Patent Publication Number: US-2002003092-A1

Title: Process for the production of refractory metal plates and expanded metal grids platinized on one side

Description:
REFERRENCE TO A RELATED APPLICATION  
     [0001] This application claims the benefit of priority based on our copending U.S. Provisional application Ser. No. 60/213,246 and German patent application 100 29 837.0 both of which are relied on and incorporated herein by reference. 
    
    
     
       INTRODUCTION AND BACKGROUND  
       [0002] This invention relates to a process for the production of refractory metals plates and expanded metal grids platinized on one side.  
       [0003] Platinum-coated refractory metal plates or sheets and expanded metal grids made therefrom are frequently used as anodes in electrolytic and electroplating processes.  
       [0004] Typical applications are, for example, as inert anodes in the electrodeposition of metal and alloy layers, in particular in the production of decorative and/or functional layers of noble metals or noble metal alloys. Further applications are electrodes for electrolytic production of chromic acid from dichromate, for chromic acid processing (oxidation of Cr3+ to Cr6+), for cyanide oxidation, for electrolytic metal recovery, for electrodialysis, cathodic corrosion protection etc.  
       [0005] Conventional methods of production for plates of refractory metals coated with platinum are based on roll-bonding, in which thin platinum foils are rolled onto one or both sides of the refractory metal. Roll-bonded strip or sheet may then be processed in known manner to yield expanded metal by providing the plates with cuts and drawing them out into expanded metal grids. For mechanical and processing reasons, this process entails a minimum platinum foil thickness of at least some 3 μm. No further reduction in thickness to economize on costly platinum is possible.  
       [0006] In principle, platinum may be electrochemically deposited onto the refractory metal in virtually any desired low thickness. Of course, the refractory metal plates suspended in the platinum electroplating bath are coated with platinum on all sides. However, for use as electrodes, platinization on just one side would be sufficient and would moreover permit economies on the costly platinum. However, it is virtually impossible to achieve effective partial coverage of the refractory metal plates for use under the aggressive conditions in platinum electroplating electrolytes and this would moreover be associated with considerable extra materials and processing costs.  
       [0007] It is extremely difficult to electroplate refractory metals from aqueous electrolytes, due to the spontaneous formation of a protective film in aqueous solutions. Prior roughening in elaborate pickling processes may bring about moderate adhesion of the coating, primarily due to mechanical adhesion. Adhesion may be somewhat increased by heat treatment in a reducing atmosphere. Co-deposition of hydrogen during platinum deposition embrittles the platinum layer due to incorporation of hydrogen into the platinum.  
       [0008] Inadequate adhesion to refractory metals and the low ductility of aqueously deposited platinum layers virtually excludes their use for applications involving severe mechanical stresses and deformation, in particular in the production of expanded metal from coated plates or sheets. An object of the present invention is to produce refractory metal sheets platinized on one side, which are moreover suitable for the production of expanded metal grids, for example for use as electrodes in electrolytic and electroplating processes.  
       SUMMARY OF THE INVENTION  
       [0009] The above and other objects of the present invention can be achieved if refractory metal plates of predetermined dimensions are tightly, form-fittingly joined together back to back, are coated with platinum on their exposed surfaces by melt electrolysis and the plates are then separated one from the other.  
       [0010] In the process according to the invention, the formfitting, tight joining of the refractory metal plates to be coated with platinum is conveniently achieved by screw fastening or welding in the edge area of the plates. The plates are of dimensions tailored to the intended application. Plates of dimensions 400×600 mm are typical. The combination of two plates are screw-fastened together to form a set of plates by providing a sufficient number of congruent assembly holes in the edge area, through which assembly screw fasteners are passed so that the plates may be joined together back to back. In the case of a welded joint, spot or continuous welds may be provided on the edge gap between the two joined plates. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 shows an enlarged view of a section of a titanium expanded metal grid platinized on one side which was produced according to the invention. The crack-free platinum overlay (light coloured), which is firmly attached to the titanium base material (dark coloured), is clearly visible. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0012] The present invention will now be described in further detail. Refractory metals which may be considered are titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten or alloys of these metals. Titanium is particularly preferred.  
