Method of forming phosphate coating on zinc

Abstract

Disclosed is an improved method for coating a zinc surface comprising treating said surface with an aqueous, acidic solution containing: (a) about 0.5 to about 4 percent phosphate ion; (b) an ion selected from the group consisting of zinc ions, manganese ions, and mixtures thereof, said ions being present at a level sufficient to form dihydrogen phosphate with substantially all of said phosphate ions; and (c) about 0.01 to about 1 percent complex fluoride ions; wherein the weight:weight ratio of complex fluoride ions:chloride ions in said solution is at a value of about 8:1 or greater. It is preferred that the ratio complex of fluoride ion to chloride ion (F - :Cl - ) in said solution to be maintained at a value of greater than about 8:1, preferably greater than about 10:1, and more preferably greater than about 14:1. The select weight:weight complex fluoride:chloride ratio substantially eliminates the abnormal crystal growth frequently found in treated surfaces while at the same time reduces the need for excess fluoride.

What is claimed is: 
     
       1. An improved method for coating a zinc surface comprising treating said surface with an aqueous, acidic solution containing: (a) about 0.5 to about 4 percent phosphate ion;   (b) an ion selected from the group consisting of zinc ions, manganese ions, and mixtures thereof, said ions being present at a level sufficient to form dihydrogen phosphate with substantially all of said phosphate ions; and   (c) about 0.01 to about 1 percent complex fluoride ions; and   (d) measuring the chloride ion concentration and maintaining the weight:weight ratio of complex fluoride ions:chloride ions in said solution at a value of about 8:1 or greater.   
     
     
       2. A method according to claim 1, wherein said solution additionally contains about 0.025 to about 2 percent nitrate ion. 
     
     
       3. A method according to claim 1 wherein said solution additionally contains about 0.01 to about 1 percent of an ion selected from the group consisting of cobalt ions, nickel ions, and mixtures thereof. 
     
     
       4. A method according to claim 1, wherein the ratio is maintained at a value of about 10:1 or greater. 
     
     
       5. A method according to claim 4 wherein the ratio is maintained at a value of about 14:1 or greater. 
     
     
       6. A method according to claim 1 wherein the phosphate ion is present at a level of about 0.5 to about 2.5 percent. 
     
     
       7. A method according to claim 4 wherein the phosphate ion is present at a level of about 0.5 to about 2.0 percent. 
     
     
       8. A method according to claim 2 wherein the nitrate ion is present at a level of about 0.05 to about 1 percent. 
     
     
       9. A method according to claim 1 wherein the chloride ion is present at a level of less than about 0.0050 percent. 
     
     
       10. A method according to claim 9 wherein the chloride ion is present at a level of less than about 0.0020 percent. 
     
     
       11. A method for coating a zinc surface comprising treating said surface with an aqueous acidic solution containing: (a) about 0.5 to about 4 percent phosphate ion;   (b) about 0.025 to about 2 percent nitrate ion;   (c) an ion selected from the group consisting of zinc ions, manganese ions, and mixtures thereof, said ions being present at a level sufficient to form dihydrogen phosphate with substantially all of said phosphate ions; and   (d) about 0.01 to about 1 percent of an ion selected from the group consisting of cobalt ions, nickel ions, and mixtures thereof; wherein said solution contains less than about 0.0050 percent chloride ion; and   (e) measuring the chloride ion concentration and maintaining the ratio of fluoride ions:chloride ions in said solution at a value of about 8:1 or greater.   
     
     
       12. A method according to claim 11 wherein the ratio is maintained at a value of about 10:1 or greater. 
     
     
       13. A method according to claim 12 wherein the ratio is maintained at a value of about 14:1 or greater. 
     
     
       14. A method according to claim 11 wherein the phosphate ion is present at a level of about 0.5 to about 2.5 percent. 
     
     
       15. A method according to claim 1 wherein all pre-treatment steps are substantially free of chloride ions. .Iadd. 
     
     
       16.  A process for suppressing white specking when phosphate-coating a zinc surface, comprising: contacting the zinc surface with an aqueous, acidic solution containing:   (a) about 0.5 to about 4 percent phosphate ion;   (b) an ion selected from the group consisting of zinc ions, manganese ions, and mixtures thereof;   (c) about 0.01 to about 1 percent complex fluoride ions; and   (d) chloride ions;   wherein the weight:weight ratio of complex fluoride ions:chloride ions in the solution is measured and maintained during coating at a value of about 8:1 or greater by adjusting the amount of complex fluoride ion in the composition thereby suppressing the white specking. .Iaddend..Iadd.   
     
