Multi-Stage Treatment for Activated Zinc Phosphating of Metallic Components

This application is a continuation of International Patent Application No. PCT/EP2023/086847 filed Dec. 20, 2023, which claims priority to European Patent Application No. 23154613.6 filed Feb. 2, 2023.

The present invention relates to a process for the anti-corrosion pre-treatment of a plurality of components in series, in which each component in the series at least partly has surfaces made of zinc and/or iron and at least parts of these surfaces are firstly targetedly activated for the purpose of subsequent zinc phosphating. The targeted activation is achieved by the controlled dispensing of an aqueous dispersion to wet the aforementioned zinc and/or iron surfaces so as to ensure resource-efficient activation (“activation wetting”). The aqueous dispersion for activation wetting contains a particulate constituent dispersed in water, which is at least partially composed of hopeite, phosphophyllite, scholzite and/or hureaulite and provided as a dispersion of these crystalline solids, which is stabilized by at least one polymeric organic compound. The phosphating quality of the acidic, aqueous composition for zinc phosphating is in turn ensured and maintained by adding a quantity of an aqueous dispersion, in particular the same aqueous dispersion that is also used for activation wetting.

Layer-forming phosphating is a process for applying crystalline anti-corrosion coatings to metal surfaces, in particular to materials of the metals iron, zinc and aluminum, which has been used for decades and has been studied in depth. The zinc phosphating process established in particular for protection against corrosion is carried out in a layer thickness of a few micrometers and is based on the corrosive pickling of the metal material in an acidic aqueous composition containing zinc ions and phosphates, during which crystallites of low solubility form near the surface that precipitate directly on the boundary surface with the metal material where they continue to grow.

Zinc phosphating is usually set in such a way that homogeneous, closed and compact crystalline coatings are obtained on the surfaces of the metals iron, zinc and aluminum. Otherwise, good corrosion protection and a good coating base cannot be achieved. Homogeneous, closed coatings are usually reliably achieved during zinc phosphating above a layer weight of 2 g/m.

In order to achieve such homogeneous, closed coatings with a high degree of compactness or a high number density of phosphate crystallites, in the prior art zinc phosphating is usually initiated by activating the metal surfaces of the component to be phosphated. The activation is a wet-chemical process step, conventionally carried out by bringing said surfaces into contact with colloidal aqueous solutions of phosphates (“activation stage”), which, insofar as they are immobilized on the metal surface, serve as a nucleus for growth for the formation of the crystalline coating in the subsequent phosphating process such that a high number density of growing crystallites is brought about, and thus in turn a compact crystalline zinc phosphate layer is produced that has excellent corrosion protection and, due to its high electrical charge transfer resistance, also has excellent electrocoating properties.

Depending on the metal surface to be phosphated, it is necessary in the zinc phosphating stage not only to adjust the performance of the activation stage described above but also the pickling process by means of the concentration of the active components, which often requires intensive activation in the immersion process and, at the same time, sufficient pickling in the presence of fluoride ions in the phosphating stage, particularly for metal components such as car bodies that consist of a mix of different materials, in order to achieve correspondingly high layer weights on all surfaces of the component, in particular zinc, iron or steel and aluminum.

Very recently, however, a process was described in WO 2022/048963 A1 in which conventional activation of the surfaces prior to zinc phosphating can even be dispensed with, provided that the zinc phosphating treatment bath itself has an activating effect and thus contains a sufficient amount of phosphates in dispersed form.

Even though this process for zinc phosphating with integrated activation can be used to reliably phosphatize different metals to form layers, the proportion of dispersed phosphates that have an activating effect in the acidic, aqueous composition of the phosphating bath must be adapted each time to the components to be treated. When components made of different metals are treated to protect them against corrosion, the metal surface that is most challenging to activate will always determine the total consumption of active components, i.e. dispersed phosphates. Accepting less optimal activation for a lower consumption of dispersed phosphates in the activation stage or phosphating stage inevitably leads to higher phosphate layer weights, especially on zinc or iron surfaces, and thus comes at the expense of a higher consumption of active components during the phosphating stage.

