Alkaline cleaning method for zinc-magnesium alloy steel strip

Abstract

The invention relates to a method for the wet chemical cleaning and conditioning of a hot-dip galvanized (ZM) steel by contacting said hot-dip galvanized (ZM) steel with an alkaline aqueous composition comprising magnesium ions dissolved in water and at least one complexing agent. Also included is a method of pre-treating a steel strip, which is hot-dip galvanized (ZM) on one or both sides, in order to protect said steel from corrosion, wherein the cleaning according to the invention is followed by a wet chemical conversion coating and a subsequent construction of a varnish layer. The invention also relates to an alkaline aqueous composition comprising both magnesium ions and iron ions, which alkaline aqueous composition is particularly suitable for cleaning and subsequent anti-corrosion surface treatments; and to a complexing agent system.

Description

[0001] The invention relates to a method for wet-chemical cleaning and conditioning of hot-dip galvanized (ZM) steel by contacting the steel with an alkaline aqueous composition comprising magnesium ions dissolved in water and at least one complexing agent. It also comprises a method for pre-treating a steel strip which has been hot-dip galvanized (ZM) on one or both sides to protect the steel from corrosion, wherein the cleaning according to the invention is followed by a wet-chemical conversion coating and subsequent construction of a clearcoat layer. The invention also relates to an alkaline aqueous composition comprising both magnesium ions and iron ions, which is particularly suitable for cleaning and subsequent anti-corrosion surface treatment; and to a complexing agent system.

[0002] In motor vehicle construction, the use of zinc coatings on magnesium alloy steels is becoming increasingly important due to the growing demand for lightweight structural bodies. Compared to other hot-dip galvanizing, coatings of zinc and magnesium provide a significantly increased corrosion protection effect, in particular excellent corrosion resistance to spalling, even after application of organic topcoats and dip lacquers. Due to these improved property characteristics, coatings with smaller layer thicknesses can be provided, but still meet the high requirements for recoatability and corrosion protection. The weight reduction associated with the smaller layer thickness makes it possible to use hot-dip galvanized (ZM) steels as resource-saving strip for the production of lightweight bodies, so that in motor vehicle construction, the surface area of ​​this material on the body is further increased in addition to the surface area of ​​other lightweight metals such as aluminum.

[0003] The metal coating applied to hot-dip galvanized (ZM) steel strip contains about 1.5-8% by weight of metallic aluminum and magnesium, the proportion of magnesium being at least 0.2% by weight. In the conventional methods established in the prior art for forming, pretreatment and coating, the basic suitability of these coatings has been recognized and proven in principle (Characteristic Properties 095E, "Continuously Hot-Dip Coated Steel Strip and Sheet", Chapters 8 and 10, 2017 edition, Wirtschaftsvereinigung Stahl), but in particular in the case of cleaning and pretreatment, in order to achieve a coating result that is as uniform and reproducible as possible and thus achieve the best anticorrosion characteristics or the desired surface function, there are special characteristics that must be taken into account based on the specific composition of the coating and the native oxide layer.

[0004] For example, it is known from the prior art that, during the cleaning process, before the anticorrosion pretreatment of hot-dip galvanized (ZM) steel strip, a change in the proportion of the oxide of the alloying component magnesium may be necessary for sufficient adhesion to the subsequently applied varnish layer. For example, US2016 / 0010216 A1 reports that a large amount of magnesium oxide in the oxide layer close to the surface of the hot-dip galvanized (ZM) steel strip can effectively suppress the appearance of bubble-like protrusions in the topcoat (known as "blistering"). For this reason, US2016 / 0010216 A1 proposes treating the steel strip with a neutral or alkaline aqueous composition containing a strong complexing agent for magnesium at the same time or after the degreasing process. The complexing agent proposed is selected from organic acids or salts thereof, and is preferably selected from glycine and pyrophosphoric acid. At the same time, a pH value of about 7-11.5, in particular 9-10, is recommended for sufficient removal of magnesium oxide species from the substrate surface and the weakly alkaline cleaning stage thereof.

