Multi-stage method for the corrosion-protective treatment of components with steel surfaces

Abstract

The present invention relates to a multi-stage process in which a series of components, each having steel surfaces, is first coated with a conversion layer based on the elements Zr and / or Ti and then dip-coated. The conversion treatment step is followed by a conditioning step in which at least the steel surfaces of each component are brought into contact with an alkaline aqueous composition containing calcium and / or magnesium ions. In the process according to the invention, excellent corrosion protection on the steel surfaces is achieved even under process conditions that typically promote corrosion defects on the steel surfaces, so that a drying step following the conversion step can be dispensed with.

Description

[0001] The present invention relates to a multi-stage process in which a series of components, each having steel surfaces, is first coated with a conversion layer based on the elements Zr and / or Ti and then dip-coated. The conversion treatment step is followed by a conditioning step in which at least the steel surfaces of each component are brought into contact with an alkaline aqueous composition containing calcium and / or magnesium ions. In the process according to the invention, excellent corrosion protection on the steel surfaces is achieved even under process conditions that typically promote corrosion defects on the steel surfaces, so that a drying step following the conversion step can be dispensed with.

[0002] In the corrosion-protective pretreatment of components with surfaces made of steel, galvanized steel, and / or aluminum, thin-film passivation based on amorphous conversion coatings based on oxides and hydroxides of the elements Zr and / or Ti has become widely established as an alternative to phosphating, which forms crystalline coatings. Efforts to further develop this type of conversion coating are primarily aimed at establishing resource-saving and chromium-free passivations that provide an excellent adhesion base for subsequently applied paint systems, particularly dip coatings, with the aim of achieving corrosion protection comparable to that achieved with trication zinc phosphating.Especially for amorphous thin films, such as those resulting from conversion treatments from acidic aqueous solutions containing water-soluble compounds of the elements Zr and / or Ti, controlled layer formation and the growth of coatings that are as defect-free as possible are of great importance. For this purpose, the prior art focuses on influencing the kinetics of layer formation, as described in WO 2023 / 275270, and proposes, for example, sequential formation of the conversion layer in several wet-chemical process steps to create the layer deposits based on the hydroxides and oxides of the elements Zr and / or Ti, which bring about the most complete conversion possible of the conversion treatment based on fluorocomplexes of the elements Zr and / or Ti. This is intended to prevent fluorides from remaining in the thin film, which can cause local layer defects upon contact with corrosive media.Another process for conversion coating formation, described as an example in EP 1 455 002 A1, aims to reduce the proportion of fluorides in the conversion coating and the associated improvement in corrosion behavior and paint adhesion to a subsequently applied dip coating. EP 1 455 002 A1 proposes adding magnesium, calcium, a Si-containing compound, zinc, or copper to the conversion solution and, alternatively or in combination, drying the conversion coating or rinsing it with an alkaline aqueous composition.

[0003] Particularly on steel surfaces, the formation of flash rust can be observed during the serial pretreatment of components by forming a conversion coating based on complex fluorides of the elements Zr and / or Ti. This occurs if the wet film originating from the conversion stage and still adhering to the surfaces of the components during transport to the painting stage is not completely removed, e.g., by blowing it off in an air stream, or if the conversion coating is not completely dried by thermal post-treatment. This latter process step is also proposed by EP 1 455 002 A1 to improve the corrosion protection of the conversion coating. However, such process steps are technically complex and involve high energy consumption.Therefore, in the prior art, the conversion stage is often followed by an intensive rinsing stage with fresh water, in which the wet film originating from the conversion stage and still directly adhering to the surfaces of the components is completely removed before the component is transferred to the painting stage. Rinsing with alkaline aqueous rinsing solutions is also proposed in EP 1 455 002 A1 to improve the corrosion protection of the conversion coating.However, even with such a process that minimizes the formation of flash rust by means of an intensive rinsing stage, it has not yet been possible to avoid a decrease in the corrosion protection on the steel surfaces after dip coating compared to processes with thermal drying of the conversion coating. In addition, irregularities in the appearance of the dip coating, the so-called "mapping", often occur in the presence of the rinsing stage and this also applies to the other metallic surfaces of the components, e.g., on galvanized steel surfaces.

[0004] The present invention therefore has the object of establishing a process for providing conversion coatings on metal surfaces, in particular steel surfaces, which are as defect-free as possible and which, during industrial pretreatment and dip-coating of a large number of components, imparts a high degree of robustness against corrosive impairment during the phase of transferring the components from the conversion treatment stage to the dip-coating stage and also delivers high-quality dip-coated components with improved corrosion protection properties. In the process, especially on steel, the quality of the conversion coating should not be negatively influenced by subsequent rinsing steps, and yet a process should be established in which the components can be transferred directly to the painting line after rinsing, i.e. without thermal post-treatment, and can be dip-coated without any loss of corrosion protection.Ideally, fluctuations in performance, corrosion protection, and irregularities in the appearance of the dip-coated components, known as "mapping," are also prevented. The process must be suitable for effectively protecting components consisting of a mix of different metals, especially steel, zinc, and aluminum, from corrosion.

