WO2015011130A1

WO2015011130A1 - Anode system for use in an electroplating cell for the coating of a moving metal strip and a method using said anode system

Info

Publication number: WO2015011130A1
Application number: PCT/EP2014/065702
Authority: WO
Inventors: Jacques Hubert Olga Joseph Wijenberg; Jacob Miedema
Original assignee: Tata Steel Ijmuiden B.V.
Priority date: 2013-07-26
Filing date: 2014-07-22
Publication date: 2015-01-29

Abstract

This invention relates to an anode system for use in an electroplating cell for the coating of a moving metal strip and a method using said anode system.

Description

ANODE SYSTEM FOR USE IN AN ELECTROPLATING CELL FOR THE COATING OF A MOVING METAL STRIP AND A METHOD USING SAID ANODE SYSTEM

[0001] This invention relates to an anode or anode system for use in an electroplating cell for the (continuous) coating of a moving metal strip with an electrodeposited layer, to the use of said anode and to a coated metal strip produced using said anode.

[0002] In electrolytic plating, deviant deposits are sometimes formed on edge portions of a metal strip to be plated. Such deviant plating phenomena are caused by local differences in electric current density. One phenomenon is known as edge overcoating wherein a deposit on the metal strip edge becomes thicker than a deposit on an intermediate portion of the metal strip. This is disadvantageous due to the differences in coating thickness, which affects weldability e.g. in three-piece can bodies, and possibly the appearance of the coating . Also, as a result of the overcoating at the edges, the amount of plating material increases and thereby the costs for material and electricity. The other phenomenon is known as edge whiskers wherein plating metal deposits and grows on the metal strip edge to form whiskers. When this takes place the metal deposit on the metal strip edge tends to spall from the metal strip substrate. Metal spalls that have separated from the substrate may deposit later on causing scratches or damages to the product.

[0003] Edge overcoating or the formation of whiskers can be avoided by using an anode which is narrower than the moving strip to be coated. However, this means that the anode must be changed every time the width of the strip changes. For instance in conventional tinning with tin anode bars hanging from an anode bridge, this means that the anode bars need to be rearranged each time a change in width of the strip occurs. Since the difference between the width of the strip and the width of the anode only differs some tens of millimetres, this rearrangement entails halting the line or loosing quality of the plating process during this rearrangement (see Figure 1).

[0004] An alternative for the prevention of an edge overcoat or edge whiskers may be to use a dimensionally stable anode (or insoluble anode) in combination with the use of edge masks e.g. in the form of U- or U-shaped profiles that are positioned around the edges of the moving strip to shield the edges of the strip.

[0005] A major drawback of this design appears in the case of a differential coating on narrow coils, because then the anode with a zero (or low) current becomes bipolar (See Figure 2). This means that the edges of this anode become cathodic and its centre becomes anodic, resulting in a tin deposit on both edges of the anode. Moreover when an insoluble anode, typically Ti with an active Ir0₂ layer, becomes cathodic, the active layer of catalytic oxide is reduced and becomes inactive. This process is irreversible and therefore the anode has to be (partially) recoated to become active again. This problem might be solved by placing an insulating wall between both anodes, but such a wall would hinder electrolyte circulation in the plating tank.

[0006] Another drawback of a U-shaped edge mask design, is that the distance between the anode and the edge mask varies in longitudinal direction, because the anodes are tilted to compensate for the voltage (IR) drop in the strip due to the resistance of the strip.

[0007] Another drawback of a U-shaped edge mask is that if the strip hits the edge mask, this would damage the edge mask and the strip. This risk is especially high when a new coil is going to be processed that is wider than the previous one, because in that case the edge mask must be shifted outwards before the strip reaches the plating cell. This requires a sophisticated positioning device with a fast response time, because for instance the strip speed in electrolytic tinning lines is typically 300 m/min or even higher.

[0008] EP1699949-B1 discloses a dimensionally stable anode provided with a shutter placed as a mask in front of the anode basket. For this system to be operational for a wide range of widths of the metal strip to be coated, the edge mask must be large enough to accommodate even the smallest width and avoid the occurrence of a bipolar anode. This means that the width of the electroplating cell limits the application of these shutters, or that the width of the electroplating cell must be very large to enable mounting of the system, leading to high costs in terms of equipment and space.