[0005] In contrast to this prior art, the object of the present invention is to make the strongly alkaline cleaners established in strip pretreatment lines usable for the treatment of hot-dip galvanized (ZM) steel strips without having to endure disadvantages in terms of varnish adhesion and in particular with regard to blistering. The technical advantage of strongly alkaline cleaners is that degreasing and cleaning of the steel strip can be carried out in only one treatment stage. In contrast, in the case of weakly 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 stage must be carried out in order to achieve the desired surface quality while ensuring good varnish adhesion. Another advantage of using a strongly alkaline cleaning and degreasing bath is that, due to the high leaching rate of aluminum (which is an alloying component and the metallic phase of hot-dip galvanizing), the coil coating line can also be easily fed with other hot-dip galvanized steel strip grades (schmelztauchverzinkten Bandstahlsorten) (e.g., (Z)), which saves resources in the pretreatment and minimizes the order-to-delivery cycle and conversion costs for the coil coating of different hot-dip galvanized steel grades (schmelztauchverzinkter Stahlsorten). The object overview of the present invention also includes the necessary compatibility of the cleaning with the amorphous inorganic conversion layer applied in the pretreatment line, which again significantly increases the adhesion of the primer coating and thus significantly increases the resistance of the topcoat to corrosion flaking and blistering.

[0006] It has now been shown that excellent blister protection is possible after a corrosion protection pretreatment based on a strongly alkaline cleaning stage upstream of the conversion coating if a minimum amount of magnesium ions and at least one complexing agent are present in the cleaning stage. Under these conditions, there is no preferential leaching of magnesium on the (ZM) substrate surface, and the alkaline leaching additionally enriches the hot-dip galvanized surface with magnesium from the aqueous phase. This active enrichment of magnesium on the (ZM) surface now surprisingly leads to improved corrosion protection and varnish adhesion after a cleaning pretreatment in a strongly alkaline medium.

[0007] Thus, in a first aspect, the present invention relates to a method for cleaning and conditioning hot dip galvanized (ZM) steel by contacting the steel with an alkaline aqueous composition comprising at least 1.0 mmol / kg of magnesium ions dissolved in an aqueous phase, and at least one water-soluble organic complexing agent, the composition having a free alkalinity in points greater than 3.0 and a pH greater than 11.5.

[0008] In the context of the present invention, hot-dip galvanizing (ZM) is a metal coating on steel which contains 1.5-8% by weight of metallic aluminum and magnesium, the proportion of magnesium in the metal coating preferably being at least 0.2% by weight.

[0009] The 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 value of 8.5 or 3.6. The amount of acid solution consumed (ml) indicates the value of the free alkalinity or total alkalinity.

[0010] The pH value is defined as the negative logarithm to the base 10 of the hydronium ion activity and can be determined directly in a sample volume of the composition using a pH-sensitive glass electrode after calibration with standard buffer solutions.

[0011] In order to achieve sufficiently high leaching rates and degreasing effects (due to the extremely short transit times through the corresponding baths or spraying zones, which are particularly important for uniform surface conditioning and cleaning results in the pretreatment of flat strip products), it is advantageous according to the invention for the total alkalinity in points to be greater than 5.0, particularly preferably greater than 10.0, particularly preferably greater than 12.0. For the same reasons, it is advantageous to set the free alkalinity in points of the composition to greater than 6.0, particularly preferably greater than 8.0. In order to ensure sufficient leaching in the continuous operation of the pretreatment line, the ratio of total alkalinity to free alkalinity (each in points) is preferably less than 3.0, particularly preferably less than 2.5, in particular less than 2.2, and is therefore an important process control variable.

[0012] In the method according to the invention, the preferred pH value of the composition is above 12.0, preferably above 12.5, so that the surface coating based on magnesium oxides / hydroxides is maintained, while the hot-dip galvanized oxides / hydroxides and metallic aluminum are effectively dissolved. In this way, the enrichment of magnesium on the hot-dip galvanized steel surface and the thus improved adhesion of the varnish after the application of primer and topcoat are promoted. Preferably, in order to avoid excessive leaching and the associated excessive removal of material on the one hand, and to avoid excessive introduction of zinc ions in the pretreatment stage on the other hand, the pH value of the composition is set according to the invention so that it is less than 13.5, particularly preferably less than 13.0.

