GB1583159A - Alkaline baths for electrolytic deposition of zinc and processes for using them - Google Patents

Description

(54) ALKALINE BATHS FOR ELECTROLYTIC DEPOSITION OF ZINC AND PROCESSES FOR USING THEM (71) We, ROQUETTE FRERES, a body corporate organised and existing under the laws of the Republic of France of 62136 - Lestrem, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention, which is due to the work of Maurice Bonnemay, Jean Royon, Anna Kouba, Jean-Claude Catonne and Rene Tournier, of the Laboratoire d'Electrochimie du Conservatoire National des Arts et Metiers, and of Michel Huchette and Michel Van Der Bruggen, of the Company Roquette Freres, relates to alkaline baths for electrolytic deposition of metals.

The invention also relates to processes for electrolytic deposition of metal using the baths according to the invention.

A large number of baths customarily employed for the electrolytic deposition of metals have extremely toxic constituents in their composition. Their use must consequently be accompanied by very restrictive safety measures. Likewise, the disposal of the effluents originating from the units in which such baths are used poses serious difficulties, having, regard to the legal obligations concerning pollution. These difficulties are particular} appreciable in the case of alkaline baths containing cyanide ions as complexing agent.

However, by reason of the quality of the electroplatings obtained, and in spite of the abode disadvantages, baths containing cyanide ions are still widely employed, in particular for the deposition of zinc, copper, cadmium, gold and silver and, furthermore, of copper-tin and copper-zinc alloys.

Nevertheless, efforts have been made to find baths capable of replacing cyanide baths with advantage. Numerous attempts have been made in this direction. In particular, attempts have been made to replace the cyanide ion by organic anions capable of forming complexes with the metals to be deposited. Among these, the use of the anions corresponding to certain weak acids, such as citric acid, tartaric acid or gluconic acid has been tried. Heretofore, these attempts have come up against many difficulties and have not enabled the conditions of industrial practice to be met in a fully satisfactory manner. In fact it is required of these baths that they result in coatings whose characteristics of appearance, structure and adhesion make them comparable with those which are obtained with conventional cyanide baths.Moreover, it is necessary that these deposits can be obtained under acceptable conditions of cost and speed, using the installations currently in service for alkaline baths, in particular those containing cyanide, for the electrolytic deposition of metals.

Therefore, an object of the invention is to provide baths for the electrolytic deposition of zinc which meet these requirements and which, both through their use and through the results to which they lead, are comparable to cyanide baths of the prior art without, however, presenting the disadvantages thereof.

According to the invention, there is provided an alkaline bath for the electrolytic deposition of zinc comprising an alkaline medium containing sodium hydroxide as alkali and a complex formed between zinc and gluconic acid, the sodium hydroxide and the gluconic acid being provided in quantities such that the molar ratio of gluconic acid/Zn++ and the molar ratio of sodium hydroxide/Zn++ are each equal to at least 2, the gluconic acid used having previously been substantially freed of reducing sugar impurities.

Industrially, gluconic acid is obtained by oxidative fermentation of glucose. The solutions originating from the enzymatic hydrolysis of starch are chiefly used for this purpose as source of glucose. The conversion reactions are not complete and various compounds remain in solution, among them, in particular, reducing sugars. Thus, in the usual commercial form of gluconic acid, which is constituted by an aqueous solution of about 50% strength, the amount of reducing sugars comes to about 6% with respect to the gluconic acid present.

The inventors have been able to discover that the presence of these impurities, and in particular of the reducing sugars, was one of the main causes of the difficulties encountered during attempts to use gluconic acid in compositions for the electrolytic deposition of metals. These have been able to establish that the elimination of the reducing sugars contained in commercial gluconic acid leads, surprisingly, to the disappearance of certain phenomena which are troublesome if not nullifying, where the use of this complexing agent in baths for the electrolytic deposition of metals is concerned, and, in particular, the rapid degradation of the baths, an increase in their viscosity to the point of forming gels, and the obtainment of inadequate faradic yields for high current densities.The baths according to the invention keep, in fact, without difficulty, show a good fluidity at useful concentrations and lead to good faradic yields.

