Method for the corrosion-protective pretreatment of components comprising surfaces of zinc-magnesium hot-dip coated steel

Abstract

The present invention relates to a method for the corrosion-protective pretreatment of a plurality of components comprising surfaces of zinc-magnesium hot-dip coated steel, in which the components each successively pass through a degreasing stage, a conversion stage and a painting stage. The conversion stage based on an acidic aqueous conversion solution of compounds of the elements Zr and / or Ti dissolved in water is adjusted in terms of pH value and free fluoride content so that homogeneous conversion layers are produced on the zinc-magnesium hot-dip coated steel, which reliably provide high paint adhesion regardless of the degree of soiling of the degreasing stage.

Description

[0001] .Process for the corrosion-protective pretreatment of components comprising surfaces of zinc-magnesium hot-dip coated steel"

[0002] The present invention relates to a method for the corrosion-protective pretreatment of a plurality of components comprising surfaces of zinc-magnesium hot-dip coated steel, in which the components each successively undergo a degreasing step, a conversion step, and a painting step. The conversion step, based on an acidic aqueous conversion solution of compounds of the elements Zr and / or Ti dissolved in water, is adjusted with regard to pH and free fluoride content such that homogeneous conversion layers are produced on the zinc-magnesium hot-dip coated steel, which reliably provide high paint adhesion regardless of the degree of contamination during the degreasing step.

[0003] In automotive manufacturing, the use of magnesium-alloyed zinc coatings on steel is gaining importance due to the growing demand for lightweight car bodies. Compared to other hot-dip galvanizing processes, a zinc and magnesium coating can provide significantly increased corrosion protection and, particularly after coating with organic topcoats and dip coatings, outstanding resistance to corrosive delamination. This improved property profile allows coatings to be provided in thinner layers that still meet the high requirements for repaintability and corrosion protection.The good properties of magnesium alloyed zinc coatings in terms of corrosion behavior, especially in edge corrosion protection and paint adhesion on formed components, combined with excellent compatibility with all common joining processes and the previously mentioned weight savings, make hot-dip galvanized (ZM) sheet steel a material of particular importance for the production of lightweight car bodies, so that the surface area of ​​this material in the car body will continue to increase alongside the surface area of ​​other light metals such as aluminum in automotive production.

[0004] Magnesium-alloyed zinc coatings on steel are commonly used in automotive manufacturing as flat strip products and hot-dip galvanized (ZM) steel strip. This type of zinc-magnesium hot-dip coated steel contains approximately 1.5 to 8 wt.% of the metals aluminum and magnesium in the metallic coating, with the magnesium content being at least 0.2 wt.%.The basic suitability of these coatings to be formed, pretreated and coated using conventional and state-of-the-art processes is generally recognized and proven (Characteristic Properties 095 E, “Continuously Hot-Dip Coated Steel Strip and Sheet”, Chapters 8 and 10, 2017 edition, German Steel Association). However, due to the special composition of the coating and the native oxide layer, special features arise which must be taken into account, particularly during cleaning and pretreatment, to achieve the most homogeneous and reproducible coating result and thus optimal corrosion protection behavior or the desired surface functionality.

[0005] For example, it is known from the prior art that during cleaning prior to corrosion-protective pretreatment of hot-dip galvanized (ZM) strip steel, a change in the oxide content of the alloying component magnesium may be necessary for sufficient adhesion to a subsequently applied coating. US 2016 / 0010216 A1 reports that the extensive removal of magnesium oxide in the near-surface oxide layer of hot-dip galvanized (ZM) strip steel can effectively suppress the appearance of blister-like elevations in the topcoat.

[0006] It is also documented in the prior art that during the serial pretreatment of a large number of components, the wetting of hot-dip galvanized (ZM) strip steel and thus the reliable cleaning of the material surfaces can be problematic. WO 2023 / 036889 A1 therefore proposes conditioning the hot-dip galvanized (ZM) surfaces after degreasing but before a corrosion-protective pretreatment, which can be a conversion coating based on the elements Zr and / or Ti, in order to counteract the aging of conventional cleaning and degreasing baths associated with high component throughput and the resulting deterioration in the wettability of the hot-dip galvanized (ZM) surfaces.

[0007] Materials with zinc-magnesium coatings therefore require sophisticated process control in order to be successfully and, above all, reliably pretreated to provide corrosion protection, especially when a large number of components need to be coated to a high quality in an automated painting line. Further approaches are needed here that support simpler process control during corrosion protection pretreatment, comprising the stages of degreasing, conversion layer formation, and painting, and that contribute to the series treatment of a large number of components, ensuring that satisfactory results are achieved largely independently of bath aging, thus exploiting the full potential of zinc-magnesium hot-dip coated materials in terms of corrosion protection. The prior art approaches of intensive bath maintenance to ensure cleaning andKeeping degreasing baths as free from contamination as possible and replacing them with fresh bath solutions as early as possible is economically disadvantageous and problematic with regard to the desirable resource-saving use of process chemicals.

[0008] The present invention therefore has the object of establishing a process for the series treatment of a large number of components which is suitable for reliably pretreating zinc-magnesium hot-dip coated steel to protect it from corrosion using a conventional process sequence. For this purpose, a conversion coating based on the elements Zr and / or Ti is used, which on the one hand can provide an excellent paint adhesion base for the metallic material mix commonly used in automotive production, but on the other hand has also proven to be particularly susceptible to declining performance in stationary operation of a pretreatment line when treating zinc-magnesium hot-dip coated steel.

[0009] Surprisingly, it has now been discovered that a specific load of the degreasing bath with non-polar hydrocarbons, which are introduced into the bath by components contaminated with anti-corrosive oils, forming oils, and drawing greases, above a critical value, leads to an initially stable subsequent conversion treatment based on the elements Zr and / or Ti no longer delivering the desired result. After the paint layer has been applied, a significant loss of paint adhesion is observed, specifically on steel substrates with a zinc-magnesium coating. The present invention addresses this problem not by modifying process steps upstream of the conversion treatment, but rather, surprisingly, through the property profile of the conversion treatment itself and by adjusting the free fluoride content.

[0010] Specifically, the present invention relates to a process for the corrosion-protective pretreatment of a large number of components in series, in which the components of the series at least partially have surfaces of zinc-magnesium hot-dip coated steel, and in which the components of the series each undergo the successive process steps i) - iii) and at least the surfaces of the zinc-magnesium hot-dip coated steel are brought into contact one after the other with the respectively provided aqueous solutions (I) - (III): i) degreasing step providing an alkaline aqueous composition (I) with a pH above 9.00; ii) conversion step providing an acidic aqueous composition (II) which has a pH in the range of 3.50 to 5.20, containing at least 0.05 mmol / kg of compounds of the elements Zr and / or Ti dissolved in water and at least 2.80 mmol / kg of free fluoride;and iii) coating step providing an aqueous dispersion (III) of an organic binder.;

[0011] A series of corrosion-protective pretreatment of components occurs when a large number of components are brought into contact with the treatment solution provided in the respective treatment stages of the process according to the invention and typically stored in system tanks. The contacting of the individual components takes place sequentially and thus at different times. The system tank is the container in which the treatment solution is located for the purpose of series-based corrosion-protective pretreatment.

