Method for alkaline cleaning of zinc-magnesium-alloyed strip steel

Abstract

The present invention relates to a method for the wet-chemical cleaning and conditioning of hot-dip galvanised (HDG) steel by contacting it with an alkaline aqueous composition containing magnesium ions dissolved in water, and at least one complexing agent. A method for pre-treating strip steel by hot dip galvanisation (HDG) on one or both sides in order to protect the steel against corrosion is also included, in which a cleaning according to the invention is followed by a wet-chemical conversion coating and a subsequent building of a varnish layer. The present invention also relates to an alkaline aqueous composition containing both magnesium ions and iron ions, which is particularly suitable for the cleaning and subsequent surface treatment for protection against corrosion, as well as to a system of complexing agents.

Description

[0001] .Process for alkaline cleaning of zinc-magnesium alloyed strip steel'

[0002] The present invention relates to a process for the wet-chemical cleaning and conditioning of hot-dip galvanized (ZM) steel by contacting it with an alkaline, aqueous composition containing magnesium ions dissolved in water and at least one complexing agent. Likewise encompassed is a process for the corrosion-protective pretreatment of strip steel with single- or double-sided hot-dip galvanization (ZM), in which a cleaning according to the invention is followed by a wet-chemical conversion coating and a subsequent paint layer buildup. Furthermore, the present invention relates to an alkaline, aqueous composition containing both magnesium ions and iron ions, as well as a system of complexing agents, that is particularly suitable for cleaning and subsequent corrosion-protective surface treatment.

[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 provides significantly increased corrosion protection and, particularly after coating with organic topcoats and dip coatings, offers excellent 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 weight savings associated with the lower layer thickness make it possible for hot-dip galvanized (ZM) steel to be used as a resource-saving strip material 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] The metallic coating applied to hot-dip galvanized (ZM) steel strip contains approximately 1.5 to 8 wt.% of the metals aluminum and magnesium, with the magnesium content being at least 0.2 wt.%. The basic suitability of these coatings for forming, pre-treating and coating 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, the special composition of the coating and the native oxide layer gives rise to special features that must be taken into account, particularly during cleaning and pre-treatment, 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 proportion of oxides of the alloying component magnesium may be necessary for sufficient adhesion to a subsequently applied paint layer. For example, 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 top coat, known as "blistering." US 2016 / 0010216 A1 proposes treating the steel strip with a neutral or alkaline, aqueous composition containing a strong complexing agent for magnesium, either during or after degreasing.The proposed complexing agents are selected from organic acids or their salts, preferably from glycine and diphosphoric acid. At the same time, a pH value in the range of 7-11.5, especially 9-10, is recommended for sufficient removal of the oxidic magnesium species from the substrate surface, thus requiring a mildly alkaline cleaning step.

[0006] In contrast to this prior art, the present invention seeks to make the strongly alkaline cleaners established in strip pretreatment lines usable for the treatment of hot-dip galvanized (ZM) strip steel without having to accept disadvantages in paint adhesion and, in particular, with regard to blistering. Strongly alkaline cleaners offer the technical advantage that the degreasing and cleaning of the strip steel can be carried out in a single treatment step. In contrast, with mildly alkaline conditioning, i.e., changing the surface quality, of hot-dip galvanized (ZM) surfaces, depending on the degree of contamination of the coil, a strongly alkaline degreasing step must be carried out prior to the conditioning process to achieve the desired surface quality while simultaneously ensuring good paint adhesion.A further advantage of using strongly alkaline cleaning and degreasing baths is that, due to the high pickling rate for aluminum, which is an alloying component and the metallic phase of hot-dip galvanizing, the coil coating line can also be easily loaded with other hot-dip galvanized strip steel grades, e.g., (Z). This conserves resources during pretreatment and minimizes lead times and conversion costs when coating different hot-dip galvanized steel grades. The task profile of the present invention also includes the necessary compatibility of the cleaning with amorphous inorganic conversion coatings applied in the pretreatment line, which significantly increase the adhesion of primer coatings and thus also the resistance of topcoats to corrosive delamination and blistering.

