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United States Patent Office Patented June 6, 1972 3,668,005 United States Patent Office Patented June 6, 1972 2 metal support in a molten bath of an oxide of a metal 3,668,005 of the platinum group under pressurized oxygen. Finally, PROCESS FOR THE COATING OF ELECTRODES coating methods have also been described which are Guy Sluse, Rixensart, and Gustave Joanaes, Abolens, Belgium, assignors to Solvay & Cie, Brussels, Belgium carried out by electroStatic pulverization under vacuum No Drawing. Filed Jan. 11, 1971, Ser. No. 105,628 or in the presence of oxygen and even by means of a Claims priority, applicag xemburg, Jan. 9, 1970, plasma generator. 168 SUMMARY OF THE INVENTION Int, Cl, B44d 1/18 U.S. C. 117. 25 13 Claims One of the objects of the present invention is to rem 0 edy difficulties encountered with known metal electrodes coated with a metal oxide of the platinum group by pro ABSTRACT OF THE DISCLOSURE viding a low over-voltage stable electrode which is espe Electrodes coated with ruthenium dioxide are manu cially suitable as an anode for the electrolysis of aqueous factured by applying an anchoring layer containing at solutions of alkali metal halides, and which also may be least one compound oxidizable by ruthenium tetroxide 5 used advantageously for other electrolytic and electro to the area of an etched metal support wherein the ru chemical techniques, such as the production of peroxide thenium dioxide is to be fixed, exposing the thus coated salts, for the protection of the cathode, for the oxidation support to ruthenium tetroxide in the gaseous state, which of organic compounds and for fuel batteries. is decomposed to ruthenium dioxide upon contact with Another object of the present invention is to furnish a the anchoring layer on which it is preferentially fixed 20 simple and relatively inexpensive process for the produc and then heating the thus treated support. Electrodes pro tion of such an electrode. duced in this manner have an adherent coating of ru A process for producing a coated electrode has been thenium dioxide which is resistant to electrolyte corrosion discovered which does not require the installation of and support high current densities. costly apparatus and which may be carried out very eco 25 nomically, particularly with respect to the labor required, compared with that of the processes commonly employed at present, especially those coating processes which in BACKGROUND OF THE INVENTION volve the application of Successive coats of material. Fur The present invention pertains to electrodes used in ther, by means of the present invention, electrodes are electrochemical processes composed of a metallic support 30 obtained with an adherent coating which is completely which conducts electricity and which is resistant to corro resistant to electrolytic corrosion, particularly to nascent sion under the conditions prevailing in an electrochemical chlorine, and which develops with the release of chlorine cell and a metal oxide coating fixed on the Support which a very low over-voltage which hardly varies with time is also resistant to electrochemical corrosion and which and, in addition, the electrodes support high current favors the exchange of electrons between the support and 35 densities. the ions of the electrolyte. More particularly, this inven According to the present invention, the coating process tion concerns a process for manufacturing a metal Sup is carried out by applying an anchoring layer containing port coated with ruthenium dioxide. at least one compound oxidizable with ruthenium tetrox Recently, various types of metal electrodes have been ide to those areas of a previously etched metal support developed having a coating which is comprised of at 40 wherein ruthenium dioxide is to be fixed, exposing the least one metal oxide of the platinum group. It has been thus coated support to an atmosphere containing ruthe observed that with usage, there is a relatively rapid de nium tetroxide in the gaseous state, the ruthenium tetrox terioration of the electrochemical characteristics of these ide being decomposed to ruthenium dioxide on contact electrodes, which causes, in addition to the necessity of with the anchoring layer on which it is preferentially their replacement, the contamination of the products of 45 fixed, and then heating the thus treated and coated the electrolysis and a decrease in the efficiency of the Support. electric current. Various methods have already been proposed for the DESCRIPTION OF THE PREFERRED production of metal oxide coatings of the platinum group, EMBODIMENTS which are obtained either directly as oxide or in the 50 The support is generally composed of a film-forming metallic state; in the latter case, the metal coating is con metal, such as titanium, tantalum, zirconium, niobium and verted to the oxide by baking in an oxidizing atmosphere, tungsten, or