The gluconic acid which has been substantially free from these reducing sugars and used according to the invention can be obtained by employing customary physical or chemical purification techniques. It is advantageous, however, to use those forms which are purified by crystallization and in particular glucono-delta-lactone, which is a crystalline form of gluconic acid. It is also possible to use the crystalline forms of the salts of gluconic acid e.g.

sodium gluconate. In order to obtain these solutions of gluconic acid from these crystallized forms, dissolution is then sufficient in the first case and, in the second case, dissolution followed by displacement of the anion of the salt.

The exact nature of the complex formed from gluconic acid and the zinc ions has not yet been completely elucidated. The tests carried out, some of which are given in detail in the Examples to follow, have nevertheless enabled it to be shown that the efficiency of the baths according to the invention is linked with the presence of these complexes. Due to these tests, it has also been possible to show clearly the conditions presiding at the formation of these complexes.

In order that all the Zn++ ions may be complexed, it is preferred that the molar ratio of gluconic acid to Zn++ is between 2 and 2.5.

As indicated, the alkalization of the bath is effected by means of sodium hydroxide. The sodium hydroxide content of the bath is chosen in such manner that the molar ratio of NaOH to Zn++ is at least equal to 2 (this generally corresponds to a pH at least equal to 11.5) and preferably is between 2.50 and 3.20. Under these conditions, it is found,. in fact, that these baths lead to high faradic yields, even at high current densities.

The baths according to the invention advantageously contain, moreover, supplementary agents already used in baths of the prior art and facilitating their use or improving the characteristics of the deposits formed.

In this respect, there are added to the baths electrolytes which are called "indifferent", because they do not intervene directly in the deposit formed.

In order to improve the conductivity of the baths and depassivate the zinc anodes, it is thus advantageous to introduce halide ions, and in particular chloride ions, into the bath.

To this end, one or more alkali or alkaline earth metal halides are introduced into the bath, preferably in the proportion to which there corresponds a maximum conductivity for the bath so formed. According to the invention, NaCI is preferably introduced at the rate of 1 to 2 moles per litre of bath.

Moreover. adjutants for performing various functions. and in particular the functions of improving the brightness, adhesion and levelling of the deposit, may be used in the electrolytic coating baths. The prior art relating to the electrolytic deposition of zinc depends on numerous aids of this type, which are often constituted by mixtures of compounds. To improve the brightness of the deposits obtained from the baths according to the invention, such brightening aids used for alkaline galvanizing baths will be introduced with advantage into the bath, for example bisulphited heliotropine (at the rate of about 3 g/l), or again mixtures thereof with polyethylene glycol compounds with a molecular weight of about 20,000 (at the rate of about 2 g per litre of each of these compounds) Particularly advantageous compositions for zinc baths according to the invention contain the following per litre of solution: - glucono-delta-lactone 0.6 to 2.2 and preferably 1.1 moles - ZnO 0.25 to 1 and preferably 0.5 mole - NaOH 0.8 to 2.8 and preferably 1.4 moles - NaCI q.s. * and preferably 1 to 2 moles and, moreover, brightening aids.

(* The sodium chloride is added in a quantity sufficient for the conductivity of the bath to be acceptable when voltages compatible with the normal conditions of industrial use are applied. In practice, the voltage imposed across the terminals of the electrolytic cell is limited in upper value. It is equal at the most to about 6 V for so-called "still bath" utilization techniques and about 15 V for so-called "roll" techniques, under the bath conditions which have been defined hereinbefore.) In a preferred manner of operation, in order to form the gluconic acid-zinc-sodium hydroxide complex, there is added to a gluconic acid solution a zinc compound soluble in this solution and the mixture obtained in this way is then alkalized.According to the invention, the gluconic acid solution is preferably prepared by dissolving crystallized glucono-delta-lactone; to this solution zinc oxide is added, or again a solution of a zinc salt, and the mixture is alkalized by adding a sufficiently concentrated sodium hydroxide solution, each constituent being introduced in the proportions defined hereinbefore. If necessary, the mixture is completed by adding indifferent electrolytes and aids. It is also possible to replace the glucono-delta-lactone by a crystallized salt of gluconic acid, for example an alkaline gluconate such as sodium gluconate, and to effect a double salt displacement by adding a zinc salt, for example zinc chloride, to the solution.It is preferable, in -all cases, to carry out the mixing of gluconic acid (or gluconate) with the zinc compound before alkalizing with the sodium hydroxide. In this way, the formation of precipitate which is troublesome in the continuation of the preparation is avoided.

