JP2024540210A - Multi-step treatment for activated zinc phosphating of metal components with zinc surfaces - Google Patents

Abstract

The present invention relates to a method for the anticorrosive pretreatment of a series of components, each of which has at least partially a zinc surface, being subjected to successive treatment steps for iron deposition and zinc phosphating, the layer formed in the treatment step of iron deposition being at least 10 mg of elemental iron per ^m2 of zinc surface, the zinc phosphating following said iron deposition being carried out with an acidic aqueous composition comprising, in addition to zinc ions, phosphate ions and free fluoride, also particulate components consisting at least partially of hopeite, phosphophyllite, scholzite and/or hyullite dispersed in water, provided with an aqueous dispersion of these crystalline solids stabilized with at least one organic polymeric compound.

Description

The present invention relates to a method for the anticorrosive pretreatment of a series of components, each of which has at least partially a zinc surface, and which is subjected to successive treatment steps of iron deposition and zinc phosphating, in which the resulting coating layer is at least 10 mg of elemental iron per ^m2 of zinc surface. The zinc phosphating following the iron deposition is carried out with an acidic aqueous composition comprising, in addition to zinc ions, phosphate ions and free fluoride, also particulate components consisting at least partially of hopeite, phosphophyllite, scholzite and/or hyullite dispersed in water, provided by means of an aqueous dispersion of these crystalline solids stabilized with at least one organic polymeric compound.

Layer-forming phosphating is a process for providing crystalline anticorrosive coatings on metal surfaces, especially metallic iron, zinc and aluminum materials, that has been used and thoroughly studied for several decades. Zinc phosphating, which is particularly well established as a corrosion inhibitor, is carried out with layer thicknesses of a few micrometers and is based on the caustic pickling of the metallic material in an acidic aqueous composition containing zinc ions and phosphate salts. During the pickling process, an alkali diffusion layer is formed on the metal surface, which extends into the interior of the solution, in which sparingly soluble microcrystals are formed. These microcrystals precipitate directly at the interface with the metallic material and continue to grow there. To support the pickling reaction on the metallic aluminum material and to mask the bath poison aluminum, which in its dissolved state prevents the formation of a layer on the metallic material, water-soluble compounds that are sources of fluoride ions are often added.

Zinc phosphating is adjusted as standard to obtain a uniform, closed, dense, crystalline coating on the surfaces of metallic iron, zinc and aluminum. Otherwise, good corrosion protection and a good coating base cannot be achieved. A uniform, closed coating in zinc phosphating is usually easily achieved from a layer weight of 2 g/ ^m2 . Depending on the metal surface to be phosphated, the concentrations of the active components in the pickling and zinc phosphating stages must be adjusted to ensure a correspondingly high layer weight on the surfaces of metallic iron or steel, zinc and aluminum.

Another property of zinc phosphating, which is important for corrosion protection and coating adhesion, especially good electrocoating properties, is that the deposition process is self-limiting, i.e. the dissolution of the phosphate layer, which occurs at the acidic pH values of zinc phosphating, is in stationary equilibrium with the growth or continued growth of phosphate crystallites, so that the layer weight no longer increases. This indicates the growth of a crystalline, but porous and therefore not densely crystalline layer coating. In technical zinc phosphating processes, this means that in the case of treatment times, which are usually around 20 seconds to 5 minutes, which are reasonable from the point of view of plant technology and cost efficiency, the formation of a homogeneous, closed, crystalline zinc phosphate coating must be completed and the self-limiting thickness of the coating must already reach the ideal value. This is ensured technically by the fact that the coating grows with the highest possible number density of phosphate crystallites, so that the layer formation reaches the self-limiting range and thus the predetermined limiting layer thickness with the lowest possible layer weight.

To obtain such a uniform closed coating containing phosphate crystallites with a high degree of compactness or number density, conventional zinc phosphating always starts with the activation of the metal surface of the component to be phosphating. This activation is usually a wet-chemical treatment step (activation stage) usually carried out by contact with an aqueous colloidal solution of phosphate. The phosphates, once immobilized on the metal surface, act as growth nuclei for the formation of a crystalline coating in the alkali diffusion layer in the subsequent phosphating, resulting in a high number density of growing crystallites, which results in the formation of a dense crystalline zinc phosphate layer that combines good corrosion resistance and, due to its high charge transfer resistance, good electrocoat properties.

Suitable dispersions in this case are colloidal, nearly neutral to alkaline aqueous compositions based on phosphate crystallites, in which there are only minor crystallographic deviations of the crystal structure from the type of zinc phosphate layer formed. In this regard, WO 98/39498 A1 teaches, inter alia, divalent and trivalent phosphates of the metals Zn, Fe, Mn, Ni, Co, Ca and Al, and teaches that it is technically preferred to use phosphates of the metal zinc for activation for subsequent zinc phosphating.

The activation stage based on dispersions of divalent and trivalent phosphates demands a high degree of process control in order to always keep the activation performance at an optimum level, especially when treating a series of metal components. To ensure a sufficient robustness of the process, foreign ions carried over from previous treatment baths or ageing treatments in aqueous colloidal solutions must not lead to a deterioration of the activation performance, which first becomes noticeable in an increase in layer weight in the subsequent phosphating and ultimately leads to the formation of defective, inhomogeneous or poorly compacted phosphate layers. Overall, layer-forming zinc phosphating with upstream activation is therefore a technically difficult multi-stage process that has been carried out so far in a resource-intensive manner, both in terms of treatment compounds and energy consumed.

In the field of automobile manufacturing, which is particularly relevant for the present invention, various metallic materials are increasingly used and joined into composite structures. Although various steels are still used in car body construction, mainly due to their specific material properties, the use of light metals such as aluminum is also increasing, especially in order to significantly reduce the weight of the entire car body. In particular in the automobile industry, it is often a challenge that the zinc surface must be particularly well activated by the zinc phosphating methods known in the prior art compared to the steel surface, in order to ensure the growth of a dense crystalline phosphate layer with a coating weight of usually less ^than 5.0 g/m2 in the zinc phosphating. Because above such a phosphate layer weight, there is no sufficient corrosion protection effect on the zinc surface and the process is not carried out resource-savingly and economically, also due to the high consumption of phosphate.

WO 2019/238573 A1 deals with a resource-saving method for zinc phosphating and indirectly also with the reduction of the difficulties of the multi-step method by providing a particularly effective activation based on divalent and trivalent phosphates dispersed in a specific way. This activation provides aqueous colloidal solutions based on divalent and trivalent phosphates that are very stable against settling and allow a uniform, closed and very dense zinc phosphate coating with a relatively low particle content in the activation stage (thus also reducing the material requirements due to the formation of layers in the zinc phosphating).

WO 98/39498 A1 WO 2019/238573 A1

However, there is still a demand to optimize the zinc phosphating pretreatment line, including the activation and phosphating stages, so that the entire process is less resource-intensive and ideally can be carried out in a simultaneous and simplified procedure. However, the properties of the zinc phosphating must not be sacrificed in order to save resources throughout the entire process. The zinc phosphating must be provided as a uniform, closed, dense, crystalline coating with high charge transfer resistance in order to allow good protection against corrosion and accordingly good coverage of the coating in the subsequent electrocoating. In particular, this must always be ensured in the most common applications, i.e. in the processing sequence of components, and for components with zinc surfaces, an economically attractive and resource-saving method must also be provided.

Surprisingly, this challenging requirement profile can be met if a predetermined amount of a granular zinc phosphate dispersion is added to the acidic aqueous composition for zinc phosphating, whereby the composition for zinc phosphating is self-activated. The activation performance of the pretreatment line for zinc phosphating of a series of components can then be maintained by the metered addition of an activator based on the above-mentioned granular zinc phosphate dispersion. This makes it possible to at least partially or completely omit the activation step upstream of the zinc phosphating wet-chemical treatment step, which allows the entire zinc phosphating process to be carried out in a manner that consumes less material and energy and reduces the technical difficulties in the form of a separate activation step, which was always required in the prior art. Of importance for the economical and resource-saving operation of the pretreatment line of the invention, which serves for the successful phosphating of a series of zinc surface-containing components, is the wet-chemical step upstream of the zinc phosphating for depositing an iron coating on the zinc surface. This significantly reduces the weight of the phosphate layer on said surface. The objective of the invention to provide a series of zinc surface-containing components with constant and satisfactory anticorrosion values at low material consumption is therefore reliably achieved.

Therefore, the present invention provides a method for pre-treating a series of a plurality of members to prevent rust, comprising the steps of:
A series of components each having at least a partially zinc surface are first subjected to a wet-chemical treatment step (i) for depositing iron on the zinc surface and then to a treatment step (ii) for zinc phosphating,
In the treatment step (i), a coating layer of elemental iron of at least 10 mg per ^m2 of zinc surface of the component is provided,
In the treatment step (ii), each component is treated to have a free acidity greater than zero points,
(A) 5-50 g/kg of phosphate, calculated as _PO4 , dissolved in water;
(B) 0.3 to 3 g/kg zinc ions;
(C) free fluoride; and (D) a water-dispersible granular component comprising a polyvalent metal cation phosphate, the phosphate being at least partially selected from hopeite, phosphophyllite, scholzite and/or hyulelite;
The acidic aqueous composition is obtained by adding the aqueous dispersion to an acidic aqueous composition containing components (A) to (C),
The aqueous dispersion is
at least one particulate inorganic compound (P1), consisting of a polyvalent metal cation phosphate at least partially selected from hopeite, phosphophyllite, scholzite and/or hyullite, and at least one organic polymeric compound (P2).
The present invention relates to a method comprising the steps of:

A series of pretreatments consists in subjecting a series of components, each of which to a treatment step for zinc phosphating according to the method of the invention, to contact with at least one of the bath liquids for zinc phosphating provided for this purpose in a system tank, one after the other and therefore at different times. In this case, this system tank is a container into which an acidic aqueous composition is introduced for the purpose of zinc phosphating by wet-chemical pretreatment. The components can be contacted with the bath liquid of the system tank in the system tank, for example by immersion, or outside the system tank, for example by spraying on the bath liquid stored in the system tank.

The components treated according to the invention can be three-dimensional structures of any shape and design resulting from the manufacturing process, and include in particular semi-finished products such as strips, sheets, rods, pipes, etc., and composite structures assembled from said semi-finished products. The semi-finished products are preferably joined together, for example by gluing, welding and/or flanging, to form the composite structure.

