Passivation composition based on mixtures of phosphoric and phosphonic acids

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

• i) phosphoric acid;
• ii) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein said polyphosphonic acid has a general formula (I):
• in which:
• n is at least 2;
• and,
• Z is a connecting organic moiety having an effective valency of n,
• said polyphosphonic acid being characterized in that at least two phosphonic groups are separated by an alkylene bridge having 1 or 2 carbon atoms (C1-C2 alkylene), wherein the composition has a molar ratio of phosphonate groups to phosphoric acid (H3PO4) in a range from 2:1 to 1:1;
• iii) at least one divalent metal cation (M2+);
• and,
• iv) at least one water-soluble or water-dispersible fluoroacid or a salt thereof, wherein said fluoroacid is defined by general empirical 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.