Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material

Definitions

• the present invention relates to a cathode and to its use in electrolytic processes based on electrochemical reactions. More specifically, the present invention refers to cathodes which active surface is made up of a certain class of metal alloys that show high corrosion resistance, and that operate under low hydrogen overvoltage, when used at water or alkali metals aqueous solutions electrolysis.
• BACKGROUND INFORMATION Electrical power consumption is responsible for the most part of the operation cost of an electrolytic cell.
• a great deal of the developments applied to electrolysis reactions aim at reducing the energy cost through the reduction of the cell voltage.
• theoretical voltage In order that the electrolysis reaction may occur, a minimum voltage is required, known as theoretical voltage, this being a function solely of other operation conditions, such as temperature, current density, kind and concentration of the electrolyte.
• a higher than the theoretical voltage should be applied, caused by the following losses: •Resistivity of the electrolytic solution;
• US patents 4,100,049, 4,871,703 and 4,975,161 teach a method for the deposition of electrocatalytic oxides which comprises applying a solution of certain precursor metals on a surface, evaporating the solution and thermally treating so as to convert the precursor into an oxide.
• US patent 4,409,085 shows another alternative where a porous crosslinked layer containing filaments of active metals is vacuum deposited on the surface of the base metal, together with crosslinked stock of inert materials such as asbestos, ceramic materials and polymer fibers. This technique is known as pre-cathode.
• duplex stainless steels are known.
• Duplex stainless steels are stainless steels containing two metal lographic phases: ferritic and austhenitic. Basically, their composition is made up of at least 40 % by weight of iron the balance of, the composition comprising also metals such as chrome, nickel, cobalt, manganese, molybdenum and silicon.
• Other metals comprised in lower amount in its composition are for example copper, tin, titanium, tantalum, niobium, zirconium and vanadium as described in US patent 4,101,347.
• small amounts of noble metals may be present. Their main use is in applications that require high corrosion resistance in saline environments, especially chlorides.
• duplex stainless steels may be cited the commercial trademarks SAF 2205, SAF 2304, SAF 2507,
• the present invention relates to a cathode to be used in electrolytic cells the active surface of which comprises employing at least one duplex stainless steel as an electrocatalytic and anticorrosion element.
• the structure of such steels contains one ferritic and one austhenitic phase. Their composition is made up of iron as main element with the remaining fraction comprising chrome, nickel and other complementary metals such as cobalt, manganese, molybdenum, silicon, copper, tin, titanium, tantalum, niobium, zirconium, vanadium and noble metals.
• the preferred forms according to the invention for applying the duplex steel in the manufacture of the cathodes are:
• the present invention comprises cathodes the active surface of which is made up of duplex stainless steels that consequently present low hydrogen overvoltage when used in the electrolysis reactions of water and of aqueous solutions of alkali metal chlorides.
• cathodes described and claimed in the present invention show high resistance to corrosion in the conditions where the said electrolysis reactions are carried out.
• the present invention provides still activated cathodes that may be easily regenerated when they become deactivated due to the presence of deposits on their surface.
• the present invention provides further the use of said cathodes in electrolysis reactions of water, for producing hydrogen and oxygen, and of aqueous solutions of alkali metal chlorides, for producing chlor-alkalies or chlorates.
• Overvoltage or cathodic overvoltage is the difference between the actual electrode voltage and the theoretical thermodynamic voltage required for the following reaction to occur:
• the improved cathodes of the present invention are advantageously used in electrolytic processes, mainly processes that comprise electrolysis reactions of water and- of aqueous alkali metal chlorides solutions. More specifically, the cathodes described and claimed in the present invention are used in electrolysis reactions of water for producing hydrogen and oxygen as well as in electrolysis reactions of aqueous solutions of alkali metal chlorides, such as in the production of chlor-alkalies and chlorates.
