Method for the anticorrosion pretreatment of parts comprising a hot dip galvannealed steel surface

Abstract

This invention relates to a method for anti-corrosion pretreatment of multiple components, including a hot-dip galvanized magnesium-plated steel surface, wherein each component sequentially passes through a degreasing stage, a conversion stage, and a painting stage. The conversion stage, based on an acidic aqueous conversion solution containing compounds of elements Zr and / or Ti dissolved in water, is adjusted in terms of pH and free fluoride content to produce a uniform conversion layer on the hot-dip galvanized magnesium-plated steel, reliably providing high paint film adhesion regardless of the degree of contamination during the degreasing stage.

Description

[0001] This invention relates to a method for pretreating multiple components, including hot-dip galvanized magnesium-plated steel surfaces, with corrosion protection, wherein each component sequentially passes through a degreasing stage, a conversion stage, and a painting stage. The conversion stage, based on an acidic aqueous conversion solution containing compounds of elements Zr and / or Ti dissolved in water, is adjusted in terms of pH and free fluoride content to produce a uniform conversion layer on the hot-dip galvanized magnesium-plated steel, reliably providing high paint film adhesion regardless of the degree of contamination during the degreasing stage.

[0002] In the automotive industry, the increasing demand for lightweight body structures has led to the growing importance of magnesium-alloyed zinc coatings on steel. Compared to other hot-dip galvanizing methods, zinc-magnesium coatings offer significantly improved corrosion resistance, particularly after the application of organic topcoats and dip-coats, maintaining excellent resistance to corrosion delamination. This improved performance allows for thinner coatings while still meeting high requirements for recoatability and corrosion protection. The superior corrosion behavior of magnesium-alloyed zinc coatings—particularly in edge corrosion protection and paint adhesion to formed parts—along with excellent compatibility with all common joining methods and the aforementioned weight reduction, makes hot-dip galvanized (ZM) steel a particularly important material for manufacturing lightweight body structures. Consequently, its surface area proportion on automotive bodies will continue to increase, alongside other lightweight metals such as aluminum.

[0003] In automotive manufacturing, magnesium-zinc alloyed steel is typically used in the form of flat strip products and so-called hot-dip galvanized (ZM) steel strips. This type of hot-dip galvanized magnesium steel contains approximately 1.5 to 8 wt.% metallic aluminum and magnesium in its metallic coating, with magnesium comprising at least 0.2 wt.%. The basic applicability of these coatings to conventional forming, pretreatment, and coating methods established in the prior art has been recognized and validated in principle (German Steel Association, “Continuously Hot-Dip Coated Steel Strips and Sheets,” 2017 edition, Chapters 8 and 10, Properties 095 E). However, based on the specific composition of the coating and the natural oxide layer, there are some characteristics that require special consideration, especially in the context of cleaning and pretreatment, to obtain the most uniform and reproducible coating effect possible, thereby achieving optimal corrosion resistance or desired surface functionality.

[0004] It is known from the prior art, for example, that during the cleaning process, prior to the anti-corrosion pretreatment of hot-dip galvanized (ZM) steel strip, it may be necessary to change the proportion of magnesium oxide in the alloy composition to ensure adequate adhesion to the subsequently applied paint layer. For example, US2016 / 0010216 A1 reports that significant removal of magnesium oxide from the oxide layer near the surface of the hot-dip galvanized (ZM) steel strip can effectively suppress the formation of blister-like protrusions in the topcoat.

[0005] The prior art also records that in the continuous pretreatment of multiple components, even the wetting of the material surface of hot-dip galvanized (ZM) steel strips and therefore reliable cleaning can be problematic. Therefore, WO 2023 / 036889 A1 proposes conditioning the hot-dip galvanized (ZM) surface after degreasing but before the anti-corrosion pretreatment (which may be a conversion coating based on elements Zr and / or Ti) to counteract the aging of conventional cleaning and degreasing baths associated with high component throughput, as well as the subsequent deterioration of the wettability of the hot-dip galvanized (ZM) surface.

[0006] Therefore, materials with zinc-magnesium coatings require complex process controls to successfully, and especially reliably, undergo anti-corrosion pretreatment, particularly when high-quality coatings for multiple components need to be delivered automatically in a paint production line. There is a need for alternative methods to support simpler process controls during anti-corrosion pretreatment, including the stages of degreasing, conversion layer formation, and painting, and to facilitate satisfactory results in the continuous processing of multiple components, largely independent of bath aging, thereby fully utilizing the anti-corrosion potential of hot-dip zinc-magnesium coated materials. Existing intensive bath maintenance methods known in the art for keeping clean and degreasing baths as uncontaminated as possible and replacing them with fresh bath solution as early as possible are economically disadvantageous and problematic in terms of the desired resource-efficient use of process chemicals.

[0007] Therefore, the object of the present invention is to establish a method for the continuous processing of multiple components, which is suitable for the reliable corrosion-protective pretreatment of hot-dip galvanized magnesium steel using conventional methods in sequence, while taking into account conversion coatings based on elements Zr and / or Ti, which on the one hand can provide an excellent paint film adhesion basis for metal mixtures commonly used in automobile manufacturing, but on the other hand has also been shown to be particularly prone to performance degradation during the fixed operation of the pretreatment production line for processing hot-dip galvanized magnesium steel.

[0008] Surprisingly, it has now been found that when the loading of nonpolar hydrocarbons in the degreasing bath exceeds a critical value, these nonpolar hydrocarbons are introduced into the bath from components contaminated by anti-corrosion oil, forming oil, and drawing grease. This means that subsequent conversion treatments based on the previously stable elements Zr and / or Ti no longer produce the desired results, and a significant loss of paint adhesion is observed after paint layer construction, particularly on steel substrates with zinc-magnesium coatings. Instead of addressing this problem by altering the upstream process steps of the conversion treatment, this invention surprisingly addresses it through the performance characteristics of the conversion treatment itself and by adjusting the content of free fluorides.

[0009] Specifically, the present invention relates to a method for pre-treating a series of multiple components for corrosion protection, wherein the series of components at least partially have hot-dip galvanized magnesium-plated steel surfaces, and wherein each of the components in the series sequentially undergoes method steps i)-iii), and at least the hot-dip galvanized magnesium-plated steel surfaces are sequentially contacted with respectively provided aqueous solutions (I)-(III): i) During the defatting stage, an alkaline aqueous composition (I) with a pH higher than 9.00 is provided; ii) In the conversion stage, an acidic aqueous composition (II) with a pH of 3.50 to 5.20 is provided, containing at least 0.05 mmol / kg of a compound of elemental Zr and / or Ti dissolved in water and at least 2.80 mmol / kg of free fluoride; and iii) In the painting stage, an aqueous dispersion of the organic binder is provided (III).

