EP3755825A1

EP3755825A1 - Process for selective phosphating of a composite metal construction

Info

Publication number: EP3755825A1
Application number: EP19703128.9A
Authority: EP
Inventors: Olaf Dahlenburg; Hardy Wietzoreck; Juergen Specht
Original assignee: Chemetall GmbH
Current assignee: Chemetall GmbH
Priority date: 2018-02-19
Filing date: 2019-02-12
Publication date: 2020-12-30
Also published as: JP2021515108A; CN111742079A; JP2024026135A; US20210062346A1; BR112020015010A2; WO2019158508A1; MX2020008651A; RU2020130225A; JP7443253B2; KR20200121310A

Abstract

The present invention relates to a process for chemical pretreatment and selective phosphating of a composite metal construction containing at least one part made of aluminium and at least one part made of zinc and optionally a further part made of iron which comprises (I) in a first step the treatment of the composite metal construction with an aqueous zinc phosphating composition which effects on the parts made of zinc and iron the formation of a surface-covering crystalline zinc phosphate layer having a coating weight in the range from 0.5 to 5g/m² but does not produce a zinc phosphate layer having a coating weight of at least 0.5g/m² on the aluminium parts and subsequently - with an intermediate water washing cycle - comprises (II) in a second step the application of an aqueous acidic passivating composition having a pH in the range from 2.0 to 5.5 on the composite metal construction, wherein the acidic passivating composition removes from the parts made of zinc and iron not more than 50% of the crystalline zinc phosphate but forms a passive layer, which is not a surface-covering crystalline phosphate layer, on the aluminium parts, wherein the zinc phosphating composition in step (I) contains a content of sodium and/or potassium ions and of aluminium ions, wherein the zinc phosphating composition in step (I) has a temperature in the range from 20°C to 65°C and contains a content of free fluoride which is at least 5 mg/l but is not more than 200 mg/l, wherein the zinc phosphating composition contains at least 0.04 g/l but not more than 3.2 g/l of boron in the form of water-soluble inorganic compounds calculated as BF4 and wherein the pointage of the free acid with added KCl in the zinc phosphating composition is at least 0.6 points. The present invention further relates to a corresponding zinc phosphating composition, to a concentrate for the production thereof, to a corresponding composite metal construction and to the use thereof.

Description

Process for the selective phosphating of a composite metal construction

The present invention relates to the corrosion-protective treatment of composite metal structures containing metallic surfaces of aluminum, zinc and possibly iron in a multi-stage process. The process according to the invention enables the selective zinc phosphating of the zinc and iron surfaces of the composite metal construction without depositing significant amounts of zinc phosphate on the aluminum surfaces. In this way, the aluminum surface is in a subsequent process step for passivation with e.g. conventional acidic and optionally silane-containing passivation compositions are available which generate a homogeneous, corrosion-protecting thin passive layer.

In the process according to the invention, on the one hand, the formation of phosphate crystal nests on the aluminum surfaces and, on the other hand, the formation of specks on zinc surfaces is prevented. Above all, however, the cryolite precipitation on the surface of the composite metal structures, in particular on their aluminum parts, and preferably the poisoning of the phosphating bath by titanium, zirconium and / or hafnium, are reduced.

Accordingly, the present invention also relates to a zinc phosphating composition containing water-soluble inorganic compounds of boron in an amount sufficient to suppress speckling but which does not exceed values for which the zinc phosphating loses its selectivity for the zinc and iron surfaces of the composite metal construction.

In the field of automotive production, which is particularly relevant to the present invention, various metallic materials are increasingly being used and joined together in composite structures. In car body construction, a large variety of steels are still used because of their specific material properties, but also increasingly light metals such as aluminum, which are particularly significant for a considerable weight reduction of the entire body. In order to take account of this development, it is necessary to develop new concepts for body protection or to further develop existing processes and compositions for the corrosion protection treatment of the bodyshell.

There is therefore a need for improved pretreatment processes of complex components, such as automobile bodies, which, in addition to parts made of aluminum, contain those made of steel and, if necessary, galvanized steel. As a result of the entire pretreatment, a conversion layer or a passivation layer is to be produced on all metal surfaces that occur, which is suitable as a corrosion-protecting lacquer base, in particular before a cathodic electrodeposition coating.

German published patent application DE 19735314 proposes a two-stage process in which initially a selective phosphating of the steel and galvanized steel surfaces of a likewise aluminum surfaces having body and then treatment of the body with a Passivierungslösung for corrosion-protective treatment of Alumi- niumteile the body. According to the teaching disclosed therein, the selective phosphating is achieved by decreasing the pickling effect of the phosphating solution. For this purpose, DE 19735314 teaches phosphating solutions with a free fluoride content of less than 100 ppm, wherein the source of the free fluoride is formed exclusively by water-soluble complex fluorides, in particular hexafluorosilicates in a concentration of 1 to 6 g / l. The prior art discloses other two-stage pretreatment processes which also conceptually follow the deposition of a crystalline phosphate layer on the steel and optionally galvanized and alloy-galvanized steel surfaces in the first step and the passivation of the aluminum surfaces in a further subsequent step. These methods are disclosed in the documents WO 1999/012661 and WO 2002/066702. In principle, the processes disclosed therein are carried out in such a way that, in a first step, selective phosphating of the steel or galvanized steel surfaces takes place, which is also retained in the post-passivation in a second process step, while no phosphate crystals are formed on the aluminum surfaces. The selective phosphating of the steel and galvanized steel surfaces is achieved by a temperature-dependent limitation of the proportion of free fluoride ions in the phosphating solutions, whose free acid contents are set in a range of 0 to 2.5 points.

The international application WO 2008/055726 discloses an at least one-step process for the selective phosphating of steel and galvanized steel surfaces of a composite construction comprising aluminum parts. This publication teaches phosphating solutions containing water-soluble inorganic compounds of the elements zirconium and titanium, the presence of which successfully prevents the phosphating of the aluminum surfaces.

European application EP 2588646 discloses a process for the selective phosphating of steel and galvanized steel in metallic components which also have aluminum surfaces. This process also avoids the formation of phosphate crystal nests on the aluminum surfaces and specks on the galvanized steel surfaces. For this purpose, silicon is added to the zinc phosphating composition in the form of water-soluble inorganic compounds.

Starting from this prior art, the selective phosphating of steel and galvanized steel in the anticorrosive treatment of mixed metallic components, i. E. To further develop composite metal structures having aluminum surfaces in such a way that an improvement of the process economy during the phosphating is achieved by a targeted control of the selectivity-controlling Badpa- parameters. In addition to avoiding the formation of phosphate crystal nests on the aluminum surfaces and of specks on the galvanized steel surfaces, in particular the reduction of the cryolite precipitation on the surface of the metallic components, in particular on the aluminum surfaces, and preferably also with regard to the quality of the corrosion-protective treatment of composite metal constructions poisoning of the phosphatizing bath with titanium, zirconium and / or hafnium.

The term "phosphate crystal nests" is understood by the person skilled in the art to mean the isolated and locally limited deposition of phosphate crystals on metal surfaces (here: aluminum surfaces). Such "crystal nests" are trapped by a subsequent coating primer and represent in-homogeneities in the coating, which both disturb the uniform visual impression of the painted surfaces and can also cause punctiform paint damage.

By "speckling", one skilled in the phosphating art understands the phenomenon of local deposition of amorphous, white zinc phosphate in an otherwise crystalline phosphate layer on the treated zinc surfaces or on the treated galvanized or alloy galvanized steel surfaces. The speckling is caused by a locally increased pickling rate of the substrate. Such point defects in the phosphating can Starting point for the corrosive delamination of subsequently applied organic paint systems, so that the occurrence of specks in practice is largely to be avoided.