[0009] DE730819 discloses a device for changing the electrolytically active surface of the anode in electroplating baths for chrome plating single objects one by one. To avoid having to change the anodes when a differently sized work piece has to be plated part of the anode is shieldable.

[0010] It is an object of the present invention to provide an apparatus for electrodepositing a metal coating onto a metal strip which prevents overcoats and/or whiskers.

[0011] It is also an object of the present invention to provide an apparatus for electrodepositing a metal coating onto a metal strip which can effectively cope with changes in width of the metal strip.

[0012] It is also an object of the present invention to provide an apparatus for electrodepositing a metal coating onto a metal strip which does not become bipolar in the case of plating a narrow strip.

[0013] It is also an object of the present invention to provide an apparatus for electrodepositing a metal coating onto a metal strip which is compact in size, particularly in width.

[0014] One or more of these objects can be achieved by an anode (1) for use in an electroplating cell for the coating of a moving metal strip (M) with an electrodeposited layer, said anode (1) comprising an anode having an active front surface (2) facing, in use, the metal strip to be coated, and a back side (3) turned away from the front surface of the anode, the active front surface of the anode being delimited by an upper and a lower edge (2a, 2b) and a left and a right edge (2c,2d), the left and right edges (2c,2d) being, in use, substantially parallel to the edges of the metal strip (M), wherein means are provided for shielding part of the front surface of the anode by means of one or more shutters (4a, 4b) to vary the width (w) of the active front surface (2) by varying the position of the left (2c) and/or right edge (2d) of the active front surface, wherein the one or more shutters is provided in the form, and having the functionality of, a roll shutter, and wherein the material of the one or more shutters is pliable in the direction of the movement of the metal strip, and not pliable in the perpendicular direction for the shutter to be able to be coiled or be guided around the edge or edges (lc, Id) of the anode to the back of the anode.

[0015] Strictly speaking the anode is only the positively charged part indicated with

1 in figure 2 to 5. So the invention is not only about the anode, but about an anode system where the anode 1 is part of the system in combination with the shutter. Positioning and control systems to control the position of the shutter(s) may be needed to exploit this anode system in practice.

[0016] With the term "pliable" the functionality of the shutter is characterised in that it can be coiled or guided around the edges as indicated in figures 3a to 5, but wherein the shutter does not bend in the perpendicular direction so that the distance between the shutter and the moving strip or the anode remains constant. Instead of pliable, the term rigid could also be used. The shutter is rigid in the direction of movement of the metal strip when the anode system is in use and not rigid (i.e. coilable or bendable) in the perpendicular direction. The figures and the explanation below should be ample clarification of the intended functionality of the shutter. [0017] Within the context of this invention a roll shutter is a type of shutter that is often used for doors, garages or furniture (e.g. the world famous "Amsterdammertje" produced by Pastoe and designed by Aldo van den Nieuwelaar), and may consist of many horizontal slats (or sometimes bars or web systems) hinged together. This hinging may e.g. be by means of a male-female connection of the slats (see figure 7a and 7b) or by the slats being incorporated in a flexible material (see figure 7c). However, the shutter may also consist of a single flexible material like in the "Amsterdammertje" where the rigidity in one direction and the pliability in the other direction is the result of the corrugated shape of the shutter.

[0018] Although it is preferable that the anode system is provided with at least one shutter on each side to obtain a symmetrical system able to delimit both edges of the active front surface, it is possible to provide the anode system with a shutter or shutters only on one side. This enables the active front surface to be delimited by the shutter(s) on one side only, so that the strip edge has to be carefully aligned with the other, non-delimited edge of the active front surface to avoid overcoats and/or whiskers.

[0019] The electrodeposited layer may be a layer of a metal, or a layer of a metal alloy, or a layer of a metal oxide, or any combination thereof, such as Cr/CrOx. The metal layer may e.g. be a chromium layer, a zinc layer, a nickel layer, a copper layer or a tin layer, or alloys thereof.

