PRETREATMENT BEFORE PAINTING OF COMPOSITE METAL STRUCTURES CONTAINING ALUMINUM PORTIONS

摘要

In a process for the chemical pretreatment before painting of composite metal structures that contain aluminum or aluminum alloy portions together with steel, galvanized steel and/or alloy-galvanized steel portions, in a first step the metal structure is treated with a zinc phosphating solution that forms a surface-covering crystalline zinc phosphate layer on steel and on galvanized or alloy-galvanized steel, but without forming a zinc phosphate layer on the aluminum portions, and then in a second step the metal structure is brought into contact with a treatment solution that does not excessively dissolve the crystalline zinc phosphate layer on steel, galvanized and/or alloy-galvanized steel, but forms a conversion layer on the aluminum portions.