[0013] Furthermore, adequate protection against blistering of the organic top layer applied to hot-dip galvanized (ZM) steel requires the presence of magnesium ions dissolved in the aqueous phase of the strongly alkaline composition according to the method of the invention. In this regard, it was found that, in order to effectively suppress blistering after the topcoat application, at least 1.0 mmol / kg, preferably at least 2.0 mmol / kg, particularly preferably at least 3.0 mmol / kg, particularly preferably at least 4.0 mmol / kg of magnesium, based on the composition, was required. It was found that, in the presence of magnesium ions, an additional layer thickness of magnesium is formed on the hot-dip galvanized steel surface, which has been shown to be beneficial for the adhesion of the varnish in the material (ZM). Higher contents are not sufficiently stable in strongly alkaline media, even in the presence of complexing agents, or require correspondingly high proportions of complexing agents, but do not significantly improve the adhesion of the subsequently applied primer and topcoat, so that according to the invention it is preferred that, in each case, based on the strongly alkaline aqueous composition, 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.

[0014] Any water-soluble magnesium salt may be used as the source of magnesium ions, but nitrates, sulfates, carbonates and / or salts of alpha-hydroxycarboxylic acids (e.g. glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acid, aldonic acid and / or glucoheptonic acid) are particularly suitable and are therefore preferred.

[0015] The additional layer thickness in the range of a few milligrams of magnesium per square meter on the hot-dip galvanized (ZM) surface achieved in the method according to the invention is supported by the presence of complexing agents and can be further increased by selecting certain complexing agents, which is then accompanied by a further optimization of the adhesion of the organic top layer applied to the (ZM) substrate.

[0016] Complexing agents within the meaning of the present invention are Lewis bases and organic compounds having a molar mass of no more than 500 g / mol and containing at least two functional groups having heteroatoms with at least one free electron pair. In a preferred embodiment of the method according to the invention, at least two functional groups of the complexing agent contained in the alkaline aqueous composition are selected from carboxylic acid groups, phosphonic acid groups, phosphoric acid groups, amino groups and / or hydroxyl groups, provided that the complexing agent containing at least one amino group or hydroxyl group also has at least one functional group selected from carboxylic acid groups, phosphonic acid groups or phosphoric acid groups. In this preferred embodiment, the complexing agent is therefore always an acid and in the context of the present invention can also be contained in the alkaline aqueous composition partially or completely in the form of the corresponding salt or added thereto.

[0017] If the complexing agent is only weakly complexed, the formation of a layer thickness of magnesium is generally favored. In the method according to the invention, the complexing agent of the alkaline aqueous composition is therefore selected from the group consisting of the complex formation constant pK for magnesium L Those below 2.0, preferably below 1.5, particularly preferably below 1.0.

[0018] The term "complex formation constant pK L " is defined within the scope of this application as the equilibrium constant K of the complex formation reaction in water at 25°C L The base 10 logarithm of the complex formation reaction, K L Given by:

[0019] K L =[MgL n ] / [Mg][L] n

[0020] Among them [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 complex formation constant pK L The result is the highest.

[0021] Suitable pK L Weak complexing agents with values ​​below 2.0 are preferably α-hydroxycarboxylic acids, which in turn are preferably selected from glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acid, aldonic acid and / or glucoheptonic acid, wherein particularly preferably at least one aldonic acid, in particular gluconic acid, and / or glucoheptonic acid, very particularly preferably gluconic acid, is selected.

[0022] If weak complexing agents are combined with stronger complexing agents in the alkaline aqueous composition in the method according to the invention, the coating of hot-dip galvanized steel surfaces with magnesium can be unexpectedly enhanced and the adhesion to primers and topcoats can thus be further improved over (ZM).

