EP0321536B1 - Process for passivating anodization of copper in a molten fluoride medium, and application to the protection of copper parts of fluorine electrolyzers - Google Patents

Abstract

In a process for obtaining a resistant, adherent, protective coating on copper parts with high recovery of substrate, by passivating anodization, said copper parts are immersed in a liquid KF, 2HF bath and subjected to a continuous or intermittent anodic current of low surface density less than 0.1 A/dm2.

Description

TECHNICAL AREA

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.

STATE OF THE ART

In the process for obtaining fluorine by electrolysis, a bath of molten fluorides is used, which is generally a mixture of hydrogen fluoride and alkali metal fluorides and / or ammonium. 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 copper anode 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 the bath and the bubbles of fluorine 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. It is then necessary to isolate them to avoid their dissolution; one can for example make sheathing resistant to the bath. A device of this kind is described in patent SU 193,454, which describes a cladding of the current leads effected by magnesium and the protection of the copper-anode contacts by a chemically inert insulator (fluorinated hydrocarbon). Such protections are difficult to implement and use expensive products.

However, the document "Electrodeposition and surface treatment" 1 (3) - 1973- p. 256-265 (Battelle), an anodic passivation treatment of copper in a KF-HF liquid bath. To form the passivating layer, this document describes a constant anodic passivation current of at least 0.4 A / dm² in a KF-HF equimolecular bath at 245 ° C., the time of application of this current being all the shorter that said current is higher; for a passivation time greater than approximately 60 min, we note that the value of the passivation current is always between 0.4 and 0.45 A / dm² (fig. 2), in other words that 0.4 A / dm² represents a minimum asymptotic value of the passivation current.
This document also describes an anodic passivation of copper in an anhydrous HF bath at 20 ° C. and in this case the minimum asymptotic value of the anodization current is approximately 0.15 A / dm².

The difficulty of significantly reducing corrosion of copper and avoiding deterioration of copper-anode contacts in KF-x HF liquid baths (throughout the description, the expression 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 fluorine electrolyzer.

OBJECT OF THE INVENTION

Thus, the Applicant has continued its research, the main object of which is to obtain a durable and effective passivation of copper in a liquid fluoride bath using a process that is simple to implement. In particular, this passivation must durably and effectively protect the copper under the conditions encountered during the electrolytic production of fluorine; it must in particular 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, friction, impact ...

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 up 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.

DESCRIPTION OF THE INVENTION

The invention is a method of passively anodizing parts in copper in KF medium, xHF (x close to 2) liquid making it possible to obtain an adherent protective layer, mechanically and electrically resistant, with a high recovery rate of the copper substrate, characterized in that said copper parts, once immersed in the KF, xHF liquid bath, are subjected to an anodic current of low surface density, calculated relative to the immersed copper surface, less than 0.1 A / dm2, this current being maintained either at a constant value depending on the time, or at a variable value. This treatment is applied for a variable duration which is always greater than a limit value dependent on the value of the anodic 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 electrolytic 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.

For this type of bath, used in the electrolytic production of fluorine, the applicant sought a process for the passivation of copper by anodization, a process which must be such that the protective layer formed resists both the action of the bath which is acid (presence of 2 HF) and to the action of fluorine which is released during electrolysis. Such a bath is essentially different from those described by Battelle which are (i) one very basic, taking into account the presence of a single HF molecule linked to the KF molecule, the dissociation giving the species F⁻ and HF⁻, (ii) the other free of KF. In such baths, the activity of the constituents is different from that encountered in the baths used in the invention and the temperatures described there are also very different.

It follows that Battelle describes anodization intensities greater than a floor value (for example 0.4 A / dm2, itself significantly higher than the maximum intensity prescribed by the applicant.

As a result, the conditions of 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 electrolysis, and which is also adherent, compact and solid over time.

According to the invention, a direct voltage is applied between the copper part to be protected and a cathode 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 variable mode.

It is advantageous to use the lowest possible current densities; in fact, for low values of current density, the recovery rate of the substrate and the compactness of the protective layer are better. Furthermore, the quality of the protective layer obtained by the anodic treatment is all the better the longer the duration of the treatment.

However, for too low current densities, 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. Thus, the current density must be less than 0.1 A / dm², but preferably less than 0.05 A / dm² and more particularly less than 0.025 A / dm². Regarding 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 / dm².
For current densities at the upper limit of 0.1 A / dm², the treatment time is generally greater than 0.5 h but for current densities of the order of 0.05 A / dm², it is d use of treatment times between 2 and 4 h.

The curve in Figure 1 gives an illustration of what may be the relationship between the current density (plotted on the ordinate) and the processing time (plotted on the x axis) 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.

In an advantageous embodiment of the invention (variable mode) the current density applied is variable as a function of time, while remaining within the limits described above. In particular, it is possible to alternate energizing sequences (non-zero current density) and relaxation sequences (zero voltage and current); 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. In this some anodizing sequences may have current densities of less than 0.01 A / dm².

This so-called 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 method according to the invention makes it possible to obtain a durable and efficient 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 level. covering the copper substrate, great compactness in the arrangement of the elementary particles, strong adhesion and high resistivity. This layer thus avoids the anodic dissolution of copper.
These properties are all the more marked the lower the current density and the longer the treatment time.

These properties are highlighted by the measurement of the leakage current passing through the protective layer formed, using a given voltage applied on either side of said layer. In general, it is measured, the part being immersed in a conductive bath, for example the passivation bath, by applying a DC voltage between said part and another plunging electrode.

Thus, a passivated copper piece, according to the prior art, by simple dipping in a KF, xHF liquid bath, has a leakage current of 25 mA / dm² at 5V.
On the other hand, a part passivated according to the method of the invention in this same type of bath has a leakage current not exceeding 5 mA / dm² at 10 V, and usually close to or less than 3 mA / dm² at 10 V.