The cryolite precipitation (deposition of solid Na _ß AIFe and / or K3AIF6) on the surface of the composite metal structures, in particular on the aluminum parts, causes a lubricating paper-like nature of this surface, which is undesirable because they are down to the surface of the finished painted Constructions continues. The result is painting defects that can only be eliminated with great effort by, for example, grinding the affected areas. This, of course, involves corresponding work effort and higher costs.

The cryolite precipitation takes place according to the following formula:

3 Na ⁺ (aq) + Al ³⁺ (aq) + 6 F (aq) -> Na ₃ AIF ₆ (s) or J.

3 K ⁺ (aq) + Al ³⁺ (aq) + 6 F ^~ (aq) -> K ₃ AIF ₆ (s) j.

The sodium ions in the phosphating composition originate from additives such as, in particular, additives and / or accelerators. As additives here are mainly sodium fluoride, sodium nitrate, sodium phosphate and sodium hydroxide to name as accelerators, especially sodium nitrite. The same applies to the potassium ions. These additives are added to the bath depending on the situation and available bathing parameters. In contrast, the aluminum ions were leached out of the aluminum parts of the composite metal constructions in the acidic environment of the phosphating bath composition.

A certain minimum amount of free fluoride (fluoride ions) in the phosphating bath is necessary to ensure adequate deposition kinetics for the zinc phosphate layer on the surfaces of iron and zinc of the composite metal structure, in particular because of the simultaneous treatment of the aluminum surfaces of the composite metal structure aluminum Io - enter the zinc phosphating composition (see above), which in turn, in an uncomplexed form, interferes with or even prevents the formation of a uniform, homogeneous zinc phosphate layer on the iron and zinc parts of the composite construction. The free fluoride can also originate from fluoro complexes, from which it is released in equilibrium reactions.

However, the higher the content of free fluoride - at a given sodium / potassium and aluminum content - at the surface of the composite metal constructions, the stronger the resulting cryolite precipitation.

A poisoning of the phosphating bath by titanium, zirconium and / or hafnium may be due in particular to encrusted conveyor systems for the composite metal constructions or in pre-adjusted aluminum parts of the constructions.

The crusts (eg phosphate crusts or non-baked coating layers) on the conveyor systems contain titanium, zirconium and / or hafnium. The latter are derived from the phosphatization (see step (I)) of the composite metal constructions subsequent passivation (see step (II)) with titanium, zirconium and / or hafnium-containing water-soluble compounds and are absorbed by the crusts due to their porosity. Since the conveyors run in a circle, the crusts in the passivation bath, which are exposed to titanium, zirconium and / or hafnium, are again exposed to the acid phosphating composition in the phosphating bath. Titanium, zirconium and / or hafnium partially dissolve there. The aluminum parts of composite metal constructions are often pre-passivated, ie they are already provided with a passivation layer containing titanium, zirconium and / or hafnium, which has already been applied to them by the supplier of the aluminum parts.

In both cases, it comes in the fluoride-containing phosphating complexing and thus dissolution of titanium, zirconium and / or hafnium from the crusts and the pre-passivation. Consequently, titanium, zirconium and / or hafnium accumulates in the phosphating bath. On the aluminum parts of the composite metal constructions, this can lead to the deposition of black and wipeable layers, which are amorphous zinc phosphate. Said layers can later lead to defects in the paint and also reduce corrosion protection.

This object described above is achieved according to the invention by a process for the chemical pretreatment of selective phosphating of a composite metal construction which contains at least one part of aluminum and at least one part of zinc and optionally a further part of iron which

(I) in a first step, the treatment of the composite metal construction with an aqueous zinc phosphating composition which on the parts of zinc and iron causes the formation of a surface-covering crystalline zinc phosphate layer having a coating weight in the range of 0.5 to 5 g / m ² does not produce a zinc phosphate layer with a coating weight of at least 0.5 g / m ² on the aluminum parts, and then - with intermediate rinsing with water -

(II) in a second step comprises applying to the composite metal structure an aqueous acidic passivating composition having a pH in the range of 2.0 to 5.5, the acidic passivating composition on the parts of zinc and Iron does not dissolve more than 50% of the crystalline zinc phosphate, but forms a passive layer on the aluminum parts which does not constitute a surface-covering crystalline phosphate layer, the zinc phosphating composition in step (I) containing sodium and / or potassium ions and aluminum Contains ions,

wherein the zinc phosphating composition in step (I) has a temperature in the range of 20 to 65 ° C and contains a content of free fluoride which is at least 5 mg / l but not greater than 200 mg / l,

being calculated in the Zinkphosphatierungszusammensetzung at least 0.04 g / l but not more than 3.2 g / l of boron in the form of water-soluble inorganic compounds as BF ₄ are included, and

wherein the score of the free acid with KCI addition in the zinc phosphating composition is at least 0.6 points.

definitions:

A "composite metal construction" in the sense of the present invention is a mixed component metallic component, in particular a body. In addition to surfaces made of zinc, surfaces of aluminum and, if appropriate, surfaces of iron, the composite metal construction may also have surfaces of other metals and / or surfaces of non-metals, in particular of plastics. According to the invention, the material "aluminum" also means its alloys. At the same time, according to the invention, the material zinc also comprises zinc alloys, for example zinc-magnesium alloys, and galvanized steel and alloy-galvanized steel, while the inclusion of iron also includes iron alloys, in particular steel. Alloys of the aforementioned materials have a Fremdatomanteil of less than 50 wt .-%.

By an "aqueous composition" herein is meant a solution or dispersion which, for the most part, i. to more than 50 wt .-%, preferably more than 70% by weight, particularly preferably more than 90 wt .-% (based on the totality of the solvents contained, dispersion media) contains water as a solvent or dispersion medium. The aqueous composition can therefore contain not only dissolved but also dispersed constituents. Preferably, however, it is a solution, i. to such a composition containing only small amounts of dispersed ingredients, preferably no dispersed ingredients at all, but only or almost only dissolved components.

A "passive layer" in the sense of the present invention is a passivating, i. Corrosion-protective, inorganic or mixed inorganic-organic thin-film coating, which is not a closed crystalline phosphate layer. Such passive layers may consist, for example, of oxide compounds of titanium, zirconium and / or hafnium.

According to the invention, by "water-soluble compounds" is meant both a water-soluble compound and two or more water-soluble compounds.

The requirement that no zinc phosphate layer may form on the aluminum parts in treatment step (I) is to be understood as meaning that no closed and sealed crystalline layer is formed there. This condition is at least satisfied when the basis weight of zinc phosphate deposited on the aluminum parts is less than 0.5 g / m ² . In the context of the present invention, aluminum parts are understood as meaning sheets and components made of aluminum and / or alloys of aluminum.

On the other hand, the formation of a closed and crystalline zinc phosphate layer on the steel, galvanized and / or alloy-galvanized steel surfaces is absolutely necessary and characterizes the process according to the invention. For this purpose, the composite metal structure in step (I) of the present process zinc phosphate layers with an area-related coating weight of preferably at least 1 on these surfaces, 0 g / m ^2, more preferably of at least 2.0 g / m ^2, but preferably not more than 4.0 g / m ² deposited.