[0023] In a preferred embodiment, in addition to the at least one weak complexing agent for magnesium mentioned above, at least one further complexing agent is therefore comprised which is selected from:

[0024] a) diphosphonic acid and / or triphosphonic acid, preferably etidronic acid and / or aminotrimethylenephosphonic acid, particularly preferably aminotrimethylenephosphonic acid,

[0025] or

[0026] b) Organic compounds having at least three carboxyl groups and at least one secondary and / or tertiary amino group, particularly preferably β-alaninediacetic acid, N-(1-carboxyethyl)iminodiacetic acid, diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid and / or iminodisuccinic acid, particularly preferably iminodisuccinic acid.

[0027] With regard to the quantitative ratios and proportions of the complexing agents in the alkaline aqueous composition, particular preference is given to a method according to the invention in which the proportion of the complexing agent in the composition is at least 2.0 mmol / kg, particularly preferably at least 4.0 mmol / kg, particularly preferably at least 5.0 mmol / kg, based in each case on the composition, but the total proportion of the complexing agent is preferably not more than 50 mmol / kg, particularly preferably not more than 40 mmol / kg, particularly preferably not more than 30 mmol / kg. It is further preferred that, based on the total proportion of the complexing agent, the constant pK for complex formation of magnesium is L The proportion of complexing agents, in particular aldonic acid and / or glucoheptonic acid, below 2.0 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 preceding paragraph, and preferably at least 10 mol % of complexing agents from the groups a) and b) mentioned in the preceding paragraph, based on the total proportion of complexing agents. In order to achieve a positive influence on the magnesium deposition due to the additionally introduced complexing agents according to the groups a) and / or b) mentioned in the preceding paragraph, the total amount of these complexing agents is preferably at least 0.2 mmol / kg, particularly preferably at least 0.4 mmol / kg, but the 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 an advantageous increase in the magnesium coating can no longer be achieved on a conventional basis and, moreover, compared with such a coating containing only weak complexing agents (pK LCompared with compositions with a viscosity lower than 2.0), degradation may occur.

[0028] Such preferred complexing agent systems in alkaline aqueous compositions are particularly suitable for adequate and uniform surface coating of hot-dip galvanized steel with magnesium. (ZM) substrates conditioned according to the method of the invention, which have been pretreated to prevent corrosion and coated with primer and topcoat, exhibit excellent adhesion after forming and do not blister, even after weeks of exposure to condensed water.

[0029] In the method according to the present invention, the manner of contacting the alkaline aqueous composition can be freely selected. However, dipping and spraying methods are preferred.

[0030] In a second aspect, the invention therefore also relates to a method for cleaning and anti-corrosion surface treatment of a steel strip having hot-dip galvanizing (ZM) on either or both sides, wherein the hot-dip galvanized (ZM) surface of the steel strip is:

[0031] i) optionally, first defatting;

[0032] ii) treated according to the method for conditioning and cleansing;

[0033] iii) subsequently conversion-coated by contact with an acidic aqueous composition comprising a water-soluble compound of the elements Zr, Ti and / or Si, the aqueous agent preferably having a pH value in the range of 1.0 to 5.0, and wherein the contacting is preferably carried out by applying a wet film of the composition, in particular by spraying / extrusion or roller coating, and the wet film is then dried in situ; and

[0034] iv) Subsequently providing a topcoat with or without a previous primer coat, or providing dip coating, in particular electrophoretic dip coating.

[0035] Method steps i) to iv) are continuous but can be interrupted by intermediate steps, which can be conventional rinsing steps, wherein the rinsing step is intended to remove from the component surface as far as possible active components from the immediately preceding wet-chemical treatment step by means of a rinsing solution, which active components adhere to the steel strip as a wet film and cannot be dried in situ.

[0036] The conversion coating in process step iii) is carried out using an acidic aqueous composition which produces an amorphous oxide / hydroxide coating based on the elements Zr, Ti and / or Si and which therefore contains compounds of the elements Zr, Ti and / or Si dissolved in water. The term "soluble in water" includes molecularly dissolved species as well as compounds which dissociate in aqueous solution and form hydrated ions.