[0010] The corrosion pretreatment of a series of components occurs when multiple components come into contact with a treatment solution provided in the corresponding treatment stage of the method according to the invention and conventionally stored in a system tank, with each component coming into contact with the others sequentially and thus at different times. In this case, the system tank is a container for holding the treatment solution for continuous corrosion pretreatment.

[0011] According to the invention, the component comprises a material steel having a zinc-magnesium coating, wherein the coating is applied by a melt of an alloy composition. This hot-dip galvanizing is known in the prior art as hot-dip galvanized (ZM) steel and is a metallic coating comprising 1.5 to 8 wt.% aluminum and magnesium, preferably with magnesium comprising at least 0.2 wt.% of the metallic coating. Hereinafter, the term hot-dip galvanized (ZM) steel is used synonymously with hot-dip galvanized magnesium steel.

[0012] Within the scope of this invention, corrosion pretreatment always refers to the pretreatment of the surfaces of a series of components formed of metallic materials. This material can be a homogeneous material or a coating. According to the invention, galvanized steel grades consist of steel and zinc, and the surface of the steel can be exposed, for example, at the cut edges and cylindrical grinding points of a car body made of galvanized steel. In this case, the steel is pretreated according to the invention. Therefore, if the invention refers to the pretreatment of components made of a particular metallic material, this includes all materials and coatings containing more than 50 at.% of the relevant elements of that material. Thus, components with a galvanized layer contain more than 50 at.% zinc in their metallic coating.

[0013] The method according to the invention is not limited to hot-dip galvanized (ZM) steel; therefore, common substrates supplied by the steel industry, such as steel, particularly cold-rolled steel (CRS), and electro-galvanized (ZE) or hot-dip galvanized (Z), alloy galvanized, particularly (ZF), (ZA), or aluminum-coated (AZ), (AS) steel, are also suitable as other components of the parts. In the method according to the invention, light metals such as aluminum and magnesium and their alloys can also be processed together with the hot-dip galvanized (ZM) steel of the parts, and are cleaned and / or undergo anti-corrosion pretreatment in the process. The method according to the invention is characterized by its suitability for anti-corrosion pretreatment of common metallic materials composed of iron, zinc, aluminum, and magnesium, i.e., providing them with a conversion coating that provides a good primer base for painting.

[0014] A particularly preferred embodiment is in which the series of components, in addition to hot-dip galvanized (ZM) steel, are also composed of galvanized steel, steel, and / or aluminum. The method according to the invention is particularly advantageous for providing a good anti-corrosion pretreatment for this material combination, consisting of cleaning, conversion layer formation, and painting, especially for components manufactured in a composite structure and assembled from different semi-finished products. Therefore, a preferred method according to the invention is one in which the series of components is a composite structure, preferably a method for automotive bodies, consisting of semi-finished hot-dip galvanized (ZM) steel and semi-finished products of galvanized steel and aluminum, and particularly preferably assembled from semi-finished hot-dip galvanized (ZM) steel and semi-finished products of galvanized steel, aluminum, and steel.

[0015] The components pre-treated according to the present invention can be three-dimensional structures of arbitrary shape and design derived from the manufacturing process, particularly semi-finished products such as strips, plates, bars, tubes, etc., as well as composite structures assembled from the aforementioned semi-finished products. Composite structures assembled from different materials are typically flat products formed by cutting, shaping, and joining by welding, bonding, and flanging. The components to be continuously pre-treated according to the present invention are preferably selected from automobile bodies or their components, heat exchangers, profiles, pipes, storage tanks, or troughs.

[0016] The concentration of an active ingredient or compound is referred to in the context of this invention as mass per kilogram, which is mass based on the weight of the corresponding total composition.

[0017] Regarding the method steps, the composition (I)-(II) or dispersion (III) is considered to be "provided" within the meaning of the method according to the invention when it is either stored in a system tank and ready for contact use, or is carried out in a defined manner during contact, as specified in the relevant processing stages (i)-(iii).

[0018] The multi-stage pretreatment according to the invention ensures that, during the conversion stage, a Zr- and / or Ti-based conversion coating is produced for multiple parts to be treated, providing good paint film adhesion and thus reliably preventing corrosive delamination of the entire paint system. This is achieved by adjusting the free fluoride content within a specified pH range, regardless of the load of contaminants removed from the part surface during the degreasing stage. In particular, the proportion of nonpolar hydrocarbons has proven critical in that if the specified proportion of free fluoride is not met during the conversion stage, above a certain fixed proportion of hydrocarbons introduced into the degreasing bath, paint film adhesion on the hot-dip galvanized (ZM) surfaces of the corresponding pretreated parts will be significantly degraded. It has also been found that the amount of free fluoride required to overcome the loss of paint film adhesion on hot-dip galvanized (ZM) surfaces during continuous treatment of parts contaminated with grease and oil depends more on the pH value of the conversion treatment stage than on the actual load of nonpolar components in the degreasing bath. At lower pH values, a greater amount of free fluoride is often required to compensate for the adverse effects on hot-dip galvanized (ZM) surfaces, which would otherwise inevitably occur during continuous processing due to the gradual accumulation of a fixed proportion of hydrocarbons during the degreasing stage.

[0019] Degreasing stage: During the degreasing stage, an alkaline aqueous composition (I) with a pH higher than 9.00 is provided for cleaning and degreasing the parts. The purpose of the degreasing stage is to ensure that the part surface is largely free of inorganic salts and organic contaminants, particularly drawing oils, forming oils, rolling oils, and anti-corrosion oils, for successful subsequent anti-corrosion pretreatment consisting of conversion and painting stages. In a preferred embodiment, the degreasing stage occurs immediately after the degreasing stage, i.e., before the conversion stage, but using deionized water (κ < 1 μS cm⁻¹). -1 After rinsing, less than 0.20 g / m 2 Especially preferred is less than 0.10 g / m 2 Carbon deposits remain on the metallic surfaces of the series of components, and preferably have a concentration of at least 0.50 g / m² before the metallic surfaces of the series of components, i.e., before undergoing method step i), such as immediately before contact with the alkaline aqueous composition (I). 2 Carbon deposits originating from the aforementioned organic pollutants. Carbon deposits retained on the surface formed by the metallic material of the component can be determined by pyrolysis. For this purpose, a representative portion of the component in a defined area is subjected to a base temperature (PMT) of 550°C in an oxygen atmosphere, and the amount of carbon dioxide released is quantitatively determined as carbon content using an infrared sensor, for example, an analytical device such as the LECO® RC-412 multiphase carbon analyzer (Leco Corp.).

