Chromium Alloy Coating with Enhanced Resistance to Corrosion in Calcium Chloride Environments

Abstract

The invention consists of a chromium electroplating solution comprising a chromium electroplating solution comprising: (1) a water soluble trivalent chromium salt; (2) at least one complexant for trivalent chromium ions; (3) a source of hydrogen ions sufficient to create a pH of from 2.8-4.2; (4) a pH buffering compound; and (5) a sulfur-containing organic compound. The chromium electroplating solution is usable in a method for producing an adherent metallic coating on a decorative article, such coating having enhanced resistance to corrosion in environments containing calcium chloride.

1 . A chromium electroplating solution comprising:
 a. a water soluble trivalent chromium salt;   b. a complexant for trivalent chromium ions comprising a dicarboxylic acid and an amino carboxylic acid;   c. a pH buffering compound; and   d. a sulfur-containing organic compound;   
       wherein the pH of the solution is from about 2.8-4.2. 
     
     
         2 . The chromium electroplating solution according to  claim 1 , comprising a water-soluble trivalent salt of chloride or sulfate selected from the group consisting of sodium, potassium and ammonium salts of chloride or sulfate and combinations of one or more of the foregoing. 
     
     
         3 . The chromium electroplating solution according to  claim 1 , wherein the water-soluble trivalent chromium salt is selected from the group consisting of chromium sulfate, chromium chloride, chromium methane sulfonate, and combinations of one or more of the foregoing. 
     
     
         4 . The chromium electroplating solution according to  claim 1 , wherein sulfur-containing organic compound comprises a divalent sulfur. 
     
     
         5 . The chromium electroplating solution according to  claim 1 , wherein the pH buffering compound is selected from the group consisting of boric acid and salts thereof, acetic acid and salts thereof, phosphoric acid and salts thereof, glycine and salts thereof, and combinations of one or more of the foregoing. 
     
     
         6 . The chromium electroplating solution according to  claim 2 , wherein the sulfur-containing organic compound comprises divalent sulfur. 
     
     
         7 . The chromium electroplating solution according to  claim 6 , wherein the sulfur-containing organic compound is selected from the group consisting of sodium thiocyanate and other salts thereof, sodium dimethyldithiocarbamate, dialkylthiocarbamate salts, thiourea and derivatives thereof, sodium mercaptopropane sulfonate, soluble mercaptoalkanesulfonate salts, and combinations of one or more of the foregoing. 
     
     
         8 . The chromium electroplating solution according to  claim 1 , wherein the complexant for trivalent chromium ions is selected from the group consisting of malic acid, aspartic acid, maleic acid, succinic acid, glycine, soluble salts of any of the foregoing, and combinations of one or more of the foregoing. 
     
     
         9 . The chromium electroplating solution according to  claim 1 , comprising an additional organic compound selected from the group consisting of saccharin, sodium allyl sulfonate, 2-butyne-1,4-diol, sodium 2-ethylhexyl sulfate, sodium dihexyl sulfosuccinate, other water soluble salts of any of the foregoing and combinations of one or more of the foregoing. 
     
     
         10 . The chromium electroplating solution according to  claim 1  wherein the concentration of the sulfur-containing organic compound in the electroplating solution is such that the chromium deposit produced by the electroplating solution comprises from 0.5% by weight to 25% by weight of sulfur. 
     
     
         11 - 21 . (canceled) 
     
     
         22 . A method of depositing a corrosion resistant chromium alloy coating on an article, the method comprising the steps of:
 a) coating the article by electrolytic or electroless means with one or more layers of metal or metal alloy, wherein said metal or metal alloy comprises palladium, tin, copper or nickel; and thereafter   b) immersing the article as a cathode in a chromium electroplating solution, wherein the chromium electroplating solution is prepared by
 i) combining (a) a water soluble trivalent chromium salt; (b) at least one complexant for trivalent chromium ions; and (c) a pH buffering compound; and thereafter 
 ii) purifying the solution; and thereafter 
 iii) adding at least one sulfur-containing organic compound comprising a divalent sulfur and, optionally, at least one additional additive to the purified solution; and 
   c) passing an electrical current through the chromium electroplating solution to deposit a chromium alloy on the article;   wherein the chromium electroplating solution has a pH of about 2.8-4.0.   
     
     
         23 . The method according to claim  28 , wherein the chromium alloy deposit comprises from 2% by weight to 20% by weight sulfur. 
     
     
         24 . The method according to claim  28 , wherein the chromium electroplating solution is purified by treating the chromium electroplating solution with hydrogen peroxide and activated carbon and then filtering the purified solution.