[0007] It has now been shown that excellent protection against blistering can be achieved after a corrosion-protective pretreatment based on a strongly alkaline solution,

[0008] Conversion coating upstream cleaning stage is possible if in the

[0009] During the cleaning stage, a minimum amount of magnesium ions and at least one complexing agent are present. Under these conditions, preferential pickling of magnesium on the (ZM) substrate surface is avoided, and additional magnesium from the aqueous phase is enriched on the hot-dip galvanized surface through the alkaline pickling process. This active enrichment of magnesium on the (ZM) surface is surprisingly responsible for improved corrosion protection and paint adhesion after cleaning pretreatment in a strongly alkaline medium.

[0010] The present invention therefore relates, in a first aspect, to a process for cleaning and conditioning hot-dip galvanized (ZM) steel by bringing it into contact with an alkaline, aqueous composition containing at least 1.0 mmol / kg of magnesium ions dissolved in the aqueous phase and at least one water-soluble organic complexing agent, wherein the free alkalinity of the composition is greater than 3.0 and the pH is greater than 11.5.

[0011] In the context of the present invention, hot-dip galvanizing (ZM) represents a metallic coating on steel which contains 1.5 to 8 wt.% of the metals aluminum and magnesium, wherein the proportion of magnesium in the metallic coating is preferably at least 0.2 wt.%.

[0012] Free alkalinity or total alkalinity is determined by titrating 2 grams of the aqueous composition, diluted to 50 ml, with 0.1 N hydrochloric acid to a pH of 8.5 or 3.6, respectively. The consumption of acid solution in ml indicates the free alkalinity or total alkalinity score.

[0013] The pH value is defined as the negative decimal logarithm of the hydronium ion activity and can be determined directly in a sample volume of the composition using pH-sensitive glass electrodes after calibration with standard buffer solutions.

[0014] For a sufficiently high pickling rate and degreasing effect, which is particularly important for homogeneous surface conditioning and cleaning results in the pretreatment of flat strip products due to the very short throughput times through corresponding baths or spray zones, it is advantageous according to the invention if the total alkalinity in points is greater than 5.0, particularly preferably greater than 10.0, and especially preferably greater than 12.0. For the same reason, it is advantageous if the free alkalinity of the composition is set in points greater than 6.0, particularly preferably greater than 8.0. The ratio of total alkalinity to free alkalinity in points is preferably less than 3.0, particularly preferably less than 2.5, and in particular less than 2.2, in order to ensure sufficient pickling in the continuous operation of a pretreatment line and is therefore an important process control variable.

[0015] The preferred pH of the composition in the process according to the invention is above 12.0, preferably above 12.5, so that a surface coating based on magnesium oxides / hydroxides is maintained, while oxides / hydroxides and metallic aluminum from the hot-dip galvanizing actively dissolve. In this way, an enrichment of magnesium on the hot-dip galvanized steel surface and thus improved paint adhesion after primer and topcoat coating is promoted. Preferably, the pH of the composition is adjusted according to the invention so that it is less than 13.5, particularly preferably less than 13.0, in order to avoid over-pickling and the associated excessive material removal on the one hand, and excessive introduction of zinc ions into the pretreatment stages on the other.

[0016] Furthermore, the presence of magnesium ions dissolved in the aqueous phase of the strongly alkaline composition of the process according to the invention is required to ensure sufficient protection against blistering of organic top coats applied to hot-dip galvanized (ZM) steel. It can be seen that, based on the composition, at least 1.0 mmol / kg of magnesium is required, preferably at least 2.0 mmol / kg, particularly preferably at least 3.0 mmol / kg, and especially preferably at least 4.0 mmol / kg, in order to effectively suppress blistering after top coat coating. It can be seen that in the presence of magnesium ions, an additional layer of magnesium forms on the hot-dip galvanized steel surfaces, which proves to be advantageous for paint adhesion in the case of material (ZM). Higher contents are not sufficiently stabilized or formed in the strongly alkaline medium, even in the presence of the complexing agent.require correspondingly high proportions of complexing agents, but do not provide any significant improvement in the adhesion of subsequently applied primers and topcoats, so that according to the invention it is preferred that not more than 0.080 mol / kg, particularly preferably not more than 0.020 mol / kg of magnesium ions dissolved in the aqueous phase are contained, in each case based on the strongly alkaline, aqueous composition.

[0017] Any water-soluble magnesium salts can be used as a source of magnesium ions, but nitrates, sulfates, carbonates and / or salts of a-hydroxycarboxylic acids, for example glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acids, aldonic acids and / or glucoheptonic acid, are particularly suitable and therefore preferred.