The invention provides according to a further aspect thereof processes for electrolytic deposition of zinc in which the aforesaid baths are used.

It has been found that by using the baths according to the invention containing the gluconic acid-zinc-sodium hydroxide complex. it is possible to conduct the operation of electrolytic deposition under particularly advantageous conditions. Thus, with the baths according to the invention, it is possible to carry out the electrolytic deposition with a high cathode current density while maintaining a high faradic yield.

In the case of zinc electrodeposition, it is possible to attain current densities going up to 8 A/dm2 or more with a suitable faradic yield; thus, at 5 A/dm2, the faradic yield is higher than 70%, or again, for less high densities, it is possible to obtain faradic yields higher than 90%.

The characteristics of the coating formed, namely in particular its more or less matt or bright appearance and its structure can be varied according to the conditions chosen for performing the electrolysis. Among the conditions intervening in this result there figure, in particular, the current density, the duration of the electrolysis, the bath temperature, the exact composition of the bath and notably the adjuvant compounds. Thus, the thickness of the deposits can be regulated by varying the duration of the electrolysis, but it is also possible to vary the current density, which enables the time of electrolysis to be reduced for the same thickness of deposit.The choice of the value of these parameters, of course, is not immaterial because, by increasing the current density, that is to say by increasing the voltage across the terminals, the passivation of the anodes and a reduction of the faradic yield are promoted. Likewise, by changing the temperature conditions of the bath, the other conditions remaining the same, however, the appearance of the deposits can be modified. Thus, by raising the temperture, the formation of a matt deposit is promoted and, conversely, by operating at low temperature, the formation of a bright deposit is promoted.

Therefore, the operating conditions will be modified according to whether a deposit which is solely protective or solely decorative is formed. These examples are not exhaustive, because many parameters can be modified to arrive at the desired result and in particular the nature and the proportions of the aids used.

Baths may be formulated according to the invention which are stable at the temperatures customarily employed in electrolytic galvanizing processes. Thus, it will be possible to operate without difficulty in the temperature range extending from room temperature to 450C.

The baths according to the invention can be employed by all the usual techniques, whether so-called "still bath" or so-called "roll" operations are concerned, or again, when the substrate to be coated consists of wire or sheets, it can be passed continuously through the electrolysis tank.

When anodes made of the metal to be deposited are used, which is the most usual case, the impoverishment of the bath at the cathode is compensated by the gradual dissolution of the anodes. In this case, in manner known per se, it is advantageous either to separate the anode from the rest of the bath by enclosing it in a filter forming a bag or trap, or to filter the bath continuously to eliminate the slime of which there is a risk of formation and which would consequently alter the satisfactory development of the process and, in particular, would harm the appearance of the deposit formed.

It is particularly remarkable to find that, contrary to some electrolytic baths containing organic complexing agents, the baths according to the invention stand up well to prolonged usage. In fact, the faradic yield at the anode never being 100%, it might have been feared that the anode would be the seat of a more or less considerable oxidation leading to the degradation of the gluconic acid. Experience has shown that this phenomenon does not occur appreciably. Moreover, even when the baths are not used, they do not give rise to any troublesome fermentation.

In the course of their use, some of the constituents of the baths according to the invention are exhausted and it is consequently necessary to make them up or readjust their composition. This is the case particularly with the adjacant agents, which have a tendency to be adsorbed in the coating formed.

The articles leaving the electrolysis tank after the deposition of metal has been effected are treated in conventional manner. It is thus possible to finish the brightening by dipping in a bath of dilute nitric acid. Moreover, a plurality of depositions of different metals may be effected by a succession of electrolysis operations in different baths. In all cases, the treated articles are subjected to washing or rinsing operations to eliminate the constituents of the baths which have been carried out with them. Also from this point of view, the baths according to the invention are particularly advantageous. In fact, the disposal of the rinsing or washing water containing gluconic acid does not pose any difficulty from the point of view of pollution, because gluconic acid, on the one hand, is not toxic and, on the other hand, is biodegradable.In connection with the foregoing, it is interesting to compare the relative toxicities of gluconic acid and of the cyanides defined under the conditions of Standard AFNOR-T-90301 (measurement of the immobilization constant IC50 on daphnia, that is to say the concentration at which 50% of a population of daphnia is killed). This toxicity is established at 20.5 g/l of gluconate ion and at 0.1 mg/l of cyanide ion, thus showing the very low toxicity of the effluents rejected in the case of the use of the baths according to the invention in comparison with conventional cyanide baths.

