Abstract:
A container method for pre-treating the inside surface of a titanium container before electroplating a metal conductive such as silver to increase adhesion for use in a lightweight alkaline battery comprising the step of applying an alkaline solution of peroxide to the surface.

Description:
TECHNICAL FIELD 
   A method is disclosed for treating titanium in order to improve the adherence of a metal subsequently applied by electroplating. 
   BACKGROUND OF THE INVENTION 
   Titanium is a readily available metal that finds use in a wide variety of applications, in particular as the structural material in aircrafts, engines, missiles, and bicycles. It is also used in chemical equipment, as X-ray tube targets, and as a material in metal-ceramic brazing. It is economical and relatively light. It is desirable in certain applications to electroplate the surface of titanium with a metal having certain desired properties. For example, it might be desirable to plate titanium with silver because the silver surface would convey electrical conductivity to the surface, since titanium is a relatively poor electrical conductor by comparison. 
   STATEMENT OF THE PRIOR ART 
   Titanium is a notoriously difficult material to electroplate. Conventional pretreatment methods are primarily acid based. “Modern Electroplating” by Frederick Lowenheim (3 rd  edition, John Wiley &amp; Sons) describes five general procedures for activating the titanium surface. These methods include etching with glycol-fluoride solutions, etching in chromic-hydrofluoric acid solutions, electrolytic activation in acetic-hydrofluoric acid solutions, etching in hot hydrochloric acid solutions and surface blasting accompanied by heat bonding of the plated coating. Silver in particular has been plated on vapor-blasted titanium followed by vacuum or argon heat treatment at 190-450 degrees Celsius for one hour. Another method calls for using an alkaline silver cyanide bath at room temperature using a vapor-blasted titanium cathode. These methods have the effect of removing the substrate oxide layer as well as roughening the surface in order to improve adhesion between the surface and the electroplated metal. Abrasive blasting, however, can lead to undesirable metallurgical changes in the substrate. 
   In U.S. Pat. No. 4,416,739, Turner exposes the titanium surface to be electroplated to an aqueous solution of hydrofluoric acid and formamides until a gray deposit appears. In practice, it has been observed that plating on titanium surfaces pretreated by acid-based activation methods do not yield robust adhesion. 
   STATEMENT OF THE INVENTION 
   This invention discloses an alkaline-based activation method which yields superior adhesion. The activated titanium surface can be coated with a conductive metal such as silver which can be used to form a lightweight battery in which the titanium does double duty as electrode substrate and battery case thereby removing the need for metal screens or sheets as electrode substrates. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic drawing showing a battery with a silver plated titanium substrate and  FIG. 2  is a cross-section taken along  2 — 2  of FIG.  1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides an alkaline-based method of activating a titanium or a titanium base alloy surface prior to electroplating. The following ranges have been found to afford an active titanium surface with good adhesion of silver: 
   
     
       
             
             
             
           
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               General 
               Subgeneral 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Alkaline 
               Group IA or 
               Preferred 
               Range 
             
             
                 
               Material 
               NH 4  hydroxide 
               KOH 
                2-8 g/l 
             
             
                 
                 
                 
               NaOH 
             
             
                 
               Peroxide 
               Hydrogen or 
               H 2 O 2   
               10-20 g/l 
             
             
                 
                 
               Group IA 
             
             
                 
                 
               peroxide 
             
             
                 
                 
             
           
        
       
     
   
   The above two substances are mixed at room temperature and the titanium is quickly placed in the mixed solution. Etching is performed for approximately one hour.
     Example   SURFACE ACTIVATION   85 g water   5.5 g sodium hydroxide   15 g hydrogen peroxide (50% by weight in water)   

   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               STRIKE BATH 
                 
             
             
                 
               Silver, as KAg(CN) 2   
                1 g/L 
             
             
                 
               Potassium Cyanide 
               100 g/L  
             
             
                 
               Potassium Carbonate 
               15 g/L 
             
             
                 
               PLATE BATH 
             
             
                 
               Silver, as KAg(CN) 2   
               30 g/L 
             
             
                 
               Potassium Cyanide 
               50 g/L 
             
             
                 
               Potassium Carbonate 
               15 g/L 
             
             
                 
                 
             
           
        
       
     
   
   The Group 1A cyanide is present in the plating bath at a concentration range of 45 to 55%. Silver cyanide is present in the plating bath at a concentration range of 25 to 35%. The Group 1A carbonate is present in the plating bath in a concentration within the range of 10 to 20% 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               Silver, as KAg(CN) 2   
               30 g/L 
             
             
                 
               Potassium Cyanide 
               50 g/L 
             
             
                 
               Potassium Carbonate 
               15 g/L 
             
             
                 
                 
             
           
        
       
     
   
   The components of the surface activation bath are mixed and stirred slowly. The etch bath warms up by itself to about 50 degrees Celsius. Titanium is placed in the activation bath and etched for one hour. The surface of the metal at first becomes shiny but subsequently exhibits a dull grey finish. After this treatment the metal is placed immediately in a dilute strike bath at 100 mA/cm 2  for one minute. Then the metal is quickly dipped in the plating bath at 400 mA/cm 2  for ten minutes. The metal surface is rinsed with silver deionized water and wipe dried. 
   The silver metal film was mechanically stressed to test adherence. Whereas the acid-treated coating attached to the electroplated layer readily peeled off, the silver coating applied to the peroxide pretreated surface remained attached to the peroxide treated surface. The silver film was also placed in 50% KOH at 50 degrees Celsius for one month. Again no peeling was observed of the silver alkaline film formed on the peroxide pretreated surface. 
   One of the proposed uses of the metal coated titanium is as a case for a lightweight alkaline zinc battery. Depositing a conductive layer on the inside of the case eliminates the need for current collector screens or expanded metal. The outside of the case need not be insulated since titanium is a poor conductor of electricity and normally is covered with a layer of poorly conducting titanium oxide. 
   Referring now to  FIGS. 1 and 2  a battery  10  has a case  12  formed of mating halves  14 ,  16  joined at the inner edges  18 ,  20  and sealed by a layer  22  of insulation such as a ceramic, an epoxy, or an injection moldable resin such as polyethylene or polypropylene. The inside surface  24 ,  26  of each half  14 ,  16  is coated with a layer  28 ,  30  of silver by the process of the invention. The silver layer  28 ,  30  protects the titanium surfaces  24 ,  26  from attack by the aqueous alkaline electrolyte present in the sheets  32  of regenerated cellulose separator present between the layer  34  of anode paste such as zinc/zinc oxide and the layer  36  of cathode paste such as Ag/Ag respectively, present on the silver layers  28 ,  30 . Battery terminals  40 ,  42  are connected to the current collecting layers  38 ,  30 . 
   It is to be realized that only preferred embodiments of the invention have been described and that numerous substitutions, modifications and alterations are permissible without departing from the spirit and scope of the invention as defined in the following claims.