Method and apparatus for the electrolytic coating of one side of a moving metal strip

Abstract: In the electrolytic coating of one side of a moving metal strip (1), the strip as cathode is in contact with a rotating roller (3) and an insoluble anode (4) is concentric with the roller (3) at a distance from the strip (1) so that a slot (5) is formed. The electrolyte flows through the slot (5) at a sufficient average velocity that turbulent flow occurs, and is fed from a nozzle (9) as a fluid jet into the slot (5) opposite to the direction of travel of the strip (1) at the end of the slot (5) at which the strip (1) exits. To improve efficiency, the conformation of the nozzle (9) is substantially uniform across the whole width of the strip and the electrolyte is fed into the slot at a velocity that nowhere varies more than 10% from the said average velocity of the electrolyte.

Description: The invention relates to a method for electrolytic coating of one side of a moving metal strip, wherein the strip as cathode is in contact with a rotating roller and an insoluble anode is positioned concentrically with the roller over a part of the circumference of the roller at a distance from the strip so that a slot is formed in which the electrolytic coating takes place, the electrolyte flowing through the slot at a sufficient average velocity that turbulent flow occurs, and the electrolyte being fed from a nozzle as a fluid jet into the slot with a tangential component opposite to the direction of travel of the strip at the end of the slot at which the strip exit and to an apparatus for electrolytic coating of one side of a moving metal strip, comprising a rotatable roller around which, in use, the strip passes, an insoluble anode concentric with said roller and providing a circumferential slot between the strip and the anode, means including an inlet nozzle for feeding electrolyte into said slot to achieve generally circumferential flow along the slot from the strip exit end thereof at an average velocity such that turbulent flow occurs and means for supplying electrical current to said strip as cathode and said anode to cause electrolytic coating. EP-A-125707 describes an electrolyte coating method in which the moving metal strip as cathode is in contact with an electrically conductive outer surface of a rotating cathode roller and an insoluble anode is positioned concentrically with the roller over a part of the circumference of the roller at a distance from the strip. A slot is thus formed over that circumference part into which electrolyte is fed and in which the coating takes place, the electrolyte flowing generally through the gap at an average velocity such that turbulent flow occurs. The electrolyte is fed as a fluid jet into the gap at one of its ends with a tangential component relative to the path of the strip. This method of electrolyte coating strip has a number of advantages compared with other known methods. EP-A-282980 discloses a similar apparatus, in which the electrolyte is fed in at the strip exit end of the slot. Where the current is fed to the strip via the roller, it does not need to be led with resistance losses along the strip, as is the case with flat, vertical or horizontal cells, but rather it may be transferred directly from the cathode roller to the strip; this advantage is of particular importance for thin strips such as for example when plating tinplate with a thickness of for example 0.17 mm. A second advantage is that (in contrast with flat, vertical or horizontal cells where the strip is led between two anodes positioned at a distance from the strip) the path of the strip is fixed, because the strip is taken around the cathode roller. This means that the gap between the strip and the anode varies less during coating, especially if the anode is an insoluble one, thereby achieving a more uniform thickness of the coating layer. In spite of the above mentioned advantages it has been found from experiments carried out by the applicant on the method of EP-A-125707 that it has a number of disadvantages. First of all the uniformity of the thickness of the coating layer is not satisfactorily across the width of the strip. Secondly, under certain conditions the efficiency of the known method may be very low especially at somewhat higher strip speeds. These disadvantages will be further illustrated below. One object of the invention is to provide an improved method and apparatus in which a better uniformity of the thickness of the coating layer may be obtained. Another object of the invention is to create a method which has a high efficiency under any conditions. In accordance with the invention the average relative velocity of the electrolyte compared with the strip in the slot is at least 5 m/sec and the electrolyte is fed into the slot at a velocity that nowhere varies more than 10 % from the said average velocity of the electrolyte from a nozzle which has a conformation which is substantially uniform across the whole width of the strip, which has a slot-shaped outlet mouth which is open uninterruptedly across the whole width of the strip and is of uniform width across the whole width of the strip and which is connected to a supply vessel extending across the width of the strip, which vessel has a large volume relative to the volume of the nozzle and is supplied with electrolyte by means of a plurality of conduits distributed across the width of the strip. The electrolyte is fed in at that end of the slots where the strip exits, with a tangential component opposite to the direction of travel of the strip. This arrangement optimises the electrolyte flow conditions into the slot between the strip and the anode, whereby a very uniform thickness of the coating layer across the width of the strip and high efficiency of the coating process is obtained. In addition the pumping energy needed for feeding the electrolyte into the slot can be low. The average relative velocity of the electrolyte in the slot is preferably at least 5 m/sec and still more preferably at least 7 m/sec. The advantage of this is that high current densities may be used when coating so that