Composition for chromium plating a substrate and chromium plating process using such a composition

Abstract

An aqueous composition for the electrolytic deposition of a chromium coating on the surface of a substrate is disclosed, in which it has a pH of between 0 and 1 and which contains a trivalent chromium salt, glycine, an alkali metal salt, an aluminium salt and, optionally, an ammonium salt. The chromium plating process according to the invention comprises the immersion of the substrate to be treated in this composition and the application of a current between this substrate and an anode. This process makes it possible to form a high-quality chromium coating on the entire surface of the substrate.

1 . An aqueous liquid composition for the electrolytic deposition of a chromium coating on the surface of a substrate, said composition containing 0.4 to 0.9 mol/l of trivalent chromium salt, 0.6 to 0.9 mol/l of glycine, an alkali metal salt, an aluminium salt and optionally an ammonium salt,
 and having a pH of between 0 and 1.   
     
     
         2 . The composition according to  claim 1 , wherein the concentration of trivalent chromium salt is between 0.7 and 0.9 mol/l. 
     
     
         3 . The composition according to  claim 1 , wherein at least one of said trivalent chromium salt, said alkali metal salt, said aluminium salt and, where applicable, said ammonium salt, is selected from a group consisting of chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, sulfonates, thiocyanates, or any one of the mixtures thereof. 
     
     
         4 . The composition according to  claim 1 , wherein the concentration of aluminium salt is between 0.06 and 0.7 mol/l. 
     
     
         5 . The composition according to  claim 1 , wherein the concentration of alkali metal salt is between 0.2 and 1.9 mol/l. 
     
     
         6 . The composition according to  claim 1 , wherein the concentration of ammonium salt is between 0 and 1.0 mol/l. 
     
     
         7 . The composition according to  claim 1 , wherein the concentration of ammonium salt is between 0.5 and 0.8 mol/l and the concentration of alkali metal salt is between 0.2 and 1.3 mol/l. 
     
     
         8 . The composition according to  claim 1 , wherein the composition is devoid of ammonium salt and the concentration of alkali metal salt is between 1.5 and 1.9 mol/l. 
     
     
         9 . A method for the electrolytic deposition of a chromium coating on the surface of a substrate, comprising:
 immersing said substrate and anode in a bath of an aqueous composition according to  claim 1 ,   and applying a current between said substrate and said anode.   
     
     
         10 . The method according to  claim 9 , further comprising applying a continuous current between the substrate and the anode. 
     
     
         11 . The method according to  claim 9 , wherein the temperature of said bath is between 20° C. and 80° C. 
     
     
         12 . The method according to  claim 9 , wherein applying a current between the substrate and the anode is carried out for a suitable duration for forming on the surface of the substrate a chromium coating with a thickness of between 5 and 500 μm. 
     
     
         13 . The composition according to  claim 3 , wherein a plurality of said trivalent chromium salt, said alkali metal salt, said aluminium salt and, where applicable, said ammonium salt, are selected in the group consisting of chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, sulfonates, thiocyanates, or any one of the mixtures thereof. 
     
     
         14 . The composition according to  claim 4 , wherein the concentration of aluminium salt is between 0.2 and 0.3 mol/l. 
     
     
         15 . The method according to  claim 10 , wherein the density of said continuous current is between 10 and 100 A/dm2.