GB2119814A - Process and plant for the continuous electrolytic deposit of a layer of zinc alloy with a high current density - Google Patents

Abstract

Process for continuous electrolytic deposit with a high current density of a layer of a zinc-based alloy on an article which has a shape of a band, wire, bar, tube and the like, wherein said article is moved inside and electrolysis cell past an anode through an electrolyte containing zinc sulfate, while maintaining in this electrolyte a pH lower than 2, a temperature between 40 DEG C and 70 DEG C and a zinc sulfate concentration of 0.2 to 2 mol/liter and an iron, nickel and/or cobalt sulfate concentration of 0.3 to 2 mol/liter according to the kind of coating layer to be produced.

Description

SPECIFICATION Process and plant for the continuous electrolytic deposit of a layer of zinc alloy with a high current density This invention concerns a process for continuous electrolytic deposit of a layer of zinc alloy with a high current density on an article having a shape of band, wire, tube, bar and the like, wherein this article is moved in an electrolysis cell past an anode through an electrolyte containing zinc sulfate.

In the known prior art processes for covering such an article with a metal layer, two quite different techniques are known.

A first technique simply consists in passing the article through one or more baths containing melted coating metals, such as for example described in U.S. patent 3,858,333 of Thompson, while according to a second technique, coating of the article is obtained by means of successive electrolytic deposits of coating metals on this article, such as for example described in DOS 2,106,226 of Michelin et Cie or in the Japanese patent application 8100297 of Showa Electric Wire and Cable Cy.

Such processes are in general very intricate and expensive, in particular due to the fact that they require a relatively high number of steps, each of them necessitating a very severe control of operation conditions.

More particularly in the prior art processes, for coating band-shaped articles, in particular steel sheets, use is made of an electrolyte having a relatively complex composition requiring a very severe control of operation conditions so as to obtain alloys of a sufficiently high purity, with commercially suitable yields. Thus this presents the risk of raising rather severe technical problems for applying these processes on an industrial scale.

Thus DE-PS 30.05159 which relates to a process for preparing a zinc-nickel alloy mentions a sulfate bath containing a rather high amount of Na2SO4, as support electrolyte. Use of such a support electrolyte is essential so that the electrolytic bath in the electrolysis cell is conducting enough at a pH of at least 2, at which the bath has to be maintained in usual practice, such as indicated in the examples described in this publication.

In other respects, DE-OS 30.11911, which also relates to a process for preparing a zinc-nickel alloy concerns use of SrSO4 as support electrolyte in an electrolytic bath, the pH of which is always higher than 2, as also illustrated by actuai examples of application described therein.

In these prior processes, the current density generally cannot in industrial practice raise above 20 A/dm2, as it is besides once more apparent from said illustrative examples.

One of the objects of the present invention is to remedy said drawbacks and to provide an extremely simple and reliable process for a continuous application on an industrial scale with a high current density and without toxic effluents being rejected.

To this end, according to the invention a pH lower than 2, a temperature between 400C and 700C and a concentration of 0.2 to 2 mol/liter of zinc sulfate and of 0.3 to 2 mol/liter of iron, nickel and/or cobalt sulfate according to the kind of alloy to be produced are maintained in the electrolyte of the electrolysis cell.

Advantageously, a pH lower than 1.2 and preferably lower than 0.4 is maintained in the electrolyte.

According to a preferred embodiment, a molar ratio M++/Zn++ lower than 1.5 is maintained in the electrolyte for a pH lower or at most equal to 1.2, M being Ni, Fe or Co, this ratio being preferably between 2 and 3 for preparing a Zn-Fe alloy.

This invention also concerns a process for continuously regenerating the electrolyte of an electrolysis cell for the electrolytic deposit with a high current density of a layer of a zinc-based alloy on an article having a shape of band, wire, tube, bar and the like, in particular of the electrolyte such as hereinabove described.

According to the invention, a pH lower than 2 is maintained in the electrolyte and at least a portion of this electrolyte is continuously extracted from the cell so as to regenerate this electrolyte by dissolving in this portion, in ratios corresponding to the desired alloy, on the one hand zinc and on the other hand nickel, iron and/or cobalt, according to the kind of the desired alloy, some so regenerated electrolyte being then readmitted, also continuously, into the electrolysis cell.

According to a particular embodiment of the invention, zinc is introduced into the electrolyte to be regenerated as a metal form.

According to a particularly advantageous embodiment of the invention, zinc and the other metal of the alloy to be produced, and used for regenerating electrolyte are introduced into separate electrolyte amounts, these amounts being then combined together before being then reintroduced into the electrolysis cell, the ratio of these amounts being controlled in terms of the desired metal ratio in the alloy to be produced in the cell.

Finally, the invention still concerns a plant for carrying out the electrolysis and regeneration process such as hereinabove described.

