Trivalent chromium plating formulations and processes

Abstract

An electrolyte solution for chrome plating from trivalent chromium is prepared by dissolving in an aqueous medium a trivalent chromium salt (e.g., chromium (III) chloride or chromium (III) sulfate), dissolving an oxalate compound (e.g., sodium oxalate, potassium oxalate, or oxalic acid), dissolving a metal salt (e.g., aluminum sulfate or aluminum chloride), dissolving an alkali metal sulfate (e.g., sodium sulfate or potassium sulfate), and dissolving an alkali metal halide (e.g., sodium fluoride or potassium fluoride). A substrate is chrome plated from trivalent chromium using the electrolyte solution by passing a current between a cathode and an anode through the electrolyte solution to deposit chromium on the substrate.

What is claimed is: 
     
         1 . A method for chrome plating a substrate using an electrolyte solution, the method comprising:
 dissolving in an aqueous medium a trivalent chromium salt in an amount ranging from about 0.1 mol to about 0.9 mol per liter of the electrolyte solution;   dissolving an oxalate compound in an amount ranging from about 0.1 mol to about 3.0 mol per liter of the electrolyte solution;   dissolving a metal salt in an amount ranging from about 0.1 mol to about 4.0 mol per liter of the electrolyte solution, an alkali metal sulfate in an amount ranging from about 0.1 mol to about 2.0 mol per liter of the electrolyte solution, and an alkali metal halide in an amount ranging from about 0.1 mol to about 0.5 mol per liter of the electrolyte solution; and   passing a current between a cathode and an anode through the electrolyte solution to deposit chromium on the substrate.   
     
     
         2 . The method of  claim 1 , wherein the step of dissolving the trivalent chromium salt comprises dissolving chromium (III) chloride and/or chromium (III) sulfate. 
     
     
         3 . The method of  claim 1 , wherein the step of dissolving the oxalate compound comprises dissolving sodium oxalate in an amount ranging from about 0.1 mol to about 1.0 mol per liter of the electrolyte solution, potassium oxalate in an amount ranging from about 0.1 mol to about 1.0 mol per liter of the electrolyte solution, and/or oxalic acid in an amount ranging from about 0.1 mol to about 3.0 mol per liter of the electrolyte solution. 
     
     
         4 . The method of  claim 1 , wherein:
 dissolving the metal salt comprises dissolving aluminum sulfate in an amount ranging from about 0.1 mol to about 0.4 mol per liter of the electrolyte solution and/or aluminum chloride in an amount ranging from about 0.1 mol to about 4.0 mol per liter of the electrolyte solution;   dissolving the alkali metal sulfate comprises dissolving sodium sulfate and/or potassium sulfate; and   dissolving the alkali metal halide comprises dissolving sodium fluoride and/or potassium fluoride.   
     
     
         5 . The method of  claim 1 , wherein the dissolving the trivalent chromium salt, the oxalate compound, the metal salt, the alkali metal sulfate, and the alkali metal halide is performed in the following order:
 (1) dissolving the trivalent chromium salt and the oxalate compound;   (2) dissolving the metal salt;   (3) dissolving the alkali metal sulfate; and   (4) dissolving the alkali metal halide.   
     
     
         6 . The method of  claim 1 , wherein the step of dissolving the oxalate compound comprises stirring the oxalate compound at a temperature ranging from about 70° C. to about 80° C. for a time ranging from about 1 hour to about 3 hours. 
     
     
         7 . The method of  claim 1 , further comprising adjusting the pH of the electrolyte solution to a pH ranging from about 2 to about 4. 
     
     
         8 . The method of  claim 1 , further comprising adding sodium lauryl sulfate and/or potassium lauryl sulfate in an amount ranging from about 0.1 g to about 1 g per liter of the electrolyte solution. 
     
     
         9 . The method of  claim 1 , further comprising adding sodium bromide and/or potassium bromide in an amount ranging from about 0.1 g to about 1 g per liter of the electrolyte solution. 
     
     
         10 . The electrolyte solution prepared by the method of  claim 1 . 
     
     
         11 . The method of  claim 1 , further comprising maintaining the electrolyte solution at a pH ranging from about 2 to about 4. 
     
     
         12 . The method of  claim 1 , further comprising maintaining the electrolyte solution at a temperature ranging from about 30° C. to about 40° C. during the step of passing the current. 
     
     
         13 . The method of  claim 1 , wherein the step of passing the current is performed using a carbonaceous anode, a platinum anode, or a platinized titanium anode, and wherein the trivalent chromium salt comprises chromium (III) sulfate. 
     
     
         14 . The method of  claim 1 , wherein the step of passing the current is performed using a carbonaceous anode, and wherein the trivalent chromium salt comprises chromium (III) chloride. 
     
     
         15 . The method of  claim 1 , wherein the step of passing the current comprises applying a pulsed current or a direct current having a current density ranging from about 5 A/dm 2  to about 50 A/dm 2 . 
     
     
         16 . The method of  claim 1 , wherein the step of the passing the current comprises applying a pulsed current having a duty cycle ranging from about 20% to about 80%. 
     
     
         17 . The method of  claim 1 , wherein the step of passing the current is performed until a chromium layer having a thickness greater than about 5 microns and hardness greater than about 800 HV is formed on the substrate. 
     
     
         18 . The method of  claim 1 , wherein the step of passing the current to deposit chromium on the substrate comprises passing the current to deposit chromium on a steel substrate, a copper substrate, a nickel substrate, a copper-coated substrate, or a nickel-coated substrate. 
     
     
         19 . The method of  claim 1 , further comprising responsive to the step of passing the current, depositing chromium on the substrate or co-depositing chromium and carbon on the substrate. 
     
     
         20 . A method for preparing an electrolyte solution for chrome plating, the method comprising:
 providing trivalent chromium by dissolving a trivalent chromium salt;   forming complexes of oxalate and trivalent chromium by dissolving an oxalate compound;   buffering the electrolyte solution by dissolving a metal salt;   increasing the conductivity by dissolving an alkali metal sulfate; and   increasing the wetting property of the electrolyte solution by dissolving alkali metal halide.