The coating of zinc phosphate is determined for all surfaces of the composite metal construction by means of gravimetric differential weighing on test plates of the individual metallic materials of the respective composite metal construction. In this case, steel sheets immediately after a step (I) for 5 minutes with an aqueous solution of 9 wt .-% EDTA, 8.7 wt .-% NaOH and 0.4 wt .-% triethanolamine at a temperature of 70 ° C in Contact and freed in this way from the zinc phosphate layer. Analogously, to determine the zinc phosphate layer coating on galvanized or alloy-galvanized steel sheets, a corresponding test sheet is immediately after one step (I) for 4 minutes with an aqueous solution of 20 g / l (NH 4) 2Cr ₂ 0 ₇ and 27.5 wt .-%. NH3 brought at a temperature of 25 ° C in contact and freed in this way from the zinc phosphate layer. By contrast, aluminum sheets are brought into contact with an aqueous 65% strength by weight HNO 3 solution at a temperature of 25 ° C. for 15 minutes immediately after step (I) and are freed from zinc phosphate portions accordingly. The difference in weight of the dry metal sheets after this respective treatment for weight of the same dry untreated metal sheet immediately before the step (I) corresponds to the coating of zinc phosphate according to this invention.

The requirement according to the invention that in step (II) not more than 50% of the crystalline zinc phosphate layer is dissolved on the steel and galvanized and / or alloy-galvanized steel surfaces can also be reconstructed on the basis of test sheets of the individual metallic materials of the respective composite metal construction. For this purpose, the test panels of steel, galvanized or alloy-galvanized steel phosphated according to step (II) of the process according to the invention are blown dry with compressed air after a rinsing step with deionized water and then weighed.

The same test sheet is then brought into contact with the acidic passivation composition in accordance with step (II) of the process according to the invention, then rinsed with deionized water, blown dry with compressed air and then weighed again. The zinc phosphating of the same test sheet is then treated with an aqueous solution of 9% by weight EDTA, 8.7% by weight NaOH and 0.4% by weight triethanolamine or with an aqueous solution of 20 g / l (NH ₄ ) ₂ Cr ₂ 0 ₇ and 27.5 wt .-% NH ₃ completely removed as described above and weighed the dried test sheet once more. From the weighing differences of the test sheet, the percentage loss of phosphate layer in step (II) of the method according to the invention is now determined.

The free acid with KCI additive (FS, KCl) of the zinc phosphating composition in points in step (I) of the method according to the invention is determined as follows (see: W. Rausch "The phosphating of metals", Eugen G. Leuze Verlag, 3rd edition , 2005, Chapter 8.1, pp. 333-334):

10 ml of the phosphating composition are pipetted into a suitable vessel, for example a 300 ml Erlenmeyer flask. If the phosphating composition contains complex fluorides, 6-8 g of potassium chloride are added to the sample. Then, using a pH meter and an electrode, it is titrated with 0.1 M NaOH to a pH of 4.0. The consumed amount of 0.1 M NaOH in ml per 10 ml of the phosphating composition gives the value of the free acid with KCI additive (FS, KCl) in points.

The zinc phosphating composition in the step (I) preferably has a temperature in the range of 30 to 60 ° C, more preferably 35 to 55 ° C.

The concentration of free fluoride in the zinc phosphating composition is determined in the method according to the invention by means of a potentiometric method. In the process, a sample volume of the zinc phosphating composition is taken and the activity of the free fluoride ions is determined with any commercial fluoride-selective potentiometric single-rod measuring chain after calibration of the combination electrode by means of fluoride-containing standard solutions without pH buffering. Both the calibration of the combination electrode and the measurement of the free fluoride are carried out at a temperature of 20 ° C.

Exceeding a limit concentration of free fluoride, which is dependent on the free acid, but in any case, the exceeding of a concentration of 200 mg / l causes the Deposition of a full-coverage crystalline zinc phosphate layer on the aluminum surfaces. However, such a layer formation is undesirable because of the substrate-specific coating properties of zinc phosphating and therefore not according to the invention. However, a certain minimum amount of free fluoride is necessary to ensure adequate deposition kinetics for the zinc phosphate layer on the surfaces of iron and zinc of the composite metal structure, in particular because of the simultaneous treatment of the aluminum surfaces of the composite metal structure Zinc phosphating come, which in turn inhibit the Zinkphosphatie- tion in uncomplexed form.

The zinc phosphating composition therefore has a free fluoride content of at least 5 mg / l but not greater than 200 mg / l, preferably at least 10 mg / l but not greater than 200 mg / l, more preferably at least 20 mg / l but not greater than 175 mg / l, more preferably at least 20 mg / l but not greater than 150 mg / l, more preferably at least 20 mg / l but not greater than 135 mg more preferably is at least 20 mg / L, but is not greater than 120 mg / L, more preferably at least 50 mg / L but not greater than 120 mg / L, and most preferably at least 70 mg / L is but not greater than 120 mg / l.

The total _{fluoride content} (F _tot ) of the zinc phosphating composition is preferably in the range of 0.01 to 0.3 mol / l, more preferably 0.02 to 0.2 mol / l.

_{(Sat F.)} Of the total fluoride content, the content of fluoride (F-) as a whole and is composed of the complex-bound fluoride and the simple fluoride. The complex-bound fluoride is the fluoride bound to Si or B but also to Al and further complexes. The single fluoride, however, is the non-complexed fluoride and is composed of the identifiable by Einstabmesskette free fluoride, ie the unbound fluoride, and the HF bound fluorine together.

The higher the content of sodium and / or potassium ions, aluminum ions and free fluoride in the zinc phosphating composition in step (I) of the process according to the invention, the stronger the cryolite precipitation on the surface of the composite metal structures, in particular on the aluminum parts.

If the content of sodium and / or potassium ions (calculated as sodium) in the range of 1 to 4 g / l, in particular from 2 to 3 g / l and the content of free fluoride in the range of 50 to 150 mg / l In particular, from 70 to 120 mg / l can be in the presence of aluminum ions, the Kry- olithfällung by the presence of boron in the form of water-soluble inorganic compounds, in particular in the form of fluoroborates such as HBF ₄ , reduce particularly effective.

The zinc phosphating composition preferably has a concentration of boron in the form of water-soluble inorganic compounds calculated as BF ₄ in the range of 0.08 to 2.5 g / l, more preferably 0.08 to 2 g / l, more preferably 0.25 to 2 g / l, more preferably from 0.5 to 1, 5 g / l and most preferably from 0.8 to 1, 2 g / l on.

The addition according to the invention of water-soluble inorganic compounds containing boron additionally acts to prevent the formation of specks on the zinc surfaces. The upper limit of 3.2 g / l calculated as BF ₄ is on the one hand due to the economy of the process and on the other hand, that the process control is made difficult by such high concentrations of water-soluble inorganic compounds containing boron, since the formation of Phosphate crystal nests on the aluminum surfaces can only be pushed back to a lesser extent by increasing the free acid content.

After completion of the paint system, the crystal nests require punctiform elevations which have to be ground for an optically uniform lacquering of the composite metal construction, for example an automobile bodywork, desired by the customer.

Surprisingly, it has been found that, for an effective inhibition of the formation of a crystalline zinc phosphate layer and of zinc phosphate crystal nests on the aluminum surfaces, the concentration of boron in the form of water-soluble inorganic compounds calculated as BF ₄ as a critical parameter is decisive for the success of the process according to the invention. If a concentration of 2.0 g / l is exceeded, the formation of at least individual zinc phosphate crystal nests on the aluminum surfaces already takes place.