In the context of the method of the invention, the component has at least one surface which is either zinc, or metallic iron or aluminum, if more than 50 at% of the metallic structure on this surface is composed of zinc or iron or aluminum, up to a material penetration depth of at least 1 μm. This generally applies to components made of the corresponding metallic material only if more than 50 at% of the metallic material is composed of zinc or iron or aluminum as a homogeneous material. However, components which include a zinc surface are also ferrous materials which are provided with a metallic coating, for example electrolytic galvanized or hot-dip galvanized steel, which can also be alloyed with iron (ZF), aluminum (ZA) and/or magnesium (ZM).

Treatment step (i) – Iron deposition:
According to the invention, the deposition of a coating layer based on elemental iron of at least 10 mg per ^m2 of zinc surface of the component is necessary. A higher coating layer of iron is advantageous for a reduction in the weight of the phosphate layer in the subsequent treatment step (ii). Thus, in the wet-chemical treatment step (i) of the method of the invention, a coating layer based on elemental iron of at least 20 mg, particularly preferably at least 40 mg, very particularly preferably at least 60 mg, on the surface of the component formed by zinc is preferred. However, it can often be observed that a significantly higher coating layer of iron on the zinc surface proves to be disadvantageous for the method of the invention, since in the context of zinc phosphating, organic topcoats reduce the adhesion. Thus, in the wet-chemical treatment step (i), it is preferred to limit the coating layer of iron to less than 150 mg, particularly preferably less than 120 mg, in each case per ^m2 of zinc surface of the component.

The wet-chemical treatment step in treatment step (i) is present when the series of components is contacted with an aqueous composition which contains the active components for iron deposition in an aqueous phase in dissolved or dispersed form. The composition is aqueous when the proportion of water is in each case at least 50% by weight, preferably at least 70% by weight, particularly preferably at least 80% by weight, based on the total composition.

Suitable wet chemical methods for iron deposition in the desired coating layer are known to those skilled in the art. Deposition typically occurs by at least contact of the zinc surface with an aqueous composition containing iron(II) and/or iron(III) ions, the proportion of iron ions dissolved in water being at least 50 mg/L, preferably at least 100 mg/L.

The coating layer of iron on the zinc surface is measured by pickling and photoanalysis. For quantitative measurement, a given sample volume of 5% by weight nitric acid solution is transferred by pipette onto a given surface of the galvanized sheet immediately after treatment step (i) with the measuring cell ring and, after 30 seconds of exposure at a temperature of 25°C, transferred to a UV measuring cuvette in which a 1.0% sodium thiocyanate solution has been previously introduced for measuring the absorption at a wavelength of 517 nm at a temperature of 25°C. Calibration of the two-point process is performed by measuring the absorption values of two standard solutions of iron(III) nitrate in 5% by weight nitric acid.

In order to avoid metal deposition of precious elements, which reduces the positive effect of the iron coating on the zinc surface for the formation of the zinc phosphate layer, and also for environmental reasons, it is preferred that the aqueous composition for iron deposition in treatment step (i) contains in total less than 10 mg/L of ionic compounds of the metals copper, nickel, cobalt, particularly preferably in total less than 10 mg/L of ionic compounds of the metals copper, nickel, cobalt, tin, manganese, molybdenum, chromium and/or cerium, particularly preferably in each case 1 mg/L of ionic compounds of the metals nickel and cobalt, based on the metal elements in the aqueous composition.

Similarly, in order to avoid the formation of a layer competing with the iron deposition in process step (i), it is preferred that the aqueous composition for iron deposition contains in total less than 20 mg/L of water-soluble compounds of the elements Zr, Ti, Hf and/or Si, particularly preferably in each case less than 5 mg/L, particularly preferably in each case less than 1 mg/L of water-soluble compounds of the elements Zr, Ti, Hf or Si.

Iron deposition on zinc from an acidic aqueous composition in the presence of iron(II) ions is described in WO 2008/135478 A1. Such a method is also suitable in the present invention and is particularly advantageous due to its compatibility with the pH value of the zinc phosphating in the subsequent process step (ii). In this respect, contact with an acidic aqueous composition having a pH value of 2.0 to 6.0 and containing at least 50 mg/L of iron(II) ions and preferably α-hydroxycarboxylic acids, preferably in a molar ratio of iron ions of 5:1 to 1:5, and particularly preferably further containing at least one reducing agent selected from phosphorus or nitrogen oxoacids and their salts (at least one phosphorus or nitrogen atom is present in the average oxidation state), hydrazine, hydroxylamine, nitroguanidine, N-methylmorpholine-N-oxide, glucoheptonate, ascorbic acid and/or reducing sugars is preferred.

Alternatively, deposition of the iron coating from an alkaline aqueous composition containing iron(II) ions and/or iron(III) ions is particularly advantageous, since it is particularly effective in reducing the weight of the phosphate layer on the zinc surface during the subsequent zinc phosphating, the pH value of which is preferably 8.5 or more, particularly preferably 9.5 or more, very particularly preferably 10.5 or more, but preferably 13.5 or less, particularly preferably 12.5 or less, very particularly preferably 11.5 or less.

In a preferred embodiment of such an alkaline aqueous composition in step (i) of the method of the present invention,
(a) at least 50 mg/L, preferably at least 100 mg/L, particularly preferably at least 200 mg/L, of iron(III) ions, and (b) at least 100 mg/L, calculated as _PO4 , of an organic compound (b1) having at least one functional group selected from COOX, _OPO3X and/or _PO3X , where X is either an H atom, an alkali metal atom and/or an alkaline earth metal atom, and/or a complexing agent (b2) selected from condensed phosphates, wherein the composition preferably has a free alkalinity of at least 1 point, but preferably less than 6 points.

By "condensed phosphates" of component (b1) is meant di-, tri- and polyphosphates (Me _n + ₂ [P _n O _{3n +1} ] or Me _n [H ₂ P _n O _{3n +1} ]) combining metaphosphates (Me n [P _n O 3n ]), isometaphosphates and crosslinked polyphosphates that are water-soluble at room temperature, where Me is either an alkali metal atom or an alkaline earth metal atom. Of course, instead of the water-soluble salts, the corresponding phosphoric condensed acids can also be used for the preparation of the alkaline aqueous composition, provided that they are adjusted to the stated free alkalinity. The mass-based proportion of the "condensed phosphates" of component (b1) is always calculated as the corresponding amount of PO _4. Similarly, when determining the molar ratio constituting the amount of condensed phosphate, this amount of condensed phosphate is always based on the equivalent amount of PO ₄ .

It has been found that the alkaline aqueous composition in step (i) of the method of the present invention results in a coating layer of iron on the zinc surface, which is suitable for the subsequent zinc phosphating, and can reliably form a dense crystalline zinc phosphate layer, particularly when the free alkalinity is less than 5 points. For the application of the alkaline aqueous composition by spraying, a suitable iron coating layer is particularly produced when the free alkalinity is less than 4 points. Surprisingly, it has been found that a high iron coating layer on the zinc surface of more than 150 mg/ ^m2 is rather disadvantageous to the method of the present invention, since, as mentioned above, it shows poorer adhesion to organic topcoats in relation to zinc phosphating. Therefore, the alkaline aqueous composition in step (i) should not have an excessively high free alkalinity. However, the free alkalinity should preferably be at least 2 points in order to form an optimal coating layer on the zinc surface of at least 20 mg/ ^m2 based on elemental iron. Alkaline aqueous compositions having a free alkalinity of more than 6 points form a high iron coating layer on the zinc surface; however, adhesion to a coating layer that may be optionally applied after step (ii) is significantly reduced by the high coating layer based on elemental iron and therefore the corrosion protection is also less effective or insufficient.

Free alkalinity is measured by titrating 2 mL of bath solution, preferably diluted to 50 mL, with 0.1 N acid (e.g. hydrochloric acid or sulfuric acid) to a pH value of 8.5. The consumption of acid solution in mL indicates the number of points of free alkalinity.

Ideally, the alkaline aqueous composition in step (i) of the method of the present invention has a pH value of at least 9.5, particularly preferably at least 10.5. At a pH value of 10.5 or less, an iron coating layer of at least 20 mg/ ^m2 is formed on the zinc surface when contacted with a composition containing iron(II) and/or iron(III) ions exclusively in the presence of a reducing agent. A composition of this type for forming an iron coating is disclosed in WO 2011/098322 A1. The composition comprises an amino acid and further comprises a reducing agent selected from phosphorus or nitrogen oxoacids and their salts. At least one phosphorus or nitrogen atom is present in the average oxidation state and is also suitable as such in the present invention.

In order to minimize pickling attack on the zinc surface of the component, it is further preferred that the pH value of the alkaline aqueous composition in step (i) of the method of the invention is not more than 13.5, particularly preferably not more than 12.5. If the component has an aluminum surface in addition to the zinc surface, it is advantageous that the pH value of the composition in step (i) of the method of the invention is not more than 11.5. Otherwise, an intensified pickling attack would lead to a strong blackening of the aluminum surface (so-called well blackness), which would have a negative effect on the effectiveness of subsequent conversion treatments, for example for the zinc phosphating in step (ii) of the method of the invention or, in the case of zinc phosphating adjusted so that no layer is formed on the aluminum in step (ii), for the acidic post-passivation based on water-soluble inorganic compounds of the elements zirconium and/or titanium according to the method of the invention.

When saturated with atmospheric oxygen, iron ions are present mainly as iron(III) ions in the alkaline aqueous composition, preferably at prevailing pH values. In step (i) of the method of the present invention, the proportion is preferably not more than 2000 mg/L. A higher proportion of iron(III) ions is disadvantageous for process control, and more favorable properties of the iron coating on the zinc surface are not achieved, since the solubility of iron(III) ions in the alkaline medium must be maintained by a correspondingly high proportion of complexing agent. However, such alkaline aqueous compositions in which the proportion of iron(III) ions is at least 100 mg/L, particularly preferably at least 200 mg/L, are preferred in step (i) of the method of the present invention, in order, on the one hand, to ensure a sufficient iron coating layer on the zinc surface in step (i) of the method of the present invention within a treatment time of less than 2 minutes, which is typical for the method of the present invention, and, on the other hand, to obtain a phosphate layer of excellent layer properties on the zinc surface in step (ii) of the method of the present invention.

