METHOD AND APPARATUS FOR ELECTROPLATING

Abstract

A process for the continuous application of metallic layers on a body comprising providing an electrically conductive body having a surface with an outer surface area; providing a plating apparatus comprising a cylindrical, hollow anode having an internal volume and an inner surface area, an electrolyte having metal ions dissolved therein, a cathode; imparting a charge on said body using said cathode, generating an electrical field by applying an electrical current to said anode, feeding said electrolyte into said internal volume of said anode, feeding said body through said anode, such that said body contacts said electrolyte, whereby said ions plate onto said body, forming a metallic layer on the surface of said body, and withdrawing said body from the anode; wherein a ratio of said anode inner surface area to said body outer surface area is in the range between 2.6:1 to 26:1.

Description

BACKGROUND OF THE INVENTION

The invention relates generally to the electroplating art, and more specifically to an apparatus for electrodepositing a layer of metal, such as zinc, from a plating bath onto a continuously moving base material.

In the present day processes for electroplating coatings on objects, such as tubes, wires, and the like, the object is usually electroplated by passing it through a tank containing the plating solution that requires considerable floor space. Electrical contact to the object is made at intervals by the use of “fingers” or bars on which the object rubs, or by metallic contact rolls, which rotate as the object passes over them. In either case, the contacts are electrical conductors and are troublesome, due to the deposition of metal on the contact material, the scratching of the coating of the object, or the contact voltage drop at high cathode current densities. It is also customary to use soluble or insoluble anodes in the bottom of the plating tank only, producing a far from uniform coating distribution around the circumference of the object, such depending on the “throwing power” of the solution used. The present-day installations for electro-coating objects have an appreciable distance from anode to cathode, causing a considerable voltage drop between the two through the solution. There is an appreciable solution drag-out and loss with the object when it leaves the plating tank, and the solution spray due to gas evolution with insoluble anodes is troublesome. The solution is open to the atmosphere and subject to oxidation and dirt accumulation. Another disadvantage, assuming that the tank anodes are placed equidistant from all points of the object (when the objects are in parallel, as is usually the case), is the variation of current density on the object in the space between the cathode current contacts.

The thickness of the deposit depends upon the current and the immersion time, and, in a continuous strip plating operation, can be controlled by adjusting the strip speed in relation to the current available for plating and the number of plating cells in the line. Consequently, when producing a deposit of a desired thickness, maximum current and high current densities are required for high line speeds. It is therefore important in a strip plating operation to control the conditions affecting current to make full use of the available current and achieve a high rate of production, it is essential to provide effective circulation and agitation of the plating solution in each cell. Maximum solution agitation reduces anode polarizations, which restrict current flow. Proper circulation of the solution between the anodes and the cathode strip is also necessary to assure an adequate supply of the coating metal ions to the strip and thereby improve the plating efficiency, and to prevent deposit burning at high current densities.

It is known in theory that the deposition rate in electrolytic transfer of material increases in proportion to increasing current densities. In practice, however, a diffusion layer forms at the cathode as current densities increase, since the transfer of matter between the anode and cathode is slower than the deposition rate of the ions in the immediate vicinity of the cathode. Thus, the greater the current density that is applied, the greater the diffusion layer around the cathode, and the slower and less complete the deposition rate of the ions on the cathode. Beyond a determined reaction speed, the delivery of metal ions at the phase limit between the material transfer region and charge passage region can no longer compensate for the consumption at the cathode. Therefore, the current density/deposition rate curve exhibits an asymptotic limiting value which occurs due to the electrically insulating diffusion layer resulting from insufficient supply of matter. Electrolyte movement can provide a solution to this problem. As experiments have shown, the thickness of the diffusion layer decreases as the intensity of electrolyte movement increases. On the other hand, metallic deposits become rough and powdery when the selected current densities approach the theoretically possible limiting current densities. Therefore, in order to obtain satisfactory coating qualities, it is necessary to select current densities that lie far below the possible limiting current density and which, as a rule, amount to roughly only one third of the limiting current density. In addition, additives and organic polymers are often added to smooth the coating. These additives accumulate in the solution, which requires cleaning and/or purging of the solution to remove the depleted additives.