If this concentration is exceeded further, the aluminum surfaces in the process according to the invention are coated with a surface-covering crystalline zinc phosphate layer. Both scenarios must be avoided for successful corrosion-protective pretreatment. Therefore, in step (I) of the process according to the invention zinc phosphating compositions are used in which the concentration of boron in the form of water-soluble inorganic compounds has a value of 3.2 g / l, particularly preferably a value of 2.0 g / l. each calculated as BF ₄ - does not exceed.

In any case, the proportion of boron according to the invention in the form of water-soluble inorganic compounds is sufficient to prevent the formation of specks on the parts of zinc treated according to the invention. Water-soluble inorganic compounds containing boron which are preferred in the process according to the invention are fluoroborates, more preferably HBF ₄ , (NH ₄ ) BF ₄ , LiBF ₄ and / or NaBF ₄ . The water-soluble fluoroborates are also suitable as a source of free fluoride and therefore serve the complexation of trivalent aluminum cations introduced into the bath solution, so that the phosphating remains ensured on the surfaces of steel and galvanized and / or alloy-galvanized steel.

When using fluoroborates in phosphating solutions in step (I) of the process according to the invention, it must of course always be ensured that the concentration of boron in the form of water-soluble inorganic compounds calculated as BF _{4 is} 3.2 g / l according to claim 1 does not exceed the present invention.

It is possible that in addition to water-soluble inorganic compounds containing boron, the zinc phosphating composition in step (I) also has a certain content of water-soluble inorganic compounds containing silicon. The latter can, for example, have been incorporated into the phosphating composition from previous process steps or-with a high silicon content of the aluminum parts-can also originate from the composite metal constructions.

However, a content of inorganic compounds containing silicon is, for the purposes of the present invention, first of all undesirable, since this enhances the cryolite precipitation on the surface of the composite metal structures, in particular on their aluminum parts, which can be explained as follows:

During the acid pickling attack on the surface of the composite metal structure, protons are consumed by being reduced to hydrogen. This leads to an increase in the pH value at the surface. Surprisingly, it has now been found that fluorosilicates, such as hexafluorosilicate, release fluoride ions (free fluoride) to a much greater extent as the pH increases, than the corresponding fluoroborates, such as tetrafluoroborate. As a result, the content of fluoride ions at the surface in the case of the fluorosilicates also increases significantly more, which finally results in a correspondingly increased cryolite precipitation with a corresponding content of sodium / potassium and aluminum ions (cf., Massenwirkungs - law).

Second, a content of inorganic compounds containing silicon is secondarily undesirable since it has now surprisingly been found that a zinc phosphating bath composition containing free silicon fluoride in the form of water-soluble inorganic compounds has much more titanium, zirconium and / or hafnium encrusted conveyor systems for the composite metal structures and / or the pre-passivated aluminum parts of the constructions can lead and thus lead to a much greater poisoning of the phosphating by titanium, zirconium and / or hafnium, as those having boron in the form of water-soluble inorganic compounds.

This can again be explained by the fact that, as already described above, the pH shift on the surface of the composite metal construction releases fluoride from fluoride complexes, which can then complex titanium, zirconium and hafnium. As a result, they remain soluble and can accumulate in the bathroom. Compared with fluorosilicates such as hexafluorosilicate, fluoroborates such as tetrafluoroborate are less likely to release fluoride ions, leaving less free fluoride available for the complexation of titanium, zirconium, and hafnium. In addition, in the case of tetrafluoroborates, the concentration of free fluoride can be better controlled.

Thirdly, a content of inorganic compounds containing silicon is undesirable, since sparingly soluble precipitates form in the presence of potassium ions, in particular in the case of fluorosilcates such as H2S1F6. The formation of such precipitates is as undesirable as the precipitation of cryolite. Potassium ions as cations in salts for adjustment of the bath parameters, such as in phosphates, hydroxides and fluorides, compared to sodium ions is advantageous since K3AIF6 is more soluble than Na _ß Aife and thus less cryolite is precipitated. Although ammonium ions which may be present in the zinc phosphating composition also have this advantage (good solubility of (NH ₄ ) 3AIF 6), they are more environmentally hazardous in wastewater than potassium ions. The formation of sparingly soluble, potassium-containing precipitates can not be determined for corresponding boron-containing compounds, in particular in the case of fluoroborates such as HBF ₄ .

The content of inorganic compounds containing silicon (calculated as SiFe) is therefore preferably below 25 mg / l, more preferably below 15 mg / l, more preferably below 5 mg / l and most preferably below 1 mg / l.

The zinc phosphating composition in step (I) preferably has a KCI addition free acid score in the range of from 0.6 to 3.0 points, preferably from 0.8 to 3.0 points, more preferably from 1.0 to 3.0 points, and most preferably from 1, 0 to 2.5 points. Maintaining the preferred ranges for the free acid ensures on the one hand a sufficient deposition kinetics of the phosphate layer on the selected metal surfaces and on the other hand prevents unnecessary pickling of metal ions, which in turn requires intensive monitoring or processing of the phosphating bath to avoid the Precipitation of sludges or disposal thereof in the continuous operation of the inventive method requires.

Furthermore, the total acid (GS) in the phosphating composition in step (I) of the process according to the invention should be in the range from 10 to 50, more preferably from 15 to 40 and particularly preferably from 20 to 35 points.

By contrast, Fischer's total acidity (GSF) should be in the range of 10 to 30, more preferably 12 to 25, and most preferably 15 to 20 points, while the S value is in the range of 0.04 to 0.20, more preferably from 0.05 to 0.15, and more preferably from 0.06 to 0.12.

Surprisingly, it has been found that both the cryolite precipitation on the surface of the composite metal constructions and the poisoning of the phosphating bath by titanium, zirconium and / or hafnium can be reduced particularly effectively if the dimensionless quotient for the zinc phosphating composition in step (I)

(S value x T) / (F _sat. X [B] x 1000) in the range of 0.13 to 22.5 preferably from 0.2 to 15 and more preferably from 0.4 to 10, said " T "for the temperature / ° C," F _tot. Stands for the total fluoride content / (mol / l) and ,, [B] "for the concentration of the element boron / (mol / l).

The free acid (diluted) (FS (dil.)), Which is needed exclusively to determine Fischer's total acidity (GSF), Fischer's total acidity (GSF), total acidity (GS) and S-value are determined as follows :

Free acid (diluted) (FS (dil.)):

(See W. Rausch "The Phosphatization of Metals", Eugen G. Leuze Verlag, 3rd Edition, 2005, Chapter 8.1, pp. 333-334)

To determine the free acid (diluted), 10 ml of the phosphating composition are pipetted into a suitable vessel, for example into a 300 ml Erlenmeyer flask. Subsequently, 150 ml of deionized water are added. Using a pH meter and an electrode, titrate with 0.1 M NaOH to a pH of 4.7. The consumed amount of 0.1 M NaOH in ml per 10 ml of the diluted phosphating composition gives the value of the free acid (diluted) (FS (dil.)) In points.

Total acid according to Fischer (GSF):

(See W. Rausch "The Phosphatization of Metals", Eugen G. Leuze Verlag, 3rd Edition, 2005, Chapter 8.2, pp. 334-336)

Following the determination of the free acid (diluted), the dilute phosphating composition is titrated to pH 8.9 after addition of potassium oxalate solution using a pH meter and electrode with 0.1 M NaOH. The consumption of 0.1 M NaOH in ml per 10 ml of the diluted phosphating composition gives the total Fischer acid (GSF) in points. Total Acid (GS):

(See W. Rausch "Phosphating of Metals", Euqen G. Leuze Verlaq, 3rd ed., 2005, Chapter 8.3, pp. 336-338)

The total acid (GS) is the sum of the divalent cations present as well as free and bound phosphoric acids (the latter being phosphates). It is determined by the consumption of 0.1 M NaOH using a pH meter and an electrode. For this purpose, 10 ml of the phosphating composition are pipetted into a suitable vessel, for example a 300 ml Erlenmeyer flask, and diluted with 25 ml of demineralized water. It is then titrated with 0.1 M NaOH to a pH of 9. The consumption in ml per 10 ml of the diluted phosphating composition corresponds to the total acid score (GS).