The complexing agent according to component (b) of the alkaline aqueous composition in step (i) of the method of the invention is preferably present in an amount such that the molar ratio of the total component (b) to iron (III) ions is greater than 1:1, particularly preferably at least 2:1, and especially preferably at least 5. It has been found to be advantageous for process control to use a complexing agent in a stoichiometric excess amount, since this maintains the proportion of iron (III) ions in the solution in the long term. This completely suppresses the precipitation of insoluble iron hydroxide, so that the alkaline aqueous composition remains stable in the long term and the iron (III) ions are not depleted. Nevertheless, at the same time, sufficient deposition of an iron ion-containing inorganic layer on the zinc surface occurs. Nevertheless, for reasons of economy and resource conservation of complexing agents, it is preferred that the molar ratio of component (b) to iron (III) ions in composition (A) is less than or equal to 10.

In a preferred embodiment, the alkaline aqueous composition may further comprise at least 100 mg/L of phosphate ions in step (i) of the method of the invention. This proportion of phosphate ions requires that in addition to iron ions, phosphate ions are also a substantial constituent of the iron-containing coating layer formed on the zinc surface in step (i). It has been found that such a layer is advantageous for the subsequent zinc phosphating and interacts with the zinc phosphating to provide good adhesion to the subsequently applied coating layer. It is therefore further preferred that in step (i) of the method of the invention, the alkaline aqueous composition contains at least 200 mg/L of phosphate ions, particularly preferably at least 500 mg/L. Since the proportion of the passivation layer is not further positively influenced when the zinc surface of the component is contacted with composition (A) in step (i) of the method of the invention above a proportion of phosphate ions of 4 g/L, for reasons of economic efficiency, the proportion of phosphate ions in the alkaline aqueous composition in step (i) of the method of the invention should preferably be 10 g/L or less.

Thereby, the ratio of iron(III) ions to phosphate ions can be varied within wide limits. The mass-based ratio of iron(III) ions to phosphate ions in the alkaline aqueous composition in step (i) of the method of the present invention is preferably in the range of 1:20 to 1:2, particularly preferably in the range of 1:10 to 1:3. An alkaline aqueous composition having such a mass ratio of iron(III) ions to phosphate ions leads, after contact with a zinc surface, to a uniform phosphate ion-containing black-grey layer with an easily adjustable coating layer in the range of 20 to 150 mg/ ^m2 , based on elemental iron.

The condensed phosphate (b2) can dissolve in an alkaline medium and retain the iron (III) ion through complexation. The type of condensed phosphate that can be used in the alkaline aqueous composition in step (i) of the method of the present invention is not particularly limited, but is preferably a condensed phosphate selected from pyrophosphate, tripolyphosphate and/or polyphosphate, particularly preferably pyrophosphate, because of its high water solubility and high availability.

As organic compounds (b1) which are also present in the alkaline aqueous composition as complexing agents or as a replacement for condensed phosphates (b2), in step (i) of the method of the invention, compounds having an acid value of at least 250 in the acid form (X = hydrogen atom) are preferred. Since a lower acid value gives the organic compound surface active properties, organic compounds (b1) having an acid value of less than 250 can have a strong emulsifying action as anionic surfactants. In this regard, it is further preferred that the organic compounds do not have a high molecular weight and have a number-average molecular weight not exceeding 5000 u, particularly preferably 1000 u. Above the preferred acid value and the optionally preferred molecular weight, the emulsifying effect of the organic compounds (b1) can become so pronounced that impurities in the form of oil and drawn grease carried over from the cleaning stage through the components can be removed exclusively from the treatment stage with a difficult separation treatment for the attachment of the iron coating, for example by metered addition of cationic surfactants. It is therefore necessary to control further treatment parameters. It is therefore more advantageous to adjust the alkaline aqueous composition in step (i) of the method of the invention to be only slightly emulsifying in order to allow for the conventional separation of the floating fats and oils. Anionic surfactants also tend to cause significant foaming, which is particularly disadvantageous during spray application of the alkaline aqueous composition. Therefore, organic complexing agents (b1) having an acid number of at least 250 are preferably used in the composition in step (i) of the method of the invention. The acid number means the amount of potassium hydroxide in mg required to neutralize 1 g of organic compound (b1) in 100 g of water according to DIN EN ISO 2114.

Preferred organic complexing agents (b1) in the alkaline aqueous composition in step (i) of the method of the present invention are selected from α-, β- and/or γ-hydroxycarboxylic acids, hydroxyethane-1,1-diphosphonic acid, [(2-hydroxyethyl)(phosphonomethyl)amino]-methylphosphonic acid, diethylenetriaminepentakis(methylenephosphonic acid) and/or amino-tris-(methylenephosphonic acid) and salts thereof, particularly preferably hydroxyethane-1,1-diphosphonic acid, [(2-hydroxyethyl)(phosphonomethyl)amino]-methylphosphonic acid, diethylenetriaminepentakis(methylenephosphonic acid) and/or amino-tris-(methylenephosphonic acid) and salts thereof.

In the present invention, such an alkaline aqueous composition exclusively comprising condensed phosphates (b2) exclusively comprises an organic complexing agent (b1), i.e. a mixture of both is expressly included in step (i) of the method of the present invention. However, the proportion of organic complexing agent (b1) in the alkaline aqueous composition can be reduced to the extent that a complexing agent (b2) selected from condensed phosphates is present. In a particular embodiment of the method of the present invention, the alkaline aqueous composition in step (i) comprises a complexing agent (b2) selected from condensed phosphates and an organic complexing agent (b1), the molar ratio of the total component (b) to iron (III) ions is greater than 1:1, and the molar ratio of component (b1) to iron (III) ions is less than 1:1, particularly preferably less than 3:4, but preferably at least 1:5. The mixture of the two complexing agents (b1) and (b2) is advantageous in that the phosphate ions consumed by the formation of the layer on the zinc surface are slowly replenished from the condensed phosphate, since the condensed phosphate in the alkaline medium is in equilibrium with the phosphate ions of the alkaline aqueous composition at high temperatures. However, on the contrary, the presence of the condensed phosphate alone is insufficient to result in a coating layer based on iron and phosphate on the zinc surface, so that the proportion of phosphate ions in the alkaline aqueous composition in step (i) of the method of the present invention is always preferred. However, in the presence of the condensed phosphate, the precipitation of less soluble phosphates, such as iron phosphate, is suppressed by interaction with the organic complexing agent (b1), especially even at high pH values above 10.5, so that the alkaline aqueous composition containing the complexing agent mixture in step (i) of the method of the present invention is preferred. It is then preferable to ensure that the molar ratio of component (b1) to iron (III) ions is at least 1:5.

To enhance the cleaning ability of the series of components treated in the method of the present invention, the alkaline aqueous composition may further comprise a non-ionic surfactant in step (i). In this case, the non-ionic surfactant is preferably selected from one or more ethoxylated and/or propoxylated C10-C18 fatty alcohols having a total of 2 to 12 alkoxy groups, particularly preferably ethoxy and/or propoxy groups, some of which may be present end-capped with alkyl functions, particularly preferably methyl, ethyl, propyl or butyl functions.

In a particular embodiment of the method of the present invention, the alkaline aqueous composition in step (i) comprises:
a) 0.05 to 2 g/L of iron(III) ions;
b) at least 0.1 g/L, calculated as _PO4 , of an organic compound (b1) having at least one functional group selected from COOX, _OPO3X and/or _PO3X , where X is either an H atom, an alkali metal atom and/or an alkaline earth metal atom, and/or a complexing agent selected from condensed phosphates (b2);
c) 0.1 to 4 g/L of phosphate ion;
d) a total of 0.01 to 10 g/L of nonionic surfactants, preferably selected from one or more ethoxylated and/or propoxylated C10-C18 fatty alcohols having a total of 2 to 12 alkoxy groups, particularly preferably ethoxy and/or propoxy groups, some of which may be present end-capped with alkyl functions, particularly preferably methyl, ethyl, propyl or butyl functions,
e) less than 10 mg/L in total of ionic compounds of the metals copper, nickel, cobalt, tin, manganese, molybdenum, chromium and/or cerium, in particular less than 1 mg/L in each case of ionic compounds of the metals nickel and cobalt, in which not more than 10 g/L of condensed phosphate (b2) is present as _PO4 , the molar ratio of the sum of components (b1) and (b2) to iron(III) ions is greater than 1:1, the free alkalinity is at least 1 point but less than 6 points and the pH value is at least 10.5.

In a preferred embodiment of the method of the invention, during the treatment sequence in step (i), the component is contacted with an aqueous composition for iron deposition, in particular with an alkaline aqueous composition containing iron(II) and/or iron(III) ions, at a temperature of at least 30° C., particularly preferably at least 40° C. but not more than 70° C., particularly preferably not more than 60° C., for a time of at least 30 seconds but not more than 4 minutes. The preferred treatment or contact time in step (i) of the method of the invention should be selected so that an iron coating layer of at least 10 mg/m ² , preferably at least 20 mg/m ^2, is obtained. The treatment and contact times to obtain a minimum coating layer of this kind vary depending on the type of application, in particular the flow of the aqueous fluid acting on the metal surface to be treated. In a method in which the composition is applied by spraying, a minimum iron coating layer develops more quickly than in a dip application.

Treatment step (ii) - Activated zinc phosphating:
The zinc phosphating of the series of components carried out in process step (ii) is effected by an acidic aqueous zinc phosphating composition comprising polyvalent metal cation phosphates dispersed as particulate components, said phosphates being at least partially selected from hopeite, phosphophyllite, scholzite and/or hyullite.The acidic aqueous zinc phosphating is therefore self-activating and does not require a separate prior activation, but is obtained by correspondingly adding a predetermined amount of the aqueous dispersion to an acidic aqueous composition comprising components (A)-(C) according to claim 1 of the present invention.

The aqueous dispersion comprises a particulate component (P) in water-dispersed form, which is
at least one particulate inorganic compound (P1), consisting of a polyvalent metal cation phosphate at least partially selected from hopeite, phosphophyllite, scholzite and/or hyullite, and at least one organic polymeric compound (P2).
Including,
The aqueous dispersion for providing an acidic aqueous composition for the zinc phosphating in process step (ii) is preferably added in an amount such that the weight proportion of phosphate from the granular component of the aqueous dispersion is at least 0.004 g/kg, preferably at least 0.01 g/kg, particularly preferably at least 0.05 g/kg and very particularly preferably at least 0.08 g/kg, based on the acidic aqueous composition comprising components (A)-(C).

