Composition and process for treating metal surfaces

• US 4,963,198 A
• Filed: 02/07/1989
• Issued: 10/16/1990
• Est. Priority Date: 02/08/1988
• Status: Expired due to Fees

• Current Assignee: Brent Chemicals International CLS Ridgeway IVER Buckinghamshire
• Original Assignee: Brent International Plc
• Inventors: Higgins, George L.
• Primary Examiner(s): Silverberg, Sam
• Application Number:
  US07/307,730
• Time in Patent Office: 616 Days
• Field of Search: 148/267
• US Class Current: 148/267
• CPC Class Codes: C23C 22/60 using alkaline aqueous solutions with pH greater than 8
• CPC Class Codes: C23C 22/67 with solutions containing hexavalent chromium

Abstract

An alkaline aqueous solution comprising hexavalent chromium, trivalent chromium, and a complexing agent for trivalent chromium sufficiently effective to solubilize at least 20 ppm of trivalent chromium forms a protective coating when contacted with zinc, aluminum, and particularly zinc-aluminum alloys.

Claims

1. An aqueous alkaline solution suitable for forming a protective coating on zinc, aluminum, and alloys thereof, comprising:

(a) dissolved hexavlent chromium in an amount in the range 8-75 ppm

(b) a dissolved complexing agent for trivalent chromium ions; and

(c) dissolved trivalent chromium in an amount in the range 20-700 ppm,

said solution having a pH above 12.

2. A solution according to claim 1, wherein the content of hexavalent chromium is between 15 60 ppm.

3. A solution according to claim 2, wherein the amount of dissolved trivalent chromium is between 30 and 700 ppm.

4. A solution according to claim 1, wherein the amount of dissolved trivalent chromium is between 100 and 500 ppm.

5. A solution according to claim 4, in which the pH is at least 12.5.

6. A solution according to claim 1, containing between 10 and 80 g/l of sodium hydroxide.

7. A solution according to claim 1, in which the complexing agent comprises an organic compound containing both hydroxy and carboxyl groups and is present in an amount between 0.2 and 10 g/l.

8. A solution according to claim 7 in which the organic compound is citric acid, heptonic acid, or gluconic acid.

9. A solution according to claim 8 additionally comprising an amino carboxylic acid in an amount between 0.5 and 10 g/l.

10. A solution according to claim 9 in which the aminocarboxylic acid is nitriloacetic acid or ethylene diamine tetraacetic acid.

11. A process for forming a protective coating on a surface of zinc, aluminum, or an alloy of zinc or aluminum, comprising

contacting the surface with a solution comprising:

(a) dissolved hexavlent chromium in an amount in the range 8-75 ppm;

(b) a dissolved complexing agent for trivalent chromium ions; and

(c) dissolved trivalent chromium in an amount in the range 20-700 ppm,

said solution having a pH above 12; and

then rinsing the surface.

12. A process according to claim 11, in which the surface contacted comprises an alloy of both aluminum and zinc.

13. A process according to claim 12, in which the surface contacted also comprises a zinc surface.