[0018] The additional layer deposit on the hot-dip galvanized (ZM) surface in the range of a few milligrams of magnesium per square meter achieved in the process according to the invention is supported by the presence of the complexing agent and can be further increased by selecting certain complexing agents, which is then accompanied by a further optimization of the adhesion of organic covering layers applied to the (ZM) substrate.

[0019] A complexing agent within the meaning of the present invention is a Lewis base and organic compound with a molar mass of not more than 500 g / mol, which comprises at least two functional groups containing heteroatoms with at least one free electron pair. In a preferred embodiment of the process according to the invention, the at least two functional groups of the complexing agent present in the alkaline, aqueous composition are selected from carboxylic acid, phosphonic acid, phosphoric acid, amino, and / or hydroxyl groups, with the proviso that complexing agents containing at least one amino or hydroxyl group also contain at least one functional group selected from a carboxylic acid, phosphonic acid, or phosphoric acid group.In this preferred embodiment, the complexing agent is therefore always an acid and, in the context of the present invention, can also be partially or entirely contained in the alkaline, aqueous composition or added to it in the form of the corresponding salt.

[0020] It is fundamentally advantageous for the formation of a layer of magnesium if the complexing agent is only weakly complexing. In the process according to the invention, the complexing agents of the alkaline, aqueous composition are therefore selected from those with a complex formation constant pKi for magnesium below 2.0, preferably below 1.5, particularly preferably below 1.0.

[0021] The term complex formation constant pKi is defined in the application as the decimal logarithm of the equilibrium constant KL of the complex formation reaction at 25°C in water, where the equilibrium constant KL of the complex formation reaction results from: K L =[MgL n ] / [Mg][L]n where [MgL n ] is the molar equilibrium concentration of the magnesium complex, [Mg] is the molar equilibrium concentration of the metal ion, [L] is the molar equilibrium concentration of the ligand and n is the number of ligands bound in the complex for which the highest complex formation constant PKL results for the respective ligand.

[0022] Suitable weak complexing agents with a PKL value below 2.0 are preferably a-hydroxycarboxylic acids, which in turn are preferably selected from the group consisting of glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acids, aldonic acids and / or glucoheptonic acid, with particular preference being given to selecting at least one aldonic acid, in particular gluconic acid and / or glucoheptonic acid, and most preferably gluconic acid.

[0023] If weak complexing agents are combined with stronger complexing agents in the alkaline, aqueous composition in the process according to the invention, the coverage of the hot-dip galvanized steel surfaces with magnesium can be surprisingly increased and thus the adhesion to primer and topcoat can be further improved.

[0024] In preferred embodiments, therefore, in addition to the at least one previously mentioned weak complexing agent for magnesium, at least one further complexing agent is included, which is selected either a) from di- and / or triphosphonic acids, preferably from etidronic acid and / or aminotrimethylenephosphonic acid, particularly preferably from aminotrimethylenephosphonic acid, or b) from organic compounds having at least three carboxyl groups and at least one secondary and / or tertiary amino group, particularly preferably from ß-alaninediacetic acid, N-(1-carboxyethyl)iminodiacetic acid, diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid and / or iminodisuccinic acid, particularly preferably from iminodisuccinic acid.

[0025] Particularly preferred with regard to the quantitative ratios and proportion of complexing agents in the alkaline, aqueous composition is a process according to the invention in which the proportion of complexing agents in the composition is at least 2.0 mmol / kg, particularly preferably at least 4.0 mmol / kg, especially preferably at least 5.0 mmol / kg, but the total proportion of complexing agents is preferably not greater than 50 mmol / kg, particularly preferably not greater than 40 mmol / kg, especially preferably not greater than 30 mmol / kg, in each case based on the composition. It is further preferred if the proportion of complexing agents with a complex formation constant pKi for magnesium below 2.0, in particular aldonic acids and / or glucoheptonic acid, based on the total proportion of complexing agent, is preferably at least 60 mol%, particularly preferably at least 80 mol.-%, and particularly preferably at least one further complexing agent selected from one of the groups a) or b) mentioned in the previous paragraph is contained, again preferably at least 10 mol% of complexing agents of the groups a) and b) mentioned in the previous paragraph based on the total proportion of complexing agents.For a positive effect on the magnesium deposition due to the additionally introduced complexing agents according to groups a) and / or b) mentioned in the previous paragraph, a total amount of these complexing agents of at least 0.2 mmol / kg, particularly preferably at least 0.4 mmol / kg is preferred, but their total amount should preferably not exceed 2.0 mmol / kg, particularly preferably 1.5 mmol / kg, very particularly preferably 1.0 mmol / kg, since otherwise the advantageous increase in the magnesium coverage can generally no longer be achieved and, in addition, a deterioration compared to compositions which contain exclusively weak complexing agents (pKi below 2.0) can occur.

