US3226315A

US3226315A - Continuous electroplating apparatus

Info

Publication number: US3226315A
Application number: US72421A
Authority: US
Inventors: Edwin J Smith; Michael G Vucich; Lowell W Austin
Original assignee: National Steel Corp
Current assignee: National Steel Corp
Priority date: 1960-06-09
Filing date: 1960-11-29
Publication date: 1965-12-28
Anticipated expiration: 1982-12-28
Also published as: DE1300414B; FR1338559A; BE639127Q; GB1015961A; US3167403A; USRE26223E

Description

Dec. 28, 1965 E, J. SMITH ETAL CONTINUOUS ELECTROPLATING APPARATUS Filed Nov. 29, 1960 FIGI FIGZ

IN V EN TORS T HC TU% W S W. L

L N HE W W EML A TTORNEYS United States Patent CONTINUUUS ELEQTRGPLATING APPARATUS Edwin J. Smith, Steubenville, Ulrio, and Michael G.

Vucich and Lowell W. Austin, Weirton, W. Va, assignors to National Steel Corporation, a corporation of Delaware Filed Nov. 29, 1960, Ser. No. 72,421 8 Claims. (Cl. 204206) This invention relates to a novel method and improved apparatus for the electrodeposition of a coating metal on a substrate.

The present invention will be described and illustrated hereinafter with specific reference tothe electrodeposition of aluminum on ferrous metal strip from a fused electrolyte containing a major proportion of aluminum chloride and the remainder one or more alkali metal chlorides such as sodium or potassium chloride. However, it will be apparent to those skilled in the art that the principles of the invention are applicable to other substrates and electrolytes which present the problems solved by the present invention.

A fused electrolyte bath composition having an aluminum chloride content of approximately 75-85% by weight and the remainder sodium and/ or potassium chloride is useful in producing aluminum electroplate. The electrolyte composition may be of the binary or ternary type, with a satisfactory binary electrolyte composition being exemplified by a composition containing by weight 80% aluminum chloride and 20% sodium chloride, and a satisfactory ternary electrolyte being exemplified by a composition containing by weight 80% aluminum chloride, 10% sodium chloride and 10% potassium chloride. Although a fused electrolyte bath containing 8085% aluminum chloride produces objectionable visible fuming in the presence of Water, an electrolyte containing more than 85% aluminum chloride fumes excessively and the plating results usually are not improved by increasing the aluminum chloride content above this value. However, if the aluminum chloride content falls below about 75% by weight, poor plating results when using unmodified direct current and in the absence of special plating conditions. When using a fuming fused aluminum chloridealkali metal chloride electrolyte bath, such as a bath containing 80% by weight aluminum chloride, 10% by weight sodium chloride and 10% by weight potassium chloride, current densities up to 100 amperes per square foot and often as high as 100-400 amperes per square foot may be used with straight direct current, and current densities for special plating conditions may be equally satisfactory or even better.

Still other aluminum electroplating baths are known to the art and may be used in practicing the present invention. For example, one or more of the chloride salts of the electrolyte baths mentioned herein may be replaced with other suitable halide salts such as aluminum bromide, sodium bromide, and potassium bromide in amounts to maintain the same molar proportions. Also, the aluminum chloride may be complexed with other complexing agents such as ammonia and organic amines, or their salts in a manner somewhat similar in effect to the complexing of aluminum chloride in the foregoing baths with potassium and/or sodium halide. An aluminum chloride-ammonia bath contains complexed aluminum chloride which is thought to be in the form of A1Cl .NH or possibly higher members of the ammonia series with varying amounts of free aluminum chloride being dissolved therein in the case of a fuming bath. Further, aluminum chloride may be complexed by means of organic amines and their salts such as ethyl pyridinium bromide to produce the complex ethyl pyridinium bromide AlCl This material may contain free dissolved aluminum chloride and it may be used as a bath for practicing the present invention. The various aluminum electroplating baths discussed above may be considered to be, broadly speaking, fused salt electroplating baths containing aluminum halide and a complexing agent therefor, with the aluminum halide complex being in the molten state and having dissolved therein free aluminum halide in proportions causing fuming at the operating temperature.

