METHOD FOR SELECTIVELY PHOSPHATING A COMPOSITE METAL CONSTRUCTION

摘要

The present invention relates to a multistage method for the anticorrosiontreatment of composite metal structures containing metallic surfaces ofaluminum, zinc and optionally iron, in which, in the first step, selectivezincphosphating of the zinc and ferrous surfaces proceeds using a phosphatingsolution which contains water-soluble inorganic compounds of silicon in aquantity sufficient to suppress white spot formation on the zinc surfaces, butinthis respect the quantity at which the zinc phosphating loses its selectivityisnot exceeded. In the following second step of the method according to theinvention, the aluminum surfaces are passivated with an acidic treatmentsolution. The present invention furthermore relates to a zinc phosphatingsolution suitable for use in the method according to the invention containingatleast 0.025 g/l, but less than 1 g/l of silicon in the form of water-solubleinorganic compounds calculated as SiF6, the product (Si/mM).cndot.(F/mM) oftheconcentration of silicon [Si in mM] in the form of water-soluble inorganiccompounds and the concentration of free fluoride [F in mM] divided by the freeacid point number being no greater than 5.