[0020] In continuous operation, i.e., during the processing of a series of multiple components, contaminants absorbed by alkaline aqueous detergents accumulate in the degreasing bath. It has now been determined that, in the method according to the invention, the adhesion of the paint film on hot-dip galvanized (ZM) surfaces can be maintained even when the proportion of nonpolar hydrocarbons in the degreasing bath exceeds a critical threshold. Therefore, the method according to the invention is advantageous when the critical threshold is reached or exceeded in the degreasing bath. Even when the concentration exceeds 0.05 kg / m³ in the degreasing bath... 3 The proportion of nonpolar hydrocarbons can lead to a significant deterioration in the paint adhesion of hot-dip galvanized (ZM) steel if the anti-corrosion pretreatment described in conversion stage ii) is not performed as specified according to the invention. In this regard, due to the continuous processing of multiple components, once at least 0.05 kg / m 3 A minimum of 0.10 kg / m³ is preferred. 3 Especially at least 0.20 kg / m 3 Nonpolar hydrocarbons accumulate in the defatting bath, and the method according to the invention achieves the full effect. The proportion of nonpolar hydrocarbons in the defatting bath can be determined in a sample of the defatting bath that has been adjusted with hydrochloric acid (methyl orange color change point) and to which equal parts (1 / 10) of sodium chloride and equal parts (1 / 4) of ethanol have been added. From this prepared defatting bath sample, a certain proportion of hydrocarbons is extracted by shaking with an equal part (1 / 1) of petroleum ether. After phase separation, the petroleum ether phase is mixed with silica gel with the aid of continuous addition of ethanol to separate polar organic components such as fatty acids, acid esters, and nonionic surfactants. After filtration, the proportion of nonpolar hydrocarbons can be determined by gravimetric analysis after the petroleum ether has been distilled off.

[0021] The critical threshold for nonpolar hydrocarbons is reached only after a certain number of components have passed. In this case, it is preferable that the series of components includes multiple components whose total surface area formed by the metallic material of the components is greater than the following terms: VB: The volume of the system tank during the degreasing stage, in meters. 3 as a unit KW crit The critical threshold for the proportion of nonpolar hydrocarbons in the system tank during the degreasing stage, expressed in kg / m³. 3 The unit is 0.05 kg / m³, where the critical threshold is 0.05 kg / m³. 3 0.1kg / m 3 0.2kg / m 3 Δm TOC The change in carbon content per unit area on the surface of the component formed of metallic material after the degreasing stage, expressed in kg / m². 2 Units.

[0022] The aforementioned properties of the degreasing stage, particularly regarding the proportion of nonpolar hydrocarbons present, always refer to the last degreasing stage prior to the conversion stage. In industry, it is very common to use multiple degreasing stages to thoroughly remove contaminants from the parts to be pretreated and to optimally condition the metal substrate for subsequent conversion stages via acid cleaning. Therefore, in order to determine the critical threshold for nonpolar hydrocarbons according to the aforementioned terminology, the amount of hydrocarbons in the system tank volume of the final degreasing stage must be considered, and similarly, variations in carbon content per unit area on the surface formed by the metal material of the parts must be considered after the final degreasing stage.

[0023] For good degreasing performance, the pH of the alkaline aqueous composition (I) is preferably at least 9.50, particularly preferably at least 10.5. However, mild to moderate acid cleaning of the zinc-magnesium coating is beneficial for maintaining good corrosion resistance of the (ZM) substrate, so the pH is preferably less than 12.50, particularly preferably less than 12.00, and very particularly preferably less than 11.50.

[0024] According to the present invention, the pH in the defatting stage corresponds to the negative decimal logarithm of the hydrated hydrogen ion activity measured at 20°C using a pH-sensitive glass electrode in an alkaline aqueous composition (I) after two-point calibration for technical buffer solutions of acetate / acetate (pH=4.0) and boric acid / borate (pH=9.0).

[0025] Preferably, the alkaline aqueous detergent has a specific buffering capacity such that, in the method according to the invention, its total alkalinity at the reaction point is at least 5.0, particularly preferably at least 8.0, very particularly preferably at least 10.0, but preferably not exceeding 30.0, particularly preferably not exceeding 20.0. The total alkalinity corresponds to the concentration obtained at 20°C in the presence of bromocresol green indicator (color change point: pH 3.6) using 50 ml of deionized water (κ < 1 μS / cm). -1 The consumption of 0.1 N hydrochloric acid, in milliliters, after titration of a 10 ml diluted alkaline aqueous composition (I) sample.

[0026] To adjust the alkalinity of the alkaline aqueous composition (I), all known builders in the art, which are alkaline reactive compounds or mixtures of such compounds, can be used. Particularly suitable and defined builders are alkaline reactive inorganic compounds, which are preferred in the context of this invention, and particularly preferably selected from water-soluble hydroxides, carbonates, borates, silicates and / or phosphates, wherein at least water-soluble phosphates are more preferably present, which are more preferably selected from orthophosphates, pyrophosphates and / or tripolyphosphates. Thus, suitable builders are alkali metal carbonates (preferably selected from potassium carbonate), alkali metal hydroxides (preferably selected from potassium hydroxide) mixed with phosphoric acid and / or with boric acid, and alkali metal tripolyphosphates (preferably selected from potassium tripolyphosphate).

[0027] The alkaline aqueous composition (I) contains at least one surfactant, preferably selected from surfactants, for effective degreasing. Surfactants within the meaning of this invention are considered to be surface-active organic compounds that, in terms of their surface activity, consist of either a hydrophilic component and at least one lipophilic molecular component or a lipophilic component and at least one hydrophilic molecular component, and the molecular weight of the surface-active organic compound does not exceed 2000 g / mol.

[0028] The surfactant used in the degreasing stage of step i) of the method according to the invention may be selected from anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants, with nonionic surfactants generally preferred. Particularly suitable nonionic surfactants for degreasing parts containing hot-dip galvanized (ZM) surfaces, as components of the alkaline aqueous composition (I), are those with an HLB value (hydrophilic-lipophilic balance) of at least 8, particularly preferably at least 10, particularly preferably at least 12, but particularly preferably not exceeding 18, and particularly preferably not exceeding 16. The HLB value is used as a quantitative reference variable for classifying nonionic surfactants in terms of their miscibility with water or their ability to form O / W emulsions. For quantification, the nonionic surfactant is broken down into lipophilic and hydrophilic groups. The HLB value is then calculated as follows, and values ​​from 0 to 20 can be assumed on any scale: HLB=20·(1-M L / M) Where M L Molar mass of the lipophilic group in a nonionic surfactant M: Molar mass of the nonionic surfactant.