It is understood that when substituting other halides for the chlorides in the electroplating baths discussed herein, the substitutions may be made in molar proportions rather than in proportions by weight so as to provide the same molar proportion of aluminum halide, the complexing agent and the free dissolved aluminum halide. Unless specifically stated to the contrary, the parts and percentages of the specific electrolyte bath ingredients are in parts or percentages by weight. If desired, substitution of other materials may be made on a molar basis, as mentioned above. When electroplating a substrate which is a conductor of electricity with aluminum in accordance with the teachings of the present invention, the above electroplating baths may be used under the same general conditions as used by the prior art with the exceptions noted hereinafter.

In conventional electroplating apparatus for use with the above'mentioned molten salt baths, in electroplating aluminum on a substrate, the contact rolls are arranged above the electrolyte and the substrate is Withdrawn from and returned to the electrolyte repeatedly as the plating progresses. With continued plating over extended periods of time, deposits of electrolyte salts and/or other substances removed from the bath build up on the contact rolls and become embedded in the roll surfaces with the result that the rolls are no longer able to perform effectively their primary function of supplying plating current to the substrate and plating difficulties invariably develop. Additionally, the surface of the electroplated strip is marred by mechanical scratching of the surface and the electrodeposit is characterized by poor adherence and color.

In accordance with one important embodiment of the present invention, it has been discovered that the abovementioned difficulties may be overcome by providing an improved arrangement for the contact and guide rolls of the electroplating apparatus and mounting of the same, including providing contact rolls at least partially submerged and by electroplating the strip in such a manner that it may be submerged in the electroplating bath from the time plating commences until it is completed, if this should be desirable.

it is an object of the present invention to provide a novel method and improved apparatus for electroplating a coating metal on a substrate.

It is a further object of the present invention to provide a novel method and improved apparatus for electroplating aluminum on a substrate from a fused salt electrolyte.

It is still a further object of the present invention to provide improved electroplating apparatus including contact rolls at least partially submerged in the electrolyte.

Still other objects and advantages of the invention will be apparent to those skilled in the art upon reference to the following detailed description and the drawings, where- 1n:

FIGURE 1 is a diagrammatic side elevational view partially in cross section, of one arrangement of apparatus suitable for use in practicing the present invention; and

FIGURE 2 is a cross sectional view taken along the line 2-2 of FIGURE 1, but omitting a showing of the strip,

electrolyte, electrical leads to the contact rolls, and electrodes for the purpose of clarity.

Referring now to FIGURES 1 and 2 of the drawings, which illustrate one embodiment of the invention, the ferrous metal strip is shown passing under roll 11 after receiving a suitable prior art cleaning treatment such as cathodic cleaning in an Ortho-Sil solution (Na SiO employing a treating time of 12 seconds at a current density of 35 ampere seconds per square foot, followed by, in sequence, brushing and spraying with water, anodic pickling in a suitable electrolyte such as aqueous 35% sulfuric acid for 2-3 seconds at 80-100 amperes per square foot, scrubbing and washing with water and drying to remove all traces of water. The ferrous metal strip then passes upward through strip conditioning unit 12 where it is thoroughly dried to remove any remaining traces of free water and then passes over

rolls

14 and 15. If desired, the ferrous metal strip 10 may be treated within strip conditioning unit 12 in a reducing atmosphere or otherwise treated for the purpose of further conditioning the strip surface and assuring absolute removal of free water, combined water and substances forming water under conditions present in the electroplating unit. It also may be desirable to preheat the strip to a temperature about the temperature of the bath at the time plating commences.

