Dark Colored Chromium Based Electrodeposits

Abstract

An aqueous trivalent chromium electrolyte comprising trivalent chromium ions and amino acids that allow for producing a dark colored hue in the trivalent chromium coating which is plated on a substrate. The amino acids described herein comprise a cationic side chain and are at least essentially free of sulfur. The cationic side chain of the amino acid further comprises nitrogen. When used in the trivalent chromium electrolyte, these amino acids allow for producing significantly darker trivalent chromium deposits. The trivalent chromium electrolyte is used in a method for producing the desired dark colored hue in the trivalent chromium coating that is produced on a substrate using electrodeposition.

1 . A trivalent chromium electrolyte comprising:
 i) trivalent chromium ions,   ii) one or more complexants capable of maintaining the trivalent chromium ions in solution; and   iii) one or more amino acids; wherein the amino acids comprise a cationic side chain comprising nitrogen and wherein the cationic side chain is at least essentially free of sulfur; and   wherein the electrolyte is substantially free of hexavalent chromium salts;   wherein the electrolyte further comprises thiocyanate ions;   wherein concentration of the amino acids in the electrolyte is such that a dark colored chromium deposit produced on a substrate, when the electrolyte is used to plate chromium on said substrate, has an L* value of 78.47 or lower as measured according to an L*a*b* colorspace system.   
     
     
         2 . A trivalent chromium electrolyte according to  claim 1 , wherein the one or more amino acids are selected from the group consisting of arginine, histidine, lysine, tryptophan, and combinations thereof. 
     
     
         3 . (canceled) 
     
     
         4 . (canceled) 
     
     
         5 . A trivalent chromium electrolyte according to  claim 1 , wherein the total concentration of amino acids is between about 5 g/L and about 10 WI. 
     
     
         6 . (canceled) 
     
     
         7 . The trivalent chromium electrolyte according to  claim 1 , wherein the thiocyanate ions are present in a concentration between about 0.2 g/L and about 5 g/L. 
     
     
         8 . The trivalent chromium electrolyte according to  claim 2 , wherein the amino acid comprises histidine. 
     
     
         9 . The trivalent chromium electrolyte according to  claim 2 , wherein the amino acid comprises arginine. 
     
     
         10 . The trivalent chromium electrolyte according to  claim 2 , wherein the amino acid comprises a mixture of histidine and arginine. 
     
     
         11 . A method of producing a dark colored chromium deposit on a substrate comprising the steps of:
 i) providing a trivalent chromium based electrolyte comprising:
 a) trivalent chromium ions, 
 b) one or more complexants capable of maintaining the trivalent chromium ions in solution, and 
 c) one or more amino acids,
 wherein the amino acids comprise a cationic side chain comprising nitrogen and wherein the cationic side chain is at least essentially free of sulfur; and 
 
 wherein the electrolyte is substantially free of hexavalent chromium salts; and 
 adding such a concentration of the amino acids to the electrolyte such that the dark colored chromium deposit produced on the substrate, after electrodepositing, has an L* value of 78.47 or lower as measured according to an L*a*b* colorspace system; and 
   ii) electrodepositing the dark colored chromium deposit on the substrate using the trivalent chromium based electrolyte.   
     
     
         12 . The method according to  claim 11 , wherein the one or more amino acids are selected from the group consisting of arginine, histidine, lysine, tryptophan, and combinations thereof. 
     
     
         13 . (canceled) 
     
     
         14 . (canceled) 
     
     
         15 . The method according to  claim 11 , wherein the total concentration of amino acids is between about 5 g/L and about 10 el. 
     
     
         16 . The method according to  claim 11 , wherein the trivalent chromium based electrolyte further comprises thiocyanate ions. 
     
     
         17 . The method according to  claim 16 , wherein the thiocyanate ions are present in a concentration between about 0.2 g/L and about 5 g/L. 
     
     
         18 . The method according to  claim 12 , wherein the one or more amino acids comprises histidine. 
     
     
         19 . The method according to  claim 12 , wherein the one or more amino acid comprises arginine. 
     
     
         20 . The method according to  claim 12 , wherein the one or more amino acids comprises a mixture of histidine and arginine. 
     
     
         21 . (canceled) 
     
     
         22 . The method according to  claim 11 , wherein the substrate comprises a nickel deposit on the substrate and the trivalent chromium is plated on the nickel deposit. 
     
     
         23 . The method according to  claim 11 , wherein the pH of the trivalent chromium electrolyte is between about 2.0 and about 5.0. 
     
     
         24 . The method according to  claim 23 , wherein the pH of the trivalent chromium electrolyte is about 3.5. 
     
     
         25 . A method of producing a dark colored chromium deposit on a substrate comprising the steps of:
 i) providing a trivalent chromium based electrolyte comprising:
 a) trivalent chromium ions, 
 b) one or more complexants capable of maintaining the trivalent chromium ions in solution, and 
 c) one or more amino acids,
 wherein the amino acids comprise a cationic side chain comprising nitrogen and wherein the cationic side chain is at least essentially free of sulfur; and 
 
 wherein the electrolyte is substantially free of hexavalent chromium salts; and 
   ii) electrodepositing the dark colored chromium deposit on the substrate using the trivalent chromium based electrolyte;   wherein the dark colored chromium deposit produced on the substrate has an L* value, measured according to an L*a*b* colorspace system, at least 5.3% lower than the trivalent chromium deposit produced by the same trivalent chromium electrolyte that does not comprise the one or more amino acids.