US3803013A

US3803013A - Electrolytic plating apparatus and method

Info

Publication number: US3803013A
Application number: US00024956A
Authority: US
Inventors: C Yates
Original assignee: Circuit Foil Corp
Current assignee: Circuit Foil USA Inc
Priority date: 1967-09-25
Filing date: 1970-03-06
Publication date: 1974-04-09
Anticipated expiration: 1991-04-09
Also published as: BE721365A; GB1251658A; FR1583016A; US3567595A; US3567596A; DE1796222B2; LU56951A1; DE1796222A1

Abstract

A method of improving the electroplating of a metal onto the surface of a cathodic strip of metal which is moved in the electrolyte in proximity to the surface of an anode, said method involving the creation of substantially uniform upward flow of electrolyte across the surface of said strip of metal using gas evolved between the anode and said strip of metal as the driving force for a pumping system to create an air lift effect. Apparatus for carrying out said method.

Description

1451 Apr. 9, 1974 United States Patent Yates I 3 537 971 11/1970 Green 204/206 2,461,556 2/1949 Lorig 204/28 3 104 221 9/1963 H111 204/237 1 1 ELECTROLYTIC PLATING APPARATUS AND METHOD Primary Examiner-F. C. Edmundson Assignee:

[22] Filed:

ABSTRACT Related U.S. Application Data [62] Division of Ser. No. 670.152. Sept. 25,

A method of improving the electroplating of a metal 1967, Pat.

onto the surface of a cathodic strip of metal which is moved in the electrolyte in proximity to the surface of an anode, said method involving the creation of substantially uniform upward flow of electrolyte across 84 Q5 a 2 8C s M6 25 Q 2 DC 4. 0n G St Um UN 55 the surface of said strip of metal using gas evolved be- [58] Field of Search.......

204/284 206-21 1 tween the anode and said strip of metal as the driving force for a pumping system to create an air lift effect.

Apparatus for carrying out said method;

[56] References Cited UNITED STATES PATENTS 2.673.836 3/1954 Vonada................,.......... 204/28 5 Claims, 5 Drawing Figures PATEN'IHI APR 9 IBM SHEET 1 OF 3 PATENTEDAPR 9 I974 3,803,013. SHEET 3 OF 3 HEAVY 017 050 204/5 ELECTROLYTIC PLATING APPARATUS AND METHOD CROSS-REFERENCE TO RELATED APPLICATION This application is a division of copending application Ser. No. 670,152, filed Sept. 25, 1967 now US. Pat. No. 3,567,595.

This invention relates to an electrolytic process and the apparatus used therein and more particularly to a process and apparatus for continuously electroplating a moving strip of metal.

Electroplating techniques are known in which a continuously moving metallic sheet is moved in serpentine fashion into and out of an electrolyte between successive planar anodes vertically arranged in parallelism with one another. By rendering the metallic sheet cathodic, metal is electrodeposited in the sheet as it traverses the electrolyte. A system of this type is illustrated and described in copending application Ser. No. 421,048, filed Dec. 24, 1964.

In such an electroplating process, it is highly desirable that the metallic deposit be of a very high degree of uniformity of thickness. Two problems are encountered in conventional methods which result in variations in coating thicknesses which are inadequate. First, electrical current which flows from the cathode to the anode in an electroplating process often leaves the desired plating surface in a non-uniform manner, which results in uneven current distribution and plating. In addition, the weight of the deposited material often varies due to changes in the deposition current efficiency resulting from an ionic imbalance in the electrolyte bath. This ionic imbalance has been found to occur because of variations in degree in flow and agitation in the electrolytic bath during the deposition process.

Problems such as the foregoing become particularly troublesome in certain electrolytic processes such as, for example, one involving the electrodeposition of chromium on a moving metallic sheet. For some electrolytic systems, establishing a uniform current distribution is adequate to obtain an adequate electrodeposit. In chrome plating, however, this is not enough. More specifically, the common chrome plating baths in commercial use have current efficiencies between and 35 percent and the efficiency has been shown to be related to a wide collection of bath variables such as chromate to sulfate ratio, foreign metal contamination, temperature, agitation, degree of physical electrolyte exchange from cathode to anode, and so forth. In order to achieve a uniform chrome deposit, it is necessary for the electrolyte bath to behave uniformly with respect to current efficiencies of deposition at different parts of the cathode. Thus, the uniformity of the flow and agitation pattern in the electrolyte is of very great importance in achieving a uniform thickness of chrome deposit.

