Patent Publication Number: US-6334943-B1

Title: Electroplating installation, electrode and support device for this installation and electroplating process

Description:
This application is a Division of application Ser. No. 09/204,545 filed on Dec. 3, 1998 now U.S. Pat. No 6,129,820. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a support device for submerged electrodes in the bath of an electroplating installation, and also to the current supply to these electrodes, particularly in the case of electrodes that are expendable, soluble anodes and that have to be exchanged during electroplating. An example of an installation of this type is an installation for zincplating steel strips in a chloride-based electrolytic bath. 
     2. Discussion of the Background 
     Cells are usually used as a zincplating installation for strips. These cells are typically referred to as “radial” cells. A shown in FIGS. 1-6, the cells may include mechanisms for passing the strip  100  to be coated through a bath  5 . Such a mechanism may include, for example, a strip carrying roller  1  at least partially submerged under the surface level of the bath  5 ; support devices  2 ,  4  serving both as support for, and as current supply to, the submerged electrodes  3 ; and mechanisms for causing an electric current to flow between the strip  100  to be coated (serving as cathode) and the electrodes  3  (serving as anodes) on said support devices  2 ,  4 . For zincplating strips in chloride medium, soluble anodes made of zinc or a zinc alloy are generally used. 
     The electrodes  3  (or soluble anodes) are formed of curved bars turned toward the roller  1  along the direction of travel of the strip  100 . The electrodes  3  are also grouped in sets of electrodes  3  positioned side by side so as to form a cylindrical generator portion partially enveloping the roller  1  in the electroplating bath, as illustrated in FIGS. 1 and 2. The arrows shown in FIG. 4 illustrate how an electroplating electric current may flow. 
     As illustrated in FIGS. 1-3, each support device  2 ,  4  is common to all of the electrodes  3  of a corresponding set. In the example given in FIGS. 1-3, each support device  2 ,  4  is formed of a beam positioned transversally to the travel path of the strip  100  on which all the electrodes  3  of a given set rest. Each electrode  3  is held against the beams by an electrode hook  31 . 
     The mechanical and electrical contact between an electrode  3  and its corresponding support device  2 ,  4  defines an interface  6  between a resting surface  6 A of the electrode  3  and a corresponding bearing surface  6 B of the support device (see FIGS.  5  and  6 ). Since the electrodes may be expendable (as in the case of soluble anodes), their thickness varies (see FIG. 3) according to the level of wear, and it may be necessary to change the electrodes  3  during electroplating as they dissolve. During electroplating, the electrodes  3  in the same set are slid along their corresponding support devices  2 ,  4  in the directions A and B respectively (see FIG. 3) in order to remove a worn electrode  3  at one end of the beam while creating a place for a new electrode  3  at the other end. To make the sliding of the electrodes  3  possible, each electrode  3  rests against its corresponding support device  2 ,  4  at the interface  6  only under the force of its own weight. Thus, the electrodes rest freely against their respective support device. 
     The support devices  2 ,  4  also serve to supply the electrodes  3  with electric current for electroplating. It has been noted that the electrical contact resistance at the interface  6  produces large energy losses. Given the weight of each electrode  3 , the pressure exerted at the interface  6  on the support device generally does not exceed 10 4  Pa or 1 Newton per cm 2  of the bearing surface. During electroplating, the circulation of the bath  5  in the installation may cause this resting pressure to fall below 0.1 Newton per cm 2  of the bearing surface (10 3  Pa). As used herein, the term “bearing surface” means the total surface area at the interface  6  between the electrode  3  and the respective support device  2 ,  4 . 
     Energy losses resulting from contact resistance at the interface  6  become particularly significant when the current density exceeds 0.025 A/mm 2  at the interface  6 , particularly when the resting pressure is less than 10  4  Pa, and even more so when the resting pressure is less than 10 3  Pa. The increasing losses in energy appear to stem from a slight lifting of the anodes under the effect of the electric current, requiring the electric supply current of the electrodes  3  to pass in transit through the bath  5  interposed at the interface and causing gas emissions, e.g., emissions of chloride, at this location. This phenomenon is schematized in FIG.  6 . 
     The beams that serve as the support devices  2 ,  4  are generally formed of resin-impregnated graphite. This material wears and deteriorates as a result of two phenomena: first, wear caused by the friction of the electrodes sliding on the beam; and second, fissuring due to heating and/or gas emissions caused by the electrical contact losses described above. 
     A graphite-based material which resists wear well is generally less resistant to fissuring and vice versa. Therefore, it is difficult to find a good compromise when choosing graphite material, and it remains necessary, regardless of this choice, to replace the beams regularly which represents a significant economic handicap. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of this invention is to reduce electrical contact losses at the interfaces between the support devices and electrodes of an electroplating installation. 
     It is another object of this invention to increase the useful lives of the support devices and the electrodes of an electroplating installation. 
