DK153166B - CLOTHED METAL ELECTRODE WITH IMPROVED BARRIER LAYER, PROCEDURE FOR PREPARING SUCH A ELECTRODE, AND USING THE ELECTRIC WIRE AS ANODE BY CHLOR ALKALIE ELECTROLYSIS - Google Patents

Abstract

An electrode for use in electrolytic processes comprises a substrate of film-forming metal such as titanium having a porous electrocatalytic coating comprising at least one platinum-group metal and/or oxide thereof possibly mixed with other metal oxides in an amount of at least about 2 g/m<2> of the platinum-group metal(s) per projected surface area of the substrate. Below the coating is a preformed barrier layer constituted by a surface oxide film grown up from the substrate. This preformed barrier layer has rhodium and/or iridium as metal or compound incorporated in the surface oxide film during formation thereof in an amount of up to 1 g/m<2> (as metal) per projected surface area of the substrate.

Description

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The invention relates to electrodes for use in electrolytic processes, the electrode comprising a film-forming metal substrate having a porous electrocatalytic coating comprising at least one metal group metal and / or oxide thereof optionally mixed with other metal oxides in a amount of at least 2 g / m 2 of platinum group metals) per projected surface area of the substrate and wherein the substrate underneath the coating has a prefabricated barrier layer formed by an oxide surface film grown from the substrate.

By "film-forming metal" is meant a metal which has the property that when connected as an anode in the electrolyte in which the coated anode is then to work, a passivating oxide film is rapidly formed which protects the underlying metal from corrosion of the electrolyte.

These metals are also often referred to as "valve metals".

Examples of substrates are titanium, tantalum, zirconium, niobium, tungsten, aluminum and alloys containing one or more of these metals, as well as 20 silicon-iron alloys.

In early proposals (see, for example, British Patent Specifications Nos. 855,107 and 869,865), a titanium electrode with a plating group metal coating was provided with an inert barrier layer of titanium oxide at the porous sites of the coating, this barrier layer being preferably formed or reinforced by a heat treatment. Later, in British Patent Specification No. 925,080, the inert barrier layer of titanium oxide was prepared by electrolytic treatment or heating of the titanium substrate 30 in an oxidizing atmosphere prior to application of the platinum group metal. The preparation of such a barrier layer was also proposed in English Patent Specification No. 1,147,422 to improve the anchoring of an active coating consisting of or containing platinum group metal oxides.

Later, the development of coatings formed from mixed crystals or solid solutions of co-deposited oxides of film-forming metals and platinum

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2 group metals (see U.S. Patent No. 3,632,498) to commercially viable electrodes that revolutionized the chlor-alkali industry and have been extensively used in other applications. With these electrodes excellent performance was achieved without the need for a reinforced or prefabricated inert barrier or anchoring layer on the substrate, and today it is widely accepted that the prefabricated or reinforced inert barrier layers are detrimental to the function. Looking back, the early proposals with prefabricated or reinforced inert barrier layers appear to have been unsuccessful attempts to avoid defects inherent in the prior coatings rather than in the substrate.

Nevertheless, some suggestions have been made to try to improve inert barrier layers, e.g. by applying a titanium oxide barrier layer 4+ from a solution containing Ti ions. Again, this has been shown to impair the functioning of the electrodes.

Another angle of attack has been to indicate a non-passivating barrier layer or intermediate layer lying beneath the active outer lining. Typical proposals have been doped tin dioxide substrates thin substrates of one or more platinum metals such as a platinum iridium metal.

alloy? substrate of cobalt oxide or lead oxide and so 9 S

continue. Although various patents have required marginal improvements at these electrodes in particular applications, none of these proposals has in practice led to any significant improvement or widespread commercial use.

The invention relates to an electrode with a film-forming metal substrate having a porous outer electrocatalytic coating containing at least about 20 cm. 2 g / m (as plate group metal per projected surface area of the substrate) of at least one plate group metal and / or

* 3 C

oxide thereof optionally mixed with other metal oxides and an improved non-passivating barrier layer constituted by an oxide surface film grown from substrate

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3, which barrier layer extends the lifetime of the electrode with a given noble metal content or reduces the amount of precious metal required to achieve a given lifespan.

