Platinum alloy electrodeposition bath and process for manufacturing platinum alloy electrodeposited product using the same
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an electrodeposition bath of platinum alloy which has characteristics superior to that of pure platinum in terms of luster and high hardness and allows thick plating and to a process for manufacturing a platinum alloy electrodeposited product using the same. 
     2. Description of the Prior Art 
     Platinum is widely used as a noble metal material for decoration. Such decorative platinum are obtained using a known platinum plating bath, for example, as disclosed in Japanese Laid-Open Patent Publication No. Hei-2-107794. 
     However, such conventional platinum plating baths have problems in that they give deposits with lusterless appearance or low hardness, cannot achieve thick plating or has inconsistent deposition efficiency, and thus they are not very preferable for decoration. In addition, industrial use of such platinum plating has been limited to the fields such as electrodes manufacturing. 
     SUMMARY OF THE INVENTION 
     The present invention is to provide a platinum alloy electrodeposition bath employing no pure platinum but an alloy of platinum and other metals, whereby allowing thick plating, giving lustrous or high-hardness platinum alloy layers, and a process for manufacturing a platinum alloy electrodeposited product using the same. 
     In the preceding and following descriptions, the term &#34;electrodeposition&#34; is used as having a broad concept which includes electroplating and electro forming. 
     These and other objects of the invention will become more apparent upon a reading of the following detailed description and embodiments. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to achieve the above-mentioned objects, the platinum alloy electrodeposition bath according to this invention contains 2 to 100 g/lit. of platinum in the form of Pt(OH) 6   2-  complex ion and at least one of Sn, Zn and Pd in an amount of 1 mg/lit. or more. 
     In this case, Sn assumes a stable,state in the form of Sn(OH) 6   2-  and allows to give excellent platinum-tin alloy layers. 
     Further, Zn assumes a stable state if it is present in the bath in the form of Zn(OH) 3   -  or Zn(OH) 4   2-  and allows to give excellent platinum-zinc alloy layers. 
     Pd may be present in the electrodeposition bath in the form of [Pd(NH 3 ) 2  ] 2+  , [Pd(NH 3 ) 2  X 4  ] 2-  or [Pd(NH 3 ) 4  ] 2+ , wherein X is a monovalent anion. Pd is stable when it is present in the bath in the form of complexion expressed by [Pd(NH 3 )n] 2+ , wherein n is 1 to 4. In this case, a halogen anion such as I - , B - , Cl -  and F -  may further be coordinated. Meanwhile, stability of Pd can further be increased by allowing amidosulfuric acid (sulfamic acid), potassium amidosulfate (potassium sulfamate) or sodium amidosulfate (sodium sulfamate) to be present in the electrodeposition bath. 
     If Pd is reacted with an oxidizing agent such as sodium peroxodisulfate and potassium peroxodisulfate prior to its addition to the electrodeposition bath, it can be present in the bath in a more stable state. Move stable complex ion can again be obtained by reacting it with a halogen ion in addition to NH 3 . 
     Subsequent reactions may proceed beneficially if Pd is used in the form of salt such as Pd(NH 3 ) 4  Cl 2 , Pd(NH 3 ) 2  Cl 2  and Pd(NH 3 ) 4  (OH) 2 . Addition of a carboxylic acid such as citric acid, oxalic acid, acetic acid, malic acid and tartaric acid or alkali metal salts of carboxylic acids to the bath effectively served to improve uniformity in the appearance of the deposits, to prevent cracking or to stabilize the bath. 
     Although not so conspicuous as in the ease of carboxylic acids, addition of an alkali metal salt of sulfuric acid or phosphoric acid brought about the same effects. 
     Referring to operational conditions, while DC power supply can of course be employed, a pulse power supply may be used to vary the electrodeposit composition and to make the metal layer appearance smooth. 
     The pH of the bath is preferably 11 or higher, and more preferably 12.5 or higher. The bath temperature is preferably 60° C. or higher, and more preferably 80° C. or higher. 
     The hardness of the electrodeposit may sometimes be increased by recrystallization, if it is subjected to heat treatment as a post-treatment. 
     It is also possible to melt the ground metal and use the resulting metal layer as a film. 
     The platinum alloy electrodeposition bath and the process for manufacturing a platinum alloy electrodeposited product using the same according to tile present invention is as described above. Thus, not only the cost of ground metal can be reduced by using the platinum alloy, but also luster and high hardness, which are the properties unattainable by use of a pure platinum plating bath or pure platinum electroforming bath, can be imparted to the deposit film. 
     The followings are descriptions of preferable embodiments according to the present invention. 
     
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First Embodiment:                                                         
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(1) Electrodeposition bath composition                                    
    K.sub.2 Pt(OH).sub.6 10 g/lit. (in terms of Pt)                       
    ZnO alkaline solution                                                 
                         0.2 g/lit. (in terms of Zn)                      
    KOH                  60 g/lit.                                        
(2) Operational condition                                                 
    Current density      1 A/dm.sup.2                                     
    Temperature          90° C.                                    
    Electrodeposition time                                                
                         120 min.                                         
(3) Result                                                                
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     A lustrous product with approximate 17-μm thickness of platinum zinc alloy was obtained. The Pt purity of the lustrous product was 96%. 
     
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Second Embodiment:                                                        
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(1) Electrodeposition bath composition                                    
    K.sub.2 Pt(OH).sub.6 10 g/lit. (in terms of Pt)                       
    K.sub.2 SnO.sub.3.3H.sub.2 O solution                                 
                         15 g/lit. (in terms of Sn)                       
    KOH                  20 g/lit.                                        
(2) Operational condition                                                 
    Current density      2 A/dm.sup.2                                     
    Temperature          90° C.                                    
    Electrodeposition time                                                
                         240 min.                                         
(3) Result                                                                
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     A semilustrous product of platinum-tin alloy with approximate 30-μm thickness was obtained. The Vickers hardness was found to be 600 to 850 Hv. The Pt purity of the semi lustrous product was 85%. 
     
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Third Embodiment:                                                         
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(1) Electrodeposition bath composition                                    
    K.sub.2 Pt(OH).sub.6 20 g/lit. (in terms of Pt)                       
    Pd(NH.sub.3).sub.4 (OH).sub.2                                         
                         0.3 g/lit. (in terms of Pt)                      
    KOH                  30 g/lit.                                        
(2) Operational condition                                                 
    Current density      3 A/dm.sup.2                                     
    Temperature          90° C.                                    
    Electrodeposition time                                                
                         120 min.                                         
(3) Result                                                                
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     A nonlustrous product platinum-palladium alloy layer with approximate 50-μm thickness was obtained. After the ground metal was melted, the deposit film was subjected to heat treatment at 350° C. for 2 hours in N 2  atmosphere. Thus, a flexible foil of Pt/Pd alloy was obtained. The Pt purity of the foil was 90%.