There is therefore a need to be able to fully exploit the undoubtedly existing process-economic advantages of the integrated process in WO 2022/048963 A1, which consist of a reduced pretreatment sequence that can be controlled with less effort, and to reduce the consumption of active components as much as possible but not at the expense of phosphating quality, which is equivalent to the resulting homogeneous, closed and as finely crystalline as possible phosphate layers having a low layer weight, in particular on zinc and/or iron surfaces, very particularly on the zinc surfaces of the components to be treated. Such a resource-saving process with activating zinc phosphating must be suitable for sufficiently activating components composed of different metal materials to form layers with high phosphating quality despite different material-specific requirements.

The task profile has been solved in the present case by the selective and controlled dispensing of an aqueous dispersion for the activating wetting of metal surfaces before the actual integrated zinc phosphating process. The activating wetting of the zinc and/or iron surfaces surprisingly makes possible a significant reduction in the zinc phosphating layer weight with the same phosphating quality and thus an equally significantly reduced consumption of active components during zinc phosphating.

Aspect 1. A process for the anti-corrosion pretreatment of a plurality of components in series, wherein each component in the series at least partly has surfaces of zinc and/or iron and first undergoes a process step (i) for activating zinc and/or iron surfaces and immediately after a process step (ii) for zinc phosphating,

Aspect 2. The process according to Aspect 1, characterized in that the process of bringing at least the zinc and/or iron surfaces of the components into contact with said dispersion in process step (i) is carried out by dispensing the aqueous dispersion from a supply such that no more than 0.50 liter, preferably no more than 0.20 liter, of the aqueous dispersion are dispensed per square meter of the zinc and/or iron surfaces of the components in the series to be activated, which surfaces are to be brought into contact with said dispersion, preferably per square meter of the surface of a component in the series.

Aspect 3. The process according to any one of the preceding Aspects, characterized in that the aqueous agent to be brought into contact with said surfaces in process step (i) is dispensed such that at least the zinc and/or iron surfaces are covered by a liquid film containing the aqueous dispersion, wherein a volume-related coating per square meter of preferably no more than 1.00 liter, particularly preferably no more than 0.50 liter, very particularly preferably no more than 0.20 liter, and particularly preferably no more than 0.10 liter, results on the zinc and/or iron surfaces.

Aspect 4. The process according to any one of the preceding Aspects, characterized in that the aqueous agent is dispensed in process step (i) as a spray, as a spray mist or as a liquid film, preferably as a spray and/or spray mist, particularly preferably as a spray mist.

Aspect 5. The process according to any one of the preceding Aspects, characterized in that, in process step (i) and/or in process step (ii), the polymeric organic compound (P2) in the particulate constituent (P) of the aqueous dispersion is at least partly composed of styrene and/or an α-olefin having no more than 5 carbon atoms, wherein the polymeric organic compound (P2) additionally has units of maleic acid, the anhydride and/or the imide thereof, and preferably additionally polyoxyalkylene units, particularly preferably polyoxyalkylene units, in its side chains, which in turn are preferably at least partially end-capped with aliphatic alkyl groups having no more than four carbon atoms.

Aspect 6. The process according to Aspect 5, characterized in that the polymeric organic compound (P2) in the particulate constituent (P) of the aqueous dispersion additionally also contains imidazole units.

Aspect 7. The process according to any one of Aspects 5 and 6, characterized in that the proportion of polyoxyalkylene units in all the polymeric organic compounds (P2) is at least 40 wt. %, preferably at least 50 wt. %, but preferably does not exceed 70 wt. %.

Aspect 8. The process according to any one of the preceding Aspects, characterized in that, in process step (i) and/or process step (ii), the proportion of phosphates, calculated as PO4, contain in the at least one particulate inorganic compound (P1), in relation to the dispersed inorganic particulate constituent of the aqueous dispersion, is at least 25 wt. %, preferably at least 35 wt. %, particularly preferably at least 40 wt. %, very particularly preferably at least 45 wt. %.

Aspect 9. The process according to any one of the preceding Aspects, characterized in that, in process step (i) and/or process step (ii), the aqueous dispersion contains at least one thickener as a further component, which is preferably selected from urea urethane resins, preferably urea urethane resins that have an amine value of less than 8 mg KOH/g, particularly preferably of less than 5 mg KOH/g, very particularly preferably of less than 2 mg KOH/g.