This plant is characterised in that it is provided for continuous regeneration of electrolyte with an apparatus comprising a buffer tank which is directly connected with the electrolysis cell by means of inlet and outlet pipes.

Other details and particularities of the invention will become apparent from the following description, as non limitative examples, of some particular embodiments of the process and plant according to the invention with reference to annexed drawings.

Fig. 1 is a block diagram of a first embodiment of the plant according to the invention, Fig. 2 is a block diagram of a second embodiment of the plant according to the invention.

In said Figures, same reference numerals relate to identical or similar elements.

The invention is more particularly relating to a process for continuous electrolytic deposit, with a high current density, in particular from 20 to 300 A/dm2, preferably from 30 to 200 A/dm2, of a layer of a zinc alloy of a thickness preferably varying between 1 and 12 y on an article having a shape of band, wire, bar, tube and the like, made of steel or soft iron.

This is a process according to which said article is moved inside one or more electrolysis cells past an anode through an electrolyte containing zinc sulfate.

The cell may be of a traditional type or correspond to that of the British patent 1,038,671, improved by means of an electrolyte circulation system, more particularly when these are articles having a shape of wire, tube or bar for example.

In other respects for coating bands, such as steel sheets, use may be in particular made of a cell corresponding to that of the U.S. patent 4,304,653.

According to this process, in the electrolyte of the electrolysis cell, a pH lower than 2, a temperature between 400C and 700C and a concentration of 0.2 to 2 mol/liter of zinc sulfate and of 0.3 to 2 mol/liter of iron, nickel and/or cobalt sulfate according to the type of alloy to be produced are maintained.

The alloy may thus be a zinc-nickel, zinc-iron, zinc-cobalt alloy or also a zinc-nickel-iron, zincnickel-cobalt, zinc-iron-cobalt alloy or still a zinciron-cobalt-nickel alloy.

More particularly, a pH lower than 1.2, preferably lower than 0.8, for example between 0 and 0.5 is maintained in the electrolyte of the electrolysis cell.

One may for example use, with a high current density, an electrolyte having an acidity higher than 1 mol/liter, in particular of about 1.5 mol/liter.

The acidity in electrolyte may be controlled by sulfuric acid addition.

Advantageously, the relative concentration of metal ions in the electrolyte is controlled so as to obtain a zinc-nickel alloy containing from 2 to 14% of nickel, a zinc-iron alloy containing from 2 to 1 5% of iron or a zinc-cobalt alloy containing from 2 to 12% of cobalt.

Contrary to some known processes, an electrolytic bath free from metal chlorides and also free from support electrolytes is used, so as to obtain an as pure as possible bath which consequently allows to ensure formation of an electrolytic deposit, on the article, of a zinc alloy of very high homogeneity and purity, without being necessary to take special cautions to this end.

Being given that the electrolyte is free from chlorides, if an insoluble anode is used, use may be made of an anode of a lead-silver alloy, with a silver content of 0.6 to 1.2% preferably.

If, on the contrary, a soluble anode is used, the latter is a zinc-based alloy or is made of an alloy corresponding to the alloy of the coating layer to be produced.

Thus if the anode is made of pure zinc, ions of the other metal or of the other metals of the alloy to be formed are added to the electrolyte.

In other respects, if the soluble anode is made of an alloy, the composition of the latter may be the same as or different from that of the coating layer to be produced.

Thus, if the anode is made of an alloy having a composition different from that of the coating layer to be produced, the composition of the electrolyte bath may be adapted by addition of ions of the metal or metals.

A soluble anode is particularly suitable for forming a coating layer made of a Zn-Fe alloy.

In other respects, thanks to the fact that the electrolyte has a very low pH, a molar ratio M++/Zn++ lower than 1.5 may be maintained, M being nickel or cobalt, and between 2 and 3 if M is iron, which thus allows to reduce the relative amount of the metal M used forforming a zinc alloy.

This may constitute an important feature of the electrolyte.

In order to allow to operate with relatively high current densities, in particular up to 300 A/dm2, such as already mentioned, the relative speed of the electrolyte with respect to the article to be coated is advantageously maintained at a value higher than 1 m/sec, and even at 2 m/sec this speed may be in some cases higher than 4 m/sec for current densities of about 200 to 300 A/dm2, according to the type and shape of the article to be coated.

In order to be able to carry out an electrolysis in a very acid bath, at a pH till 0 or still lower, a continuous regeneration of the electrolyte is associated to the electrolysis, this regeneration being such that the pH may be stabilized in the cells, the regeneration using this strong acidity for dissolving metals which are deposited on the cathode, in the electrolyte to be regenerated.

