the fact that the use of uncoated anodes of titanium or tantalum in electrolytic processes is uneconomical.

When titanium or tantalum anodes are coated with platinum or platinum group metal conductors, they are also passivated, which causes a rapid potential rise after their brief use at a sufficiently high current intensity to evolve chlorine. This potential rise indicates that the anodic oxidation of dissolved chlorine ion to molecular chlorine gas will only proceed at higher overpotentials due to a reduction in the catalytic activity of the electrode surface.

Similarly, attempts to counteract this passivation, even after a short period of use, by providing titanium or tantalum anodes with a platinum group metal coating, which was applied by electroplating or thermal processes, but which essentially covered the entire surface of the titanium anode facing the cathode, were not successful, such coatings were not always adequately adherent, the platinum group metal consumption was high and the results were unsatisfactory.

It has long been known that rutile or titanium dioxide and tantalum oxide have semiconducting properties when either doped with traces of other elements or compounds that disrupt the lattice structure and change the conductivity of the titanium dioxide or tantalum oxide, or when the lattice is disrupted by removing oxygen from the titanium dioxide or tantalum oxide crystal. Titanium dioxide has been doped with tantalum, niobium, chromium, vanadium, tin, nickel, and iron oxides, as well as other materials, to change the electrical conductive or semiconducting properties of titanium dioxide, or the stoichiometric equilibrium has been altered by removing oxygen from the crystal lattice. Similarly, the conductivity of tantalum oxide films has been modified by ultraviolet radiation or other methods, but the use of doped titanium dioxide or tantalum oxide to form a conductive or semiconducting surface on a valve metal electrode for use in electrochemical reactions has not yet been proposed. Other metal oxides, especially mixtures of metal oxides belonging to adjacent groups in the periodic table, also have the property of forming semiconductors when thoroughly mixed and heated.

Various theories have been developed to explain the conductive or semiconducting properties of doped or undoped titanium dioxide and also of tantalum oxide. Reference is made, for example, to Grant: Review of Modern Physics, 1, p. 646 (1959), Frederikse: Journal of Applied Physics, Supplement to Vol. 32, No. 10, p. 221 (1961), and Vermilyea: Journal of the Electrochemical Society 104, p. 212 (1957), but there seems to be no unanimous conclusion as to what gives doped titanium dioxide and tantalum oxide their semiconducting properties. When other mixed oxides are used to form oxides, it is possible that the oxides of one metal belonging to an adjacent group in the periodic table penetrate the crystal lattice of the other metal oxide by dissolving in the solid phase and act as a doping oxide that disrupts the stoichiometric crystal structure of one of the metal oxides and gives the mixed oxide coating its semiconducting properties.

The invention provides an electrode with a chlorine-resistant metal core made of titanium and tantalum, wherein this core is provided with a semiconductor coating according to the invention, consisting of at least one oxide of at least one platinum group metal, at least one oxide from the group comprising titanium dioxide and tantalum oxide, and at least one doping oxide from the group comprising oxides of silver, gold, tin, chromium, tantalum, lanthanum, aluminum, cobalt, antimony, molybdenum, nickel, iron, tungsten, vanadium, phosphorus, boron, beryllium, sodium, calcium, strontium, lead, copper and bismuth and mixtures thereof, wherein the weight fraction of the doping oxide or oxides is in the range from 0.10 to 50% of the mass of titanium and/or tantalum in the titanium dioxide or tantalum, and the ratio of the mass of metals of the group The mass ratio of platinum to the mass of other metals in the oxide coating is from 20:100 to 85:100.

Such an electrode core coating has conductive or semiconductive properties sufficient to conduct electrolytic current from the core to the electrolyte for long periods of time without passivation occurring.

The supporting oxides preferably consist of tin oxide and at least one metal oxide from the group consisting of tantalum, lanthanum, chromium and aluminum.

According to another embodiment of the invention, the coating contains oxides of two platinum group metals, namely ruthenium oxide and iridium oxide.