the apparatus used for coating may be compact. Preferably the nozzle has a slot-shaped outlet mouth which is open substantially uninterrupted across the width of the strip and is of uniform width across the width of the strip. The nozzle may be a conveying nozzle. Suitably, the nozzle is supplied from a vessel extending across the width of the strip, which vessel has a large volume relative to the volume of the nozzle and is supplied with electrolyte by means of plurality of conduits distributed across the width of the strip. In this case, it is preferable that the discharge directions of the conduits are not aligned with the nozzle and that a core body should be fitted in the vessel. Furthermore, the nozzle makes an acute angle α with the tangential direction of the slot, which angle is preferably less than 45°, and still more preferably about 30°. The feed of the supply vessel for the nozzle through a number of conduits gives reduced yet still considerable variations in velocity in the vessel. By directing the supply flows from the conduits towards a closed side of the vessel, these variations are damped out. For example the feed conduits are positioned at right angles to the outlet opening of the vessel to the outlet opening of the vessel. The velocity variations are also reduced by partially filling the vessel with the core body. In the vessel the flow velocities are relatively low because of the comparatively large volume of the vessel. This means that the velocity variations become proportionately smaller. Also the non-radial velocity components in the vessel are smaller. which means that a uniform quantity distribution occurs across the outlet opening. The velocity variations are further reduced in the nozzle. The electrolyte is also injected into the slot by the nozzle at a small angle. The small angle and the narrowing of the nozzle close to where the electrolyte comes out produce a small under-pressure in the exit opening of the strip thus reducing leakage of the electrolyte through that exit opening. With the method in accordance with the invention and for an 850 mm wide strip, a uniform velocity can be attained which does not deviate more than +6 % and -7 % from the average velocity. In another aspect the invention is embodied in an apparatus on which said nozzle has a substantially uniform conformation across the width of the strip, has a slot-shaped outlet mouth which is open uninterruptedly across the whole width of the strip and is of uniform width across the whole width of the strip and is connected to a supply vessel extending across the width of the strip, which vessel has a large volume relative to the volume of the nozzle and plurality of conduits distributed across the width of the strip for supply with electrolyte so that the electrolyte is fed into the slot at a velocity which nowhere deviates by more than 10 % from the average velocity of the electrolyte in the slot. The invention will now be illustrated by way of a non-limitative embodiment described below with reference to the drawings, in which; Fig. 1 shows schematically a radial jet cell embodying the invention for use in the method embodying the invention, Fig. 2 is a cross-section of the slot of the cell of Fig. 1, Fig. 3 is a view corresponding to arrow III of Fig. 2, Fig. 4 is a graph with experimental results relating to the coating weight, and Fig. 5 is a graph giving a line of action of the method in accordance with the invention at optimum process efficiency. In the schematic drawing of the radial jet cell of Fig. 1 a metal strip 1 is shown which is in contact with an electrically conductive part 2 of the outer surface of a rotating cathode roller 3 as it is led through a slot 5 formed by the insoluble anode 4 concentric with the roller 3, in the direction indicated by arrows. The cathode roller 3 is connected to the negative terminal and the anode to the positive terminal of a source of rectified voltage. The electrolyte is fed at en acute angle α (see Fig. 2) into the slot 5 from a vessel 8 extending across the whole width of the strip 1 and provided with a central core body 7 through a slit-shaped converging nozzle 9 as a liquid jet distributed uniformly across the width of the strip at the strip exit end of the slot, in such a way that a tangential component is obtained opposite to the direction of travel of the strip. An average velocity in the gap is achieved such that turbulent flow occurs. The electrolyte is fed into the vessel 8 through four feed pipes 6 spaced across the width of the strip and out of line with the nozzle 9. The nozzle 9 has an outlet mouth of uniform width and open uninterruptedly across the width of the strip 1. After it has passed through the slot 5, the electrolyte is discharged through a duct 10, and then the metallic ion concentration in the electrolyte is brought back to the desired level (this is not shown in drawing) and finally the electrolyte is pumped again through the feed pipes 6. Fig. 2 shows that the pipes 6 are not aligned with the nozzle 9, but are at right angles to it. At the same time Fig. 2 shows that the nozzle 9 joins the slot 5 at an acute angle α; the angle α shown is 30°. Furthermore, Fig. 2 shows that the volume of the vessel 8 is large compared with the volume of the nozzle 9. Fig. 2 also shows that the nozzle 9 is connected leak-free to the anode 4 at the exit end of the slot 5. Finally, Fig. 2 shows the exit opening 11 of the strip at the nozzle. In this, a small under pressure is generated through the nozzle because of the small angle α, thus limiting leakage of the electrolyte through the exit opening. Fig. 4 shows some experimental results relating to the coating weight in tinplating. The graph gives vertically the recorded coating weight W m and horizontally the theoretical coating weight W t . The results relate to trials in which the direction of flow of the electrolyte into the gap was the same as the direction of travel of the strip, that is to say as in the process of EP-A-125707, and using various combinations of strip and electrolyte velocities. It was found that with many combinations the recorded coating weight