In order to ensure this electrolyte regeneration, a pH lower than 2, preferably lower than 1.2 is maintained in this electrolyte, and some electrolyte is continuously extracted from the cell in order to dissolve therein, in ratios corresponding to the desired alloy, on the one hand zinc and on the other hand iron and/or cobalt, the so regenerated electrolyte being then reintroduced, also continuously, into the electrolysis cell.

In order to prevent undissolved particles issuing from this regeneration to be carried along into the electrolytic bath itself and thus to prevent a staining of the electrolytic deposit on the article to be covered and also to allow a flexible control of the regeneration procedure in terms of operative conditions, the addition of these metals is made in separate amounts of the electrolyte to be regenerated, outside the electrolysis cell, these amounts being then again mixed, before being reintroduced into the cell, at the time when all the metals are completely dissolved and possibly after decantation of insoluble impurities issuing from said regeneration.

The fact of isolating controllable amounts of electrolyte, to which metals to be dissolved for regeneration are then added separately, allows to control dissolution conditions of these metals, independently of the electrolyte circulation in the electrolysis cell itself, while carrying out an electrolysis and a regeneration in a continuous manner which is quite adapted to this electrolysis.

Advantageously, zinc is introduced into the electrolyte to be regenerated as a metallic form, which excludes or minimizes staining of the electrolyte bath.

Nickel and cobalt are advantageously added to the electrolyte as carbonates while iron is advantageously added as a metallic form or as ferric hydroxide or as a combination of both of them.

For the formation of Zn-Fe alloys, in terms of operating conditions, it is useful and often necessary to take some precautions so as to prevent a substantial decrease of the total electrolysis yield due to the formation of ions Fe+++ at the anode, from ions Fe++.

Thus, according to the invention, ammonium sulfate is added to the electrolyte in order to stabilize ions Fe++ as complexes.

Moreover, at least a portion of the electrolyte is advantageously subjected to a reduction operation, so as to transform ions Fe+++ into ions Fe++.

In this regard, it has been found that SO2 is particularly suitable as reducing agent. It allows to form ions Fe++ and SO;-. It may then be regenerated by means of an ion exchange resin.

Fig. 1 is a block diagram of a first embodiment of a plant for continuous electrolytic depositing with a high current density, of a layer of a zincbased alloy on an article which has a shape of band, wire, bar, tube and the like.

This plant comprises an electrolysis cell 1 wherein one or more of said articles simultaneously move past an insoluble anode through an electrolyte containing zinc sulfate.

This or these articles, as well as the anode and the cathode, have not been shown on this figure, being given that it may be an electrolysis cell of any type known per se for this kind of operation.

The article thus acts as a cathode, while if an insoluble anode is used, the latter is preferably made of a lead-silver alloy, the silver content of which is comprised between 0.6 and 1.2%.

This plant is characterized in that it is provided with a continuous regeneration apparatus for the electrolyte, connected with the electrolysis cell 1.

This regeneration apparatus comprises a buffer tank 2 directly connected with this cell, by means of an inlet pipe 3 and an outlet pipe 4, and two dissolution reactors 5 and 6, which are mounted in parallel to the tank 2, means being provided in order to control the relative electrolyte output through said reactors in terms of the metal ratio in the alloy to be produced in cell 1.

For example, these means consist of valves 7 and 8, provided upstream the reactors 5 and 6 respectively, the latter being connected by an intake pipe 9 and a return pipe 1 to tank 2.

Tank 2 may possibly act as a decanter in case insoluble materials issuing from metal dissolution in reactors 5 and 6 would be introduced into said tank by return pipe 1 0.

The circulation between cell 1 and tank 2 is ensured by means of a pump 12, for example provided in pipe 4, while circulation between tank 2 and reactors 5 and 6 is obtained by means of a pump 13, for example provided in pipe 9.

For preparing a Zn-Fe alloy, this plant may moreover comprise a reduction reactor 1 8 connected through pipes 19 and 20 and a pump 21, for example of adjustable output, on the buffer tank 2.

A reducing agent, such as SO2, is then introduced into said reactor so as to transform ions Fe+++ into ions Fe++.

An ion exchange resin, for example as a grid in reactor 18 allows to regenerate the reducing agent.

The operation of this plant may be described as follows.

Due to the electrolysis reaction in cell 1, zinc and one or more other metals intended to form an alloy with zinc are deposited on the article or articles moving through the cell. This has as a consequence a depletion of electrolyte in metal ions and an increase of bath acidity. In normal operation, a pH lower than 2, preferably lower than 1.2 is aimed to be maintained in the bath, as already mentioned hereinbefore.

Electrolyte generally flows in the cell at relative speeds of 1 to 4 m/sec or possibly higher, with respect to said article, according to the desired current density and is extracted from the cell by inlet pipe 3 in order to be introduced into buffer tank 2, wherein a relatively stable pH substantially corresponding to that desired in the cell is maintained.