[0026] Such a preferred complexing agent system, in its alkaline, aqueous composition, is particularly suitable for sufficient and homogeneous surface deposition of magnesium on hot-dip galvanized steel. Substrates conditioned accordingly (ZM) in the process according to the invention, which are pretreated for corrosion protection and coated with a primer and topcoat, exhibit excellent adhesion after forming, and no blistering occurs even after weeks of exposure to condensing moisture.

[0027] The method of contacting the alkaline, aqueous composition can be freely selected in the process according to the invention. However, immersion and spraying methods are preferred.

[0028] In a second aspect, the present invention therefore also relates to a method for the cleaning and corrosion-protective surface treatment of strip steel which has a hot-dip galvanizing (ZM) on either one or both sides, in which the hot-dip galvanized (ZM) surface of the strip steel i) if necessary.is first degreased; ii) treated according to a conditioning and cleaning process as previously described; iii) then a conversion coating is carried out by bringing into contact with an acidic, aqueous composition containing water-soluble compounds of the elements Zr, Ti and / or Si, wherein the aqueous agent preferably has a pH in the range from 1.0 to 5.0, and wherein the bringing into contact is preferably carried out by applying a wet film of the composition, in particular by spraying / squeezing or rolling, and the wet film is then dried; and iv) then provided either with a topcoat with or without prior primer coating or with an immersion paint, in particular electrocoat.

[0029] The process steps i) - iv) are consecutive, but can be interrupted by intermediate steps, which can regularly represent rinsing steps, whereby rinsing steps are intended to remove as far as possible from the surface of the component by means of a rinsing solution active components from an immediately preceding wet-chemical treatment step, which adhere to the strip steel as a wet film and cannot be dried out.

[0030] The conversion coating in process step iii) is applied using acidic, aqueous compositions that produce an amorphous oxide / hydroxide coating based on the elements Zr, Ti, and / or Si and contain compounds of the elements Zr, Ti, and / or Si dissolved in water. The term "dissolved in water" encompasses molecularly dissolved species and compounds that dissociate in aqueous solution and form hydrated ions.

[0031] Fluoroacids are particularly suitable as water-soluble compounds of the elements Zr, Ti, or Si. Typical examples of water-soluble compounds include hexafluorotitanic acid (H2TiFe), hexafluorozirconic acid (H2ZrFe), and hexafluorosilicic acid (H2SiF6) and their salts, as well as titanyl sulfate (TiO(SC>4)), titanyl nitrate (TiO(NO3)2), and ammonium zirconium carbonate ((NH4)2ZrO(CO3)2).

[0032] Organosilanes are also suitable as water-soluble compounds of the element Si, although they are preferably present in addition to one or more compounds of the elements Zr and / or Ti. Such organosilanes preferably have at least one hydrolyzable, aliphatic radical, which is preferably selected from alkoxy groups having preferably no more than 2 carbon atoms and particularly preferably at least one non-hydrolyzable, aliphatic radical, which is preferably selected from alkyl groups having preferably no more than 6 carbon atoms, which particularly preferably additionally have at least one primary amine, glycidyl, or hydroxyl group. Typical representatives of these organosilanes are (3-glycidyloxypropyl)trimethoxysilane and 3-aminopropyltriethoxysilane.

[0033] A conversion coating based on the fluoro acids of the element Ti is particularly preferred in the process according to the invention according to the second aspect of the present invention, since such layers form optimally when formed by drying (“dry-in-place” process) and have a high compatibility for additivation with polymeric components in the acidic, aqueous medium, as described in DE 102006039633 A1, and therefore provide particularly good paint adhesion, so that depending on the requirement profile, a primer coating in step vi) can be dispensed with.

