Method for chromating treatment of zinc coated steel

A method for the chromate treatment of zinc coated steel, said method comprising steps of:

• (1) providing an aqueous liquid chromate containing composition as made by substeps (1.1)-(1.2) or by substeps (1.1')-(1.3'), where substeps (1.1)-(1.2) are:
• (1.1) preparing a preliminary aqueous liquid composition which consists essentially of water and:
• (1.1.1) a source of ions containing hexavalent chromium to provide from 3.5 to 50.0 grams per liter (hereinafter "g/L") of dissolved hexavalent chromium;
• (1.1.2) a source of trivalent chromium ions to provide from 2.0 to 40.0 g/L of trivalent chromium:
• and
• (1.1.3) a source of phosphate ions to provide from 1.0 to 100 g/L of phosphate ions;
• and, optionally,
• (1.1.4) the residue from a reducing agent added to reduce some of the hexavalent chromium originally present to trivalent chromium,
• said preliminary aqueous liquid composition having a weight ratio of trivalent chromium to hexavalent chromium in the range from 0.25 to 1.5 and a weight ratio of phosphate ions to total chromium ion in the range from 0.1 to 1.2,;
• and
• (1.2) adding to the preliminary aqueous liquid composition prepared in step (1.1):
• (1.2.1) an amount of colloidally dispersed silica that provides a ratio of from 0.1 to 1.2 for the weight of dispersed silica to total weight of chromium ions in the resulting composition;
• and
• (1.2.2) an amount of silane coupling agent that provides a ratio of the moles of silane coupling agent in the resulting composition to the moles of hexavalent chromium in the resulting composition in the range from 0.05 to 0.3;
• and substeps (1.1') - (1.3') are:
• (1.1') preparing a first aqueous partial composition comprising a source of hexavalent chromium and a source of trivalent chromium and, optionally, also comprising the residue from a reducing agent added to reduce some of the hexavalent chromium originally present to trivalent chromium;
• (1.2') preparing a second aqueous partial composition comprising phosphate ions, dispersed colloidal silica, and a silane coupling agent;
• and
• (1.3') mixing said first and second aqueous partial compositions to produce an aqueous liquid chromate containing composition that could have been prepared by steps (1.1)-(1.2);
• (2) covering the surface of the zinc coated steel with a layer of the aqueous liquid chromate containing composition provided in step (1), said layer containing from 10 to 150 milligrams of total chromium per square meter of zinc coated steel surface covered;
• and
• (3) drying into place on the coated steel surface the covering liquid put in place in step (2).