[0005] This task is solved by a process for the corrosion-protective pretreatment of components in series comprising steel surfaces, in which each component undergoes the successive treatment stages i) - iii): i) conversion treatment step comprising bringing into contact with an acidic aqueous composition (I) containing a) at least 0.05 mmol / kg of fluorocomplexes of the elements Zr and / or Ti calculated as the amount of the elements Zr and / or Ti, and b) an amount of free fluoride; ii) conditioning step comprising bringing into contact with an alkaline aqueous composition (II) having a pH of at least 7.50, but preferably below 12.00, containing magnesium ions and / or calcium ions dissolved in water in an amount such that at least one of the two following conditions is met: (1) amount of dissolved magnesium ions calculated as mg / kg is greater than 20 divided by the pH of the composition (II reduced by the value 7), and / or (2) amount of dissolved calcium ions calculated as mg / kg is greater than 50 divided by the pH of the composition (II reduced by the value 7);iii) coating step comprising dip coating by bringing into contact with an aqueous dispersion (III) of an organic binder; wherein process step ii) immediately follows process step i).

[0006] A series corrosion-protective treatment of components occurs when a large number of components are brought into contact with the treatment solution provided in the respective treatment stages i)-iii) of the process according to the invention and usually stored in system tanks, with the individual components being brought into contact sequentially and thus separated in time. The system tank is the container in which the respective treatment solution, i.e., the acidic aqueous composition (I) of the conversion treatment stage, the alkaline aqueous composition (II) of the conditioning stage, and the aqueous dispersion (III) containing the organic binder of the coating stage, is located for the purpose of the series corrosion-protective treatment according to the present invention.

[0007] If, within the scope of the present invention, the treatment of a component composed of a metallic material is referred to, in particular, the surfaces of the steel material to be treated in the inventive method, this encompasses all materials that contain the respective element, i.e., iron in the case of steel, at more than 50 at.%, preferably at more than 80 at.%, particularly preferably at more than 90 at.%. A corrosion-protective treatment always concerns the surfaces of the component that are formed by metallic materials. The material can be a uniform material or a coating.Thus, according to the invention, galvanized steel grades consist of both steel and zinc, whereby at the cutting edges and grinding points, for example of an automobile body made of galvanized steel, steel surfaces can be exposed and, according to the invention, the steel material is then pretreated.

[0008] The method according to the invention is not limited to application on the steel surfaces of series components, so that in addition to steel, in particular cold-rolled steel (CRS), the common substrates provided by the steel industry such as electrolytically galvanized (ZE) or hot-dip galvanized (Z), alloy-galvanized, in particular (ZM), (ZF), (ZA), or aluminum-coated (AZ), (AS) steel can also be considered as further components of the components. Light metals such as aluminum and magnesium and their alloys can also be pretreated with corrosion protection in the method according to the invention together with the steel surfaces. The method according to the invention is characterized precisely by the fact that it is suitable for pretreating common metallic materials composed of iron, zinc, aluminum and magnesium with corrosion protection, i.e. for providing them with a conversion coating that provides a good paint adhesion base.

[0009] Particularly preferred is an embodiment in which the components of the series are composed not only of steel, but also of hot-dip galvanized steel and / or aluminum. The suitability of the method according to the invention for this material mix to provide a good corrosion-protective pretreatment consisting of conversion layer formation and painting is particularly advantageous for components that are manufactured as a composite structure and are assembled from different semi-finished products. According to the invention, therefore, preferred is a method in which the components of the series represent composite structures, preferably automobile bodies, that are composed of semi-finished steel products and semi-finished products of galvanized steel and aluminum, particularly preferably semi-finished steel products and semi-finished products of galvanized steel and aluminum.

[0010] The components pretreated according to the present invention can be any spatial structure of any shape and design originating from a manufacturing process, in particular semi-finished products such as strips, sheets, rods, pipes, etc., and composite structures assembled from the aforementioned semi-finished products. The composite structures assembled from different materials are usually in the form of cut, formed, and joined flat products by welding, gluing, and flanging. The components to be pretreated in series according to the present invention are preferably selected from automobile bodies or parts thereof, heat exchangers, profiles, pipes, tanks, or tubs.

[0011] Insofar as in the context of the present invention the concentration of an active component or compound is stated as an amount of substance per kilogram, this is the amount of substance based on the weight of the respective total composition.

[0012] The inventive implementation of a conditioning step based on an alkaline, aqueous composition containing calcium and / or magnesium ions in the specified minimum amount brings about a significant improvement in both the corrosion protection on steel and the coating result on the metallic surfaces of the components in an otherwise conventional corrosion-protective pretreatment process comprising a conversion treatment step based on the elements Zr and / or Ti and a coating step based on dip coating. Furthermore, these improvements are achieved without additional process-technologically and energy-intensive process steps, in particular without a drying step prior to dip coating.

