Zr-/Ti-containing phosphating solution for passivation of metal composite surfaces

#The present invention relates to an aqueous composition and to a method for the anticorrosion conversion treatment of metallic surfaces. The aqueous composition is particularly suitable for treating various metallic materials which are assembled in composite structures, inter alia of steel or galvanized or alloy-galvanized steel and any combinations of these materials, the composite structure being composed at least in part of aluminum or the alloys thereof. In the remainder of the text, mention of “aluminum” always includes alloys consisting of more than 50 atom % of aluminum. Depending on how the method is carried out, the metallic surfaces of the composite structure treated according to the invention may be coated in subsequent dip coating uniformly and with excellent adhesion properties, such that it is possible to dispense with post-passivation of the conversion-treated metallic surfaces. The clear advantage of the aqueous composition according to the invention for treating metallic surfaces consists in selectively coating different metal surfaces with a crystalline phosphate layer in the case of steel or galvanized or alloy-galvanized steel surfaces and with a noncrystalline conversion layer on the aluminum surfaces in such a manner that excellent passivation of the metallic surfaces and adequate coating adhesion for a subsequently applied coating are obtained. Using the aqueous composition according to the invention therefore enables a one-step process for the anticorrosion pretreatment of metal surfaces assembled into a composite structure.

#In the field of automotive production which is of particular relevance to the present invention, increasing use is being made of different metallic materials assembled into composite structures. In body construction, use is predominantly made of many different steels due to their specific material properties, but increasing use is also made of light metals, which are of particular significance in terms of a considerable reduction in weight of the entire body. The average proportion of aluminum in an automotive body has risen in recent years from 6 kg in 1998 to 26 kg in 2002 and a further rise to approx. 50 kg is forecast for 2008, an amount which would correspond to a proportion by weight of approx. 10% of the unfinished body of a typical mid-range automobile. In order to take account of this development, it is appropriate to develop new approaches to body protection or to further develop existing methods and compositions for the anticorrosion treatment of the unfinished body.

#In conventional phosphating baths, an accumulation of aluminum ions in the bath solution results in considerable impairment of the phosphating process, in particular of the quality of the conversion layer. A uniform crystalline phosphate layer is not formed on steel surfaces in the presence of trivalent cations of aluminum. Aluminum ions thus act as a bath poison in phosphating and, in the case of standard treatment of vehicle bodies which in part comprise aluminum surfaces, must be effectively masked by appropriate additives. Suitable masking of aluminum ions may be achieved by the addition of fluoride ions or fluoro complexes for example SiF62−″, as disclosed in U.S. Pat. No. 5,683,357. Depending on the strength of the pickling attack due to the additional input of fluoride ions, hexafluoroaluminates, for example in the form of cryolite, may be precipitated from the bath solution and make a significant contribution to sludge formation in the phosphating bath, so considerably complicating the phosphating process. Moreover, a phosphate layer is only formed on the aluminum surface at elevated pickling rates, thus at a relatively high concentration of free fluoride ions. Controlling defined bath parameters, in particular free fluoride content, is here of considerable significance to adequate anticorrosion protection and good coating adhesion. Inadequate phosphating of the aluminum surfaces always entails post-passivation in a subsequent processing step. In contrast, once priming is complete, visible blemishes caused by a non-uniformly deposited phosphate layer are in principle irreparable.

#FIG. 1 shows a scanning electron microscope (SEM) micrograph of an aluminum sheet (AC120) conversion-treated in an aqueous composition at a content of free fluoride of 55 ppm, a zirconium content of 0 ppm and a λ value that is not defined.

#FIG. 2 shows a scanning electron microscope (SEM) micrograph of an aluminum sheet (AC120) conversion-treated in the aqueous composition according to the invention at a content of free fluoride of 55 ppm, a zirconium content of 10 ppm and a λ value of 8.7.

#FIG. 3 shows a scanning electron microscope (SEM) micrograph of an aluminum sheet (AC120) conversion-treated in the aqueous composition according to the invention at a content of free fluoride of 55 ppm, a zirconium content of 20 ppm and a λ value of 5.6.

#The object of the present invention is accordingly to identify those conditions under which a bath solution based on the teaching of DE10322446 is suitable for conversion treatment of metallic surfaces assembled in a composite structure, which surfaces, in addition to steel and galvanized steel surfaces, at least in part comprise aluminum surfaces for producing a uniform continuous conversion layer on all surfaces which permits immediately subsequent coating with an organic dip coating without intermediate post-passivation and overcomes the above-stated technical problems caused by excessive pickling rates.

#The present invention therefore relates to an aqueous composition for the anticorrosion conversion treatment of metallic surfaces, which comprises surfaces of steel or galvanized steel or alloy-galvanized steel or aluminum and any combinations thereof, which composition contains (a) 5-50 g/l phosphate ions, (b) 0.3-3 g/l zinc(II) ions, (c) in total 1-200 ppm of one or more water-soluble compounds of zirconium and/or titanium relative to the element zirconium and/or titanium, wherein a quantity of free fluoride of 1-400 ppm, measured with a fluoride-sensitive electrode, is additionally present in the aqueous composition.

#In order to ensure in this bath composition a minimum pickling rate, which is in particular determined by the proportion of free fluoride ions, and simultaneously selective phosphating of the steel and/or galvanized and/or alloy-galvanized steel surfaces, the aluminum surfaces merely receiving a noncrystalline zirconium- and/or titanium-based passivation layer, the concentration of the free fluoride ions should not be optimized independently of the concentration of the zirconium and/or titanium compound.