[0034] Typically, the coating thicknesses of Zr, Ti and / or Si, especially Zr and / or Ti, in processes for cleaning and corrosion-protective surface treatment of steel strip must be set within a very narrow range, since otherwise the coating thickness is either too low for an inorganic, corrosion-protective paint primer, or above a tolerable threshold, a significant deterioration in adhesion to the primer and topcoat is already achieved. An advantage of the process according to the invention, however, is that a broader application window is made available for the conversion coating, since only at coating thicknesses above 20 mg / m 2With respect to the elements Zr, Ti and / or Si, the performance with regard to paint adhesion drops significantly. Accordingly, it is preferred that the contacting in process step ii) takes place for such a duration or in such an application quantity that, after the drying step iv), a layer of Zr, Ti and / or Si of at least 1 mg / m² each is formed on the hot-dip galvanized (ZM) surfaces of the steel strip. 2 , particularly preferably at least 2 mg / m 2 based on the respective element, but in total preferably not more than 20 mg / m 2 , particularly preferably not more than 12 mg / m 2 based on the elements Zr, Ti and / or Si.

[0035] In a final aspect, the present invention also relates to an alkaline, aqueous composition for cleaning and conditioning hot-dip galvanized (ZM) steel which is particularly suitable in the context of the previously described inventive processes and which establishes a system of complexing agents optimized for the formation of a homogeneous layer of magnesium.

[0036] This alkaline, aqueous composition according to the invention has a free alkalinity in points of greater than 3.0 and a pH of greater than 11.0 and contains a) at least 1.0 mmol / kg of magnesium ions dissolved in water, and c) a system of complexing agents consisting of more than 2.0 mmol / kg, preferably more than 5.0 mmol / kg, but less than 50.0 mmol / kg, preferably less than 30.0 mmol / kg of complexing agents with a complex formation constant pKi for magnesium of less than 2.0, preferably selected from glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acids, aldonic acids and / or glucoheptonic acid, particularly preferably from aldonic acids and / or glucoheptonic acid, especially preferably from gluconic acid, and c1) more than 1.0 mmol / kg, but less than 10.0 mmol / kg of at least one di- and / or triphosphonic acid, preferably etidronic acid and / or aminotrimethylenephosphonic acid, particularly preferably aminotrimethylenephosphonic acid, and / or c2) more than 1,0 mmol / kg, but less than 10.0 mmol / kg of at least one organic compound with at least three carboxyl groups and at least one secondary and / or tertiary amino group, preferably ß-,

[0037] Alaninediacetic acid, N-(1-carboxyethyl)iminodiacetic acid, diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid, iminodisuccinic acid, particularly preferably iminodisuccinic acid.

[0038] As preferred embodiments of the composition according to the invention, all those variants of the alkaline, aqueous composition for cleaning and conditioning mentioned in the context of the first aspect of the present invention are considered to be equally preferred, insofar as they do not explicitly relate to the system of complexing agents.

[0039] The composition according to the invention, like the processes according to the invention, can also be used for conditioning and cleaning all other hot-dip galvanized steel grades, in particular hot-dip galvanized (Z), alloy galvanized, especially (ZF), (ZA), or aluminum-coated (AZ), (AS) steel. This opens up the possibility of operating the pretreatment stages of a coil coating plant with precisely these hot-dip galvanized steel grades without having to wet-chemically "convert" the respective stages, in particular the cleaning and degreasing stage, when changing the strip steel grade, e.g., from (ZM) to (Z) and vice versa.

[0040] All objects and aspects relating to the present invention, be it the process for conditioning and cleaning or the process for cleaning and corrosion-protecting surface treatment or the alkaline, aqueous composition for conditioning and cleaning itself, have in common that the respective alkaline, aqueous composition can be additived, but certain additives mentioned below can have a negative effect on the performance of the conditioning and should therefore preferably not be included or only be included in small amounts.

[0041] These include some electropositive metal ions which, when present in the alkaline environment of cleaning and conditioning, deposit in metallic form on the hot-dip galvanized steel and can thereby disrupt or even prevent the formation of a layer of magnesium required for improved paint adhesion.

[0042] Accordingly, it is preferred for all objects of the present invention that the proportion of water-soluble compounds of the elements Ni or Co, preferably the proportion of water-soluble compounds of metal elements which have a more positive standard reduction potential than iron, in the composition is in each case less than 10 mg / kg, preferably less than 5 mg / kg, particularly preferably less than 1 mg / kg, in each case as a proportion of the element and based on the composition.