Aspect 10. The process according to any one of the preceding Aspects, characterized in that the water-dispersed, particulate constituent (P) of the aqueous dispersion in process step (i) amounts to at least 0.060 g/kg, preferably at least 0.100 g/kg, but preferably no more than 5.0 g/kg, particularly preferably no more than 1.0 g/kg, in relation to the aqueous dispersion.

Aspect 11. The process according to any one of the preceding Aspects, characterized in that the aqueous dispersion in method step (i) for activating the zinc surfaces has a pH value above 6.0, preferably above 6.5, but preferably does not exceed a pH value of 9.0, particularly preferably of 8.5, very particularly preferably of 8.0 and particularly preferably of 7.5.

Aspect 12. The process according to any one of the preceding Aspects, characterized in that, in process step (ii), such an amount of the aqueous dispersion is added that the weight proportion of the phosphates of the water-dispersed particulate constituent (D) is, based on the acidic aqueous composition, at least 0.1 mg/kg, preferably at least 0.5 mg/kg, particularly preferably at least 1.0 mg/kg and very particularly preferably at least 2.0 mg/kg.

Aspect 13. The process according to any one of the preceding Aspects, characterized in that the acidic aqueous composition for zinc phosphating in process step (ii) has a pH value below 3.6, preferably below 3.4, particularly preferably below 3.2, wherein the free acid is preferably greater than 0.5 points, particularly preferably greater than 0.8 points, and particularly preferably greater than 1.0 points.

Aspect 14. The process according to any one of the preceding Aspects, characterized in that the components in the series at least partly have zinc surfaces and preferably also iron surfaces and very particularly preferably also aluminum surfaces.

Aspect 15. The process according to any one of the preceding Aspects, characterized in that the components in the series at least partly have zinc surfaces and preferably also iron surfaces and very particularly preferably also aluminum surfaces.

The present invention specifically relates to a process for the anti-corrosion pretreatment of a plurality of components in series, wherein each component in the series at least partly has zinc and/or iron surfaces and first undergoes a process step (i) for activating zinc and/or iron surfaces and immediately afterwards a process step (ii) for zinc phosphating,

A pretreatment in series occurs when the individual components in the series undergo process steps (i) and (ii) of zinc phosphating one after the other, and thus at separate times, in accordance with the process according to the invention and are therefore brought into contact with the corresponding aqueous compositions stored in system tanks in immediate succession, as intended. The system tank in process step (i) is the container in which the aqueous dispersion is held for the purpose of activating the zinc and/or iron surfaces by wetting, and the system tank in process step (ii) is correspondingly the container containing the acidic aqueous composition for zinc phosphating. The components can be brought into contact with the acidic aqueous composition in process step (ii) inside the system tank, for example by immersion, or outside the system tank, for example by spraying-on the bath acidic aqueous composition stored in the system tank. The zinc and/or iron surfaces of each component in the series are brought into contact with the aqueous dispersion in process step (i) by dispensing a defined volume of the dispersion from the supply onto the surfaces to be activated, preferably in such a way that the volume of aqueous dispersion dispensed once for each component is not returned to the system tank holding the dispersion, for example by wetting the surfaces to be activated outside the system tank from where the stored aqueous dispersion is dispensed for each component.

The components treated according to the present invention can be three-dimensional structures of any shape and design that originate from a manufacturing process, in particular also including semi-finished products such as strips, sheets, rods, pipes, etc., and composite structures assembled from said semi-finished products, the semi-finished products preferably being interconnected by means of adhesion, welding and/or flanging to form a composite structure.

The process according to the invention is particularly effective for producing compact, closed and crystalline phosphate coatings on the zinc surfaces such that preferred components in the series are those which at least have zinc surfaces. The process according to the invention is also well-suited for the layer-forming phosphating of aluminum such that even components having a mixed construction, e.g. automobile bodies, composed of the materials zinc, iron and aluminum can be phosphated effectively and in a resource-saving manner in accordance with the present invention. However, aluminum surfaces generally do not require pre-activation in process step (i) and sufficient layer formation occurs when the aluminum surfaces are brought into contact with the acidic aqueous composition in process step (ii). In a particular embodiment of the process according to the invention, the components in the series, which at least partially have zinc and/or iron surfaces, additionally also have surfaces of the metal aluminum, wherein the surfaces of the metal aluminum are preferably not brought into contact with the dispensed aqueous dispersion in process step (i) but are brought into contact with the acidic, aqueous composition in process step (ii).