[0020] With the anode according to the invention the width of the active front surface, which is a part of the complete front surface of the anode, can be used to plate a moving metal strip that acts as a cathode. The metal ions dissolved in the plating solution in which the anode(s) and cathode are immersed, will be electrodeposited onto the cathode, thus forming an electrodeposited layer onto the cathode (i.e. on the metal strip). By moving the shutter, the edges of the active front surface move, and the surface (in m²) of the active front surface changes accordingly. If the width of the strip changes, then the position of the edges of the active front surface can be positioned to accommodate that change and achieve an even electrodeposited layer thickness on the strip. If the strip to be plated is very narrow, then the shutters close to a narrow width. Because the shutters are provided in the form of a suitably dimensioned and suitably placed roll- shutter, the outer edges of the anode remain shielded, and no bipolarity of the anode can occur.

[0021] If the width of the strip changes, then the shutter needs to react quickly to avoid damage differences in thickness of the electrodeposited layer. However, since the shutters do not surround the strip, but the anode, there is no risk of a sudden increase in width of the strip damaging the shutters, such as could be the case when using edge masks that shield the edges of the strip which may be hit by a sudden increased width of the strip.

[0022] The construction of these roll shutters and the system to move them are designed in such a way that they can be operated from a safe distance from the plating line excluding labour intensive and possibly dangerous work. The movement of the roll shutters can for instance be guided by a width gauge to measure the incoming strip width coupled to a control system of the shutter position which uses the output of the width gauge to control and guide the position of the roll shutter, and thereby the position and size of the active front surface of the anode. The movement of the shutters can be done by suitably dimensioned and positioned motors.

[0023] The anode according to the invention also effectively deals with tilted anodes to compensate for IR-drop (ohmic drop) because the anodes are shielded, and not the cathode (i.e. the strip). Furthermore, because of the use of a roll shutter, the dimensions of the anode system remain limited, because the excess shutter length is 'storable' in a compact way whereas in case of a rigid shutter the excess shutter length requires a much wider electroplating cell or an electroplating cell that is not able to deal with all widths of metal strip properly.

[0024] In an embodiment the anode system is intended for the continuous coating of a moving metal strip (M) with an electrodeposited layer, e.g. in an electrolytic tinning line.

[0025] In an embodiment the anode is provided with two shutters: a shutter to shield part of the left hand side of the front surface of the anode and a shutter to shield part of the right hand side of the front surface of the anode. By having two shutters, the system can easily compensate a sideways fluctuation in the position of the strip. Although strip plating lines are dimensioned such that the strip is guided over the centreline of the line, it is still possible that there is a sideways fluctuation. By using the input from the width gauge the position of the strip can be determined and the shutters can be positioned accordingly. Preferably two shutters per anode can be positioned independently from each other.

[0026] In a preferable embodiment guiding means are provided to guide the roll shutters, preferably in a curved, preferably circularly curved fashion, around the left and right edges of the anode and substantially parallel to the anode along the back of the anode, which is the surface turned away from the active surface. This embodiment 'stores' the excess shutter length around the back of the anode, and this is the most space effective roll shutter solution. The edges of the anode are completely shielded so no bipolarity can occur at all. This is the most compact embodiment width wise.

[0027] In a preferable embodiment the part of the roll shutter not involved in shielding the anode front surface is coiled. In this embodiment the excess shutter length is stored by coiling it, just like in the cassette of a roll shutter mounted on a window. This coiled form is also very compact, thereby limiting the width of the plating cell. Since the coiled shutter has to be next to the anode, the edges of the anode are completely shielded.

[0028] In an embodiment the distance (w) (Figure 3b) between the left edge and right edge of the active front surface is variable between 0 and the width of the anode. If it is 0, then the shutter is completely closed. The width of the anode determines the maximum width (W) of the metal strip to be plated. In most cases w is smaller than W by a distance between 0 and 100 mm, preferably between at least 5 and/or at most 50 mm.

[0029] The material of the shutter(s) must be an electrical insulator, and it must be able to withstand the conditions of the plating bath (temperature, chemicals, pH). It must also be sufficiently pliable to be able to be coiled or be guided around the edge of the anode to the back of the anode. A shutter made of a flexible (fibre reinforced) plastic material, a rubber lined metal or a rubber lined material strengthened with strips of rigid material, or a collection of interlocking slats, which may e.g. be produced by extrusion casting. Examples are given in Figure 7. For the shutter to be coilable, the material must be rigid in one direction, and pliable in the perpendicular direction. Figure 7a shows interlocking slats which can be coiled . Figure 7b shows a schematic representation of a slat which is interlockable with other slats of the same type. This type of slats may be produced from an extruded polymer, and may be hollow, or the hollow parts may be filled, e.g. with a (closed) foam. Figure 7c shows a cross section of a flexible shutter strengthened with rigid material (indicated with the white oval). These examples are not intended to be limiting.