[0029] In the degreasing stage of the method according to the invention, the preferred nonionic surfactant is selected from those selected from alkoxylated alkyl alcohols, alkoxylated fatty amines, and / or alkyl polysaccharides, particularly preferably selected from those selected from alkoxylated alkyl alcohols and / or alkoxylated fatty amines. In this case, for defoaming effect, the alkoxylated alkyl alcohols and / or alkoxylated fatty amines are preferably end-capped, particularly preferably having an alkyl group, which preferably has no more than 8 carbon atoms, particularly preferably no more than 4 carbon atoms. In the degreasing stage of the method according to the invention, the alkoxylated alkyl alcohols and / or alkoxylated fatty amines used as nonionic surfactants are particularly preferred to be those present in ethoxylated and / or propoxylated forms, and the number of epoxide units is preferably no more than 16, particularly preferably no more than 12, particularly preferably no more than 10, but particularly preferably greater than 4, particularly preferably greater than 6.

[0030] Regarding the lipophilic component of the aforementioned nonionic surfactant, in the degreasing stage of the method according to the invention, the preferred alkoxylated alkyl alcohols and / or alkoxylated fatty amines as nonionic surfactants are those in which the alkyl group is saturated and preferably straight-chain, and the number of carbon atoms in the alkyl group is preferably greater than 6, particularly preferably at least 10, particularly preferably at least 12, but preferably not greater than 20, particularly preferably not greater than 18, and particularly preferably not greater than 16.

[0031] In general, it is evident that long-chain nonionic surfactants are well-suited and preferred for the effective cleaning and degreasing of conventional drawing oils, forming oils, rolling oils, and anti-corrosion oils, such that in another preferred embodiment of the method according to the invention, alkoxylated alkyl alcohols and / or alkoxylated fatty amines are preferred as surfactant components of the alkaline aqueous composition (I), especially those alkoxylated alkyl alcohols whose lipophilic alkyl groups contain 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 has an HLB value of 12 to 16.

[0032] Preferred representatives of alkoxylated alkyl alcohols are selected from, for example... Tetra- to octa-ethoxylated or propoxylated C6-C12 fatty alcohols Octa- to dodeoxylated C12-C18 fatty alcohols Hexa- to tetradecyl propoxylated C12-C18 fatty alcohols, Hexa- to deca-ethoxylated and propoxylated C12-C14 fatty alcohols, It can also exist in a form with methyl, butyl or benzyl end groups closed.

[0033] The cloud point, as determined according to DIN 53 917 (1981), is another suitable selection criterion for nonionic surfactants used in the degreasing stage, the nonionic surfactants being selected from alkoxylated alkyl alcohols, alkoxylated fatty amines and / or alkyl polysaccharides, preferably above 20°C, but particularly preferably below the application temperature of the alkaline aqueous composition (I) in the degreasing stage, particularly preferably above 5°C but not exceeding 10°C, and below the separately selected application temperature of the alkaline aqueous composition (I) used for degreasing.

[0034] The surfactant, particularly the nonionic surfactant, is preferably present in a proportion greater than 0.01 wt.%, particularly preferably greater than 0.10 wt.%, particularly preferably greater than 0.20 wt.%, but preferably not greater than 2.00 wt.%, in each case based on the alkaline aqueous composition (I).

[0035] Furthermore, the alkaline aqueous composition (I) preferably contains an alkaline builder selected from phosphates, pyrophosphates, phosphoric acid, silicates, carbonates and hydroxides, as well as mixtures of these builder substances, to provide its alkalinity.

[0036] For resource-saving methods and for economic reasons, it is preferred that the alkaline aqueous composition (I) in the conversion stage of the method according to the present invention contains... (a) In each case, less than 10 mg / kg of a compound of metal Bi, Ni, Co and / or Cu dissolved in water (in the amount of the corresponding element in the aqueous composition), preferably less than 10 mg / kg of a compound of such metal dissolved in water (in the amount of the corresponding element in the aqueous composition) having a standard reduction potential greater than -0.40 V (SHE). (b) A copolymer comprising less than 0.01 g / L (based on solid additives), wherein the copolymer has monomer units containing at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group and monomer units without acid groups in an alternating configuration, preferably a copolymer comprising less than 0.1 g / L (based on solid additives), wherein the copolymer has monomer units containing at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group, particularly preferably an organic polymer comprising less than 0.1 g / L, wherein the organic polymer is not any of the above-mentioned surfactants with a molecular weight not greater than 2000 g / mol.

[0037] The application and thus contact of the alkaline aqueous composition (I) is preferably carried out at at least 30°C, particularly preferably at least 40°C, but preferably below 60°C. The alkaline aqueous composition (I) in the degreasing stage can be contacted with the series of components by means of application types established in the prior art. These specifically include impregnation, rinsing, spraying, and / or atomizing, wherein impregnation of the series of components occurs in a system tank containing the corresponding alkaline aqueous composition (I) during the degreasing stage and / or by spraying alkaline aqueous composition (I) stored in the system tank. As previously discussed, there may be multiple degreasing stages prior to the conversion stage, which in turn feed the alkaline composition via their own storage system tanks.

[0038] Transformation phase: According to the present invention, the conversion treatment in step ii) must be carried out with an acidic aqueous composition containing at least 2.80 mmol / kg of free fluoride; otherwise, it cannot be guaranteed that the adhesion of the resulting paint film to the hot-dip galvanized (ZM) surface is largely unaffected by the nonpolar hydrocarbons enriched during the degreasing stage.