Electrodes of this type are suitable for the electrolysis of chlorides, bromides and iodides of lithium, sodium and potassium, in general for the electrolysis of halides, longer for the electrolysis of other salts which decompose under the conditions of electrolysis, for the electrolysis of hydrochloric acid solutions and for the electrolysis of water, etc. These electrodes are also useful for other purposes, such as other processes where an electrolytic current is passed through the electrolyte for the purpose of decomposition of the electrolyte, for the purpose of carrying out organic oxidations and reductions for the protection of the cathode and in other electrolytic processes.

They can be used in mercury cells, in perforated vessel cells and can also have other shapes than those described by way of example. They are also well suited as anodes for the electrolysis of sodium chloride in mercury cells and perforated vessel cells, since they have the ability to release chlorine at low anode voltages for substantially the entire life of the semiconductor coating of titanium dioxide or tantalum oxide or other metal oxide. They also wear out little, that is, they have a low wear factor, measured as the loss of conductive metal per ton of chlorine generated.

The electrode according to the invention can be used in general in any liquid or gaseous electrolyte, in particular in aqueous salt solutions or in molten salts. These electrodes are dimensionally stable and do not corrode during the electrolytic process and when used in electrolytes containing alkali metal halides, such as sodium chloride solutions used for the production of chlorine and sodium hydroxide, the electrodes according to the invention forming anodes and cathodes are formed, for example, of mercury, steel, or other suitable conductive material.

Further features and advantages of the invention will become apparent from the following description.

The coating is usually prepared by preparing a solution of a semiconducting metal and a dopant which, when applied and fired on a bare electrode in the case of titanium or tantalum metal, forms a solution of titanium dioxide and a dopant oxide or tantalum oxide and a dopant oxide, and this substance is sinteringly applied to the core in several layers, so that a solid solution of titanium dioxide or tantalum oxide and a dopant oxide is formed on the surface of the electrode, which will have the desired semiconducting properties and will continuously release chlorine without increasing the overvoltage for long periods of time. Any solvents or compounds which, when fired, form a coating consisting of titanium dioxide or tantalum oxide and a dopant oxide can be used, such as chlorides, nitrates, sulfides, etc., and in addition to the solutions given below as examples.

The overvoltage referred to above is the voltage exceeding the reversible or equilibrium electromotive force that must be applied to cause the reaction at the electrode to proceed at the desired rate. The overvoltage of chlorine varies with the anode material and its physical state. It increases with the anode current density, but decreases with increasing temperature.

Titanium dioxide and tantalum oxide forming a semiconducting surface can be formed by doping titanium dioxide or tantalum oxide in the form of crystals with various dopants, or by breaking the etiechiometric lattice by removing oxygen from this lattice, so that titanium dioxide or tantalum oxide becomes semiconducting. As a result of the tendency of titanium dioxide or tantalum oxide crystals to reoxidize, it is expedient to form semiconducting surfaces on electrodes by using dopants which, when fired, form solid solutions with titanium dioxide or tantalum oxide in the form of crystals, which are more resistant to changes during electrolytic processes. However, the electrodes of the invention may also employ semiconducting coatings formed by removing oxygen from titanium dioxide or tantalum oxide lattices, thereby inducing defects or imperfections in the lattice.

To increase the conductivity and electrocatalytic properties of the titanium dioxide or tantalum oxide layer on the electrode, various doping materials can be used that introduce impurities into the titanium dioxide and tantalum oxide crystals to induce their conductivity, such as WO ₂ , PgOg, ®^2θ5' ^v 2°5' ^Ta 2°5'^2θ5' ^B 2°3' ^Cr 2°3>

NagO, CaO, SrO, RuOg, IrOg, PbOg, OsOg, PtOg, AuOg, AgOg, SnOg, AlgO^, and mixtures thereof. The best results are obtained with those dopants for titanium dioxide which have a tetragonal structure with similar unit cell dimensions and with approximately the same cation radii (0.68 A°). RuOg (0.65 A°) and IrOg (0.66 A°) are particularly suitable dopants, as are other platinum group metal oxides (i.e. platinum, palladium, osmium, and rhodium). IrOg forms solid complexes in titanium dioxide up to about 5 mol. % IrOg when heated together to 1040 °C. At lower temperatures, the amount of IrO2 that will form solid solutions in titanium dioxide is lower, but the amount of platinum group metal oxide that is not in solid solution continues to act as a catalyst for the release of chlorine.