According to the invention, this barrier layer is a pre-prepared oxide surface film and with rhodium and / or iridium incorporated into the oxide surface film during its formation in an amount of up to 1 g / m projected surface area of the substrate.

1q The oxide surface film of the barrier layer is rendered non-passivating by the uptake of rhodium and / or iridium as metal or as a compound generally the oxide or a partially oxidized compound.

The invention further relates to a method for producing such an electrode which is characterized by the characterizing part of claim 11.

The invention further relates to a method of producing such an electrode which is characterized by the characterizing part of claim 13.

The dilute acid solution used, or certain solution indicating the film-formed metal substrate, will hereinafter be referred to as "paint".

The paint used will typically include an organic solvent such as isopropyl alcohol, an acid (especially HCl, HBr or HI) or another agent (e.g.

NaF), which attacks the film-forming metal and promotes the formation of film-forming metal oxide during the subsequent heat treatment, and one or more thermon-degradable salts of iridium and / or rhodium. Generally, this solution will contain about 1 to 15 g / liter of iridium and / or rhodium (as metal) and be at least five times more diluted and preferably approx. diluted ten or more times (in terms of its noble metal content) than the paint solution that can be used to prepare the outer porous electrocatalytic oxide coatings; this means that the amount of iridium and / or rhodium will be reduced e.g. to 1/5 or 1/10 or even 1/100 of the amount of the corresponding plate group.

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4 metal in the paint used for the manufacture of outer coatings for about the same amount of solvent and acid.

The action of the acid or other agent which attacks or corrodes the film-forming metal and promotes the formation of the oxide film during the subsequent heat treatment is very important, without a suitable agent producing this effect, formation of the oxide surface film on the film-forming metal in it is substantially prevented or inhibited.

It has been noted that by applying a layer of a given solvent / acid mixture to a film-forming metal base previously subjected to the usual purification and etching treatment, and then heating ζ after drying to remove the solvent, a given amount of film-forming metal oxide being produced. This procedure can be repeated a number of times (generally four or five times for 4 ml of HCl in 60 ml of isopropyl alcohol applied to a titanium base, dried and heated to 500 ° C for ten minutes) before the growth of film-forming metal oxide during successive treatments is inhibited. . The first layer of the formed integral oxide surface film will be relatively porous.

This allows the then applied layer of o c: the acid paint to penetrate this first porous layer during the drying phase, so that the acid attacks the underlying film-forming metal. Thus, ions of the film-forming metal are released from the base to convert to oxide during the subsequent heating, this being

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oxide partially forms in the pores of the first layer. Thus, the porosity of the resulting oxide film is reduced after each coating cycle until no more film-forming metal can be converted from the base to oxide. Thus, an extremely stable relatively compact and impervious film of film-forming metal oxide can be prepared by the application of a limited number of coatings of acid paint followed by drying and heating.

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To prepare barrier layers in the electrode of the invention, each applied paint layer includes such a small amount of the iridium and / or rhodium compound that the electrocatalyst formed by the thermocouple 5 is completely incorporated into the integral surface film of film-forming metal oxide formed each time. In general, each applied paint layer will contain at most approx. 0.2 g / m iridium and / or rhodium per ml. projected surface area of the base, generally 10 much smaller. Further application of additional layers of the diluted paint is stopped after the number of coatings after which the growth of the oxide surface film on the film-forming metal ceases or is inhibited. Thus, the optimum amount of electrocatalytic agent in the diluted paint and the optimum number of layers to be applied to produce a satisfactorily compact impermeable barrier layer can be determined quite easily for any particular substrate, solvent / acid and electrolytic material. In many cases, 20 to 10 layers of the very diluted paint will each be applied, followed by drying and heating from approx. 400 to 600 ° C for approx. 5 to 10 minutes, with the possible exception of the final layer, which can be heated for a prolonged period of time - possibly several hours or 2 ^ days at 450-600 ° C in air or in a reducing atmosphere (eg, ammonia / hydrogen).