In the context of the process according to the invention, a component has at least one surface made of zinc and/or iron if more than 50 at. % of the metal structure on this surface, up to a material penetration depth of at least one micrometer, is composed of zinc and/or iron. This frequently applies to components made of corresponding metal materials, insofar as more than 50 at. % of the metal materials are composed of zinc and/or iron as uniform materials. Components comprising surfaces of zinc are, however, also iron materials provided with metallic coatings, such as, for example, electrolytically galvanized or hot-dip galvanized steel, which can also be alloyed with iron (ZF), aluminum (ZA) and/or magnesium (ZM).

For the resource-saving operation of the corrosion-protective process pre-treatment based on zinc phosphating in step (ii), the invention provides that process step (ii) immediately follows the activation in step (i). In this way, on the one hand, the degree of activation of the zinc and/or iron surfaces of the components for the zinc phosphating stage is maximally maintained and, on the other hand, the zinc phosphating treatment stage is resharpened with essential—since they have an activating effect—particulate phosphates, since these are introduced into the phosphating stage via the wet film adhering to the component.

Accordingly, within the context of the present invention, the direct sequence of activation and zinc phosphating provided according to the invention means that the components undergo after the process step (i) without an intermediate rinsing step or other treatment step which either involves further contact, in particular between the zinc or iron surfaces of the components and an aqueous dispersion containing a water-dispersed, particulate constituent (P) in the manner of process step (i) or preferably contact, in particular between the zinc or iron surfaces of the components and an aqueous dispersion for activation for zinc phosphating or particularly preferably contact with an aqueous composition, wherein in each case preferably no drying step is carried out after process step (i) or before process step (ii). A rinsing step in this context can involve one or more immediately successive process steps which serve to remove as completely as possible soluble residues, particles and/or active components which inevitably remain on the surfaces of the components after they have been discharged from previous wet-chemical process steps, for example by rinsing with city water. A drying step in this context is a process of drying the components, caused by controllable technical provisions, for example by supplying heat or by means of a directed air supply.

A further advantage of the compact, closed and crystalline coatings accessible on all these metal surfaces using the process according to the invention is their excellent electrocoatability, by means of which high wrap-around behavior can be realized. In this respect, it is preferred if step (ii) is followed by electrocoating, particularly preferably cathodic electrocoating. In principle, the process can be accompanied by any kind of coating with an organic topcoat system that is customary in the prior art, in particular a powder coating, since an excellent undercoat is provided.

The aqueous dispersion is brought into contact in order to activate at least the zinc and/or iron surfaces by dispensing it from a supply. The dispensing of the aqueous dispersion from a supply for the contacting requires, within the meaning of the present invention, the use of a device for removing a volume of liquid from a supply, for example a container which holds a quantity of the aqueous dispersion sufficient for a plurality of components, and a device for dispensing the volume of liquid removed onto the surfaces of one or more components which are to be brought into contact therewith. Consequently the components are not brought into contact in the stored aqueous dispersion, i.e. not by immersion in the stored aqueous dispersion, but, for example, by direct application using rollers or by spraying/misting with a partial volume of the stored aqueous dispersion taken from the supply. Furthermore, according to the invention, the volume of the aqueous dispersion dispensed from the supply for contact is limited and should be less than 1.00 liter per square meter of the surface of the component or preferably only of the zinc and/or iron surfaces of the component. This ensures that a significantly greater liquid volume of the aqueous dispersion is not dispensed than would be required for complete wetting of the zinc and/or iron surfaces with a liquid film of the aqueous dispersion. It is therefore advantageous in principle if the aqueous dispersion is applied to the surfaces to be treated as effectively as possible and without any excess. In a preferred embodiment of the process according to the invention, contact with the zinc and/or iron surfaces is made by dispensing the aqueous dispersion from a supply such that no more than 0.50 liter, preferably no more than 0.20 liter, of the aqueous dispersion is dispensed per square meter of the surfaces of the component, preferably only the zinc and/or iron surfaces of the component that are to be activated and thus brought into contact therewith.

