EP0254703B1 - Process and apparatus for electroplating zinc on a steel strip - Google Patents

Abstract

The steel strip (2) acts as a cathode facing a series of anodes (5, 6), while a stream of electrolyte carrying zinc is circulated between the cathode and the anodes (5', 6'). Each anode (5', 6') is fed independently of the others and the quantity of current passing through each anode (5', 6') is adjusted to obtain, from each of them, the same electrolysis current density at all points, in order to obtain on the strip (2) a zinc deposit of a compact, monooriented crystalline structure. The invention also relates to an apparatus for applying this process. <IMAGE>

Description

The invention relates to a method of electrogalvanizing a steel strip (2) in which a steel strip (2) acting as a cathode is passed in front of at least one series of anodes, while that a flow of zinc-charged electrolyte is circulated between the cathode and the anodes.

The industry, in particular the automobile industry, is making use of increasingly large quantities of galvanized steel sheets.

The qualities of the electrogalvanized sheets depend on the morphology of the zinc coating, which itself depends on the composition of the electrolytic fluid, the dynamic conditions of the electrolytic fluid, the density of the current applied and the type of electrolytic cell used.

It is mainly on this morphology that the corrosion resistance qualities of these sheets depend. It is particularly interesting to produce compact mono-oriented crystal structures of the zinc deposit.

It is known to electrogalvanize a steel strip by means of an apparatus essentially comprising a rotary drum of insulating material around which the steel strip to be electrogalvanized is driven, a series of anodes disposed peripherally around of a part of the drum and a flow of electrolyte (containing zinc, in the form of salt) created between these anodes and the steel strip playing the role of cathode. In this known device, the anodes are arranged side by side and mounted in parallel.

• hétérogénéité des anodes;
• résistance électrique de la bande d'acier;
• teneur variable en oxygène du liquide électrolytique, et
• variations dans la position des anodes.

It has been found that this device does not make it possible to obtain a deposit of zinc with a compact mono-oriented crystal structure. This is due to the fact that not only is the electrolyte flow not unidirectional, but also because the distribution of electric current between anodes and cathode is not uniform. The causes of this non-uniformity are as follows:

• heterogeneity of the anodes;
• electrical resistance of the steel strip;
• variable oxygen content of the electrolytic liquid, and
• variations in the position of the anodes.

The anodes consist of a cast lead-silver alloy body with a copper core. The connection between the copper and the alloy is not always perfect, so that the resistance to the passage of current can differ from one anode to another.

The resistance of the steel strip varies throughout it, so that the current passing through the lowest placed anode meets a greater resistance than it which passes through the highest placed anode. .

Oxygen is released in large quantities on the anodes. The oxygen concentration in the electrolyte increases from the lowest anode to the highest anode and there are therefore different conditions for the current to flow from one anode to another.

The position of the anodes is obviously variable from one to the other.

Whatever the flow rate of the electrolyte flow and the way in which this is produced between the steel sheet constituting the cathode and the series of anodes, there will therefore inevitably be a variation in the conditions of deposition of zinc on the strip, from one anode to another.

US Pat. No. 3,900,383 discloses an electrogalvanizing drum provided with several conductive segments insulated from one another. These segments are arranged so as to be in electrical contact with a metal strip immersed in an electrolytic bath. A switch connects the segments one after the other to a source of electric current, so that only the segments of the drum which are fully in contact with the strip are supplied with current. This drum prevents the formation of electric arcs between the strip and the drum.

This switch can also be made up of independent cathodes connected to anodes by electrical sources, so as to allow the passage of different electrical currents between cathodes and anodes.

Thus the electrogalvanizing current can be, for example, gradually increased.

The device represented in FIG. 4 of this document US-A-3900383 comprises a switch made up of cathodes connected to anodes by adjustable electrical sources. The anodes are mounted in an electric circuit delivering in each of them an electric current, this circuit comprising a current rectifier and a voltage adjustment device.

Document US-A-4,500,400 describes a counter-current device for an electroplating apparatus. He teaches that it is preferable to use a high current density so as to obtain a high deposition speed and that if too high a current density is used problems appear at the edges of the metal strip.

