Method for adjusting the brightness l* of an electroplated chromium layer

Abstract

The present invention relates to a method for adjusting the brightness L* of an electroplated chromium layer, in particular of a dark electroplated chromium layer. The method includes an aqueous trivalent chromium electroplating bath comprising colloidal particles and/or agglomerates thereof and a step of fully or partially removing same with optionally adding colloidal particles.

1 . A method for adjusting the brightness L* of an electroplated chromium layer, the method comprising the steps:
 (a) providing a substrate,   (b) providing an aqueous trivalent chromium electroplating bath comprising:
 (i) trivalent chromium ions, 
 (ii) one or more than one complexing agent for said trivalent chromium ions, 
 (iii) colloidal particles and/or agglomerates thereof, and 
 (iv) one or more than one sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below, 
   (c) contacting the substrate with said electroplating bath and applying an electrical current such that a chromium layer is electrolytically deposited on the substrate with a lightness value L1* according to the L*a*b* color-space system,   (d) treating the aqueous trivalent chromium electroplating bath obtained from step (c) such that a treated aqueous trivalent chromium electroplating bath results, by
 (d1) removing fully or partially said colloidal particles and/or said agglomerates thereof, 
 and optionally 
 (d2) adding colloidal particles, and 
   (e) contacting another substrate with the treated aqueous trivalent chromium electroplating bath and applying an electrical current such that a chromium layer is electrolytically deposited on the another substrate with a lightness value L2* according to the L*a*b* color-space system,   
       wherein L2* is higher or lower than L1*. 
     
     
         2 . The method of  claim 1 , wherein the colloidal particles comprise one or more than one chemical element selected from the group consisting of silicon, aluminum, and carbon. 
     
     
         3 . The method of  claim 1 , wherein said colloidal particles comprise nano-particles. 
     
     
         4 . The method of  claim 1 , wherein said colloidal particles comprise at least particles with a particle size of 100 nm or less. 
     
     
         5 . The method of  claim 1 , wherein the colloidal particles comprising the chemical element silicon, comprise silica. 
     
     
         6 . The method of  claim 1 , wherein the colloidal particles comprising the chemical element aluminum, comprise aluminum oxide. 
     
     
         7 . The method of  claim 1 , wherein the colloidal particles comprising the chemical element carbon, comprise nanodiamonds. 
     
     
         8 . The method of  claim 1 , wherein in step (b) or after step (d2) said colloidal particles are present in a total amount ranging from 0.05 g/L to 100 g/L, based on the total volume of the respective aqueous trivalent chromium electroplating bath. 
     
     
         9 . The method of  claim 1 , wherein (iv) comprises thiocyanate anions. 
     
     
         10 . The method of  claim 1 , wherein (iv) comprises at least a sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below and additionally comprising a nitrogen atom. 
     
     
         11 . The method of  claim 1 , wherein L1* is 55 or less. 
     
     
         12 . The method of  claim 1 , wherein said method comprises step (d1) and step (d2). 
     
     
         13 . The method of  claim 1 , wherein, in step (d1), the removing is carried out by filtration, centrifugation, or sedimentation, or by a combination of two or more of filtration, centrifugation, and sedimentation. 
     
     
         14 . The method of  claim 1 , wherein L2* is higher than L1*. 
     
     
         15 . The method of  claim 1 , wherein L2* is more than 55.