Pretreatment before painting of composite metal structures containing aluminum portions

A process for chemical pretreatment, before an organic coating, of a composite metal structure that contains at least one aluminium or aluminium alloy portion together with at least one steel, galvanized steel or alloy-galvanized steel portion, said process comprising steps of:

• I) treating in a first step the composite metal structure with a zinc phosphating solution, wherein the zinc phosphating solution has a free acid value of between 0 and 2.5 points and contains an amount of free fluoride, expressed in g/l, that is not greater than a quotient of the number 8 divided by the solution temperature in ° C. for a sufficient time to thereby form on steel and on galvanized and alloy-galvanized steel a surface-covering crystalline zinc phosphate layer having a coating weight in the range from 0.5 to 5 g/m2, but without forming a surface-covering zinc phosphate layer on the aluminium portions;
• and subsequently, with or without an intermediate rinsing with water,
• (II) contacting in a second step the composite metal structure with a treatment solution, comprising organic polymer, hexafluorotitanate and/or hexafluorozirconate ions, having a DH of 2.5-10 and a temperature in a range from 20 to 70° C. such that the treatment solution does not dissolve more than 60% of the crystalline zinc phosphate layer on steel, galvanized and/or alloy-galvanized steel, but does produce a conversion layer on the aluminum portions.