The importance of a uniform chrome layer takes on added significance when considering the use of a product such as chrome plated copper foil as a bimetallic lithographic or offset plate. In processing such a plate, starting with a chrome plated copper sheet, part of the chrome surface is masked with a photosensitive polymer leaving the areas of chrome which it is desired to remove exposed, such that they may be attacked by an etchant. As the chrome is attacked in the etching step,

there is a tendency for the etchant to attack sideways as well as vertically causing the area of the base copper which is finally exposed to be somewhat larger than the area of chrome originally exposed. This problem is called dot growth, its name coming from the manifestation of the effect in the etching of half tone dots where the eye has been shown to be sensitive to infinitesimal variations in dot diameter which in turn cause variations in the shading or color density of an area.

In developing transparent positives for use in bimetal plate making, the camera attendant normally adjusts the dot size of the transparency to allow for a given degree of dot growth. The problem arises in that the degree of dot growth is a direct function of the chrome thickness, and chrome plates made in the conventional sheet by sheet manner suffer from all of the undesirable edge effects and non-uniform flow and agitation effects of a still bath. Thus, the printer in using such plates is forced to accept an undesirable degree of variation in dot growth which, in multicolor work, seriously diminishes the quality of the result. In this particular usage, uniformity of chrome thickness assumes a unique degree of importance not often necessary in other sorts of chrome plating. 1

The present invention provides a process and apparatus for continuously electroplating a sheet of metal foil wherein the above-mentioned disadvantages are eliminated. An even deposition of the electrodeposited layer is obtained by utilizing a novel electrically insulating shield and anode barrier arrangement in the plating tank which permits the control of the flow and agitation of the electrolytic bath during deposition.

It is accordingly a primary object of the present invention to provide a novel continuous electroplating process which is extremely effective to form high quality, uniformly thick, metallic electrodeposits on a sheet of base metal.

It is a further object of the present invention to provide a novel electroplating process and apparatus for carrying out such process whereby a sheet of metal may be continuously electroplated in which the electrolyte circulation is controlled by a unique anode and insulating barrier construction. I

It is still another important object of the present invention to provide a novel anode structure which makes possible the obtaining of electrodeposits which are extremely uniform in thickness.

These and further objects and advantages of the present invention will become more apparent through reference to the following description and appended claims, as well as to the accompanying drawings wherein:

FIG. 1 illustrates a longitudinal section of an electrolytic apparatus in'accordance with the present invention taken along line 1-1 in FIG. 2;

FIG. 2 is a plan view partially in section of a portion of the apparatus in FIG. 1;

FIG. 3 is a fragmentary sectional perspective view of a portion of the electroltyic apparatus of FIG. 1;

FIG. 4 is a perspective view of the anode used in the present invention; and

FIG. 5 illustrates in schematic form part of a plating bath in the area near the edge of an anode and a cathode.

As previously indicated, the novel method and apparatus of the present invention are applicable to the electroplating of any metal on a base metal where uniformity of thickness of the electrodeposit is desired, such as copper on copper, chrome on copper, copper on steel, or the like. Since such novel method and apparatus are particularly advantageous in chrome electroplating systems, however, the ensuing description will be couched in terms of the electroplating of chrome on a continuous strip of copper, though the broad essence of the invention is not to be deemed being so restricted.

The electroplating apparatus illustrated in FIG. 1 comprises a tank containing an electrolyte inlet 12 and outlet 14 for entrance and withdrawal of electrolyte 16 from the tank 10. A positive bus bar 18 is secured to one longitudinal edge of the tank 10 by means of an insulating strip 20. Supported on the bus bar 18 and insulating strip 20 by means of copper bars 22 are a plurality of anodes 24. These anodes 24 are shown in the form of thick planar lead plates, whose physical configuration forms an important feature of the present invention, as hereinbelow described. A terminal lead 25, adapted to be connected to the positive terminal of a power source (not shown), is connected to the bus bar 18. A support frame 26 is mounted on the tank 10. in order to guide a sheet of copper 54 to be plated, a series of upper electrically conductive idler rollers identified by the

numbers

28, 30, 32, 34 and 36 are mounted on and are in electrical contact with the support frame 26, and a series of polyvinyl

chloride idler rolls

38, 40, 42 and 44 are rotatably mounted on

stainless steel bars

46, 48, and 52, which in turn are connected to the supporting frame 26 at their upper ends. By virtue of the electrical contact between

rollers

28, 30, 32, 34 and 36 and support frame 26, and. by connecting the latter to the negative terminal of a power source (not shown), copper sheet 54 may be. rendered cathodic.