In this connection, for the surface-area-related volume of the aqueous dispersion dispensed, the surface area of a component in the series is the surface of the polyhedron that has 12 surfaces, preferably 6 surfaces, and is particularly preferably the cuboid which in each case completely encompasses the component and in so doing has the smallest surface area, each surface of the polyhedron touching the component at at least one point. If the component is an automobile body, its surface area in connection with the surface-area-related dispensing of the aqueous dispersion for conditioning purposes is preferably that of the cuboid that has the smallest surface area that completely encompasses the automobile body, each surface of the cuboid touching the automobile body at at least one point. In the preferred embodiment of the process according to the invention, the upper limit for the surface-area-related aqueous dispersion volume dispensed onto the zinc and/or iron surfaces is standardized. The geometric area of the surfaces of the component made of zinc and/or iron to be activated must then be taken into account. In the treatment of flat products such as strip steel, it can therefore be the entire outer surface of the flat product that already needs to be pre-activated in process step (i), whereas, in the series treatment of automobile bodies in the preferred embodiment, only those outer surfaces of the body that are made from strip steel after forming and joining need to be included, since often only these can be optimally phosphated to form layers and thus pre-activated correspondingly.

The dispensing of the aqueous dispersion for bringing it into contact with, and thus activating, the zinc and/or iron surfaces at the same time requires and demands that the quantity dispensed from the supply also at least partially reaches these surfaces. In a preferred embodiment, the dispensing of the aqueous dispersion for the contact in process step (i) for sufficient activation is therefore carried out so as to ensure that at least the zinc and/or iron surfaces are covered by a liquid film containing the aqueous dispersion, resulting in a volume-related coating of preferably no more than 1.00 liter, particularly preferably no more than 0.50 liter, very particularly preferably no more than 0.20 liter and particularly preferably no more than 0.10 liter per square meter on the zinc and/or iron surfaces. In contrast to the volume of the aqueous dispersion dispensed for the contact, the volume coating here does not refer to the surface of the component approximated by polyhedra but to the actual geometric surface of the zinc and/or iron surfaces of the components in the series, whereby the volume coating can be determined by differential weighing after blowing off the liquid film, assuming a density of the liquid adhering to the surfaces of 1 g/cmcan be determined.

It should be borne in mind that the components are often already wetted with a liquid film, for example formed by rinsing water from a rinsing step immediately preceding the activation, when they are transferred to the activation stage according to process step (i), before the contact with the zinc and/or iron surfaces then takes place according to the invention by absorption of liquid volumes of the aqueous dispersion in the wet film already adhering to these surfaces. Such a process variant can be particularly advantageous because the active components absorbed by the wet film adhering to the component are better absorbed by the pre-wetted surfaces of the component and then distributed more homogeneously thereon, which in turn promotes uniform activation for the subsequent zinc phosphating step.

If the device for dispensing and creating contact is alone sufficient to achieve as complete wetting of the zinc and/or iron surfaces to be activated as possible, it may again be advantageous for reasons of efficiency to remove the wet film adhering to the components from previous treatment steps immediately before process step (i) or immediately before the zone in which the aqueous dispersion is dispensed for bringing it into contact with the surfaces, for example by blowing off or wiping, in order to use as efficiently as possible only those aqueous dispersions whose particulate content is relatively low but still just enough to bring about the desired activation.