Two pairs of sealing flaps 16 and 17 are provided near the entrance and exit ends, respectively, of strip conditioning unit 12 to prevent undue loss of dry gaseous treating agent which may be employed therein, as well as to aid in preventing entry of atmospheric water vapor. The dry gaseous treating agent may be supplied to strip conditioning unit 12 by means of conduit 20 and withdrawn by means of conduit 21. The flow rate of dry gaseous treating agent via conduit 20 may be controlled by means of valve 22 while the withdrawal rate via conduit 21 may be controlled by means of valve 23. Any suitable dry gaseous treating agent may be employed depending upon the nature of the treatment desired. However, the treatment should be such so as to insure the absence from the surface of strip 10 of free water, combined water such as hydrated ferrous metal salts, or substances reacting under conditions present in the electroplating zone to form water. In addition, the resultant dry strip should not be subjected to any source of water while passing from the drying step to the electrolyte. It is preferred to surround the strip while passing from the drying step to the electroplating bath with a dry medium, such as a dry non-reactive gas. In instances where a special conditioning treatment is not necessary, the gaseous treating agent may be dry air, nitrogen, carbon dioxide, argon, etc. Usually, it is preferred that the gaseous treating agent be supplied through strip conditioning unit 12 at about atmospheric pressure.

The strip 10 is passed downward from unit 12 into electroplating tank 24 which contains a fused aluminum electroplating electrolyte such as herein described. The fused electrolyte 25 may contain a predominant proportion of aluminum chloride such as for example 80% aluminum chloride, 10% sodium chloride and 10% potassium chloride on a weight basis, and when such an electrolyte is used under the operating conditions it fumes in the presence of water or water vapor. A top 30 is positioned above electroplating tank 24. The exit end 31 of strip conditioning unit 12 extends downward a short distance through opening 32 in top 30 and, similarly, the entrance end 33 of unit 36 extends downward a short distance through opening 34 in top 30, thereby providing an entrance and exit respectively to electroplating tank 24. The top 30 is joined to strip conditioning unit 12 and unit 36 in airtight relationship, and since sealing flaps 17 and 38 are provided near

ends

31 and 33, respectively, this combination of elements substantially seals off electrolyte 25 from the surrounding atmosphere. A gas or gaseous mixture such as dry nitrogen, carbon dioxide, argon or air is fed by means of conduit 39 into space 40 which is formed within electroplating tank 24 above level 41 of electrolyte 25 and the under surface of top 30. A valve 42 is provided in conduit 39 for the purpose of controlling the feed rate of dry gas to space 40.

Electroplating tank 24 is provided with spaced pairs of aluminum anodes 45 which may be of high purity aluminum. The strip 10 passes downward from roll 15 into fused electrolyte 25 and is subsequently passed between the spaced pairs of anodes 45 by means of a path established by rolls 4658. The anodes 45 are electrically connected to the positive side of generator 64 (or other suitable source of direct current), while strip 10 is made electro-negative between

contact rolls

15, 48, 52 and 56. The strip 10 may be electroplated with a coating of aluminum of a desired thickness while passing through electroplating tank 28.

When the electrolyte 25 contains by weight aluminum chloride, 10% sodium chloride and 10% potassium chloride, it may have a temperature of about 250-400 F. and, preferably, about 300-350 F. Current density preferably should be about 20 amperes per square foot. However, under optimum conditions of operation current densities up to about 100400 amperes per square foot are possible and may be used when desired under such operating conditions. Except as specifically stated herein, the electroplating conditions for use in accordance with the present invention may not differ from those of the prior art when electroplating an aluminum containing coating.

