Passivation composition based on trivalent chromium

An aqueous passivation composition for the treatment of zinc or zinc alloy coatings comprising:

• i) a source of trivalent chromium (Cr(III)) ions, present in a concentration of from 0.005 to 0.1 moles/liter;
• ii) at least one α-hydroxycarboxylic acid, wherein said at least one α-hydroxycarboxylic acid is selected from the group consisting of glycolic acid, lactic acid, 2-hydroxybutanoic acid, 2-hydroxypentanoic acid and 2-hydroxyhexanoic acid, and wherein having regard to the carboxylic acid groups provided by said at least one α-hydroxycarboxylic acid, a molar ratio of said carboxylic acid groups to chromium (Cr) is in a range from 1:10 to 1:2;
• iii) phosphoric acid;
• iv) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, wherein said polyphosphonic acid is selected from a group consisting of aminotris(methylene phosphonic acid) (ATMP), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), hexamethylene diamine tetra(methylene phosphonic acid) (HDTMP), diethylenetriamine penta(methylene phosphonic acid), diethylenetriamine penta(methylenephosphonic acid (DTPMP) and mixtures thereof;
• and wherein said at least one polyphosphonic acid or the water soluble salt thereof is present in the composition in an amount such that a molar ratio of phosphonate groups to phosphoric acid (H3PO4) in the composition is in a range from 1:0.75 to 1:1.25;
• and
• v) at least one divalent metal cation selected from the group consisting of Mg2+, Ca2+, Mn2+, Sr2+, Ba2+ and Zn2+, wherein a molar concentration of the divalent metal cations in the aqueous passivation composition is in a range from 0.01 to 1 moles/liter.