     These and other objects of the present invention are achieved according to a novel apparatus and method in which an electrode and a support device form an interface. The interface forms a plurality of grooves that open into the bath. The grooves may be formed in the electrode, the support device, or both the electrode and the support device. The presence of the grooves in the interface increases current density and reduces the contact resistance at the interface. As a result, the useful lives of the support devices and the electrodes are extended. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 shows a perspective view of a strip carrying roller  1 ; 
     FIG. 2 shows a side view of the strip carrying roller  1  and the electrodes  3  of a radial cell; 
     FIG. 3 shows a cross sectional view, taken along the plane of the axis of the roller  1 , showing two sets of electrodes  3  on either side of the roller  1 ; 
     FIG. 4 shows a side view of a continuous strip electroplating installation (i.e., a radial cell) for electroplating a strip  100 , with arrows indicating the flow of the electric current; 
     FIG. 5 shows an electrode  3  resting on a support device  2  at an interface  6 ; 
     FIG. 6 shows a gap between a resting surface  6 A and a contact surface  6 B likely to cause an increase in the contact resistance; 
     FIG. 7A shows a perspective view of the electrode support device  2  having a grooved resting surface  6 B in accordance with the invention; 
     FIG. 7B shows an enlarged view of the circled area A, in FIG. 7A; 
     FIG. 8 shows a conventional electrode or support device resting surface  102  having a planar surface; and 
     FIGS. 9 through 13 show various embodiments of an electrode resting surface  6 B (or, alternatively, a support device resting surface  6 A) constructed in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and more particularly to FIG. 7A thereof, a support device  2  for an electrode  3  is shown. A contact surface or bearing surface  6 B of the support device  2  has grooves  7 . 
     FIG. 7B shows the details of the grooves  7  in the contact surface  6 B of the support device  2 . Preferably, the grooves  7  are not closed, even when the electrodes  3  are resting against the contact surface  6 B. Therefore, the grooves  7  open into the electroplating bath  5  when the installation is operating. 
     Preferably the width and the density of the grooves  7  are suitably adapted such that the total surface area of the grooves  7  does not represent more than 30% of the contact surface  6 B. For a rectangular contact surface  6 B of 200 cm×50 cm, the grooves  7  may have a width, w, of 0.5 mm and may be spaced apart at intervals, d, of 3 cm. The direction of the grooves  7  preferably makes an acute angle a with the small side of the rectangle of the contact surface  6 B. 
     FIGS. 9-13 illustrate various examples of contact surfaces  6 B provided with grooves  7  in accordance with the present invention. The grooves  7  in the contact surfaces  6 B shown in FIGS. 9-13 contrast with the flat, smooth support surface  102  of the conventional contact surface shown in FIG.  8 . 
     The present invention also includes the formation of grooves  7  on the resting surface  6 A of the electrode  3 . Accordingly, the surfaces shown in FIGS. 9 through 13 may be resting surfaces  6 A of an electrode  3  rather than contact surfaces  6 B of a support device  2 . 
     Further, the present invention includes the formation of the interface  6  by the resting surface  6 A of the electrodes  3  and/or the bearing surface  6 B of the support devices  2 ,  4  such that grooves  7  provide an opening for the bath  5  to enter the interface  6 . Accordingly, the electroplating installation of the present invention includes a support interface  6  having grooves  7  that open into the bath  5 . 
     Counterintuitively, the presence of grooves  7  at the interface  6  increases the real current density at the interface  6  even though the real electrical contact surface is reduced. Further, the contact resistance declines appreciably at a constant resting force of the electrodes  3  against the support device  2 . Thus, the grooves  7  made in the support surface  6 A, the contact surface  6 B, and/or the interface  6  reduce the electrical contact resistance between the electrodes  3  and the support devices  2 ,  4 . Further, electrical losses arising from the electrical contact resistance between the electrodes  3  and the support devices  2 ,  4  are also reduced, particularly when the current density at the interface  6  exceeds 0.025 A/mm 2 . 
     Since heating and gas emission at the interface  6 , even at high current densities, are limited by the present invention, the support and electrode contact material is less susceptible to cracking as in the case of graphite-based material used in conventional devices. Accordingly, the present invention allows for the use of graphite-based materials that are much more resistant to wear and greatly improve the useful life of the electrode support devices  2 , 4  with little or no drawbacks. The invention therefore makes it possible to increase the useful life of the electrode support devices  2 ,  4 , and also, if necessary, the useful lives of the electrodes  3  themselves. 
     The present invention is based on French Patent Application No. 97 15 179, filed Dec. 3, 1997, incorporated by reference herein. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. The invention is applicable to all types of electroplating installations where the electrical contacts between the electrodes and their corresponding support devices are submerged in a bath. The present invention may be applied to any variety of plating processes and apparatuses by providing grooves that open the electrical interface to the bath. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.