The continuous sheet of copper 54 which is to be electroplated is shown fed from a supply spool 56 mounted on the support frame 26, over the first upper idler 36 and into the electrolytic bath 16. The continuous sheet 54 is guided into and out of the electrolyte 16 in serpentine fashion by means of the remaining

idler rollers

38, 40, 42 and 44 at the bottom of the tank 10 and the

upper idler rollers

28, 30, 32 and 34. As the plated sheet 54 emerges from the electrolytic tank 10 it passes by a pair of

nozzle elements

62 and 64 which are connected to a source of wash solution (not shown) to permit the sheet 54 to be spray washed on both sides. The plated sheet metal then is fed across idler roller 66, past asqueegee device 68 which removes any excess material, past a pair of

infrared lamps

70 and 72 to dry the plated material, onto an additional idler roller 74 and finally to the take-up roll 76 which is provided with a suitable drive mechanism (not shown) which provides the necessary force to draw the sheet of copper 54 through the system.

The system thus far broadly described is conventional. Details of the essence of the present invention are shown specifically in FIGS. 2-4. FIG. 2 illustrates a section of the electroplating system as viewed from the top. This section includes an anode 24, a cathode (which is the copper sheet 54 to be plated), a bottom idler roll 38 and a bottom roll support tube 46.

Secured to each side edge of anode 24 at 92 is one leg of a reverse S-shaped barrier 90, which is preferably constructed of a non-conductive material such as polyvinyl chloride. The other leg of each barrier is secured at 94 to one edge of a back shield 96 which is preferably constructed of polyvinyl chloride or similar non-conductive material. Back shield 96 is mounted by means of non-conductive brackets 98 (preferably made of polyvinyl chloride) each of which has a slot 100 which engages an edge of back shield 96. Each of brackets 98 is, in turn, secured to a support tube 46 at 99. As shown in FIG. 2, the edges 101 of copper sheet 54 are positioned within a channel 103 formed within barrier 90 but are'not secured to barrier 90 so that movement of sheet 54 will not be obstructed.

As will be readily seen from the drawings, by virtue of the presence of barriers 90, the electrolyte between each anode 24 and the portion of sheet 54 which is parallel to and adjacent the planar face of the anode is effectively segregated from the remainder of the electrolyte in tank 10 to form channels 102 which are confined by four walls (barriers 90, an anode 24 and sheet 54) but which are open in a slot of uniform width at the bottom and open at the top. By virtue of the parallelism between anodes 24 and the adjacent portions of sheet 54, confined channels 102 each have a substantially constantcross-section from the bottom of the top of anodes 24. As will be seen hereafter, the establishment of these confined channels 102 forms an essential aspect of the present invention.

Details of anodes 24 are shown in FIG. 4. Each anode 24 is comprised of a slab of conductive material (preferably lead in a system in which chrome is plated on a copper sheet) and is provided with a pair of conductive projections 22 (which may appropriately comprise the ends of a single copper bar) which are used to mount anode 24 on bus bar 18 so as to establish an electrical connection between the power source and the anodes. Positioned near the top of each anode 24 but slightly below projections 22 is a longitudinal and horizontally extending slot 1 l0. Reinforcing bars 1 12 mounted over bracket slot provide structural strength to the anode. Secured to one face of each anode 24 byconventional means (not shown) is a non-conductive sheet 113, which may appropriately be made of polyethylene.

The operation of the system of the present invention may be described as follows. A sheet of metal 54 of suitable width is fed from supply roll 56 into and out of the electrolyte solution 16 over the

upper idler rollers

28, 30, 32, 34 and 36 and the

lower rollers

38, 40, 42 and 44, past

idler rollers

66 and 74 to the take-up reel 76. Upon energizing the system, the motor driven take-up reel 76 drives the sheet of metal 54 at a desired speed through the electrolytic bath 16 while a positive voltage is applied to the anodes 24 and a negative voltage to the metal sheet 54 acting as a cathode.