Whether or not a liquid film containing the aqueous dispersion is formed on the zinc and/or iron surfaces in process step (i) can be checked by means of fluorescent markers added to the aqueous dispersion supply. The detection can then be carried out by irradiation with UV light and correspondingly recording the fluorescence by means of suitable cameras, which make possible an imaging control of the wetting of the component surfaces with the aqueous dispersion. This is particularly helpful when components with complex surface geometries have to be pre-treated and the type of dispensing of the aqueous dispersion, e.g. the relative orientation and spacing of a spray lance with respect to the component, first has to be adjusted in an iterative process in such a way that the zinc and/or iron surfaces are brought into contact with the aqueous dispersion, in particular in such a way that these surfaces are covered with a liquid film containing the aqueous dispersion. The latter preferred condition does not have to be directly fulfilled by bringing the aqueous dispersion from the supply into contact with the surfaces, i.e. directly by the application device, but it is sufficient if, for example by rotating, pivoting or tilting the components, a liquid film containing the aqueous dispersion that is in contact with the zinc and/or iron surfaces is produced before process step (ii), i.e. before the components are introduced into the zinc phosphating process, preferably at least 5 seconds, particularly preferably at least 10 seconds, very particularly preferably at least 20 seconds, before bringing them into contact with the acidic aqueous composition in process step (ii).

For the controlled dispensing of the aqueous dispersion required in the process according to the invention for the activation wetting of the zinc and/or iron surfaces, it is advantageous and therefore further preferred if in process step (i) the aqueous dispersion is dispensed as a spray, as a spray mist or as a liquid film, particularly preferably as a spray and/or spray mist, particularly preferably as a spray mist. The aqueous dispersion is brought into contact with the surfaces of the component to be activated as a spray and/or spray mist by means of spraying and misting processes established in the prior art and can be carried out in a locally limited manner by means of a spraying lance, and/or in a manner encompassing the component, at least in part, by means of a spraying ring in which a plurality of atomizer nozzles can be installed. The spraying devices to be used for the dispensing of a spray and/or spray mist are, for example, pressure atomizers, rotary atomizers or two-substance atomizers. A liquid film can be applied to the component by direct application by means of rollers, cloths, brushes, paint brushes or similar tools for applying liquids, depending on the complexity and geometry of the components in the series.

Preferred controlled and efficient activation wetting with the aqueous dispersion is achieved by setting a spray that is targetedly directed at the zinc and/or iron surfaces to be wetted and/or by providing a spray mist through which the component is transported together with the conveyor frame and which, at a given volume flow, is realized over such a transport path that the surfaces of the component to be wetted are preferably exposed to a closed liquid film containing the aqueous dispersion before the component is brought into contact with the acidic aqueous composition for zinc phosphating in the process step (ii) that immediately follows.

For example, in order to dispense, for example, as much aqueous dispersion as is necessary for forming a liquid film that covers the surfaces of the component and thus for effective activation wetting, it is preferred according to the invention for the dispersion dispensed as the spray and/or the spray mist in process step (i) to have a mean droplet size of less than 100 μm, particularly preferably of less than 60 μm, particularly preferably of less than 40 μm. In the case of average droplet sizes below 40 μm, the aqueous dispersion is atomized so strongly that the boundary region to aerosols is exceeded and a spray mist is formed. If the aqueous dispersion is further atomized and the average droplet size is reduced, the droplets will increasingly be held in suspension and not follow gravity. The spray mist held in suspension is then also moved and possibly swirled due to the air masses displaced while the component is being transported through the spray chamber, and therefore a directed impact on the zinc and/or iron surfaces to be activated is more likely to be thwarted and the component surfaces are less evenly wetted by a liquid film. It is therefore preferred if the dispensed aqueous dispersion in process step (i) has a mean droplet size of not less than 5 μm, particularly preferably of not less than 10 μm.

It is also advantageous for the formation of a closed liquid film containing the aqueous dispersion on the surfaces of the components to be contacted if the spray and/or spray mist of the aqueous dispersion is dispensed in such a way that the average speed of the liquid droplets which have the average droplet size is less than 5 m/s, preferably less than 2 m/s, and particularly preferably less than 1 m/s. This applies in particular to sprays and/or spray mists whose average droplet size is less than 100 μm, particularly preferably less than 60 μm, particularly preferably less than 40 μm.

According to the invention, the average droplet size and average speed of the droplets of a spray or spray mist is determined at the location of the geometric center of gravity of the polyhedron surrounding the component, which is also used for determining the quantity of the agent that is dispensed per surface area of the component, as described above. The determination can be carried out by means of light scattering and the phase Doppler anemometry.

