Compositions and processes for producing chromium conversion coatings on surfaces of zinc or cadmium

Abstract

In a process for producing a chromium protective coating on a zinc or cadmium surface the surface has applied thereto an aqueous composition including chromium, perchlorate and fluoride ions with the chromium ion being the only metal ion present before application to the surface. The apparatus by which the composition is applied to a strip moving through an application station includes a provision for rinsing the surface if the strip is stopped for longer than a predetermined time in the station.

We claim: 
     
       1. A composition for producing a chromate protective coating on a surface comprising zinc or cadmium the composition .Iadd.having a pH less than 1.15 and .Iaddend.consisting of .[.an aqueous solution of pH less than 1.15 containing.]. .Iadd.demineralised water, .Iaddend.a source of chromium ion, .Iadd.chromium being the only metallic ion present, .Iaddend.the chromium ion being present in the amount of 5-15 grams per liter, a source of perchlorate ion, the perchlorate ion being present in the amount .[.of 0.5 to 0.85 grams per liter.]. .Iadd.corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter.Iaddend., and a source of fluoride ion, the fluoride ion being present in the amount .[.of 0.1 to 0.27 grams.]. .Iadd.corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter. 
     
     
       2. A composition as claimed in claim 1, wherein the pH of the solution is in the range 1.15 to 0.85. 
     
     
       3. A composition as claimed in claim 2, wherein the pH of the solution is in the range 1.0 to 0.9. 
     
     
       4. A composition as claimed in claim 1, wherein the chromium ion is present in the amount of 9 to 13 grams per liter. 
     
     
       5. A composition as claimed in claim 1, wherein the fluoride ion is present in the amount .[.of 0.15 to 0.27 grams.]. .Iadd.corresponding to from 0.15 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter. 
     
     
       6. A composition as claimed in claim 1, wherein the pH of the solution is in the range 1.15 to 0.85, wherein the chromium ion is present in the amount of 5 to 15 grams per liter, wherein the perchlorate ion is present in the amount .[.of 0.5 to 0.85 grams.]. .Iadd.corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid .Iaddend.per liter, and wherein the fluoride ion is present in the amount .[.of 0.1 to 0.27 grams.]. .Iadd.corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter. 
     
     
       7. A composition as claimed in claim 1, wherein the pH of the solution is in the range 1.0 to 0.9, wherein the chromium ion is present in the amount of 9 to 13 grams per liter, wherein the perchlorate ion is present in the amount .[.of 0.5 to 0.85 grams.]. .Iadd.corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid .Iaddend.per liter and wherein the fluoride ion is present in the amount .[.of 0.15 to 0.27 grams.]. .Iadd.corresponding to from 0.15 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter. 
     
     
       8. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 150° F to 200° F a composition .Iadd.having a pH in the range 1.0 to 0.9 and .Iaddend.consisting of .[.an aqueous solution of pH in the range 1.0 to 0.9 containing.]. .Iadd.demineralised water, .Iaddend.a source of chromium ion, the chromium ion being present in the amount of 5-15 grams per liter.Iadd., chromium being the only metallic ion present.Iaddend., a source of perchlorate ion, the perchlorate ion being present in the amount .[.of 0.5 to 0.85 grams.]. .Iadd.corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid .Iaddend.per liter, and a source of fluoride ion, the fluoride ion being present in the amount .[.of 0.1 to 0.27 grams.]. .Iadd.corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter. 
     
     
       9. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 105° F to 200° F a composition .[.containing an aqueous solution of pH less than 1.15 containing.]. .Iadd.having a pH less than 1.15 and consisting of demineralised water, .Iaddend.a source of chromium ion, the chromium ion being present in the amount of 5 to 15 grams per liter, .Iadd.chromium being the only metallic ion present, .Iaddend.a source of perchlorate ion, the perchlorate ion being present in the amount .[.of 0.5 to 0.85 grams.]. .Iadd.corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid .Iaddend.per liter, and a source of fluoride ion, the fluoride ion being present in the amount .[.of 0.1 to 0.27 grams.]. .Iadd.corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter. 
     
     
       10. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 105° F to 200° F a composition .Iadd.having a pH less than 1.15 and .Iaddend.consisting of .[.an aqueous solution of pH less than 1.15 containing.]. .Iadd.demineralised water, .Iaddend.a source of chromium ion, the chromium ion being present in the amount of 9 to 13 grams per liter, .Iadd.chromium being the only metallic ion present, .Iaddend.a source of perchlorate ion, the perchlorate ion being present in the amount .[.of 0.5 to 0.85 grams.]. .Iadd.corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid .Iaddend.per liter, and a source of fluoride ion, the fluoride ion being present in the amount .[.of 0.1 to 0.27 grams.]. .Iadd.corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter. 
     
     
       11. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 105° F to 200° F a composition .Iadd.having a pH less than 1.15 .Iaddend.consisting of .[.an aqueous solution of pH less than 1.15 containing.]. .Iadd.demineralised water, .Iaddend.a source of chromium ion, the chromium ion being present in the amount of 5-15 grams per liter, .Iadd.chromium being the only metallic ion present, .Iaddend.a source of perchlorate ion, the perchlorate ion being present in the amount .[.of 0.5 to 0.85 grams.]. .Iadd.corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid .Iaddend.per liter, and a source of fluoride ion, the fluoride ion being present in the amount .[.of 0.15 to 0.27 grams.]. .Iadd.corresponding to from 0.15 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter. 
     
     
       12. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 150° F to 200° F a composition .Iadd.having a pH in the range 1.15 to 0.85, .Iaddend.consisting of .[.an aqueous solution wherein the pH of the solution is in the range 1.15 to 0.85,.]. .Iadd.demineralised water, wherein .Iaddend.chromium ion is present in the amount of 5 to 15 grams per liter, .Iadd.chromium being the only metallic ion present, .Iaddend.perchlorate ion is present in the amount .[.of 0.5 to 0.85 grams.]. .Iadd.corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid .Iaddend.per liter, and fluoride ion is present in the amount .[.of 0.1 to 0.27 grams.]. .Iadd.corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter. 
     
     
       13. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 170° F to 190° F a composition .Iadd.having a pH in the range 1.0 to 0.9 .Iaddend.consisting of .[.an aqueous solution wherein the pH of the solution is in the range 1.0 to 0.9,.]. .Iadd.demineralised water, wherein .Iaddend.chromium ion is present in the amount of 9 to 13 grams per liter, .Iadd.chromium being the only metallic ion present, .Iaddend.perchlorate ion is present in the amount .[.of 0.5 to 0.85 grams.]. .Iadd.corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid .Iaddend.per liter and fluoride ion is present in the amount .[.of 0.15 to 0.27 grams.]. .Iadd.corresponding to from 0.15 to 0.25 ml of 48 percent hydrofluoric acid .Iaddend.per liter.