Upon passing under roll 58, strip 10 passes upward through electrolyte 25 and after emerging therefrom passes between squeegee rolls 65, which reduce the amount of drag out. The aluminum coated ferrous metal strip 10 then continues to pass upward through unit 36 and over roll 66. A pair of sealing flaps 67 are provided at the exit end 35 of unit 36 for the purpose of sealing off the exit. This allows a slight suction to be applied to unit 36 through exhaust conduit 68 when this may be desired, as well as preventing entry past flaps 38 to space 40 of atmospheric water vapor which would have a detrimental effect on the aluminum chloride content of the electrolyte. The exhaust conduit 68 may be provided with valve 69 for the purpose of controlling exhaust of unit 36, in instances where unit 36 is to be exhausted. The aluminum plated strip 10 passes from the exit end 35 of unit 36 and may be washed with water and dried or given other subsequent treatments as are conventional in the art. However, the washed and dried electroplate is generally very satisfactory for use directly in the manufacture of containers of the type for which tinplate is generally used.

For the purpose of minimizing loss of heat, electroplating tank 24 may be surrounded by an insulating material 70. Additionally, and in order to maintain the fused electrolyte 25 at the proper operating temperature, heating means (not shown) may be employed as is conventional in the art. For example, immersion heating may be employed in the conventional manner wherein the heating units are immersed directly in the electrolyte, or the insulating material 70 may be spaced from the electroplating tank 24 and the tank surrounded within the space thus formed by suitable heating means such as fin-type strip electrical heaters.

The gas or gaseous mixture fed through conduit 39 to space 40 must be dry. Also, preferably, it should be heated to an elevated temperature such as at least about 40 F. below the temperature of the electrolyte bath, or higher. The pressure employed within space 40 may beatmospheric pressure or slightly above, although any suitable convenient pressure may be applied. It is only necessary to replace the atmosphere within space 40 with a dry gas and a slight suction may be maintained, if desired, on

units

12 and 36 by means of exhaust conduits 21 and 68, respectively, to aid in the escape of gas fed to space 40 together with any moisture. The composition of the specific dry gas or dry gaseous mixture supplied to space 40 may vary widely. The dry gas or gaseous mixture should be substantially non-reactive with the electrolyte 25, the strip and the apparatus contacted by the gas or gaseous mixture under the conditions of operation of the electroplating line.

As best seen in FIGURE 2, the guide rolls 46, 50, 54 and 58 are fixedly mounted on shafts 72-75, respectively, and the shafts 7275 in turn are rotatably mounted within electroplating tank or vessel 24 by bearings 76-79, respectively. Similarly, guide rolls 47, 49, 51, 53, 55 and 57 are fixedly mounted on shafts 8045, respectively, and shafts 80-85 are rotatably mounted within electroplating tank 24 in bearings 86491, respectively. The contact rolls 48, S2 and 56 are fixedly mounted on shafts 9294, respectively. The shafts 92-94 pass through openings 99 in the walls of electroplating tank 24 and are rotatably mounted in mounting means 9597, respectively, and driven by driving means such as electrical motors 98 which may be arranged outside of the electroplating tank 24.

Upon reference to FIGURES 1 and 2, it may be noted that guide rolls 47 and 49 are arranged slightly under the level 41 of electrolyte 25 and spaced apart horizontally so as to pass the strip 18 along a path near the surface of the electrolyte while maintaining it submerged at all times. Additionally, the contact roll 48 is supported on shaft 92 and shaft 92 is mounted above level 41 of electrolyte 25, and thus only a vapor seal is required for the openings 99 in the walls of electroplating tank 24. The contact roll 48 is of a diameter whereby its lower peripheral surface is in electrical contact with the portion of the strip 10 passed over guide rolls 4'7 and 49, and the contact roll 48 may be mounted in opposition to at least one of the guide rolls, and preferably both as shown, whereby a driving relationship exists thereby allowing the strip 10 to be pulled through the electroplating apparatus upon driving shaft 92 by means of motor 98. The contact roll 48 -:is shown as being submerged Within the electrolyte 25 to about A to /3 of its diameter, and this is the preferred practice in most instances. However, it is possible to partially submerge the contact roll 48 to a greater depth or completely submerged, if this should be desirable.

The relationship above discussed for contact roll 48 and guide rolls 47 and 49 also exists for contact roll 52 and guide rolls 51 and 53, and contact roll 56 and guide rolls 84 and 85.