[0012] According to the invention, the components comprise steel material provided with a metallic zinc-magnesium coating, the coating being applied from a melt of the alloy components. Such hot-dip galvanizing processes are known in the art collectively as hot-dip galvanized (ZM) steel and represent metallic coatings containing 1.5 to 8 wt.% of the metals aluminum and magnesium, with the proportion of magnesium in the metallic coating preferably being at least 0.2 wt.%. In the following, the term hot-dip galvanized (ZM) steel is used synonymously with zinc-magnesium hot-dip coated steel.

[0013] Corrosion-protective pretreatment within the meaning of the present invention always relates to the pretreatment of the surfaces of the series components formed by the metallic materials. The material can be a uniform material or a coating. For example, galvanized steel grades according to the invention consist of both steel and zinc, whereby at the cut edges and grinding points of, for example, an automobile body made of galvanized steel, steel surfaces can be exposed, and according to the invention, a pretreatment of the steel material then takes place. Therefore, if the present invention refers to the pretreatment of a component composed of a specific metallic material, this includes all materials and coatings that contain more than 50 at.% of the respective element of the named material.A component with a galvanized coating therefore contains more than 50 at.% zinc in the metallic coating.

[0014] The process according to the invention is not limited to hot-dip galvanized (ZM) steel, so that the substrates commonly provided by the steel industry, such as steel, in particular cold-rolled steel (CRS), as well as electrolytically galvanized (ZE) or hot-dip galvanized (Z), alloy galvanized, in particular (ZF), (ZA), or aluminum-coated (AZ), (AS) steel, can also be considered as additional components of the components. Light metals such as aluminum and magnesium, as well as their alloys, can also be treated in the process according to the invention together with the hot-dip galvanized (ZM) steel of the component and can be cleaned and / or pretreated to protect against corrosion.The method according to the invention is characterized precisely by the fact that it is suitable for pretreating common metallic materials composed of iron, zinc, aluminum and magnesium to protect them from corrosion, i.e. for providing them with a conversion coating that provides a good paint adhesion base.

[0015] Particularly preferred is an embodiment in which the components of the series are composed of galvanized steel, steel and / or aluminum in addition to hot-dip galvanized (ZM) steel. The suitability of the method according to the invention for this material mix to provide a good corrosion-protective pretreatment consisting of cleaning, conversion layer formation and painting is particularly advantageous for components that are manufactured in a composite construction and are assembled from different semi-finished products. According to the invention, therefore, preferred is a method in which the components of the series represent composite structures, preferably automobile bodies, that are composed of semi-finished products of hot-dip galvanized (ZM) steel and of semi-finished products of galvanized steel and aluminum, particularly preferably of semi-finished products of hot-dip galvanized (ZM) steel and of semi-finished products of galvanized steel, aluminum and steel.

[0016] The components pretreated according to the present invention can be any spatial structure of any shape and design originating from a manufacturing process, in particular semi-finished products such as strips, sheets, rods, pipes, etc., and composite structures assembled from the aforementioned semi-finished products. The composite structures assembled from different materials are usually in the form of cut, formed, and joined flat products by welding, gluing, and flanging. The components to be pretreated in series according to the present invention are preferably selected from automobile bodies or parts thereof, heat exchangers, profiles, pipes, tanks, or tubs.

[0017] Insofar as in the context of the present invention the concentration of an active component or compound is stated as an amount of substance per kilogram, this is the amount of substance based on the weight of the respective total composition.

[0018] With regard to the process steps, a composition (I)-(II) or dispersion (III) is considered to be "provided" within the meaning of the process according to the invention if, as specified in the respective treatment stage (i)-(iii), it is either stored in system tanks and kept ready for application for bringing into contact or is realized as defined during bringing into contact.

[0019] The multi-stage pretreatment according to the present invention ensures that, for a wide variety of components to be treated, conversion coatings based on Zr and / or Ti are produced in the conversion stage. These coatings provide good paint adhesion and thus reliably protect against corrosive delamination of the entire paint coating system. This is achieved by adjusting the free fluoride content within the specified pH range, regardless of the load of the degreasing stage with contaminants removed from the component surfaces.In particular, the proportion of non-polar hydrocarbons has proven critical in the sense that, above a certain steady-state proportion of hydrocarbons introduced into the degreasing bath, a significant deterioration in paint adhesion on the hot-dip galvanized (ZM) surfaces of the correspondingly pretreated components results if the specified proportion of free fluoride is not met in the conversion stage. It has also been shown that the amount of free fluoride required to overcome a loss of paint adhesion on the hot-dip galvanized (ZM) surfaces that occurs during the series treatment of components contaminated with grease and oil depends less on the actual load of non-polar components in the degreasing bath than on the pH value of the conversion stage.At lower pH values, a larger amount of free fluoride will tend to be required to compensate for the adverse effect on the hot-dip galvanized (ZM) surfaces, which would otherwise inevitably occur during series treatment due to the gradual build-up of a stationary proportion of hydrocarbons in the degreasing stage.

[0020] Degreasing level:

[0021] In the degreasing stage, an alkaline aqueous composition (I) with a pH value above 9.00 is provided for cleaning and degreasing the components of the series. The aim of the degreasing stage is to ensure that the component surface is largely free of inorganic salts and organic contaminants, in particular drawing, forming, rolling, and corrosion protection oils, for subsequent successful corrosion protection pretreatment consisting of the conversion and painting stages. In a preferred embodiment, immediately after passing through the degreasing stage, i.e. before the conversion stage, but after a rinsing step with deionized water (K ​​< I pScrrr 1 ), on the surface of the components of the series formed by the metallic materials, a carbon coating of less than 0.20 g / m 2 , particularly preferably less than 0.10 g / m 2, whereas the surface of the components of the series formed by the metallic materials previously, i.e. before passing through process step i), rather than immediately before being brought into contact with the alkaline aqueous composition (I), preferably has a carbon coating of at least 0.50 g / m 2 which originates from the aforementioned organic contamination. The carbon layer remaining on the surface of the component formed by the metallic materials can be determined by pyrolytic decomposition. For this purpose, a representative component section of a defined area is heated to a substrate temperature (PMT) of 550°C in an oxygen atmosphere, and the amount of released carbon dioxide is quantitatively measured as the amount of carbon using an infrared sensor, for example, using the LECO® RC-412 Multiphase Carbon Determinator (Leco Corp.).