Other characteristics of the invention will appear in the description of a number of characterization tests and of particular modes of use of galvanizing baths according to the invention, which are given by way of example and with reference to the accompanying drawings, in which: Figure 1 shows the variations in pH (curve 2) and in conductivity (curve 1), of a solution which is 0.2 molar in gluconate ion and 0.1 molar in Zn++ ion, as a function of the molar ratio of OH-/Zn++ when increasing amounts of a molar sodium hydroxide solution are gradually added; Figure 2 shows the variations at room temperature in the faradic yield in %, for three values of current density, namely 1, 3 and 5 A/dm2, as a function of the concentration of sodium hydroxide in the bath according to the invention, which is made up, for each litre of solution, from:: - glucono-delta-lactone 1.1 moles - zinc oxide 0.5 moles - sodium chloride 1 mole - brightening aids marketed by the Galvanotechnik Company under the

names ( 2000 M2 6 cc ( 2000 M 3.75 cc - sodium hydroxide respectively: 1 0.85 mole 2 1.0 mole 3 1.15 moles 4 1.25 moles 5 1.40 moles 6 1.60 moles 7 1.75 moles Figure 3 is another representation of the variations given in Figure 2. showing as ordinates the faradic yield in 6ho and as abscissae the concentration of sddium hydroxide, in moles per litre, the curves 1. 2 and 3 corresponding respectively to the current densities 1, 3 and 5 A/dm-.

In the following Examples, the tests are carried out with cathodes constituted by sheets of cold-rolled mild steel. Before use, they are degreased, pickled and rinsed. The anodes are of zinc of electrolytic quality.

Fore these studies there is used either a cell with a constant electric field or a so-called Hull electrolytic cell (cell of trapezoidal cross-section in which the current density at the cathode varies in a defined manner as a function of the geometrical position of the considered cathode portion).

Example 1 (formation of the complex) ++ In this Example, the behaviour of the solutions containing Zn ions and gluconic acid on the addition of sodium hydroxide was studied.

In a first stage, an equimolar solution of zinc ion and gluconate ion was formed. It was found that, for an addition of 0.5 to 0.6 equivalent of sodium hydroxide, a precipitate was formed. Under these conditions, there was therefore no formation of a stable complex soluble in alkaline medium.

On repeating this test with a solution containing gluconate ions and zinc ions in a molar ratio of gluconate to Zn++ equal at least to 2, precipitation was no longer found when sodium hydroxide was added.

Simultaneously, the variations in pH and conductivity of these solutions were followed as the sodium hydroxide was added. In this way there were found (Figure 1) a great variation in pH (curve 2) for about 1.7 to 2 equivalents of sodium hydroxide (with respect to Zn++) and a point of change of slope in the conductivity curve (curve 1) for about 2 equivalents.

These tests showed that the proportions maintained, within the limits of the present invention, for the constituents of the galvanizing baths, corresponded to the appearance of a particular ion form.

The presence of an alkaline complex of zinc and gluconic acid in these solutions was confirmed by the appearance of an absorption band in the ultra-violet range in the vicinity of 266 nm. As can be confirmed by these curves, the complex formed is stable, even in the presence of an excess of sodium hydroxide with respect to the required minimum (namely, 2 equivalents of sodium hydroxide for 1 of Zn++).

Example 2 The influence of the sodium hydroxide content on the behaviour of the electrolytic galvanizing baths was determined from the results of the foregoing Example.

The bath used contained in all cases, referred to one litre: - 1.1 moles of glucono-delta-lactone - 0.5 mole of ZnO - 1 mole of NaCl - brightening aids: 6 cc of 2000 M2 3.75 cc of 2000 M3 The faradic yield at the cathode was studied as a function of the sodium hydroxide content of the mixture and as a function of the cathode current density. The sodium hydroxide contents studied were: 0.85 - 1 - 1.15 - 1.25 - 1.40 - 1.60 - 1.75 moles/litre.