In zinc deposition especially, an increased current density leads to unusable zinc deposits at the body that is to be coated owing to the present diffusion layer and the resulting poor transfer of matter. If a zinc anode is used in addition to the zinc ions in the electrolyte so as to maintain constant the percentage of metal ions for the duration of the galvanizing process, passivity effects occur at the zinc anode, since the anodic current density increases at the anode due to the dissolution process at the anode.

An arrangement of metal anodes on both sides of, or adjacent to, the cathode also does not lead to an improvement in the deposition rate or thickness of the coating because this arrangement produces eccentric deposits. I.e. more zinc plates along the sides of the tube leaving the top and bottom with thinner coatings, which means the excess material along the sides is wasted.

SUMMARY OF THE INVENTION

A process for the continuous application of metallic layers on a body comprising providing an electrically conductive body having a surface with an outer surface area; providing a plating apparatus comprising a cylindrical, hollow anode having an internal volume and an inner surface area, an electrolyte having metal ions dissolved therein, a cathode; imparting a charge on said body using said cathode, generating an electrical field by applying an electrical current to said anode, feeding said electrolyte into said internal volume of said anode, feeding said body through said anode, such that said body contacts said electrolyte, whereby said ions plate onto said body, forming a metallic layer on the surface of said body, and withdrawing said body from the anode; wherein a ratio of said anode inner surface area to said body outer surface area is in the range between 2.6:1 to 26:1.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which:

FIG. 1 is side view, partially in cross-sectional view, of the electroplating apparatus of the present invention;

FIG. 2 is a top view of the apparatus of FIG. 1;

FIG. 3 is cross-sectional view of the anode employed in the apparatus of FIG. 1;

FIG. 4 is an end view of the anode of FIG. 3

FIG. 4A is an enlarged view of FIG. 4;

FIG. 5 is an enlarged view of FIG. 1 showing the plating apparatus in greater detail; and

FIG. 6 is a schematic diagram showing plating several apparatuses in a plating production line.

DETAILED DESCRIPTION OF THE INVENTION

Our invention is directed to an improved apparatus for electrodepositing a layer of metal, such as zinc, from a plating bath onto a continuously moving base material, and more specifically, to copper tubing. We found that using a stationary cylindrical anode with a diameter that is between 2.6 to 26 times the diameter of the body to be plated resulted in a more uniform metal coating, higher production speeds, and less equipment when compared to other plating methods used in present industry. For convenience, the invention is shown and described as applied to the electroplating of copper tubing with zinc, but it is within the scope of the invention that the process and apparatus is applicable, with slight modification, to the electroplating of wire or the like base materials, with other metals, where the base materials are of a variety of other metals, such as steel or the like.

Further, the electroplating step can be part of a series of other steps, which are not shown, but are known in the art, such as steps to prepare the base material for electroplating, including, but not limited to, straightening the base material as it is fed from feed rolls, cleaning, pickling, washing, and drying the base material, and post treatments, including further washing, drying and coating the electroplated base with plastic protective coating(s).

Generally, the method employed in the present invention includes preparing a body to be plated, for example, forming a copper tube, preparing the tube for the electroplating step by subjecting it to electro-cleaning and pickling, electroplating the tube, cleaning the tube, drying the tube, and recovering the tube. During the electroplating step, the length of tubing is fed into the electroplating apparatus through an end cover aperture of a hollow cylindrical anode, wherein the anode is in the shape of an electrically conductive pipe, which has a diameter slightly larger than the tubing. As the tubing passes through the anode, an appropriate electrolyte fluid is circulated continuously through the interior of the anode. By applying a current, including alternating polarities, to the anode, and removing the tubing from the apparatus through a aperture in the end cover of the anode, the entirety of the above-noted steps are carried out in a continuous process.

Because the body passes through the center of the cylindrical anode, the metal coating is a generally uniform cylindrical coating. This is an improvement over prior art and current industry practices, which tend to produce coatings that over-accumulate and bulge at the sides and have less coating along the top and bottom. Because these other processes still need to meet a customer's specifications, the process is run until the top and bottom coating thickness meets the requirements, which means the coating on the sides is extra and wasted material. By eliminating this wasted material, the present invention saves not only the added material, but the time and energy used to plate the thinner section, i.e. the top and bottom of the body to be plated.