[0030] In an embodiment of the invention the anode is a dimensionally stable anode (DSA) made of an electrically conducting material that is inert to the electroplating conditions, and wherein the anode has the form of a plate. Although the invention can also be used in the conventional process using dissolving tin anode bars as anode as in figure 1 of EP1699949-B1, the system is more suitable in combination with a DSA. In this embodiment this DSA has the form of a plate, preferably a rectangular plate having a width which is of the same order of magnitude as the width of the widest strip to be coated in the electroplating cell. Since the anodes are not dissolving, measures must be taken to replenish the electroplating solution with the metal ions that are being deposited onto the metal strip and to maintain the solution in good conditions in respect to pH and composition.

[0031] In an embodiment of the invention the anode is a basket made of an electrically conducting material that is inert to the electroplating conditions, containing the metal to be plated onto the metal strip in its metallic form, preferably wherein the metal to be plated is provided in the form of pellets or suitably sized chunks for dissolving to replenish the electroplating solution with metal ions to be deposited onto the metal strip.

[0032] It may be useful to provide the anode basket with an anode bag to prevent small metal fines entering the electrolyte.

[0033] The term "pellets or suitably sized chunks" should be interpreted as any shape suitable for placing it in the anode basket and dissolving it to replenish the electroplating solution with metal ions to be deposited onto the metal strip. Whether these pellets or suitably sized chunks are spherical, angular or any other shape is not relevant as long as they are suitably size to be able to go into the anode basket and easy to handle. Preferably the pellets or chunks are provided in a hopper system that regulates the addition of pellets or chunks to the basket depending on the consumption in the line.

[0034] The material of the DSA is preferably a titanium based material with a catalytic coating of platinum, iridium oxide or a mixed metal oxide (e.g. a mixture of tantalum oxide and iridium oxide). The material of the anode basket is typically titanium.

[0035] In case of electrolytic tinning lines, in which tinplate for packaging applications is produced, traditionally soluble tin bars on anode bridges are used as anodes as depicted in Figure 1.

[0036] In an embodiment of the invention the strip is provided with an electrodeposited layer on both sides. Consequently, an anode has to be placed on either side of the metal strip. When considering an electroplating cell consisting of a metal strip being moved downwardly between two anodes and then, after having been turned around a roll, upwardly between two anodes, means that such an electroplating cell comprises four anodes, and thus, potentially, eight shutters. One or more of these electroplating cells may be placed in succession, each with their own electroplating solution which may be the same in all electroplating cells, but may also be different to provide a metal strip with layers of different metals or metal alloys on top of each other.

[0037] It should be noted that, although the electroplating cell comprising the anode thus far was described as if the strip moves in a vertical direction past the anode, that the system can also be embodied in an electroplating cell where the strip moves in a horizontal direction past the anode, and wherein the anode is therefore also horizontal, or indeed any direction in between. From a control and maintenance point of view, the preferable option is the electroplating cell wherein the strip moves in a vertical direction past the anode, because then the motors for controlling the shutters can be positioned above the electroplating solution. An anode basket also functions more effectively when mounted vertically and can be simply filled with pellets.

[0038] The invention is also embodied in the dependent claims.

[0039] In an embodiment wherein the anode (or anode system) is provided with a shutter (4a) to shield part of the left hand side of the front surface of the anode and a shutter (4b) to shield part of the right hand side of the front surface of the anode.

[0040] Although the invention is also embodied in an anode (or anode system) where only one edge of the anode is shieldable by a shutter, and the other is not, the control of the quality of the coating, it is preferable to potentially shield both edges. This way the moving strip can always be threaded through the coating line by the centre line of the coating line, which is beneficial for strip stability during the process.

[0041] In an embodiment guiding means are provided to guide the roll shutters around the left and right edges (lc, Id) of the anode and substantially parallel to the anode along the back side (3) of the anode, which is the surface turned away from the active surface (2).