In an alternative or preferred embodiment, in step (ii) of the method of the present invention, the series of members comprises:
(A) 5-50 g/kg of phosphate dissolved in water, calculated as _PO4 ;
(B) 0.3 to 3 g/kg zinc ions, and (C) free fluoride and free acid at above 0 points, wherein in the treatment step (ii) for zinc phosphating, an activator is added continuously ^or discontinuously to the acidic aqueous composition in an amount sufficient to maintain the properties of the acidic aqueous composition to form, under the selected conditions of the zinc phosphating treatment step (ii), a zinc phosphate layer on the hot-dip galvanized steel surface (Z) having a layer weight of less than 5.0 g/m ² , preferably less than 4.5 g/m ² , particularly preferably less than 4.0 g/m ² , very particularly preferably less than 3.5 g/m 2, wherein the activator comprises a granular component (P) in a water-dispersed form, the granular component (P) being
at least one particulate inorganic compound (P1), consisting of a polyvalent metal cation phosphate at least partially selected from hopeite, phosphophyllite, scholzite and/or hyullite, and at least one organic polymeric compound (P2).
Includes.

The ^inventive feature of the acidic aqueous composition for zinc phosphating in treatment ^step (ii) on hot-dip galvanized steel surfaces (Z) resulting in the growth of a zinc phosphate layer having a total weight of less than 5.0 g/ ^m2 , preferably less than 4.5 g/ ^m2 , particularly preferably less than 4.0 g/m2 and very particularly preferably less than 3.5 g/m2 (hereinafter referred to as "phosphating property") is observed on substrates that are cleaned and degreased (Z) and that have not been subjected to further wet-chemical pretreatment steps before contact with the acidic aqueous composition of the inventive method in step (ii) and after the deposition of iron on the zinc surface in step (i). To verify the phosphatizing properties of the acidic aqueous composition, a hot-dip galvanized steel (Z) is first cleaned by immersion for 5 minutes with an alkaline cleaner prepared as 2 wt. % Bonderite® C-AK 1565 A and 0.2 wt. % Bonderite® C-AD 1270 in deionized water (κ<1 μScm ⁻¹ ) at pH 11.0 and 55° C. The thus cleaned and degreased substrate (Z) is rinsed with deionized water (κ<1 μScm ⁻¹ ) at room temperature and then subjected to the treatment stages of process steps (i) and (ii) according to the selected treatment conditions. "According to selected treatment conditions" means using the same application time and bath circulation at the relevant temperature and wet-chemical treatment step, in which the phosphating properties defined according to the invention are applied, i.e. the resulting hot-dip galvanized steel (Z) top layer weight is less than 5.0 g/m ² , preferably less than 4.5 g/m ² , particularly preferably less than 4.0 g/m ² , very particularly preferably less than 3.5 g/m ^2. The phosphating properties can therefore be determined in the present method of the invention by introducing a cleaned and degreased hot-dip galvanized steel sheet (Z) together with a series of components for treatment steps (i) and (ii) and determining the total weight of zinc phosphate on said sheet, and thus the phosphating properties of the acidic aqueous composition for zinc phosphating, in treatment step (ii). In order to ensure that the flow conditions during the transport of the components together with the transport frame through the phosphating bath are reproduced as similarly as possible to the test sheet, the cleaned and degreased hot-dip galvanized steel sheet (Z) in its function as a test sheet for determining the phosphating properties is preferably firmly joined to the components or to the transport frame. For this purpose, the test sheet must ideally be joined to the component or the transport frame in such a way that the transport of the test sheet with the component and the transport frame is insensitive to the possible flow conditions compared to the transport of the component and the transport frame without such a test sheet and that the flow conditions are in each case substantially identical and therefore substantially correspond to the flow conditions of at least a part of the series of components. This can be achieved, for example, by adapting the dimensions and/or shape of the test sheet to those of the component and/or the transport frame which are in each case arranged adjacent to the test sheet. In this case, in particular when the test sheet is arranged on an outer surface portion of the component or the transport frame, it is conceivable to make the dimensions of the test member accordingly smaller than the dimensions of said surface portion, for example in order to prevent the test member from protruding beyond the surface portion. Alternatively or additionally, the test member may follow the curvature or other planar deviations of the surface portion or the transport frame. It has been found to be particularly advantageous to select a sheet portion which is sufficiently small compared to the dimensions of the relevant outer surface of the component. The outer surface is particularly suitable when it is located at the position of least curvature or at a position of particularly low curvature of the component, and the test sheet metal is mounted substantially parallel to and spaced along the surface normal of such outer surface. The phosphating properties are directly obtained during the course of processing of such a component, which also has a hot-dip galvanized steel (Z) surface as the zinc surface. In a preferred embodiment of the method, such a component is preferred.

It is further preferred that the phosphating properties are such that, when the contact is extended for 1 minute, the total weight on the hot-dip galvanized steel (Z) increases by no more than 0.2 g/m ² , so that the layer formation under the selected conditions is already within the self-limiting range, and thus the properties of the acidic aqueous composition for zinc phosphating to form a dense crystalline zinc phosphate layer in step (ii) of the method of the present invention are ensured. It is therefore preferred to add an amount of activator sufficient to maintain the properties of the acidic aqueous composition in the zinc phosphating step to form, under the selected conditions of the zinc phosphating step of the method of the present invention, a zinc phosphate layer with a layer weight of less than 5.0 g/m ² , preferably less than 4.5 ^g /m ² , particularly preferably less than 4.0 g/m ² , very particularly preferably less than 3.5 g/m 2 on the hot-dip galvanized steel surface (Z). Here, when the contact time with the acidic aqueous composition is extended for 60 seconds, the layer weight achieved under the selected conditions of the zinc phosphating step (ii) of the method of the present invention increases by no more than 0.2 g/m ² .

Typically, the phosphating properties are determined and monitored in the method of the present invention by subjecting hot-dip galvanized steel (Z), cleaned and degreased as described above, to a zinc phosphating treatment step at regular intervals during the treatment sequence and then subjecting it to layer weight measurements. As described above, the phosphating properties are obtained directly during the treatment sequence of such components, which also have at least one hot-dip galvanized steel (Z) surface as a zinc surface. As long as the phosphating properties of the acidic aqueous composition are ensured by the metered addition of activator, a uniform, closed, dense, crystalline zinc phosphate coating is formed on components having metallic zinc, iron and aluminum surfaces in normal treatment times of 20 seconds to 5 minutes.

The zinc phosphate layer weight is measured within the scope of the present invention by removing the zinc phosphate layer using a 5 wt. % _CrO3 aqueous solution as pickling solution contacting the predefined area of the phosphating material or component immediately after zinc phosphating for 5 minutes at 25° C., rinsing with deionized water (κ<1 μScm ⁻¹ ), and then measuring the phosphorus content in the same pickling solution using ICP-OES. The zinc phosphate layer weight can be determined by multiplying the amount of phosphorus relative to the surface area by 6.23.

In the method of the present invention, an activator is added to the acidic aqueous composition for zinc phosphating with the aim of maintaining the phosphating properties in the zinc phosphating treatment step (ii). To maintain the phosphating properties in the treatment process series, the addition can be carried out by continuous or discontinuous metering into the system tank. Continuous metering is preferred when the pretreatment of a series of components follows each other directly, the decrease in the phosphating properties over time can be measured, and the amount of activator can be metered continuously over time. This process has the advantage that after the start-up of the pretreatment line and the determination of the material flow for the metered addition of activator and other active ingredients, the phosphating properties do not need to be further verified, as long as the treatment series does not change in terms of the timing and properties of the components to be treated and the treatment parameters of the zinc phosphating treatment step (ii). However, discontinuous metering of activator can be advantageous and even preferred if a constant operating mode in the treatment series cannot be ensured or is not favorable for the system. In this case, the phosphating property of the acidic aqueous composition is preferably monitored discontinuously or at defined intervals in step (ii) and a defined amount of activator is metered in when the total weight on the hot-dip galvanized steel (Z) reaches a value of less than 5.0 g/m ² , preferably less than 4.5 g/m ² , particularly preferably less than 4.0 g/m ² , very particularly preferably less than 3.5 g/m ^2. Continuous or quasi-continuous measurements of the phosphating property at defined time intervals can also be carried out with proxy data that correlate with the actual zinc phosphate layer weight. Non-destructive measurements of the layer thickness, for example using eddy current methods or non-contact optical measuring methods such as ellipsometry or spectroscopic reflectometry, provide suitable proxy data of the zinc phosphate layer weight. This data can be reliably measured on the zinc surface of the component in the pretreatment line and can be correlated with the actual layer weight on the hot-dip galvanized component steel (Z). Since the higher the layer weight on hot-dip galvanized steel (Z) the lower the number density of the crystallites but the larger they are relative to each other, and therefore the roughness increases with layer weight, the measurement of the crystallite size and therefore the roughness by optical profilometry can also provide a proxy for the layer weight.

It has been found that the phosphating properties are already adequate in most cases if the activator is metered continuously or discontinuously during the pretreatment of the series of elements in an amount suitable for maintaining a steady-state amount of particulate component (P) in the acidic aqueous composition of preferably at least 0.001 g/kg, particularly preferably at least 0.005 g/kg, more particularly preferably at least 0.01 g/kg. This applies in particular to contacting the acidic aqueous composition by spraying, whereas in the case of immersion application, a steady-state amount of particulate component (P) of preferably at least 0.002 g/kg, particularly preferably at least 0.01 g/kg, more particularly preferably at least 0.02 g/kg should be contained in the acidic aqueous composition for zinc phosphating.

The present invention therefore surprisingly shows that by directly metering an activator, as known in the prior art and described, for example, in WO 98/39498 A1, into the acidic aqueous treatment solution for zinc phosphating in step (ii), activation of the metal surface occurs, so that a uniform, closed, dense, crystalline zinc phosphate coating with high charge transfer resistance grows on the metal surface. At this time, due to the iron deposition occurring in treatment step (i), such a highly rust-resistant and adherent phosphate coating is formed on the zinc surface. The present invention makes use of this effect in that the series of component treatments is based on zinc phosphating with an acidic aqueous composition which, in addition to zinc ions, phosphate ions and free fluoride, also contains a granular component (P) dispersed in water, and/or on maintaining the phosphating properties by metering an activator into the acidic aqueous composition for zinc phosphating. In this case, for the desired phosphating properties, it is possible to switch to the sole addition of an aqueous dispersion containing the particulate component (P) or the metered addition of an activator containing the particulate component (P) before the zinc phosphating process, without passing the entire series through a wet-chemical activation stage, for example based on an aqueous dispersion or activator. This makes it possible for the first time to operate a pretreatment line for zinc phosphating in an extremely resource-saving and economical manner, since the entire process step, including the necessary bath maintenance, circulation, temperature control and chemical addition, for example with water-soluble condensed phosphates, can be omitted.