A portion of electrolyte of tank 2 is sent by intake pipe 9 to both dissolution reactors 5 and 6, also called regenerators. In one of the reactors, for example reactor 5, necessary zinc to counterbalance zinc depletion in the electrolytic bath of cell 1 is admitted, while in the other reactor, the other metal intended to form an alloy with zinc in cell 1 is admitted.

The relative output of electrolyte through these reactors 5 and 6 is regulated by means of valves 7 and 8, so that the ratio of amounts of dissolved metals, brought back by return pipe 10 to the tank, corresponds to the concentration ratio of these metals, which is needed so as to obtain the desired alloy. The arrow 11 indicates entry of water into tank 2 in order to counterbalance water consumption during electrolysis reaction.

As already mentioned hereinabove, zinc may be admitted as a metal, for example as bars or ingots, into reactor 5.

When forming a Zn-Fe alloy, a portion of electrolyte coming into the buffer tank 2 is sent to the reactor with an output that is a function of the concentration in ions Fe+++ in the electrolyte.

Electrochemical reactions at electrodes, on which electrolysis and regeneration of electrolyte such as hereinabove mentioned, are based, as well as preferential operating conditions, for electrolytic deposit of a zinc-metal, zinc-iron and zinc-cobalt alloy are described more detailedly hereinafter.

10 Zinc-nickel a) electrolyte bath composition in cell 1: ZnSO4: 0.5 to 2 mol/liter NiSO4: 0.2 to 2 mol/liter pH: 0 to 1.2 (by addition of HzSO4) b) operating parameters of the electrolysis cell: temperature: 40 to 70 C current density: 20 to 300 A/dm2 relative flow speed of electrolyte: 1 to 8 m/sec c) regeneration products introduced into reactors 5 and 6: Zn and NiCO3. 2 Ni (OH)2 d) composition of the alloy such as obtained: 2to 14%ofNi.

e) electrochemical reactions: -at the cathode: Zn+++2eeZn Ni+++2eeNi 2H++2eeH2 at the anode: 2H20e02+4H++4e 20 Zinc-iron a) electrolytic bath composition in cell 1: ZnSO4: 0.3 to 2 mol/liter FeSO4: 0.6 to 2 mol/liter pH: 0 to 1.2 (by addition of H2SO4) Optionally (NH4)2SO4: 0.5 to 1 mol/liter.

b) operating parameters of the electrolysis cell: temperature: 40 to 70 C current density: 20 to 300 A/dm2 relative flow speed of electrolyte: 1 to 8 m/sec c) composition of the alloy such as obtained: 2to 15%ofFe.

A. Insoluble anode (Pb-Ag 0.6% for example) 1) regeneration products introduced into reactors 5 and 6: Zn and Fe (possibly Fe+Fe(OH3) 2) electrochemical reactions: -at the cathode: Zn+++2e-tZn Fe+++2eeFe Fe++++e Fe++ 2H++2eeH2 -at the anode: 2H2OoO2+4H ++4e Fe++oFe++++e 3 reduction of ions Fe+++ into ions Fe++: lons Fe+++ produced at the anode are reduced into ions Fe++, at the outlet of electrolysis cell 1 thanks to three mechanisms: a.-in the zinc reactor 5: Zn+2Fe+++eZn+++2 Fe++ or

b-In iron reactor 6:: Fe+2H+oFe+++H2 H2+2 Fe+++ < 2 H++2 Fe++ Fe+2 Fe+++s3 Fe++ c.-ln reduction reactor 17: 2 Fe++++SO2+2H2Oo2 Fe+++H2SO4+2H+ In terms of operating conditions, sizes of the three reactors are determined so as to maintain a constant composition in Fe++ and Zn++, and a low content of ions Fe+++.

B. Soluble anode 1) anode: Zn-Fe alloy of the composition of the wished coating layer; or pure zinc with iron regeneration in an auxiliary reactor.

2) electrochemical reactions: -at the cathode: Fe+++2eoFe Zn+++2eeZn 2H++2eeH2 -at the anode: FeoFe+++2e ZneZn+++2e 3) reductions of ions Fe+++ into ions Fe++: Thanks to the use of a soluble anode, formation of Fe+++ at the latter does not produce and these is in principle only deriving from an air oxidation of ions Fe++.

This has for a consequence that the amount of Fe+++ formed in the electrolyte is relatively low and does not generally require addition of a reducing agent of the SO2 type for example.