[0042] In this embodiment the part of the shutter that is not used to shield the edge of the anode is safely stored away from the moving strip at the other side of the anode (see e.g. figure 4a and 4b), but it is readily available in case the strip width changes.

[0043] In an embodiment the part of the roll shutter that is not involved in shielding the anode front surface is coiled.

[0044] This embodiment is an alternative where the part of the roll shutter that is not involved in shielding the anode front surface is not stored around the back of the anode, but in coiled form beside the edge of the anode. [0045] In an embodiment the means (4a) for shielding part of the left hand side of the front surface of the anode can be positioned independently from the means (4b) for shielding part of the right hand side of the front surface of the anode.

[0046] This embodiment enables the independent positioning of the left and right hand side shutter, e.g. in case the moving strip is not moving over the centre line of the coating line, or in case a non-symmetrical coating layer is desired where the coating on one edge of the moving strip is different in thickness than the coating on the other edge of the moving strip. This embodiment may also be useful to correct an undesired difference in thickness between the left and the right hand side of the strip over the width.

[0047] In an embodiment the distance (w) between the left edge (2c) and right edge (2d) of the active front surface is variable between 0 and the width of the anode.

[0048] The embodiment allows the plating of a very narrow strip.

[0049] In an embodiment the roll shutters are made of an electrically insulating material which is resistant to the electroplating conditions.

[0050] In an embodiment the shutter or shutters are made of a flexible plastic material, optionally a fibre reinforced plastic material, a rubber lined metal or a rubber lined material strengthened with strips of rigid material other rigid material, or a collection of interlocking rigid or flexible slats.

[0051] In an embodiment the anode is a dimensionally stable anode made of an electrically conducting material that is inert to the electroplating conditions, and wherein the anode has the form of a plate.

[0052] This is constructionally the simplest embodiment, although the anode may also be a gauze or grid. In this system the depletion of the electrolyt of metal ions to be plated onto the moving strip has to be counteracted by additions of these ions, e.g. in the form of a salt comprising these ions.

[0053] In an embodiment wherein the anode is a dimensionally stable anode made of an electrically conducting material that is inert to the electroplating conditions, and wherein the anode has the form and functionality of an anode basket for containing the metal to be plated onto the metal strip, preferably wherein the metal to be plated is provided in the form of pellets or suitably sized chunks for dissolving to replenish the electroplating solution with metal ions to be deposited onto the metal strip.

[0054] In this embodiment no salt additions are needed.

[0055] In an embodiment the anode basket is provided with an anode bag to contain the pellets or suitably sized chunks.

[0056] In this embodiment any fine particles resulting from the dissolution of the pellets or chunks can come into the electrolyte and potentially deteriorate the plating layer.

[0057] In an embodiment the anode is a soluble tin anode or consists of soluble tin anodes. In this embodiment the anode as such dissolves, i.e. the anode is not a dimensionally stable anode.

[0058] In an embodiment the anode is made of titanium, optionally wherein the titanium anode is provided with a catalytic coating.

[0059] According to a second aspect, the invention is also embodied in a method for electrodepositing a metal coating layer from an electroplating solution on a metal strip M having a width W in an electroplating cell comprising one or more anodes (or anode systems) according to the invention, wherein the distance w between the left and right edge of the active front surface of the anode or anodes is variable and dependent on the width W of the strip.

[0060] In an embodiment a plurality of anodes or anode systems is placed in succession. This plurality of anodes or anode systems may be located in one plating cell comprising a single electrolyte, or in a plurality of plating cells each comprising an electrolyte for depositing the same metal, or in a plurality of plating cells each comprising an electrolyte for depositing the different metals, such as (for instance) a cobalt layer on top of a nickel layer as described in EP303035-A1.

[0061] In an embodiment the anode system according to any one of the claims is provided with positioning and control means to monitor and control the position of the shutter(s) in relation to the width of the strip to be plated. These monitor means may be coupled to means for measuring the thickness of the coating layer which enables to control and adapt the position of the shutter if the coating layer after plating does not have the desired thickness at the edges of the strip.

[0062] The invention will now be described with reference to the following non- limiting figures.

