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
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/28—Cleaning or pickling metallic material with solutions or molten salts with molten salts
  • C23G1/32—Heavy metals
• • 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
  • C25B15/00—Operating or servicing cells

Definitions

• the present invention relates to stripping of coated titanium electrodes for re-coating. More particularly it relates to a method for the removal of a coating comprising one or more platinum group metal oxides from a titanium support which carries the coating, in a manner which renders the titanium surface suitable for re-coating and without excessive loss of titanium from the support.
• elec trodes particularly as anodes in cells electrolyzing aqueous solutions of alkali metal chloride, a combination of a film-forming metal support and a coating thereon comprising oxides of one or more of the platinum group metals, the coating optionally also containing one or more oxides of non-noble metals, for example an oxide of one of the film-forming metals, especially titanium dioxide (see for instance British Pat. Specifications No. 1,147,442 and No. 1,195,871), or other oxides that are resistant to anodic attack, for instance oxides of tin, antimony and germanium as taught in Belgian Pat. Specification No. 740,242.
• the film-forming metal of the support is most suitably titanium.
• Electrodes of the aforesaid type have advantages over prior art electrodes when employed as anodes in the electrolysis of alkali metal chloride solutions in that they have both low chlorine overpotential characteristics and high resistance to electrochemical attack in use.
• the electrodes have a very low wear rate in the cell, their useful lives are not infinite and in time they must be replaced or re-coated in order to maintain optimum performance.
• the electrodes When the electrodes are taken out of service they still retain an appreciable amount of the original coating andif the titanium support is to be used as a support for a new coating it is preferable first to remove the remains of the old coating. This presents the problem of removing the old coating cheaply and easily without at the same time removing an unacceptable amount of titanium metal from the support.
• a method for stripping the coating from an electrode comprising a titanium support and a coating comprising a platinum group metal oxide thereon which comprises immersing the electrode for at least seconds, preferably at least 30 seconds, in a bath of molten alkali metal hydroxide containing at least 0.1 percent by weight of alkali metal hydride and then washing the treated electrode in water.
• alkali metal hydroxide is most suitably sodium hydroxide
• alkali metal hydride is most suitably sodium hydride
• the desired concentration of alkali metal hydride may suitably be established in a bath of molten alkali metal hydroxide by suspending in the molten hydroxide a sufficient amount of alkali metal and passing a stream of hydrogen or cracked ammonia gas into the bath. Baths prepared in this way may be used in the method of the invention whether or not all the added alkali
• the bath of molten alkali metal hydroxide containing alkali metal hydride may be used at any convenient temperature upwards of a minimum of approximately 10 C. above the temperature at which the bath begins to freeze on cooling but we have found no advantage in working at temperatures more than about C. above the freezing range of the bath. Moreover, we have found that with bath temperatures above 450 C.
• the rate of attack on the titanium substrate of the electrode can become unacceptably high.
• the preferred temperature range for baths based on sodium hydroxide is 345-380 C.
• the alkali metal hydride content of these baths is in the range 0.5-2 percent sodium hydride by weight.
• the treatment time in these preferred baths is most suitably in the range l-3 minutes.
• the washing step in water may most suitably be carried out by immediately quenching the hot electrode in water.
• the electrode after the said washing step in water it is preferred to give the electrode a dip for a few seconds in hot dilute mineral acid, suitably sulphuric acid of about 10 N concentration, followed by again washing in water. Most suitably the electrode is immersed in the 'acid until effervescence begins and is then immediately withdrawn and washed in water. We have found that this acid dip improves the condition of the titanium surface for recoating.
• hot dilute mineral acid suitably sulphuric acid of about 10 N concentration
• the electrodes to be treated in accordance with the invention may have coatings in which the whole of the platinum group metal content is in the form of platinum group metal oxides.
• the method of the invention is, however, also applicable to coatings wherein a minor amount of the platinum group metal content is in the form of free platinum group metals, the remainder and major amount being in the form of oxides.
• the platinum group metal content thus defined may consist of any one or more of the metals platinum, iridium, rhodium, osmium, ruthenium and palladium.
• the invention is applicable to electrodes in which the coating consists entirely of platinum group metals
• the coating consists of platinum group metals, mainly in the oxidized state, in association with oxides of other metals, e.g., oxides of the film-forming metals titanium, zirconium, niobium, tantalum and tungsten, tin dioxide, germanium dioxide and oxides of antimony.
• oxides of the film-forming metals titanium, zirconium, niobium, tantalum and tungsten tin dioxide, germanium dioxide and oxides of antimony.
• Specific examples of the latter type of coating are coatings of ruthenium dioxide in association with titanium dioxide and coatings of ruthenium dioxide in association with 'mixtures of tin dioxide and oxides of antimony.
• the invention is further illustrated by the following examples on the stripping of coatings consisting substantially of ruthenium dioxide in admixture with titanium dioxide (Examples l-8), with tin dioxide (Example 9) and with a mixture of tin dioxide and oxides of antimony (Example 10), all on titanium supports.
• Example 7 Example 8 min 2 2.5 all coating removed After this treatment the titanium substrate appeared to be clean enough for re-coating. It was, however, given a short etching treatment in sulphuric acid before applying a new coating.
• EXAMPLE 9 A strip of titanium coated with a mixture of ruthenium dioxide and tin dioxide (approximate composition 40%RO:60%S b 'ht a df minute ina bath Pmoi te rf sQJdQur n co ii tz t i ii ing 1.5 percent by weight sodium hydride held at a temperature of 360 C. and was then quenched in water.
• a strip of titanium coated with a mixture of ruthenium dioxide, tin dioxide and oxides of antimony was treated in the manner of Example 9. This treatment removed all the coating and left the titanium surface in a suitable condition forre-coating.
• a method for stripping the coating from an elec trode comprising a titanium support and a coating comprising a platinum group metal oxide thereon which comprises immersing the electrode for at least 10 seconds in a bath of molten alkali metal hydroxide containing at least 0.1 percent by weight of alkali metal hydride and then washing the treated electrode in water.
• a method according to claim 8 wherein the electrode is immersed in the said acid until effervescence begins and is then washed in water.
• a method according to claim 1, wherein the coating of the electrode is a mixture of ruthenium dioxide and titanium dioxide.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Electrochemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)
• Battery Electrode And Active Subsutance (AREA)
• ing And Chemical Polishing (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=10317183

Family Applications (1)

Country Status (10)

Cited By (9)

Families Citing this family (1)

Citations (4)

• 1970
  • 1970-06-26 GB GB3104870A patent/GB1312375A/en not_active Expired
• 1971
  • 1971-06-11 CA CA115472A patent/CA919560A/en not_active Expired
  • 1971-06-16 IL IL37073A patent/IL37073A/xx unknown
  • 1971-06-18 US US154611A patent/US3706600A/en not_active Expired - Lifetime
  • 1971-06-24 SU SU1673266A patent/SU490279A3/ru active
  • 1971-06-25 BE BE769041A patent/BE769041A/xx not_active IP Right Cessation
  • 1971-06-25 JP JP4577671A patent/JPS5529153B1/ja active Pending
  • 1971-06-25 FR FR7123269A patent/FR2099942A5/fr not_active Expired
  • 1971-08-24 ZA ZA714129A patent/ZA714129B/xx unknown
• 1974
  • 1974-12-30 MY MY198/74A patent/MY7400198A/xx unknown

Patent Citations (4)

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