The above described arrangement of rolls provides many advantages over prior art practices. For instance, once the strip 10 enters the electrolyte 25 and electroplating of aluminum thereon is commenced upon passage between rolls and 46, the strip continues through the electrolyte and a plurality of passes is made between spaced pairs of anodes 45 without the strip being removed from the electrolyte until the electroplating is completed. Thus, any given portion of the strip is submerged within the electrolyte from the time the electroplating of aluminum thereon commences until it is completed. This method of plating has been found to produce a greatly improved product as a more tightly adherent, non-powdery electrodeposit of substantially better color is obtained than when using the usual hairpin arrangement of rolls wherein the strip after electroplating has commenced is passed out of the electrolyte and then returned to the electrolyte for further electroplating.

The guide rolls 46, 47, 49, 50, 51, 53, 54, 55, 57 and 58 are all completely submerged within the fused electrolyte 25 and mounted by means of their respective shafts and bearing supports Within the electroplating tank 24. Thus, the rolls are maintained at the temperature of the electrolyte and there is no tendency for solidified salts to buildup. Since the above guide rolls are supported within the electroplating tank, the walls of the electroplating tank 24 are not pierced by their respective shafts and liquid seals are eliminated. The contact rolls 48, 52 and 56 are partially submerged in the electrolyte to obtain the benefits noted above and mounted on shafts 9294 which pass through the walls of the electroplating tank 24 via openings 99 at a point above the electrolyte level, and liquid seals are not necessary and vapor seals may be used. Thus, liquid seals are not needed for any of the rolls' As shown in the drawings, the strip 10 may be passed through the electrolyte along a path whereby it is in electrical contact with the lower peripheral surfaces of contact rolls 48, 52 and 56, and with the strip 10 being between the contact rolls and the guide rolls and anodes. This results in important additional advantages as the strip 18 shields the contact rolls from stray currents and assures that a minimum of any solid material present in the electrolyte will be carried to the contact rolls and embedded therein. Also, arrangement of the contact rolls above both the anodes and the strip further reduces the amount of solid material from the working anodes which is deposited on and embedded in the surfaces of the contact rolls.

The various guide rolls may be glass-coated rolls which are highly resistant to corrosion and have substantially no tendency for solids from the working anodes 45 or other sources to become embedded in the roll surfaces and thereby damage the strip surface, while the contact rolls 48, 52 and 56 may be constructed of a non-contaminating substance such as aluminum. When metal strip is electroplated in accordance with the present invention with the the strip being submerged and remaining submerged throughout the electroplating process, the coating is adherent, uniformly bright, smooth and free of surface blemishes including scratches. Since the strip is contacted only by rolls submerged or partially submerged within the electrolyte, the electrolyte has the additional advantage of acting as a lubricant and the usual friction between the rolls and the strip is greatly reduced and smoother and brighter electroplate surface is produced.

When the electroplating of aluminum is referred to herein, it is understood that the invention is equally applicable to the electroplating of alloys of aluminum with another metal such as manganese, titanium, cobalt, chromium, molybdenum, etc., and combinations thereof,

The foregoing detailed description and the following specific examples are for purposes of illustration only, and are not intended as being limiting to the spirit or scope of the appended claims.

EXAMPLE I Ferrous metal strip subjected to a prior art cleaning step to remove surface oil, oxides, etc., is fed to electroplating apparatus similar to that illustrated in FIGURES 1 and 2 of the drawings at a speed of about 10 feet per minute. The strip is heated to a temperature of about 325 375 F. and treated in a reducing atmosphere for the purpose of completely removing free water, combined water, such as hydrated metal salts, and substances reacting under conditions present in the electroplating zone to form water, and it is in this condition at the time of entering the electrolyte.

The electrolyte contains by Weight aluminum chloride, 10% sodium chloride and 10% potassium chloride, and it is maintained in the fused condition at an operating temperature of about 325 F. A 0.18 pound per base box coating weight of aluminum is electrodeposited on the strip employing high purity aluminum anodes and current conditions of about 42.5 amperes per square foot.