[0063] Figure 1 shows the use of conventional tin anode bars, used in the classical way as described in EP1699949-B1, where the steel strip is wider (13 mm on both sides in fact) than the anode width which results in an even layer of the deposited tin layer. This method is inflexible and requires significant manual labour under unpleasant working conditions. The words 'IN' and OUT' and the broken arrows indicate the 'movement' of the tin anode bars (1) over the course of time. The thick fresh bars are placed in the cell at the 'IN' side and as they gradually become thinner, they are rearranged until they become too thin and the worn bars are discarded for recycling at the OUT' side. This change in thickness is the reason why the anode bridges to which the anode bars are attached are skewed with respect to the strip.

[0064] Figure 2a and 2b show a cross section of the plating cell with an anode on each side of the metal strip M. The strip moves perpendicularly to the drawing. If the strip is coated on both sides, the top anode is used for the top side and the bottom anode is used for the bottom side. Figure 2 also illustrates the problem of bipolarity of the anode when using edge marks to shield the edges of the metal strip M . In Figure 2a this works fine, but when the metal strip becomes too narrow as depicted in Figure 2b, the bottom anode becomes bipolar and metal is deposited onto the edges of the bottom anode. Also, for specific anode types such as Ti with an active Ir0₂-layer, when the anode becomes bipolar the active layer is reduced and becomes inactive on the cathodic parts of the bipolar anode. This process is irreversible and the anode must subsequently be sent for recoating. Note that the terms 'top' and 'bottom' are used as relative indications of positions of anodes in relation to the strip for the benefit of explaining the drawings and are not intended to be restricting the applicability of the invention.

[0065] Figure 3b shows a schematic representation of the active front surface 2

(the hatched area) of the anode 1, wherein the left and right hand side of the anode are shielded by shutters 4a and 4b. The two dark areas are the shielded parts of the anode and the actual size of the anode is schematically indicated by the hatched lines behind shutters 4a and 4b. The hatched area (active front surface 2) is delimited by the top and bottom edge of the anode 2a and 2b, and the left and right edge of the shutters 2c and 2d. If the shutter opens (indicated by the two-pointed arrows on shutter 4a and 4b), then 2d shifts to the right, and 2c shifts to the left, thereby increasing the hatched area. The moving strip (not shown), passes the hatched area as schematically drawn in figure 3a. In this image only one side of the metal strip is coated with an electrodeposited layer. It is clear from the combination of figure 3a and 3b that the shutter around edges Id and lc are pliable around these edges and rigid (or not pliable) in the direction perpendicular to W, requiring a pliability of the shutter in one direction and non-pliability in the direction perpendicular to the said one direction.

[0066] Figure 4 shows an anode according to the invention for plating a metal strip

M on both sides. Figure 4a shows the position for a wide strip, and figure 4b for a narrow strip. The excess roll shutter length is guided around the left and right edges (lc and Id) of the anode and then substantially parallel to the anode along the back (3) of the anode.

[0067] In figure 5 the anode with the spirally wound excess shutter length is schematically shown. In this figure only one anode and shutter combination is shown, but it will be clear that by mirroring this in the metal strip a system for coating a metal strip on both sides is obtained.

[0068] Figure 6 shows a calculated example of a system with DSA's on both sides of the cathode (strip M) where the bottom anode (indicated with 1') has a low or zero current to deposit a differential coating (a coating only on one side of a narrow strip, or a difference in coating thickness on both sides of the strip), or at least a lower current than the top anode. In this example the metal that is deposited is assumed to be tin (Sn). When using edge masks, a current flows from the top anode (1) to the bottom anode ( ) on the extremities of the anode for a narrow strip when the edge masks are between the anodes leaving free ends of the anode (in the dashed circle) and therefore tin (Sn) is deposited on both edges of the bottom anode ( )- In Figure 6 only the tin deposit on the right edge of the bottom anode is shown, because the plating cell is cut into two along its symmetry axis. This makes it easy to close the geometry. By doing so, also the numerical resolution is doubled for the same amount of elements. Due to the cathodic currents at both edges of the bottom anode, the middle part of the bottom anode facing the strip becomes anodic (7). As a result of this anodic current, tin is also deposited on the bottom of the cathode (M). Both phenomena (a metal deposit on the bottom side of the cathode, and metal deposits on the edges of the bottom anode) are unwanted. Furthermore, the cathodic currents at the edges of the bottom anode may destroy the active layer on the DSA. This is further explained below.