Viewed with the naked eye or under a microscope, barrier layers made in this way on an etched or non-etched titanium base generally retain the same set of distinctive features as titanium oxide films made in the same manner and which do not contain iridium and / or the rhodium electrocatalyst, typically a clear blue, yellow and / or red "interference" film color.

The dilute acidic paint solution used to prepare the barrier layer by the process of the invention preferably includes only one thermosetting agent.

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6, degradable iridium and / or rhodium compound since the film-forming metal oxide component is obtained from the base.

However, the diluted paint may contain small amounts of other components such as other platinum group metals 5 (ruthenium, palladium, platinum, osmium, especially ruthenium), gold, silver, tin, chromium, cobalt, antimony, molybdenum, iron, nickel, manganese, tungsten, vanadium, titanium, tantalum, zirconium, niobium, bismuth, lanthanum, tellurium, phosphorus, boron, beryllium, sodium, lithium, calcium , 10 strontium, lead and copper compounds and mixtures thereof. If a small amount of a film-forming metal compound is used, in order to contribute to doping the surface film, it will generally be a metal different from the film-forming metal substrate. Excellent results have been obtained with iridium / ruthenium compounds having a weight ratio of approx. 2: 1 as metal. Of course, when such additives are included in the diluted paint composition, they will be present in an amount compatible with the small amount of the main electrocatalyst, ie. an iridium and / or rhodium compound, so that substantially all of the main electrocatalyst and additive are incorporated into the surface film of film-forming metal oxide. In any case, the total 25 amount of iridium and / or rhodium and other metals is below 1 g / m and generally below 0.5 g / m, and the additional metal will be present in a smaller amount than the rhodium and / or iridiummet. These iridium / rhodium compounds and other metal compounds may be thermally degradable to form the metal or oxide, but in neither case is it necessary to proceed to complete decomposition, e.g. has barrier layers containing partially degraded iridium chloride containing up to approx. 5% by weight of the original chlorophyll 35 excellent properties. Barrier layers containing as little as 0.1 to 0.3 g / m (as metal) iridium and / or rhodium oxide / chloride in their surface film provide

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7 great results. Studies have shown that a 2 barrier layer containing 0.5 to 0.6 g / m (as metal) iridium produces an optimum effect expressed by the increased life of the coated electrodes.

Increasing the amount of iridium beyond these values does not further increase the service life.

If a titanium substrate is used, the oxide surface film has been found to be mostly rutile-titanium dioxide, presumably catalyzing the formation of rutile e.g. at about. 400-500 ° C of the rhodium and / or iridium in the diluted coating solution.

After forming the improved barrier layer impermeable to electrolyte and oxygen developed, the porous outer electrocatalytic coating is applied using standard techniques, e.g. by applying on top of the prefabricated barrier layer a plurality of layers of a relatively concentrated solution containing a thermally degradable plate group metal compound and heating. Each applied outer layer will contain at least 20 0.4 g / m 2 of the plate group metal per The projected area of the substrate and the coating procedure are repeated to build an effective outer coating containing at least approx. 2 g / m of plate group metal (clay): generally in oxide form. The coating components may be selected to provide a coating consisting essentially of a solid solution of at least one oxide of a film-forming metal and at least one plate group metal oxide as disclosed in U.S. Patent No. 3,632,498. The coating is advantageously a solid solution of ruthenium and titanium-5 ° oxides with a ruthenium: titanium atane ratio of from 1: 1 to 1: 4. In this case, the coating consists of several superimposed layers which typically have a micro-cracked appearance and are quite porous. The use of an improved barrier layer in the electrode of the invention together with such cladding significantly improves the functioning of the electrode during standard accelerated lifetime studies under oxygen evolution conditions.

