Process for preparing chromium conversion coatings for iron and iron alloys

Abstract

Process of coating iron alloys to improve the corrosion resistance and adhesive bonding strengths. The process comprises treating the iron or iron alloys such as steel with an acidic aqueous solution comprising, per liter of solution, from about 0.01 to 22 grams of a water soluble trivalent chromium compound, about 0.01 to 12 grams of hexafluorozirconate, at least one tetrafluoroborate and/or hexafluorosilicate, at least one water soluble divalent zinc compound and from 0.0 to 10 grams of water soluble thickeners and/or water soluble surfactants.

1 . Process for preparing zirconium-chromium conversion coatings on iron and iron alloys to improve the corrosion resistance and adhesive bonding strength which comprises treating iron and iron alloys with an acidic aqueous solution having a pH ranging from about 2.5 to 5.5; said acidic aqueous solution comprising, per liter of solution, from about 0.01 to 22 grams of a trivalent chromium compound, about 0.01 to 12 grams of a hexafluorozirconate, about 0.0 to 12 grams of at least one fluorocompound selected from the group consisting of tetrafluoroborate, hexafluorosilicate and mixtures thereof, from about 0.0 to 10 grams of at least one divalent zinc compound, from 0.0 to about 10 grams of at least one water soluble thickener and from 0.0 to about 10 grams of at least one water soluble surfactant.  
   
   
       2 . The process of  claim 1  wherein the pH of the aqueous solution ranges from about 3.7 to 4.0 and the iron alloy is steel.  
   
   
       3 . The process of  claim 1  wherein the trivalent chromium is a water soluble compound ranging from about 0.01 to 10 grams, the hexafluorozirconate is a water soluble compound ranging from about 0.01 to 8.0 grams, and the fluorocompounds are water soluble compounds ranging from about 0.01 to 1.2 grams.  
   
   
       4 . The process of  claim 3  wherein the thickener ranges from about 0.5 to 1.5 grams and the surfactant ranges from about 0.5 to 1.5 grams.  
   
   
       5 . Process for preparing zirconium-chromium conversion coatings on iron and iron alloys to improve corrosion resistance and adhesive bonding which comprises treating iron and iron alloys with an acidic aqueous solution having a pH ranging from about 3.7 to 4.0; said acidic aqueous solution comprising, per liter of solution, from about 0.01 to 10 grams of a trivalent chromium salt, about 0.01 to 8.0 grams of an alkali metal hexafluorozirconate, about 0.01 to 1.2 grams of at least one fluorocompound selected from the group consisting of alkali metal tetrafluoroborates, alkali metal hexafluorosilicates and mixtures thereof, from about 0.001 to 10 grams of at least one divalent zinc compound, from 0.5 to about 1.5 grams of at least one water soluble thickener and from 0.5 to about 1.5 grams of at least one water soluble surfactant.  
   
   
       6 . The process of  claim 5  wherein the iron alloy is steel.  
   
   
       7 . The process of  claim 6  wherein the thickener is a cellulose compound.  
   
   
       8 . The process of  claim 7  wherein the chromium salt is trivalent chromium sulfate.  
   
   
       9 . The process of  claim 8  wherein the alkali metal zirconate is potassium hexafluorozirconate.  
   
   
       10 . The process of  claim 5  wherein the trivalent chromium salt ranges from about 5.0 to 7.0 grams, the hexafluorozirconate ranges from about 0.01 to 8.0 grams, and the tetrafluoroborate ranges from about 0.12 to 0.24 grams.  
   
   
       11 . The process of  claim 10  wherein the thickener is a water soluble alkyl cellulose compound.  
   
   
       12 . The process of  claim 11  wherein the zinc compound is zinc acetate.  
   
   
       13 . The process of  claim 11  wherein the chromium salt is trivalent chromium sulfate and the divalent zinc compound is zinc sulfate.  
   
   
       14 . The process of  claim 13  wherein the surfactant is selected from the group consisting of water soluble non-ionic, anionic and cationic surfactants.  
   
   
       15 . The process of  claim 14  wherein the zinc sulfate is present in the aqueous solution in an amount ranging from about 0.001 to 10 grams.  
   
   
       16 . The process of  claim 15  wherein the zinc compound is a divalent zinc sulfate ranging from about 0.1 to 5.0 grams.  
   
   
       17 . The process of  claim 5  wherein the chromium salt is trivalent chromium sulfate present in the aqueous solution in an amount ranging from 5.0 to 7.0 grams, and the mixture of the alkali metal tetrafluoroborates and hexafluorosilicates are present in the aqueous solution in an amount ranging from about 0.5 to 1.5 grams.  
   
   
       18 . The process of  claim 17  wherein the zinc compound is divalent zinc sulfate present in the aqueous solution in an amount ranging from about 0.001 to 10 grams.  
   
   
       19 . Coated iron alloys obtained by the process of  claim 2 .  
   
   
       20 . Coated iron alloys obtained by the process of  claim 6.