Process for selective phosphating of a composite metal construction

A method of chemical pretreatment and selective phosphation of a composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, wherein the method comprises:

• (I) treating the composite metal construction with an aqueous zinc phosphation composition that results in a formation of a surface-covering crystalline zinc phosphate layer having a coat weight in a range from 0.5 to 5 g/m2 on the parts made of iron and zinc, but does not produce a zinc phosphate layer having a coat weight of at least 0.5 g/m2 on the aluminum parts, and then—with an intervening water rinse operation—
• (II) applying an aqueous acidic passivation composition having a pH in a range from 2.0 to 5.5 to the composite metal construction, where the aqueous acidic passivation composition removes not more than 50% of the crystalline zinc phosphate on the parts made of zinc and iron, but forms a passive layer on the aluminum parts that does not constitute a surface-covering crystalline phosphate layer,
• wherein the aqueous zinc phosphation composition in step (I) comprises a content of sodium and/or potassium ions and of aluminum ions,
• wherein the aqueous zinc phosphation composition in step (I) has a temperature in a range from 20 to 65° C. and comprises a free fluoride content of at least 5 mg/l but not greater than 200 mg/1,
• where wherein at least 0.04 g/l but not more than 3.2 g/l of boron in a form of water-soluble inorganic compounds, calculated as BF4, is present in the aqueous zinc phosphation composition, and
• wherein a points number of free acid with added KCl in the aqueous zinc phosphation composition is at least 0.6 points.