[0022] In continuous operation, i.e., when treating a large number of components in a series, the soils absorbed by the alkaline aqueous cleaner accumulate in the degreasing bath. It has now been determined that, in the process according to the invention, paint adhesion to hot-dip galvanized (ZM) surfaces can be maintained even when the proportion of non-polar hydrocarbons in the degreasing bath exceeds a critical threshold. The process according to the invention is therefore advantageous when the critical threshold is reached and exceeded in the degreasing bath. Even above a proportion of 0.05 kg / m 3The presence of non-polar hydrocarbons in the degreasing bath can lead to a significant deterioration in the paint adhesion of the hot-dip galvanized (ZM) steel if a corrosion-protective pretreatment as described in conversion stage ii) is not carried out as specified in the invention. Therefore, the process according to the invention achieves its full effectiveness as soon as non-polar hydrocarbons have accumulated in the degreasing bath due to the serial treatment of a large number of components, preferably at least 0.05 kg / m 3 , particularly preferably at least 0.10 kg / m 3 , in particular at least 0.20 kg / m 3of non-polar hydrocarbons. The proportion of non-polar hydrocarbons in the degreasing bath can be determined in a sample of the degreasing bath adjusted to hydrochloric acid (methyl orange transition point), to which an aliquot part (1 / 10) of sodium chloride and an aliquot part (1 / 4) of ethanol have been added. From this prepared sample of the degreasing bath, the hydrocarbon content is extracted by shaking with an aliquot part (1 / 1) of petroleum ether. After phase separation, alternatively achieved by successive addition of ethanol, the petroleum ether phase is mixed with silica gel to separate polar organic components such as fatty acids, acid esters, and nonionic surfactants. After filtration, the proportion of non-polar hydrocarbons can be determined gravimetrically after distilling off the petroleum ether.

[0023] The critical threshold for non-polar hydrocarbons is only reached after a certain number of components have been processed. In this context, it is preferred that the series of components includes a number of components whose total surface area formed by the metallic materials of the components is greater than the following term:

[0024] VB ■ KW crit Um T0C ' 1 / 2

[0025] VB: Volume of the system tank of the degreasing stage in m 3

[0026] KWcrit: critical threshold value for the proportion of non-polar hydrocarbons in the system tank of the degreasing stage in kg / m 3 , where the critical threshold is 0.05 kg / m 3 , preferably 0.1 kg / m 3 , particularly preferably 0.2 kg / m 3 amounts

[0027] Amroc: Change in the area-related carbon content on the surfaces of the components formed by the metallic materials after the degreasing stage in kg / m 2

[0028] The aforementioned properties of the degreasing stage, particularly with regard to the proportion of non-polar hydrocarbons therein, always refer to the last degreasing stage, which precedes the conversion stage. In industry, it is quite common for multiple degreasing stages to be used, on the one hand, to sufficiently remove contaminants from the components to be pretreated and, on the other hand, to optimally condition the metal substrates for the subsequent conversion stage by pickling. Therefore, when determining the critical threshold value for non-polar hydrocarbons, according to the aforementioned term, the amount of hydrocarbons in the volume of the system tank of the last degreasing stage must be taken into account. Similarly, when changing the area-related carbon content on the surfaces of the components formed by the metallic materials, the change after the last degreasing stage must be taken into account.The pH of the alkaline aqueous composition (I) is preferably at least 9.50, more preferably at least 10.5, for good degreasing performance. However, only a slight to moderate pickling of the zinc-magnesium coating may be advantageous for maintaining the good corrosion-protective properties of the (ZM) substrate, so that the pH is preferably less than 12.50, more preferably less than 12.00, and most preferably less than 11.50.

[0029] In the context of the degreasing stage, the pH value corresponds according to the invention to the negative decadic logarithm of the hydronium ion activity measured in the alkaline aqueous composition (I) at a temperature of 20 °C using a pH-sensitive glass electrode after two-point calibration against technical buffer solutions of acetic acid / acetate (pH = 4.0) and boric acid / borate (pH = 9.0).

[0030] Preferably, the alkaline aqueous cleaner is provided with a specific buffer capacity so that in the process according to the invention it has a total alkalinity in points of at least 5.0, particularly preferably of at least 8.0, very particularly preferably of at least 10.0, but preferably a total alkalinity of 30.0, particularly preferably of 20.0 is not exceeded. The total alkalinity corresponds to the consumption of 0.1 N hydrochloric acid in milliliters after titration of a solution of 50 ml of deionized water (K ​​< 1 pScrrr 1 ) diluted sample volume of 10 ml of the alkaline aqueous composition (I) in the presence of the indicator bromocresol green (transition point: pH 3.6) at a temperature of 20 °C.

[0031] To adjust the alkalinity of the alkaline aqueous composition (I), any builders known in the art that represent alkaline-reacting compounds or a mixture of such compounds can be used. Particularly suitable and established builders are alkaline-reacting inorganic compounds, which are preferred in the context of the present invention and are particularly preferably selected from water-soluble hydroxides, carbonates, borates, silicates, and / or phosphates, with at least water-soluble phosphates preferably being present, which in turn are preferably selected from orthophosphates, pyrophosphates, and / or tripolyphosphates.Suitable builders are therefore alkali metal carbonates, preferably selected from potassium carbonate, mixtures of alkali metal hydroxides, preferably selected from potassium hydroxide, with phosphoric acid and / or with boric acid and alkali metal tripolyphosphates, preferably selected from potassium tripolyphosphate.

[0032] The alkaline aqueous composition (I) contains at least one surface-active substance, preferably selected from surfactants, for effective degreasing. Surfactants within the meaning of the present invention are considered to be surface-active organic compounds that, for their surface activity, are composed of a hydrophilic and at least one lipophilic molecular component or of a lipophilic and at least one hydrophilic molecular component, wherein the molecular weight of the surface-active organic compound does not exceed 2000 g / mol.

[0033] The surfactants used in the degreasing step in process step i) of the process according to the invention can be selected from anionic surfactants, cationic surfactants, zwitterionic surfactants, and nonionic surfactants, with the use of nonionic surfactants generally being preferred. Particularly suitable nonionic surfactants as constituents of the alkaline aqueous composition (I) for degreasing components comprising hot-dip galvanized (ZM) surfaces are those whose HLB value (hydrophilic-lipophilic balance) is at least 8, more preferably at least 10, especially preferably at least 12, but particularly preferably not more than 18, especially preferably not more than 16. The HLB value serves as a quantitative reference value for classifying nonionic surfactants with regard to their miscibility with water or their ability to form O / W emulsions. For quantification, the nonionic surfactant is broken down into a lipophilic and a hydrophilic group.The HLB value is then calculated as follows and can take values ​​from zero to 20 on the arbitrary scale:.