[0013] In the following, the individual process steps i)-iii) of the process according to the invention as well as preferred embodiments, also with regard to the process control, are explained in detail. Conversion treatment stage:

[0014] In the conversion treatment stage of the process according to the invention, an amorphous oxidic / hydroxidic coating based on the elements Zr and / or Ti, preferably the element Zr, is to be brought about and the compounds of the elements Zr and / or Ti dissolved in water are accordingly contained, wherein a minimum concentration of 0.05 mmol / kg of compounds of the elements Zr and / or Ti dissolved in water is required in order to be able to achieve a sufficient conversion layer coating in customary contact times of 10-300 seconds at a preferred temperature of the acidic aqueous composition (I) in the range of 10-60 °C and customary application methods such as dipping or spraying.For this purpose, it is preferred according to the invention if the proportion of compounds of the elements Zr and / or Ti dissolved in water in the acidic aqueous composition (I) in process step i) is preferably at least 0.10 mmol / kg, particularly preferably at least 0.30 mmol / kg, especially preferably at least 0.40 mmol / kg.

[0015] For process economic reasons, the contents of compounds of the elements Zr and / or Ti dissolved in water should preferably be below 5.0 mmol / kg, particularly preferably below 3.0 mmol / kg and most preferably below 2.0 mmol / kg based on the elements Zr and / or Ti.

[0016] In a preferred embodiment, in order to achieve the most sufficient conversion of the metallic surfaces, in particular the steel surfaces, the components are brought into contact with the acidic aqueous composition (I) for at least a duration sufficient to produce a layer thickness of at least 20 mg / m 2 , particularly preferably at least 40 mg / m 2 , on the steel surfaces. However, the contacting preferably does not last so long that a layer thickness of more than 200 mg / m 2 , particularly preferably more than 150 mg / m 2 , very particularly preferably more than 100 mg / m 2 , especially preferably more than 80 mg / m 2 , in each case based on the elements Zr and / or Ti, results on the steel surfaces. The layer thicknesses can be determined by means of X-ray fluorescence analysis (XRF).

[0017] As already mentioned, the conversion treatment stage is intended to create an amorphous oxide / hydroxide coating based on the elements Zr and / or Ti, and accordingly, compounds of the elements Zr and / or Ti dissolved in water are included. The term "dissolved in water" encompasses molecularly dissolved species and compounds that dissociate in aqueous solution and form hydrated ions. Typical representatives of these compounds suitable for conversion layer formation from acidic aqueous solutions are titanyl sulfate (TiO(SO4)), titanyl nitrate (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. Preferably, the compounds dissolved in water in the acidic aqueous composition (I) are selected from fluoro acids and / or fluoro complexes of the elements Zr and / or Ti and their water-soluble salts.The formation of conversion layers based on fluoro acids and / or fluoro complexes of the element Zr is particularly preferred since such conversion layers provide improved paint adhesion.

[0018] According to the invention, the conversion treatment in process step i) must be carried out with acidic aqueous compositions (I) that also contain at least a certain amount of free fluoride, since otherwise sufficient pickling of the steel substrate and the subsequent layer formation cannot generally be ensured. Typically, the amount of free fluoride in the acidic aqueous composition (I) is at least 1.00 mmol / kg.For the treatment of components made from a mix of different metallic materials, and in particular galvanized steel surfaces, higher contents of free fluoride can be useful in order to increase the conversion layer formation kinetics and to generate sufficient layer coverage, so that for the treatment of components that also have surfaces of zinc, in particular galvanized steel, very particularly preferably hot-dip galvanized (ZM) steel, a proportion of at least 2.00 mmol / kg, very particularly preferably at least 3.00 mmol / kg of free fluoride is preferred.

[0019] However, it turns out that high levels of free fluoride can in turn be detrimental to paint adhesion on steel, so that the acidic aqueous composition (I) of the conversion treatment stage in process step i) is therefore preferably less than 7.50 mmol / kg, particularly preferably less than 6.00 mmol / kg and most preferably less than 5.00 mmol / kg.

[0020] Suitable sources of free fluoride for the acidic aqueous composition (I) are water-soluble complex fluorides of the elements Zr, Ti and / or Si, preferably of the elements Zr and / or Ti, particularly preferably of the element Zr, and / or hydrofluoric acid, ammonium bifluoride and / or water-soluble alkali metal fluorides.

[0021] The amount of free fluoride is to be determined potentiometrically at 20°C in the respective acidic aqueous composition (I) provided after calibration with fluoride-containing buffer solutions without pH buffering using a fluoride-sensitive measuring electrode.

[0022] With regard to the pH of the acidic aqueous composition in the conversion treatment stage, it must first be noted that the compounds of the elements Zr and / or Ti dissolved in water do not form brines due to hydrolysis, which are no longer available for conversion layer formation. At the same time, the pickling rate for common metallic materials should be sufficiently high to form homogeneous, closed conversion layers; this applies in particular to the steel substrate. According to the invention, it is preferred for this purpose that the acidic aqueous compositions (I) do not have a pH above 5.20 and the pH is preferably less than 5.10, more preferably less than 5.00, most preferably less than 4.90, and especially preferably below 4.80. At the same time, increased pickling and rapid layer formation kinetics can be detrimental to the formation of suitable conversion coatings.Particularly on hot-dip galvanized steel, relatively high coating weights based on the elements Zr and / or Ti are obtained in the more acidic environment, which are more defective due to the higher layer formation kinetics. According to the invention, it is therefore preferred, particularly for the pretreatment of components that have zinc surfaces in addition to steel surfaces, if the pH of the acidic aqueous composition (I) is greater than 3.50, more preferably greater than 4.00, especially more preferably greater than 4.20, and most preferably greater than 4.40.