To create semiconductors and electrocatalytic coatings on titanium and tantalum electrodes, metal oxides from Group VIII of the periodic table of elements, as well as metal oxides from Group VB, Group VIB, metal oxides from Group IB and oxides of elements from Group VA, as well as mixtures of these oxides, which are capable of forming solid solution crystals with titanium dioxide and tantalum oxide upon firing and disrupting the crystal lattice of titanium dioxide and tantalum oxide, can be used.

In forming semiconducting coatings on titanium and tantalum electrodes from other metal oxides, it is advantageous to use mixed metal oxides or materials that form mixed metal oxides from adjacent groups of the periodic table, such as iron and rhenium; titanium, tantalum and vanadium; titanium and lanthanum. Other oxides that can be used are manganese and tin oxides; molybdenum and iron; cobalt and antimony; rhenium and manganese, as well as other mixtures of metal oxides.

The weight percentage of dopants can vary from 0.10% to 50% of the amount of titanium or tantalum in the titanium dioxide or tantalum oxide, and a surprising increase in the conductivity of the titanium dioxide or tantalum oxide surface can be achieved by adding as little as 0.25% to 1% of the dopant to the titanium dioxide or tantalum oxide in the conductive surface of the electrode. However, it is preferred to use a sufficient excess of the dopant metal oxide to form a coating on the anodes that will catalyze the evolution of chlorine without appreciable overpotential.

The conductive coating of the invention can be applied in various ways and on various shapes of titanium or tantalum-based anodes, such as solid rolled solid titanium plates, perforated plates, cut, sheet-like titanium plates, titanium platinum and titanium ingot braids, titanium wire cloth or screen, titanium rods and bars, or similar plates and shapes of tantalum and other metals.

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The preferred method of application is chemical deposition in the form of solutions applied by painting, dipping or spraying or by screen or electrostatic spraying, followed by firing of such coatings on an anodic base, but other methods of application can also be used, such as electrophoretic deposition or electroplating. Care must be taken to ensure that no air bubbles remain in the coating and that the heating temperature remains below the temperature that causes deformation of the base material.

The spectrum of doped titanium dioxide samples shows that a foreign ion replaces a titanium ion at its regular lattice position and induces a splitting of the hyperfine line in agreement with the nuclear spin of the replacing element.

For all applications, the titanium or tantalum core is first cleaned and freed from oxide or other coatings. This cleaning can be done mechanically or chemically, such as by sandblasting, etching, pickling, or the like.

Several examples of embodiments of the invention will now be provided for the purpose of illustration, while of course the composition and form of the solutions, the firing procedure and other operations can be varied within the scope of the invention.

Example 1

The sponge titanium anode is cleaned and etched and then coated with a liquid coating containing the following materials:

ruthenium as RuC1 ₃ .3H ₂ O titanium as TiClj tantalum as TaClj

0.8 mg/cm (metal) 0.89 mg/cm ² (metal) 0.089 mg/cm ² (metal)

The coating mixture is obtained by mixing the dry ruthenium salt in a commercial hydrochloric acid solution containing 15% TiClj. Tantalum is then added in the above ratio and in the form of a solution of 50 g/l TaClj in 20% HCl. The blue color of the solution is changed to orange by adding the necessary amount of hydrogen peroxide, after which isopropyl alcohol is added as a thickener. The coating mixture is applied to both sides of the titanium anode core by electrostatic spraying in four successive layers. The number of layers can be varied and it may sometimes be advantageous to apply several layers to the surface facing the cathode and only one layer, preferably the first, to the surface facing away from the cathode. After each layer is applied, the anode is heated in a forced air oven at a temperature between 300 and 350 °C for 10 to 1.5 minutes, followed by rapid natural cooling in air between each of the first three layers, and after the fourth layer is applied, the anode is heated to 450 °C for one hour under forced air circulation and then cooled.

