EP0969124A1 - Process and apparatus for the deposition of a zinc-nickel alloy on a substrate - Google Patents

Abstract

The substrate is moved with respect to an anode in an electrolyte containing zinc sulfate, nickel sulfate and sulfuric acid. The concentrations of zinc, nickel and sulfuric acid are 50-80, 70-120 and 5-20 g/l, respectively, and the electrolyte is circulated at 0.5-10 m/s. The temperature of the electrolyte is 50-70 degrees C, preferably 55-63 degrees C, and the electrolyte is substantially chlorine-free. Preferably, the concentrations of zinc, nickel and sulfuric acid are 50-65, 70-90 and 5-15 (more preferably 8-12) g/l, respectively. The nickel/zinc ratio is 1-2, particularly 1.2-1.8, and more preferably 1.25-1.5. Preferably, the electrolyte is circulated at a rate of 1.5-1.7 (more preferably 2.5-5) m/s. Sodium concentration in the electrolyte is kept at 10 g/l, preferably 5 g/l. Current density between the substrate and the anode is maintained at 40-200, preferably 60-150, more preferably 70-140, A/dm<2>. A turbulent substantially homogeneous flow of electrolyte is directed against the anode face in a direction perpendicular to the direction of displacement of substrate in the electrolyte. An Independent claim is given for the electrolyte cell used to implement the above process.

Description

The present invention relates to a method for continuous electrolytic deposition of a zinc and nickel alloy on an electrically conductive substrate, such as a sheet of steel, moving opposite an anode in an electrolyte containing zinc sulfate, nickel sulfate and sulfuric acid.

In the hitherto known methods, making use of an electrolytic bath of the aforementioned type, a current density of at most 108 A / dm ² can be achieved on continuous plating lines. This limit imposes a relatively significant brake on productivity and is located relatively far from what could be supported by current electrogalvanizing lines. In addition, the cathodic yields at these current densities are generally below 85%.

Furthermore, in these known methods, a problem arises poses to properly control the homogeneity of the deposits of a zinc and nickel alloy as well as to guarantee the characteristics grip.

One of the essential aims of the present invention is to propose a method to remedy the various problems cited above.

Thus, the invention relates more particularly to a productivity increase of at least 40% by increasing significantly the current density while improving efficiency cathodic beyond 90%.

To this end, according to the invention, it is maintained in the bath electrolytic, a zinc concentration of 50 to 80 g / l, a nickel concentration from 70 to 120 g / l and an acid concentration sulfuric from 5 to 20 g / l and the electrolyte is circulated relative to the substrate at a speed of 0.5 to 10 m / s.

Advantageously, the electrolyte is maintained at a temperature from 50 to 70 ° C.

According to a particularly embodiment advantageous, use is made of an electrolyte substantially free of chloride and / or sodium.

According to a preferred embodiment of the invention, it is creates a turbulent and substantially homogeneous electrolyte current against the face of the substrate directed towards the anode in a direction perpendicular to the direction of movement of the substrate in the electrolyte.

The invention also relates to an electrolysis cell, in particular for the implementation of the above method.

This cell includes a fixed anode, means for moving an electrically conductive substrate, forming cathode, we look at this anode, and means to circulate the electrolyte in a space that looks like a slot between the substrate and the anode and having a substantially constant thickness.

This electrolysis cell is characterized by the fact that it includes an injector arranged at at least one of the open ends of this space and extending over substantially the entire width of this last, this injector being such as to be able to distribute the electrolyte in the form a relatively thin and substantially homogeneous layer forming turbulent current over the entire length and width of the space separating the anode from the substrate.

La figure 1 est une vue schématique et en coupe des parties essentielles d'une cellule radiale d'électrolyse pouvant être utilisée pour l'application du procédé suivant l'invention.

La figure 2 est, à plus grande échelle, une vue de face, suivant la ligne II-II de la figure 1 d'une partie essentielle de la cellule radiale, telle que montrée en coupe à la figure 1.

La figure 3 est une représentation sous forme d'un graphique d'un exemple de répartition de vitesse de l'électrolyte entre le substrat et l'anode.

La figure 4 est une vue schématique d'une cellule d'électrolyse associée à une installation de régénération de l'électrolyte.

Other details and particularities of the invention will emerge from the description, given below, by way of nonlimiting example of some particular embodiments of the method and of the device according to the invention, with reference to the attached drawings.