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8 ser. It can be predicted that under the conditions of normal commercial production of chlorine, the improved electrode will have a significantly longer life, since it is known that one of the causes of the breakdown of these electrodes after extensive use in chlorine production is due to the effect of oxygen on the substrate. It will also be possible to achieve the same lifetime with a noticeable reduction in the thickness of the outer garment, which allows for a reduction in the amount of clothing used and in the work and energy consumption used for manufacturing.

The outer covering may also be formed from one or more sheet metal metals, e.g. a platinum-iridium alloy useful for chlorate production ^ and to a limited extent in partition or membrane cells for chlorine production. With conventional Pt / Ir coated electrodes, the coatings should be relatively thick (at least about 5 g / m) to avoid passivation problems. With the present improved barrier layer, thinner and more porous layers of the platinum metals can be used without any problems due to oxidation of the substrate or the disadvantages associated with the known passive barrier layers of titanium oxide.

It is also possible to apply the outer coating by plasma spraying of a solid solution of an oxide of a film-forming metal and a platinum group metal oxide.

Eg. For example, a solid solution powder can be prepared by flame spraying as described in U.S. Patent No. 3,677,975, and this powder is then sprayed onto the base. Alternatively, the coating is applied by plasma spraying of at least one oxide of a film-forming metal on top of the prefabricated barrier layer with subsequent incorporation of plate group metal (s) and / or oxides thereof into the plasma-sprayed oxide of film-forming metal, e.g. in accordance with the method of U.S. Pat.

4140813. The improved barrier layer again increases the service life and enables a reduction in the precious metal content

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9 in the garment.

In a preferred method of mass-making the electrodes, a set of electrode substrates is subjected to a series of pretreatments including etching and formation of the barrier layer by dipping the substrate set into said diluted solution and heating the substrate set / and then applying the outer electrocatalytic coating to the substrates. . This procedure removes the disadvantage associated with commercial electrode apparel systems with a "bottleneck" between the etching bath and the clothing line. In the usual mass-making procedure, a set of substrates with sandblasting is pre-treated followed by etching, cleaning and drying, and these substrates are then individually coated on a coating / baking line. Thus, it has been necessary to synchronize the etching with the coating / baking because the etched substrates cannot be left for long periods (more than about 2 days) without damage to the electrode function due to air oxidation of the substrate prior to coating, especially if dust or dirt is anchored in the thin oxide film. By pre-coating the substrate sets with an improved barrier layer immediately after etching, this bottleneck effect is avoided and the surface-treated substrates can be stored without the risk of further oxidation. Any dust or dirt that can settle on the barrier layer can easily be blown off prior to cladding as it is not anchored in the film.

Further, the dip application procedure is one

O A

substrate sets stacked against each other satisfactorily to produce the enhanced barrier layer oxide film growing from the substrate. Corresponding handling is not satisfactory for the application of the conventional coatings, where an added thickness must be built up over and above the film-forming metal base and its very thin oxide surface film with each applied coat.

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The electrode base may be a sheet of any film-forming metal, although titanium is preferred for cost reasons. Rods, tubes and expanded grids of titanium or other film-forming metals can also be surface treated by the method of the invention. Titanium or other film-forming metal applied to a conductive core may also be used. For most applications, the base will be etched prior to the surface treatment to provide an uneven surface that provides good front coverage of the subsequent electrocatalytic coating applied. It is also possible to surface-treat porous, sintered or surface-sprayed titanium with the diluted paint solution in the same way, but the porous titanium will preferably merely be a surface layer 15 on a non-porous base.

The electrodes with an improved barrier layer according to the invention are excellent as anodes for chlor-alkali electrolysis. These electrodes have also shown excellent performance when used for electro-recovery in a mixed chloride-sulfate electrolyte providing mixed chlorine and oxygen evolution.

The invention is further illustrated in the following examples.