[0039] However, it has been found that a higher proportion of free fluoride for adhesion of the paint film to the hot-dip galvanized (ZM) surface of the component treated according to the method of the invention does not adversely affect corrosion protection, while even moderately increasing the free fluoride content further increases the reliability of the method according to the invention, because the proportion of nonpolar hydrocarbons that can be enriched in the degreasing bath can be significantly increased without loss of performance. In a preferred embodiment, the acidic aqueous composition (II) in the conversion stage of step ii) therefore contains at least 3.00 mmol / kg, particularly preferably at least 3.20 mmol / kg, and very particularly preferably at least 3.40 mmol / kg of free fluoride. However, for the compatibility of the method according to the invention with other metallic materials (especially steel), it is preferred that the proportion of free fluoride in the acidic aqueous composition (II) be less than 7.50 mmol / kg, particularly preferably less than 6.00 mmol / kg, and very particularly preferably less than 5.00 mmol / kg. Since technical components made of galvanized (ZM) steel that are continuously subjected to corrosion protection treatment according to the method of the present invention typically also have steel surfaces, such as on the cut edges or worn-through areas of semi-finished products made of galvanized (ZM) steel, which are formed into body parts, for example, in the form of cut sheet metal, limiting the free fluoride content in the conversion stage to the aforementioned upper limit is generally beneficial for fully satisfactory corrosion protection, even for components made solely of hot-dip galvanized (ZM) steel, to avoid flash rust formation on the aforementioned exposed steel surfaces.

[0040] The amount of free fluoride in each stage of the pretreatment according to the invention can be determined by potentiometric measurement at 20°C in the provided solution after calibration with a fluoride-containing buffer solution without pH buffer.

[0041] If a slightly higher free fluoride content is present at a lower pH value, and preferably a minimum amount of free fluoride in mmol / kg, then the pH of the acidic aqueous composition (II) during the conversion stage is also significant and generally favorable for obtaining reliable and good paint film adhesion to the hot-dip zinc-magnesium (ZM) surface, wherein the content is calculated according to the method of the present invention in ascending order of priority as follows: , , , , , In each case, the pH of the acidic aqueous composition (II) is referred to as "pH".

[0042] Alternatively, depending on the pH of the acidic aqueous composition (II), a preferred minimum amount of free fluoride may be used, calculated according to the following formula: , In each case, the pH of the acidic aqueous composition (II) is referred to as “pH”, which is preferably less than 5.00 herein.

[0043] In a particularly preferred embodiment, the minimum amount of free fluoride that should be present in the acidic aqueous composition for optimal film adhesion to the (ZM) surface is adjusted according to the pH and the ratio of nonpolar hydrocarbons as follows: in pH: The pH of the acidic aqueous composition (II) is less than 4.80, preferably greater than 4.00, otherwise corrosion may occur on the exposed steel surfaces of hot-dip galvanized (ZM) steel processed into parts due to the high minimum fluoride content. KW: Dimensionless proportion of nonpolar hydrocarbons (in kg / m³) in the alkaline aqueous composition (I) in the system tank during the degreasing stage. 3 It is estimated that it is at least 0.10 kg / m³. 3 However, it is preferable to have a concentration not exceeding 0.80 kg / m³. 3 0.60kg / m 3Otherwise, due to the high minimum fluoride content, corrosion may occur on the exposed steel surface of hot-dip galvanized (ZM) steel processed into parts.

[0044] Suitable free fluoride sources for the acidic aqueous composition (II) are elements Zr, Ti and / or Si, preferably elements Zr and / or Ti, particularly preferably water-soluble complexed fluorides of element Zr, and / or hydrofluoric acid, ammonium hydrogen fluoride and / or water-soluble alkali metal fluorides.

[0045] Regarding the pH of the acidic aqueous composition in the conversion stage, it should be noted that compounds of elemental Zr and / or Ti dissolved in water do not form a brine solution due to hydrolysis, and therefore can no longer be used to form the conversion layer. Simultaneously, the acid cleaning rate of common metallic materials should be sufficiently high to form a uniform, sealing conversion layer; this is particularly applicable to hot-dip galvanized (ZM) steel substrates. Therefore, according to the invention, the acidic aqueous composition (II) needs to have a pH higher than 5.20, and the pH is preferably less than 5.10, particularly preferably less than 5.00, very particularly preferably less than 4.90, and particularly preferably less than 4.80. Meanwhile, increased acid cleaning and rapid layer formation kinetics may be detrimental to the formation of a suitable conversion coating. In the lower pH range according to the invention, particularly on hot-dip galvanized (ZM) steel, relatively high layer weights based on elemental Zr and / or Ti are achieved, but these are less dense and are themselves susceptible to corrosion at low pH values, potentially leading to the formation of pitting defects in the conversion coating. Therefore, according to the present invention, it is preferred that the pH value of the acidic aqueous composition (II) is greater than 4.00, particularly preferred to be greater than 4.20, and very particularly preferred to be greater than 4.40.

[0046] According to the present invention, during the conversion phase, pH corresponds to the negative decimal logarithm of the hydrated hydrogen ion activity measured at 20°C using a pH-sensitive glass electrode in an acidic aqueous composition (II) after two-point calibration relative to technical buffer solutions of acetate / acetate (pH=4.0) and phosphate (pH=7.0).

[0047] Therefore, during the conversion stage, the objective is to construct a conversion coating that is as uniform and dense as possible, based on elemental Zr and / or Ti, preferably Zr oxide / hydroxide compounds. Thus, in a preferred embodiment, the contact operation continues at least until a minimum of 20 mg / m² is formed on the hot-dip zinc-magnesium (ZM) steel surface. 2 A minimum of 40 mg / m² is preferred. 2 Layered sediments, but contact operations are preferably not sustained for too long to cause layered sediments to exceed 250 mg / m³. 2 Specially selected for concentrations exceeding 150 mg / m² 2 Very special preference for doses exceeding 100 mg / m³ 2Especially preferred is a concentration exceeding 80 mg / m³. 2 All of the above are based on the elemental Zr and / or Ti on the surface of hot-dip zinc-magnesium (ZM) galvanized steel. The layer deposit can be determined by X-ray fluorescence analysis (XRF). The preferred treatment time for the layer deposit, i.e., the duration of contact with the acidic aqueous composition (II) at a preferred temperature of 10-60°C, should be between 10 seconds and 300 seconds. To ensure this, it is preferred that, according to the method of the invention, the proportion of elemental Zr and / or Ti compounds dissolved in water in the acidic aqueous composition (II) in step ii) is preferably at least 0.10 mmol / kg, particularly preferably at least 0.30 mmol / kg, and especially preferably at least 0.40 mmol / kg. For process economy reasons, the content of elemental Zr and / or Ti compounds dissolved in water should preferably be less than 5.0 mmol / kg, particularly preferably less than 3.0 mmol / kg, and very particularly preferably less than 2.0 mmol / kg.