30 Zinc-cobalt a) electrolytic bath composition in cell 1: ZnSO4: 0.3 to 2 mol/liter CoSO4: 0.6 to 2 mol/liter pH: 0 to 2 (by addition of H2SO4) b) operating parameters of electrolysis cell: temperature: 40 to 70 C current density: 20 to 300 A/dm2 relative flow speed of electrolyte: 1 to 8 m/sec; c) regeneration products introduced into reactors 5 and 6: Zn and CoCO3. 2 Co(OH)2 d) composition of the alloy such as obtained: 2to 12%ofCo e) electrochemical reaction: -at the cathode: Zn+++2eeZn Co+++2e-tCo 2H++2eeH2 -at the anode: 2H20o02+4H++4e As a conclusion, it has thus been found that, for these three kinds of alloys, there is at the electrolysis stage, in addition to a metal ion consumption for the alloy to be produced, a H2O consumption, a ion H+ production and an oxygen release.

Morever, in particular in the case of a zincnickel alloy, the molar ratio Ni++/Zn++=a is lower than 1.5 and generally comprised between 0.5 and 1.2 in the electrolytic bath.

Production of ions H+, such as mentioned before, thus leads to an acidification of the electrolytic bath in the cell.

As already mentioned hereinabove, an essential feature of the invention consists of using said ions H+ such as produced in order to regenerate the electrolyte.

Thus, pH in the electrolytic bath of the cell may be maintained above a minimum value and at the same time it is possible to replace in the latter metal ions which have been extracted from this bath due to electrolytic deposit, without being necessary to use particular means in order to form these ions.

The metals or metal compounds used for regeneration in reactors 5 and 6 must preferably be easily soluble in acid baths without leaving insoluble traces risking to pollute the electrolytic bath. Thus, metals in a metallic state, such as zinc and iron, and nickel cobalt and iron carbonates or hydroxides are particularly suitable. A preference is for example given to basic carbonates of nickel and cobalt and to ferric hydroxide optionally mixed with metal iron particles.

In the case of use of carbonates, the CO2 release stirs up the solution and thus allows to increase electrolyte homogeneity in reactors 5 and 6.

The invention process is hereinafter illustrated by a few practical embodiments.

Example 1 In order to produce an electrolytic deposit of zinc-nickel alloy on a steel sheet, electrolysis of an electrolytic bath containing 1.2 mol/liter of NiSO4 and 1 mol/liter of ZnSO4, maintained at a pH of about 1 and at a temperature of about 500C was carried out.

The extraction current density was about 100 A/dm2, while the relative flow speed of the electrolyte was 1 m/sec with respect to the sheet.

Used anodes were of lead-silver, with a silver content of 1.2%.

The deposit thickness was 5 y.

The pH of the electrolyte at the outlet of the cell and passing through pipe 3 to buffer tank 2 was of about 0.8, while pH of the electrolyte, having been used for dissolution, passing through return pipe 10 had a value of about 1.5. The obtained alloy contained about 8% of nickel.

Example 2 In the electrolysis cell, a bath of 0.5 mol/liter of ZnSO4 and of 0.6 mol/liter of NiSO4, maintained at a temperature of 50"C and at a pH of 1.2 by addition of H2SO4was used, in order to coat a steel sheet with an electrolytic deposit of a Ni-Zn alloy.

The density of the cathodic extraction current was about 30 A/dm2, while the flow speed of electrolyte was about 1 m/sec.

For regeneration in reactors 5 and 6, metal zinc and basic nickel carbonate were used. The electrolytic deposit such as obtained was made of a nickel-zinc alloy containing 11% of nickel and having a thickness of about 58.

The deposit aspect was of an even clear grey colour.

Example 3 The used electrolytic bath comprised 2 mol/liter of ZnSO4 and 2 mol/liter of NiSO4 and was maintained at a temperature of about 500C and at a pH of 0 by addition H2S04, so as to coat a steel sheet with an electrolytic deposit of a Ni-Zn alloy. The cathodic current density was 300 A/dm2, while the relative speed of electrolyte with respect to the sheet to be protected with this alloy was of 4 m/sec.

Regeneration is carried out by addition of metal zinc and basic nickel carbonate to the electrolyte.

The alloy comprised 8% of nickel and had an even brilliant grey aspect.

Example 4 The used electrolytic bath contained 0.5 mol/liter of NiSO4, 0.5 mol/liter of ZnSO4 and 1.5 mol/litre of H2 SO4 and was maintained at a temperature of 500C in order to coat a steel sheet with an electrolytic deposit of a Zn-Ni alloy.

The cathodic current density was 200 A/dm2, while the relative speed of the electrolyte with respect to the sheet to be coated with the alloy was 4 m/sec.

The regeneration was carried out by addition of metal zinc, or basic carbonate of nickel to the electrolyte. The alloy contained 8% of nickel and had a metal grey aspect.

Example 5 In order to produce an electrolytic deposit of a zinc-nickel alloy on a steel wire having a diameter of 1 mm, an electrolysis of an electrolytic bath containing 0.5 mol/liter of NiSO4 and 0.7 mol/liter of ZnSO4, maintained at a pH of about 0.3 by addition of sulfuric acid and at a temperature of about 500C was carried out. The extraction current density was 30 A/dm2. The voltage across the cell was of 4 volts and the cathodic yield was of 74%. The electrolytic flow speed was 3 m/sec, whilst the wire speed, in the opposite direction to that of said flow, was of 1 m/sec.