25

Example 1

7.5 x 2 cm titanium plates available for (5) under the trade name "Contimet ^ 30" were degreased, purified in water, dried and etched for 1/2 hour in oxalic acid. A paint solution consisting of 6 ml of n-propanol, 0.4 ml of HCl (concentrated) and 0.1 g of iridium and / or rhodium chloride was then applied with a brush on both sides of the plates in four thin layers. The plates were dried to evaporate the solvent and then heated in air to 500 ° C for 10 minutes after each of the first three layers and for 30 minutes after the final layer. This gives a rhodium and / or iridium content (calculated as metal) of approx. 0.2 to 0.3 g / m in

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11, the barrier layer depending on the amount of solution in each applied layer determined by weight measurement.

A titanium oxide-ruthenium oxide solid solution with a titanium: ruthenium atane ratio of approx. 2: 1 was then applied 5 by brushing with a solution consisting of 6 ml of n-propanol, 0 ° 4 ml of HCl (concentrated), 3 ml of butyl titanate and 1 g of RaCl 2 and heating in air at 400 ° C for 5 minutes. . (Note: This solution is ten times more concentrated in precious metal: propanol pan solvent than the diluted solution used to prepare the barrier layer). This procedure was repeated until the garment had a thickness of approx. 10 g / m (i.e. about 4 g / m Ru metal).

Electrodes thus prepared are subjected to 15 comparative electrochemical studies with corresponding electrodes (a) with a TiO 2 barrier layer prepared by the same procedure but with a paint consisting solely of 6 ml of n-propanol and 0.4 ml of HCl (concentrated) and ( b) without any barrier layer. The first 20 results suggest that the electrode of the invention is increased by a factor of 2-10 by accelerated lifetime studies as anodes under oxygen evolution conditions and should have a life span many times longer than the comparison anode (a) and by chlor-alkali electrolysis; significantly longer than comparison anode (b).

Example 2

A titanium plate was degreased, purified in water, dried, etched and then surface treated as in Example 1 with a paint solution containing 2: 1 by weight iridium and ruthenium chlorides (as metal). The treatment was repeated four times until the titanium dioxide film formed contained an amount of 0.2 g / m Ir 55 and 0.1 g / µl Ru, both calculated as metal. The heat treatment was carried out at 400 ° C for 10 minutes after each layer applied. Then an outer covering was applied

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TiC ^ .RuC ^ as in Example 1. The same electrochemical comparison studies have yielded the same promising initial results as in Example 1.

Example 3

Titanium plates were degreased, purified in water, dried and etched as in Example 1 and treated with an iridium chloride solution similar to that of Example 1. The solution was applied in four thin layers and the plates were dried to evaporate the solvent and then heated to room temperature. 480 ° C for 10 minutes at the end of each application. The iridium concentration was varied to give a content of 0.3, 0.6 and 0.8 g / m iridium (calculated as metal) in the barrier layer.

15

A titanium dioxide-ruthenium dioxide solid solution coating was then applied as in Example 1, except that the coating thickness was 20 g / m (about 8 g / m Ru metal). These electrodes were subjected to accelerated lifetime studies under oxygen evolution conditions. The lifetime increase in comparison with the life of a corresponding electrode without a barrier layer (or with a barrier layer of TiC 2 containing no iridium) was as follows:

Ir in barrier layer Lifetime increase 25 (g / m 2) (factor) 0.3 2 0.6 5 0.8 5

In comparison, a similar coated electrode without barrier layers, but with the addition of 0.6 g of iridium dispersed in the coating, shows only a marginal lifetime increase.

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Example 4

Electrodes were prepared in a similar manner to Example 1, but using snow-diluted paint containing various platinum group metals including palladium, platinum and ruthenium alone, as well as rhodium and iridium as described previously to prepare the barrier layer. These electrodes were subjected to lifetime comparison studies such as oxygen evolution anodes. Only the electrodes with a bar-10 barrier layer containing Rh and / or Ir showed a marked increase in lifetime in this study; combinations of Rh and / or Ir with smaller amounts of the other platinum group metals or their compounds, in particular Ru and Pd, also produced significant improvements.