In order to achieve uniform deposition of the chrome layer on the copper sheet 54, it is essential that a uniform flow distribution of electrolyte material between the cathode and anode be achieved. Because of gas generation which takes place at the anode interface (generally shown by the bubbles at in FIG. 3), the channel 102 between the anode 24 and the copper sheet 54 is filled with a foamy solution whose average density is less than the average density of the remainder of electrolyte 16. This causes a Pohle air lift effect which carries the liquid electrolyte 16 up towards the top of anodes 24. By mounting anodes 24 so that the lower edge 1 14 of slot 1 10 is substantially level with the surface of the electrolyte 16, the foamy electrolyte is caused to rise up in channel 102 and to flow through slot 110 as shown in FIG. 3. The result is that a circulation pattern is formed from the bottom to the top of channel 102 and through slot 110 of anode 24.

Because this circulation pattern is confined within channel 102, the pumping action of the generated gas causes the whole electrolyte body 16 to move upwardly at substantially the same velocity across the width of the metal sheet 54, and to flow out over the anode 24 through the slot 110 at a uniform rate across the entire width of the anode 24. This creates a uniform flow pattern in the desired direction, with no eddies and no still zones. All of the agitation and pumping effects of the generated gas occur in the same manner and cause the same results across the entire width of the plating area of the metal sheet 54 because the flow pattern is entirely unidirectional and there is no factor or effect which would tend to disturb this uniformity. The result is uniform thickness of deposit across the width of sheet 54.

By virtue of the unique arrangement described above, the nature of the electrodeposit on the metal sheet can be readily modified by control of electrolyte circulation within confined channel 102. Since the electrolyte circulation within channel 102 is directly related to the amount of electrolyte which is caused to flow through slots 110 of anodes 24 as a result of the gassing of the electrolyte as aforesaid, the electrolyte circulation can be modified by adjustment of the position of slots 110 relative to the electrolyte surface. By,

raising anodes 24 above the level of the electrolyte solution 16, the flow of electrolyte over the anode slot 110 may be impeded somewhat. By bringing the anode 24down below the level of the electrolyte solution 16, it is easier for the electrolyte to be lifted by the gassing effect and to pass through the slot 1 10, thereby increasing electrolyte flow. Thus, the flow rate in the plating area can be readily adjusted by adjusting the height of the anode 24.

The exact position of the anode 24 must be determined for each specific application and depends on a number of variables including the distance between the cathode and anode, the materials utilized, the electrolyte solution, the rate of movement of the sheet of material to be plated and the thickness of the plating desired. If the anode is placed too high above the level of the electrolyte solution 16, there will not be a sufficient flow rate and the electrolyte will become depleted in terms of its chromate content (in systems in which chrome plating is taking place), which will result in a loss of current efficiency and produce poor electroplating. If the anode 24 is placed too far below the level of the electrolyte solution 16 there will be virtually a still flow and the gasses will come up and simply bubble out of the electrolyte and not cause the desired lifting effect of the electrolyte across slots 1 of the anodes 24.

In the preferred embodiment, the anode 24 has been shown with a slot 110 through which the electrolyte solution l6 flows, with separate projections 22 being provided above the lower edge 114 of slot 110 for mounting of the anodes. This particular construction facilitates the handling and mounting of the anode 24 and eliminates the necessity for sealing of the tank 10 at the points at which projections 22 come in contact with the supporting tank portion. While this arrangement is thus highly advantageous, the broad advantages of the present invention may still be achieved through the use of an anode without a slot by mounting such anode with its upper edge substantially level with the electrolyte surface. In such case, the upper edge of the anode performs the function of the lower edge 114 of slot 110. As will be apparent, this arrangement requires that special precautions be taken to seal the tank at the points at which the anodes are mounted.