[0048] In the conversion stage of the method according to the invention, an amorphous oxide / hydroxide coating based on elemental Zr and / or Ti, preferably elemental Zr, is achieved, and thus compounds of elemental Zr and / or Ti dissolved in water are present. The term "dissolved in water" includes both molecularly dissolved substances and compounds that dissociate in aqueous solution and form hydrated ions. Typical representatives of these compounds are titanium oxysulfate (TiO(SO4)), titanium oxynitrate (TiO(NO3)2) and / or hexafluorotitanic acid (H2TiF6) and their salts, or ammonium zirconium carbonate ((NH4)2ZrO(CO3)2) and / or hexafluorozirconic acid (H2ZrF6) and their salts. The compounds dissolved in water in the conversion stage are preferably selected from fluorescent acids and / or fluorine complexes of elemental Zr and / or Ti and their water-soluble salts. The formation of a conversion layer based on fluorescent acids and / or fluorine complexes of elemental Zr is particularly preferred because such a conversion layer provides improved film adhesion.

[0049] To repair point defects in conversion coatings grown on the surface of galvanized steel, particularly hot-dip galvanized (ZM) steel, the presence of copper ions in step ii) of the method, in which a rapid overall layer formation occurs, may be advantageous, as their local displacement deposition in the point defects provides improved corrosion protection. Therefore, it is preferred that the acidic aqueous composition (II) of the conversion stage in step ii) further contains copper ions dissolved in water, preferably at least 0.05 mmol / kg, but more preferably less than 4.0 mmol / kg, and particularly preferably less than 2.0 mmol / kg. 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).

[0050] Other additives known to those skilled in the art of surface treatment may be present, such as promoters like nitrate ions, nitrite ions, nitroguanidine, N-methylmorpholine N-oxide, free or bound forms of hydrogen peroxide, free or bound forms of hydroxylamine, reducing sugars, and / or wetting agents such as nonionic surfactants, and / or polymers such as polyamidoamines, and / or cationic / compound forms of elements Mg, Ca, Al, Si, Sn, Bi, and / or Mo, to improve layer formation kinetics, wettability, and corrosion resistance. For resource-saving methods and for economic reasons, it is preferred that the acidic aqueous composition (II) in the conversion stage of the method according to the invention contains... (a) Chromium-containing compounds, in terms of total chromium content, less than 100 mg / kg, preferably less than 50 mg / kg, particularly preferably less than 10 mg / kg, and especially preferably less than 1 mg / kg. (b) A total of less than 100 mg / kg, preferably less than 10 mg / kg, of water-soluble phosphates, preferably water-soluble phosphorus compounds, in each case based on the amount of phosphorus. (c) A total of less than 100 mg / kg, preferably less than 10 mg / kg, of hydrolyzable organosilanes and siloxanes (calculated as Si(OCH2CH3)4), preferably compounds of elemental silicon dissolved in water (calculated as the amount of Si), and / or (d) A copolymer having a total of less than 0.01 g / L (based on solid additives), wherein the copolymer has monomer units containing at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group and monomer units without acid groups in an alternating configuration, preferably a copolymer having a total of less than 0.1 g / L (based on solid additives), wherein the copolymer has monomer units containing at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group, and particularly preferably an organic polymer that is not a polyamide amine having a total of less than 0.1 g / L.

[0051] The application and thus contact of the acidic aqueous composition (II) is preferably carried out at at least 30°C, particularly preferably at least 40°C, but preferably below 60°C. The acidic aqueous composition (II) in the conversion phase can be applied to the series of components by means of application types established in the prior art. These particularly include immersion, rinsing, spraying, and / or atomizing, with application by immersion and / or spraying methods being preferred, particularly by immersing the series of components in a system tank containing the corresponding acidic aqueous composition (II).

[0052] Painting stage: In the painting stage, the surfaces of components formed at least of hot-dip galvanized (ZM) steel and coated in step ii) of method, preferably all surfaces formed of metallic materials, are coated with a first paint system by contacting the components or at least the coated hot-dip galvanized (ZM) steel surfaces with an aqueous dispersion (III) containing an organic binder. Thus, the paint system is deposited directly from the aqueous phase, as a coating of the organic binder of the aqueous dispersion (III) deposited on the aforementioned surfaces, which is typically post-treated by heat to form a film and cure. Coating in the painting stage is preferably performed by dip coating, particularly preferably by electrocoating, and further preferably by cathodic electrocoating. For this purpose, the organic binder of the aqueous dispersion (III) is preferably based on an amine-modified film-forming polyepoxide, which preferably additionally contains an organic compound containing capped and / or uncapped isocyanate groups as a curing agent. Inorganic pigments are also typically components of the aqueous dispersion and preferred additives for improving corrosion protection. The aqueous phase preferably contains compounds containing trace amounts of yttrium and / or bismuth dissolved or dispersed in water, which have a positive effect on crosslinking and film formation.

[0053] The preferred pH of the aqueous dispersion (III) in the coating stage is 5.0 to 6.0, particularly preferably 5.4 to 5.8. According to the invention, in the coating stage, the pH corresponds to a two-point calibration at 20°C using a pH-sensitive glass electrode with respect to technical buffer solutions of acetate / acetate (pH=4.0) and boric acid / borate (pH=9.0), followed by deionized water (κ<1 μScm). -1 The negative decimal logarithm of the hydrated hydrogen ion activity measured in an aqueous dispersion (III) diluted 10 times.

[0054] The application of the aqueous dispersion (III) to the surface of the hot-dip galvanized (ZM) steel component, or at least the conversion coating, into contact with the component is preferably carried out at a temperature of at least 30°C, particularly preferably at least 40°C, but more preferably below 60°C. The aqueous dispersion (III) in the painting stage can be brought into contact with a series of components by means of application types established in the prior art. These particularly include dip coating, spraying, and roller coating, with dip coating being preferred, specifically by immersing a series of components into a system tank containing the corresponding aqueous dispersion (III), and in the case of dip coating, the type of paint system has been predetermined.

[0055] program: Preferred embodiments of the method according to the invention are shown and explained below with respect to the various processing stages and implementation methods, which are particularly advantageous to the purpose on which the invention is based.

[0056] Processing stages i)-iii) of the method according to the invention each include at least one processing step that brings a series of components into contact with an aqueous composition, which is specific to and more precisely defined by the processing stage. These characteristic compositions are stored or held in a system tank for contact purposes, and contact can be made within the system tank, for example by immersion in the composition held therein, or outside the system tank, for example by atomizing the composition stored in the system tank in an atomization chamber, depending on the specific requirements or preferences of the relevant method steps.