Acid value (S value):

(See W. Rausch "The phosphating of metals", Eugen G. Leuze Verlag, 3rd edition, 2005, chapter 8.4, p. 338)

The so-called acid value (S value) stands for the ratio of the free acid with KCI additive to the total acid according to Fischer, ie (FS, KCl): GSF, and is obtained by dividing the value of the free acid with KCI additive (FS, KCl) by the value of total acid according to Fischer (GSF).

The phosphating composition in step (I) preferably has a pH in the range of 2.5 to 3.5.

For an optimum phosphating result of metallic components which, in addition to aluminum surfaces, also have surfaces of steel and galvanized and / or alloy-galvanized steel, zinc phosphating compositions are preferred in step (I) of the process according to the invention, which total not more than 20 ppm, preferably no more than 15 ppm, more preferably not more than 10 ppm, more preferably not more than 5 ppm and most preferably not more than 1 ppm total of water-soluble compounds of zirconium, titanium and / or hafnium, based on the elements zirconium, Titanium and / or hafnium and in particular preferably no water-soluble compounds of zirconium, titanium and / or hafnium.

Consequently, in particular, the addition of water-soluble compounds of zirconium, titanium and / or hafnium to the phosphating compositions in step (i) of the process according to the invention is completely dispensed with.

It is known from WO 2008/055726 that the presence of water-soluble compounds of these elements in a phosphating step can also effectively suppress the formation of crystalline phosphate layers on aluminum surfaces. However, it has been found that in the presence of water-soluble compounds of zirconium, titanium and / or hafnium, in particular when the phosphating composition is applied by spraying, an inhomogeneous amorphous zirconium-, titanium- and / or hafnium-based conversion coating on the aluminum parts results, which in subsequent organic varnishing leads to the occurrence of "mapping".

By "mapping" the expert in the dip coating of metallic components understands a flecked optical impression of the paint coating due to an inhomogeneous coating thickness after firing the dip. In order to keep the concentration of zirconium, titanium and / or hafnium low, it has proved to be advantageous to reduce the free fluoride concentration or the total fluoride concentration, resulting in suppressing the film formation on the aluminum parts and better controlling the process can.

As already described above, poisoning of the phosphating bath by titanium, zirconium and / or hafnium can be caused, above all, by encrusted conveying systems for the composite metal structures or in pre-passivated aluminum parts of the structures.

As explained further above, the content of boron in the form of water-soluble inorganic compounds, in particular by fluoroborates, in particular tetrafluoroborate, can reduce the poisoning of the phosphating bath by titanium, zirconium and / or hafnium.

In addition, at high levels of free fluoride leaching from the crusts and the pre-passivation takes place in the acidic medium stronger. At the same time, the titanium and / or zirconium is stabilized by the high content of free fluoride. It comes to the formation of the fluorocomplexes of titanium and / or zirconium. By lowering the content of free fluoride, titanium and / or zirconium is destabilized in its solubility, precipitates and thus no longer poisoning of the phosphatizing bath.

By lowering the content of free fluoride, the formation of a zinc phosphate layer on the aluminum parts is indeed reduced. Through a downstream passivation - cf. Step (II) - however, this is not a problem. On the contrary, a low zinc phosphating on the aluminum parts is conducive in the carried out corrosion and paint adhesion tests.

According to a first preferred embodiment of the method according to the invention, the conveyor systems for the composite metal constructions have crusts containing titanium, zirconium and / or hafnium and preferably from the subsequent phosphatization in step (I) passivation of the composite metal structures with titanium, zirconium and / or Haf - water-soluble compounds containing sodium in step (II) come. The conveyor systems preferably run in a circle, so that the crusts applied to titanium, zirconium and / or hafnium in step (II) are again exposed to the acid phosphating composition in step (I).

According to a second preferred embodiment, the aluminum parts of the composite metal constructions are pre-passivated, i. they are already provided with a passivating layer containing titanium, zirconium and / or hafnium.

According to a third preferred embodiment, the conveyor systems for the composite metal constructions have crusts which contain titanium and / or zirconium and which preferably from the phosphatization in step (I) subsequent passivation of the composite metal constructions with titanium, zirconium and / or hafnium-containing water-soluble compounds in step (II) originate. The conveyor systems preferably run in a circle, so that the crusts produced in step (II) are again exposed to the acid phosphating composition in step (I). In addition, the aluminum parts of the composite metal constructions are pre-passivated, ie they are already provided with a passivation layer containing titanium, zirconium and / or hafnium.

The zinc phosphating composition in step (I) of the process according to the invention preferably contains at least 0.3 g / l, more preferably at least 0.8 g / l, but preferably not more than 3 g / l, more preferably not more than 2 g / l of zinc ions. The content of phosphate ions in the phosphating solution is preferably in the range of 5 to 50 g / l, more preferably 8 to 25 g / l, and particularly preferably 10 to 20 g / l, each calculated as P2O5.

The zinc phosphating composition of the process according to the invention may contain, in addition to the abovementioned zinc ions and phosphate ions, additionally at least one of the following accelerators:

0.07 to 0.25 g / l nitrite (calculated as sodium nitrite),

about 0.1 to 2 g / l nitroguanidine

8 to 51 mg / l hydrogen peroxide.

Such accelerators are familiar in the prior art as components of phosphating baths and fulfill the function of "hydrogen scavengers" by directly oxidizing the hydrogen produced by the acid attack on the metallic surface and thereby reducing it itself. The formation of a homogeneous crystalline zinc phosphate layer is greatly facilitated by the accelerator, which prevents the formation of gaseous hydrogen on the metal surface.

The zinc phosphating composition of the process according to the invention may contain, in addition to the abovementioned zinc ions and phosphate ions, additionally at least one of the following anions:

0 to 2 g / l sulphate,

0.01 to 500 mg / L (for nickel-free zinc phosphating composition) or 0.01 to 20 g / L (for nickel-containing zinc phosphating composition) nitrate.

The amount of up to 5 g / l, preferably up to 3 g / l of nitrate customary in nickel-containing phosphating facilitates the formation of a crystalline homogeneous and closed phosphate layer on steel as well as on galvanized and alloy-galvanized steel surfaces.

In nickel-free phosphating, however, the acceleration of the nitrate layering reaction is critical. Such leads to lower coating weights and above all to a reduced incorporation of manganese into the crystal. However, since the phosphate coating does not contain nickel, too low a manganese content of the phosphate coating is at the expense of its alkali resistance.

The alkali resistance in turn plays a decisive role in a subsequent cathodic electrodeposition deposition. This leads to the substrate surface to a Electrolytic splitting of water: Hydroxide ions form. This causes the pH at the interface of the substrate to increase. It is true that only then can the electrocoat be agglomerated and separated; however, the increased pH can also damage the crystalline phosphate layer.