[0034] HLB = 20 (1 -ML / M) with Mi.: Molar mass of the lypophilic group of the nonionic surfactant

[0035] M: Molar mass of the nonionic surfactant

[0036] In terms of material, nonionic surfactants selected from alkoxylated alkyl alcohols, alkoxylated fatty amines, and / or alkyl polyglycosides are preferred in the degreasing stage of the process according to the invention, particularly preferably from alkoxylated alkyl alcohols and / or alkoxylated fatty amines, especially preferably from alkoxylated alkyl alcohols. The alkoxylated alkyl alcohols and / or alkoxylated fatty amines are preferably end-capped for a defoaming effect, particularly preferably with an alkyl group, which in turn preferably has no more than 8 carbon atoms, particularly preferably no more than 4 carbon atoms.Particularly preferably, those alkoxylated alkyl alcohols and / or alkoxylated fatty amines which are present in ethoxylated and / or propoxylated form are used as nonionic surfactants in the defatting stage of the process according to the invention, wherein the number of alkylene oxide units is preferably not greater than 16 in total, particularly preferably not greater than 12, especially preferably not greater than 10, but particularly preferably greater than 4, especially preferably greater than 6.

[0037] With regard to the lipophilic component of the aforementioned nonionic surfactants, those alkoxylated alkyl alcohols and / or alkoxylated fatty amines are preferred as nonionic surfactants in the defatting stage of the process according to the invention whose alkyl group is saturated and preferably unbranched, wherein the number of carbon atoms in the alkyl group is preferably greater than 6, particularly preferably at least 10, especially preferably at least 12, but preferably not greater than 20, particularly preferably not greater than 18, especially preferably not greater than 16.

[0038] Overall, it can be seen that longer-chain nonionic surfactants are very well suited and preferable for effective cleaning and degreasing with conventional drawing, forming, rolling and corrosion protection oils, so that in a further preferred embodiment of the process according to the invention, as a surfactant component of the alkaline aqueous composition (I), preference is given to those alkoxylated alkyl alcohols and / or alkoxylated fatty amines, in particular the alkoxylated alkyl alcohols, whose lipophilic alkyl group comprises at least 10 carbon atoms, particularly preferably at least 12 carbon atoms, wherein the longest carbon chain in the alkyl group consists of at least 8 carbon atoms and an HLB value in the range from 12 to 16 is realized.

[0039] Preferred representatives of the alkoxylated alkyl alcohols are selected, for example, from

[0040] - four to eight times ethoxylated or propoxylated C6-C12 fatty alcohols,

[0041] - eight to twelve times ethoxylated C12-C18 fatty alcohols,

[0042] - six to fourteen times propoxylated C12-C18 fatty alcohols,

[0043] - six to ten times ethoxylated and propoxylated C12-C14 fatty alcohols, which in turn can be end-capped with methyl, butyl or benzyl groups.

[0044] Another suitable selection criterion for the nonionic surfactant to be used in the degreasing stage, which is selected from alkoxylated alkyl alcohols, alkoxylated fatty amines and / or alkyl polyglycosides, is the cloud point determined according to DIN 53 917 (1981), which is preferably above 20°C, but particularly preferably below the application temperature of the alkaline aqueous composition (I) in the degreasing stage, particularly preferably more than 5°C, but not more than 10°C below the respectively selected application temperature of the alkaline aqueous composition (I) for degreasing.

[0045] The proportion of surfactants, in particular nonionic surfactants, in the alkaline aqueous composition (I) of the degreasing stage is preferably above 0.01% by weight, particularly preferably above 0.10% by weight, especially preferably above 0.20% by weight, but preferably not above 2.00% by weight, in each case based on the alkaline aqueous composition (I).

[0046] Furthermore, the alkaline aqueous composition (I) preferably contains alkaline builders selected from phosphates, pyrophosphates, phosphoric acid, silicates, carbonates and hydroxides, as well as mixtures of these builder substances, to provide its alkalinity. For a resource-saving process and for reasons of economic efficiency, it is preferred if the alkaline aqueous composition (I) of the conversion stage in a process according to the invention

[0047] (a) less than 10 mg / kg of compounds of the metals Bi, Ni, Co and / or Cu dissolved in water, calculated as the amount of the respective element in the aqueous composition, preferably less than 10 mg / kg of compounds of such metals dissolved in water which have a standard reduction potential greater than -0.40 V (SHE), calculated as the amount of the respective element in the aqueous composition,

[0048] (b) a total of less than 0.01 g / L (calculated as solids addition) of copolymers which contain, in alternating configuration, monomer units which have at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group and monomer units which do not have an acid group, preferably a total of less than 0.1 g / L (calculated as solids addition) of copolymers which contain monomer units which have at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group, particularly preferably less than 0.1 g / L of organic polymers which do not represent any of the aforementioned surface-active substances with a molecular weight of not more than 2000 g / mol.

[0049] The application and thus the contacting of the alkaline aqueous composition (I) preferably takes place at at least 30°C, particularly preferably at at least 40°C, but preferably below 60°C. The alkaline aqueous composition (I) of the degreasing stage can be brought into contact with the components of the series using application methods established in the prior art. These include, in particular, immersion, rinsing, spraying, and / or atomizing, whereby the immersion of the components of the series into a system tank of the degreasing stage containing the corresponding alkaline aqueous composition (I) takes place and / or by spraying on the alkaline aqueous composition (I) stored in a system tank. As already discussed above, the conversion stage can be preceded by several degreasing stages, which in turn are fed with an alkaline composition from their own system tanks.

[0050] Conversion level:

[0051] According to the invention, the conversion treatment in process step ii) must be carried out with acidic aqueous compositions containing at least 2.80 mmol / kg of free fluoride, since otherwise it is generally not guaranteed that the resulting paint adhesion on the hot-dip galvanized (ZM) surfaces will be largely indifferent to the non-polar hydrocarbons enriched in the degreasing stage.

[0052] However, it has been shown that higher proportions of free fluoride for paint adhesion on the hot-dip galvanized (ZM) surfaces of the components treated in the process according to the invention do not have a detrimental effect on corrosion protection. At the same time, however, even moderate increases in the free fluoride content further increase the reliability of the process according to the invention by tolerating even significant increases in the proportion of non-polar hydrocarbons enriched in the degreasing bath without any loss of performance. In preferred embodiments, the acidic aqueous composition (II) of the conversion stage in process step ii) therefore contains at least 3.00 mmol / kg, particularly preferably at least 3.20 mmol / kg, and most preferably at least 3.40 mmol / kg of free fluoride.However, for reasons of compatibility of the process according to the invention with other metallic materials, in particular steel, it is preferred if the proportion of free fluoride in the acidic aqueous composition (II) is less than 7.50 mmol / kg, particularly preferably less than 6.00 mmol / kg and most preferably less than 5.00 mmol / kg.Since technical components made of galvanized (ZM) steel, which are treated in series for corrosion protection according to the method according to the invention, usually also have steel surfaces, for example on the cut edges or ground-through points of the semi-finished products made of galvanized (ZM) steel, which are present, for example, as cut sheets and formed into body components, a limitation of the free fluoride content in the conversion stage to the aforementioned upper limits is generally advantageous for fully satisfactory corrosion protection, even for components made solely from hot-dip galvanized (ZM) steel, in order to avoid flash rust on the aforementioned exposed steel surfaces.