The process of electrogalvanizing a steel strip in which a steel strip acting as a cathode is passed in front of at least one series of anodes, while one circulates between the cathode and the anodes a flow of electrolyte charged with zinc is characterized in that the flow of electrolyte is forced to flow between a series of anodes and the strip in a direction and at a relative speed with respect to the strip which are everywhere the same, in that each anode of said series is supplied independently of the other anodes with electric current and in that the voltage of each anode is adjusted to pass a determined quantity of electric current through each anode to obtain from each of these anodes the same density of electrolysis current at all points so as to obtain on the steel strip (2) a deposit of zinc with a compact mono-oriented crystal structure.

The installation according to the invention for continuously coating in a cell a steel strip of a zinc deposit with a compact mono-oriented structure comprises a drum around which the strip serving as cathode passes and driving it. series of anodes placed one next to the other opposite the cathode and separated from the latter by a continuous flow of electrolyte, said anodes being mounted in an electrical circuit supplying in each of them an electric current, each circuit comprising a current rectifier and a voltage adjustment device.

This installation is characterized in that the drum is made of an insulating material and in that the strip closes the electrical circuit so that the resistance to the passage of current presented by this strip varies continuously from an anode to an adjacent anode , the individual assembly making it possible to obtain that the current delivered in each branch corresponds to the same current density in each of the branches respectively comprising an anode and a corresponding portion of the steel strip which moves in front of it and in that '' a variable speed pump intended to bring the electrolyte through a convergent conduit and ramp or nozzle in the cell and / or a variable speed motor intended to drive in rotation a drum on which the band is wound up allow to adjust the relative speed of the fluid with respect to the strip so as to obtain a deposit of zinc with a compact mono-oriented crystal structure.

In a particular embodiment of the installation, the anodes are distributed around the circumference of the drum and surrounded by an external envelope so as to constitute around the drum two channels symmetrical with respect to a vertical plane passing through the axis of the drum , these channels being traversed by an electrolyte containing zinc, the supply of electrolyte being done by an upper end of one of the channels, and the evacuation of the spent electrolyte, by the end of the other.

A preferred embodiment of the installation according to the invention is an installation in which below the drum is located, between the two lower ends of the two symmetrical channels, a vane rotor driven by a motor.

The drawings attached to this memo do better understand the invention.

• figure 1, représentés schématiquement, les traits essentiels d'une installation connue;
• figure 2, une vue schématique d'une installation suivant l'invention;
• figure 3, une vue analogue à celle de la figure 2, modifiée suivant un développement de l'invention, dans une forme de réalisation préférée;
• figure 4, le détail d'un rotor à aubes et de rampes qui lui sont associées, et
• figures 5 et 6, en perspective, des formes de rampes.

We see in

• Figure 1, shown schematically, the essential features of a known installation;
• Figure 2, a schematic view of an installation according to the invention;
• Figure 3, a view similar to that of Figure 2, modified according to a development of the invention, in a preferred embodiment;
• FIG. 4, the detail of a vane rotor and of ramps associated therewith, and
• Figures 5 and 6, in perspective, shapes of ramps.

In Figure 1, we see in cross section a drum 1 of insulating material, freely rotating about its horizontal axis. Around the drum 1 is the steel strip 2 delivered by a roller 3 and returned to a roller 4 in an arrangement which makes it possible to hold the strip 2 stretched around the drum 1. In front of the drum 1, at its lower part, are located, arranged on either side and distributed peripherally in the same way, two groups of contiguous anodes 5 and 6. These anodes are supplied respectively by DC circuits 7 and 8, the sources of which are indicated by 9 and 10. The strip 2 closes the rectifier circuit 9 by its brain 2 'and the roller assembly 11, and the rectifier circuit 10 by its brain 2 "and the roller assembly 12. The lower part of the drum 1 is surrounded by an envelope 13 forming around the drum 1 a pair of channels 14 and 15 joined at their lower part. At the meeting point of the two channels 14 and 15 opens a pipe 16 for supplying the system with electrolyte. The electrolyte flows according to the arrows F₁, F₂.

As the electrodes of the input assembly 5 and the output assembly 6 are supplied in parallel, we are faced with the problems mentioned in the introduction to the description. This means that it is impossible to obtain the same current density everywhere and, consequently, that it is excluded to carry out on the steel strip a deposit of zinc with a compact mono-oriented crystal structure.