• the present invention finds use in any kind of electrode design and of electrolytic cell configuration.
• electrodes of varied geometry such as rectangular or cylindrical electrodes, as well as .electrolysers of construction arrangements of monopolar or bipolar kind that contain one or more anode-cathode pairs.
• the cathodes herein presented may be used in electrolytic cells with or without separating elements between the anode and cathode compartments.
• the cathodes herein claimed are advantageously applied in electrolytic cells that use asbestos diaphragms, synthetic diaphragms or membranes as separators between the anode and cathode chambers.
• the cathodes that are the object of the invention comprise employing in their active surface at least one duplex stainless steel as electrocatalytic and anticorrosion element.
• the structure of such cathodes may contain as its main portion the same materials employed in the active surface or even other duplex stainless steels.
• internal elements for current distribution may be used, that are made up of copper or any other material of high electrical conductivity.
• the cathodes may be ,built from conventional materials such as mild steel and one-phase stainless steel, such as for example austhenitic steel, and recovered with one or more of the following shapes that contain at least one kind of duplex stainless steel:
• the specific area of the steel surface may be increased with the aid of blasting, sandblasting, chemical attack by acids or electrochemical surface activation, these techniques being used alone or in combination, so as to increase even more their activity.
• the structure of the stainless steels useful for the purposes of the present invention is biphase, fen ⁇ tic-austhenitic, the steels being made up of iron as main element, the remaining fraction comprising chrome, nickel and other complementary metals such as cobalt, manganese, molybdenum and silicon.
• the duplex stainless steels that contain other metals in their composition such as for example copper, tin, titanium, tantalum, niobium, zirconium, vanadium and noble metals, are also suitable for the present invention. Nitrogen and carbon may be present in small concentrations.
• SAF 2205 and Uranus 45 N steels are used.
• cathodes Another advantage intrinsic to the herein described cathodes is their high corrosion resistance, specially in conditions under which are carried out the electrolytic reactions, even when the electrodes are not polarized.
• One of the problems related to the cathode activation with the aid of the deposition of an electrocatalytical layer is the occurrence of metal deposits, basically iron, besides other impurities, on the cathode surface.
• duplex steels in the manufacture of cathodes allows the easy withdrawal of the deposits potentially formed, this being done by techniques of blasting, sandblasting, removal through chemical agents such as solvents and acids, specially hydrochloric acid, or with the aid of ultrasound, the techniques being used alone or in combination, without harming the active surface due to the high hardness and high chemical resistance of these materials.
• a cell having flat electrodes was used, that had a platinum electrode the surface of which was 202 m 2 .
• the reference electrode was of the kind Hg/HgO,OH ' (1.00 MNaOH).
• the cathode area that contacted the electrode was 73 mm 2 .
• the cathodes were sandblasted with a 100 to 1200 mesh sandblast, followed by polishing with ⁇ -alumina 1.0 ⁇ m, isopropanol immersion for 5 minutes, HC1 5% treatment for 5 minutes and washing with deionized water.
• the electrolyte was a 1.0 M NaOH aqueous solution.
• the example using the duplex stainless steel shows a reduction in potential for all the current densities.
• the reduction in potential measured for the current density of 345 mA/cm 2 is higher than 0.30 V when compared to mild steel SAE 1020 and higher than 0.20 when compared to the austhenitic stainless steel 904L or 316L or to a chrome-nickel alloy.
• the comparison to these two latest stocks demonstrates that the presence of high concentrations of elements such as nickel, chrome and molybdenum is not a sufficient condition for these stocks to present low hydrogen overvoltage when they are employed as cathodes.
• Cathodes of low hydrogen overvoltage and high durability having an active surface that is made up of at least one duplex stainless steel;

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)

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Citations (2)

• 1999
  • 1999-12-30 BR BR9906021A patent/BR9906021A/pt not_active Application Discontinuation
• 2000
  • 2000-12-29 WO PCT/BR2000/000154 patent/WO2001088224A1/en active Application Filing

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