As will be apparent from the preceding description, the arrangement of the present invention is not only effective to establish uniform electrolyte'circulation patterns between the anodes and adjacent moving cathodic metallic sheets (resulting in extremely uniform electrodeposits) but provides a significant secondary advantage. This secondary advantage results from the use of barriers and their associated back shields 96 and can best be explained by reference to FIG. 5, which illustrates schematically current flow between a cathode and anode in an electrolyte both with no edge shield (such as barrier 90) being present. In FIG. 5, the path of the current across the bath is represented by dotted lines 200 whose spacing indicates the current density across the electrolyte. In this right-hand area 202 of the sketch, where vast continuous areas of anode and cathode are exposed to one another at a uniform spacing, a uniform current density exists in the electrolyte assuming that the resistivity of the electrolyte is the same throughout its volume. In the area of the edge 204 of the sheet, the current will proceed directly across the electrolyte in the same manner and with the same density as in the right-hand area, but in addition to that current, the volume of the electrolyte to the left of the edge not directly between the anode and cathode is also capable of conducting current, and additional current will leave the anode in the area near the edge to take advantage of the conductivity of this extra volume of electrolyte. This extra flow of current is represented in FIG. 5 as dotted lines 206 extending well out into the side body of electrolyte. The conducting path may be somewhat longer, and as a result the actual current density across the electrolyte will fall off dramatically as one moves away from the anode edge to the left into the electrolyte volume, but the net effect as far as the cathode is concerned is a considerably higher total amount of current discharged at that edge, and consequently a thicker deposit.

In order not to confuse separate effects, the current lines which would normally emanate from the back of the anode (which is also at a potential relative to the cathode and the electrolyte, and which would also inevitably be the source of additional current) have not been shown in FIG. 6. This additional current, due to the greater exposed anode area which is often assumed to be the sole cause of edge build-up, is really only one of several contributing factors. For example, if the anode were insulated on its back and along its edges, entirely masking all of its surface other than that facing the cathode, there would still be a considerable edge build-up due to the greater volume of electrolyte available for carrying current near the edges of the electrodes.

In the system of the present invention, barriers 90 cut off all extraneous current paths immediately before the edges of the moving cathodic sheet 54, so that current flow is not permitted to loop out into the side body of electrolyte. Except for a very narrow band of no more than l-2 cm. in width, therefore, the edges of thesheet will see no more current flow than the center of the sheet. In addition, the back shields 96 will serve as a rear current barrier to prevent electrodeposition on the rear surface of sheet 54. These functions are, as indicated, performed in addition to the primary function of establishing uniform electrolyte circulation patterns in conjunction with the unique anode of the present invention.

As previously indicated, each of anodes 24 is provided with a non-conductive sheet 113 on one of its faces. This non-conductive sheet has the effect of making each anode a one-sided anode so as to avoid gas evolution on both sides of the anodes. Unless this were done, the air lift effect on one side of a given anode would cancel out that on the reverse side and the advantages of the present invention would not be obtained. A similar effect could be obtained from placing two non-insulated anodes back-to-back in order to create a still volume of electrolyte between them.

The operating conditions employed in the carrying out of the process of the present invention will vary through wide limits depending on a number of variables including but not restricted to the nature of the base metal as well as plating metal, the thickness of the base metal, the thickness, smoothness and uniformity of the plated metal which is desired, the speed of movement of the base metal sheet throughout the electrolyte, and so forth. Those skilled in the art will have no difficulty adapting the proper conditions of the present invention to the production of a particular bimetallic product.

By way of example of the practice of the present invention, a matte chrome surface may be electrodeposited on a moving copper sheet approximately 60 inches wide using an electrolyte approximately as follows:

Bath Composition Grams pcr Liter Chromic Acid 200 Sulfuric Acid 2 -29C, (preferably C.) 400 amps per square foot 8-8 volts 3/4 inch 8 inches per minute 2-4 minutes Temperature Cathode Current Density Average Tank Voltage Anode-Cathode Spacing Sheet Speed Bath Time Under the operating conditions set forth above, the thickness of a chrome plating on a copper sheet 5 mils thick will be approximately 2 microns.

The circulation of the electrolyte within the tank (other than that provided by the novel barrier-anode arrangement previously described) need not exceed more than 2 or 3 gallons per minute. This flow is sufficient to keep the electrolyte from being depleted but does not affect the flow characteristics caused by the Pohle air lift effect described above.

By means of the novel process and apparatus of the present invention, metal sheets can be produced having a continuously electrodeposited coating the thickness of which is extremely uniform across the entire width of the sheet. For example, while prior art techniques often result in coating thickness variations across the width of the sheet which vary 100% or more, the present invention can reduce this thickness variation figure to 50% and even to 10-30% and less.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment and processes are therefore to be considered in all aspects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