The amounts of the three metals in the coating correspond to the weight ratios of 45%Ru, 50%Tu, 5%Ta.

In accelerated test operation, this anode shows practically zero mass loss after two current reversal cycles and two amalgam immersions. Each current reversal cycle consists of a sequence of five anodic polarizations at 1A/ ^cm2 , each lasting two minutes, followed by cathodic polarization at the same current density and for the same time. After more than 1,500 hours of operation at 3A/ ^cm2 in saturated sodium chloride solution, the anode potential is 1.41 V. Example 2

The sponge titanium anode is cleaned and etched and then coated with a liquid coating containing the following materials:

ruthenium as RuClj.SHjO titanium as TiClj tin as SnCl^

0.6 mg/cm (metal) 0.94 mg/cm ² (metal) 0.17 mg/cm ² (metal).

The coating is prepared by first mixing the dry ruthenium salt with 15% TiCl2 in a commercial hydrochloric acid solution. Tin tetrachloride is then mixed into the mixture in the above ratio, after which sufficient hydrogen peroxide is added to turn the blue color of this solution to orange. Isopropyl alcohol is then added as a carburizing agent. The coating mixture is applied to both sides of the eye wall and etched titanium anode in four successive layers, and each layer is subjected to the usual heat treatment described in Example 1.

The amounts of the three metals in the coating correspond to the weight ratios of 35% Ru, 55% Ti, 10% Sn. In accelerated test operation, the anode shows a weight loss of 0.09 mg/cm after two cycles of current cycling described in Example 1, and a mass loss of 0.01 mg/cm after one immersion in amalgam. After more than 1,500 hours of operation in a concentrated NaCl solution at 2A/cc and at 60°C, the anode potential is 1.42 V.

Example 3

The cleaned titanium anode plate is coated with a mixture consisting of a hydrochloric acid solution containing the following salts:

ruthenium as RuCl ₃ .3HgO titanium as TiClj aluminum as Α101 ₃ .6Η ₂ 0

0.8 mg/cm (metal) 0.96 mg/ci/ (metal) 0.018 mg/ci/ (metal)

The mixture is prepared by mixing the ruthenium and titanium salts in a solution of TiCl^ in commercial hydrochloric acid as described above. Aluminum chloride is then added in the above ratio, followed by treatment with hydrogen peroxide as in Example 1, after which isopropyl alcohol is added as a thickening agent. The mixture is applied to the cleaned and etched titanium core anode in four successive layers, the coating being applied to both sides of the substrate and to the exposed areas between the upper and lower surfaces of the anode substrate. After the application of each layer, a heat treatment is followed as in Example 1.

The amount of the three metals in the coating corresponds to the weight ratios of 45% Ru, 54% Ti and 1% Al.

After one cycle of current reversal and two wettings in amalgam, the total mass loss is 2

0.1 mg/cm . After operation for more than 1,500 hours in concentrated sodium chloride solution at 60 °C and an anode current density of 3 A/ci/, the anode potential is 1.42 V.

Example 4

The sponge tantalum anode plate is cleaned and etched and then coated with a liquid coating containing the following materials:

Ruthenium as RuC1 ₃ .3HjO 0.8 mg/cm (metal) Titanium as TiCl^ 0.89 mg/ci/ (metal) Tantalum as TaCl^ 0.089 mg/ci/ (metal)

The coating mixture is prepared by mixing the dry ruthenium salt in a commercial hydrochloric acid solution containing 15% TiCl2. Tantalum is then added in the above ratio and in the form of a solution of 50 g/l TaCl2 in 20% HCl. The blue color of the solution is changed to orange by adding the required amount of hydrogen peroxide, after which isopropyl alcohol is added as a thickener. The coating mixture is applied by brush to both sides of the tantalum anode in four successive layers. After the application of each layer, the anode is heated in a forced air oven at a temperature between 300 and 350 °C for 10 to 15 minutes, followed by rapid natural cooling in air between each of the first three layers, and after the application of the fourth layer, the anode is heated to 450 °C for one hour under forced air circulation and then cooled.

