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
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/60—Constructional parts of cells
  • C25B9/65—Means for supplying current; Electrode connections; Electric inter-cell connections
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/34—Anodisation of metals or alloys not provided for in groups C25D11/04 - C25D11/32

Definitions

• the present invention relates to a method of passively anodizing copper pieces in the middle of molten fluorides forming an adherent protective layer with a high recovery rate; this process is particularly, but not exclusively, applicable to the protection of copper parts used in electrolysers for the production of fluorine.
• a molten fluoride bath which is generally a mixture of hydrogen fluoride and alkali metal and / or ammonium fluorides.
• the anodes made of carbonaceous material are immersed vertically in the bath and are supplied with electric current by current leads usually made of copper.
• the Cuivreanode junction which represents a weak point, is usually carried out at the top of the anode, in this case the copper current supply and the Copper-anode junction are partially immersed in the bath and are subjected to the action of bath and fluorine bubbles released at the anode.
• a passivation of the copper occurs on the one hand due to the soaking in the bath of liquid fluorides, on the other hand by anodization during the tensioning of the electrolysis cell, but the properties of the layer obtained are very insufficient for effective protection of copper.
• a dissolution of the copper thus occurs, leading to the deterioration of the slow and regular copper-anode contact, necessitating the stopping and refurbishing of the electrolysis cell, in particular the repair of the current leads and the change of the anode.
• This refurbishment takes place approximately once a year.
• the copper-anode junction can also and advantageously be carried out from below. In this case, the copper current leads pass through the total thickness of the bath before being connected to the feet of the anodes.
• KF-x HF will mean a mixture where the number of moles d 'HF is exclusively equal to or close to 2) for the electrolytic production of fluorine, currently limits " the development and development of more efficient fluorinated electrolyzers.
• the Applicant has continued its research, the main object of which is to obtain a durable and efficient passivation of copper in a liquid fluoride bath using a process that is simple to implement.
• this passivation must durably and effectively protect the copper under the conditions encountered during the electrolytic production of fluorine; in particular, it must resist the action of the KF, xHF electrolysis baths, the fluorine produced and the electrolysis current.
• Another object is the obtaining or the controlled development of a protective layer of copper in the middle of molten fluorides which is tight and which has a strong adhesion to the copper substrate and a high recovery rate of said substrate.
• Another object is to obtain an electrically insulating layer.
• Another object is to obtain a layer which is thin while having, thanks to the strong cohesion of the particles which constitutes it, good mechanical characteristics, in particular resistance to abrasion, to friction, to shocks ...
• Another object of the invention is to use an electrochemical process which allows this passivation to be carried out in the tanks and on the production site, opening the production of said tanks.
• Another object is to avoid the slow dissolution of copper and the degradation of the copper-anode bonds during the electrolysis of liquid fluoride baths and particularly of the KF, xHF bath.
• the invention is a method of passively nodizing parts copper mid KF, xHF medium (x close to 2) liquid to obtain an adherent protective layer, mechanically strong "and electrically high recovery rate of the copper substrate, characterized in that said copper parts, once immersed in the liquid KF xHF bath, are subjected to an anodic current of low surface density, calculated with respect to the submerged copper surface, less than 0.1 A / dm 2. This treatment is applied for a variable duration which is always greater than a limit value dependent on the value of the anode current density.
• the bath consists of a liquid KF, xHF mixture, the HF content of which is preferably between 38 and 42.5%; this mixture is usually used as a bath for the electrol ical production of fluorine.
• the bath should be liquid; it is advantageous to operate under such conditions (of temperature and of concentration) that the vapor pressure of HF does not exceed 50 mm of mercury, or that there is not more than 7% (weight) of HF driven by gases. Thus, it is advantageous to operate at a temperature between 85 and 105 ° C.
• Battelle describes anodization intensities greater than a floor value (for example 0.4 A / dm 2 ), it- even much higher than the maximum intensity prescribed by the applicant.
• the conditions for the formation (in particular of nucleation, of growth, etc.) of the passivating layer, described by Battelle are very different and provide said layer with properties, for example of homogeneity of adhesion density, also very different.
• These operating conditions cannot therefore be used to predict the conditions for the formation of a protective layer in a KF, xHF medium, meeting the requirements of the applicant, a layer which must be resistant to bathing, to the release of fluorine and to electrical conditions during of electrolysis, and which is also adherent, compact and solid over time.
• a direct voltage is applied between the copper part to be protected and a cathode made of any conductive material, for example steel, also immersed in the bath.
• This voltage as well as the shape, location, spacing, etc. of the cathode are such that the current density at all points of the surface to be protected is uniform and maintained at a low value.
• the low current density applied to the surface to be protected can be maintained at a constant value as a function of time, and throughout the duration of the treatment, in this case the anodization treatment is said to be in constant mode; it can also have a variable value in this case the processing is said to be of variable mode.
• the duration processing increases exponentially and becomes prohibitive; similarly, for a given current density, the quality of the protective layer formed practically does not change any more when the duration of treatment is excessively extended.