     
       17.  The process of claim 16 wherein the weight ratio of complex fluoride ions:chloride ions in the solution is maintained above about 10:1. .Iaddend..Iadd.18. The process of claim 16 wherein the weight ratio of complex fluoride ions:chloride ions in the solution is maintained above about 14:1. .Iaddend..Iadd.19. The process of claim 16 wherein the 
     
     
        solution contains manganese ions. .Iaddend..Iadd.20.  The process of claim 17 wherein the solution contains manganese ions. .Iaddend..Iadd.21. The process of claim 18 wherein the solution contains manganese ions. .Iaddend..Iadd.22. The process of claim 16 wherein the solution contains from about 0.025 to about 2 percent of the solution of nitrate ions. .Iaddend..Iadd.23. The process of claim 17 wherein the solution contains from about 0.025 to about 2 percent of the solution of nitrate ions. .Iaddend..Iadd.24. The process of claim 18 wherein the solution contains from about 0.025 to about 2 percent of the solution of nitrate ions. .Iaddend..Iadd.25. The process of claim 18 wherein the solution contains from about 0.01 to about 1 percent of the solution of an ion selected from the group consisting of cobalt ions, nickel ions, and mixtures thereof. .Iaddend..Iadd.26. The process of claim 17 wherein the solution contains from about 0.01 to about 1 percent of the solution of an ion selected from the group consisting of cobalt ions, nickel ions, and mixtures thereof. 
     
     
        .Iaddend..Iadd.27.  The process of claim 18 wherein the solution contains from about 0.01 to about 1 percent of the solution of an ion selected from the group consisting of cobalt ions, nickel ions, and mixtures thereof. .Iaddend..Iadd.28. The process of claim 16 wherein the solution contains from about 0.5 to about 2.5 percent of phosphate ions. .Iaddend..Iadd.29. The process of claim 17 wherein the solution contains from about 0.5 to about 2.5 percent of phosphate ions. .Iaddend..Iadd.30. The process of claim 18 wherein the solution contains from about 0.5 to about 2.5 percent of phosphate ions. .Iaddend..Iadd.31. The process of claim 16 wherein the solution contains less than about 0.005 percent of chloride ions. .Iaddend..Iadd.32. The process of claim 17 wherein the solution contains 
     
     
        less than about 0.005 percent of chloride ions. .Iaddend..Iadd.33.  The process of claim 18 wherein the solution contains less than about 0.005 percent of chloride ions. .Iaddend..Iadd.34. The process of claim 22 wherein the solution contains from about 0.05 to about 1.0 percent nitrate ions. .Iaddend..Iadd.35. The process of claim 23 wherein the solution contains from about 0.05 to about 1.0 percent nitrate ions. .Iaddend..Iadd.36. The process of claim 24 wherein the solution contains from about 0.05 to about 1.0 percent nitrate ions. .Iaddend..Iadd.37. The process of claim 28 wherein the solution contains from about 0.025 to about 2 percent nitrate ions. .Iaddend..Iadd.38. The process of claim 29 wherein the solution contains from about 0.025 to about 2 percent nitrate ions. .Iaddend..Iadd.39. The process of claim 30 wherein the solution contains from about 0.025 to about 2 percent nitrate ions. 
     
     
        .Iaddend..Iadd. 0.  The process of claim 37 wherein the solution contains from about 0.025 to about 0.25 percent complex fluoride ions. .Iaddend..Iadd.41. The process of claim 38 wherein the solution contains from about 0.025 to about 0.25 percent complex fluoride ions. .Iaddend..Iadd.42. The process of claim 39 wherein the solution contains from about 0.025 to about 0.25 percent complex fluoride ions. .Iaddend..Iadd.43. The process of claim 37 wherein the solution contains manganese ions. .Iaddend..Iadd.44. The process of claim 38 wherein the solution contains manganese ions. .Iaddend..Iadd.45. The process of claim 39 wherein the solution contains manganese ions. .Iaddend..Iadd.46. The process of claim 37 wherein the solution contains from about 0.025 to about 0.25 percent of complex fluoride ions. .Iaddend..Iadd.47. The process of claim 38 wherein the solution contains from about 0.025 to 
     
     
        about 0.25 percent of complex fluoride ions. .Iaddend..Iadd.48.  The process of claim 39 wherein the solution contains from about 0.025 to about 0.25 percent of complex fluoride ions. .Iaddend..Iadd.49. The process of claim 16 which further comprises contacting the zinc surface with at least one pretreating solution wherein the at least one pretreating solution contains less than about 0.01 percent chloride ions. 
     
     
        .Iaddend..Iadd.50.  In a method for phosphate-coating a zinc surface wherein the surface is contacted with an aqueous, acidic solution, the improvement comprising: suppressing the formation of white specking in said coating by contacting the zinc surface with an aqueous, acidic solution containing:   (a) about 0.5 to about 4 percent phosphate ion;   (b) an ion selected from the group consisting of zinc ions, manganese ions, and mixtures thereof;   (c) about 0.01 to about 1 percent complex fluoride ions; and   (d) chloride ions, wherein the weight:weight ration of complex fluoride ions:chloride ions in the solution is at a value of about 8:1 or above; and   maintaining said ratio at a value of about 8:1 or above during formation of the coating by adjusting the amount of complex fluoride ion in the solution thereby suppressing white specking. .Iaddend.