Coating processes – With pretreatment of the base – Metal base
Reexamination Certificate
2000-03-10
2004-04-13
Mulcahy, Peter D. (Department: 1713)
Coating processes
With pretreatment of the base
Metal base
C427S388500, C427S435000, C427S354000, C148S251000, C148S253000
Reexamination Certificate
active
06720032
ABSTRACT:
BACKGROUND OF THE INVENTION
For many reasons, such as weight, rigidity or recyclability, aluminum is increasingly used in vehicle construction. In the context of this invention the expression “aluminum” refers not only to pure aluminum but also to aluminum alloys whose main component is aluminum. Examples of commonly used alloying elements are silicon, magnesium, copper, manganese, chromium and nickel, the total proportion by weight of these alloying elements in the alloy normally not exceeding 10%. Whereas engine and gear parts, wheels, seat frames, etc. already contain large amounts of aluminum, the use of aluminum in bodywork construction is presently still restricted to parts such as hoods, rear trunk lids, inner door parts and various small parts as well as truck cabins, side walls of transporters or attachments to minivans. Overall, worldwide less than 5% of the metal surface of automobile bodies is made of aluminum. The increased use of aluminum in this sector is being intensively investigated by the aluminum and automobile industries.
This invention relates to a process for the corrosion-prevention pretreatment before painting of composite metal structures that contain aluminum and/or aluminum alloy portions in addition to steel and/or galvanized steel portions. The process is particularly intended for use in automobile manufacturing. In automobile manufacturing, car bodies or car body parts that contain structural portions of aluminum and/or its alloys in addition to structural portions of steel and/or galvanized steel are subjected to a conversion-chemical pretreatment before they are painted. In this connection a cathodic electro-dip-coating is conventionally used at the present time as the first painting stage. The process according to the invention is particularly suitable as a pretreatment for this stage.
The process differs from previous conventional pretreatment processes in automobile manufacturing in that a surface-covering zinc phosphate layer is deposited in a first step on the steel and/or galvanized steel surfaces, without coating the aluminum surfaces to any appreciable extent. A second step comprises a treatment with a solution that does not excessively attack the previously formed zinc phosphate layer, and indeed preferably even enhances its corrosion-prevention action, and which simultaneously forms a surface layer on the aluminum surfaces.
A two-stage process is thus involved, whose first stage comprises a conventional zinc phosphating. It is a necessary condition, of course, that a zinc phosphating solution is used that does not form a layer on aluminum. Such zinc phosphating solutions are known in the prior art and are referred to by the way of example hereinafter. In the second stage solutions with constituents that are effective to form a protective layer on aluminum are used. In this connection the nature and concentration of these solutions should be chosen so that on the one hand a layer is reliably formed on the aluminum surfaces, but on the other hand the crystalline zinc phosphation layers formed on the iron and/or zinc surfaces are not excessively damaged.
The aim of phosphating metals is to produce firmly adhering metal phosphate layers on the metal surface that per se already improve the corrosion resistance, and in conjunction with paints or other organic coatings contribute to a substantial improvement of the coating adhesion and resistance to creepage under corrosive stress. Such phosphating processes have been known for a long time. For the pretreatment before painting, especially before electro-dipcoating, low zinc phosphating processes, in which the phosphating solutions contain relatively small concentrations of zinc ions, for example 0.5 to 2 grams per liter, hereinafter usually abbreviated as “g/l”, are particularly suitable. A basic parameter in these low zinc phosphating baths is the weight ratio of phosphate ions to zinc ions, which is normally above 8 and may reach values of up to 30.
It has been found that phosphate layers with substantially improved corrosion-prevention and paint adhesion properties can be formed by the co-use of other polyvalent cations in the zinc phosphating baths. For example, low zinc processes with the addition of, e.g., 0.5 to 1.5 g/l of manganese ions and, e.g., 0.3 to 2.0 g/l of nickel ions are widely used as so-called “tri-cation” processes for preparing metal surfaces for painting, for example for cathodic electro-dipcoating of car bodies.
Since nickel and its alternative cobalt also are classed as hazardous from the toxicological and effluent treatment aspects, efforts are being made at the present time to find phosphating processes that are just as effective as the tri-cation processes but employ significantly lower bath concentrations of nickel and/or cobalt and preferably even dispense with these two metals altogether.
EP-A-459 541 describes phosphating solutions that are essentially free of nickel and that contain, in addition to zinc and phosphate, 0.2 to 4 g/l of manganese and 1 to 30 milligrams per liter, hereinafter usually abbreviated as “mg/l”, of copper. From DE-A-42 10 513 nickel-free phosphating solutions are known that contain, in addition to zinc and phosphate, 0.5 to 25 mg/l of copper ions as well as hydroxylamine as accelerator. These phosphating solutions optionally also contain 0.15 to 5 g/l of manganese.
German patent application DE 196 06 017.6 describes a phosphating solution, with a decreased heavy metal concentration, which contains 0.2 to 3 g/l of zinc ions, 1 to 150 mg/l of manganese ions, and 1 to 30 mg/l of copper ions. This phosphating solution may optionally contain up to 50 mg/l of nickel ions and up to 100 mg/l of cobalt ions. A further optional constituent is lithium ions in amounts of between 0.2 and 1.5 g/l.
DE 195 38 778 describes controlling the coating weight of phosphate layers by the use of hydroxylamine as accelerator. The use of hydroxylamine and/or its compounds in order to influence the form of the phosphate crystals is known from a number of publications. EP-A-315 059 discloses as a special effect of the use of hydroxylamine in phosphating baths the fact that on steel the phosphate crystals still occur in the desired columnar or nodular form, even if the zinc concentration in the phosphating bath exceeds the conventional range for low zinc processes. In this way it is possible to operate the phosphating baths with zinc concentrations up to 2 g/l and with weight ratios of phosphate to zinc of as low as 3.7. The required hydroxylamine concentration is given as 0.5 to 50 g/l, preferably 1 to 10 g/l.
WO 93/03198 discloses the use of hydroxylamine as accelerator in tri-cation phosphating baths with zinc contents of between 0.5 and 2 g/l and nickel and manganese contents of in each case 0.2 to 1.5 g/l, specific weight ratios of zinc to the other divalent cations having to be maintained. In addition, these baths contain 1 to 2.5 g/l of a “hydroxylamine accelerator”, which according to the description denotes salts of hydroxylamine, preferably hydroxylamine ammonium sulfate.
In order to improve the corrosion prevention produced by the phosphate layer, a so-called passivating post-rinsing, also termed post-passivation, is generally employed in this technology. Treatment baths containing chromic acid are still widely used for this purpose. For reasons of work safety and environmental protection there is a tendency, however, to replace these chromium-containing passivating baths by chromium-free treatment baths. Organo-reactive bath solutions containing complexing substituted poly(vinylphenols) are known for this purpose. Examples of such compounds are described in DE-C-31 46 265. Particularly effective polymers of this type contain amine substituents and may be obtained by a Mannich reaction between poly(vinylphenols) and aldehydes and organic amines. Such polymers are described for example in EP-B-91 166, EP-B-319 016 and EP-B-319 017. Polymers of this type are also used within the scope of the present invention, and accordingly the contents of the immediate
Brouwer Jan-Willem
Cormier Gerald J.
Enke Volkhard
Geke Jurgen
Hamacher Matthias
Cameron Mary K.
Harper Stephen D.
Henkel Kommanditgesellschaft auf Aktien
Mulcahy Peter D.
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