[0053] The amount of free fluoride in the respective stages of the pretreatment according to the invention is to be determined potentiometrically at 20°C in the respective provided solution after calibration with fluoride-containing buffer solutions without pH buffering using a fluoride-sensitive measuring electrode.

[0054] Furthermore, the pH value of the acidic aqueous composition (II) in the conversion stage is important and generally advantageous for a reliably good paint adhesion on the hot-dip galvanized (ZM) surfaces, if at lower pH values ​​somewhat higher free fluoride contents are present and preferably a minimum amount of free fluoride in mmol / kg is contained, which is calculated with increasing preference for the process according to the invention according to the following terms:

[0055] 4.4 - g - (pH - 3.5),

[0056] 4.4 - g - (pH - 3.5),

[0057] 4.4 - g - (pH - 3.5), 4.4

[0058] 4.4 - g - (pH - 3.5), where "pH" is the pH value of the acidic aqueous composition (II).

[0059] Alternatively, depending on the pH of the acidic aqueous composition (II), a preferred minimum amount of free fluoride can be used, which is calculated according to the following term:

[0060] 2 + 1.85 ■ 5.2 - pH, where again the pH value of the acidic aqueous composition (II) is to be used for the “pH”, which in this context is preferably less than 5.00.

[0061] In a particularly preferred embodiment, the minimum amount of free fluoride that should be contained in the acidic aqueous composition for optimal paint retention on the (ZM) surfaces is to be adjusted as a function of the pH value and the proportion of non-polar hydrocarbons as follows:

[0062] 1.2 + 5 ■ 70.1 ■ KW + 1.85 ■ 75.2 - pH with pH: pH value of the acidic aqueous composition (II), which is less than 4.80, and preferably greater than 4.00, since otherwise corrosion may occur on exposed steel surfaces of the hot-dip galvanized (ZM) steel processed into components due to the high minimum fluoride content

[0063] KW: dimensionless proportion of non-polar hydrocarbons in the alkaline aqueous composition (I) in the system tank of the degreasing stage in kg / m 3 which is at least 0.10 kg / m 3 but preferably 0.80 kg / m 3 , particularly preferably 0.60 kg / m 3 is not exceeded, since otherwise corrosion may occur on exposed steel surfaces of the hot-dip galvanized (ZM) steel processed into components due to the high minimum fluoride content.

[0064] Suitable sources of free fluoride for the acidic aqueous composition (II) include water-soluble complex fluorides of the elements Zr, Ti, and / or Si, preferably of the elements Zr and / or Ti, particularly preferably of the element Zr, and / or hydrofluoric acid, ammonium bifluoride, and / or water-soluble alkali metal fluorides. Regarding the pH value of the acidic aqueous composition of the conversion stage, it is important to note that the compounds of the elements Zr and / or Ti dissolved in water do not form brines due to hydrolysis, which are no longer available for conversion layer formation. At the same time, the pickling rate for common metallic materials should be sufficiently high to form homogeneous, closed conversion layers; this applies particularly to the substrate hot-dip galvanized (ZM) steel.According to the invention, it is therefore necessary that the acidic aqueous compositions (II) do not have a pH above 5.20 and that the pH is preferably less than 5.10, more preferably less than 5.00, most preferably less than 4.90, and especially preferably below 4.80. At the same time, increased pickling and rapid layer formation kinetics can be detrimental to the formation of suitable conversion coatings. Particularly on hot-dip galvanized (ZM) steel, comparatively high layer weights based on the elements Zr and / or Ti are achieved in the lower pH range according to the invention. These elements, in turn, are less compact and tend to self-corrode at low pH values, so that point defects can form in the conversion coating.According to the invention, it is therefore preferred if the pH of the acidic aqueous composition (II) is greater than 4.00, particularly preferably greater than 4.20, most preferably greater than 4.40.

[0065] In the context of the conversion stage, the pH value corresponds according to the invention to the negative decadic logarithm of the hydronium ion activity measured in the acidic aqueous composition (II) at a temperature of 20 °C using a pH-sensitive glass electrode after two-point calibration against technical buffer solutions of acetic acid / acetate (pH = 4.0) and phosphate (pH = 7.0).

[0066] In the conversion stage, the aim is to create a conversion coating based on oxidic / hydroxidic compounds of the elements Zr and / or Ti, preferably the element Zr, that is as homogeneous and compact as possible. In a preferred embodiment, the contacting takes place for at least a period of time for which a layer thickness of at least 20 mg / m² is formed on the surfaces of the hot-dip galvanized (ZM) steel. 2 , particularly preferably at least 40 mg / m 2 is brought about, but the contact is preferably not prolonged so that a layer thickness of more than 250 mg / m 2 , particularly preferably more than 150 mg / m 2 , most preferably more than 100 mg / m 2 , particularly preferably more than 80 mg / m 2in each case based on the elements Zr and / or Ti on the hot-dip galvanized (ZM) surfaces. The layer deposits can be determined by X-ray fluorescence analysis (XRF). The treatment time required for the preferred layer deposits, i.e. the duration of contact with the acidic aqueous composition (II) at a preferred temperature in the range of 10-60 °C, should be in the range of 10 seconds to 300 seconds. To ensure this, a process is preferred according to the invention in which the proportion of compounds of the elements Zr and / or Ti dissolved in water in the acidic aqueous composition (II) in process step ii) is preferably at least 0.10 mmol / kg, more preferably at least 0.30 mmol / kg, especially preferably at least 0.40 mmol / kg.For process economic reasons, the contents of compounds of the elements Zr and / or Ti dissolved in water should preferably be below 5.0 mmol / kg, particularly preferably below 3.0 mmol / kg and most preferably below 2.0 mmol / kg based on the elements Zr and / or Ti.

[0067] In the conversion stage of the process according to the invention, an amorphous oxidic / hydroxidic coating based on the elements Zr and / or Ti, preferably the element Zr, is to be produced, and accordingly, the compounds of the elements Zr and / or Ti dissolved in water are included. The term "dissolved in water" encompasses molecularly dissolved species and compounds that dissociate in aqueous solution and form hydrated ions. Typical representatives of these compounds are titanyl sulfate (TiO(SO4)), titanyl nitrate (TiO(NO3)2) and / or hexafluorotitanic acid (H2TiFe) and their salts, or ammonium zirconium carbonate ((NH4)2ZrO(CO3)2) and / or hexafluorozirconic acid (H2ZrFe) and their salts. The compounds dissolved in water in the conversion stage are preferably selected from fluoro acids and / or fluoro complexes of the elements Zr and / or Ti and their water-soluble salts.The formation of conversion layers based on fluoro acids and / or fluoro complexes of the element Zr is particularly preferred since such conversion layers provide improved paint adhesion.