Figure 1 is a schematic sectional view of the essential parts of a radial electrolysis cell that can be used for the application of the method according to the invention.

FIG. 2 is, on a larger scale, a front view, along line II-II of FIG. 1, of an essential part of the radial cell, as shown in section in FIG. 1.

FIG. 3 is a graphical representation of an example of the speed distribution of the electrolyte between the substrate and the anode.

FIG. 4 is a schematic view of an electrolysis cell associated with an installation for regenerating the electrolyte.

In the different figures, the same figures of reference relate to identical or analogous elements.

The method according to the invention relates first of all to a electrolytic process for the continuous deposition of an alloy of zinc and nickel on an electrically conductive substrate, in particular a strip continuous sheet steel, from an electrolysis bath maintained at a temperature of 50 to 70 ° C and consisting essentially of sulphate zinc with a zinc concentration of 50 to 80 g / l, nickel sulfate with a nickel concentration of 70 to 120 g / l and sulfuric acid with a concentration of 5 to 20 g / l.

This electrolyte is put in circulation in the bath with a relative speed from 0.5 m / s to 10 m / s relative to the substrate.

The electrically conductive substrate moves at a certain speed we look at a fixed anode, insoluble in the electrolyte. This speed can generally vary between 0 and 200 m / min, from preferably between 100 and 200 m / min. Regarding the anode ,. he may, for example, be an anode on platinum, made of iridium oxide or Stainless steel.

Thus, the speed of circulation of the electrolyte we look at the substrate depends on the running speed of the substrate and the direction according to which this circulation takes place.

Advantageously, the relative speed of the electrolyte by ratio to the substrate is 1.5 to 7 m / s with an optimum between 2.5 and 5 m / s.

Regarding the composition of the electrolytic bath, this is a sulfate electrolyte which advantageously consists of 50 to 65 g / l of zinc, 70 to 90 g / l of nickel and 5 to 15 g / l of acid sulfuric, preferably 8 to 12 g / l sulfuric acid.

In addition, the sodium content is at most 10 g / l and advantageously less than 5 g / l with a pronounced preference for a sodium-free electrolytic bath.

Furthermore, using suitable instruments, known per se and not shown in the figures, a nickel / zinc ratio is maintained between 1 and 2, more particularly between 1.2 and 1.8 with a preferably in the range of 1.25 to 1.5 which gives a deposit having excellent properties.

By the process according to the invention, the current density can vary from 40 to 200 A / dm ² on an industrial line, but is generally between 60 and 150 A / dm ² , giving yields greater than 90% for a density between 70 and 140 A / dm ² .

By making use of an electrolytic bath, as defined above, subjected to a uniformly distributed flow velocity and constant between the fixed anode and the substrate moving at a distance constant opposite this anode and forming cathode, it is possible to deposit a zinc-nickel alloy on the surface of the substrate directed to the anode containing between 10.5 and 13% of nickel and what whatever the thickness of the layer of this deposit, which can vary between wide limits, in particular from 1 to 10 µm.

For regeneration of the electrolyte and maintain permanently the composition of zinc sulfate, nickel sulfate and sulfuric acid within the limits given above, use is made of metallic zinc particles dissolved by the addition of a catalyst, such as chromium and molybdenum ions in the form of chromium sulfate and sodium molybdate.

In this regard, according to the invention, it has been found that obtains a rapid zinc enrichment of the electrolytic bath despite the presence of nickel sulfate acting as an inhibitor in the reaction of zinc dissolution.

The molybdenum content is generally maintained at a concentration below 50 ppm and that of chromium at a concentration below 50 ppm.

As indicated above, the zinc is dissolved in granules of metallic zinc while the nickel is measured in the form of a hydrated NiCO ₃ paste.

The attached figures relate to a cell which is particularly suitable for the application of process as described above.

It is a radial cell comprising a drum 1 rotating around a horizontal axis 2 whose cylindrical wall 3 is made of an electrically conductive material, such as steel stainless steel.

This cylindrical wall 3 acts as a cathode and has been set to a negative potential.

From a deflector roller 4, the substrate 5, on which the deposit of the zinc-nickel alloy must be formed and which is generally made of sheet steel, is applied against the lower part of the surface of the cylindrical wall 3 rotating in the direction of arrow 6 to through an electrolyte 7 contained in a semi-cylindrical tank 8 made of an electrically conductive material and insoluble in electrolyte 7. This tank forms an anode and extends coaxially around the drum 1 at a constant distance from the cylindrical wall 3 of this last, for example of the order of 2 to 5 cm, thus sparing between this tray 8 and the substrate 5 a space 10 in the form of a slot.