[0057] In the method of the present invention, steps i)-iii) are performed sequentially, i.e., in a specified order, and preferably, between two steps i)-iii), the component undergoes no other wet chemical treatment steps other than a rinsing step. In this context, the rinsing step is primarily, preferably, only used to remove the wet film adhering to the component in the previous wet chemical method step, thereby completely or partially removing soluble residues, particles, and active components that would otherwise adhere to the component and be carried from the previous wet chemical method step to the next processing stage.

[0058] In a preferred method according to the invention, a so-called rinsing stage, comprising at least one rinsing step, follows both the degreasing and conversion stages. During the rinsing stage, the series of components are freed from the wet film adhering from the degreasing and conversion stages to prevent the active components from being carried over to the next processing stage. For this purpose, the rinsing stage consists of one or more rinsing steps that proceed sequentially. Here, the rinsing steps are also performed sequentially if the components are not subjected to another wet chemical treatment step (which is not a rinsing step). For the practical function of the rinsing stage, namely preventing the active components from being carried over to subsequent wet chemical treatment stages, it is advantageous and therefore preferred in the invention that the rinsing stage, as described above, comprises multiple sequentially rinsing steps so that the series of components, in each case, comes into contact with the rinsing solution stored in the system tank of the relevant rinsing step.

[0059] For the rinsing stage after the degreasing stage and before the conversion stage, it is preferable that the specific conductivity in the system tank during a single or final rinsing step is less than 100 μScm. -1 .

[0060] For the rinsing stage, which occurs after the conversion stage and before the painting stage, it is preferable that the specific conductivity in the system tank during a single or final rinsing step is less than 40 μScm. -1 .

[0061] To achieve this objective, a specific conductivity of less than 10 μS / cm is preferably supplied to each rinsing stage. -1Fresh water, wherein the volumetric flow rate of the supplied fresh water is large enough to avoid exceeding the maximum specific conductivity specified for the rinsing phase in each case during the treatment of the series of components.

[0062] During the rinsing stage, the wet film from the previous wet chemical treatment stage should be removed as thoroughly as possible. Therefore, the rinsing stage is carried out with a rinsing solution that itself does not contain any kind or amount of any active component, which would otherwise be problematic and must be prevented from being carried into subsequent treatment stages. However, if desired, the rinsing solution may contain small amounts of redox-active compounds (“depolarizers”), such as hydrogen peroxide, or other surfactants such as nonionic surfactants, to improve the wettability of the rinsing solution. This is particularly applicable to rinsing stages prior to the conversion and painting stages. However, the addition of additives should not imply an increase in the specific conductivity specified in a single or final rinsing step of the rinsing stage. In particular, the presence of elements and compounds that may adversely affect corrosion protection performance should be avoided. Similarly, the addition of additives should always be omitted if it does not hinder the main purpose of the rinsing stage but does not provide a significant improvement in the performance of the fundamental aspects of the method according to the invention, and is therefore economically unreasonable. Therefore, it is preferred that the rinsing solution of a single or final rinsing step in the rinsing stage of the method according to the invention, preferably any rinsing solution of all rinsing steps in the rinsing stage, contains... (a) In each case, less than 10 mg / kg of a compound of metal Bi, Ni, Co and / or Cu dissolved in water, based on the amount of the corresponding element in the aqueous composition; preferably, in each case, less than 10 mg / kg of a compound of such metal dissolved in water having a standard reduction potential greater than -0.40 V (SHE), based on the amount of the corresponding element in the aqueous composition. (b) The total amount of surfactant is less than 100 mg / kg, preferably less than 50 mg / kg, particularly preferably less than 10 mg / kg, preferably a surface-active organic compound, and particularly preferably an organic compound. (c) Total less than 100 mg / kg, preferably less than 10 mg / kg of organosilanes and siloxanes (calculated as Si(OCH2CH3)4), preferably compounds of elemental silicon dissolved in water (calculated as the amount of Si). (d) A total of less than 20 μmol / kg, preferably less than 10 μmol / kg, and particularly preferably less than 5 μmol / kg of compounds of Zr and / or Ti dissolved in water. (e) Total sodium and / or potassium ions less than 50 mg / kg, preferably less than 10 mg / kg each. (f) Total zinc ion concentration less than 50 mg / kg, preferably less than 10 mg / kg. (g) A total of less than 100 mg / kg, preferably less than 10 mg / kg, of phosphates dissolved in water, preferably phosphorus-containing compounds dissolved in water, in each case in terms of the amount of phosphorus, and / or (h) A copolymer having a total concentration of less than 0.01 g / L (based on solid additives), wherein the copolymer has monomer units containing at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group and monomer units without acid groups in an alternating configuration, preferably a copolymer having a total concentration of less than 0.1 g / L (based on solid additives), wherein the copolymer has monomer units containing at least one carboxylic acid group, phosphonic acid group and / or sulfonic acid group, and particularly preferably an organic polymer having a concentration of less than 0.1 g / L. The pH of the rinsing solution is preferably between 5.0 and 8.5.

[0063] The standard reduction potential is the value relative to the standard hydrogen electrode H2 / H at a metal ion activity of 1 mol / L and a temperature of 20 °C. + Electrochemical half-cell Me / Me ratio measured at pH=0 n+ The reduction potential.

[0064] Example: Corrosion protection treatment was performed on several sections (each 10cm x 20cm) of hot-dip galvanized steel sheets of type ZM (CR180 ZM 40 / 40-EWO from Voetalpine AG) and type HDG (Gardobond® MBZ from Chemetall), using the following process steps. 1.0-2.0 g / m 2 Anticorrosive oil (Anticorit® RP 4107 LV from Fuchs Europe Schmierstoffe GmbH) was applied to the steel plate.