The temperature of the bath was room temperature.

The depositions were effected at the current densities of 1, 3 and 5 A/dm2.

The results of these tests are given in Figures 2 and 3, which show, respectively, the variation in the faradic yield as a function of the current density for different sodium hydroxide contents (Figure 2), and the variation in the faradic yield as a function of the sodium hydroxide content for each of the current densities (Figure 3). Figure 3 is particularly significant and shows that, in order to have an industrially acceptable yield under the conditions maintained, it is necessary to use a bath containing more than 1.25 moles of sodium hydroxide per litre, and that, still under these conditions, it is possible to operate with a current density of 5 A/dm2, or even with higher densities.

In all these tests, the depositions were effected with the aid of these baths in a cell with a constant field and on a cathode the area of which was 1 dm2. The deposit formed, which had a thickness of about 20 Il, was even and presented a bright metallic appearance. For current densities of 5 A/dm2, the duration of the electrolysis was about 20 minutes.

Example 3 The behaviour of the baths according to the invention in the course of prolonged usage was studied. Still for baths of 1 litre and a cathode area of 1 dm2, a succession of depositions was effected at a current density of 5 A/dm2 with a faradic yield higher than 70%.

The appearance of the deposits began to deteriorate only after a use corresponding to more than 20 Ah. If the amount of adjuvant agents required for restoring their starting concentration is added, the bath regains its initial properties and the deposits regain their bright metallic appearance.

For satisfactory operation of the process, it is therefore advisable to readjust the composition for working corresponding to about 10 Ah per litre of bath. The deposits obtained under these conditions are quite homogeneous.

Example 4 In this Example, a comparison was made between baths containing gluconic acid originating from the dissolution of crystallized glucono-delta-lactone and baths prepared from the 50% gluconic acid solutions such as are found commercially and which contain of the order of about 3% of reducing sugars.

The tests were carried out under conditions similar to those of Example 2. As in this Example, the faradic yield of the baths was determined as a function of the current density for 1, 3 and 5 A/dm2.

The baths were identical, except for the quality of the gluconic acid used for forming these baths.

It was found that the baths prepared according to the invention from glucono-deltalactone, that is to say practically free from the natural impurities of gluconic acid, lead to faradic yields higher by about at least 30%, the other conditions being moreover identical.

This being the case, and whatever the mode of carrying into effect adopted, baths and processes for electrolytic deposition of metals are thus available, the characteristics of which are sufficiently apparent from the foregoing for it to be unnecessary to dwell on the subject, and which have numerous advantages in comparison with those already in existence, in particular those of using non-toxic and biodegradable complexing agents without, however, harming the qualities of the coatings obtained or requiring disadvantageous operating conditions.

WHAT WE CLAIM IS: 1. An alkaline bath for the electrolytic deposition of zinc comprising an alkaline medium containing sodium hydroxide as alkali and a complex formed between zinc and