[0023] In the context of the conversion treatment stage, the pH value corresponds according to the invention to the negative decadic logarithm of the hydronium ion activity measured in the acidic aqueous composition (I) at a temperature of 20 °C using a pH-sensitive glass electrode after two-point calibration against technical buffer solutions of acetic acid / acetate (pH = 4.00) and phosphate (pH = 7.00).

[0024] In order to heal defects in the conversion layer growing in process step i) on the surfaces of steel, but also in particular on the surfaces of hot-dip galvanized steel, the presence of copper ions can be advantageous, as their local cementation in the area of ​​the layer defects then in turn provides improved corrosion protection. In this context, it is preferred that the acidic aqueous composition (I) of the conversion treatment stage in process step i) additionally contains copper ions dissolved in water, preferably at least 0.05 mmol / kg, but again preferably less than 4.0 mmol / kg, particularly preferably less than 2.0 mmol / kg of copper ions dissolved in water. Suitable sources of copper ions dissolved in water are water-soluble salts such as copper nitrate (Cu(NO 3 ) 2 ), copper sulfate (CuSO 4 ) and copper acetate (Cu(CH 3 COO) 2 ).

[0025] Other additives known to those skilled in the art of surface treatment, such as accelerators such as nitrate ions, nitrite ions, nitroguanidine, N-methylmorpholine N-oxide, hydrogen peroxide in free or bound form, hydroxylamine in free or bound form, reducing sugars, and / or wetting agents such as nonionic surfactants, and / or polymers such as polyamidoamines, and / or cations / compounds of the elements Mg, Ca, Al, Si, Sn, Bi and / or Mo, can be contained in the acidic aqueous composition (I) to improve the layer formation kinetics, wettability and corrosion-protective properties. However, for a resource-saving process and for reasons of cost-effectiveness, organic silicon compounds can largely be dispensed with.In a preferred embodiment of the process according to the invention, the acidic aqueous composition (I) in the conversion treatment stage is therefore substantially free of hydrolyzable organic silanes / siloxanes and preferably contains less than 10 mg / kg of hydrolyzable organic silanes / siloxanes calculated as Si(OCH 2 CH 3 ) 4 .

[0026] The application and thus the contacting of the acidic aqueous composition (I) in process step i) for conversion layer formation preferably takes place at at least 30°C, particularly preferably at at least 40°C, but preferably below 60°C. The acidic aqueous composition (II) of the conversion stage can be brought into contact with the components of the series using application methods established in the prior art. These include, in particular, immersion, rinsing, spraying, and / or spraying, with application by immersion and / or spraying methods and, in particular, immersion of the components of the series in a system tank containing the corresponding acidic aqueous composition (I) being preferred. Conditioning level:

[0027] The success of the process according to the invention depends firstly on at least one of the following conditions being met with regard to the alkaline aqueous composition (II) in the process step: (1) The amount of dissolved magnesium ions calculated as mg / kg is greater than 20 divided by the pH of composition (II) reduced by 7, and / or (2) The amount of dissolved calcium ions calculated as mg / kg is greater than 50 divided by the pH of composition (II) reduced by 7.

[0028] Condition (1) is equivalent to the statement that the threshold value of magnesium ions to be exceeded in milligrams per kilogram, preferably rounded to the nearest whole number, based on the alkaline aqueous composition (II) corresponds to the following term: 20 pH − 7

[0029] Condition (2) is equivalent to the statement that the threshold value of calcium ions to be exceeded in milligrams per kilogram, preferably rounded to the nearest whole number, based on the alkaline aqueous composition (II) corresponds to the following term: 50 pH − 7 where the pH value of the alkaline aqueous composition (II) is to be used for the variable "pH".

[0030] Once the amount of calcium and / or magnesium ions is set above the threshold value, a significant improvement in the corrosion protection properties of the freshly deposited amorphous conversion coating based on the elements Zr and / or Ti is achieved after dip coating. In a preferred embodiment, which often results in a further improvement in said corrosion protection performance, at least one of the following two conditions is met for the alkaline aqueous composition (II) in the process step: (1) The amount of dissolved magnesium ions calculated as mg / kg is greater than 40, particularly preferably greater than 50, most preferably greater than 60 divided by the pH value of the composition (II) reduced by the value 7, and / or (2) The amount of dissolved calcium ions calculated as mg / kg is greater than 70, particularly preferably greater than 90 and most preferably greater than 100 divided by the pH value of the composition (II) reduced by the value 7.

[0031] However, with regard to the property of the alkaline aqueous composition (II) to improve the corrosion protection performance of freshly deposited amorphous conversion coatings based on the elements Zr and / or Ti, saturation occurs above a certain amount of calcium and / or magnesium ions. Therefore, for economic reasons and to reduce the carryover of these ions into the subsequent coating stage of process step (iii), it is sensible and therefore preferred if the amount of magnesium ions dissolved in water and calcium ions dissolved in water in the alkaline aqueous composition (II) does not exceed 200 mg / kg each, and particularly preferably, the total amount of magnesium and calcium ions dissolved in water does not exceed 200 mg / kg.