The used anodes were of lead-silver with a silver content of 0.6%. For the regeneration in reactors 5 and 6, metal zinc and basic carbonate of nickel were used.

The thickness of the deposit so obtained was of 4,u.

The so obtained alloy comprised about 13% of nickel.

The deposit aspect was of an even and brilliant, light grey.

Example 6 A steel wire of a diameter of 3 mm was coated with a zinc-nickel alloy.

To this end, in the electrolysis cell, a bath of 1 mo/liter of ZnSO4 and 0.8 mol/liter of NiSO4, maintained at a pH of 0.5 by addition of H2SO4 and at a temperature of 50 C was used. The cathodic current density of extraction was of about 50 A/dm2. The voltage across the cell was of 5.3 volts and the cathodic current yield was of 74%. The flow speed of the electrolyte was of 3 m/sec, while the displacement speed of the wire in the opposite direction to that of the electrolyte was of about 6 m/sec.

For regeneration in reactors 5 and 6, metal zinc and basic carbonate of nickel were used.

The electrolytic deposit so obtained was formed of a nickel-zinc alloy containing 11% of nickel and having a thickness of about 1 y.

The deposit aspect was of an even light grey.

Example 7 A steel wire of a diameter of 3.5 mm was coated with a zinc-nickel alloy.

The used electrolytic bath contained 2 mol/iiter of ZnSO4 and 2 mol/liter of NiSO4 and was maintained at a pH of 0.2 by addition of H2SO4 and at a temperature of about 500C.

The cathodic current density was of 1 50 A/dm2. The voltage across the cell was of 12 volts and the cathodic current yield was of 74%.

The electrolyte speed was of 4 m/sec, while that of the wire, in the opposite direction to that of the electrolyte was of 3.3 m/sec.

The anodes used were of lead-silver with a silver content of 0.6%.

The regeneration was made by addition of nickel-zinc or basic carbonate of nickel to the electrolyte.

The alloy contained 13% of nickel and had a brilliant grey aspect.

Example 8 A steel tube having an outside diameter of 5/8 inch and an inside diameter of 14.5 mm was internally and externally coated with a zinc-nickel alloy.

The used electrolytic bath contained 1 mol/liter of NiSO4 and 1 mol/liter of ZnSO4, and was maintained at a pH of 0.8 and at a temperature of about 500 C.

The cathodic current density was of 50 A/dm2.

The voltage across the cell was of 5.3 volts and the cathodic current yield was of 76%. The flow speed of the electrolyte was of 2 m/sec, while that of the tube, in the opposite direction to that of the electrolyte, was of 1.2 m/sec.

The used anodes were of lead-silver with a silver content of 0.6%.

The regeneration was carried out by addition of metal-zinc and basic nickel carbonate to the electrolyte. The alloy contained 10% of nickel and had an even clear grey aspect. The deposit thickness was of about 5 u.

Example 9 A steel sheet was covered with a deposit of a Zn-Fe alloy in an electrolytic bath the composition of which was as follows: Zn++: 25 g/l Fe++: 65,3 g/l Fe+++: 3 g/l H2SO4: 46 g/l (NH4)2SO4: 80 g/l The used anode was of lead-silver, with a silver content of 0.6%.

The current density was of 60 A/dm2.

The relative speed of the electrolyte in the electrolysis cell was of 1 m/sec, while the displacement speed of the steel sheet was of 8 m/min. The current yield was of 88% and the anode-cathode voltage was of 9.7 volts.

The so obtained electrolytic deposit was formed of a zinc-iron alloy, containing 92% of zinc and 8% of iron.

The deposit was clear, ductile, even and very well covering, and presenting a fine crystallization.

Example 10 A steel sheet was coated with a deposit of a Zn-Fe alloy in an electrolytic bath, the composition of which was maintained as follows: Zn++: 23 g/l Fe: 68 g/l Fe+++: 0.2 g/l H2SO4: 50 g/l (NH4)2SO4: 120 g/l The used anode was a soluble anode made of pure zinc. The current density was of 70 A/dm2.

The relative speed of the electrolyte in the cell was of 1 m/sec while the displacement speed of the sheet was of 8 m/mn.

The cathodic current yield was of 90% and the anode-cathode voltage was of 8.2 volts.

The electrolytic deposit was formed of a zinciron alloy containing 91% of zinc and 9% of iron.

This deposit was clear, even, very well covering and of a fine crystallization.