Means are provided for circulating the electrolyte to through this space 10. This circulation can be co-current, counter-current or even co-current and counter-current with respect to the direction of displacement of the substrate 5 in this space 10.

Preference is however given to traffic counter current as shown by arrow 11 in Figure 1. At this regard, the electrolyte 7 is injected at high speed using injectors specials 9 in this space 10 where the substrate 5 leaves the tank 8 to be recovered, by overflow, in a collector 12 at the edge opposite of tank 8 and then be recirculated, by a pipe and a system pumping, towards the injector 9.

This injector 9 comprises a tubular element, formed of a horizontal supply pipe 13, one end of which 14 is connected to an inlet for recycled electrolyte and the other end of which 15 is closed. This supply pipe 13 is connected laterally to an elongated buffer box 16, substantially the same length as the supply pipe 13 and extending along the latter, by tubes 13 'connector distributed at constant distances between the pipe supply 14 and the box 16 along their length. Box 16 includes a funnel-shaped portion 16 'also extending over its entire length. The bottom of this part 16 ′ has a slot of distribution, forming an injection head, through which the electrolyte can leave the box 16 in the form of a homogeneous blade. In this way, an electrolyte flow is created at a speed and a flow rate constants at all points of this slit over the entire width of the space 10 formed between the drum 1 and the electrolyte tank 8.

Figure 3 is a graph showing the distribution of speed over the width of space 10. On the ordinate is given the speed relative in m / min of a substrate formed by a continuous strip and, in abscissa, the width of this strip. The ordinate indicates the axis 26 of the strip, while the two dashed lines 29 on both sides of this axis 28 and parallel to the latter indicate the two banks of the bandaged

Figure 4 is a schematic representation of a electrolysis cell combined with a regeneration plant the electrolyte.

This installation includes two separate reactors 17 and 18 each connected to a storage tank 27 by a pipe 19 respectively 21 to which a flow pump 20 is connected variable, respectively 22.

Reactor 17 contains for example conditioned zinc in the form of granules of given particle size. By means of the pump 20, the electrolyte from the storage tank 27 is pumped in the reactor 17 and is recirculated to this tank 27 by a pipe 23.

With the addition of a combination of chromium, under in the form of sulphate, and of molybdenum, in the form of sodium molybdate, the zinc is dissolved, despite the presence of nickel sulfate acting as a reaction inhibitor. This way we get a permanent and regular enrichment on zinc ions of the electrolysis bath in storage bin 27.

In the second reactor 18, which is also connected to the storage tank 27 by a line 21 to which is connected a pump 22, electrolyte depleted in zinc and nickel is introduced and nickel is carefully dissolved therein in the form of hydrated NiCO ₃ paste.

As for reactor 17, the electrolyte thus enriched in nickel is recycled to the storage tank 27 through a pipe 24.

Thanks to the fact that relatively intense traffic is created through the tank 8 by the injector 9, which introduces the electrolyte to high speed in space 10, and by the manifold 12, which is connected to this injector 9 via a line 25 to which a pump is connected 26, a homogeneous composition of the electrolyte is maintained in space 10.

Below are given concrete examples of implementation to further illustrate the process according to the invention.

Example 1

This example was carried out in a radial cell, of the type as shown in the figures, on a steel strip with a thickness of 0.8 mm and a width of 1256 mm.

The steel used for this test had a yield strength of 140 N / mm ² . Work settings Zn ²⁺ concentration 57 g / l Ni ²⁺ concentration 82 g / l J in A / dm ² 140 Additives other than Zn and Ni No Electrolyte flow about 4 m / s with homogeneous distribution with controlled turbulence and against the current. Temperature 60 ° C Results Actual yield from 82 to 90% depending on the temperature for layer weights of 40 g / m ² and from 90 to 97% in 20 g / m ² % Ni 12.3 Crystal texture Texture range Ni ₅ Zn ₂₁ Grip Excellent, nothing comes off by 180 ° bending test "Diamond shot testing" excellent without pre-coating (maximum 2.5 mm ² of paint removed)

Salt spray resistance according to ASTM B117-90: More than 80 h / µ.