[0043] Furthermore, the presence of phosphates can have a detrimental effect on the performance of the conditioning, i.e. the desired surface coverage with magnesium, so that their total proportion comprising orthophosphate and metaphosphates such as pyrophosphate in the alkaline, aqueous composition and in the context of all subject matters of the present invention is preferably less than 0.100 g / kg, particularly preferably less than 0.050 g / kg, especially preferably less than 0.010 g / kg and very particularly preferably less than 0.005 g / kg calculated as PO4 and based on the composition.

[0044] The same applies to the presence of water-dispersed or water-dissolved polymeric organic compounds having a molecular weight above 500 g / mol, which are not surfactants, preferably not nonionic surfactants, so that their total proportion in the alkaline, aqueous composition and in the context of all subject matters of the present invention is preferably less than 0.100 g / kg, particularly preferably less than 0.050 g / kg, especially preferably less than 0.010 g / kg and very particularly preferably less than 0.005 g / kg based on the composition.

[0045] However, the presence of iron ions can improve the formation of the conversion coating in process step iii) of the process according to the second aspect of the present invention on the hot-dip galvanized surfaces and have a positive effect on conditioning and paint adhesion. Therefore, in the context of all aspects of the present invention, the alkaline, aqueous composition preferably additionally contains iron ions dissolved in water, preferably at least 0.4 mmol / kg, particularly preferably at least 1.0 mmol / kg, very particularly preferably at least 1.5 mmol / kg of iron ions, but preferably less than 0.040 mol / kg, very particularly preferably less than 0.010 mol / kg of iron ions, in each case based on the composition.A suitable source of iron ions dissolved in water are the nitrates, sulfates, carbonates and / or salts of a-hydroxycarboxylic acids, for example glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acids, aldonic acids and / or glucoheptonic acid.

[0046] In the context of all subject-matter of the present invention, suitable additives for the alkaline, aqueous composition for cleaning and conditioning are surfactants. In preferred embodiments, the alkaline, aqueous composition additionally contains at least one surfactant, which is preferably selected from non-ionic surfactants, preferably with an HLB value of at least 8, more preferably of at least 10, especially preferably of at least 12, but preferably not more than 18, particularly preferably not more than 16, wherein the total proportion of the surfactants, preferably of the non-ionic surfactants, is preferably greater than 0.050 g / kg, more preferably greater than 0.100 g / kg, especially preferably greater than 0.200 g / kg, but the total proportion of the surfactants is preferably not more than 5.0 g / kg, more preferably not more than 2.0 g / kg, in each case based on the composition.The HLB value (hydrophilic-lipophilic balance) is calculated as follows and can take values ​​from zero to 20 on an arbitrary scale:.

[0047] HLB = 20 (1-Mi / M) with Mi: molecular mass of the lypophilic group of the nonionic surfactant

[0048] M: Molar mass of the nonionic surfactant

[0049] Preferred nonionic surfactants are those selected from alkoxylated alkyl alcohols, alkoxylated fatty amines, and / or alkyl polyglycosides, 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 are added as nonionic surfactants which are ethoxylated and / or propoxylated, wherein the total number of alkylene oxide units is preferably not greater than 16, particularly preferably not greater than 12, especially preferably not greater than 10, but particularly preferably greater than 4, especially preferably greater than 6.

[0050] With regard to the lipophilic component of the aforementioned nonionic surfactants, preferred nonionic surfactants for additives are those alkoxylated alkyl alcohols and / or alkoxylated fatty amines 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. Examples:

[0051] To illustrate the effect of compositions according to the invention in processes according to the invention for suppressing so-called blistering, hot-dip galvanized (ZM) sheet sections (ZM120 MC, d = 0.58 mm) were first conditioned by cleaning and then pretreated to protect against corrosion and provided with a polyurethane-based primer and topcoat.

[0052] Conditioning was carried out with cleaners of different alkalinity according to Table 1 using a spray method at 1 bar for 20 seconds at an application temperature of the composition of 50°C.