Examples 1 to 8 were carried out again simply by replacing NiSO4 by FeSO4 and CoSO4 respectively in order to produce zinc-iron and zinc-cobalt alloys. Example 10 was carried out again by replacing ions Fe++ by ions Ni++ and ions Co++ respectively and by deleting (NH4)2SO4 and ions Fe+++ in order to produce Zn-Ni and Zn-Co alloys.

For the regeneration, in case of production of zinc-iron alloy, use was made of metal zinc and of an iron and goethite mixture and also of iron alone.

In case of production of a zinc-cobalt alloy, the regeneration was carried out by means of metal zinc and basic cobalt carbonate.

Other parameters and operating conditions were maintained similar to those of these eight examples for preparing a zinc-nickel alloy.

Fig. 2 shows a block diagram of a second embodiment of a plant for continuous electrolytic deposit, with a high current density, of a layer of a zinc-based alloy on an article having a shape of band, wire, bar, tube and the like.

In this embodiment, the whole electrolyte was continuously circulated through a closed circuit 15, wherein a circulation pump 1 6 was provided, in order to obtain the necessary flow speed in the electrolysis cell 1 and so allow to obtain a sufficiently high cathodic current density.

Only a fraction of electrolyte was subjected to said regeneration in an apparatus which comprised a buffer tank 2, the latter also acting as a dissolution reactor, in a same manner as reactors 5 and 6 of the first embodiment of Fig. 1.

This tank 2 is directly connected to cell 1 by means of an intake pipe 3 and an outlet pipe 4, a circulation pump 12 and a valve 1 7 for controlling the output being provided in said pipe 4.

This allows to make flow in this separate circuit independent from flow through the electrolysis cell itself, which would have as an advantage to allow to create, for example in tank 2 thus acting as dissolution reactor, the necessary dissolution conditions for metals intended to be used for regenerating the electrolytic bath.

From the description and in particular from hereinabove given practical examples, it results that the process according to the invention has the advantage that a coating of an article having a shape of band, wire, bar, tube and the like with an alloy is obtained in only one operation, contrary to which is for example the case in known process wherein it is for example necessary to pass through a series of successive baths of coating metals which are separated by washing baths and followed by a final thermal treatment.

According to this known technique, a coating made of at least two successive layers between which a potential difference can occur is obtained.

This can thus give rise to a material migration and so to the formation of corrosion centers in the coating.

It has to be understood that the invention is not limited to hereinabove described embodiments of the process and plant according to the invention.

Thus for example in some cases, one could limit for regenerating the electrolyte to the addition, in buffer tank 2, of necessary metals to compensate bath exhaustion in the electrolysis cell without using the closed circuit 1 5 of Fig. 2, so that the whole electrolyte of the cell would pass through this tank.

However, the possibility exists that, in such an embodiment, particular cautions have to be taken in order to prevent solid particles coming from regeneration being carried away into the electrolysis cell through pipe 4.

In other respects, according to the type of desired alloy or the operating conditions, only one reactor or two or more reactors could be provided in parallel to the buffer tank 2, in combination or not with a closed circuit of the same type as circuit 1 5 of Fig. 2, in order to ensure flow inside the cell and, in case of need, a reduction reactor 18.

Finally, the process and plant according to the invention applies to coating of any type of band, wire, tube, bar and the like, made of an electricityconducting material, in particular of steel.

Claims (28)

Claims

1. A process for continuous electrolytic deposit with a high current density of a layer of a zincbased alloy on an article which has a shape of band, wire, bar, tube and the like wherein this article is moved in an electrolysis cell past an anode through an electrolyte comprising zinc sulfate, characterized in that a pH lower than 2, a temperature between 400C and 70 C and a concentration of 0.2 to 2 mol/liter of zinc sulfate and of 0.3 to 2 mol/liter of iron, nickel and/or cobalt sulfate according to the kind of coating layer to be produced are maintained in the electrolyte of the electrolysis cell.

2. A process as claimed in claim 1, characterised in that a pH lower than 1.2 and preferably lower than 0.4 is maintained in the electrolyte.

3. A process as claimed in either of claims 1 and 2, characterised in that pH is controlled in the electrolyte by addition of sulfuric acid.

4. A process as claimed in claim 3, characterised in that a concentration of sulfuric acid higher than 1 mol/liter is maintained in the electrolyte of the electrolysis cell.

5. A process as claimed in any of claims 1 to 4, characterised in that the metal concentration in the electrolyte is controlled so as to obtain a zincnickel alloy containing 2 to 14% of nickel, a zinciron alloy containing 2 to 15% of iron or a zinccobalt alloy containing 2 to 12% of cobalt.

6. A process as claimed in any of preceding claims, characterised in that an electrolytic bath which is substantially free from chlorides is used.

7. A process as claimed in any of preceding claims, characterised in that a molar ratio M++/Zn++ lower than 1.5, M being Ni or Co, is maintained in the electrolyte of the electrolysis cell.