The amounts of the three metals in the coating correspond to the weight ratios of 45% Ru, 50% Ti, % Ta.

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X-ray diffraction analysis shows that the coatings on the above anodes are in the form of semiconducting rutile, with the dopant oxides dispersed in the rutile crystals by solid-state dissolution, so that the valve metal anode core has a semiconducting rutile surface that is capable of oxidizing dissolved chloride ions to molecular chlorine gas. Coatings can be applied and stabilized on tantalum electrode cores in a similar manner.

Although semiconducting surfaces can be deposited on titanium or tantalum cores with other dopants, the above tests show that using the described procedures, the presence of titanium or tantalum oxide and iridium alone, i.e. without ruthenium oxide, results in a coating with low activity and high chlorine release potential.

Example 5

Electrodes with five different types of coating are made, each consisting of a four-component mixture of salts including a ruthenium salt:

Sample No. 1 ruthenium as RuC1 ₃ .3Hj,O

Sample No. 2 tin as SnCl.,, ·5Η ₂ ^Ο ruthenium as RuC1 ₃ .3H ₂ O

Sample No. 3 lanthanum as La/NO^/jS^O tin as SnCl^.5H ₂ O ruthenium as RuCl ₃ .3H ₂ O

Sample No. 4

(business) ',14 2 mg/cm (metal) i HCl (commercial) 0.71 mg/ ^cm2 (metal) . (business) 0.017 mg/ ^cm2 (metal) 0.53 mg/ ^cm2 (metal) (business) 1.06 2 mg/cm (metal) . (business) 0.088 mg/ ^cm2 (metal) 0.088 mg/ ^cm2 (metal) 0.53 mg/ ^cm2 (metal) (business) 0.96 2 mg/cm (metal) 0.71 mg/ ^cm2 (metal) 0.25 mg/ ^cm2 (metal) 0.53 mg/ ^cm2 (metal) (business) 1.07 2 mg/cm (metal) 0.088 mg/ ^cm2 (metal) 0.088 mg/ ^cm2 (metal) 0.53 mg/ ^cm2 (metal) (business) 0.88 2 mg/cm (metal) 0.088 mg/ ^cm2 (metal) 0.088 mg/cm ⁴ (metal) 0.071 mg/cnr (metal)

titanium as TiCl^ in HCl solution chromium as Cr/NOy ₃ ·8Η ₂ 0 tin as SnC1^.5H ₂ O ruthenium as RuCl ₃ .3H ₂ 0

Sample No. 5 titanium as TiCl^ in HCl solution aluminum as AlClj.6HjO tin as SnCl^.SH^O ruthenium as RuCl^.1H^O

Each sample is prepared by mixing the ruthenium salt into a solution of TiCl^ in commercial hydrochloric acid and adding hydrogen peroxide in the amount necessary to change the color from blue to red. To this mixture are added the other salts in the indicated proportions along with 0.56 ml

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With each mg of total metal. The above pSt mixtures are applied to five separate titanium plates in five successive layers. Between each coating and the next coating, a heat treatment is carried out at 350 °C for ten minutes. The last coating is followed by a final treatment at 450 °C for one hour.

Anodic tests are performed in a saturated NaCl solution at 60 °C and a current density of 1 A/cm ² . The measured electrode potentials have the following values:

Sample No. 1 Sample No. 2 Sample No. 3 Sample No. 4 Sample No. 5

1.48V 1.40V 1.39V 1.44V 1.39V

Example 6

Four types of coatings are tested, each consisting of a four-component salt mixture, including a precious metal salt.