If, for example, each group of anodes 5, 6, supposed to have eight anodes, is supplied by a current of 10,000 amperes, one would expect that, in each of the branches of the circuit, leading to any anode 5 ', 6', circulates a current of 1250 amps. However, it is found that the currents flowing in the different branches present, compared to the average value of 1250 amperes, percentage deviations indicated in the second line of Table I below, the first line of this table containing the enumeration of the various anodes input 1E, 2E, etc ..., and various output anodes 1S, 2S, etc ...

It is obviously impossible to vary the flow rate of the electrolytic fluid at the right of each anode, so as to achieve a current density everywhere the same. It is therefore on the electrical circuit, according to the invention, that action must be taken to obtain a uniform current density.

In the installation according to the invention, the principle of which is shown in FIG. 2, it can be seen that instead of two groups of contiguous anodes 5 and 6, a series of anodes is provided in passages 14 and 15 5 'and 6' separated from each other, each of the anodes 5 'and 6' being mounted in a branch of the circuit 17 containing an independent current source 18, adjusted individually to obtain the same current density in each electrolysis cell , that is to say that in each branch 17 of the circuit will circulate a current of the same intensity, of 1250 amps for example. The various anodes 5 'and 6' are separated from each other by a layer of insulating material 27. The voltage of each anode is adjusted so as to allow the passage of a determined quantity of electric current through each anode to obtain from each of them the same density of electrolysis current at all points.

et plus précisément une relation de proportionnalité

${\text{i = KcR}}_{\text{e}}{\text{}}^{\text{0,7}}$

dans laquelle K est une constante dépendant de divers facteurs, notamment du type de morphologie à obtenir et de la forme géométrique des cellules, tandis que c désigne la concentration en zinc de l'électrolyte. Le nombre de Reynolds est égal à

où V_r est la vitesse relative du fluide par rapport à la bande 2; D est le diamètre hydraulique équivalent du conduit à travers lequel passe le fluide électrolytique et ν est la viscosité cinématique de ce fluide.It is known that when compact mono-oriented crystal structures are produced, there exists between the current density i and the Reynolds number R _e characterizing the flow of the fluid relative to the steel strip 2, a relationship ${\text{i = f (R}}_{\text{e}}\text{)}$

and more precisely a relation of proportionality

${\text{i = KcR}}_{\text{e}}{}^{\text{0.7}}$

in which K is a constant depending on various factors, in particular the type of morphology to be obtained and of the geometric shape of the cells, while c denotes the zinc concentration of the electrolyte. Reynolds number is equal to

where V _r is the relative speed of the fluid with respect to the strip 2; D is the equivalent hydraulic diameter of the conduit through which the electrolytic fluid passes and ν is the kinematic viscosity of this fluid.

Q est le débit du fluide électrolytique et S est la section du conduit de passage du fluide. Cette section est égale au produit de la longueur du tambour 1 et de la distance entre une anode quelconque 5', 6' et la bande d'acier 2; le signe ± est à choisir suivant le sens d'écoulement du fluide par rapport au sens du défilement de la bande 2. Ce défilement a lieu à une vitesse indiquée par V_D.We obviously have:

Q is the flow rate of the electrolytic fluid and S is the section of the fluid passage duct. This section is equal to the product of the length of the drum 1 and the distance between any anode 5 ', 6' and the steel strip 2; the sign ± is to be chosen according to the direction of flow of the fluid with respect to the direction of travel of the strip 2. This travel takes place at a speed indicated by V _D.

qui montre que, toutes choses étant égales par ailleurs, on pourra agir sur la grandeur du débit Q pour obtenir une structure cristalline mono-orientée compacte déterminée.So ultimately we have a relation of the form $\text{i = f (Q)}$

which shows that, all other things being equal, we can act on the quantity of the flow rate Q to obtain a determined compact mono-oriented crystal structure.

soit partout satisfaite, il est indispensable que le sens de l'écoulement du flot d'électrolyte autour de la bande servant de cathode soit partout le même, et que la vitesse relative de ce flot par rapport à la bande 2 soit aussi partout la même.So that the condition ${\text{i = Kc (R}}_{\text{e}}{\text{)}}^{\text{-0.7}}$

either everywhere satisfied, it is essential that the direction of flow of the electrolyte flow around the strip serving as the cathode is everywhere the same, and that the relative speed of this flow with respect to the strip 2 is also everywhere the same.

