Abstract: An aqueous passivation composition, having a pH of less than 3, comprising: i) a source of trivalent chromium ions; ii) at least one ?-hydroxycarboxylic acid having 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; iii) phosphoric acid; iv) at least one water-soluble polyphosphonic acid or a water-soluble salt thereof, said polyphosphonic acid having general formula (II): Z—[—P?O(OH)2]n (II); in which: n is at least 2; and Z is a connecting organic moiety having an effective valency of n, said polyphosphonic acid including at least two phosphonic groups separated by an alkylene bridge having 1 or 2 carbon atoms (C1-C2 alkylene); and v) at least one divalent metal cation; wherein said composition is substantially free of: nitrate and fluoride anions, and hexavalent chromium.