[0068] In order to heal point defects in the conversion coating growing in process step ii) on the surfaces of galvanized steel, in particular on the surfaces of hot-dip galvanized (ZM) steel, where an overall rapid layer formation occurs, the presence of copper ions can be advantageous, as their local cementation in the point defects provides improved corrosion protection. Accordingly, it is preferred that the acidic aqueous composition (II) of the conversion stage in process step ii) additionally contains copper ions dissolved in water, preferably at least 0.05 mmol / kg, but again preferably less than 4.0 mmol / kg, particularly preferably less than 2.0 mmol / kg of copper ions dissolved in water. Suitable sources of copper ions dissolved in water are water-soluble salts such as copper nitrate (Cu(NO3)2), copper sulfate (CUSO4*), and copper acetate (Cu(CH3COO)2).

[0069] Further additives known to those skilled in the art of surface treatment, such as accelerators such as nitrate ions, nitrite ions, nitroguanidine, N-methylmorpholine N-oxide, hydrogen peroxide in free or bound form, hydroxylamine in free or bound form, reducing sugars, and / or wetting agents such as nonionic surfactants, and / or polymers such as polyamidoamines, and / or cations / compounds of the elements Mg, Ca, Al, Si, Sn, Bi and / or Mo may be included to improve the layer formation kinetics, wettability and corrosion protection properties. For a resource-saving process and for reasons of cost-effectiveness, it is preferred if the acidic aqueous composition (II) of the conversion stage in a process according to the invention

[0070] (a) a total of less than 100 mg / kg, preferably less than 50 mg / kg, particularly preferably less than 10 mg / kg, especially preferably less than 1 mg / kg of compounds containing chromium, calculated as the amount of chromium,

[0071] (b) a total of less than 100 mg / kg, preferably less than 10 mg / kg, of phosphates dissolved in water, preferably of phosphorus-containing compounds dissolved in water, each calculated as the amount of phosphorus,

[0072] (c) a total of less than 100 mg / kg, preferably less than 10 mg / kg, of hydrolysable organic silanes and siloxanes calculated as Si(OCH2CH3)4, preferably of compounds of the element silicon dissolved in water calculated as the amount of Si, and / or

[0073] (d) a total of less than 0.01 g / L (calculated as solids addition) of copolymers which contain, in alternating configuration, monomer units which have at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group and monomer units which do not have an acid group, preferably a total of less than 0.1 g / L (calculated as solids addition) of copolymers which contain monomer units which have at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group, particularly preferably less than 0.1 g / L of organic polymers which are not polyamidoamines.

[0074] The application and thus the contacting of the acidic aqueous composition (II) preferably takes place at at least 30°C, particularly preferably at at least 40°C, but preferably below 60°C. The acidic aqueous composition (II) of the conversion stage can be brought into contact with the components of the series using application methods established in the prior art. These include, in particular, immersion, rinsing, spraying, and / or spraying, with application by immersion and / or spraying, and in particular, immersion of the components of the series in a system tank containing the corresponding acidic aqueous composition (II), being preferred.

[0075] Painting stage:

[0076] In the coating stage, at least the surfaces of the components formed by the hot-dip galvanized (ZM) steel and conversion-coated in process step ii), preferably all surfaces formed by metallic materials, are coated with a first coating system by contacting the component or at least said surfaces of the conversion-coated hot-dip galvanized (ZM) steel with the aqueous dispersion (III) containing the organic binder. The coating system is therefore deposited directly from the aqueous phase as a coating of the organic binder of the aqueous dispersion (III) precipitated onto said surfaces, which is typically subjected to thermal post-treatment for film formation and curing. The coating in the coating stage is preferably applied by dip coating, particularly preferably by electrocoating, and again preferably by cathodic electrocoating.For this purpose, the organic binder of the aqueous dispersion (III) is preferably based on amine-modified film-forming polyepoxides, which preferably additionally comprise blocked and / or unblocked organic compounds containing isocyanate groups as hardeners. Inorganic pigments are also often a component of the aqueous dispersion and a preferred additive for improving corrosion protection. The aqueous phase also preferably contains small amounts of compounds of the elements yttrium and / or bismuth, dissolved or dispersed in water, which have a positive effect on crosslinking and film formation.

[0077] The preferred pH of the aqueous dispersion (III) of the coating stage is in the range from 5.0 to 6.0, particularly preferably in the range from 5.4 to 5.8. In the context of the coating stage, the pH corresponds according to the invention to the negative decadic logarithm of the hydronium ion activity measured in a solution diluted by a factor of 10 with deionized water (K<1 pScm -1 ) diluted aqueous dispersion (III) at a temperature of 20 °C using a pH-sensitive glass electrode after two-point calibration against technical buffer solutions of acetic acid / acetate (pH = 4.0) and boric acid / borate (pH = 9.0).

[0078] The application and thus the bringing into contact of the components or at least the said surfaces of the conversion-coated hot-dip galvanized (ZM) steel with the aqueous dispersion (III) preferably takes place at at least 30 °C, particularly preferably at at least 40 °C, but preferably below 60 °C. The aqueous dispersion (III) of the painting stage can be brought into contact with the series components using application methods established in the state of the art. These include, in particular, dipping, spraying, and roller application, whereby application by dipping and, in particular, immersing the series components in a system tank containing the corresponding aqueous dispersion (III) is preferred and, in the case of dipping paints, is already predetermined by the type of paint system.

[0079] Procedure:

[0080] In the following, preferred embodiments of the method according to the invention are described and explained with regard to the individual treatment stages and the method management, which are particularly advantageous with regard to the object underlying the invention.

[0081] Treatment stages i)-iii) of the process according to the invention each comprise at least one treatment step that involves bringing the components of the series into contact with a more precisely defined aqueous composition characteristic of the treatment stage. For the purpose of contacting, these characteristic compositions are either stored or kept in system tanks. Contacting can take place either in the system tank, for example, by immersion in a composition stored there, or outside the system tank, for example, by spraying a composition stored in the system tank in a spray chamber, depending on the specific requirement or preference of the respective process step.

[0082] In the process according to the invention, process steps i)-iii) follow one another, i.e., in the specified order, and preferably such that the components are not subjected to any wet-chemical treatment step other than one that constitutes a rinsing step between two process steps i)-iii). In this context, a rinsing step serves primarily, preferably exclusively, to remove the wet film adhering to the components from the respective preceding wet-chemical process step and thus to completely or partially remove soluble residues, particles, and active components that would otherwise be carried over from the preceding wet-chemical process step adhering to the component into the next treatment stage.

[0083] In a preferred process according to the invention, both the degreasing stage and the conversion stage are followed by a so-called rinsing stage comprising at least one rinsing step. During the rinsing stage, the components in the series are freed of any wet film adhering to them from the degreasing stage and the conversion stage in order to prevent the carryover of active components into the next treatment stage. For this purpose, the rinsing stage consists of one or more immediately consecutive rinsing steps. Here, too, rinsing steps follow one another if the components are not subjected to another wet-chemical treatment step that is not a rinsing step in the meantime.For the actual function of a rinsing stage, which consists in preventing the carryover of active components into downstream wet-chemical treatment stages, it is beneficial and therefore also preferred within the scope of the present invention if the rinsing stage, as already described, comprises several immediately successive rinsing steps for bringing the components of the series into contact with a rinsing solution stored in the system tank of the respective rinsing step.

