Cr(III) based dry-in-place coating composition for zinc coated steel

An aqueous passivation composition for treating zinc or zinc alloy coatings, said composition having a pH of less than 3 and comprising:

• i) a source of trivalent chromium (Cr(III)) ions, wherein the trivalent chromium ions (Cr(III)) are present in a concentration of from 0.05 to 0.50 moles/liter;
• ii) at least one α-hydroxycarboxylic acid represented by the General Formula (I):
• R1CH(OH)COOH (I)
• wherein:
• R1 represents a hydrogen atom, a C1-C4 alkyl group, a C2-C6 alkenyl group, a C1-C6 alkoxy group, a C3-C6 cycloalkyl group or a C6-C10 aryl group;
• wherein ii) comprises glycolic acid;
• iii) phosphoric acid;
• iv) from 1 to 10 wt. %, based on weight of the composition, of at least one compound having both a thiol group (—SH) and a carboxyl group (—CO2H);
• and,
• v) from 1 to 10 wt. %, based on the weight of the composition, of at least one water-soluble or water-dispersible fluoroacid or a salt thereof, wherein said fluoroacid is defined by the General Formula (II):
• HpTqFrOs (II)
• wherein:
• each of q and r represents an integer from 1 to 10;
• each of p and s represents an integer from 0 to 10;
• and,
• “T” represents an element selected from the group consisting of Ti, Zr, Hf, Si, Sn, Al, Ge, and B;
• wherein said composition is substantially free of nitrate anions and is substantially free of hexavalent chromium (Cr(VI));
• wherein said composition comprises from 30 to 80 wt. %, based on the weight of the composition, of water;
• and is obtainable through mixing a portion comprising hexavalent chromium dissolved in water with a molar excess of the at least one α-hydroxycarboxylic acid according to component ii), forming a mixture comprising the trivalent chromium ions of b) by reduction of said hexavalent chromium;
• and thereafter, adding the components iii) to v) to said mixture.