[0037] As water-soluble compounds of the elements Zr, Ti or Si, their fluoric acids are particularly suitable. Typical representatives of water-soluble compounds are hexafluorotitanic acid (H2TiF6), hexafluorozirconic acid (H2ZrF6) and hexafluorosilicic acid (H2SiF6) and their salts, as well as titanyl sulfate (TiO(SO4)), titanyl nitrate (TiO(NO3)2) and ammonium zirconium carbonate ((NH4)2ZrO(CO3)2).

[0038] Organosilanes are also suitable as water-soluble compounds of the element Si, which, however, are preferably contained in addition to one or more compounds of the elements Zr and / or Ti. Such organosilanes preferably have at least one hydrolyzable aliphatic group, which is preferably selected from alkoxy groups having preferably not more than 2 carbon atoms, and particularly preferably at least one non-hydrolyzable aliphatic group, which is preferably selected from alkyl groups having preferably not more than 6 carbon atoms, and particularly preferably also have at least one primary amino group, glycidyl group or hydroxyl group. Typical representatives of these organosilanes are (3-glycidyloxypropyl)trimethoxysilane and 3-aminopropyltriethoxysilane.

[0039] In the method according to the invention according to the second aspect of the invention, conversion coatings based on fluoric acid of elemental Ti are particularly preferred, since such layers are formed optimally when formed by in situ drying (“in situ drying” method) and have a high compatibility with the addition of polymer components in acidic aqueous media (as described in DE 102006039633 A1), thus imparting particularly good clearcoat adhesion, making it possible, depending on the requirements, to dispense with the primer application in step vi).

[0040] Usually, in methods for cleaning and anticorrosion surface treatment of steel strips, the layer thickness of Zr, Ti and / or Si, in particular Zr and / or Ti, must be set within a very narrow range, since otherwise the layer thickness is too low for inorganic primer coatings for protection against corrosion, or above the permissible threshold would already lead to a significant deterioration of the adhesion to primers and topcoats. However, the method according to the invention has the advantage that a wider application window is achieved for the conversion coating, since the performance with regard to clearcoat adhesion is only above 20 mg / m 2 based on elemental Zr, Ti and / or Si. 2 Therefore, it is preferred that the contacting in method step ii) is carried out for such a time or in such an amount that after the drying step iv) at least 1 mg / m 2 of the corresponding element is produced on the hot-dip galvanized (ZM) surface of the steel strip. 2 , particularly preferably in each case at least 2 mg / m 2 , but preferably not more than 20 mg / m in total based on the elements Zr, Ti and / or Si 2, especially preferably not more than 12 mg / m 2 The layer thickness of Zr, Ti and / or Si is .

[0041] In a final aspect, the 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 above-described method according to the invention and establishes a complexing agent system optimized for forming a uniform layer thickness of magnesium.

[0042] This alkaline aqueous composition according to the invention has a free alkalinity of greater than 3.0 points and a pH value of greater than 11.0 and comprises:

[0043] a) at least 1.0 mmol / kg of magnesium ions dissolved in water, and

[0044] c) a complexing agent system, the complexing agent system consisting of a complex formation constant pK for magnesium 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 L less than 2.0 of a complexing agent, the complexing agent is preferably selected from glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acid, aldonic acid and / or glucoheptonic acid, particularly preferably from aldonic acid and / or glucoheptonic acid, particularly preferably from gluconic acid, and

[0045] c1) more than 1.0 mmol / kg but less than 10.0 mmol / kg of at least one diphosphonic acid and / or triphosphonic acid, preferably etidronic acid and / or aminotrimethylenephosphonic acid, particularly preferably aminotrimethylenephosphonic acid, and / or

[0046] c2) more than 1.0 mmol / kg but less than 10.0 mmol / kg of at least one organic compound having at least three carboxyl groups and at least one secondary and / or tertiary amino group, preferably β-alaninediacetic acid, N-(1-carboxyethyl)iminodiacetic acid, diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid, iminodisuccinic acid, particularly preferably iminodisuccinic acid.

[0047] As preferred embodiments of the compositions according to the invention, all those variants of the alkaline aqueous compositions for cleaning and conditioning mentioned in the context of the first aspect of the invention are regarded as being equally preferred, as long as they do not explicitly involve complexing agent systems.