[0069] When the current of the top anode is different from the current of the bottom anode, both anodes will interact with each other in case a differential coating weight (i.e. a different coating weight on each side of the strip) is applied and a U-shaped edge mask is used. This is demonstrated by the example in Figure 6, in which the so-called primary current distribution is calculated. In the primary current distribution only the geometry (i.e. the dimensions and the positions of the electrodes, edge masks and plating cell walls) of the problem are considered and activation and concentration overpotentials at the electrodes are neglected. It is known that the primary current distribution closely approximates the actual current distribution for many practical situations, like tin plating from the usual commercial MSA and PSA electrolytes and nickel plating from a Watts type electrolyte.

For calculating the primary current distribution, two differential equations have to be solved numerically. These equations are the Laplace equation ( ν²φ = 0 ) and Ohm's law ( i = - κνφ ), where 'φ' is the local potential [V], Y is the current density [A/m²] and V is the electrolyte conductivity [S/m] .

Both differential equations were solved numerically by using a Boundary Element Method. The calculations were performed with the software package EISy2D Version 7.0 for a closed geometry consisting of line segments. Each line segment becomes either an insulator or an electrode as defined by the user. The line segments are divided into a number of elements. The distribution of the elements is refined towards the edge of the strip (indicated by the smaller distances between the vertical lines in figure 6). The local current density is calculated for each element.

[0070] In order to obtain a unique numerical solution, at least one electrode should receive an imposed potential. It is most practical to define a potential of 0 V for all electrodes representing the strip M (being an equipotential surface). The cut edge of the strip is neglected in the calculations (thus, the cut edge is defined to be an insulator).

[0071] In this calculation, a current of 1 A was imposed on the top anode and a value of 1 S/m was taken for the conductivity of the electrolyte. It should be noted an arbitrary value has been chosen for the current on the top anode and the conductivity of the electrolyte, because these values have no influence on the current distribution.

[0072] A current of 0 A was imposed on the bottom anode that is 'idle', meaning that the current rectifier connected to this anode is switched off. Being exposed to a potential field, this anode can become 'floating' or 'bipolar' (i.e. partly anodic, partly cathodic). Obviously, the sum of the currents leaving and entering a bipolar electrode has to be zero.

[0073] The chosen values of all parameters are given in Table 1.

[0074] Table 1 : Overview of parameters.

parameter value

distance of edge mask from anode 5 mm

thickness of edge mask 10 mm

amount of shielding of strip edge 10 mm

plating cell length 1720 mm

plating cell width 550 mm

anode width 1150 mm

anode thickness 9 mm

strip width 850 mm

strip thickness 0.2 mm [0075] In Figure 6, the current distribution along the electrodes is plotted. Clearly, a significant current flows from the unshielded edges of the top anode to the unshielded edges of the bottom anode. As a result, metal (e.g. Sn) is deposited at the edges of the bottom anode. Not only is metal deposited at the edges of the bottom anode, but in case this anode would have been a titanium anode with a catalytic Ir0₂ coating, the Ir0₂ coating might be damaged irreversibly, because it is polarised cathodically.

[0076] The cathodic current at the edges of the bottom anode is counterbalanced by an anodic current at the part of the bottom anode that is facing the bottom side of the strip. As a result of this anodic current metal is deposited at the bottom side of the strip. In this case this is an unwanted effect because the current of the bottom anode was set to 0.

[0077] In this case, a significant amount of metal (ca. 9 %) is deposited at the bottom side of the strip. This not only results in a cost increase due to unwanted plating, but potentially also in damage of the anodes. When anodes with shutters according to the invention are used, no interaction between both occurs, because the edges are shielded so no current can flow between them. So, no metal is deposited anymore at the edges of the bottom anode. Also, no metal is deposited anymore at the bottom side of the strip. A small deposition at the strip edges is unavoidable in any case, because some current still wraps around the corner (a phenomenon known as 'backplating')-

[0078] Figure 6 shows half of the plating cell with a metal strip M between two anodes 1. The current is indicated with I and with an arrow to point its direction. The U-shaped edge mask is indicated on the right. Those areas where metal is deposited are indicated with a vertical hatch, and those areas that act as an anode and where, in case an inert anode would be used, oxygen would be generated, are indicated with a skewed hatch.