Sample No. 1

titanium as TiCl^ in HCl solution (commercial) lanthanum as La/NOy^.eHgU tin as SnCl ₄ .5H ₂ O platinum as PtCl^.nHgO (commercial) 0.7 0.088 0.15 0.85 o mg/cm mg/em ² o mg/cm mg/cm ² (metal) (metal) (metal) (metal) Sample P. 2 titanium as TiCl^ in HCl solution (commercial) 0.7 2 mg/cm (metal) lanthanum as La/NOy^.eHgO 0.068 mg/ ^cm2 (metal) tin as SnCl ₄ .5H ₂ O 0.15 mg/ ^cm2 '(metal) rhodium as /NH^/jKhClg 0.85 mg/ ^cm2 (metal) Sample No. 3 titanium as TiCl ₃ in HCl solution (commercial) 0.7 2 mg/cm (metal) aluminum as AlClj.óHjO 0.088 mg/ ^cm2 (metal) tin as SnCl ₄ .5H ₂ O 0.15 mg/cm (metal) iridium as IrCl ₄ 0.85 mg/ ^cm2 (metal) Sample No. 4 titanium as TiCl ₃ in HCl solution (commercial) 0.7 o mg/cm (metal) aluminum as A1C1 ₃ .6H ₂ O 0.088 mg/ ^cm2 (coy) tin as SnCl ₄ .5H ₂ O 0.088 mg/ ^cm2 (metal) palladium as PdClg 0.85 mg/ ^cm2 (metal) These four mixtures are applied to five separate titanium plates and to five separated

tantalum plates in five successive coats. Intermediate and final heat treatments are carried out as in Example 5. The anode potentials, measured under the same conditions as in the previous example, are as follows:

Sample No. 1 1.45 Sample No. 2 1.85 Sample No. 3 1.37 Sample No. 4 1.39

Table 1

Accelerated mass loss tests

Sample Current reversal mg/em ² Wetting in amalgam mg/cm ² Total B (ex. II) 2 Ir 0.2 mg/cm Bu 0.2 mg/cm ² Ti 1.12 mg/cm ² 0 0.152 0.152 C (ex. IV) Ir 0.2 mg/cm ² Bu 0.2 mg/cm ² Ti 1.12 mg/cm ² with titanium core processing to black core oxide 0 0.068 0.068 D (ex. V) Ir 0.2 mg/cm ² Bu 0.2 mg/cm ² Ta 1.6 mg/cm ² 0.020 7 0-.013 8 0.034 5 B (ex. VI) o Au 0.075 mg/cm Bu 0.225 mg/cm ² Ti 1.2 mg/cm ² only coating on Ti ta- 0.030 0.043 0.073 new nucleus 2 Bu 1 mg/cm 0.2 0.73 0.93

Table I shows the advantages of anodes made according to Examples 1 to 10 compared to titanium core anodes coated with platinum group metals by electroplating or chemical deposition.

The mass losses on the samples prepared according to the method of the invention were determined in a simulated operation and compared with the mass losses determined under the same conditions on the samples with a titanium base coated with an alloy of platinum and iridium. The tests were carried out in a saturated sodium chloride solution at 65 ° C and at an anode current density of 1 A/cm ² . The anode potentials were measured with a Luggin tip against a saturated calomel electrode and converted to the normal electrode scale. The results are summarized in Table II. The integrated weight change, compiled in the penultimate paragraph, was positive, i.e. increased, for most of the samples prepared according to the invention. This is an indication that the coating, instead of gradually wearing away and thus reducing its noble metal oxide content, tends to acquire an additional amount of protective semiconducting surface which reaches stability after a short period of service, as illustrated by Example C. The results summarized in Table I, on the contrary, show that even the best noble metal alloy coatings suffer a greater rate of wear in service; while such wear rate is not necessarily attributable to noble metal chipping, it certainly represents a substantial reduction in the noble metal content of the coating. The amount of noble metal in such noble metal alloy coatings required to achieve satisfactory anode activity and a sufficiently long service life is five to ten times greater than in the rutile or tantalum oxide semiconducting coatings prepared according to the invention.