[0048] The composition according to the invention and the method 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) steel, alloy galvanized steel (especially (ZF) steel, (ZA) steel), or aluminized (AZ), (AS) steel. This opens up the possibility of operating the pretreatment stages of coil coating plants precisely with these hot-dip galvanized steel grades, without having to "rearrange" individual stages, in particular cleaning and degreasing stages, in a wet-chemical manner when changing the steel strip grade (for example, from (ZM) to (Z) or vice versa).

[0049] All objects and aspects related to the present invention (whether it is a method for conditioning and cleaning, a method for cleaning and anti-corrosion surface treatment, or the alkaline aqueous composition itself for conditioning and cleaning) have in common that the corresponding alkaline aqueous composition can be added with additives, but some of the additives mentioned below may have a negative impact on the conditioning performance and should therefore preferably not be included or only included in small amounts.

[0050] These include some electropositive metal ions which, when present in a clean and conditioned alkaline environment, may deposit in metallic form on hot dip galvanized steel, thereby disrupting or even preventing the formation of the layer thickness of magnesium required for improved varnish adhesion.

[0051] It is therefore preferred for all purposes of the present invention that the proportion of water-soluble compounds of the elements Ni or Co, preferably of water-soluble compounds of metal elements having 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 elements and based on the composition.

[0052] Furthermore, the presence of phosphates may also have a deleterious effect on the properties of conditioning (i.e. the desired surface coating with magnesium), so that in the alkaline aqueous composition and in the context of all objects of the present invention, the total proportion of phosphates (comprising orthophosphates and metaphosphates, such as pyrophosphates), calculated as PO4 and based on the composition, is preferably less than 0.100 g / kg, particularly preferably less than 0.050 g / kg, especially preferably less than 0.010 g / kg, very particularly preferably less than 0.005 g / kg.

[0053] The same applies to the presence of water-dispersible or water-soluble polymeric organic compounds with a molar mass of more than 500 g / mol, which are not surfactants, preferably not nonionic surfactants, so that in the alkaline aqueous composition and in the context of all objects of the present invention, their total proportion, based on the composition, is preferably less than 0.100 g / kg, particularly preferably less than 0.050 g / kg, especially preferably less than 0.010 g / kg, very particularly preferably less than 0.005 g / kg.

[0054] However, the presence of iron ions can improve the formation of the conversion coating on the hot-dip galvanized surface in method step iii) of the method according to the second aspect of the invention and can have a positive effect on conditioning and varnish adhesion. In this respect, in the context of all objects of the present invention, the alkaline aqueous composition preferably also 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. Suitable sources of iron ions dissolved in water are likewise nitrates, sulfates, carbonates and / or salts of α-hydroxycarboxylic acids (e.g. glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acid, aldonic acid and / or glucoheptonic acid).

[0055] However, in the context of all the purposes of the present invention, suitable additives for alkaline aqueous compositions for cleaning and conditioning are surfactants. In a preferred embodiment, the alkaline aqueous composition also comprises at least one surfactant, which is preferably selected from nonionic surfactants, the HLB value of which is preferably at least 8, particularly preferably at least 10, particularly preferably at least 12, but preferably not more than 18, particularly preferably not more than 16, wherein the total proportion of surfactants, preferably nonionic surfactants, based on the composition in each case is preferably greater than 0.050 g / kg, particularly preferably greater than 0.100 g / kg, particularly preferably greater than 0.200 g / kg, but the total proportion of surfactants is preferably not more than 5.0 g / kg, particularly preferably not more than 2.0 g / kg. The HLB value (hydrophile-lipophile balance) is calculated as follows and can assume a value of 0 to 20 on an arbitrary scale:

[0056] HLB=20·(1-M l / M)

[0057] Among them, M l : Molar mass of the lipophilic group of nonionic surfactant

[0058] M: molar mass of nonionic surfactant.