15

Example 5

Titanium sheets were provided with barrier layers containing 2 approx. 0.2 g / m iridium and / or rhodium according to the procedure of Example 1. They were then ground with a solution containing 0.5 g iridium chloride and 1 g platinum chloride in 10 ml isopropyl alcohol and 10 ml linalool, and heated in a oven to 350 ° C. An ammonia / hydrogen mixture was then passed for approx. 30 seconds to produce a garment containing 70% Pt and 30% 25 Ir. The coating procedure was repeated to build a coating containing 4 g / m of the Pt / Ir alloy.

2

For similar electrodes coated with less than 7 g / m of the Pt / Ir alloy, but without the improved barrier layer, it has been reported that operation at elevated 30 current density causes passivation and that at least 27 g / m must be applied to achieve satisfactory operation for extended periods. This problem is apparently solved with the electrode according to the invention, which works 2 satisfactorily with a coating of 4 g / m.

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Example 6

Titanium sheets were provided with barrier layers containing o. 0.2 g / m iridium and / or rhodium according to the procedure of Example 1. Thereafter, a plasma spray of a layer of approx. 400 g / m 2 of titanium oxide on the barrier layer using standard techniques. The plasma sprayed titanium oxide layer was then coated with layers containing 2 g / m 2 (as metal) ruthenium oxide and / or iridium oxide in various ratios by painting with a solution of 6 ml of propanol and 1 g of RuCl 2 and / or IrCl 2 and heating in air to 500 C for 10 minutes after each layer. Initial electrochemical studies suggest that these electrodes should have an extremely long life as anodes in mercury-chloro-alkaline cells operating at high current densities. From the data published in U.S. Patent No. 4,140,813, it appears that the electrode of the invention will achieve the same excellent service life with as little as 1/5 of the precious metal content.

20

Example 7

Titanium sheets were provided with barrier layers containing approx. 0.3 g / m iridium, rhodium and iridium / ruthenium in a 2: 1 weight ratio according to the procedure of Example 1 (except that in some cases the final heating was prolonged for several hours.

An aqueous solution containing iridium chloride and tantalum chloride (with Ir and Ta metals in equal weight ratio) was applied with brush on both sides of the plates in 5, 10 and 15 layers. Each applied layer contained approx.

0.5 g / m iridium. After each layer, the plates were dried and heated in air for 10 minutes at 450 ° C and for 1 hour after the final layer. The resultant coating was a solid solution of iridium and tantalum oxides containing approx. 2.5, 5 and 7.5 g / m 2 iridium. The electrodes were tested as anodes in 10% sulfuric acid at 60 ° C

Claims (22)