[0053] Table 1

[0054] VB1 based on the cleaner Bonderite® C-AK 703 (Henkel AG & Co. KGaA)

[0055] FA free alkalinity

[0056] GA Total Alkalinity

[0057] P Phosphorus from tripolyphosphates

[0058] HEDP Etidronic acid

[0059] ATMP trimethylenephosphonic acid

[0060] Following cleaning and conditioning in the same process step, the sheet sections (ZM) were pretreated with corrosion protection based on the commercial acid passivation Bonderite® M-NT 1455 T (Henkel AG & Co. KGaA). Pretreatment was carried out by roller application and drying of a wet film of the acid passivation, resulting in a titanium deposit of 5 or 1 mg / m², respectively. 2 After application of the PU primer (PU020PB0070, Akzo Nobel NV) in a dry film thickness of 20 μm and the PU topcoat system (PU747TX80024, Akzo Nobel NV) in a dry film thickness of 30 μm, the blistering was examined in the so-called “Q-Panel Condensation Test” (QCT) according to DIN EN 13523-26:2014-08, in which the appropriately pretreated and coated (ZM) panel sections were exposed to water (K ​​< 1 pScm -1 ) saturated air at 70 °C for 6 weeks.

[0061] It was shown that with conditioning according to the invention (B1) already at a layer thickness of 1 mg / m 2 Ti, blistering can be almost completely suppressed, whereas on all other substrates (VB1-VB3) strong blistering was observed. At a coating layer of 5 mg / m 2 Although blistering is significantly reduced with the mildly alkaline cleaner containing magnesium (VB3) and with the strongly alkaline cleaner (VB1), no blistering is observed at all in the inventive example (B1). The inventive process therefore enables particularly economical operation, significantly reducing the material required for the corrosion-protective pretreatment.

Claims

Claims:

1. Method for cleaning and conditioning galvanized (ZM) steel by contacting it with an alkaline aqueous composition containing at least 1.0 mmol / kg of magnesium ions dissolved in the aqueous phase and at least one water-soluble organic complexing agent, wherein the free alkalinity of the composition is greater than 3.0 points and the pH is greater than 11.

5.

2. Method according to claim 1, characterized in that the total alkalinity in points is greater than 5.0, preferably greater than 10.0, particularly preferably greater than 12.0, wherein the ratio of total alkalinity to free alkalinity in points is preferably less than 3.0, preferably less than 2.5, particularly preferably less than 2.

2.

3. Method according to one or both of the preceding claims, characterized in that the pH value of the composition is greater than 12.0, preferably greater than 12.5, but preferably less than 13.5, particularly preferably less than 13.

0.

4. Method according to one or more of the preceding claims, characterized in that the proportion of magnesium ions dissolved in the aqueous phase is at least 2.0 mmol / kg, preferably at least 3.0 mmol / kg, particularly preferably at least 4.0 mmol / kg, but preferably not more than 0.080 mol / kg, particularly preferably not more than 0.020 mol / kg based on the composition.

5. A method according to one or more of the preceding claims, characterized in that the composition additionally contains iron ions dissolved in the aqueous phase, preferably at least 0.4 mmol / kg of iron ions, particularly preferably at least 1.0 mmol / kg, most preferably at least 1.5 mmol / kg, but preferably less than 0.040 mol / kg, most preferably less than 0.010 mol / kg of iron ions.