For the dispersed particulate component (P) and at least one particulate inorganic compound (P1) or organic polymeric compound (P2), the definitions and preferred specifications set out below apply, regardless of whether the dispersed particulate component (P) is an activator for maintaining the activation performance of the acidic aqueous composition in process step (ii) of the method of the present invention or part of an aqueous dispersion resulting in a self-activating acidic aqueous composition for zinc phosphating. In the following, for the sake of brevity, only the activator is mentioned. Thus, the statements regarding the activator also apply, where appropriate, to the aqueous dispersion for providing a self-activating acidic aqueous composition for zinc phosphating.

The activator that can be used in the present invention, i.e. that maintains the phosphating properties upon metered addition to the acidic aqueous composition for zinc phosphating or that initially results in an acidic aqueous composition for zinc phosphating in step (ii), is an aqueous dispersion and therefore comprises a granular component (P) in water-dispersed form. The granular component (P) comprises at least one granular inorganic compound (P1) consisting of a polyvalent metal cation phosphate at least partially selected from hopeite, phosphophyllite, scholzite and/or hyullite, and at least one organic polymeric compound (P2).

The use of polyvalent metal cations in the form of phosphates is responsible for the good activation performance or suitability of the activator for maintaining the phosphating properties of the acidic aqueous composition for zinc phosphating, so that said phosphates should be present in the activator in a sufficiently high proportion in the dispersed granular component (P). The proportion of phosphates contained in the at least one granular inorganic compound (P1), based on the dispersed granular component (P) in the activator, is therefore preferably at least 25% by weight, particularly preferably at least 35% by weight, more particularly preferably at least 40% by weight, very particularly preferably at least 45% by weight. The dispersed granular component (P) of the activator is the solids remaining after drying the retentate of ultrafiltration of a defined partial volume of the activator at a nominal cut-off limit of 10 kD (NMWC: nominal molecular weight cut-off). Ultrafiltration is carried out by adding deionized water (κ<1 μScm ⁻¹ ) until the conductivity of the filtrate is less than 10 μScm ⁻¹ . The inorganic particulate components in the activator are those that remain when the particulate component (P) obtained from drying the ultrafiltration residue is pyrolyzed in the reactor by supplying a _CO2 -free oxygen flow at 900 ° C without mixing of catalysts or other additives until the infrared sensor outputs the same signal as the _CO2 -free carrier gas (blank value) at the reactor outlet. The phosphates contained in the inorganic particulate components are measured as the phosphorus content by atomic emission spectrometry (ICP-OES) directly from the acid digestion after acid digestion for 15 minutes at 25 ° C with a 10 wt % HNO3 _aqueous solution.

The active components of the activator, which, as soon as they are added in a sufficient amount to the acidic aqueous composition for zinc phosphating, promote the formation of a uniform, closed and dense crystalline phosphate coating on the metal surface, in particular on the zinc surface, and in this sense activate the metal surface, are, as already mentioned, mainly composed of phosphates at least partially selected from hopeite, phosphophyllite, scholzite and/or hyullyte, preferably at least partially selected from hopeite, phosphophyllite and/or scholzite, particularly preferably at least partially selected from hopeite and/or phosphophyllite, very particularly preferably at least partially selected from hopeite. The maintenance of the phosphating properties in the acidic aqueous composition is therefore substantially based on the granular metered-in phosphates contained in the activator. Without taking into account the water of crystallization, hopeite stoichiometrically comprises Zn ₃ (PO ₄ ) ₂ and the nickel- and manganese-containing variants Zn ₂ Mn(PO ₄ ) ₃ , Zn ₂ Ni(PO ₄ ) ₃ , while phosphophyllite is composed of Zn ₂ Fe(PO ₄ ) ₃ , scholzite is composed of Zn ₂ Ca(PO ₄ ) ₃ and hyureulite is composed of Mn ₃ (PO ₄ ) _2. The presence of the crystalline phases hopeite, phosphophyllite, scholzite and/or hyureulite in the activator can be demonstrated using X-ray diffraction (XRD) after separation of the particulate component (P) using ultrafiltration with a nominal cut-off of 10 kD (NMWC: nominal molecular weight cut-off) and drying the residue to constant mass at 105° C., as described above.

Since the presence of a phosphate containing zinc ions and having a certain degree of crystallinity is preferred, in the process of the invention, for the formation of a tightly adherent crystalline zinc phosphate coating, it is preferred that the activator contains at least ₂₀ % by weight, particularly preferably at least 30% by weight, more particularly preferably at least 40% by weight of zinc in the inorganic particulate component, calculated as PO4, based on the phosphate content in the inorganic particulate component.

However, it is preferred that the activator does not additionally contain titanium phosphates, since these do not have a positive effect on the phosphating properties when metered in. In a preferred embodiment of the method of the invention, the proportion of titanium in the inorganic particulate components of the activator is therefore less than 0.01% by weight, particularly preferably less than 0.001% by weight, based on the activator. In a particularly preferred embodiment, the activator contains less than 10 mg/kg, particularly preferably less than 1 mg/kg, of titanium in total.

The organic polymeric compound (P2) that stabilizes the granular component has a significant influence on the effectiveness of the granular component (P) that is metered in via the activator. It has been found that the choice of organic polymeric compound determines the degree of activation of the metal surface in the acidic aqueous composition for zinc phosphating in step (ii). This activation is known to be brought about by dispersed polyvalent phosphates and, as the present invention shows, can surprisingly occur simultaneously with the layer formation.

In the present invention, an organic compound is a polymer if the weight-average molecular weight is greater than 500 g/mol. In this case, the molar mass is determined using the molar mass distribution curve of a sample of relevant reference values. This curve is experimentally obtained at 30°C by size-exclusion chromatography with a concentration-dependent refractive index detector and calibrated against polyethylene glycol standards. The average molar mass is evaluated computer-aided according to the strip method using a third calibration curve. Hydroxylated polymethacrylate is suitable as the column material, and an aqueous solution of 0.2 mol/L sodium chloride, 0.02 mol/L sodium hydroxide, and 6.5 mol/L ammonium hydroxide is suitable as the eluent.

It has been found that the maintenance of the phosphating properties when contacted with an acidic aqueous composition, i.e. the activation of the metal surface in the zinc phosphating treatment step, is particularly well achieved, i.e. with a relatively small amount of the active component of the activator, when the organic polymeric compound (P2) used to disperse the particulate inorganic compound (P1) is at least partially composed of styrene and/or α-olefins having not more than 5 carbon atoms (wherein the organic polymeric compound (P2) further comprises in its side chain units of maleic acid, its anhydride and/or imide, preferably polyoxyalkylene units, particularly preferably polyoxyalkylene units). Such organic polymeric compounds (P2) are therefore preferred in the particulate component (P) of the activator according to the invention.

The α-olefin in this case is preferably selected from ethene, 1-propene, 1-butene, isobutylene, 1-pentene, 2-methyl-but-1-ene and/or 3-methyl-but-1-ene, particularly preferably isobutylene. It will be clear to those skilled in the art that the organic polymeric compound (P2) contains these monomers as structural units in unsaturated form covalently bonded to each other or to other structural units.

Suitable commercial examples of organic polymeric compounds (P2) are, for example, Dispex® CX 4320 (BASF SE), a maleic acid-isobutylene copolymer modified with polypropylene glycol, Tego® Dispers 752 W (Evonik Industries AG), a maleic acid-styrene copolymer modified with polyethylene glycol, or Edaplan® 490 (Muenzing Chemie GmbH), a maleic acid-styrene copolymer modified with EO/PO and imidazole units.

In the present invention, an organic polymer compound (P2) at least partly composed of styrene is preferred.

The organic polymeric compound (P2) used for colloidal stabilization of the granular component (P) of the activator preferably has polyoxyalkylene units, preferably composed of 1,2-ethanediol and/or 1,2-propanediol, particularly preferably both 1,2-ethanediol and 1,2-propanediol, the proportion of 1,2-propanediol in the total of the polyoxyalkylene units being preferably at least 15% by weight, particularly preferably not more than 40% by weight, based on the total of the polyoxyalkylene units. The polyoxyalkylene units are also preferably contained in the side chains of the organic polymeric compound (P2). A proportion of polyoxyalkylene units in the total of the organic polymeric compound (P2) of preferably at least 40% by weight, particularly preferably at least 50% by weight, but preferably not more than 70% by weight, is advantageous for the dispersibility of the compound.

In order to fix the organic polymeric compound (P2) with the inorganic particulate component (P1) of the activator formed at least in part from polyvalent metal cations in the form of phosphates selected from among hopeite, phosphophyllite, scholzite and/or hyullite, and for improved stability and ability of the particulate component (P) to be activated in the acidic aqueous composition of zinc phosphating, the organic polymeric compound (P2) also has imidazole units, preferably in the side chains, particularly preferably as constituents of the polyoxyalkylene units of the organic polymeric compound (P2).

In a preferred embodiment, the amine value of the organic polymeric compound (P2) is at least 25 mg KOH/g, particularly preferably at least 40 mg KOH/g, but preferably less than 125 mg KOH/g, particularly preferably less than 80 mg KOH/g. Thus, in a preferred embodiment, the total of the organic polymeric compounds in the granular component (P) of the activator also has these preferred amine values. The amine value is in each case determined by weighing out about 1 g of the relevant reference value (total of the organic polymeric compounds in the granular component (P) or the organic polymeric compound (P2)) in 100 ml of ethanol and titrating it with 0.1 N HCl titration solution against the indicator bromophenol blue until the color turns yellow at the temperature of the ethanolic solution at 20° C. The amount of HCl titration solution used (in ml) multiplied by the factor 5.61 divided by the exact mass of the weight in grams corresponds to the amine value (in mg KOH/g) of the relevant reference value.

In order to ensure a sufficient number of polyoxyalkylene units, it has also been found to be advantageous for the organic polymeric compounds (P2), preferably also the total of the organic polymeric compounds in the granular component (P), to have an acid number according to DGF C-V 2 (06) (April 2018) of at least 25 mg KOH/g, but preferably less than 100 mg KOH/g, particularly preferably less than 70 mg KOH/g. It is also preferred that the organic polymeric compounds (P2), preferably also the total of the organic polymeric compounds in the granular component (P), have a hydroxyl number, in each case measured according to method A of European Pharmacopoeia 9.0 01/2008:20503, of less than 15 mg KOH/g, particularly preferably less than 12 mg KOH/g, more particularly preferably less than 10 mg KOH/g.