] claim: I

1. Apparatus for electroplating a metal from an electrolyte solution onto the surface of an elongated strip of metal comprising, a tank for containing electrolyte, at least one anode in said tank, said anode being substantially planar, provided with an elongated slot near its upper edge and being positionable in said tank so that gas evolution between said strip and said anode will cause electrolyte to circulate through said elongated slot adjacent the surface of said electrolyte, means for connecting said anode to a positive terminal of an electric power source, means for passing and guiding said strip of metal into and out of said electrolyte in proximity to said anode, means for rendering said strip of metal cathodic in the electrolytic circuit during its passage through the electrolyte, and electrically insulating barriers shaped so as to surround the edge of said strip of metal, one portion of each of said barriers abutting said anode and another portion of said barriers abutting an electrically insulating shield adjacent the side of said strip of metal remote from said adjacent anode, the barriers serving to confine a portion of the electrolyte between said strip of metal and said anode, preventing the migration of electrolyte transversely across the edges of said strip, and creating a substantially uniform upward flow of electrolyte past the surface of and substantially throughout the width of 4. Apparatus as defined in claim 2 wherein said confining means comprises electrically insulating barriers at each edge of said strip of metal and extending between said strip of metal and its adjacent anode; said anodes being substantially planar and being substantially vertically arranged in parallel manner; said anodes each being provided near its upper edge with an elongated slot; the lower edge of said slot being positionable so as to serve as said anode portion adjacent the surface of said electrolyte; said anodes each being provided with means supporting it in said tank at a point above the surface of said electrolyte; said passing and guiding means being adapted to move said strip of metal in said tank in serpentine fashion so as to travel alternatively upwards and downwards between successively arranged anodes.

5. Apparatus as defined in claim 1 wherein said anodes are each provided with means supporting it in said tank at a point above the surface of said electrolyte.

Luz-cu: No. .QG' m 2 Dated 4/9/74 3 Charles rams ;n.u.. \a

It is certified that :zror appears in :h above idez;tified.patez=: and 51:22: said Letters i'atent" e: carrecteci as shown below:

Calumn 4, line 23 deletes 05' anci sibstitute -ta-. j 1 Col-and 6 line 23, delete 'TJLS" and substltucc :.qe.-

Ii ll Calm-m1 6, line '52; delete 0 and 'subs-cituze -5-- Page 7, '..l ine 48 delete 8-8 volts" and substitute --ii6 Volts-' Signed seald this 10th day 1 September 191%. i

; Attest:

attesting Officerr; Commissioner of Patents

Claims

1. Apparatus for electroplating a metal from an electrolyte solution onto the surface of an elongated strip of metal comprising, a tank for containing electrolyte, at least one anode in said tank, said anode being substantially planar, provided with an elongated slot near its upper edge and being positionable in said tank so that gas evolution between said strip and said anode will cause electrolyte to circulate through said elongated slot adjacent the surface of said electrolyte, means for connecting said anode to a positive terminal of an electric power source, means for passing and guiding said strip of metal into and out of said electrolyte in proximity to said anode, means for rendering said strip of metal cathodic in the electrolytic circuit during its passage through the electrolyte, and electrically insulating barriers shaped so as to surround the edge of said strip of metal, one portion of each of said barriers abutting said anode and another portion of said barriers abutting an electrically insulating shield adjacent the side of said strip of metal remote from said adjacent anode, the barriers serving to confine a portion of the electrolyte between said strip of metal and said anode, preventing the migration of electrolyte transversely across the edges of said strip, and creating a substantially uniform upward flow of electrolyte past the surface of and substantially throughout the width of said strip of metal by means of gas evolved between said anode and said strip of metal.

2. Apparatus as defined in claim 1 wherein a plurality of anodes spaced from one another are positioned in said tank in the manner recited.

3. Apparatus as defined in claim 2 wherein said anodes are substantially planar and are substantially vertically arranged in parallel manner; said passing and guiding means being adapted to move said strip of metal in said tank in serpentine fashion so as to travel alternatively upwards and downwards between successively arranged anodes.

4. Apparatus as defined in claim 2 wherein said confining means comprises electrically insulating barriers at each edge of said strip of metal and extending between said strip of metal and its adjacent anode; said anodes being substantially planar and being substantially vertically arranged in parallel manner; said anodes each being provided near its upper edgE with an elongated slot; the lower edge of said slot being positionable so as to serve as said anode portion adjacent the surface of said electrolyte; said anodes each being provided with means supporting it in said tank at a point above the surface of said electrolyte; said passing and guiding means being adapted to move said strip of metal in said tank in serpentine fashion so as to travel alternatively upwards and downwards between successively arranged anodes.