pH Stable Trivalent Chromium Coating Solutions

Abstract

A stable trivalent chromium coating solution can be prepared by heating the chromium coating solution and adjusting the pH of the chromium coating solution. The coating solution can be heated to at least 35° C. for no more than 24 hours. While the solution is hot, a basic or acidic material can be added to increase or reduce the pH to a target value, respectively. In some embodiments, the chromium coating solution can have a pH of 3.5 to 4.0 and the pH can vary no more than ±0.15 while in storage for at least 20 days. The trivalent chromium coating solution can include a fluoride compound, a zirconate compound, a corrosion inhibitor compound, a chelating agent, a phosphorous compound, and/or other compounds.

1 . A method comprising:
 heating a trivalent chromium coating solution to at least 35° C.;   adjusting the pH of the trivalent chromium coating solution while the temperature of the trivalent chromium coating solution is at least 35° C.; and   cooling the trivalent chromium coating solution to ambient temperature resulting in a pH stable trivalent chromium coating solution;   wherein the pH stable trivalent chromium coating solution is formed separately from a metal surface treatment process.   
     
     
         2 . The method of  claim 1  comprising heating the trivalent chromium coating solution to at least 40° C. 
     
     
         3 . The method of  claim 1  comprising heating the trivalent chromium coating solution to at least 45° C. 
     
     
         4 . The method of  claim 1  comprising heating the trivalent chromium coating solution to at least 50° C. 
     
     
         5 . The method of  claim 1  comprising heating the trivalent chromium coating solution to at least 35° C. for no more than 24 hours. 
     
     
         6 . The method of  claim 1  comprising heating the trivalent chromium coating solution to at least 35° C. for no more than 12 hours. 
     
     
         7 . The method of  claim 1  comprising heating the trivalent chromium coating solution to at least 35° C. for no more than 4 hours. 
     
     
         8 . The method of  claim 1  comprising adjusting the pH of the trivalent chromium coating solution to approximately 3.5 to approximately 4.0 while the temperature of the trivalent chromium coating solution is at least 35° C. 
     
     
         9 . The method of  claim 1  wherein adjusting the pH of the trivalent chromium coating solution comprises adding an alkaline material to the trivalent chromium coating solution to raise the pH of the trivalent chromium coating solution. 
     
     
         10 . The method of  claim 1  comprising mixing the trivalent chromium coating solution while adjusting the pH. 
     
     
         11 . The method of  claim 1  comprising filtering precipitates from the trivalent chromium coating solution after adjusting the pH. 
     
     
         12 . The method of  claim 1  wherein adjusting the pH of the trivalent chromium coating solution includes repeatedly raising the pH of the trivalent chromium coating solution. 
     
     
         13 . The method of  claim 1  wherein the pH of the pH stable trivalent chromium coating solution varies no more than ±0.15 while in storage for at least 60 days; and 
     
     
         14 . The method of  claim 1  wherein the pH of the pH stable trivalent chromium coating solution varies no more than ±0.07 while in storage for at least 60 days. 
     
     
         15 . The method of  claim 1  wherein the pH of the pH stable trivalent chromium coating solution varies no more than ±0.15 while in storage for at least 120 days. 
     
     
         16 . The method of  claim 1  wherein the pH stable trivalent chromium coating solution is free of hexavalent chromium. 
     
     
         17 . The method of  claim 1  wherein the pH stable trivalent chromium coating solution comprises a fluoride compound. 
     
     
         18 . The method of  claim 1  wherein the pH stable trivalent chromium coating solution comprises a fluorozirconate compound. 
     
     
         19 . The method of  claim 1  wherein the pH stable trivalent chromium coating solution comprises a chelating agent. 
     
     
         20 . The method of  claim 1  wherein the pH stable trivalent chromium coating solution comprises a corrosion inhibitor. 
     
     
         21 . The method of  claim 1  wherein the corrosion inhibitor comprises 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, 2-mercaptobenzoxazole, and/or benzotriazole. 
     
     
         22 . The pH stable trivalent chromium coating solution produced by the process of  claim 1 . 
     
     
         23 . A method comprising:
 heating a trivalent chromium coating solution to at least 40° C.;   adjusting the pH of the trivalent chromium coating solution while the temperature of the trivalent chromium coating solution is at least 40° C.; and   cooling the trivalent chromium coating solution to ambient temperature resulting in a pH stable trivalent chromium coating solution.   
     
     
         24 . The method of  claim 23  comprising heating the trivalent chromium coating solution to at least 45° C. 
     
     
         25 . The method of  claim 23  comprising heating the trivalent chromium coating solution to at least 50° C.