For a stable dispersion of the inorganic particulate components in the activator, it is sufficient that the total proportion of the organic polymeric compounds (P2), preferably the organic polymeric compounds in the particulate component (P), is at least 3% by weight, particularly preferably at least 6% by weight, but preferably not more than 15% by weight, based on the particulate component (P). The dispersed particulate component (P) of the activator is the solids remaining after drying the retentate of ultrafiltration of a defined partial volume of the activator at a nominal cut-off limit of 10 kD (NMWC: nominal molecular weight cut-off). Ultrafiltration is carried out by adding deionized water (κ<1 μScm ⁻¹ ) until the conductivity of the filtrate is less than 10 μScm ⁻¹ .

The activator preferably comprises not more than 40% by weight of granular components (P), based on the agent, since otherwise the technical handling behavior and dispersion stability for continuous or discontinuous metered addition of the agent by means of a metering pump to the acidic aqueous composition for zinc phosphating are no longer ensured or at least become difficult. This is particularly true with respect to the low total amount of granular components (P) required to maintain the phosphating properties of the reference amount of acidic aqueous composition for zinc phosphating. However, it is advantageous for the activator to be provided as a dispersion that is as stable as possible and at the same time as highly concentrated as possible. This can be achieved in particular when using the preferred organic polymeric compounds (P2) for dispersing the granular inorganic compounds (P1), so that it is preferable to use activators that comprise at least 5% by weight, but preferably not more than 30% by weight, of granular components (P), based on the agent.

In such highly concentrated aqueous dispersions of activator, i.e. aqueous dispersions containing 5% by weight of particulate component (P) based on the agent, the activator may additionally be characterized in the process of the present invention by its D50 value of more than 10 μm, which is correspondingly preferred. The agglomerates of dispersed particles contained in the dispersion result in thixotropic flow properties which are advantageous for the handling behavior of the activator. The tendency of the agglomerates to become more viscous at low shear is advantageous for long-term storage, while the decrease in viscosity on shear allows pumping. Good flow properties are also obtained when the dispersion does not significantly exceed a D90 value of 150 μm; therefore, in the present invention, D90 values of the aqueous dispersion less than 150 μm, preferably less than 100 μm μm, in particular less than 80 μm, are preferred. In the present invention, the D50 value or D90 value respectively refers to the particle size at or below which 50% or 90% by volume of the particulate components contained in the aqueous dispersion are equal to or less than said value. According to ISO 13320:2009, the D50 or D90 values can be determined from the volume-weighted cumulative particle size distribution by scattered light analysis according to the Mie theory immediately after dilution of the activator in concentrated aqueous dispersion with a corresponding amount of deionized water (κ<1 μScm ⁻¹ ) at 20° C. to 0.05% by weight of dispersed particulate content with spherical particles and a scattering particle refractive index nD=1.52-i·0.1. The dilution is carried out by adding the concentrated dispersion equivalent to a volume of 200 mL of ion-exchanged water to the sample vessel of a particle size distribution analyzer LA-950 V2 manufactured by Horiba Ltd. and circulating it mechanically from there to the measurement chamber (settings of the circulation pump of the LA-950 V2: level 5=1167 rpm with a volume flow rate of 3.3 L/min). The particle size distribution is measured within 120 seconds after the dispersion has been added to the dilution volume.

The presence of a thickening agent may be advantageous to prevent irreversible aggregation of the primary particles of the particulate component (P), especially when the activator is present as said concentrated dispersion. Thus, in a preferred embodiment of the method of the present invention, the activator is preferably present in an amount that gives the activator a maximum kinematic viscosity of at least 1000 Pa· ^s , but preferably less than 5000 Pa·s, at a temperature of 25° C., in the shear rate range of 0.001 to 0.25 s −1, preferably in an amount such that it results in shear thinning behavior, i.e. a decrease in viscosity with increasing shear rate occurring at 25° C. at shear rates above the shear rate present at the maximum kinematic viscosity, so that the activator has an overall thixotropic flow behavior. In this case, the viscosity over a given shear rate range can be measured using a cone and plate viscometer with a cone diameter of 35 mm and a gap width of 0.047 mm.

A thickener within the meaning of the present invention is a polymeric compound or a defined mixture of compounds which, when included as a component of 0.5% by weight in deionized water (κ<1 μScm ⁻¹ ) at a temperature of 25° C., exhibits a Brookfield viscosity of at least 100 mPa·s at a shear rate of 60 rpm (= revolutions per minute) using a size 2 spindle. When determining the properties of this thickener, the mixture must be mixed with water in such a way that the corresponding amount of polymeric compound is added to the aqueous phase with stirring at 25° C., the mixture is homogenized in an ultrasonic bath to remove air bubbles and is left to stand for 24 hours. The viscosity measurement is then read immediately after the application of a shear rate of 60 rpm using a number 2 spindle within 5 seconds.

The activator preferably comprises one or more thickeners in a total amount of at least 0.5% by weight, but preferably not more than 4% by weight, particularly preferably not more than 3% by weight, and the total proportion of organic polymeric compounds in the non-particulate component of the aqueous dispersion is more preferably not more than 4% by weight (based on the dispersion). The non-particulate component is the solids content of the aqueous dispersion in the permeate of said ultrafiltration after drying to constant mass at 105°C, i.e. the solids content after separation of the particulate component by ultrafiltration.

Certain polymeric compounds are particularly suitable thickeners and are readily available on the market. Thus, the thickener is preferably selected from organic polymeric compounds, preferably from polysaccharides, cellulose derivatives, aminoplasts, polyvinyl alcohols, polyvinylpyrrolidones, polyurethanes and/or urea-urethane resins, and particularly preferably from urea-urethane resins, in particular urea-urethane resins which are mixtures of polymeric compounds resulting from the reaction of polyisocyanates with polyols and mono- and/or diamines. In a preferred embodiment, the urea urethane resin is obtained from a polyvalent isocyanate, preferably selected from 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2(4),4-trimethyl-1,6-hexamethylene diisocyanate, 1,10-decamethylene diisocyanate, 1,4-cyclohexylene diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, 2,6-toluene diisocyanate, 2,4-toluene diisocyanate and mixtures thereof, p- and m-xylylene diisocyanate, and 4-4'-diisocyanatodicyclohexylmethane, particularly preferably selected from 2,4-toluene diisocyanate and/or m-xylylene diisocyanate. In a particularly preferred embodiment, the urea urethane resin is obtained from a polyol, preferably selected from polyoxyalkylene diols, particularly preferably selected from polyoxyethylene glycols, composed of oxyalkylene units, preferably at least 6, particularly preferably at least 8, more particularly preferably at least 10, but preferably less than 26, particularly preferably less than 23.

Particularly suitable, and therefore preferred in the present invention, urea-urethane resins can be obtained by first reacting a diisocyanate, for example toluene-2,4-diisocyanate, with a polyol, for example polyethylene glycol, to give an NCO-terminated urethane prepolymer, which is then further reacted with a primary monoamine and/or a primary diamine, for example m-xylylenediamine. Urea-urethane resins having neither free nor blocked isocyanate groups are particularly preferred. As a component of the activator, such urea-urethane resins promote the formation of loose aggregates of primary particles, which are protected from further aggregation and dissociate into primary particles when metered into the acidic aqueous composition for zinc phosphating. To further promote this property profile, it is preferred to use urea-urethane resins having neither free nor blocked isocyanate groups nor terminal amine groups as thickeners. Thus, in a preferred embodiment, the thickener, which is a urea-urethane resin, has an amine value, in each case measured according to the method described above for the organic polymeric compound (P2), of less than 8 mg KOH/g, particularly preferably less than 5 mg KOH/g, more particularly preferably less than 2 mg KOH/g. Since the thickener is substantially dissolved in the aqueous phase of the activator and can therefore be allocated to the non-granular component, while component (P2) is substantially bound in the granular component (P), activators are preferred in which the total of the organic polymeric compounds in the non-granular component preferably has an amine value of less than 16 mg KOH/g, particularly preferably less than 10 mg KOH/g, more particularly preferably less than 4 mg KOH/g. It is further preferred that the urea-urethane resin has a hydroxyl value, measured according to method A of European Pharmacopoeia 9.0 01/2008:20503, in the range from 10 to 100 mg KOH/g, particularly preferably in the range from 20 to 60 mg KOH/g. With regard to the molecular weight of the urea-urethane resin, it is advantageous and therefore preferred in the present invention that the weight-average molar mass, experimentally measured in each case as described above in connection with the definition according to the present invention of the organic polymeric compound, is in the range of 1000 to 10000 g/mol, preferably in the range of 2000 to 6000 g/mol.

The activator is preferably an aqueous dispersion having a pH value in the range of 6.5 to 8.0, particularly preferably free of any pH-adjusting water-soluble compounds having a pK _S value of less than 6 or a pK _B value of less than 5.

The activator may also contain auxiliaries, selected for example from preservatives, wetting agents and antifoaming agents, in the amounts required for the relevant function. The proportion of auxiliaries, particularly preferably not thickeners, other compounds in the non-granular components is preferably less than 1% by weight.

The activator is preferably
i) grinding 10 parts by weight of inorganic particulate compound (P1) with 0.5 to 2 parts by weight of organic polymeric compound (P2) in the presence of 4 to 7 parts by weight of water, for example using a Zetasizer® Nano ZS from Malvern Panalytical GmbH, until a D50 value measured by dynamic light scattering after a 1000-fold dilution with water of less than 1 μm is reached, to obtain a pigment paste;
ii) The pigment paste can be obtained as a concentrated aqueous dispersion by diluting it with at least 5% by weight of dispersed particulate component (P) and with water, preferably deionized water (κ<1 μScm ⁻¹ ) or tap water and thickener in such an amount that a maximum dynamic viscosity of at least 1000 Pa·s at a temperature of 25° C. is set at shear rates in the range of 0.001-0.25 s ⁻¹ . Preferred embodiments of the activator are here obtained in a similar manner by selecting the corresponding components (P1), (P2) and thickener in the amounts that may be provided or required in each case. Such concentrated aqueous dispersions have excellent stability and, due to their thixotropic flow behavior, also have good pumpability, so that the concentrated dispersions can be metered in a controlled manner directly into the tank of the zinc phosphating system.

In order to form a uniform and closed zinc phosphate layer, it is necessary in step (ii) of the method of the present invention that the acidic aqueous composition for zinc phosphating contains at least
(A) 5 to 50 g/kg of phosphate, calculated as _PO4 , dissolved in water;
(B) zinc ion of 0.3 to 3 g/kg; and (C) free fluoride and free acidity of greater than 0 points.