Corrosion protection and paint adhesion of the crystalline zinc phosphate layers produced with an aqueous phosphating composition according to the invention are according to experience improved if additionally one or more of the following cations is contained:

0.3 to 2 g / l, preferably 0.5 to 1.5 g / l manganese (II),

0.3 to 2 g / l, preferably 0.8 to 1.2 g / l nickel (II),

0.001 to 0.2 g / l, preferably 0.005 to 0.1 g / l of iron (III).

On the one hand, iron (III) cations provide improved corrosion protection on steel as well as on zinc and zinc alloys. On the other hand, iron (III) cations lead to improved stability of the corresponding phosphating bath. The addition of iron (III) cations also makes it possible to generate a sludge flake that is easier to filter so that it is easier to remove cryolite from the bath.

Aqueous conversion-conversion compositions which contain both manganese ions and manganese ions are known to the person skilled in the art of phosphating as trication-phosphating solutions and are also well suited for the purposes of the present invention.

In addition to the abovementioned cations, which are incorporated into the phosphate layer or at least have a positive effect on the crystal growth of the phosphate layer, the phosphating solutions in step (I) of the process according to the invention also contain, in addition to sodium and / or potassium ions. Ions which enter the phosphating solution to adjust the free acid content.

The flushing of water between step (I) and step (II) is obligatory since otherwise the phosphate ions contained in the phosphating composition as well as metal cations would be carried off into the passivation bath. This would lead to a difficult-to-control change in the bath parameters and to the precipitation of titanium, zirconium and / or hafnium as poorly soluble phosphates.

The water purging process can be a water purging but also a total of at least two pours of water.

Each water rinse can be carried out with demineralised water or city water. In each case at least one surfactant may also be present.

After working up the rinsing water enriched with the components of the phosphating bath according to step (I), a selective recycling of the components into the phosphating bath can be carried out.

The composite metal structure treated and rinsed according to step (I) and is contacted in step (II) with the acidic passivation composition by dipping or by spraying the solution. In step (II) of the method, by contacting the composite metal structure with the acid passivation composition according to the invention, the formation of a passive layer on the aluminum surfaces, wherein the zinc phosphate layer on the steel surfaces, galvanized and / or alloy-galvanized steel surfaces during in-contact -Ring with the passivation composition to not more than 50%, preferably not more than 20%, particularly preferably not more than 10% is dissolved. In the context of the present invention, passivating layers of aluminum are passivating inorganic or mixed inorganic-organic thin-film coatings which do not comprise closed crystalline phosphate layers. While the pH of the acidic passivation composition ranges from 2.0 to 5.5, preferably from 2.5 to 5.5, more preferably from 3.0 to 5.5, and most preferably from 3.5 to 5, 3, essentially ensuring that no more than 50% of the zinc phosphate layer is dissolved on the steel surfaces, galvanized and / or alloy galvanized steel surfaces, the corresponding passive layers on the aluminum surfaces of the composite metal construction are typically produced by chromium-free acid passivation compositions.

The passivating composition contains at least one water-soluble compound selected from the water-soluble compounds of the metal ions of Mo, Cu, Ag, Au, Ti, Zr, Hf, Pd, Sn, Sb, Li, V and Ce. The water-soluble compounds are in each case present in a concentration in the following preferred, particularly preferred and very particularly preferred ranges (in each case calculated as the corresponding metal):

The metal ions contained in the water-soluble compounds are deposited either in the form of a salt which preferably contains the corresponding metal cation (eg molybdenum or tin) in at least two oxidation states - in particular in the form of an oxide hydroxide, a hydroxide, a spinel or a defect spinel - or elemental on the surface to be treated (eg copper, silver, gold or palladium). According to a first preferred embodiment, the passivating composition contains molybdenum ions and optionally further metal ions - each in the form of water-soluble compounds. These are preferably added as molybdate, more preferably as ammonium heptamolybdate and particularly preferably as ammonium heptamolybdate x 7H 2 O of the passivation composition. The molybdenum ions can also be added as sodium molybdate.

However, molybdenum ions can also be added to the passivation composition in the form of at least one molybdenum cation-containing salt such as molybdenum chloride, for example, and then oxidized to molybdate by a suitable oxidizing agent, for example by the accelerators described above. In such a case, the passivation composition itself contains a corresponding oxidizing agent.

More preferably, the passivating composition contains molybdenum ions in combination with copper ions, tin ions or zirconium ions.

It particularly preferably contains molybdenum ions in combination with zirconium ions and, if appropriate, a polymer or copolymer, in particular selected from the group consisting of the polymer classes of polyamines, polyethyleneamines, polyanilines, polyimines, polyethyleneamines, polythiophenes and polypryrenes, and their mixtures and copolymers and polyols - lyacrylic acid.

The content of molybdenum ions is preferably in the range from 20 to 225 mg / l, particularly preferably from 50 to 225 mg / l and very particularly preferably from 100 to 225 mg / l (calculated as Mo) and the content of zirconium ions in the range from 50 to 300 mg / l, more preferably from 50 to 150 mg / l (calculated as Zr).

According to a second preferred embodiment, the passivation composition contains copper ions, in particular copper (II) ions and optionally other metal ions - in each case in the form of water-soluble compounds. The passivation composition then preferably contains a content of copper ions in the range from 100 to 500 mg / l, more preferably from 150 to 225 mg / l.

According to a particularly preferred embodiment, the passivation composition contains Ti, Zr and / or Hf in complexed form, preferably in a concentration in the range from 20 to 500 mg / l, more preferably from 50 to 300 mg / l and particularly preferably from 50 to 150 mg / l (calculated as Ti, Zr and / or Hf), and optionally further metal ions in the form of water-soluble compounds. These are preferably fluoro complexes, more preferably hexafluorocomplexes. In addition, the passivating composition then preferably comprises 10 to 500 mg / l, more preferably 15 to 100 mg / l, and most preferably 15 to 50 mg / l of free fluoride.

According to a first very particularly preferred embodiment, in addition to Ti, Zr and / or Hf in complexed form, in particular as fluorocomplexes, the passivating composition additionally contains at least one water-soluble compound selected from the water-soluble compounds of the metal ions of molybdenum, copper, in particular copper (II), silver, gold, palladium, tin and antimony as well as lithium, preferably of molybdenum and copper, in particular copper (II). The water-soluble compounds are each present in a concentration in the following preferred ranges (in each case calculated as the corresponding metal):

According to a second very particularly preferred embodiment, the passivation composition contains, in addition to Ti, Zr and / or Hf in complexed form, in particular as fluorocomplexes, additionally at least one organosilane and / or at least one hydrolysis product thereof, ie an organosilanol, and / or at least one condensation product thereof, ie an organosiloxane / polyorganosiloxane, preferably in a concentration in the range from 5 to 200 mg / l, more preferably from 10 to 100 mg / l and particularly preferably from 20 to 80 mg / l (calculated as Si ).

The at least one organosilane preferably has at least one amino group. It is particularly preferred that one which can be hydrolyzed to an aminopropylsilanol and / or to 2-aminoethyl-3-amino-propyl-silanol and / or a bis (trimethoxysilylpropyl) amine.