[0032] Typical and suitable sources for the magnesium and calcium ions contained in the alkaline aqueous composition (II) are the respective nitrates and hydroxides, preferably hydroxides, as well as the corresponding salts of α-hydroxycarboxylic acids such as lactic acid, citric acid, tartaric acid and gluconic acid.

[0033] The pH of the alkaline aqueous composition is at least 7.50, but preferably below 12.00, for adequate conditioning of the conversion coating applied to the steel surfaces in step i). Although the desired conditioning of the conversion coating on steel surfaces is possible at higher alkalinity, such a process is less suitable for conditioning the conversion coating on galvanized steel and aluminum substrates, since defects in the conversion coating are heavily pickled on these substrates, which is often accompanied by a deterioration in corrosion protection.Therefore, for the pretreatment of components comprising not only steel surfaces but also those of zinc and / or aluminum, it is particularly preferred if the pH is below 12.00 and particularly preferably below 11.50, very particularly preferably below 10.50 and especially preferably below 10.00. At the same time, improved corrosion values ​​can regularly be achieved if the pH of the alkaline aqueous composition (II) is moderately increased above the value of 7.50. The resulting improvement is often significant, but still requires the presence of calcium and / or magnesium ions as specified in the invention. However, as the pH is increased, the minimum quantities required for conditioning decrease just as moderately.According to the invention, it has been found to be preferable for achieving optimum corrosion protection on the surfaces of steel and also for avoiding mapping if the pH of the alkaline aqueous composition (II) is set above 8.00, particularly preferably above 8.50 and especially preferably above 9.00.

[0034] In the context of the conditioning stage, the pH value corresponds according to the invention to the negative decadic logarithm of the hydronium ion activity measured in the alkaline aqueous composition (II) at a temperature of 20 °C using a pH-sensitive glass electrode after two-point calibration against technical buffer solutions of acetic acid / acetate (pH = 4.00) and boric acid / borate (pH = 9.00).

[0035] The subject of process step ii) is merely to condition the freshly deposited conversion coating and, as such, to prevent or heal layer defects in the amorphous passive layer consisting of oxides, hydroxides of the elements Zr and / or Ti as well as fluoride-containing hydrolysis products of the compounds of the elements Zr and / or Ti that bring about the conversion of the steel surface. It is undesirable for a further significant deposition of further active components to take place in the conditioning stage and, moreover, it is also disadvantageous from a process engineering perspective if significant amounts of other active components than those that serve to build up the minimum amount of magnesium and / or calcium ions are present, the carryover of which into the aqueous dispersion (III) of the subsequent dip coating would have to be prevented. In this respect, it is preferred according to the invention that the alkaline aqueous composition (II) in the conditioning stage (a) less than 50 mg / kg in each case, preferably less than 100 mg / kg in total, particularly preferably less than 50 mg / kg in total of compounds of metal elements dissolved in water whose standard reduction potential (Me 0< / Me n+< ) is greater than that of iron (Fe 0< / Fe 2+< ), calculated as the amount of the respective element in the composition (II), (b) less than 50 µmol / kg in each case, preferably less than 20 µmol / kg in each case, particularly preferably less than 10 µmol / kg in each case of compounds of the elements Cu, Ni, Co, Bi or Ag dissolved in water, calculated as the amount of the respective element in the composition (II), (c) less than 100 mg / kg in total, preferably less than 50 mg / kg in each case, particularly preferably less than 10 mg / kg in each case of surfactants or preferably of surface-active organic compounds or particularly preferably of organic compounds,which are not polymeric organic compounds with a molecular weight above 500 g / mol, (d) a total of less than 50 mg / kg, preferably less than 10 mg / kg, particularly preferably less than 5 mg / kg, especially preferably less than 1 mg / kg of organic polymeric compounds with a molecular weight above 500 g / mol or preferably of organic compounds, (e) a total of less than 100 mg / kg, preferably less than 10 mg / kg, particularly preferably less than 5 mg / kg, especially preferably less than 1 mg / kg of organosilanes and / or siloxanes calculated as Si(OCH 2 CH 3 ) 4 or a total of less than 100 mg / kg, preferably less than 10 mg / kg of compounds of the element silicon dissolved in water, (f) a total of less than 50 µmol / kg, preferably less than 20 µmol / kg, especially preferably less than 10 µmol / kg of in water dissolved compounds of the elements Zr and / or Ti,(g) a total of less than 50 mg / kg, preferably less than 10 mg / kg, of zinc ions, and / or (h) a total of less than 100 mg / kg, preferably less than 10 mg / kg, of phosphates dissolved in water or, preferably, of phosphorus-containing compounds dissolved in water, contains.

[0036] The standard reduction potential is the reduction potential of the electrochemical half-cell Me / Me n+< determined against the standard hydrogen electrode H 2 / H +< (pH=0) at a metal ion activity of 1 mol / l and a temperature of 20 °C.

