Method for treating coated ferrous substrates

Claim 2

In the method for the treatment of ferrous metal substrates coated with thin layers of Zn base alloys the improvement which comprises, applying to said coated substrate a solution consisting essentially of, (a) 2.0 to 12.0 gms/l of hexavalent chromium, (b) 0.7 to 4.0 gms/l of a bifluoride selected from the group consisting of NaHF 2 , NH 4 HF 2 and mixtures thereof, and (c) less than 0.5 gms/l of phosphate, said solution adjusted to a pH within the range 2.7 to 5.5, metering the amount of solution remaining on the surface of said substrate to provide a final coating thickness of 0.8 to 2.0 mg. of Cr per square foot of substrate surface and thereafter; drying the thus coated substrate to remove substantially all of the physically bound water in said film.

Claim 3

The method of claim 1, in which said pH is 3.5 to 4.7, and said phosphate is less than 0.1 gms/l.

Claim 4

The method of claim 2, in which a major portion of said hexavalent chromium is supplied by a dichromate selected from the group consisting of Na 2 Cr 2 O 7 , K 2 Cr 2 O 7 or mixtures thereof.

Claim 5

The method of claim 3 in which said dichromate concentration is 12-22 gms/l.

Claim 6

In the method for the treatment of ferrous metal substrates coated with thin layers of a metal selected from a group consisting of Zn, Al and Pb base alloys; the improvement which comprises, applying to said coated substrate a solution consisting essentially of, (a) 2.0 to 12.0 gms/l of hexavalent chromium, and (b) 0.7 to 4.0 gms/l of a bifluoride selected from the group of NaHF 2 , NH 4 HF 2 and mixtures thereof, (c) less than 0.5 gms/l of phosphate, said solution adjusted to a pH within the range 2.7 to 5.5; metering the amount of solution remaining on the surface of said substrate to provide a final coating thickness of 0.8 to 2.0 mg. of Cr per square foot of substrate surface and thereafter; drying the thus coated substrate to remove substantially all of the physically bound water in said film.

Claim 7

In the method for the treatment of ferrous metal substrates coated with thin layers of a metal selected from a group consisting of Zn, Al and Pb base alloys; the improvement which comprises, applying to said coated substrate a solution consisting essentially of, (a) 2.0 to 12.0 gms/l of hexavalent chromium, and (b) 0.7 to 4.0 gms/l of a bifluoride selected from the group of NaHF 2 , NH 4 HF 2 and mixtures thereof, (c) less than 0.5 gms/l of phosphate, said solution adjusted to a pH within the range 2.7 to 5.5; metering the amount of solution remaining on the surface of said substrate to provide a final coating thickness of 0.8 to 2.0 mg. of Cr per square foot of substrate surface and thereafter; drying the thus coated substrate to remove substantially all of the physically bound water in said film.

Claim 8

The method of claim 5, wherein the pH of said solution is within the range 3.5 to 4.7 and said phosphate is less than 0.1 g/l.

Claim 9

The method of claim 6, wherein a major portion of said hexavalent chromium is supplied by a dichromate selected from the group consisting of Na 2 Cr 2 O 7 , K 2 Cr 2 O 7 or mixtures thereof.

Claim 10

The method of claim 7, wherein said application is accomplished by passing the substrate through said solution maintained at a temperature of 45° to 95°C, for an immersion time in excess of 0.25 seconds.

Claim 11

The method of claim 8, wherein said immersion time is not greater than about 5 minutes, and said drying is accomplished by the blowing of hot air at a temperature of 50° to 120°C.