PASSIVATION COMPOSITION BASED ON MIXTURES OF PHOSPHORIC AND PHOSPHONIC ACIDS

The present invention provides 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 the 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-C2alkylene);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 the following 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.