According to a third very particularly preferred embodiment, in addition to Ti, Zr and / or Hf in complexed form, in particular as fluorocomplexes, the passivating composition additionally contains at least one water-soluble compound selected from the water-soluble compounds of the metal ions of molybdenum, copper, in particular copper (II), silver, gold, palladium, tin and antimony, and lithium, preferably of molybdenum and copper, in particular copper (II), and also at least one organosilane and / or at least one hydrolysis product thereof, ie an organosilanol, and / or at least one condensation product thereof, ie an organosiloxane / polyorganosiloxane, preferably in a concentration in the range from 5 to 200 mg / l, more preferably from 10 to 100 mg / l and particularly preferably from 20 to 80 mg / l (calculated as Si). The water-soluble compounds are each present in a concentration in the following preferred ranges (in each case calculated as the corresponding metal):

The method according to the invention for the anticorrosive treatment of composite metal structures assembled from metallic materials which at least partly also have aluminum surfaces is carried out after cleaning and activation of the metallic surfaces by first contacting the surfaces with the zinc phosphating composition of step (I), eg by spraying or dipping, at temperatures in the range of 20-65 ° C and for a time interval matched to the type of application. Experience has shown that speck formation on the galvanized and / or alloy-galvanized steel surfaces is particularly pronounced in conventional immersion phosphating processes, so that the phosphating in step (I) of the process according to the invention is particularly suitable for phosphating plants which operate on the principle of the immersion process. since the speck formation in the method according to the invention is suppressed.

It is advantageous to first clean, in particular degrease, the composite metal construction prior to treatment with the zinc-phosphating composition in step (I) in an aqueous cleaning composition. For this purpose, it is possible in particular to use an acidic, neutral, alkaline or strongly alkaline cleaning composition, but optionally also an acidic or neutral pickling composition.

An alkaline or strongly alkaline cleaning composition has proven to be particularly advantageous.

The aqueous cleaning composition may contain, in addition to at least one surfactant, if desired, a scaffold and / or other additives such as e.g. Complexing agent included. The use of an activating cleaner is also possible.

After cleaning / pickling, at least flushing of the composite metal construction with water then takes place, whereby the water may optionally also contain an additive dissolved in water, such as water. As a nitrite or surfactant may be added.

Before treating the composite metal structure with the zinc phosphating composition in step (I), it is advantageous to treat the composite metal structure with an activating composition. The activation composition serves to deposit a plurality of ultrafine phosphate particles as seed crystals on the surface of the zinc and iron parts. These help in the subsequent process step, in contact with the Phosphatierzu- composition - preferably without interim flushing - form a particular crystalline phosphate layer with the highest possible number of densely arranged fine phosphate crystals or a largely closed phosphate layer on the parts of zinc and iron.

In particular, acidic or alkaline compositions based on titanium phosphate or zinc phosphate may be considered as activating compositions.

However, it can also be advantageous to add activating agents, in particular titanium phosphate or zinc phosphate, in the cleaning composition, ie to carry out purification and activation in one step.

In a further step following step (II), the composite metal construction can be provided with a basecoat, preferably with an organic dip paint, in particular an electrodeposition paint - preferably without prior oven drying of the According to the invention treated component. After the primer coat, a top coat can be applied, which may be a powder paint or a wet paint.

The present invention also relates to a zinc phosphating composition for selectively phosphating steel, galvanized and / or alloy galvanized steel surfaces in a metallic composite construction comprising a portion of aluminum, the zinc phosphating composition having a KCI addition free acid rating of at least 0.6 points and a pH preferably in the range of 2.5 to 3.5, and

(a) 5 to 50 g / L of phosphate ions calculated as P2O5,

(b) 0.3 to 3 g / l zinc ions,

(c) at least 5 mg / l, but not more than 200 mg / l of free fluoride, and

(d) at least 0.04 g / l, but not more than 3.2 g / l of boron in the form of water-soluble inorganic compounds calculated as BF ₄ , the zinc phosphating composition additionally containing sodium and / or potassium Ions and aluminum ions.

Preferably, the content of free fluoride is in the range of 10 to 200 mg / l, more preferably from 20 to 175 mg / l, more preferably from 20 to 150 mg / l, further preferably from 20 to 135 mg / l, further preferably from 20 to 120 mg / l, more preferably from 50 to 120 mg / l and most preferably from 70 to 120 mg / l.

Surprisingly, it has been found that both the cryolite precipitation on the surface of the composite metal constructions and the poisoning of the phosphating bath by titanium, zirconium and / or hafnium can be reduced particularly effectively if the dimensionless quotient for the zinc phosphating composition is used

(S value x T) / (F _tot. X [B] x 1000) is in the range from 0.13 to 22.5, preferably from 0.2 to 15 and particularly preferably from 0.4 to 10, where " T "for the temperature / ° C," F _tot. Stands for the total fluoride content / (mol / l) and ,, [B] "for the concentration of the element boron / (mol / l).

In a preferred variant, the zinc phosphating composition according to the invention contains not more than 20 ppm, preferably not more than 15 ppm, more preferably not more than 10 ppm, more preferably not more than 5 ppm and most preferably not more than 1 ppm total of water-soluble Compounds of zirconium, titanium and / or hafnium based on the elements zirconium, titanium and / or hafnium and in particular no water-soluble compounds of zirconium, titanium and / or hafnium.

Further advantageous embodiments of the zinc phosphating composition according to the invention have already been explained above in the process according to the invention.

The present invention also relates to a concentrate from which the zinc phosphating composition according to the invention can be obtained by dilution with a suitable solvent and / or dispersing medium, preferably with water, and, if appropriate, adjusting the pH. The dilution factor here is preferably between 2 and 100, preferably between 5 and 50.

Finally, the present invention relates to a corrosion-protected composite metal construction which contains at least one part of aluminum and at least one part of zinc and optionally another part of iron and is obtainable by the process according to the invention.

The composite metal structures protected against corrosion according to the method according to the invention are preferably used in the automotive supplier industry, in automotive production in body construction, in agricultural machinery, in shipbuilding, in the construction industry, and in the production of white goods. The present invention will become apparent from the following non-limiting exemplary embodiments and comparative examples.

Examples

Aqueous zinc phosphating solutions were prepared, each containing a score of the free acid with KCl addition of 1.1 points, a total Fischer acid of 19.5 points, an S value of 0.056, and a pH in the range of 2.5 to 3.5. Said phosphatizing solutions were nickel-free and each contained 15 g / l of phosphate ions calculated as P2O5, 0.8 g / l of zinc ions, 1.0 g / l of manganese ions, varying amounts of free fluoride (see Table 1) ), 1, 0 g / l of boron in the form and calculated as BF ₄ and 34 mg / l of the accelerator hydrogen peroxide. The phosphating solutions were brought to a temperature of 45 ° C.

Cleaned test panels made of aluminum (AA 6016) were sprayed uniformly with in each case one phosphating solution, rinsed and immersed at room temperature for 30 s in an acidic, aqueous passivation composition which contained 150 mg / l of H ₂ ZrF ₆ calculated as Zr. Without previous oven drying, the sheets were still provided with an electrodeposition paint and finally with a wet paint (Standardautomobillackaufbau).

The test panels were subjected to a 144-hour CASS test according to DIN EN ISO 9227, 2017-07 and a 1008-hour Filiform test according to DIN EN 3665, 1997-08. The corrosion results are summarized in the following table (Ff _rei = content of free fluoride).

Table 1 :

As shown in Tab. 1, lowering the content of free fluoride in the phosphating solution in both the CASS and the Filiform test leads to significantly reduced corrosion on aluminum.

From a running zinc phosphating bath of a customer with fluctuating content of free fluoride, the applicant took samples over a period of 2.2 years and determined the content of free fluoride (Ff _re i) and of zirconium (Zr). For this purpose, a fluoride-selective potentiometric combination electrode was used or an ICP (Inductively Coupled Plasma) - analysis performed.