[0084] For a rinsing stage following the degreasing stage and preceding the conversion stage, it is preferred if the specific conductivity in the system tank of the single or last rinsing step is below 100 pScm -1 lies.

[0085] For a rinsing stage following the conversion stage and preceding the painting stage, it is preferred if the specific conductivity in the system tank of the only or last rinsing step is below 40 pScm1 lies.

[0086] To achieve this purpose, the respective rinsing stage is carried out with fresh water with a specific conductivity of preferably less than 10 pScrrr 1 fed, whereby the volume flow of fresh water fed in is large enough not to exceed the maximum specific conductivity specified for the rinsing stage during the treatment of the series of components.

[0087] During the rinsing stage, the wet film from the preceding wet-chemical treatment stage should be removed as best as possible. The rinsing stage is therefore carried out using rinsing solutions that do not contain any active components of the type or quantity required, as their carryover into the subsequent treatment stage would be problematic and must be prevented. If necessary, however, the rinsing solution may contain small amounts of redox-active compounds ("depolarizers"), such as hydrogen peroxide, or, to improve wettability with the rinsing solution, additional surface-active compounds such as nonionic surfactants. This applies in particular to the rinsing stage preceding the conversion stage and the coating stage. However, the addition of additives should not result in the prescribed specific conductivity being exceeded in the only or final rinsing step of the rinsing stage.In particular, the inclusion of elements and compounds that could adversely affect corrosion protection performance should be avoided. Likewise, additives should always be avoided if, while they do not defeat the primary purpose of the rinsing stage, they do not significantly improve performance in an essential aspect of the process according to the invention and would therefore not be economically justified. Therefore, it is preferred if the rinsing solutions of the only or final rinsing step of a rinsing stage in a process according to the invention, preferably each rinsing solution of all rinsing steps of a rinsing stage, contain rinsing solutions of the same type.

[0088] (a) less than 10 mg / kg of compounds of the metals Bi, Ni, Co and / or Cu dissolved in water, calculated as the amount of the respective element in the aqueous composition, preferably less than 10 mg / kg of compounds of such metals dissolved in water which have a standard reduction potential greater than -0.40 V (SHE), calculated as the amount of the respective element in the aqueous composition,

[0089] (b) a total of less than 100 mg / kg, preferably less than 50 mg / kg, particularly preferably less than 10 mg / kg of surfactants, preferably of surface-active organic compounds, particularly preferably of organic compounds,

[0090] (c) a total of less than 100 mg / kg, preferably less than 10 mg / kg, of organic silanes and siloxanes calculated as Si(OCH2CH3)4, preferably of compounds of the element silicon dissolved in water calculated as the amount of Si,

[0091] (d) a total of less than 20 pmol / kg, preferably less than 10 pmol / kg, particularly preferably less than 5 pmol / kg of compounds of the elements Zr and / or Ti dissolved in water,

[0092] (e) less than 50 mg / kg in total, preferably less than 10 mg / kg each, of sodium and / or potassium ions,

[0093] (f) a total of less than 50 mg / kg, preferably less than 10 mg / kg, of zinc ions, (g) a total of less than 100 mg / kg, preferably less than 10 mg / kg, of phosphates dissolved in water, preferably of phosphorus-containing compounds dissolved in water, each calculated as the amount of phosphorus, and / or,

[0094] (h) a total of less than 0.01 g / L (calculated as solids addition) of copolymers which contain, in alternating configuration, monomer units which have at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group and monomer units which do not have an acid group, preferably a total of less than 0.1 g / L (calculated as solids addition) of copolymers which contain monomer units which have at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group, particularly preferably less than 0.1 g / L of organic polymers, wherein the pH of the rinsing solutions is preferably in the range from 5.0 to 8.5.

[0095] The standard reduction potential is that against the standard hydrogen electrode H2 / H + (pH=0) determined reduction potential of the electrochemical half-cell Me / Me n+ at a metal ion activity of 1 mol / l and 20 °C.

[0096] Examples of implementation:

[0097] A large number of hot-dip galvanized steel sheet sections (each 10 cm x 20 cm) of the types ZM (CR180 ZM 40 / 40-EWO from Voestalpine AG) and HDG (Gardobond® MBZ from Chemetall) were treated for corrosion protection using the following process steps. The sheets were coated with 1.0-2.0 g / m 2 a corrosion protection oil (Anticorit® RP 4107 LV from Fuchs Europe Schmierstoffe GmbH).

[0098] A. Immersion cleaning at a bath temperature of 55°C for 180 seconds:

[0099] 2 g / l BONDERITE® C-AK 2011, 1 g / l BONDERITE® C-AD 1270 (each from Henkel AG & Co. KGaA) in deionized water (K ​​< I pScrrr 1 ) pH: 11.5 (determined with glass electrode at 25°C) FA: 5 points (10 ml sample, pH 8.5) GA: 12 points

[0100] PO4: 2.5 g / l

[0101] Bath volume: 5 liters

[0102] B. Rinse in the syringe with deionized water (K ​​< 1 pScrrr 1 ) at 20°C for 30 seconds

[0103] C. Rinse by immersion with deionized water (K ​​< 1 pScrrr 1 ) at 20°C for 30 seconds

[0104] D. Conversion treatment by immersion at a bath temperature of 35°C for 120 seconds

[0105] Zr: 150 mg / kg

[0106] Cu: 10 mg / kg

[0107] Zn: 0.6 g / kg

[0108] NO3: 6 g / kg pH-1: 4.7 (determined with glass electrode at 25°C) pH-2: 4.2 (determined with glass electrode at 25°C)

[0109] F-free: 40 - 80 ppm (determined with ion-sensitive electrode at 35°C)

[0110] The achieved layer weight of Zr on HDG was approximately 76-79 mg / m 2 , while on ZM 61-64 mg / m 2 Zr coating were realized (each measured by X-ray fluorescence analysis).

[0111] E. Rinse by immersion with deionized water (K ​​< 1 pScrrr 1 ) at 20°C for 30 seconds

[0112] F. Rinse by immersion with deionized water (K ​​< 1 pScrrr 1 ) at 20°C for 30 seconds

[0113] G. Drying by blowing off with compressed air and storage in a drying cabinet at 50°C H. Cathodic dip coating with Cathoguard® 800 (BASF SE) in a dry film thickness of 28-30 pm

[0114] I. Top coat build-up with (thin-film filler ALG 697 172; BC: Brilliant Black ALD 091 Y9B; CC: ALD 096 100, each from Audi AG) in a dry film thickness of 105-115 pm

[0115] To simulate a series of corrosion protection treatments, a large number of sheets were cleaned sequentially (alternating between ZM and HDG) and an increasing concentration of hydrocarbons was adjusted. The corrosion protection results are summarized in Tables 1 and 2.