During the above electroplating operation, no difficulty is experienced due to poor electrical contact between the strip and the contact rolls. Additionally, after extended operation, the contact rolls as Well as the remaining rolls in the apparatus are free of salts which tend to collect on the roll surfaces of prior art apparatus and damage the strip surface and/or result in poor electrical contact. Additionally, other mechanical difficulties such as leakage of electrolyte around bearing surfaces for the roll shafts is completely eliminated.

The resultant plated product produced in accordance with the above process possesses a uniformly bright, tightly adherent, dense mat deposit of fine grain which is pleasing in appearance and free of surface blemishes, including surface blemishes such as scratches and an off color coating. Thus, the process produces a product which is commercially acceptable in all respects.

EXAMPLE 11 Upon electroplating ferrous metal strip under the conditions of Example I, with the exception of using a conventional roll arrangement wherein the contact rolls are above the surface level of the electrolyte, the resultant electroplated strip has surface blemishes resulting from mechanical scratching of the strip as well as surface blemishes due to operating conditions which are be- :lieved to be caused by removing the strip from the electrolyte after plating has commenced and then returning the same back to the electrolyte for further electroplating.

In addition to the above difficulties which result in a product having an unsatisfactory appearance, the electroplate is not as adherent. Also, it is very difficult to maintain proper electrical contact between the electrical contact rolls and the strip surface due to build-up of salts on the contact roll surfaces and to embedding of the salts in the roll surfaces.

What is claimed is:

1. Apparatus for electroplating a coating metal on metal strip comprising vessel means, the vessel means being adapted to be filled to a predetermined level with an electrolyte for electroplating the coating metal, means including a series of rolls for passing metal strip through the vessel means along a predetermined path which is below the said electrolyte level, the series of rolls including a first electrical contact r-oll rotatably mounted in the vessel means to rotate around a substantially horizontal axis, the series of rolls also including second and third rolls rotatably mounted in the vessel means in spaced relationship and contiguous to said first contact roll, the spaced second and third rolls being mounted below the horizontal axis of the first contact roll and on opposite sides of a vertical plane passing through the horizontal axis of the first contact roll, the lower peripheral surface of the first contact roll being below the said electrolyte level and the upper peripheral surfaces of the second and third rolls, the first contact roll and the second and third rolls being constructed and arranged in such a manner that the strip may be threaded over the peripheral surfaces of the second and third rolls and under the peripheral surface of the first contact roll whereby the strip may be maintained immersed in the electrolyte in effective electrical contact with the peripheral surface of said first contact roll and the first contact roll may be shielded from stray electrical currents, anode means mounted in the vessel means below the said electrolyte level, and means for passing an clectrolyzing electric current between the anode means and the first contact roll.

2. The apparatus of claim 1 wherein the first contact roll is .a driven roll and the means for passing the metal strip through the vessel means includes driving means for driving the first contact roll.

3. The apparatus of claim 1 wherein the vessel means includes means defining an entrance and an exit for the metal strip and sealing means is provided for substantially sealing off the interior of the vessel means from the ambient atmosphere.

4. The apparatus of claim 1 wherein the said first contact roll is rotatably mounted on mounting means including a shaft arranged above the said electrolyte level and extending outside of the vessel means, the means for passing the metal strip through the vessel means includes driving means outside of the vessel means, and the shaft of the first contact roll is connected to the driving means at a point outside of the vessel means whereby the contact roll is driven.

5. The apparatus of claim 1 wherein the second and third rolls are rotatably mounted in the vessel means on mounting means including shafts which are arranged entirely within the vessel means.

6. The apparatus of claim 5 wherein the first contact roll is a driven roll and the means for passing the metal strip through the vessel means includes driving means for driving the first contact roll.