As shown in Table 2, a decrease in the content of free fluoride was accompanied by a decrease in the zirconium content in the bath, albeit possibly with a time delay (compare 10 to 26 months). Conversely, an increase in the content of free fluoride - although possibly delayed (see 4.5 to 14.5 months) - was accompanied by an increase in the zirconium content in the bath. A low content of free fluoride thus led to a lower bath poisoning by zirconium. Table 2:

Aqueous zinc phosphating solutions (ZPL Nos. 1-8) were prepared, each having a free acid score with KCI addition of 1.5 points. Said phosphating solutions were nickel-free and contained in each case 15 g / l of phosphate ions calculated as P2O5, 0.8 g / l of zinc ions, 1.0 g / l of manganese ions, 100 mg / l of free fluoride and different amounts of sodium and potassium ions as well as either tetrafluoroborate or hexafluorosilicate. The phosphating solutions were brought to a temperature of 45 ° C and stirred in a beaker by means of stirring at medium speed. By means of ICP analysis, the dissolved concentrations of aluminum, silicon and boron were determined at the beginning (0 h) and after 24 h. As the concentration of dissolved aluminum dropped, this was due to precipitation in the form of cryolite. By contrast, the concentration of dissolved silicon decreased as a result of precipitation in the form of K2S1F6.

As shown in Tab. 3, a content of 5.0 g / l of sodium ions led to a decrease in the dissolved aluminum concentration, ie to the precipitation of cryolite (ZPLs 2 and 6). In the presence of tetrafluoroborate (ZPL 2), the cryofit precipitation was slightly less pronounced than with hexafluorosilicate (ZPL 6). The same applies to a content of 2.5 g / l of sodium ions and 2.5 g / l of potassium ions (ZPL 4 vs. ZPL 8). At 2.5 g / l sodium ion (ZPLs 1 and 5) or 5.0 g / l potassium ion (ZPLs 3 and 7), however, cryolite precipitation did not occur.

In the case of hexafluorosilcate, in the presence of potassium ions, there was a decrease in the concentration of dissolved silicon, ie precipitation of K2S1F6 (ZPLs 7 and 8). The education a corresponding precipitate could not be determined for tetrafluoroborate. Here, the concentration of dissolved boron did not decrease accordingly (ZPLs 3 and 4).

Table 3:

Claims

claims

1 . A process for the chemical pretreatment and selective phosphating of a composite metal construction which contains at least one part of aluminum and at least one part of zinc and possibly another part of iron, characterized in that the process

(II) in a second step comprises applying an aqueous acidic passivating composition having a pH in the range of 2.0 to 5.5 to the composite metal construction, wherein the acidic passivating composition is supported on the parts of zinc and Iron does not dissolve more than 50% of the crystalline zinc phosphate, but forms a passive layer on the aluminum parts, which is not a surface-covering crystalline phosphate layer,

wherein the zinc phosphating composition in step (I) contains a content of sodium and / or potassium ions as well as of aluminum ions,

wherein at least 0.04 g / l but not more than 3.2 g / l of boron in the form of water-soluble inorganic compounds calculated as BF _{4 is} contained in the zinc phosphating composition, and

2. The method according to claim 1, characterized in that the zinc phosphating composition in step (I) has a free fluoride content in the range of 10 to 200 mg / l, preferably from 20 to 150 mg / l, more preferably from 20 to 120 mg / l, more preferably from 50 to 120 mg / l and most preferably from 70 to 120 mg / l.

3. The method according to claim 1 or 2, characterized in that the Zinkphosphatierungs- composition in step (I) a concentration of boron in the form of water-soluble inorganic compounds calculated as BF ₄ in the range of 0.08 to 2.5 g / l, on preferably from 0.08 to 2 g / l, more preferably from 0.25 to 2 g / l, particularly preferably from 0.5 to 1.5 g / l and very particularly preferably from 0.8 to 1.2 g / l.

4. The method according to any one of the preceding claims, characterized in that in the Zinkphosphatierungszusammensetzung in step (I), the content of sodium and / or potassium ions (calculated as sodium) in the range of 1 to 4 g / l, in particular from 2 to 3 g / l and the content of free fluoride in the range of 50 to 150 mg / l, in particular from 70 to 120 mg / l.

5. The method according to any one of the preceding claims, characterized in that in the Zinkphosphatierungszusammensetzung in step (I), the content of inorganic compounds containing silicon (calculated as SiFe) below 25 mg / l, preferably below 15 mg / l, more preferably below 5 mg / l and most preferably below 1 mg / l.

6. The method according to any one of the preceding claims, characterized in that the zinc phosphating composition in step (I) a score of free acid with KCI addition in the range of 0.6 to 3.0 points, preferably from 0.8 to 3.0 Points, more preferably from 1, 0 to 3.0 points, and most preferably from 1, 0 to 2.5 points.

Process according to any one of the preceding claims, characterized in that, for the zinc phosphating composition in step (I), the dimensionless quotient

(S value x T) / (F _tot x [B] x 1000)

in the range of 0.13 to 22.5, preferably from 0.2 to 15.

8. The method according to any one of the preceding claims, characterized in that in the Zinkphosphatierungszusammensetzung in step (I) additionally one or more of the following cations is contained:

0.3 to 2 g / l, preferably 0.5 to 1.5 g / l manganese (II),

0.3 to 2 g / l, preferably 0.8 to 1.2 g / l nickel (II),

0.001 to 0.2 g / l, preferably 0.005 to 0.1 g / l of iron (III).

9. The method according to any one of the preceding claims, characterized in that the pH of the passivation composition in step (II) in the range of 2.5 to 5.5, more preferably from 3.0 to 5.5, and most preferably from 3.5 to 5.5.

10. The method according to any one of the preceding claims, characterized in that the passivation composition in step (II) Ti, Zr and / or Hf in complexed form, preferably in a concentration in the range of 20 to 500 mg / l, more preferably of 50 to 300 mg / l and more preferably from 50 to 150 mg / l (calculated as Ti, Zr and / or Hf), and optionally further metal ions in the form of water-soluble compounds.

1 1. Method according to one of the preceding claims, characterized in that the passivation composition in step (II) additionally at least one organosilane and / or at least one hydrolysis product thereof and / or at least one condensation product thereof, preferably in a concentration in the range of 5 to 200 mg / l, more preferably from 10 to 100 mg / l and particularly preferably from 20 to 80 mg / l (calculated as Si).

12. A zinc phosphating composition for the selective phosphating of steel, galvanized and / or alloy-galvanized steel surfaces in a metallic composite construction comprising an aluminum part according to any one of claims 1 to 8, characterized in that it has a KCI-added free acid score at least 0.6 points and a pH preferably in the range of 2.5 to 3.5, and

(a) 5 to 50 g / L of phosphate ions calculated as P2O5,

(b) 0.3 to 3 g / l zinc ions,

(c) at least 5 mg / l, but not more than 200 mg / l of free fluoride, and

(d) at least 0.04 g / l but not more than 3.2 g / l of boron in the form of water-soluble inorganic compounds calculated as BF ₄ ,

contains, wherein the Zinkphosphatierungszusammensetzung also contains a content of sodium and / or potassium ions and aluminum ions.

13. Concentrate, from which by dilution with a suitable solvent and / or dispersing medium, preferably with water, and optionally adjusting the pH, the zinc phosphating composition according to claim 12 can be obtained.

14. Composite metal construction containing at least one part of aluminum and at least one part of zinc and possibly another part of iron, characterized in that it is obtainable by a method according to one of claims 1 to 11.

15. Use of the composite metal construction according to claim 14 in the automotive supplier industry, in the automotive production in the body shop, in agricultural machinery, in shipbuilding, in the construction industry or for the production of white goods.