[0116] In the present test series, the critical threshold value of hydrocarbons above which a significant deterioration (Gt value > 2) of the corrosion protection on the (ZM) sheets could be observed was approximately 0.10 kg / m 3 , which corresponded to approximately 12 penetrated sheets. However, it was shown that by increasing the minimum amount of free fluoride, good protection against corrosive delamination of the coating system could be maintained. Overall, it was also shown that lower pH values ​​in the conversion stage, with the same bath load of nonpolar hydrocarbons, require a higher free fluoride content to maintain good corrosion protection.

[0117] Table 1 Results of the cross-cut test (DIN EN ISO 2409) after corrosion-protective treatment (conversion stage with pH-1 = 4.70)

[0118] * Number of sheets already penetrated at the time of corrosion protection treatment

[0119] # Amount of non-polar hydrocarbons in the degreasing stage determined according to the corresponding number of sheets passed through according to the analytical method stated in the description nb undetermined

[0120] Table 2 Results of the cross-cut test (DIN EN ISO 2409) after corrosion-protective treatment (conversion stage with pH-2 = 4.20)

[0121] * Number of sheets already penetrated at the time of corrosion protection treatment

[0122] # Amount of non-polar hydrocarbons in the degreasing stage determined according to the corresponding number of sheets passed through according to the analytical method stated in the description nb undetermined

Claims

Claims:

1. A process for the corrosion-protective pretreatment of a large number of components in series, in which the components of the series at least partially have surfaces of zinc-magnesium hot-dip coated steel, and in which the components of the series each undergo the successive process steps i) - iii) and at least the surfaces of the zinc-magnesium hot-dip coated steel are brought into contact one after the other with the respectively provided aqueous solutions (l) - (lll): i) degreasing stage providing an alkaline aqueous composition (I) with a pH above 9.00; ii) conversion stage providing an acidic aqueous composition (II) which has a pH in the range of 3.50 to 5.20, containing at least 0.05 mmol / kg of compounds of the elements Zr and / or Ti dissolved in water and at least 2.80 mmol / kg of free fluoride;and iii) coating step providing an aqueous dispersion (III) of an organic binder.; 2. Process according to one or both of the preceding claims, characterized in that the alkaline aqueous composition (I) of the degreasing stage in process step i) contains more than 0.05 kg / m 3 , preferably more than 0.10 kg / m 3 , particularly preferably more than 0.20 kg / m 3 of non-polar hydrocarbons.

3. Process according to one or more of the preceding claims, characterized in that the acidic aqueous composition (II) of the conversion stage in process step ii) contains at least 3.00 mmol / kg of free fluoride, but preferably less than 7.50 mmol / kg, particularly preferably less than 6.00 mmol / kg and most preferably less than 5.00 mmol / kg of free fluoride.

4. Process according to one or more of the preceding claims, characterized in that the acidic aqueous composition (II) of the conversion stage in process step ii) contains a minimum amount of free fluoride in mmol / kg, which is calculated according to the following term: 4.4 - g - (pH - 3.5), preferably calculated according to the following term: 4.4 - g - (pH - 3.5), particularly preferably calculated according to the following term: 4.4 - g - (pH - 3.5), most preferably calculated according to the following term: 4.4 and particularly preferably calculated according to the following term: 4.4 - g - (pH - 3.5), where the pH value of the acidic aqueous composition (II) is to be used for the pH.

5. Process according to one or more of claims 2 to 3, characterized in that the acidic aqueous composition (II) of the conversion stage in process step ii) contains a minimum amount of free fluoride in mmol / kg, which is calculated according to the following term: 1.2 + 5 ■ 70.1 ■ KW + 1.85 ■ 75.2 - pH with pH: pH of the acidic aqueous composition (II) which is less than 4.80, and preferably greater than 4.00 KW: dimensionless proportion of non-polar hydrocarbons in the alkaline aqueous composition (I) in the system tank of the degreasing stage in kg / m 3 which is at least 0.10 kg / m 3 but preferably 0.80 kg / m 3 does not exceed.

6. Process according to one or more of the preceding claims, characterized in that the acidic aqueous composition (II) of the conversion stage in process step ii) contains the corresponding fluoro acids and / or their water-soluble salts as a source for the compounds of the elements Zr and / or Ti dissolved in water.

7. Process according to one or more of the preceding claims, characterized in that the acidic aqueous composition (II) of the conversion stage in process step ii) contains at least 0.10 mmol / kg, preferably at least 0.30 mmol / kg, particularly preferably at least 0.40 mmol / kg, but preferably less than 5.0 mmol / kg, particularly preferably less than 3.0 mmol / kg, very particularly preferably less than 2.0 mmol / kg of compounds of the elements Zr and / or Ti dissolved in water, based on the elements Zr and / or Ti.

8. Process according to one or more of the preceding claims, characterized in that the acidic aqueous composition (II) of the conversion stage in process step ii) has a pH of greater than 4.00, preferably greater than 4.20, particularly preferably greater than 4.40, but preferably less than 5.10, particularly preferably less than 5.00, very particularly preferably less than 4.90, especially preferably less than 4.

80.

9. Process according to one or more of the preceding claims, characterized in that the acidic aqueous composition (II) of the conversion stage in process step ii) additionally contains copper ions dissolved in water, preferably at least 0.05 mmol / kg, but preferably less than 4.0 mmol / kg, particularly preferably less than 2.0 mmol / kg of copper ions dissolved in water.

10. Method according to one or more of the preceding claims, characterized in that the contacting with the conversion solution (II) in the conversion stage takes place at least for a period of time for which a layer of at least 20 mg / m 2 , particularly preferably at least 40 mg / m 2 is brought about, but the contact is preferably not prolonged so that a layer thickness of more than 250 mg / m 2 , particularly preferably more than 150 mg / m 2 , most preferably more than 100 mg / m 2 , particularly preferably more than 80 mg, each based on the elements Zr and / or Ti, on these surfaces.

11. Process according to one or more of the preceding claims, characterized in that the contacting, the coating stage in process step iii) represents a dip coating, preferably an electrocoating and particularly preferably a cathodic electrocoating, and as cathodic electrocoating, in turn preferably comprises an amine-modified polyepoxide as an aqueous dispersion of an organic binder (III), wherein the aqueous dispersion preferably additionally contains water-soluble or water-dispersible salts of yttrium and / or bismuth.

12. Method according to one or more of the preceding claims, characterized in that an intermediate rinsing takes place during the transfer of the components from method step i) or ii) to the next and the method steps i)-iii) otherwise follow one another directly and preferably no drying step is used.

13. Method according to one or more of the preceding claims, characterized in that the components of the series are composite structures, preferably automobile bodies, which are made of semi-finished products of zinc-magnesium hot-dip coated steel and are composed of semi-finished products of galvanized steel and aluminum, particularly preferably of semi-finished products of zinc-magnesium hot-dip coated steel and of semi-finished products of galvanized steel, aluminum and steel.