Example 2

This example was applied in a vertical cell with an insoluble titanium iridium oxide anode on the side of a steel strip intended to form the visible face of an automobile body. Work settings Cell type Vertical (co-current) Zn ²⁺ concentration 62 g / l Ni ²⁺ concentration 92 g / l J in A / dm ² 140 Flux 3 m / s Acidity 11.5 g / l Temperature 60 ° C Results Yield 98% for 20 g / m ² , 90% for 40 g / m ² % Ni 11.1 for 20 g / m ² , 12.5 for 40 g / m ² Crystal texture Gamma texture Ni ₅ Zn ₂₁ Grip Excellent, nothing comes off by 180 ° bending test "Diamond shot testing" excellent without pre-coating (maximum 3.2 mm ² of paint removed)

Example 3

This example was also carried out in a vertical cell, this time on a strip of rephosphorus steel. Work settings Cell type vertical Zn concentration 52 g / l Ni concentration 71.5 g / l J in A / dm ² 60 Sodium 0 g / l Flux 1 m / s (co-current) Temperature 60 ° C. Results Yield 94% for a shower weight of 40 g / m ² %Or 11.9 Texture Ni ₅ Zn _{21 range} Grip Successful stamping tests

Salt spray resistance according to ASTM B117-90: More than 80h / µ.

It is understood that the invention is not limited to a process applied in a specific electrolysis cell but can principle be carried out in any type of electrolysis cell suitable for high current density electrolysis.

Claims (13)

Method and device for continuous deposition by channel electrolytic alloy of zinc and nickel on a conductive substrate of electricity, such as a sheet of steel, moving opposite an anode in an electrolyte containing zinc sulfate, nickel sulfate and sulfuric acid, characterized in that the concentration of zinc from 50 to 80 g / l, that of nickel from 70 to 120 g / l and that of acid sulfuric from 5 to 20 g / l and in that the electrolyte is circulated by relative to the substrate at a speed of 0.5 to 10 m / s. Method according to claim 1, characterized in that the electrolyte is maintained at a temperature of 50 to 70 ° C. Process according to either of Claims 1 and 2, characterized in that use is made of a substantially electrolyte chloride-free. A method according to any of claims 1 to 3, characterized in that the acid concentration is maintained sulfuric between 5 and 15 g / l and preferably between 8 and 12 g / l. A method according to any of claims 1 to 4, characterized in that the zinc concentration of the electrolyte between 50 and 65 g / l and that of nickel between 70 and 90 g / l. Method according to claim 5, characterized in that a nickel / zinc ratio of between 1 and 2, more particularly between 1.2 and 1.8 and preferably between 1.25 and 1.5. A method according to any of claims 1 to 6, characterized in that the temperature of the electrolyte is maintained between 55 and 63 ° C. A method according to any of claims 1 to 7, characterized in that one maintains in the electrolyte a sodium concentration less than 10 g / l and preferably less than 5g / l. Process according to any one of Claims 1 to 8, characterized in that a current density between the substrate and the anode is maintained from 40 to 200 A / dm ² , more particularly from 60 to 150 A / dm ² and preferably from 70 to 140 A / dm ² . Method according to any of claims 1 to 9, characterized in that the electrolyte is circulated at a speed of 1.5 to 7 m / s and preferably between 2.5 and 5 m / s. Method according to any of claims 1 to 10, characterized in that a turbulent electrolyte current is created against the face of the substrate directed towards the anode and substantially homogeneous in a direction perpendicular to the direction of displacement of the substrate in the electrolyte. Electrolyte cell, in particular for use the method according to any one of claims 1 to 11, comprising a fixed insoluble anode, means making it possible to move an electrically conductive substrate, forming a cathode, opposite of this anode, means being provided for circulating the electrolyte in a slot formed between the anode and the substrate, substantially parallel to the direction of movement of the latter, characterized in that it comprises at least one injector arranged at least one ends of this slot and extending over substantially the entire width of the latter, this injector being such as to be able to distribute under forms a blade of the electrolyte in a substantially homogeneous manner forming a turbulent current over the entire length of this slit separating the anode from the substrate. Cell according to claim 12, characterized in that that the injector comprises a tubular element, one of the ends of which is connected to an inlet for the electrolyte and the other end of which is closed, this tubular element being connected laterally to a box elongated pad extending along this tubular element, by connecting tubes distributed along this element and the next box their length, a distribution slot extending parallel to the axis of this box through which the electrolyte can leave the box in the form homogeneous blade at constant electrolyte speed and flow in all points of this slot.

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