of an alloy consisting principally of at least by heating at high frequency under vacuum, by electroly one of the foregoing metals, i.e. containing at least 50% sis in pulsated current or by immersion in a molten bath by weight of one or more of the foregoing metals. of an oxygenated salt. 55 The etching of the support is effected by any known In most of the proposed coating methods, the coating means such as electrolysis, immersion in a molten bath is carried out in the liquid phase, generally by applying of alkali salts or their mixtures, or in an aqueous solution a solution or Suspension of a compound of the platinum of alkali or organic or inorganic acid; but, generally, im group to the metal support by repeated painting by im mersion in an aqueous solution of oxalic acid or hydro mersion or by spraying, after which the oxide is precipi 60 chloric acid is preferred. tated by chemical, thermal or electrical means. Oxides of The composition applied to form the anchoring or the platinum group of metals can also be deposited di adhesive type layer must have a syrupy, i.e. slightly to rectly on the metal support starting with these solutions moderately viscous, liquid consistency so as to form a or suspensions by electrolysis with an alternating cur continuous layer, i.e. a coating without gaps, on the sur rent, or by electrophoresis, and also by immersion of the 65 face of the metallic support which is to be coated eventual 8,668,005 3 4 ly with ruthenium dioxide. The adhesive may be com RuO2 in the final layer, an anchoring layer which is thin posed of any compound or mixture thereof which is oxi ner than specified generally does not provide a coating of dized by ruthenium tetroxide. Ruthenium tetroxide is a RuO of the preferred thickness and, when the anchoring strong oxidizing agent and is known to oxidize a wide layer is too thick, an excessive deposit of RuO2 is often variety of organic and inorganic compounds, as indicated obtained which causes a decrease in the electrical polari by numerous publications, for example, Berkowitz et al., zation of the electrode. The thickness of the final coating, J. Am. Chem. Soc. 80, 6682 (1958); Benyon et al., Proc. after thermal treatment, is preferably about 0.1 to 6 g./m. Chem. Soc. 1964 (Oct.), 342; Caputo et al. Tetrahedron and more preferably about 0.5 to 4 g/m. when obtained Lett. 1967 (47), 4729; Benyon et al., Carbohyd. Res. in a single series of operations. 1968, 6(4), 431; Rylander, Engelhard Ind. Tech. Bull. O The sequence of steps according to the invention, i.e. 1969,9(4), 135 and U.S. Pat. No. 3,278,558. application of the anchoring layer, exposure to gaseous Thus, the anchoring layer may contain any compound ruthenium tetroxide, may be repeated several times so as or mixture thereof which is oxidized by ruthenium te to obtain a coating of the desired thickness. When the troxide such as aromatic compounds, olefins, alcohols, sequence of steps is repeated, the final thermal treatment aldehydes, amides, ethers, sulphides, hydrides, plasticizers 5 can be carried out driectly after the final application of for polymers, oils such as coriander oil, paraffin oil, sili the anchoring layer, i.e. the step of exposing the Support cone oil, greases such as silicone grease, hydrocarbon provided with the anchoring layer to ruthenium tetroxide, greases, or mixtures of these substances. The oxidizability may be omitted from the last sequence of steps. of a variety of specific compounds in the aforementioned In a particularly preferred embodiment of the invention, classes by means of ruthenium tetroxide is readily avail 20 the metal support material is titanium or one of its alloys able in the literature, several examples of which have al having anodic polarization properties similar to those of ready been specified. titanium. Such an electrode is particularly suitable as the In order to facilitate their application onto the support, anode in the electrolysis of aqueous solutions of alkali when desirable, any of the foregoing substances may be metal halides. placed in solution or in dispersion in a solvent such as 25 The examples which follow further illustrate the best titanium tetrachloride, carbon tetrachloride, trichloroeth mode currently contemplated for carrying out the inven ylene, perchloroethylene, methylene chloride, benzene, tion but must not be construed as restricting the invention toluene, petroleum ether, gasoline or petroleum. The main in any manner. role of the solvent is to reduce the viscosity of the coating EXAMPLE 1. in order to provide a homogeneous and continuous coating 30 of the support. The solvent may likewise play a role with Small plates of titanium which have been etched by respect to the reduction of gaseous ruthenium tetroxide. immersion over a period of 5 hours at approximately The coating can be carried out by any adequate tech 100 C.
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