In this regard, the amount of phosphate ions includes the anions of orthophosphoric acid and orthophosphate dissolved in water, calculated as _PO4 .

The proportion of free acid (in points) in the zinc phosphating acidic aqueous composition in step (ii) of the method of the present invention is preferably at least 0.4, but preferably not more than 3.0, particularly preferably not more than 2.0. The proportion of free acid (in points) is determined by diluting a sample volume of 10 mL of the acidic aqueous composition to 60 mL and titrating with 0.1 N sodium hydroxide solution to a pH value of 3.6. The amount of sodium hydroxide solution consumed (in mL) indicates the number of points of free acid.

The preferred pH value of the acidic aqueous composition is usually greater than 2.5, particularly preferably greater than 2.7, but is preferably less than 3.5, particularly preferably less than 3.3. The "pH value" as used in the present invention corresponds to the negative decimal logarithm of the hydronium ion activity at 20°C and can be measured using a pH-sensitive glass electrode.

A certain amount of free fluoride or a source of free fluoride ions is essential for the zinc phosphating treatment to form a layer. Insofar as components which contain iron or aluminum surfaces in addition to zinc surfaces are zinc phosphating in a layer-forming manner, as is necessary, for example, in the zinc phosphating of at least partially aluminum-formed car bodies, it is advantageous for the amount of free fluoride in the acidic aqueous composition in step (ii) to be at least 0.5 mmol/kg, particularly preferably at least 2 mmol/kg. The concentration of free fluoride should not exceed a value at which the phosphate coating becomes weakly adherent and can be easily wiped off, since this defect cannot often be compensated for by increasing the metered addition of activator or by increasing the steady-state amount of granular component (P) in the acidic aqueous composition for zinc phosphating. It is therefore advantageous and therefore preferred in step (ii) of the method of the invention for the concentration of free fluoride in the acidic aqueous composition for zinc phosphating to be less than 15 mmol/kg, particularly preferably less than 10 mmol/kg, more particularly preferably less than 8 mmol/kg.

The amount of free fluoride is measured potentiometrically with a fluoride-sensitive measuring electrode at 20° C. in the relevant acidic aqueous composition after calibration with a fluoride-containing buffer solution without pH buffering. Suitable sources of free fluoride ions are hydrofluoric acid and its water-soluble salts, such as ammonium hydrogen fluoride and sodium fluoride, as well as complex fluorides of the elements Zr, Ti and/or Si, in particular complex fluorides of the element Si. Thus, in the phosphating process of the invention, the source of free fluoride is preferably selected from hydrofluoric acid and its water-soluble salts and/or complex fluorides of the elements Zr, Ti and/or Si. Salts of hydrofluoric acid are water-soluble within the meaning of the invention if their solubility, calculated as F, in deionized water (κ<1 μS cm ⁻¹ ) at 60° C. is at least 1 g/L.

In order to suppress so-called "pinholes" on the surface of the metallic material consisting of zinc, it is preferred in such a treatment according to the invention that the source of free fluoride in step (ii) is at least partially selected from complex fluorides of the element Si, in particular from hexafluorosilicic acid and its salts. To those skilled in the art of phosphating, the term pinholes is understood to mean the phenomenon of local deposition of amorphous white zinc phosphate in a crystalline phosphate layer on the treated zinc surface or on the treated galvanized or alloy galvanized steel surface.

For more rapid layer formation, accelerators known in the art can be added to the acidic aqueous composition in the process of the invention. These accelerators are preferably selected from 2-hydroxymethyl-2-nitro-1,3-propanediol, nitroguanidine, N-methylmorpholine-N-oxide, nitrites, hydroxylamine and/or hydrogen peroxide. It has been found that when nitroguanidine or hydroxylamine is used as accelerator, a relatively low metered addition of activator is required or a lower steady-state amount of the granular component (P) is reliably maintained in the acidic aqueous composition for zinc phosphating in step (ii). Therefore, nitroguanidine or hydroxylamine, in particular nitroguanidine, are particularly preferred as accelerators in the acidic aqueous composition in step (ii) of the process of the invention, in view of the particularly low amount of activator used to maintain the phosphating properties.

Embodiments in which the acidic aqueous composition for zinc phosphating in step (ii) of the method of the present invention contains less than 10 ppm nickel and/or cobalt ions in total are particularly preferred from an ecological point of view.

Furthermore, the method of the present invention allows for the use of additivations known in the art in zinc phosphating processes.

Optional processing steps:
Thus, in a preferred embodiment of the method of the invention, the train of elements is not contacted with a colloidal aqueous activation solution which contains hopeite, phosphophyllite, scholzite and/or hyleulite, preferably polyvalent metal cation phosphates, or sparingly soluble salts of the element Ti in the particulate components, prior to contact with the acidic aqueous composition in zinc phosphating treatment step (ii). Prior to contact with the acidic aqueous composition in zinc phosphating treatment step (ii), the train of elements is particularly preferably not contacted with a colloidal aqueous solution which activates the surfaces of the elements for zinc phosphating, and very particularly preferably the train of elements is not passed through an activation stage which activates the surfaces of the elements for zinc phosphating before the contact in treatment step (ii).

However, it is usually not possible to carry out cleaning and degreasing steps as process steps upstream of the zinc phosphating in step (ii) and the iron deposition in step (i). In order to achieve a layer coating that is as uniform and reproducible as possible, in a preferred embodiment of the method of the invention, at least the metal surface of the component is cleaned and, if necessary, degreased prior to process step (i) in a separate cleaning step or together with process step (i). Cleaning is preferably carried out by contact with a preferably neutral or alkaline cleaning agent. Process step (i) is carried out immediately after the cleaning step, with or without an intermediate rinsing step, preferably without an intermediate rinsing step.

With regard to the cleaning and optional degreasing of components, alkaline cleaning is characterized in that metal surfaces, in particular surfaces containing metallic aluminum as a material or as an alloying component of hot-dip galvanized steel, are pickled with an acid. This leads to a further standardization of the metal surface and thus favors the growth of a uniform zinc phosphate coating.

Since, as mentioned above, activation of the metal surface prior to zinc phosphating in step (ii) can be carried out in the process of the invention, the cleaning step (or the combined cleaning and iron-donating step) is preferably not carried out by contact with a preferably neutral or alkaline aqueous cleaning agent containing particulate components containing hopeite, phosphophyllite, scholzite and/or hyulite or sparingly soluble salts of elemental Ti. The rinsing step after cleaning is optional as already mentioned and is employed in the present invention exclusively to completely or partially remove from the component to be treated soluble residues, particles and active components brought in by deposition on the component from the previous wet-chemical treatment steps (in this case the cleaning and degreasing steps). The active components based on metal elements or on metalloid elements are already consumed by the mere contact of the rinsing liquid with the metal surface of the component and are not contained in the rinsing liquid itself. For example, the rinsing liquid may simply be city water or deionized water or, if necessary, a rinsing liquid containing surface-active compounds to improve wettability by the rinsing liquid.

Materials:
Since the phosphating properties of the method of the invention on hot-dip galvanized steel are technically optimized, according to the invention, a method is naturally also preferred in which a series of components having at least partially a zinc surface also has a hot-dip galvanized steel surface. Essentially, due to the phosphating properties of the acidic aqueous composition, which are maintained in step (ii) by the addition of an activator, components made of multiple metal structures, such as car bodies, can also be zinc phosphated with very good properties, and a very uniform, closed and dense zinc phosphate coating can be obtained even on iron and aluminum surfaces. Therefore, in the method of the invention, it is also preferred that the series of components have a metallic iron surface or, in particular in the case of lightweight construction in car body construction, additional aluminum. In a particularly preferred embodiment, especially in car body construction, the components have metallic zinc, iron and aluminum surfaces adjacent to each other.

In the method of the present invention, it is preferred to contact the series of components with the acidic aqueous composition in step (ii) for at least a sufficient time to deposit a total weight of at least 1.0 g/ ^m2 on the zinc surface, in order to ensure that a sufficiently uniform closed zinc phosphate coating is formed on the entire metal surface of the component selected from zinc, iron or aluminum. Thus, the method of the present invention is preferred, in which a zinc phosphate layer having a layer weight of at least 1.0 g/ ^m2 , preferably at least 1.5 g/ ^m2, is formed on the zinc surface. The phosphating properties of the acidic aqueous composition for zinc phosphating in step (ii) are preferably maintained as a controlled variable in the method of the present invention, or the acidic aqueous composition has an inherent activation performance, so that it is also always ensured that the zinc surface of the component has a uniform closed dense crystalline zinc phosphate layer, the layer thickness of which is within a self-limiting range. Therefore, in the present invention, the total weight of the zinc phosphate layer on the zinc surface of the component, as required by the object spectrum, is preferably less than 5.0 g/ ^m2 , preferably less than 4.5 g/ ^m2 , particularly preferably less than 4.0 ^g /m2 and very particularly preferably less than 3.5 g/ ^m2 .

In the process of the present invention, a good coating base for subsequent dip-coating or powder-coating is prepared in the course of applying the substantially organic cover layer. Thus, in a preferred embodiment of the process of the present invention, the zinc phosphating in step (ii) with or without intermediate rinsing and/or drying steps, preferably with rinsing but without drying steps, is followed by dip-coating or powder-coating, particularly preferably electrodeposition coating, more particularly preferably cathodic electrodeposition coating, preferably comprising a dispersing resin comprising an amine-modified polyepoxide as well as a water-soluble or water-dispersible salt of yttrium and/or bismuth.