The process uses a modular apparatus, in the form of a plating unit, to perform the steps described above. As the tubing leaves a first plating unit, it enters a subsequent unit to repeat the steps. This modular arrangement can be repeated with the apparatus connected in series indefinitely until the tubing has a coating of desired thickness and quality. This modular design makes it easier to extend or shorten a production line as product specifications require. It also makes it easier the change, replace, or repair an individual apparatus unit, shutting down the line only for as long as it takes to remove a specific plating unit from the line. Once removed, the line can continue to operate while the unit is repaired.

Turning now to the figures, FIG. 1 shows an individual plating unit 1, comprising a process vat 10 and which receives a body 12, in this case a copper tube, to be galvanized by continuous application of a metallic layer on the body 12. The body 12 is continuously guided through the process vat 10. An electrolyte 18 is fed to the process vat 10 via an electrolyte feed 22 and pump 20 in the form of pipe connections. The electrolyte contains dissolved zinc, which provides the source of zinc ions to plate the body 12. The exiting electrolyte flows back into the electrolyte storage tank 24 via return line 26. The flow rate of the electrolyte can be controlled by the pump 20. The flow rate through the anode 30 is about 41 gallons per minute (GPM) to about 72 GPM.

The zinc concentration can range from about 8 to 22 ounces of zinc per gallon of electrolyte, with a preferred range of about 16 to 22 oz./gal. The electrolyte comprises a zinc sulfate and sulfuric acid in aqueous solution. The zinc level is maintained in the electrolyte solution by zinc source 29, such as pellets of zinc, stored in the electrolyte storage tank 24. As the zinc ions that are dissolved in the electrolyte solution plate onto the body 12, the pH of the electrolyte drops, becoming more acidic. As the electrolyte becomes more acidic, the zinc source 29 dissolves more, adding zinc ions back into the solution, keeping the electrolyte solution at equilibrium. As the zinc source 29 is depleted, additional units, such as additional pellets, are added to the system to maintain zinc levels. Prior art and current industry practices often use additives to smooth the coating, or help the ions plate more effectively. As used herein, the term “additives” means organic polymers, brighteners, grain enhancers, and any other components that are added to the solution to smooth the zinc coating or help the zinc dissolve in the electrolyte. These additives accumulate in the electrolyte, and eventually reduce the effectiveness of the electrolyte to the point where these additives need to be filtered out, or the electrolyte replaced. This adds time and cost to the process. No liquid additives or additional acid are needed to maintain the electrolyte, which is a cost savings. It is also a safety issue since no personnel are required to handle hazardous acids and additives.

While zinc is the focus of this process, the process could be adapted to use other ions, such as copper, silver, gold, cobalt, tin, nickel, iron, lead, and/or cadmium—essentially any element with a valence of 2. It would be possible to use a mixture comprising more than one ion, such as for example, zinc and nickel, and zinc and cobalt.

As shown in FIGS. 1 and 5, the electrolyte feed 22 is connected with an input end 32 of the anode 30 which is provided with an O-ring seal 39 held in place via electrolyte feed body 32. A similar arrangement is provided on the electrolyte discharge end 38 of the anode 30 via O-ring seal 40 and discharge orifice plate 42 to create an annular space to contain electrolyte which is collected and returned to the electrolyte storage container 24.

As will be seen from FIGS. 1 and 2, the tube or body 12 is driven in its direction of travel 14 by a rotary drive wheel 28 so that it passes into the vat 10 and through the anode 30 where the tube 12 receives an electroplated coating of zinc. Prior to entering the vat, the body may pass through an optional pre-treatment stage 11, where it is washed with water to remove any dirt or other contaminants, as well as helping prevent oxidation of the body 12. The body 12 is prone to oxidize due to its proximity to the high current densities used in the process. Oxidation is disfavored because the ions do not properly adhere to or plate on oxidized sections of the body. The wash water drains from the pre-treatment stage via wash drain 27, where it is collected and recycled. The drive wheel 28 also serves as an electrical contact/connector to the copper tube 12 so that it acts as a cathode in the electroplating process, and zinc ions from the dissolved zinc in the electrolyte fluid plate on the body 12 as it passes through and in proximity to the tubular anode 30. The electrolyte output from the anode is recycled via return line 26 and electrolyte storage tank 24.