[0037] The application and thus the contacting of the alkaline aqueous composition (II) in process step ii) of the conditioning stage preferably takes place at at least 20 °C, particularly preferably at at least 30 °C, but preferably below 60 °C. The alkaline aqueous composition (II) of the conditioning stage can be brought into contact with the components of the series using application methods established in the prior art. These include, in particular, immersion, spraying, rinsing by splashing, or hosing the components. Application of the corresponding acidic aqueous composition (I) by immersion, spraying, and / or spraying is preferred. Procedure:

[0038] In the following, preferred embodiments of the method according to the invention are described and explained with regard to the individual treatment stages and the method management, which are particularly advantageous with regard to the object underlying the invention.

[0039] Treatment stages i)-iii) of the process according to the invention each comprise at least one treatment step that involves bringing the components of the series into contact with a more precisely defined aqueous composition characteristic of the treatment stage. For the purpose of contacting, these characteristic compositions are either stored or kept in system tanks. Contacting can take place either in the system tank, for example, by immersion in a composition stored there, or outside the system tank, for example, by spraying a composition stored in the system tank in a spray chamber, depending on the specific requirement or preference of the respective process step.

[0040] In the process according to the invention, process steps i)-iii) follow one another, i.e., in the specified order, and preferably such that the components are not subjected to any wet-chemical treatment step other than one that constitutes a rinsing step between two process steps i)-iii). In this context, a rinsing step serves primarily, preferably exclusively, to remove the wet film adhering to the components from the respective preceding wet-chemical process step and thus to completely or partially remove soluble residues, particles, and active components that would otherwise be carried over from the preceding wet-chemical process step adhering to the component into the next treatment stage.

[0041] According to the invention, process step ii), i.e., the conditioning step, directly follows process step i) of the conversion stage. This means that the process steps follow one another in such a way that the components, with the wet film of the acidic aqueous composition (I) adhering from process step i) of the conversion stage, are transferred to the conditioning stage for contacting with the alkaline aqueous composition (II), i.e., without an intermediate wet-chemical treatment or rinsing step and without an intermediate drying step. A drying step within the meaning of the present invention is a process step that aims at drying the component surfaces by supplying thermal energy and / or an air stream using technical means, e.g., by means of a fan or a hot-air oven.

[0042] In a preferred process according to the invention, the conditioning stage is followed by a so-called rinsing stage comprising at least one rinsing step or a cascade of rinsing steps. Within the rinsing stage, the components of the series are freed from the wet film of the alkaline aqueous composition (II) adhering from the conditioning stage in order to prevent the carryover of alkalinity into the painting stage. For this purpose, the rinsing stage consists of one or more immediately consecutive rinsing steps, a so-called cascade of rinsing steps. Within the cascade, the rinsing steps are in immediate succession if the components are not subjected to another wet-chemical treatment step that is not a rinsing step, or to a drying step, in the meantime.For the actual function of a rinsing stage, which consists in preventing the carryover of alkalinity into the subsequent painting stage, it is beneficial and therefore also preferred within the scope of the present invention if the rinsing stage, as already described, comprises several immediately successive rinsing steps, i.e. a cascade of rinsing steps, for bringing the components of the series into contact with a rinsing solution stored in the system tank of the respective rinsing step.

[0043] During the rinsing stage following the conditioning stage, the wet film of the alkaline aqueous composition (II) should be removed as far as possible. The rinsing steps within the rinsing stage are therefore carried out using a freshwater-based rinsing medium that preferably does not contain any active components, either in terms of type or quantity, whose carryover into the subsequent painting stage would be problematic and must be prevented. If necessary, however, the rinsing medium can contain small amounts of redox-active compounds ("depolarizers"), such as hydrogen peroxide, or, to improve the wettability of the surfaces for dip painting, additional surface-active compounds such as nonionic surfactants. However, the addition of additives should not result in the preferred maximum specific conductivity of 40 µScm -1 being exceeded in the only or final rinsing step of the rinsing stage.In particular, it is important to avoid elements and compounds in the rinsing medium that could adversely affect corrosion protection performance. Therefore, it is preferred if the rinsing medium of the only or last rinsing step of a rinsing stage in a process according to the invention, preferably each rinsing medium of all rinsing steps of a rinsing stage, contains 1,000 μl of acetic acid. (a) less than 50 mg / kg in each case, preferably less than 100 mg / kg in total, particularly preferably less than 50 mg / kg in total of compounds of metal elements dissolved in water whose standard reduction potential (Me 0< / Me n+< ) is greater than that of iron (Fe 0< / Fe 2+< ), calculated as the amount of the respective element in the rinsing medium, (b) less than 50 µmol / kg in each case, preferably less than 20 µmol / kg in each case, particularly preferably less than 10 µmol / kg in each case of compounds of the elements Cu, Ni, Co, Bi or Ag dissolved in water, calculated as the amount of the respective element in the rinsing medium, (c) less than 100 mg / kg in each case, preferably less than 50 mg / kg in each case, particularly preferably less than 10 mg / kg in each case of surfactants or preferably of surface-active organic compounds or particularly preferably of organic compounds which do not contain polymeric organic compounds with a molecular weight above 500 g / mol,(d) a total of less than 50 mg / kg, preferably less than 10 mg / kg, particularly preferably less than 5 mg / kg, especially preferably less than 1 mg / kg of organic polymeric compounds having a molecular weight above 500 g / mol or preferably of organic compounds, (e) a total of less than 100 mg / kg, preferably less than 10 mg / kg, particularly preferably less than 5 mg / kg, especially preferably less than 1 mg / kg of organosilanes and / or siloxanes calculated as Si(OCH 2 CH 3 ) 4 or a total of less than 100 mg / kg, preferably less than 10 mg / kg of compounds of the element silicon dissolved in water, (f) a total of less than 20 µmol / kg, preferably less than 10 µmol / kg, especially preferably less than 5 µmol / kg of compounds of the elements Zr and / or Ti dissolved in water, (g) a total of less than 50 mg / kg, preferably less than 10 mg / kg of sodium and / or potassium ions,(h) contains a total of less than 50 mg / kg, preferably less than 10 mg / kg, of zinc ions, and / or (i) a total of less than 100 mg / kg, preferably less than 10 mg / kg, of phosphates dissolved in water or, preferably, of phosphorus-containing compounds dissolved in water, wherein the pH value of the rinsing medium is preferably in the range of 5.0 to 8.5.