Table II

Sample Coating fabric Hours of operation at - Anode potential in volts Integrated mass change Speed of wear 1 A/cnr (N. H. E.) mg/ ^cm2 grams per ton Clj

3 (Ex. II) IrOg (Irish 0.2 2 mg/m ) 0 1.62 0 (increase in mass) - RuOg (Ru 0.2 O mg/cm ) 792 1.53 +0.3 (increase in mass) 0 _TiO2 (Those 1.12 mg/cm ) 2 000 1.59 +0.1 (increase in mass) 0 C (Ex. III) Ir0 ₂ (Irish 0.4 2 mg/cm ) 0 1.35 - - _RuO2 (Ru 0.4 o mg/cm ) 860 1.36 +0.9 (increase in mass) 0 _TiO2 (Those 0.96 2 mg/cm ) 2 300 1.38 +0.9 (increase in mass) 0 D (Ex. IV) IrO ₂ (Irish 0.2 2 mg/cm ) 0 1.50 - - _RuO2 (Ru 0.2 mg/ ^cm2 ) 552 1.44 +0.75 (increase in mass) 0 ₂ people (Those 1.12 2 mg/cm ) 816 1.50 +0.4 (increase in mass) 0 E (Ex. V) IrO ₂ (Irish 0.2 2 mg/cm ) 0 1.45 - - _RuO2 (Ru 0.2 mg/ ^cm2 ) 514 1.45 -0.097 (decline) 0.15 TaO ₂ (Now 1.6 mg/ ^cm2 ) F (Ex. VI) Au ₂ ° ₃ (Ouch 0.075 mg/ ^cm2 ) 0 1.48 - (increase) - _RuO2 (Ru 0.225 mg/ ^cm2 ) 514 1.48 +0.2 0 Ti0 ₂ (Those 1.2 2 mg/cm ) G Fri ( 1.44 2 mg/cm ) 0 1.36 - Irish ( 3.36 2 mg/cm ) 1 032 1.48 -0.25 (decline) 0.26 H Fri ( 3.68 mg/ ^cm2 ) 0 1.39 - - Irish ( 0.92 2 mg/cm ) 926 1.35 - - 2 940 1.39 -0.6 0.18

The average thickness of the finished coating is about 1.45 microns and the ratio of the mass of platinum group metals to the mass of other metals in the oxide coatings of the catalytically active semiconducting coatings of Examples 1 to 4 is between 20:100 and 85:100.

If some theories are given above to explain the effect of the invention, we do not consider them to be binding in the event that it turns out that the invention does not operate in exact accordance with these theories.

Claims (3)

What is claimed is: 1. An electrode with a chlorine-resistant metal core formed of titanium or tantalum, wherein the metal core is provided with a semiconductor coating comprising at least one oxide of at least one platinum group metal, at least one oxide of titanium dioxide group; tantalum pentoxide, and at least one doping oxide selected from the group consisting of silver, gold, tin, chromium, tantalum, lanthanum, aluminum, cobalt, antimony, molybdenum, nickel, iron, tungsten, vanadium, phosphorus, boron, beryllium, sodium, calcium, strontium, lead, copper and bismuth and mixtures thereof, wherein the proportion by weight of doping oxide or oxides is in the range of 0.10 to 50% by weight of titanium and / or tantalum in titanium dioxide or tantalum; the amount of other metals in the oxide coating is from 2.0: 1 00 to 85: 100. 2. The electrode of claim 1 wherein the doped oxides consist of tin oxide and at least one metal oxide selected from the group consisting of tantalum, lanthanum, chromium and aluminum. 3. The electrode of claim 1 wherein the coating comprises platinum-containing oxides of two metals, namely ruthenium oxide and iridium oxide.