By means of the preferred embodiments mentioned here regarding how the aqueous dispersion can be dispensed for bringing it into contact with at least the zinc and/or iron surfaces, an extremely efficient process is available in which the amount of aqueous dispersion dispensed from the supply is essentially only applied to the zinc and/or iron surfaces of the components that are to be activated. At the same time, the proportion of the aqueous dispersion introduced by the component into the zinc phosphating treatment stage serves to at least partially compensate for the particulate proportion of the acidic aqueous composition for zinc phosphating that is consumed during the activated zinc phosphating process and removed from the zinc phosphating treatment stage. For the same purpose, in process step (i), the proportion of the aqueous dispersion components which are dispensed but do not remain on the component can also be combined and transferred to the zinc phosphating treatment stage in order to maintain the activation performance. Accordingly, a process is preferred according to the invention in which the proportions of the aqueous dispersion which are dispensed in process step (i) to be brought into contact with at least the zinc and/or iron surfaces of the components, but which do not remain on the component as a wet film until they are brought into contact with the acidic aqueous composition for zinc phosphating in process step (ii), because they sink to the bottom as excess spray, for example, or run off the component and thus remain in the spray chamber in process step (i), are at least partially combined and added to the acidic aqueous composition in process step (ii), and in any case are preferably neither partially nor completely returned to the supply.

For a sufficient pre-activation of at least the zinc and/or iron surfaces of the components in the series, it is necessary for the aqueous dispersion used to contain a water-dispersed, particulate constituent (P) composed of phosphates of polyvalent metal cations (P1) and a polymeric organic compound (P2) contributing to stabilizing the dispersion.

It should be emphasized as this point that the preferred proportion of phosphates, calculated as PO, contained in the at least one particulate inorganic compound (P1) is, based on the dispersed inorganic particulate constituent (P1) of the aqueous dispersion, at least 25 wt. %, particularly preferably at least 35 wt. %, particularly preferably at least 40 wt. %, very particularly preferably at least 45 wt. %. Further preferred embodiments of the inorganic particulate constituent (P1) can, as already explained, be taken from the corresponding preferred embodiments of the inorganic particulate constituent (P1) of the aqueous dispersion in process step (ii).

It should also be emphasized that, for excellent dispersion stability, the polymeric organic compound (P2) in the particulate constituent (P) of the aqueous dispersion is at least partly composed of styrene and/or an α-olefin having no more than 5 carbon atoms, wherein the polymeric organic compound (P2) additionally has units of maleic acid, its anhydride and/or its imide, and preferably additionally polyoxyalkylene units, particularly preferably polyoxyalkylene units in its side chains which are in turn preferably at least partially end-capped with aliphatic alkyl groups having no more than four carbon atoms. Furthermore, it is particularly advantageous if the polymeric organic compound (P2) in the particulate component (P) of the aqueous dispersion additionally comprises imidazole units. The proportion of polyoxyalkylene units in the polymeric organic compounds (P2) as a whole is preferably at least 40 wt. %, particularly preferably at least 50 wt. %, but is preferably not above 70 wt. %. Further preferred embodiments of the polymeric organic compound (P2) can, as already explained, be taken from the corresponding preferred embodiments of the polymeric organic compound (P2) of the aqueous dispersion in process step (ii).

Besides and in addition to the aforementioned embodiments of the particulate component (P) of the aqueous dispersion in process step (i), the presence of a thickener is advantageous for providing a stable dispersion which can be stored in the system tank of process step (i) over a relatively long period of time. In a preferred embodiment of the process according to the invention, the aqueous dispersion in process step (i) therefore contains at least one thickener as a further component, which is preferably selected from urea urethane resins, particularly preferably from urea urethane resins which have an amine value of less than 8 mg KOH/g, preferably of less than 5 mg KOH/g, particularly preferably of less than 2 mg KOH/g, Further preferred embodiments of the thickener are described in connection with the aqueous dispersion added in process step (ii) of zinc phosphating, which are also advantageous and also included here as preferred embodiments with regard to the aqueous dispersions used for pre-activation.

Further preferred embodiments of the aqueous dispersion containing the water-dispersed, particulate constituent (P) used in step (i) according to the invention can be found in the description of particularly suitable activating aids.