[0044] The standard reduction potential is the reduction potential of the electrochemical half-cell Me / Me n+< determined against the standard hydrogen electrode H 2 / H +< (pH=0) at a metal ion activity of 1 mol / l and a temperature of 20 °C.

[0045] For a rinsing step following the conditioning step and preceding the coating step, it is preferred if the specific conductivity of the rinsing medium in the system tank of the single rinsing step or in the system tank of the last rinsing step of the cascade, i.e. of the rinsing medium in the system tank of the rinsing step immediately preceding process step iii), is not above 40 µScm -1<.

[0046] To achieve this purpose, the rinsing stage or the system tank of the single rinsing step or at least one system tank of the cascade of rinsing steps can be fed with fresh water with a specific conductivity of preferably less than 10 µScm -1<, whereby the volume flow of fresh water fed in should be large enough not to exceed the maximum specific conductivity of 40 µScm -1< preferred for the rinsing stage during the treatment of the series of components.

[0047] Furthermore, for reasons of process economy, it is preferred if the conditioning stage according to process step ii) and optionally the rinsing stage immediately following process step ii), which, as already explained, preferably comprises at least one rinsing step or a cascade of rinsing steps, each with a freshwater-based rinsing medium, is in turn immediately followed by process step iii), preferably in such a way that the components are transferred to the coating stage for contacting with the aqueous dispersion (III) with the wet film adhering from the conditioning stage or optionally with the wet film adhering from the only or last rinsing step of the rinsing stage, i.e. without an intermediate drying step, and in this way a "wet-on-wet" procedure is established for the components of the series across all process steps i)-iii) of the process according to the invention.

[0048] Furthermore, it should be noted that in the process according to the invention, the components are first cleaned and degreased in a degreasing stage before process step i) and thus before the formation of the conversion layer.

[0049] In the degreasing stage, an alkaline aqueous composition with a pH above 9.00 and preferably containing at least one surface-active compound selected from anionic surfactants, cationic surfactants, zwitterionic surfactants, and / or nonionic surfactants can be provided for cleaning and degreasing the components of the series. The objective of the degreasing stage is to ensure that the component surface is largely free of inorganic salts and organic contaminants, particularly drawing, forming, rolling, and corrosion protection oils, for the subsequent conversion treatment stage, and to ensure that the amorphous oxide / hydroxide coating based on the elements Zr and / or Ti can be produced as homogeneously as possible.In a preferred embodiment, immediately after passing through the degreasing stage, i.e. before the conversion treatment stage, but after a rinsing step with deionized water (κ < 1 µScm -1< ), a carbon coating of less than 0.20 g / m 2< , particularly preferably less than 0.10 g / m 2< , remains on the surfaces of the components in the series formed by the metallic materials. The layer of carbon remaining on the surface of the components formed by the metallic materials can be determined by means of pyrolytic decomposition. For this purpose, a representative component section of a defined area is brought to a substrate temperature (PMT) of 550°C in an oxygen atmosphere and the amount of carbon dioxide released is quantitatively recorded as the amount of carbon using an infrared sensor, for example using the LECO ®< RC-412 Multiphase Carbon Determinator (Leco Corp.).

[0050] According to the invention, the pH value of an optional degreasing step corresponds to the negative decadic logarithm of the hydronium ion activity measured in the alkaline aqueous composition (I) at a temperature of 20 °C using a pH-sensitive glass electrode after two-point calibration against technical buffer solutions of acetic acid / acetate (pH = 4.00) and boric acid / borate (pH = 9.00).