[0065] A. Clean by immersion in a bath at 55°C for 180 seconds: In deionized water (κ<1μScm) -1 The 2g / l BONDERITE® C-AK 2011 and 1g / l BONDERITE® C-AD 1270 in the product (both from Henkel AG&Co. KGaA) pH: 11.5 (measured using a glass electrode at 25°C) FA: 5 points (10ml sample, pH 8.5) GA: 12 points PO4: 2.5g / l Bath volume: 5 liters B. By using deionized water (κ<1μScm) -1 Rinse by spraying for 30 seconds at 20°C. C. By using deionized water (κ<1μScm) at 20℃ -1 Soak for 30 seconds and then rinse. D. Conversion treatment by immersion in a bath at 35°C for 120 seconds. Zr: 150mg / kg Cu: 10mg / kg Zn: 0.6g / kg NO3: 6g / kg pH-1: 4.7 (measured using a glass electrode at 25°C) pH-2: 4.2 (measured using a glass electrode at 25°C) F-free: 40-80 ppm (measured at 35°C using an ion-sensitive electrode). The achieved Zr layer weight on HDG is approximately 76-79 mg / m³. 2 While achieving 61-64 mg / m³ on ZM 2 Zr deposits (each measured by X-ray fluorescence analysis) E. By using deionized water (κ<1μScm) at 20℃ -1 Soak for 30 seconds and then rinse. F. By using deionized water (κ<1μScm) at 20℃ -1 Soak for 30 seconds and then rinse. G. Dry by blowing with compressed air and store in a drying oven at 50°C. H. Perform cathode dip coating with Cathoguard® 800 (BASF SE) to a dry layer thickness of 28-30 μm. I. Topcoat structure (using thin film filler ALG 697 172; base coat: gloss black ALD 091 Y9B; clear coat: ALD 096 100, all from Audi AG), dry layer thickness is 105-115μm.

[0066] To continuously simulate corrosion protection treatments, multiple steel plates were cleaned one after another (first with ZM, then alternating with HDG), and increasing hydrocarbon concentrations were set in this manner. The results regarding corrosion protection are summarized in Tables 1 and 2.

[0067] In this series of tests, the critical threshold for hydrocarbons was approximately 0.10 kg / m³. 3This corresponds to a throughput of approximately 12 plates. Above this critical threshold, significant deterioration of corrosion protection on (ZM) plates can be observed (cross-cut rating >2). However, it was found that by increasing the minimum amount of free fluoride, good protection against corrosive stratification of the paint system can be maintained. Overall, it was also found that at the same bath load as nonpolar hydrocarbons, the lower pH during the conversion phase necessitates a higher proportion of free fluoride to maintain good corrosion protection.

Claims

1. A method for pretreating a series of multiple components for corrosion protection, wherein the series of components at least partially have hot-dip galvanized magnesium-coated steel surfaces, and wherein each of the series of components sequentially undergoes method steps i)-iii), and at least the hot-dip galvanized magnesium-coated steel surfaces are sequentially contacted with respectively provided aqueous solutions (I)-(III): i) During the defatting stage, an alkaline aqueous composition (I) with a pH higher than 9.00 is provided; ii) In the conversion stage, an acidic aqueous composition (II) with a pH of 3.50 to 5.20 is provided, said acidic aqueous composition (II) containing at least 0.05 mmol / kg of a compound of elemental Zr and / or Ti dissolved in water and at least 2.80 mmol / kg of free fluoride; and iii) In the painting stage, it provides an aqueous dispersion of the organic binder (III).

2. The method according to any one of the preceding claims, characterized in that, The alkaline aqueous composition (I) in the defatting stage of method step i) contains more than 0.05 kg / m 3 Preferably greater than 0.10 kg / m 3 A concentration greater than 0.20 kg / m³ is preferred. 3 Nonpolar hydrocarbons.

3. The method according to any one of the preceding claims, characterized in that, The acidic aqueous composition (II) in the conversion stage of method 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 very particularly preferably less than 5.00 mmol / kg of free fluoride.

4. The method according to any one of the preceding claims, characterized in that, The acidic aqueous composition (II) in the conversion stage of method step ii) contains a minimum amount of free fluoride in mmol / kg, said minimum amount being calculated according to the following: ， The preferred method is calculated based on the following: ， Particularly preferred calculation is based on the following: ， Very particularly preferably, it is calculated based on the following: ， And particularly preferably, it is calculated based on the following: ， In each case, the pH of the acidic aqueous composition (II) is used for the pH.

5. The method according to any one of claims 2 to 3, characterized in that, The acidic aqueous composition (II) in the conversion stage of method step ii) contains a minimum amount of free fluoride in mmol / kg, said minimum amount being calculated according to the following: in pH: The pH of the acidic aqueous composition (II) is less than 4.80, and preferably greater than 4.

00. KW: The dimensionless proportion of nonpolar hydrocarbons in the alkaline aqueous composition (I) in the system tank during the degreasing stage, expressed in kg / m³. 3 It is estimated that it is at least 0.10 kg / m³. 3 However, it is preferable to have a concentration not exceeding 0.80 kg / m³. 3 .

6. The method according to any one of the preceding claims, characterized in that, The acidic aqueous composition (II) in the conversion stage of method step ii) contains the corresponding fluoric acid and / or its water-soluble salt as a source of the compound of the dissolved elements Zr and / or Ti.

7. The method according to any one of the preceding claims, characterized in that, The acidic aqueous composition (II) in the conversion stage of method step ii) contains a compound of element Zr and / or Ti dissolved in water at a concentration of 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, and very particularly preferably less than 2.0 mmol / kg.

8. The method according to any one of the preceding claims, characterized in that, The pH of the acidic aqueous composition (II) in the conversion stage of method step ii) is 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, and particularly preferably less than 4.

80.

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

10. The method according to any one of the preceding claims, characterized in that, During the conversion stage, contact with the conversion solution (II) continues at least until a minimum of 20 mg / m² is formed on the hot-dip galvanized magnesium steel surface. 2 Especially preferred is at least 40 mg / m² 2 The layers of deposits, but the contact is preferably not prolonged enough to result in a concentration greater than 250 mg / m³ of Zr and / or Ti on these surfaces in each case. 2 Especially preferred is greater than 150mg / m³ 2 A very special preference is given to those with a concentration greater than 100 mg / m². 2 Especially preferred is a concentration greater than 80 mg / m³ 2 Layer of sediments.

11. The method according to any one of the preceding claims, characterized in that, In step iii), the contact during the coating stage is dip coating, preferably electrocoating, particularly preferably cathodic electrocoating, and further preferably an aqueous dispersion comprising an amine-modified polyepoxide as an organic binder (III), wherein the aqueous dispersion preferably additionally contains a water-soluble or water-dispersible salt of yttrium and / or bismuth.

12. The method according to any one of the preceding claims, characterized in that, An intermediate rinse is performed during the transfer of the component from method step i) or ii) to the next step, except that method steps i)-iii) are performed sequentially, and a drying step is preferably not used.

13. The method according to any one of the preceding claims, characterized in that, The components of the series are composite structures, preferably automobile bodies, which are composed of semi-finished products of hot-dip galvanized magnesium steel and semi-finished products of galvanized steel and aluminum, and are particularly preferably assembled from semi-finished products of hot-dip galvanized magnesium steel and semi-finished products of galvanized steel, aluminum and steel.

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