Electroplating bath for depositing a black chromium layer and method for electroplating a black chromium layer on a substrate

Abstract

The present invention relates to a very specific electroplating bath for depositing a black chromium layer, and a respective method for electroplating such a layer. The electroplating bath comprises two specific groups of compounds defined as (D) and (E), present in a particularly defined molar ratio ranging from 0.9 to 2.65, based on (E):(D). The black chromium layer is excellently suited for decorative purposes.

1 . An electroplating bath for depositing a black chromium layer, the electroplating bath comprising:
 (A) trivalent chromium ions;   (B) one or more than one complexing agent for said trivalent chromium ions;   (C) optionally, one or more than one pH buffer compound for said electroplating bath;   (D) one or more than one compound comprising at least one —SCN moiety, salts, esters, and/or isoforms thereof, in a total amount ranging from 100 mmol/L to 750 mmol/L, based on the total volume of the electroplating bath; and   (E) one or more than one organic compound, including sulfoxides thereof, comprising at least one —SH moiety and/or at least one —S—(CH 2 ) k —CH 3  moiety, wherein k is an integer ranging from 0 to 4,   characterized in that   (E) and (D) are present in a molar ratio ranging from 0.9 to 2.65, based on (E):(D).   
     
     
         2 . The electroplating bath according to  claim 1 , wherein the electroplating bath further comprises sulfate ions. 
     
     
         3 . The electroplating bath according to  claim 1  further comprising halogen anions. 
     
     
         4 . The electroplating bath according to  claim 1  further comprising Fe(II) ions. 
     
     
         5 . The electroplating bath according to  claim 1 , wherein (C) comprises boric acid. 
     
     
         6 . The electroplating bath according to  claim 1 , wherein said bath comprises (D) in a total amount ranging from 100 mmol/L to 600 mmol/L, based on the total volume of the electroplating bath. 
     
     
         7 . The electroplating bath according to  claim 1 , wherein said bath comprises (E) in a total amount ranging from 1 mmol/L to 950 mmol/L, based on the total volume of the electroplating bath. 
     
     
         8 . The electroplating bath according to  claim 1 , wherein (E) comprises at least a compound of formula (I), salts, and/or sulfoxides thereof
   R 1 —S—(CH 2 ) n —CH(NH 2 )—R 2   (I),
   wherein
 R 1  is a branched or unbranched C1 to C4 alkyl, 
 R 2  is selected from the group consisting of COOH, salts thereof, and (CH 2 ) m —OH, 
 n is an integer ranging from 1 to 4, and 
 m is an integer ranging from 1 to 4. 
   
     
     
         9 . The electroplating bath according to  claim 8 , wherein R 1  is methyl, ethyl, n-propyl, or iso-propyl. 
     
     
         10 . The electroplating bath according to  claim 8 , wherein R 2  is COOH and/or salts thereof. 
     
     
         11 . The electroplating bath according to  claim 8 , wherein n is 1 or 2. 
     
     
         12 . The electroplating bath according to  claim 1 , wherein (E) comprises at least methionine. 
     
     
         13 . The electroplating bath according to  claim 1 , wherein the molar ratio of (E):(D) is ranging from to 2.6. 
     
     
         14 . A method for electroplating a black chromium layer on a substrate, the method comprising the steps
 (a) providing the substrate,   (b) contacting the substrate with an electroplating bath according to  claim 1 ,   (c) applying an electrical current such that the black chromium layer is electroplated onto the substrate,   (d) heat-treating the substrate obtained from step (c) at a temperature ranging from 30° C. to 100° C.   
     
     
         15 . The method of  claim 14 , wherein in step (d) the heat-treating is carried out in water.