[0059] In terms of substances, nonionic surfactants are preferred which are selected from alkoxylated alkyl alcohols, alkoxylated fatty amines and / or alkyl polyglycosides, particularly preferably from alkoxylated alkyl alcohols and / or alkoxylated fatty amines, particularly preferably from alkoxylated alkyl alcohols. In this case, in order to achieve a defoaming effect, the alkoxylated alkyl alcohols and / or alkoxylated fatty amines are preferably end-capped, particularly preferably with an alkyl group which in turn preferably has not more than 8 carbon atoms, particularly preferably not more than 4 carbon atoms. Particularly preferably, such alkoxylated alkyl alcohols and / or alkoxylated fatty amines are added as nonionic surfactants in ethoxylated and / or propoxylated form, the number of alkylene oxide units preferably being not more than 16, particularly preferably not more than 12, particularly preferably not more than 10, but particularly preferably more than 4, particularly preferably more than 6 in total.

[0060] Regarding the lipophilic component of the above-mentioned nonionic surfactant, the following alkoxylated alkyl alcohols and / or alkoxylated fatty amines are preferred as nonionic surfactants for addition: their alkyl groups are saturated and preferably unbranched, 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 more than 20, particularly preferably not more than 18, particularly preferably not more than 16. Example:

[0061] In order to demonstrate the effect of the composition according to the invention in suppressing so-called blistering in the method according to the invention, a hot-dip galvanized (ZM) sheet metal section (ZM120 MC, d=0.58 mm) was first cleaned and conditioned, then pretreated to prevent corrosion and provided with a polyurethane-based primer and topcoat.

[0062] The spraying method was used at 1 bar and a composition application temperature of 50° C. in each case using the

[0063] The detergents of different alkalinity in Table 1 were conditioned for 20 seconds.

[0064]

[0065] After cleaning and conditioning in the same process step, use a commercial acid passivator M-NT1455T (Henkel AG & Co. KGaA) was used to pre-treat the metal sheet sections (ZM) to prevent corrosion. The pre-treatment was carried out by roller application and drying of a certain amount of acid passivation wet film, which produced an amount of 5 mg / m 2 and 1 mg / m 2 The layer thickness of titanium.

[0066] After application of a PU primer (PU020PB0070, Akzo Nobel NV) with a dry film thickness of 20 μm and a PU topcoat system (PU747TX80024, Akzo Nobel NV) with a dry film thickness of 30 μm, blistering was investigated in the so-called “Q-Panel Condensation Test” (QCT) according to DIN EN 13523-26:2014-08, wherein the correspondingly pretreated and coated (ZM) sheet metal sections were exposed to water at 70° C. (κ<1 μScm -1 ) in saturated air for 6 weeks.

[0067] It was found that, with the treatment according to the invention (E1), even at 1 mg / m 2 At a layer thickness of 5 mg / m Ti, blistering can also be almost completely suppressed, whereas strong blistering is observed on all other substrates (CE1-CE3). 2 At a layer thickness of 1.50 Ti, foaming was significantly reduced on weakly alkaline cleaners containing magnesium (CE3) and when using strongly alkaline cleaners (CE1); however, foaming was not observed at all only in Example (E1) according to the invention. The method according to the invention thus enables particularly economical operation, wherein the material requirements in the corrosion protection pretreatment can be significantly reduced.

Claims

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

5.

2. The 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 the total alkalinity in points to the free alkalinity is preferably less than 3.0, preferably less than 2.5, particularly preferably less than 2.

2.

3. The 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. The 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. Method according to one or more of the preceding claims, characterized in that The composition further comprises iron ions dissolved in the aqueous phase, 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.

6. Method according to one or more of the preceding claims, characterized in that The complexing agent is selected from the group consisting of magnesium complex formation constant pK L less than 2.0, preferably less than 1.5, particularly preferably less than 1.0, the complexing agent being preferably selected from α-hydroxycarboxylic acids, which in turn are preferably selected from glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acid, aldonic acid and / or glucoheptonic acid, wherein preferably at least one aldonic acid, in particular gluconic acid and / or glucoheptonic acid, very particularly preferably gluconic acid, is selected.