[0079] Figure 7 gives examples of how the shutter can be constructed. Figure 7a shows interlocking slats which can be coiled or guided along the edges of the anode to the back of the anode. Figure 7b shows a cross-section of another design of interlocking slat. Figure 7c shows a design where strengthening slats (white ovals) are enveloped in a rubber compound (black) which is flexible and can be coiled or or guided along the edges of the anode to the back of the anode.

[0080] The examples given are by no means intended to be limiting.

Claims

An anode system (1) for use in an electroplating cell for the coating of a moving metal strip (M) with an electrodeposited layer, said anode system (1) comprising an anode having an active front surface (2) facing, in use, the metal strip to be coated, and a back side (3) turned away from the front surface of the anode,

the active front surface of the anode being delimited by an upper and a lower edge (2a, 2b) and a left and a right edge (2c,2d),

the left and right edges (2c,2d) being, in use, substantially parallel to the edges of the metal strip (M),

wherein means are provided for shielding part of the front surface of the anode by means of one or more shutters (4a, 4b) to vary the width (w) of the active front surface (2) by varying the position of the left (2c) and/or right edge (2d) of the active front surface, wherein the one or more shutters is provided in the form, and having the functionality of, a roll shutter, and wherein the material of the one or more shutters is pliable in the direction of the movement of the metal strip, and not pliable in the perpendicular direction for the shutter to be able to be coiled or be guided around the edge or edges (lc, Id) of the anode to the back of the anode.

Anode system according to claim 1 wherein the anode is provided with a shutter (4a) to shield part of the left hand side of the front surface of the anode and a shutter (4b) to shield part of the right hand side of the front surface of the anode.

Anode system according to claim 1 or 2 wherein guiding means are provided to guide the roll shutters around the left and right edges (lc, Id) of the anode and substantially parallel to the anode along the back side (3) of the anode, which is the surface turned away from the active surface (2).

Anode system according to claim 1 or 2 wherein the part of the roll shutter that is not involved in shielding the anode front surface is coiled.

Anode system according to any one of the preceding claims wherein the means (4a) for shielding part of the left hand side of the front surface of the anode can be positioned independently from the means (4b) for shielding part of the right hand side of the front surface of the anode.

Anode system according to any one of the preceding claims wherein the distance (w) between the left edge (2c) and right edge (2d) of the active front surface is variable between 0 and the width of the anode.

Anode system according to any one of the preceding claims wherein the roll shutters are made of an electrically insulating material which is resistant to the electroplating conditions.

8. Anode system according to any one of the preceding claims wherein the shutter or shutters are made of a flexible plastic material, optionally a fibre reinforced plastic material, a rubber lined metal or a rubber lined material strengthened with strips of rigid material other rigid material, or a collection of interlocking rigid or flexible slats.

9. Anode system according to any one of the preceding claims wherein the anode is a dimensionally stable anode made of an electrically conducting material that is inert to the electroplating conditions, and wherein the anode has the form of a plate.

10. Anode system according to any one of claims 1 to 8 wherein the anode is a dimensionally stable anode made of an electrically conducting material that is inert to the electroplating conditions, and wherein the anode has the form and functionality of an anode basket for containing the metal to be plated onto the metal strip, preferably wherein the metal to be plated is provided in the form of pellets or suitably sized chunks for dissolving to replenish the electroplating solution with metal ions to be deposited onto the metal strip.

11. Anode system according to claim 10 wherein the anode basket is provided with an anode bag to contain the pellets or suitably sized chunks.

12. Anode system according to any one of claims 1 to 8, wherein the anode is a soluble tin anode or consists of soluble tin anodes.

13. Anode system according to any one of claims 1 to 11, wherein the anode is made of titanium, optionally wherein the titanium anode is provided with a catalytic coating.

14. Method of electrodepositing a metal coating layer from an electroplating solution on a metal strip (M) in an electroplating cell comprising one or more anode systems according to any one of claims 1 to 13 in an electroplating cell, wherein the distance (w) between the left and right edge of the active front surface of the anode or anodes is variable.

15. Method according to claim 14 wherein a plurality of anode systems is placed in succession.