• the current density must generally be less than 0.1 A / dm 2 , but preferably less than 0.05 A / dm 2 and more particularly less than 0.025 A / dm 2 .
• the duration of treatment practically but not limited to, it does not exceed 20 h and preferably 15 h, and consequently we avoid using, in constant mode, a current density less than 0.01 A / dm 2 .
• the treatment time is generally greater than 0.5 h but for current densities of the order of 0.05 A / dm 2 , it It is * customary to use treatment times between 2 and 4 hours.
• the curve of Figure 1 gives an illustration of what can be the relationship between the current density (plotted on the ordinate) and the processing time (plotted on the abscissa) to obtain the same protective layer in the case where the current density (or intensity) is kept constant during the treatment, for a KF, xHF bath, containing 40.5% by weight of HF.
• the current density applied is variable as a function of time, while remaining within the limits described above.
• the values of the current densities used during each anodization sequence can be constant or variable, they can be the same or be different from one sequence to another; the durations of each anodization sequence can be the same or be different; the durations of each relaxation sequence may be the same or be different and are independent of the durations of the anodization sequences.
• some anodizing sequences may have current densities of less than 0.01 A / dm 2 .
• variable embodiment of the invention makes it possible to reduce the total duration of the treatment compared to the so-called constant mode and also makes it possible to reduce the value of the current density used with each anodization sequence.
• the process according to the invention makes it possible to obtain a durable and effective passivation of copper in molten fluoride baths by obtaining a protective layer formed essentially of a mixed copper fluoride, which appears to have a high rate covering the copper substrate, high compactness of the arrangement of the elementary particles, strong adhesion and high resistivity. This layer thus prevents the anodic dissolution of copper.
• a piece of passivated copper according to the prior art, by simple dipping in a KF, xHF liquid bath, has a leakage current of 25 mA / dm 2 at 5V.
• a part passivated according to the method of the invention in this same type of bath has a leakage current not exceeding 5 mA / dm 2 at 10 V, and usually close to or less than 3 mA / dm 2 at 10 V.
• the protective layer is also mechanically resistant, moreover it is very thin so that it does not alter ⁇
• the method according to the invention is applicable to the passivation of all kinds of copper parts to be subsequently used in the medium of fluorides, molten or in aqueous solution.
• Copper parts passivated using the process according to the invention offer very good resistance to chemical corrosion in all environments containing fluorides, in particular molten fluoride baths and more particularly baths containing at least fluoride. hydrogen and an alkali or ammonium fluoride. Because the protective layer has good adhesion and significantly improved mechanical properties, it is possible to use the passivated parts in a calm or agitated, homogeneous or heterogeneous environment.
• the process finds its particular field of application in the passivation and protection of copper parts, in particular current supply bars to the electrodes, implanted in fluorine electrolysers using liquid baths KF, xHF as electrolyte, thanks to the quality improved layer which resists bathing, fluorine and current well.
• the fact that these parts are under tension does not alter their resistance to corrosion.
• the wear of passivated parts can be measured according to the method of the invention by immersing them in the molten bath and subjecting them to an anode voltage for one week, as mentioned above, and by weighing the part before and after treatment. .
• the following results have thus been noted on cylindrical discs with a diameter of 35 mm, the edges of which have been rounded and in a KF, xHF bath: - for a part passivated by simple soaking according to the prior art and subjected to an anode voltage of 5 V, the leakage current is 25 mA / dm 2 , the weight loss corresponds to wear of 3 mm / year; - for a part passivated according to the method of the invention, subjected to an anode voltage of 10 V: if the leakage current is 3 mA / dm 2 , the weight loss corresponds to a wear of 0.35 mm / year , if the leakage current is 3.5 mA / dm 2 , the
• the very good quality of the passivation obtained allows in the application to the electrolysis of fluorine to increase the lifespan of said copper parts up to at least five years, and to implement new cell technologies.
• electrolysis in particular the supply of * anodes from below, knowing that copper parts passivated according to the process can be immersed and energized without problem.
• a copper disc of type Cu a 1 with a diameter of 35 mm, with a total area of 0.2 dm 2 is subjected to an anode voltage such that the intensity is kept constant at a value of 3 mA (0.015 A / dm 2 ) for 12:30 min, with a steel cathode identical to the anode, in a KF, xHF bath containing 40.5% by weight of HF, at 95 ° C.
• the leakage current observed at a voltage of 10 V is 3.5 mA / dm 2 .
• the leakage current observed at 10 V is only 2.9 mA / dm 2 , while the treatment time is only 10 h.
• the leakage current observed at a voltage of 10V is 1 mA / dm 2 , which results in corrosion of 0.12 mm per year.
• Example 2 A copper disk, a bath and a temperature identical to those of Example 1 are used. The intensity is maintained at a value of 0.08 A / dm 2 for 0.5 h.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)
• Electroplating Methods And Accessories (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=9352918

Family Applications (1)

Country Status (8)

Family Cites Families (4)

• 1987
  • 1987-06-26 FR FR8709574A patent/FR2617200B1/fr not_active Expired - Fee Related
• 1988
  • 1988-06-23 US US07/343,145 patent/US4892630A/en not_active Expired - Fee Related
  • 1988-06-23 WO PCT/FR1988/000334 patent/WO1988010328A1/fr not_active Ceased
  • 1988-06-23 DE DE88905883T patent/DE3882948T2/de not_active Expired - Fee Related
  • 1988-06-23 CA CA000570242A patent/CA1323596C/fr not_active Expired - Fee Related
  • 1988-06-23 EP EP88905883A patent/EP0321536B1/de not_active Expired - Lifetime
  • 1988-06-23 JP JP63505522A patent/JP2680393B2/ja not_active Expired - Lifetime
  • 1988-06-24 ZA ZA884547A patent/ZA884547B/xx unknown

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events