Process for the deposition of platinum-rhodium layers having improved whiteness

Abstract

The present invention relates to an electrochemical process for the deposition of coatings comprising an alloy of platinum and rhodium on, in particular, decorative articles. The process of the invention is characterized in that defined conditions are employed under which the electrolytically deposited layer has, contrary to expectations, a high whiteness which comes extraordinarily close to the appearance of silver.

1 . Process for the electrolytic deposition of an alloy of platinum and rhodium on, in particular, decorative articles, where the rhodium content of the alloy ranges from at least 40% by weight to not more than 85% by weight,
 wherein,   the electrolysis is carried out at a pH of ≦1 and a current density of ≧2 A/dm 2  in an aqueous solution comprising:   a) 0.4-5.0 g/l of platinum ions,   b) 1.0-5.0 g/l of rhodium ions and optionally   c) one or more additives selected from the group consisting of supporting electrolytes, brighteners, surfactants, wetting agents, ligands which complex noble metals and stabilisers   as electrolytic bath.   
     
     
         2 . Process according to  claim 1 ,
 wherein,   the aqueous electrolytic bath is acidified with up to 100 ml/l of conc. sulphuric acid.   
     
     
         3 . Process according to  claim 1 ,
 wherein,   the temperature is in the range from 20 to 70° C. during the deposition.   
     
     
         4 . Process according to  claim 1 ,
 wherein,   a mixed metal oxide anode is used as anode.   
     
     
         5 . Process according to  claim 1 ,
 wherein,   the current density is not more than 7.0 A/dm 2 .   
     
     
         6 . Process according to  claim 1 ,
 wherein,   the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.   
     
     
         7 . Process according to  claim 2 ,
 wherein,   the temperature is in the range from 20 to 70° C. during the deposition.   
     
     
         8 . Process according to  claim 2 ,
 wherein,   a mixed metal oxide anode is used as anode.   
     
     
         9 . Process according to  claim 3 ,
 wherein,   a mixed metal oxide anode is used as anode.   
     
     
         10 . Process according to  claim 2 ,
 wherein,   the current density is not more than 7.0 A/dm 2 .   
     
     
         11 . Process according to  claim 3 ,
 wherein,   the current density is not more than 7.0 A/dm 2 .   
     
     
         12 . Process according to  claim 4 ,
 wherein,   the current density is not more than 7.0 A/dm 2 .   
     
     
         13 . Process according to  claim 2 ,
 wherein,   the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.   
     
     
         14 . Process according to  claim 3 ,
 wherein,   the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.   
     
     
         15 . Process according to  claim 4 ,
 wherein,   the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.   
     
     
         16 . Process according to  claim 5 ,
 wherein,   the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.