In an installation such as that of FIG. 2 where there is, on either side of the drum 1, each time a channel for the electrolyte, it is therefore necessary that one of these channels, 14, for example, is supplied with electrolyte at its upper part and the spent electrolyte escapes from the other channel 15 through its upper end. However, unless there are special provisions, the flow conditions in the second channel may not be equal to those prevailing in the first. To overcome this drawback, the system can be produced according to FIG. 3 where it can be seen that below the drum 1, between the lower ends of the channels 14, 15, a rotor 19 with blades 22 has been installed. This rotor the shaft 20 of which is driven at the end by a variable speed motor 21 (see FIG. 4) comprises symmetrical radial blades 22, the distance between the ends of the blades 22 and the periphery of the drum 1 being the same as the distance from the anodes 5 ′, 6 ′ to this drum 1. The function of this rotor 19 is to provide the electrolyte which descends along one of the channels, for example 15, with additional energy to cause the electrolyte to rise in the other channel, in this case 14, the blades 22 being designed to distribute this additional energy uniformly over the entire width of the channel and guarantee a homogeneous circulation of the electrolyte downstream of the rotor 19, thereby creating symmetrical flow conditions ( see arrows F and F "in FIG. 3) relative to a vertical plane passing through the axis of the drum 1. The lower ends of the channels 14, 15 are limited, on the side opposite that of the drum 1, by deflecting surfaces or ramps 23, 24 whose extreme edge, such as 25 in FIGS. 4 and 5, extends parallel to the axis 20 of the rotor 19 and over the same length as the latter. This edge 25 can advantageously be wavy (FIG. 6). The arrangement of these ramps 23, 24 is still symmetrical with respect to the vertical plane passing through the axis 20 of the rotor 19 and through the axis of the drum 1, so that the operation of the device is the same, regardless of the direction of rotation of the drum 1. Depending on the direction of inclination of the limit surface 23 ′, 24 ′ of the ramps 23, 24, the speed of flow of the liquid downstream of these can be increased or decreased.

Although it is possible to give the electrolyte a flow direction identical to the direction of rotation of the drum 1, experimental studies have shown that it is advantageous, in order to have a suitable deposit, to give the electrolyte a reverse flow direction (see arrows F and F "in FIG. 3) of the direction of rotation of the drum 1 (arrow F ′), a condition which is achieved for example in FIG. 3. It has also been found that, unlike to what has been said above, it may be advantageous to inject electrolyte into the channels using a ramp or converging nozzles (26, FIGS. 2 and 3), distributed over the length of the drum 1, in order to give the liquid a speed greater than the speed of movement or movement of the strip 2 playing the role of cathode.

It is obvious that the ramp or the converging nozzles 26 can be mounted on the left side of the drum 1 rather than on the right side of the drum as shown in FIGS. 2 and 3.

Finally, experience has also shown that it is necessary, under certain conditions which are a function of the width of the strip 2 and of its running speed, to rotate the rotor 19 in the opposite direction to that of the flow of the liquid in channels 14, 15.

In both cases, the rotor 19 intervenes to uniformly distribute the additional positive or negative energy which must be communicated to the electrolyte to guarantee a relative speed uniformly distributed downstream of the rotor 19, to satisfy, according to the tape running speed and the current density, under the conditions of a zinc deposit of the compact mono-oriented type.

• à obtenir une même densité du courant d'électrolyse en tous points grâce à une alimentation indépendante de chaque anode 5', 6' en courant électrique, et au réglage de la tension de chaque anode et
• à régler la vitesse relative du fluide par rapport à la bande 2 pour réaliser des structures cristallines mono-orientées compactes.

The process according to the invention therefore consists:

• to obtain the same density of the electrolysis current at all points by means of an independent supply of each anode 5 ′, 6 ′ with electric current, and by adjusting the voltage of each anode and
• to adjust the relative speed of the fluid with respect to the strip 2 to produce compact mono-oriented crystal structures.

• en changeant le débit d'électrolyte dans les passages 14, 15, c'est-à-dire en changeant la vitesse de rotation de la pompe 28 (voir figures 2 et 3) amenant l'électrolyte par le conduit 29 et la rampe ou l'ajutage convergent 26 dans la cellule et/ou
• en changeant la vitesse de défilement de la bande d'acier 2, c'est-à-dire en changeant la vitesse du moteur 30 entraînant en rotation un tambour 31 sur lequel vient s'enrouler la bande d'acier 2 (voir figure 3).