7. The apparatus of claim 6 wherein the first contact roll is rotatably mounted on mounting means including a shaft arranged above the said electrolyte level and extending outside of the vessel means, the means for passing' the metal strip through the vessel means includes driving means outside of the vessel means, and the shaft of the first contact roll is connected to the driving means at a point outside of the vessel means whereby the contact roll is driven.

8. The apparatus of claim 7 wherein the vessel means includes means defining an entrance and an exit for the metal strip and sealing means is provided for substantially sealing off the interior of the vessel means from the ambient atmosphere.

References Cited by the Examiner UNITED STATES PATENTS 893,814 7/1908 Schmitz 204-206 1,437,003 11/1922 M eller 204-28 1,488,553 4/1924 Peacock 204-39 1,515,092 11/1924 Cowper-Coles 204-28 1,793,914 2/1931 Dorsey 204-206 2,317,242 4/1943 Allen et al 204-206 2,341,158 2/1944 Nachtman 204-206 2,576,074 11/1951 Nachtman 204-28 2,993,848 7/1961 Brennan 204-206 3,007,954 11/1961 Smith et al 204-28 WINSTON A. DOUGLAS, Primary Examiner.

JOSEPH REBOLD, JOHN H. MACK, Examiners,

Claims

1. APPARATUS FOR ELECTROPLATING A COATING METAL ON METAL STRIP COMPRISING VESSEL MEANS, THE VESSEL MEANS BEING ADAPTED TO BE FILLED TO A PREDETERMINED LEVEL WITH AN ELECTROLYTE FOR ELECTROPLATING THE COATING METAL, MEANS INCLUDING A SERIES OF ROLLS FOR PASSING METAL STRIP THROUGH THE VESSEL MEANS ALONG A PREDETERMINED PATH WHICH IS BELOW THE SAID ELECTROLYTE LEVEL, THE SERIES OF ROLLS INCLUDING A FIRST ELECTRICAL CONTACT ROLL ROTATABLY MOUNTED IN THE VESSEL MEANS TO ROTATE AROUND A SUBSTANTIALLY HORIZONTAL AXIS, THE SERIES OF ROLLS ALSO INCLUDING SECOND AND THIRD ROLLS ROTATABLY MOUNTED IN THE VESSEL MEANS IN SPACED RELATIONSHIP AND CONTIGUOUS TO SAID FIRST CONTACT ROLL, THE SPACED SECOND AND THIRD ROLLS BEING MOUNTED BELOW THE HORIZONTAL AXIS OF THE FIRST CONTACT ROLL AND ON OPPOSITE SIDES OF A VERTICAL PLANE PASSING THROUGH THE HORIZONTAL AXIS OF THE FIRST CONTACT ROLL, THE LOWER PERIPHERAL SURFACE OF THE FIRST CONTACT ROLL BEING BELOW THE SAID ELECTROLYTE LEVEL AND THE UPPER PERIPHERAL SURFACES OF THE SECOND AND THIRD ROLLS, THE FIRST CONTACT ROLL AND THE SECOND AND THIRD ROLLS BEING CONSTRUCTED AND ARRANGED IN SUCH A MANNER THAT THE STRIP MAY BE THREADED OVER THE PERIPHERAL SURFACES OF THE SECOND AND THIRD ROLLS AND UNDER THE PERIPHERAL SURFACE OF THE FIRST CONTACT ROLL WHEREBY THE STRIP MAY BE MAINTAINED IMMERSED IN THE ELECTROLYTE IN EFFECTIVE ELECTRICAL CONTACT WITH THE PERIPHERAL SURFACE OF SAID FIRST CONTACT ROLL AND THE FIRST CONTACT ROLL MAY BE SHIELDED FROM STRAY ELECTRICAL CURRENTS, ANODE MEANS MOUNTED IN THE VESSEL MEANS BELOW THE SAID ELECTROLYTE LEVEL, AND MEANS FOR PASSING AN ELECTROLYZING ELECTRIC CURRENT BETWEEN THE ANODE MEANS AND THE FIRST CONTACT ROLL.