The inherent velocity impressed on the body 12 to be coated acts in the throughput direction 14. The preferred throughput speed ranges from about 30 meters/min to about 60 meters/min to achieve a zinc coating of 25 μm, which is the typical thickness for use in automotive applications. However, the feed rate can be as low as a few cm/min to over 1000 m/min, depending on the desired thickness of the coating on the body. Even at the low end of 30 m/min, the feed rate is about double that of current industry plating processes, which operate at about 15 m/min using a production line that is about twice the length of the present invention. In other words, the present invention achieves twice the speed using half the space as compared to current industry. Current industry uses wire baskets, which cannot withstand the high current densities of the present invention. The baskets can only function up to about 9 volts, and begin to disintegrate or dissolve at higher voltages. The present invention, however, can operate from very low voltage of about 0.1 volts up to about 20 volts. The preferred range is between about 13 v to about 18 v.

The electrolyte 18 is under pressure as it passes, counter-current, through the hollow body of a stationary anode 30. The anode is shown in more detail in FIGS. 3 and 4. The anode 30 comprises a titanium tube that is approximately 1.5 inches in its outside diameter. The tube is coated with a polymeric protective coating, such as iridium oxide, and may have an acrylic urethane protective coating or any other appropriate coating to minimize any corrosion. The tube is approximately 20.5 inches long, but the precise length is determined by the dimensions of the electrolysis container. The anode has a connector 50 to connect it to the electric current power source so that it functions as an anode. As shown in FIG. 4A, the anode 30 has an inner surface 31 and an inner diameter d_A 33. The body 12 has a surface and an outer diameter d_B 34. The anode inner diameter d_A is about 1.04 inches, the body diameter d_B can range between about 0.04 to 0.4 inches. For optimum performance, the ratio of anode inner surface area SA_A to body outer surface area SA_B, SA_A:SA_B can range from a minimum of about 2.6:1 to a maximum of about 26:1. As used herein, the SA_B is the effective surface area, meaning the surface area of that portion of the body that is inside or passing through the anode at a given moment. It would not include the entire length of the body, such as the portions between plating units and the portions on the coils/rolls.

At the minimum 2.6 to 1, this ratio gives the minimum gap between the surface of the body 12 to be plated to the surface of the anode 30. This ratio allows enough space for the circulation of the electrolyte between the anode and cathode/body without having any negative impact to either anode or cathode, such as reduced solution flow, higher solution pressures, burning of the body or erosion of the anode surface coating. This is the best minimum ratio from cost perspective. With any further reduction to the ratio, the anode and body come into close proximity and there is the possibility that the body could sag inside the anode due to gravity, and make contact with the anode, which would be catastrophic to the plating operation since it would create a short circuit. In order to hold the tube in the center consistently at a lower ratio would require significant capital for implementing a means to maintain the body in the centerline of the anode, and to prevent it from burning and breaking apart.

At a maximum ratio of 26 to 1, this ratio gives the maximum gap between the surface of the body 12 to the surface of the anode 30. At higher ratios, the space between the cathode and anode gets too large and causes a reduction in plating efficiencies, due to higher resistance. This leads to lower line speeds and increases costs.

As shown in FIG. 5, the electrolyte enters the anode via input orifice 21 at the input end 32, flows through the anode 30, and discharges via the discharge orifice 36 at the discharge end 38 with the electrolyte flowing counter-current 37 to the body's direction of the travel 14. This process enhances the treated body 12, such as wire or tube, to get uniform coatings regardless of its diameter or surface qualities. The anode 30 connects to a ceramic insert guide 43, which aligns and centers the body 12 to provide even, uniform current densities. The current density is regulated to 10 to 400 A/dm², corresponding to the process to be carried out, via circuit elements, where the circuit elements are known in the art. The O-ring seals 39, 40 seal the insoluble anode 30 to the feed assembly to prevent electrolyte leakage. The seals 39, 40 also allow for quick replacement of components, where the anode assembly can be removed as a unit and replaced with another unit. Discharge orifice 36 and pump 20 control the flow through insoluble anode assembly 30.

As noted above, the Zn deposition rate increases with current density. For example, a current density of 100 amps/ft² (asf) would give a plating thickness of about 3 μm/min. The Table 1 shows how increasing the current density results in a linear increase in the plating rate, at 100% efficiency (theoretical maximum). The present invention achieves plating efficiency using Zn close to 97% of the theoretical maximum yield.