The relative speed of the fluid with respect to the strip 2 can be modified:

• by changing the electrolyte flow in the passages 14, 15, that is to say by changing the speed of rotation of the pump 28 (see FIGS. 2 and 3) supplying the electrolyte through the conduit 29 and the ramp or the converging nozzle 26 in the cell and / or
• by changing the running speed of the steel strip 2, that is to say by changing the speed of the motor 30 driving in rotation a drum 31 on which the steel strip 2 is wound up (see FIG. 3 ).

Claims (14)

1. Process for electro-plating zinc on a steel strip (2) in which a steel strip (2) acting as cathode runs in front of at least one serie of anodes (5, 6, 5', 6') while a flow of electrolyte loaded with zinc circulates between the cathode and the anodes (5', 6'), characterized in that the electrolyte flow is obliged to circulate between one serie of anodes and the strip in a direction and with a relative speed with respect to the strip which are everywhere the same, in that each anode of said serie (5', 6') is independently fed of the other anodes with electric current and in that the voltage of each anode is regulated so that a determined amount of electric current passes through each anode (5', 6') so as to obtain from each of said anodes an electrolysis current density which is identic everywhere so as to obtain on the steel strip (2) a zinc deposit having the compact mono oriented crystalline structure.
2. Zinc electro-plating plant for continuously coating, according to the process according to anyone of the preceding claims, in a cell a steel strip (2) with a deposit of zinc having the compact mono oriented structure, this plant comprising a drum around which the strip (2) acting as cathode runs while driving it and a serie of anodes located the one near the other and in front of the cathode from which it is separated by a continuous flow of electrolyte, said anodes being connected to an electric circuit which feeds each of them with an electric current, each circuit comprising a current rectifier and a device for regulating the voltage, characterized in that the drum is made of an insulating material and, in that the strip (2) closes the electric circuit, the individual assembly allowing to obtain that the current fed in each branch (17) corresponds to the same current density for each branch comprising respectively an anode (5', 6') and the corresponding part of the steel strip (2) which is moved in front of it and, in that a varying velocity pump intended to bring the electrolyte by a pipe (29) and a ramp or convergent nozzle (26) in the cell and/or a varying speed motor intended to rotate a drum on which the strip (2) is rolled up allow to adjust the relative speed of the fluid with respect to the strip (2) so as to obtain a deposit of zinc having the compact mono oriented structure.
3. Plant according to claim 2, characterized in that the anodes (5', 6') are separated from each other by a layer of insulating material (27).
4. plant according to claim 2, characterized in that the anodes (5', 6') are distributed around the circumferency of the drum (1) and surrounded by an outside envelope (13) so as to form around the drum (1) two channels (14, 15) which are symmetric with respect to a plane passing through the axis of the drum (1), an electrolyte which contains zinc flowing in these channels, the feed of electrolyte being made at an upper end of one channel (14) and the outlet of the used electrolyte being made at the end of the other channel (15).
5. Plant according to claim 4, characterized in that under the drum (1) and between the lower ends of the symmetric channels (14, 15), there is a rotor (19) with blades (22) driven by a motor (21).
6. Plant according to claim 5, characterized in that the lower ends of the channels (14, 15) are bound on one side by the drum (1) and on the other side by ramps (23, 24).
7. Plant according to claim 6, characterized in that the extreme edge (25) of the ramps (23, 24) located on the side which is nearest to the rotor (19) is undulated.
8. Plant according to claim 7, characterized in that the blades (22) of the rotor (19) are radial.
9. Plant according to claim 5, characterized in that the ends of the blades (22) and the surfaces of the anodes (5', 6') directed towards the drum (1) are at a same distance from the circumference of the drum (1).
10. Plant according to claim 4, characterized in that the feed of the channels (14, 15) is made by injection of electrolyte in these channels by means of ramps or convergent nozzles (26).
11. Plant according to anyone of the claims 2 to 10, characterized in that the strip (2) acting as cathode is moved in the same direction than that of the flow of the electrolyte along the strip (2).
12. Plant according to anyone of the claims 4 to 10, characterized in that the strip (2) acting as cathode is moved in a direction opposed to that of the flow of electrolyte along the strip (2).
13. Plant according to anyone of the claims 5 to 12, characterized in that the rotor (19) rotates in the same direction than the drum (1).
14. Plant according to anyone of the claims 5 to 12, characterized in that the rotor (19) rotates in a direction opposed to that of the drum (1).

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