7. The method according to claim 6, characterized in that At least one additional complexing agent is also contained, which is selected from diphosphonic acids and / or triphosphonic acids, preferably from etidronic acid and / or aminotrimethylenephosphonic acid, particularly preferably from aminotrimethylenephosphonic acid.

8. The method according to claim 6 or 7, characterized in that: It also contains at least one additional complexing agent, which is 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 β-alanine diacetic acid, N-(1-carboxyethyl) iminodiacetic acid, iminodisuccinic acid, diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid, preferably iminodisuccinic acid.

9. Method according to one or more of the preceding claims, characterized in that The proportion of complexing agent 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, in each case based on the composition, but the total proportion of complexing agent is preferably not more than 50 mmol / kg, particularly preferably not more than 40 mmol / kg, particularly preferably not more than 30 mmol / kg; wherein, based on the total proportion of the complexing agent, the complex formation constant pK for magnesium is L The proportion of complexing agents, in particular aldonic acid and / or glucoheptonic acid, below 2.0 is preferably at least 60 mol %, particularly preferably at least 80 mol %, and particularly preferably comprises at least one further complexing agent selected from one of the groups according to claim 7 or 8.

10. Method according to one or more of the preceding claims, characterized in that The composition further comprises at least one surfactant, which is preferably selected from nonionic surfactants, the HLB value of which is preferably at least 8, particularly preferably at least 10, particularly preferably at least 12, but preferably not more than 18, particularly preferably not more than 16; wherein the total proportion of the surfactants, preferably the nonionic surfactants, 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 the surfactants is preferably not more than 5.0 g / kg, particularly preferably not more than 2.0 g / kg, based in each case on the composition.

11. 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 of water-soluble compounds of metal elements having 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 elements and based on the composition.

12. Method according to one or more of the preceding claims, characterized in that The total proportion of phosphate dissolved in water in the composition, calculated as PO4 and based on the composition, is less than 0.100 g / kg, preferably less than 0.050 g / kg, particularly preferably less than 0.010 g / kg, particularly preferably less than 0.005 g / kg.

13. A method for cleaning and anti-corrosion surface treatment of steel strip having hot dip galvanizing (ZM) on either or both sides, wherein the hot dip galvanized (ZM) surface of the flat strip product is: i) optionally, first defatting; ii) treated according to the method of the preceding claim; iii) subsequently conversion-coated by contact with an acidic aqueous composition comprising a water-soluble compound of the elements Zr, Ti and / or Si, said composition preferably having a pH in the range of 1.0 to 5.0 and particularly preferably comprising a certain amount of free fluoride, and wherein said contacting is preferably carried out by applying a wet film of said composition, in particular by spraying / extrusion or roller coating, and said wet film is then dried in situ; and v) Subsequently providing a topcoat with or without a previous primer coat, or providing a dip-coat, in particular an electrophoretic dip-coat.

14. The method according to claim 13, characterized in that In method step ii), the contacting is carried out for such a time or in such an amount that, after drying step iv), at least 1 mg / m 2 of the corresponding element is produced on the hot-dip galvanized (ZM) surface of the steel strip. 2 , preferably in each case at least 2 mg / m 2 , but preferably not more than 20 mg / m in total based on the elements Zr, Ti and / or Si 2 , especially preferably not more than 12 mg / m 2 The layer thickness of Zr, Ti and / or Si is .

15. An alkaline aqueous composition having a free alkalinity in points greater than 3.0 and a pH greater than 11.0, the alkaline aqueous composition comprising: a) at least 1.0 mmol / kg of magnesium ions dissolved in water, and b) a complexing agent system, the complexing agent system consisting of a complex formation constant pK for magnesium 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 L A complexing agent composition of less than 2.0, the complexing agent is preferably selected from glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acid, aldonic acid and / or glucoheptonic acid, particularly preferably from aldonic acid 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 diphosphonic acid 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 having at least three carboxyl groups and at least one secondary and / or tertiary amino group, preferably β-alaninediacetic acid, N-(1-carboxyethyl)iminodiacetic acid, diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid, iminodisuccinic acid, particularly preferably iminodisuccinic acid.

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