6. A method according to one or more of the preceding claims, characterized in that the complexing agent is selected from complexing agents having a complex formation constant pKi for magnesium below 2.0, preferably below 1.5, particularly preferably below 1.0, which are preferably selected from α-hydroxycarboxylic acids, which in turn are preferably selected from the group consisting of glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acids, aldonic acids and / or glucoheptonic acid, wherein preferably at least one aldonic acid, in particular gluconic acid and / or glucoheptonic acid and most preferably gluconic acid is selected. A method according to claim 6, characterized in that it additionally contains at least one further complexing agent selected from di- and / or triphosphonic acids, preferably from etidronic acid and / or aminotrimethylenephosphonic acid, particularly preferably from aminotrimethylenephosphonic acid. A method according to claim 6 or 7, characterized in that it additionally contains at least one further complexing agent selected from organic compounds having at least three carboxyl groups and at least one secondary and / or tertiary amino group, which in turn is preferably selected from the group consisting of β-alanine diacetic acid, N-(1-carboxyethyl)iminodiacetic acid, iminodisuccinic acid, diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid, preferably iminodisuccinic acid.Method according to one or more of the preceding claims, characterized in that the proportion of complexing agents in the composition is at least 2.0 mmol / kg, preferably at least 4.0 mmol / kg, particularly preferably at least 5.0 mmol / kg, but the total proportion of complexing agents is preferably not greater than. 50 mmol / kg, particularly preferably not greater than 40 mmol / kg, particularly preferably not greater than 30 mmol / kg, in each case based on the composition, wherein the proportion of complexing agents with a complex formation constant pKi for magnesium below 2.0, in particular aldonic acids and / or glucoheptonic acid, based on the total proportion of complexing agents, is preferably at least 60 mol%, particularly preferably at least 80 mol%, and particularly preferably includes at least one further complexing agent selected from one of the groups according to claim 7 or 8.A method according to one or more of the preceding claims, characterized in that the composition additionally contains at least one surfactant, which is preferably selected from niotenes, preferably with an HLB value of at least 8, particularly preferably with at least 10, particularly preferably with at least 12, but preferably with no more than 18, particularly preferably with no more than 16, wherein the total proportion of surfactants, preferably of niotenes, is greater than 0.050 g / kg, preferably greater than 0.100 g / kg, particularly preferably greater than 0.200 g / kg, but the total proportion of surfactants is preferably not above 5.0 g / kg, particularly preferably not above 2.0 g / kg, in each case based on the composition.Method according to one or more of the preceding claims, characterized in that the proportion of water-soluble compounds of the elements Ni or Co, preferably the proportion of water-soluble compounds of metallic elements, which have a more positive effect. The standard reduction potential of iron is less than 10 mg / kg, preferably less than 5 mg / kg, and particularly preferably less than 1 mg / kg, as a proportion of the element and based on the composition. A method according to one or more of the preceding claims, characterized in that the total proportion of phosphates dissolved in water in the composition is less than 0.100 g / kg, preferably less than 0.050 g / kg, particularly preferably less than 0.010 g / kg, and particularly preferably less than 0.005 g / kg, calculated as the amount of PO4 and based on the composition. A method for the cleaning and corrosion-protective surface treatment of strip steel, which has hot-dip galvanizing (ZM) on one or both sides, wherein the hot-dip galvanized (ZM) surface of the flat strip product is i) optionallyfirst, is degreased; ii) is treated according to a method of the preceding claims; ii) is subsequently subjected to a conversion coating by contacting with an acidic aqueous composition containing water-soluble compounds of the elements Zr, Ti and / or Si, wherein the composition preferably has a pH value in the range of 1.0 to 5.0, and particularly preferably contains an amount of free fluoride, and wherein the contacting is preferably carried out by applying a wet film of the composition, in particular by spraying / squeezing or rolling, and the wet film is then dried; and v) is subsequently provided either with a topcoat with or without a prior primer coating or with an immersion coating, in particular an electrocoat.Method according to claim 13, characterized in that in process step ii) the contacting is carried out for such a duration or in such a quantity that after drying step iv) a layer of Zr, Ti and / or Si of at least . 1 mg / m 2 , preferably at least 2 mg / m³ each 2 relating to the respective element, but preferably not more than 20 mg / m³ in total. 2 , particularly preferably not more than 12 mg / m³ in total 2 with respect to the elements Zr, Ti and / or Si. Alkaline aqueous composition, whose free alkalinity is greater than 3.0 and whose pH is greater than 11.0, containing a) at least 1.0 mmol / kg of magnesium ions dissolved in water, and c) a system of complexing agents consisting of more than 2.0 mmol / kg, preferably more than 5.0 mmol / kg, but less than 50.0 mmol / kg.preferably less than 30.0 mmol / kg of complexing agents with a complex formation constant pKi for magnesium of less than 2.0, preferably selected from glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acids, aldonic acids and / or glucoheptonic acid, particularly preferably from aldonic acids and / or glucoheptonic acid, particularly preferably from gluconic acid, and c1) more than 1.0 mmol / kg but less than 10.0 mmol / kg of at least one di- and / or triphosphonic acid, preferably etidronic acid and / or aminotrimethylenephosphonic acid, particularly preferably aminotrimethylenephosphonic acid, and / or c2) more than 1.0 mmol / kg but less than 10.0 mmol / kg of at least one organic compound with at least three carboxyl groups and at least one secondary and / or tertiary amino group, preferably β-alanine diacetic acid, N-(1-Carboxyethyl)iminodiacetic acid, Diethylenetriaminepentaacetic acid, Ethylenediaminetetraacetic acid, Nitrilotriacetic acid, Iminodisuccinic acid,especially preferential iminodisuccinic acid.

Images