An electrode for use in electrolytic processes, said electrode comprising a film-forming metal substrate having a porous electrocatalytic coating comprising at least one plate group metal and / or oxide thereof optionally mixed with other metal oxides in an amount of at least approx. 2 g / m of plating group cement (clay) per projected surface area of the substrate and wherein the substrate underneath the coating has a prefabricated barrier layer formed by an oxide surface film grown from the substrate, characterized in that the prefabricated barrier layer has rhodium and / or iridium incorporated into the oxide surface. the surface film during its formation in an amount of up to 2 1 g / m projected surface area of the substrate. Electrode according to claim 1, characterized in that the porous electrocatalytic coating consists of a plurality of superimposed layers of microcracked configuration. Electrode according to claim 2, characterized in that the porous electrocatalytic coating consists essentially of a solid solution of at least one oxide of a film-forming metal and at least one platinum group metal oxide. Electrode according to claim 3, characterized in that the porous electrocatalytic coating is a fixed solution of ruthenium and titanium oxides having a ruthenium: titanium atomic ratio of from 1: 1 to 1: 4. An electrode according to claim 1, characterized in that the porous electrocatalytic coating consists essentially of one or more plate group metals. An electrode according to claim 5, characterized in that the porous electrocatalytic coating is a platinum-iridium alloy. An electrode according to claim 1, characterized in that the porous electrocatalytic coating is a plasma-sprayed layer of at least one oxide of a film-forming metal incorporating plate group metal (s) and / or oxides thereof. Electrode according to any one of the preceding claims, characterized in that the oxide surface film in the barrier layer contains at least one additional added metal in addition to rhodium and / or iridium, but in lesser quantity than the rhodium and / or iridium, the total metal content of the barrier layer. is up to 1 g / m2. 35 BK 153166B Electrode according to claim 8, characterized in that said film contains up to 0.5; g / m iridium and ruthenium in a weight ratio of approx. 2 :: first An electrode according to any one of the preceding claims, characterized in that the substrate is of titanium and that the oxide surface film is essentially rutile-titanium dioxide, with rhodium and / or iridium incorporated herein. A method of producing an electrode for use in electrolytic processes, comprising forming a barrier layer on a film-forming metal substrate and applying on top of the barrier layer a porous outer electrocatalytic coating comprising at least one plate group metal and / or electrolytic coating. 15 clays of oxide thereof optionally mixed with other oxides in 2 an amount of at least 2 g / m platinum group metal (clay) per projected surface area of the substrate, characterized in that the barrier layer is prepared by applying the substrate to one or more layers of a very dilute (as defined below) acid solution containing a thermally degradable compound of rhodium and / or iridium as the layer or each layer is dried; heated to form a film film metal oxide surface film on the substrate and at the same time at least partially decomposing said compounds, the layer or each layer of said highly diluted solution containing an amount of said compound which is substantially completely is absorbed into the surface film formed during heating and the number of layers applied is such that the resulting barrier layer 2 contains up to 1.0 g / m rhodium and / or iridium per annum. projected surface area of the substrate. DK 153166 B Process according to claim 11, characterized in that each applied layer of the solution 2 contains up to 0.2 g / m rhodium and / or iridium metal per liter. projected surface area of the substrate. A method of producing an electrode for use in electrolytic processes, which comprises forming a barrier layer on a film-forming metal substrate and applying on top of the barrier layer a porous outer electrocatalytic 1Q coating comprising at least one sheet metal base metal and / or oxide thereof optionally mixed with other oxides in an amount of at least approx. 2 g / m of plating group powder metal (clay) · pr. projected surface area of the substrate, characterized in that the barrier layer is formed - by applying to the substrate several layers, each containing up to 0.2 g / m (as metal per projected surface area of the substrate), of a thermally breakable compound of rhodium and / or iridium in a solution which attacks the film-forming metal substrate, and that, after drying, each layer is heated to produce an oxide barrier layer of the film-forming metal, 2 which contains up to 1.0 g / m iridium and / or rhodium. Process according to any one of claims 11-13, characterized in that 2 to 5 layers of the diluted solution are applied, each followed by heating to between approx. 300 and 600 ° C for approx. 5 to 15 minutes, optionally heating the final layer for a longer period of time. Process according to any of claims 11-14, characterized in that the heating is carried out to completely decompose said compound. Process according to any of claims 11-15, characterized in that the porous DK 153166 B outer electrocatalytic coating is formed by applying on top of the prefabricated barrier layer a plurality of layers of a relatively concentrated solution containing a thermally breakable plate group metal compound. , and up-5 heat. Method according to claim 16, characterized in that each applied outer layer contains at least 0.4 g / m platinum group metal per second. projected area of the substrate base. A method according to any one of claims 11-15, characterized in that the porous outer electrocatalytic coating is applied by plasma spraying. Process according to any one of claims 11-15, characterized in that the porous outer electrocatalytic coating is applied by plasma spraying of at least one oxide of a film-forming metal on top of the prefabricated barrier layer and then incorporating plate group metal (clay) and / or oxides 20 thereof in the plasma-sprayed oxide of film-forming metal. A method according to any one of claims 11-19, characterized in that a set of electrode substrates is subjected together to a series of pretreatments including etching and formation of the barrier layer by immersing the substrate set in said diluted solution and heating the substrate set. the outer electrocatalytic coating is applied to the substrates one at a time. An electrode, characterized in that it is made by a method according to any one of claims 11-20. Use of the electrode according to any one of claims 1-10 or 21 as an anode for chlor-alkali electrolysis. 35