Exemplary embodiments In order to illustrate the advantages of the method according to the invention, the method sequence detailed below has been applied to the layer-forming phosphating of various metal substrates to illustrate its suitability for the treatment of components made up of a range of these metal substrates.

a) Alkaline cleaning with 0.8% Bonderite® M-FE 2020 MU, 1.2% Bonderite® M-AD ZN-2, 0.5% Bonderite® M-AD FE-1 and 0.5% Bonderite® C-AD 1561 (each of these are treatment compounds from Henkel AG & Co. KGaA) and 3.5% Bonderite® C-AK 2020-1 mixed with fully deionized water (κ<1 μScm ⁻¹ ) was applied in the proportions indicated. After setting the pH value to 11.8-11.9 and the temperature to 55° C., the sheets were first spray degreased at a pressure of 1 bar for 1 minute and then
a1) Variation without iron formation: Degreasing by immersion in the same cleaning solution for 3 minutes with stirring, or
a2) Variant with iron formation: Immersion degreasing was applied for 2 minutes in the indicated proportions by immersing the sheets under stirring in 0.8% by weight of Bonderite® M-FE 2020 MU, 0.7% by weight of Bonderite® M-AD ZN-2, 0.5% by weight of Bonderite® M-AD FE-1 and 0.5% by weight of Bonderite® C-AD 1561 (each of which is a treatment compound from Henkel AG & Co. KGaA) as well as 3.6% by weight of Bonderite® C-AK 2020-1 mixed with fully deionized water (κ<1 μS cm ⁻¹ ). The treatment was carried out after setting the pH value to 12.0 and the temperature to 60° C.

b) The substrate was then thoroughly washed with fully deionized water (κ<1 μScm ⁻¹ ) for approximately 1 min. Under these conditions, a closed water layer remained on the substrate for at least 30 s, indicating the absence of grease or oil on the substrate.

c) The substrate surface was then wetted with water and, without treatment in a separate activation bath, directly immersed in a hydroxylamine-accelerated phosphating bath based on fully deionized water (κ<1 μScm ⁻¹ ) and 4.6 wt. % Bonderite® M-Zn 1994 MU-1 and 1 wt. % Bonderite® M-AD 565 (each of these are processing compounds from Henkel AG & Co. KGaA), applying deionized water (κ<1 μScm ⁻¹ ) in the stated proportions for 3 minutes at 52° C. under stirring (free acid: 1.1 points, total acid: 26.5 points, zinc content: 0.13 wt. %, accelerator content: 0.1 wt. %). For this reason,
c1) 1 g/L of an aqueous zinc phosphate dispersion of Bonderite® M-AC 3000 (Henkel AG & Co. KGaA), prepared as described in the examples of WO 2021/104973 A1, was added (this corresponds to a proportion of 0.2 g/kg of granular zinc phosphate, based on the phosphating bath), or
c2) 3 g/L of an aqueous zinc phosphate dispersion of Bonderite® M-AC 3000 (Henkel AG & Co. KGaA), prepared as described in the examples of WO 2021/104973 A1, was added (this corresponds to a rate of 0.6 g/kg of granular zinc phosphate, based on the phosphating bath).

d) The substrate was then thoroughly washed under fully deionized water (κ<1 μScm ⁻¹ ) for approximately 1 minute.

e) Blow compressed air at room temperature and dry in an oven at 50°C.

The metal substrates coated according to this method sequence were sheets of cold rolled steel (CRS), electrolytically galvanized steel (EG), hot-dip galvanized steel (HDG), zinc-magnesium hot-dip galvanized steel (ZM) and aluminium (AA6014), which were cleaned after process step a1) or a2) and then pre-treated after the subsequent process steps b) to e).

In all cases, a closed, uniform zinc phosphate layer was formed.

In the process sequence including the iron application step (a2-b-c1-de/a2-b-c2-de), coating layers in the range of 82-105 mg/ ^m2 of iron were formed on HDG, EG and ZM sheets. The iron coating was quantitatively measured after pickling of the substrates with ₅ wt. % HNO3, followed by optical density measurements based on the formation of a colored thiocyanate complex.

In the method according to the invention, which comprises an iron application step prior to the activated zinc phosphating, a further reduction in the phosphate layer weight could be achieved on all galvanized sheets (see Table 1). This is advantageous both in terms of the process economy and in terms of the downstream electrocoating and the improved coverage that can be achieved there. At the same time, the layer weight formed on aluminum and cold-rolled steel sheets remains approximately constant or even increases slightly, making the method according to the invention suitable for zinc phosphating of components made of the aforementioned material mixtures.

Claims (15)

A method for rust prevention pretreatment of a series of a plurality of members, comprising the steps of:
A series of components each having at least a partially zinc surface are first subjected to a wet-chemical treatment step (i) for depositing iron on the zinc surface and then to a treatment step (ii) for zinc phosphating,
In the treatment step (i), a coating layer of elemental iron of at least 10 mg per ^m2 of zinc surface of the component is provided,
In the treatment step (ii), each component is treated to have a free acidity greater than zero points,
(A) 5-50 g/kg of phosphate, calculated as _PO4 , dissolved in water;
(B) 0.3 to 3 g/kg zinc ions;
(C) free fluoride; and (D) a water-dispersible granular component comprising a polyvalent metal cation phosphate, the phosphate being at least partially selected from hopeite, phosphophyllite, scholzite and/or hyulelite;
The acidic aqueous composition is obtained by adding the aqueous dispersion to an acidic aqueous composition containing components (A) to (C),
The aqueous dispersion is
at least one particulate inorganic compound (P1), consisting of a polyvalent metal cation phosphate at least partially selected from hopeite, phosphophyllite, scholzite and/or hyullite, and at least one organic polymeric compound (P2).
The method comprises the steps of: The method according to claim 1, characterized in that the aqueous dispersion for providing the acidic aqueous composition for the treatment step (ii) is added in an amount such that the weight proportion of phosphate from the granular component (P) of the aqueous dispersion is at least 0.004 g/kg, preferably at least 0.01 g/kg, particularly preferably at least 0.05 g/kg, very particularly preferably at least 0.08 g/kg, based on the acidic aqueous composition. An activator is added continuously or discontinuously to the acidic ^aqueous composition in the zinc phosphating process step (ii) comprising a particulate component (P) in water-dispersed form in an amount sufficient to maintain the properties of the acidic aqueous composition which, under the selected conditions of process step ( ⁱⁱ ), forms a zinc phosphate layer on the hot ^- dip galvanized steel surface (Z) with a layer weight of less than 5.0 g/ ^m2 , preferably less than 4.5 g/m2, particularly preferably less than 4.0 g/m2 and very particularly preferably less than 3.5 g/m2, the particulate component (P) being
at least one particulate inorganic compound (P1), consisting of a polyvalent metal cation phosphate at least partially selected from hopeite, phosphophyllite, scholzite and/or hyullite, and at least one organic polymeric compound (P2).
3. The method according to claim 1 or 2, characterized in that it comprises: The method according to any one of claims 1 to 3, characterized in that in process step (ii), the organic polymeric compound (P2) in the aqueous dispersion or in the granular component (P) of the activator is at least partially composed of styrene and/or an α-olefin having more than 5 carbon atoms, the organic polymeric compound (P2) further comprises maleic acid, its anhydride and/or imide units in its side chain, preferably further comprises polyoxyalkylene units, particularly preferably polyoxyalkylene units, and the organic polymeric compound (P2) further comprises preferably imidazole units in its side chain. The method according to any one of claims 1 to 4, characterized in that in process step (ii) the acidic aqueous composition for zinc phosphating has a pH of less than 3.6, preferably less than 3.4, particularly preferably less than 3.2, and the free acid is preferably more than 0.5 points, particularly preferably more than 0.8 points, more particularly preferably more than 1.0 points. The method according to any one of claims 1 to 5, characterized in that in process step (ii) the acidic aqueous composition for zinc phosphating contains a source of free fluoride, preferably at least 10 mg/kg, particularly preferably at least 40 mg/kg but preferably not more than 200 mg/kg of free fluoride. 7. The method according to claim ¹ , characterized in that in the wet-chemical treatment step (i) a coating layer based on elemental iron of at least 20 mg, preferably at least 40 mg, very particularly preferably at least 60 mg, but preferably less than 150 mg, particularly preferably less than 120 mg, per m 2 of zinc surface of the component is provided on the surface of the component formed by zinc. The method according to any of claims 1 to 7, characterized in that the wet-chemical treatment step (i) is preferably carried out by contacting at least the zinc surface of the series of components with an aqueous composition containing iron (II) ions and/or iron (III) ions, the proportion of iron ions dissolved in water being at least 50 mg/L, preferably at least 100 mg/L but preferably less than 10 mg/L in total of ionic compounds of the metals copper, nickel, cobalt, tin, manganese, molybdenum, chromium and/or cerium, in particular less than 1 mg/L of ionic compounds of the metals nickel and cobalt, in each case relative to the metal element, is contained in the aqueous composition. It has a pH value which is preferably 8.5 or more, particularly preferably 9.5 or more, very particularly preferably 10.5 or more, but preferably 13.5 or less, particularly preferably 12.5 or less, very particularly preferably 11.5 or less,
_{9. The} method according to claim ₈ , characterized in that the wet-chemical treatment step (i) is carried out by contact with an alkaline aqueous composition containing (a) at least 50 mg/L, preferably at least 100 mg/L, particularly preferably at least 200 mg/L, of iron(III) ions, and (b) at least 100 mg/L, calculated as _PO4 , of organic compounds (b1) having at least one functional group selected from COOX, OPO3X and/or PO3X, where X is either an H atom, an alkali metal atom and/or an alkaline earth metal atom, and/or a complexing agent selected from condensed phosphates (b2), which composition preferably has a free alkalinity of at least 1 point, but preferably less than 6 points. The method according to claim 9, characterized in that the alkaline aqueous composition further contains at least 100 mg/L, preferably at least 200 mg/L, particularly preferably at least 500 mg/L, but not more than 10 g/L, of phosphate ions, and the mass ratio of iron(III) ions to phosphate ions in the alkaline aqueous composition is in the range of 1:20 to 1:2. The method according to claim 9 or 10, characterized in that the molar ratio of all components (B) to iron(III) ions in the alkaline aqueous composition is more than 1:1, preferably at least 2:1, particularly preferably at least 5. The method according to any one of claims 1 to 11, characterized in that the series of components is not contacted with an aqueous colloidal solution containing hopeite, phosphophyllite, scholzite and/or hyullite, preferably polyvalent metal cation phosphates or sparingly soluble salts of element Ti in the granular component, prior to contact with the acidic aqueous composition in the process step (ii) for zinc phosphating, preferably is not contacted with an aqueous colloidal solution for activating the surface of the components for zinc phosphating, particularly preferably is not subjected to an activation step for activating the surface of the components for zinc phosphating, prior to contact with the acidic aqueous composition in the process step (ii). The method according to any one of claims 1 to 12, characterized in that before or together with treatment step (i), the series of parts is cleaned and optionally degreased in a cleaning step, in particular by contact with an aqueous, preferably alkaline, cleaning agent, and that a cleaning step is carried out immediately after treatment step (i), with or without an intermediate rinsing step, preferably without an intermediate rinsing step. The method according to any one of claims 1 to 13, characterized in that the series of parts further has a metallic aluminum surface, particularly preferably a metallic aluminum and iron surface. A method according to any of the preceding claims, characterised in that a zinc phosphate layer is formed on the zinc surface having a layer weight of at least 1.0 g/m ² , preferably at least 1.5 g/m ² .