FR2514375A1 - Electroplating from fused fluoride baths - contg. hydrogen fluoride-evolving salt to purify bath - Google Patents

Abstract

In the fused salt electrolytic deposition of molybdenum, tungsten, tantalum, niobium, zirconium, hafnium, vanadium, silicon and uranium from fluoride electrolytes of approx. eutectic compsn., the electrolyte contains a salt in which hydrofluoric acid is combined with a substance which is neutral relative to the deposition, the salt releasing hydrogen fluoride gas within the electrolyte. The substance is pref. one or more of the bath constituents, the original amt. of the constituent(s) in the electrolyte being reduced by the amt. supplied by the salt. The process is useful in the prodn. of X-ray tube anodes. The salt releases fine bubbles of HF which eliminate chlorides, oxides, sulphides and other impurities, resulting in prodn. of compact, sound, finely crystalline, metal deposits, which are free of dendrites and excrescences, and in much improved stability of the electrolyte w.r.t. atmospheric air. Decompsn. prods. of the salt do not contaminate the electrolyte and the time required for electrolytic pre-conditioning of the electrolyte is considerably reduced.

Description

ELECTROLYTIC DEPOSIT PROCESS FOR
IGNITION ELECTROLYSIS.

 The present invention relates to a method of electrolytic deposits for igneous electrolysis.

 Obtaining pure metals by igneous electrolysis has been known for the last century. Sainte Claire Deville obtained aluminum in 1854, in baths of molten salts containing fluorides. The alkali metals are obtained by analogous methods. However, all these processes are carried out at temperatures above the melting point of the metal to be produced and this is therefore obtained in the liquid state. Attempts to obtain more refractory metals, such as: Mo, W, Ta, Nb, Zr, Hf, V, Si, U, for which the electrolysis is carried out at a temperature below that of the melting point of the product, For a long time, only powdery deposits containing metal powders mixed with salts from the electrolyte have been obtained. The processes described more recently show dense, compact, fine-grained deposits obtained by the electrolysis of fluorides. molten alkalis containing a salt of the metal to be deposited in solution, give random results. Indeed, the presence of traces of chloride or oxide in the bath leads to dendritic deposits, saturated with salts. However, if it is already difficult to obtain salts of sufficient purity, these, hygroscopic, absorb moisture easily and in a variable manner with atmospheric conditions.

 The preliminary purification of the solvent by bubbling anhydrous hydrofluoric acid, if it improves the results, is not always satisfactory and remains extremely dangerous to handle, both by the corrosion of the installations and by its pathological and toxicological effects creating a constraint that is difficult to bear. in an industrial environment by the additional cost that it involves in security installations and qualified personnel.

 The present invention proposes to improve the conditions for depositing: Mo, W, Ta, Nb, Zr, Hf, V, Si, U, both from the point of view of the results obtained and the ease of carrying them out.

Indeed, we were surprised to note that the addition to the salts intended to be melted under inert gas and to serve as solvent (ternary eutectic FLiNaK, binary eutectic FLiNa or any other mixture of fluorides leading to a liquid whose temperature of solidification is sufficiently low) of a certain amount of
KH F2 or Na H F2 allowed, after enrichment of the metal bath to be deposited in the appropriate valence state, the production of compact, healthy, finely crystallized metal deposits, free of dendrites or growths.

 In addition, the stability of the baths thus produced is exceptionally increased: the alteration by atmospheric air, very rapid in prior techniques to the point that the parts to be introduced had to pass through an intermediate airlock which was purged by means inert bath protection gas, under penalty of damaging it, has been considerably reduced.

 Thus it was possible to keep in perfect working order an electrolysis bath for depositing tungsten for more than two months, with almost daily introductions of samples to be covered, introduced without passing through 'an airlock but by simply opening the cover of the enclosure containing the bath.

 The stability obtained is such that it was even possible to remove an accidentally dropped part from the liquid bath, using pliers and accessing the enclosure cover which thus remained open for a long time, subsequent electrolysis not having been affected.

These positive results are explained by the fact that hydrofluoric acid, released "in situ" by the decomposition of KH F2 or
Na H F2, displaces chlorides, oxides, sulphides or other impurities likely to preexist in the salts and eliminates them in volatile form or the complexes in solvated form.

 Being evolved within the liquid itself, nascent hydrofluoric acid is more active than anhydrous hydrofluoric acid passing through bubbling, and concerns the entire volume of salts, because a multitude of small bubbles of hydrofluoric acid are formed at all points. , in the same way that a gas in solution in a liquid under pressure is released in a large number of small bubbles as soon as the overpressure is removed, giving a sparkling liquid.

 It is also possible that this will lead to the solubilization of a certain quantity of hydrofluoric acid, which will be available during the subsequent introduction of impurities provided by the immersed parts and the concomitant air intake, prolonging the stability. of the bath.