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. The bath used contained in all cases, referred to one litre: - 1.1 moles of glucono-delta-lactone - 0.5 mole of ZnO - 1 mole of NaCl - brightening aids: 6 cc of 2000 M2 3.75 cc of 2000 M3 The faradic yield at the cathode was studied as a function of the sodium hydroxide content of the mixture and as a function of the cathode current density. The sodium hydroxide contents studied were: 0.85 - 1 - 1.15 - 1.25 - 1.40 - 1.60 - 1.75 moles/litre. The temperature of the bath was room temperature. The depositions were effected at the current densities of 1, 3 and 5 A/dm2. The results of these tests are given in Figures 2 and 3, which show, respectively, the variation in the faradic yield as a function of the current density for different sodium hydroxide contents (Figure 2), and the variation in the faradic yield as a function of the sodium hydroxide content for each of the current densities (Figure 3). Figure 3 is particularly significant and shows that, in order to have an industrially acceptable yield under the conditions maintained, it is necessary to use a bath containing more than 1.25 moles of sodium hydroxide per litre, and that, still under these conditions, it is possible to operate with a current density of 5 A/dm2, or even with higher densities. In all these tests, the depositions were effected with the aid of these baths in a cell with a constant field and on a cathode the area of which was 1 dm2. The deposit formed, which had a thickness of about 20 Il, was even and presented a bright metallic appearance. For current densities of 5 A/dm2, the duration of the electrolysis was about 20 minutes. Example 3 The behaviour of the baths according to the invention in the course of prolonged usage was studied. Still for baths of 1 litre and a cathode area of 1 dm2, a succession of depositions was effected at a current density of 5 A/dm2 with a faradic yield higher than 70%. The appearance of the deposits began to deteriorate only after a use corresponding to more than 20 Ah. If the amount of adjuvant agents required for restoring their starting concentration is added, the bath regains its initial properties and the deposits regain their bright metallic appearance. For satisfactory operation of the process, it is therefore advisable to readjust the composition for working corresponding to about 10 Ah per litre of bath. The deposits obtained under these conditions are quite homogeneous. Example 4 In this Example, a comparison was made between baths containing gluconic acid originating from the dissolution of crystallized glucono-delta-lactone and baths prepared from the 50% gluconic acid solutions such as are found commercially and which contain of the order of about 3% of reducing sugars. The tests were carried out under conditions similar to those of Example 2. As in this Example, the faradic yield of the baths was determined as a function of the current density for 1, 3 and 5 A/dm2. The baths were identical, except for the quality of the gluconic acid used for forming these baths. It was found that the baths prepared according to the invention from glucono-deltalactone, that is to say practically free from the natural impurities of gluconic acid, lead to faradic yields higher by about at least 30%, the other conditions being moreover identical. This being the case, and whatever the mode of carrying into effect adopted, baths and processes for electrolytic deposition of metals are thus available, the characteristics of which are sufficiently apparent from the foregoing for it to be unnecessary to dwell on the subject, and which have numerous advantages in comparison with those already in existence, in particular those of using non-toxic and biodegradable complexing agents without, however, harming the qualities of the coatings obtained or requiring disadvantageous operating conditions. WHAT WE CLAIM IS:

1. An alkaline bath for the electrolytic deposition of zinc comprising an alkaline medium containing sodium hydroxide as alkali and a complex formed between zinc and

gluconic acid, the sodium hydroxide and the gluconic acid being provided in quantities such that the molar ratio of gluconic acid/Zn++ and the molar ratio of sodium hydroxide/Zn++ are each equal to at least 2, the gluconic acid used having previously been substantially freed of reducing sugar impurities.

2. A zinc bath according to Claim 1, wherein the gluconic acid from which the impurities have been removed is produced by dissolving crystalline gluconic acid or a crystalline derivative thereof capable of yielding gluconic acid or gluconate ions in aqueous solution.

3. A zinc bath according to Claim 2, wherein the gluconic acid from which the impurities have been removed is produced by dissolving glucono-delta-lactone.

4. A zinc bath according to Claim 2, in which the gluconic acid from which the impurities have been removed is produced by dissolving sodium gluconate.

5. A zinc bath according to any preceding claim, wherein the gluconic acid/Zn++ molar ratio is between 2 and 2.5 and the NaOH/Zn++ molar ratio is between 2.50 and 3.20.

6. A zinc bath according to any preceding claim additionally containing in solution a conducting salt consisting of an alkali metal halide or an alkaline-earth metal halide.

7. A zinc bath according to Claim 6, wherein the conducting salt is NaCl in a quantity of 1 to 2 moles/l.

8. A zinc bath according to any preceding claim additionally containing levelling agents, adhesion improving agents or brightening agents.

9. A process for the production of a galvanic zinc bath according to any preceding claim which comprises adding a zinc compound to a solution of a crystalline form of gluconic acid from which the impurities consisting mainly of reducing sugars have been removed, the zinc compound being soluble in said solution, and the resulting mixture is then rendered alkaline by the addition of sodium hydroxide, the quantities of sodium hydroxide and gluconic acid being such that the molar ratio of gluconic acid/Zn++ and the molar ratio of sodium hydroxide/Zn++ are each equal to at least 2.

10. A zinc bath whenever produced by the process of Claim 9.

11. A zinc bath according to Claim 1 and substantially as hereinbefore described.

12. Process for the electrolytic deposition of zinc, which comprises using as electrolyte a bath according to any one of Claims 1 to 8 and 10.