     [0013] Before the set of plates is joined and coated, it may be advantageous to remove oxides from the surfaces thereof and activate said surfaces, in particular those which are to be coated with platinum. Sand blasting and pickling in acid, for example in a nitric acid/hydrofluoric acid mixture, is convenient.  
     [0014] The assembled pairs of plates are provided with a platinum coating on their exposed surfaces by melt electrolysis by being placed in a conventional electroplating unit suitable for melt electrolysis processes and being connected therein as the cathode. Platinization is performed in a melt of sodium cyanide and potassium cyanide at temperatures of between 500 and 600° C., wherein sheets of pure platinum are provided as the soluble counter-anodes to the refractory metal plates which are to be coated. The melt electrolyte preferably consists of approx. 50 wt.% sodium cyanide and approx. 50 wt.% potassium cyanide and, when in operation, has a platinum content of approx.  0 . 5  to  3  wt.%. A current density setting of 1 to 5 A/dm 2  is conveniently used. The platinum layer thickness which is to be obtained is controlled by means of the current density and duration of the coating operation. Coating thicknesses of 0.5 to 2.5 μm are preferably produced. In individual cases, it is also straightforwardly possible to achieve greater layer thicknesses, for example of up to 10 μm.  
     [0015] Once the plate pairs have been platinized, the plates are separated by undoing the screw fastenings and/or trimming off the edges. For convenience, generally the plates are identical in size and shape.  
     [0016] After per se known and conventional cleaning and drying operations, the desired refractory metal plates or sheets platinized on one side are obtained and may be put to their intended use or be further processed.  
     [0017] Once coating is complete, one subsequent processing operation of the separated plates may involve heat treatment at 500 to 650° C. in an atmosphere containing oxygen in order to oxidize the surface of the sides without the platinum coating. Heat treatment may typically last from 1 to 60 minutes. This operation firstly passivates the unplatinized side, which is important for use as electrodes in electrolytic processes because only the platinized side is intended to be the functional layer. On the other hand, heat treatment oxidizes the uncoated side of the refractory metal, so imparting thereto an appearance which differs distinctly from that of the platinized side. With titanium as the base material, an attractive, easily recognizable blue colour of the non-functional side may be obtained. As a result, this ensures that the functional and nonfunctional side of such an electrode may readily be distinguished in practical use.  
     [0018] Another processing operation is that the plates are provided with cuts and drawn out into expanded metal grids platinized on one side. The production of expanded metal is known and need not be explained in greater detail here. Typical mesh dimensions are approx. 10×5×1×1 mm. In the case of use as electrodes in the form of expanded metal grids, it is convenient to perform the heat treatment which passivates and identifies the non-functional side once the expanded metal grid has been produced. Depending upon the requirements of the application, the expanded metal grids may then be cut to shape and/or shaped into appropriate electrode geometries.  
     [0019] Quite apart from the platinum savings achieved by the reduction in layer thickness in comparison with conventional roll-bonding, the process according to the invention permits platinum savings of approx. 70% in comparison with electroplating of expanded metal grids on all sides and of approx. 48% in comparison with electroplating of solid refractory metal plates on all sides. Platinization by melt electrolysis imparts excellent adhesion to the platinum layer, especially in the case of the intended one-sided coating. This adhesion is in particular manifested on exposure to severe mechanical stresses, such as the flexural stresses and drawing/stretching operations which typically occur in the production of expanded metal grids.  
     [0020] The refractory metal plates platinized on one side produced according to the invention and the expanded metal grids platinized on one side produced therefrom may highly advantageously be used as anodes in electrolytic and electroplating processes.  
     [0021] Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.