The decomposition of KH F2 or Na H F2 further producing
KF or Na F, which are constituents of the basic solvent, does not in any way modify its own qualities, provided that this contribution has been taken into account by consequently reducing the amount of KF and Na F in the initial mixture. But it should be noted that a deviation from the eutectic composition is by no means critical.

 No further treatment is therefore necessary to remove the decomposition products. It goes without saying that any substance capable of releasing hydrofluoric acid without contamination of the bath can be used instead of K H F2 or Na H F2.

 Finally, an appreciable consequence of the purification obtained by this invention is the notable reduction in the preelectrolysis time which was made necessary inter alia by the presence of various impurities in the bath, preventing from immediately obtaining a satisfactory deposit.

 All that remains is a short pre-electrolysis aimed at bringing the metal in solution to an appropriate valence state.

 It should be noted that one can also add KH F2 or Na H F2 to a bath already melted and even to a bath already enriched in metal to be deposited, with the same advantages, although then the operation is d '' a realization more rnalaisais that the addition to salts in the solid state before fusion.

 On the other hand, conventional baths often suffered from a poor anode / cathode surface ratio due to the bulk of the anode or cathode parts, from which it frequently resulted in poor anode efficiency, an increase in the average valence of the species in solution, and a poor cathodic yield associated with a deposit of poor quality.

 The problem can be resolved by leaving a large metal plate to be deposited in the bath, which will serve as a soluble anode.

 The advantage is twofold: by improving the anode / cathode surface ratio, the valence elevation of the species in solution is reduced, which, as we have just seen, gradually leads to defective deposits.

On the other hand, even in the event of an increase in this valence during the electrolysis, there is during the stopping phases a spontaneous return to the state of thermodynamic equilibrium of the system element in solution - metallic element by dissolution of a little of the metal of the anode according to the reaction: M + (nl) Mn ~> nM (n 1) +
If necessary, it may be wise to accelerate the return to this equilibrium by raising the bath temperature, which will increase the kinetics of returning to equilibrium and may even move it in a favorable direction.

 This thermal cycle, the amplitude of which can reach 1500 C.

cannot be prolonged excessively to preserve the service life of the installations and that of the bath itself, since its vapor pressure is not zero, there is evaporation of salts and condensation in the colder parts. This is all the more the higher the temperature of the salts.

 The movements of natural convection, created by the temperature differences, will increase the speed of return to the thermodynamic equilibrium of the system. Mechanical agitation before and during electrolysis also acts favorably.

 In this sense, it is particularly favorable and simple to rotate the part to be covered, which has the effect of homogenizing the bath, and of reducing the limit layer of diffusion of the electroactive species leading to a more homogeneous deposit at all points.

 Leaving the anode permanently in the bath simplifies handling, but also makes it possible to keep the surface in an ideal state of reactivity thereby eliminating the pre-electrolysis which was necessary before any deposit of satisfactory quality in order to decontaminate the surface of the anodes.

 The combination of the two previously described improvements adds up their beneficial effects. But in addition, the introduction of the soluble anode from before the melting and the release of hydrofluoric acid makes it possible to clean the surface of all the impurities introduced during its preparation and to passivate it. The anode then serves as a source of raw material in the chemical reduction process, which, as we have seen, brings the metal in solution to the valence state resulting from thermodynamic equilibrium.

 As a result, the pre-electrolysis of a newly manufactured bath, one of whose aims is to bring the metal to the favorable valence state, becomes superfluous. Obtained immediately, after enrichment of the metal solution to be deposited, a usable system.

 The deposition of metal can be carried out without interruption, producing a healthy, compact, well crystallized, well covering coating, even at great thickness. It is thus possible to obtain tungsten deposits exceeding 3 mm in thickness and to produce electroformed parts by subsequent removal of the substrate.

 The method according to the invention finds application in technology of anodes of X-ray tubes.

Claims (4)

 1. Process for electrolytic deposition by igneous electrolysis of Molybdenum, Tungsten, Tantalum, Niobium, Zirconium, Hafnium, Vanadium, Silicon, Uranium, from an electrolyte at least close to a fluoride eutectic, on a conductive cathode, under an inert atmosphere, at predetermined pressure, and characterized in that the electrolyte comprises a salt where hydrofluoric acid is composed with a neutral body relative to the deposit, intended to release a gaseous flow of hydrofluoric acid HF in the molten electrolyte, so as to purify the bath.  2. Method according to claim 1, characterized in that a large soluble anode is used, permanently immersed in the electrolyte, and put in place prior to the melting of the electrolyte.  3. Method according to claim 2, characterized in that it is carried out in thermal cycles modifying the electrochemical properties of the bath.  4. Method according to the preceding claims, characterized in that the salt composed of hydrofluoric acid is with at least one of the constituents of the electrolyte, the original amounts of these constituents in the fluorides then being reduced by the amount provided by the salt composed of hydrofluoric acid.