8. A process as claimed in any of claims 1 to 6, characterised in that a molar ratio Fe++/Zn++ between 2 and 3 is maintained in the electrolyte for preparing a Zn-Fe alloy.

9. A process as claimed in any of preceding claims, characterised in that an insoluble anode formed of a lead-silver alloy the silver content of which is preferably between 0.6 and 1.2% is used in the electrolysis cell.

10. A process as claimed in any of claims 1 to 8, characterised in that a soluble zinc-based anode is used, and in case of need ions of the other metal or metals of the alloy to be produced are added to the electrolyte.

11. A process as claimed in claim 10, characterised in that a soluble anode made of an alloy having substantially the same composition as the coating layer to be produced is used.

12. A process as claimed in claim 11, characterised in that the anode is made of a Zn-Fe alloy.

13. A process as claimed in any of claims 1 to 6 and 8 to 12, characterised in that in case of formation of a coating layer of a Zn-Fe alloy, at least a portion of the electrolyte is subjected to a reduction, so as to transform ions Fe+++ into ions Fe++.

14. A process as claimed in claim 13, characterised in that a reducing agent comprising SO2 is used so as to form ions Fe++ and SQ-. and said reducing agent is regenerated, in particular by using an ion exchange resin.

15. A process as claimed in any of claims 1 to 6 and 8 to 14, characterised in that in the case of formation of a coating layer of a Zn-Fe alloy, ammonium sulfate is added to the electrolyte so as to stabilize ions Fe++.

16. A process as claimed in claim 14, characterised in that an ammonium sulfate concentration between 0.5 and 1 mol/liter is maintained in the electrolyte.

1 7. A process for continuous electrolytic deposit with a high current density of a layer of a zinc-based alloy on an article having a shape of band, wire, bar, tube and the like, wherein said article is moved in at least an electrolysis cell past an anode through an electrolytic bath comprising zinc sulfate, in particular a process as claimed in any of preceding claims, characterised in that a pH lower than 2 is maintained in the electrolyte and that some electrolyte is continuously extracted from the cell for regenerating it by dissolving in this electrolyte, in ratios corresponding to the desired alloy, on the one hand zinc and on the other hand nickel, iron and/or cobalt, according to the kind of coating layer, so regenerated electrolyte being then reintroduced also continuously into the electrolysis cell.

18. A process as claimed in claim 17, characterised in that zinc is introduced as a metallic form in the electrolyte to be regenerated.

19. A process as claimed in any of claims 17 and 18, characterised in that nickel and cobalt are introduced into the electrolyte to be regenerated as carbonates, while iron is admitted into the electrolyte to be regenerated as a metal and/or as ferric hydroxide.

20. A process as claimed in any of claims 17 to 19, characterised in that zinc and the other metal of the alloy to be produced, and used for regeneration of the electrolyte are introduced into separate electrolyte amounts, these amounts being then combined together before being reintroduced into the electrolysis cell, the ratio of these amounts being controlled in terms of the desired metal ratio in the alloy to be produced in the cell.

21. A plant for continuous electrolytic deposit with a high current density of a layer of a zincbased alloy on an article with a shape of band, wire, bar, tube and the like, comprising a cell wherein at least one said article may move continuously past an anode and through an electrolyte containing zinc sulfate, this plant being characterised in that it is provided for continuous regeneration of the electrolyte with an apparatus comprising a buffer tank directly connected to the electrolysis cell by inlet and outlet pipes.

22. A plant as claimed in claim 21, characterised in that it comprises at least a distinct dissolution reactor connected to the buffer tank and through which electrolyte coming from buffer tank may flow.

23. A plant as claimed in claim 22, characterised in that it comprises at least two dissolution reactors mounted in parallel to said tank and through each one of them electrolyte coming from buffer tank may flow, means being provided for controlling the relative electrolyte output through said reactors in terms of the metal ratio in the alloy to be produced in the cell.

24. A plant as claimed in any of claims 21 to 23, characterised in that it comprises a closed circuit of electrolyte flow on the cell, which is independent from above mentioned regeneration apparatus.

25. A plant as claimed in any of claims 21 to 24, characterised in that for forming a Zn-Fe alloy, it comprises a reduction reactor for transforming ions Fe+++ into ions Fe++ in the electrolyte.

26. An article as a band, wire, bar, tube and the like, coated with a zinc-based alloy according to the process or by means of the plant such as hereinabove described or shown by enclosed drawings.

27. A process for the continuous electrolytic deposit with a high current density of a layer of a zinc-based alloy on an article having a shape of band, wire, bar, tube and the like, such as hereinabove described or shown by enclosed drawings.

28. A plant for continuous electrolyte deposit with a high current density, such as hereinabove described or schematically shown by enclosed drawings.