The protective layer is also mechanically resistant, moreover it is very thin so that it does not alter significantly the dimensions of the passivated parts, nor their geometry.

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 media containing fluorides, in particular molten fluoride baths and more especially 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.

However, the process finds its particular field of application in the passivation and protection of copper parts, in particular current supply bars to the electrodes, installed in fluorine electrolysers using liquid baths KF, xHF as electrolyte, thanks to the quality improved layer that resists bathing, fluorine and current well. The fact that these parts are under tension does not alter their resistance to corrosion.

• pour une pièce passivée par simple trempage selon l'art antérieur et soumise à une tension anodique de 5 V, le courant de fuite est de 25 mA/dm², la perte de poids correspond à une usure de 3 mm/an;
• pour une pièce passivée selon le procédé de l'invention, soumise à une tension anodique de 10 V :
  • . si le courant de fuite est de 3 mA/dm², la perte de poids correspond à une usure de 0,35 mm/an,
  • . si le courant de fuite est de 3,5 mA/dm², l'usure correspondante est de 0,4 mm/an,
  • . si le courant de fuite est de 5 mA/dm², l'usure correspondante est inférieure à 0,6 mm/an.

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 were thus noted on cylindrical discs with a diameter of 35 mm, the edges of which were 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 / dm², 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 / dm², the weight loss corresponds to wear of 0.35 mm / year,
  • . if the leakage current is 3.5 mA / dm², the corresponding wear is 0.4 mm / year,
  • . if the leakage current is 5 mA / dm², the corresponding wear is less than 0.6 mm / year.

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 the anodes from the bottom knowing that copper parts passivated according to the process can be immersed and energized without problem.

EXAMPLES

The following examples illustrate, without limitation, various operating conditions of the process according to the invention.

. Example 1

Passivation using a current of constant intensity.
A copper disc of type Cu a 1 with a diameter of 35 mm, with a total surface area of 0.2 dm² is subjected to an anode voltage such that the intensity is kept constant at a value of 3 mA (0.015 A / dm²) for 12 h 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.
After treatment, the leakage current observed at a voltage of 10 V is 3.5 mA / dm².

. Example 2

Passivation in steps of decreasing anodizing current density, alternated with relaxation times (variable mode).

• tension anodique telle que l'intensité soit maintenue à une valeur de 10 mA (0,05 A/dm²) pendant 3 h,
• tension nulle (relaxation) pendant 30 min,
• tension anodique telle que l'intensité soit maintenue à une valeur de 2,8 mA (0,014 A/dm²) pendant 3 h,
• relaxation pendant 30 min,
• tension anodique telle que l'intensité soit maintenue à une valeur de 1 mA (0,005 A/dm²) pendant 3 h.

The copper disc and the bath are identical to those of Example 1. The treatment procedure is as follows:

• anode voltage such that the intensity is maintained at a value of 10 mA (0.05 A / dm²) for 3 h,
• zero tension (relaxation) for 30 min,
• anode voltage such that the intensity is maintained at a value of 2.8 mA (0.014 A / dm²) for 3 h,
• relaxation for 30 min,
• anode voltage such that the intensity is maintained at a value of 1 mA (0.005 A / dm²) for 3 h.

After treatment, the leakage current observed at 10 V is only 2.9 mA / dm², while the treatment time is only 10 h.

. Example 3

Passivation using a constant intensity current in a bath of another composition. A disk identical to that of Example 1 is used. The bath is an HF-KF mixture containing 38% by weight of HF at 85 ° C. The copper part is passivated under an anode current of 3 mA (i.e. 0.015 A / dm²) for approximately 3 h 30 min.

After treatment, the leakage current observed at a voltage of 10V is 1 mA / dm², which results in corrosion of 0.12 mm per year.

. Example 4

Passivation using a constant current applied for an insufficient time.
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 / dm² for 0.5 h.

After treatment, the leakage current observed is 13 mA / dm², which corresponds to an average wear of 1.5 mm / year. This mediocre value should be compared to 3 mm / year for a part passivated by simple soaking. However, it leads to a reduction in copper corrosion which remains insufficient for those skilled in the art.

Claims (7)

1. A process for the passivating anodisation of copper parts in a liquid KF, xHF medium (x close to 2), which makes it possible to produce a mechanically and electrically strong, adherent protective layer, with a high rate of covering of the copper substrate, characterised in that, once said parts are immersed in the liquid KF, xHF bath (x close to 2), they are subjected to an anodic current of a surface-related density, calculated with respect to the immersed surface area of copper to be treated, of lower than 0.1 A/dm², said current being maintained either at a constant value in dependence on time or at a variable value, and being applied for a variable period which is always greater than a limit value dependent on the value of said anodic current, and such that there is obtained a protective layer having a leakage current which does not exceed 5 mA/dm² at 10 V.
2. A process according to claim 1 characterised in that the surface-related current density is preferably lower than 0.05 A/dm².
3. A process according to either one of claims 1 and 2 characterised in that, when the anodic current is maintained at a constant limit value of 0.1 A/dm², the period for which said current is applied is at least 0.5 hour.
4. A process according to either one of claims 1 and 3 characterised in that the anodic current density is maintained at a constant value during the treatment time.
5. A process according to any one of claims 1 to 4 characterised in that the anodic current density is of a variable value in the course of the treatment.
6. A process according to claim 5 characterised by alternating anodisation sequences with a current density which is not zero and relaxation sequences with a zero current density, the value of the current density of the anodisation sequences preferably decreasing from one sequence to the next.
7. A layer for protecting copper parts, produced in accordance with any one of claims 1 to 6, characterised in that the leakage current measured across said layer is less than 3 mA/dm² at 10 V.

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