TABLE 1
Current density (asf) Plating rate (μm/min)
100                   3
300                   9.1
400                   12.1
1000                  30.2
2000                  60.4
3000                  90.6
4000                  120.8

As the current density increases, the temperature within the process increases as well. The preferred current density is about 1500 asf to about 2000 asf. The electrolyte solution, in addition to supplying the Zn ions, also acts as a coolant, maintaining the process temperature. If the current density gets too high, it can cause burning or carbon charring on the body, which prevents Zn from plating on the affected area. The operational temperature range of the process is about 104° F. to about 180° F., with a preferred operating temperature range of 135° F.±2° F. The process uses plastic components that tend to melt at temperatures over 140° F. Replacing the plastic tubes with higher grade polymer or metal would allow for higher operational temperature range, but these components would increase equipment costs.

As shown in FIG. 6, a number of plating units 1 are connected in series, where the first plating unit P₁ feeds into the second plating unit P₂, which feeds to P₃, etc., up to n number of plating units, where n is limited by capital investment and plant floor space. The uncoated body 12 is stored on a feed coil 5, and is fed through the series of plating units 1. The coated body 12 is collected on an uptake coil 6. An optional pre-treatment station 2 can be installed at the beginning of the process to remove any dirt or other contaminants as discussed previously. As noted above, the feed rate of the body can be increased or decreased, as can the current density, which affects the number of plating units required for the process. For example, a feed rate of x and a current density of y might require n plating units, but a feed rate of 2x and current density of 2y might only require n/2 or even n/3 plating units.

The last plating unit, P_n, could be left as an auxiliary unit or back-up unit, i.e. only P_n-1 units are required to meet plating specifications. So, for example, if P₂ fails, the line can be temporarily shut down, P₂ can be deactivated, or removed from the line, and P_n can be activated, and the line restarted, while P₂ is repaired. This way, the number of plating units remains the same, so there is no need to adjust feed rates or current densities to compensate for the reduction in the number of plating units.

The foregoing embodiments of the present invention have been presented for the purposes of illustration and description. These descriptions and embodiments are not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above disclosure. The embodiments were chosen and described in order to best explain the principle of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in its various embodiments and with various modifications as are suited to the particular use contemplated.

Claims

1. A process for the continuous application of metallic layers on a body comprising

providing an electrically conductive body having a surface with an outer surface area;

providing a plating apparatus comprising a cylindrical, hollow anode having an internal volume and an inner surface area, an electrolyte having metal ions dissolved therein, a cathode;

imparting a charge on said body using said cathode,

generating an electrical field by applying an electrical current to said anode,

feeding said electrolyte into said internal volume of said anode,

feeding said body through said anode, such that said body contacts said electrolyte, whereby said ions plate onto said body, forming a metallic layer on the surface of said body, and

withdrawing said body from the anode;

wherein a ratio of said anode inner surface area to said body outer surface area is in the range between 2.6:1 to 26:1.

2. The process of claim 1 wherein said electrolyte has a flow direction and said body has a feed direction, wherein said flow direction and said feed direction move in opposite directions relative to each other.

3. The process of claim 1 wherein said anode comprises hollow cylindrical titanium body having an iridium oxide coating thereon.

4. The process of claim 1 wherein said anode is stationary.

5. The process of claim 1 wherein said body comprises a metallic material.

6. The process of claim 1 wherein said body comprises a hollow copper tube.

7. The process of claim 1 wherein said cathode comprises at least a pair of electrically conductive wheels that contact said body and move said body through said anode.

8. The process of claim 1 wherein said metal ions are selected from the group consisting of zinc, nickel, cobalt, copper, silver, gold, tin, cadmium, lead, and iron.

9. The process of claim 1 wherein said body is fed through said anode as a rate of about 5 meters per minute to about 1000 meters per minute.

10. The process of claim 1 wherein said body is fed through said anode as a rate of about 30 meters per minute to about 60 meters per minute.

11. The process of claim 1 wherein said metallic layer is at least 25 μm thick.

12. The process of claim 1 wherein the electrical current applied to the anode imparts a current density between about 10 to about 400 A/dm2 on the surface of the body.

13. The process of claim 1 wherein the voltage of the process is between about 0.1 to about 20 volts.

14. The process of claim 1 wherein the voltage of the process is between about 13 to about 18 volts.

15. The process of claim 1 wherein said electrolyte is zinc sulfate.

16. The process of claim 1 wherein the electrolyte contains no organic polymers.