which means that the efficiency of the coating process is high. However, with certain combinations (in the cross-hatched area) the recorded coating weight is much lower than the theoretical coating weight; there the efficiency of the coating weight is 50 % and less. It was found that this low efficiency occurs with combinations in which the average velocity of the electrolyte V₁ is roughly as high as the strip velocity V b , that is to say where V₁/V b is about 1, or in other words within the range set out in EP-A-125707. It was found from these experimental results that the relative velocity of the electrolyte compared with the strip in an important parameter in the coating process and one which should not be too small. In the present invention, by selecting the direction of flow of the electrolyte a low relative velocity of the electrolyte is avoided. Fig. 5 shows a correlation of experimental results concerning the method in accordance with the invention in tinplating with a coating process efficiency of 95 % and above under equal conditions of concentration and temperature of the electrolyte. It was found that there is a unique linear relationship between the applied electrical currency density i (vertical axis in the graph of Fig. 5) and the relative velocity V r of the electrolyte compared with the strip (horizontal axis). The line drawn in the graph is a line of action for tinplating in accordance with the invention at an efficiency of 95 % and above of steel strip with differing coating weights. Preference is given to the application of an average relative velocity of the electrolyte into the gap of at least 5 m/sec and, more preferably at least 7 m/sec. Using such a high relative velocity of the electrolyte means that the installation may be compact. In the experiments described above, 850 mm wide steel strips were tinplated using the method in accordance with the invention with tin coating weights of between 0.5 and 2.8 g/m². In most cases it was found that the tin coating weight did not spread more than ± 0.04 to ± 0.02 g/m². When adopting the measures in the method in accordance with the invention a coated product is obtained with a coating layer which is very uniform and which has a good morphology.

Claims: Method for electrolytic coating of one side of a moving metal strip (1), wherein the strip as cathode is in contact with a rotating roller (3) and an insoluble anode (4) is positioned concentrically with the roller (3) over a part of the circumference of the roller at a distance from the strip (1) so that a slot (5) is formed in which the electrolytic coating takes place, the electrolyte flowing through the slot (5) at a sufficient average velocity that turbulent flow occurs, and the electrolyte being fed from a nozzle (9) as a fluid jet into the slot (5) with a tangential component opposite to the direction of travel of the strip (1) at the end of the slot (5) at which the strip (1) exits, characterized in that the average relative velocity of the electrolyte compared with the strip (1) in the slot (5) is at least 5 m/sec and in that the electrolyte is fed into the slot at a velocity that nowhere varies more than 10% from the said average velocity of the electrolyte from a nozzle (9) which has a conformation which is substantially uniform across the whole width of the strip, which has a slot-shaped outlet mouth which is open uninterruptedly across the whole width of the strip and is of uniform width across the whole width of the strip (1) and which is connected to a supply vessel (8) extending across the width of the strip (1), which vessel (8) has a large volume relative to the volume of the nozzle (9) and is supplied with electrolyte by means of a plurality of conduits (6) distributed across the width of the strip. Method in accordance with Claim 2, wherein the average relative velocity of the electrolyte compared with the strip (1) in the slot (5) is at least 7 m/sec. Method in accordance with any of Claims 1 or 2, wherein said nozzle (9) joins the slot (5) at an acute angle α Method in accordance with Claim 3, wherein the discharge directions of said conduits (6) are not aligned with the nozzle (9). Method in accordance with Claim 3 or 4 wherein a core body (7) is arranged within the vessel (8). Method in accordance with any one of Claims 3 to 5, wherein said acute angle α is less than 45 °. Method in accordance with any one of Claims 3 to 6, wherein said acute angle α is approximately 30 °. Method in accordance with any one of Claims 1 to 7, wherein the nozzle (9) is connected in an essentially leak-free manner to the anode (4) at the strip end exit of the slot (5). Apparatus for electrolytic coating of one side of a moving metal strip (1), comprising a rotatable roller (3) around which, in use, the strip passes, an insoluble anode (4) concentric with said roller and providing a circumferential slot (5) between the strip and the anode, means including an inlet nozzle (9) for feeding electrolyte into said slot (5) to achieve generally circumferential flow along the slot from the strip exit end thereof at an average velocity such that turbulent flow occurs and means for supplying electrical current to said strip (1) as cathode and said anode (4) to cause electrolytic coating characterised in that said nozzle (9) has a substantially uniform conformation across the width of the strip has a slot-shaped outlet mouth which is open uninterruptedly across the whole width of the strip and is of uniform width across the whole width of the strip (1) and is connected to a supply vessel (8) extending across the width of the strip (1), which vessel (8) has a large volume relative to the volume of the nozzle (9) and plurality of conduits (6) distributed across the width of the strip (1) for supply with electrolyte so that the electrolyte is fed into the slot at a velocity which nowhere deviates by more than 10 % from the average velocity of the electrolyte in the slot.

Inventors: PARAMANATHAN BALA KUMARAN DR, PARAMANATHAN, BALA KUMARAN, DR.
Applicants: HOOGOVENS GROEP BV, HOOGOVENS GROEP B.V.
Application year: 1989
Main classification: C25D7
IPC codes: C25D7, C25D5