[0051] The process in the context of the present invention also includes the possibility of applying a corrosion-protective coating, i.e., conversion-treated and dip-coated, to components which, in addition to steel, also comprise other metallic materials, preferably components which are joined together in a composite construction, for example automobile bodies, and which, in addition to the aforementioned steel surfaces, also have zinc surfaces, particularly preferably zinc and aluminum. Suitable metallic materials whose surfaces can be pretreated with corrosion protection in the process according to the invention are, in addition to steel: zinc, electrolytic (ZE), hot-dip galvanized (Z), alloy galvanized (ZA), (ZF), and (ZM), and aluminum-coated (AZ), (AS) strip steel, as well as the light metals aluminum and magnesium and their alloys. Coating level:

[0052] In the coating stage, at least the surfaces of the components formed by the steel and conversion-coated in process step ii), preferably all surfaces formed by metallic materials, are coated with a first coating system by contacting the component or at least said conversion-coated steel surfaces with the aqueous dispersion (III) containing the organic binder. The coating system is therefore deposited directly from the aqueous phase as a coating of the organic binder of the aqueous dispersion (III) precipitated onto said surfaces, which is typically subjected to thermal post-treatment for film formation and curing. The coating in the coating stage is applied by dip coating, preferably by electrocoating, and again preferably by cathodic electrocoating.For this purpose, the organic binder of the aqueous dispersion (III) is preferably based on amine-modified film-forming polyepoxides, which preferably additionally comprise organic compounds containing blocked and / or unblocked isocyanate groups as hardeners. Inorganic pigments are also often a component of the aqueous dispersion and a preferred additive for improving corrosion protection. The aqueous phase also preferably contains small amounts of compounds of the elements yttrium and / or bismuth, dissolved or dispersed in water, which have a positive effect on crosslinking and film formation.

[0053] The preferred pH of the aqueous dispersion (III) of the coating stage is in the range from 5.0 to 6.0, particularly preferably in the range from 5.4 to 5.8. In the context of the coating stage, the pH corresponds according to the invention to the negative decadic logarithm of the hydronium ion activity measured in an aqueous dispersion (III) diluted by a factor of 10 with deionized water (κ<1µScm -1< ) ​​at a temperature of 20 °C using a pH-sensitive glass electrode after two-point calibration against technical buffer solutions of acetic acid / acetate (pH = 4.0) and boric acid / borate (pH = 9.0).

[0054] The application and thus the bringing into contact of the components or at least the said conversion-coated steel surfaces with the aqueous dispersion (III) is preferably carried out at at least 30 °C, particularly preferably at at least 40 °C, but preferably below 60 °C. The aqueous dispersion (III) of the coating stage can be brought into contact with the components of the series by means of immersion application methods established in the prior art, with particular preference being given to immersing the components of the series in a system tank containing the corresponding aqueous dispersion (III).

Claims

1. A process for the corrosion-protective pretreatment of components in series comprising steel surfaces, in which each component undergoes the successive treatment stages i) - iii): i) conversion treatment stage comprising bringing into contact with an acidic aqueous composition (I) containing a) at least 0.05 mmol / kg of compounds of the elements Zr and / or Ti dissolved in water, calculated as the amount of the elements Zr and / or Ti, and b) an amount of free fluoride;ii) conditioning step comprising bringing into contact with an alkaline aqueous composition (II) having a pH of at least 7.50 containing magnesium ions and / or calcium ions dissolved in water in an amount such that at least one of the two following conditions is met: (1) amount of dissolved magnesium ions calculated as mg / kg is greater than 20 divided by the pH of the composition (II) reduced by the value 7, and / or (2) amount of dissolved calcium ions calculated as mg / kg is greater than 50 divided by the pH of the composition (II) reduced by the value 7; iii) coating step comprising dip coating by bringing into contact with an aqueous dispersion (III) of an organic binder, wherein process step ii) immediately follows process step i).

2. Method according to claim 1, characterized in thatProcess step ii) is immediately followed by a rinsing stage comprising at least one rinsing step or a cascade of rinsing steps, preferably with a fresh water-based rinsing medium, and preferably this rinsing step or this cascade of rinsing steps is immediately followed by process step iii), wherein the specific conductivity of the rinsing medium in the system tank of the rinsing step immediately preceding process step iii) is preferably less than 40 µScm -1 , particularly preferably less than 10 µScm -1 , most preferably less than 10 µScm -1 is.

3. Method according to one or more of the preceding claims, characterized in that process step iii) follows process step ii) without an intermediate drying step.

4. Method according to one or more of the preceding claims, characterized in thatthe components of the series are first cleaned and / or degreased before the conversion treatment stage in process step i).

5. Method according to one or more of the preceding claims, characterized in that the alkaline aqueous composition (II) of the conditioning in process step ii) has a pH above 8.00, preferably above 8.50, particularly preferably above 9.00, but preferably below 12.00 and particularly preferably below 11.50, very particularly preferably below 10.50 and especially preferably below 10.

00.

6. Method according to one or more of the preceding claims, characterized in thatthe amount of magnesium ions dissolved in water and of calcium ions dissolved in water in the alkaline aqueous composition (II) of the conditioning in process step ii) does not exceed 200 mg / kg each and preferably the total amount of magnesium and calcium ions dissolved in water does not exceed 200 mg / kg.

7. Method according to one or more of the preceding claims, characterized in that the amount of compounds of the element Zr and / or Ti dissolved in water in the alkaline aqueous composition (II) of the conditioning in process step ii), calculated as Zr and / or Ti, is in each case below 50 µmol / kg, preferably below 20 µmol / kg, particularly preferably below 10 µmol / kg.

8. Method according to one or more of the preceding claims, characterized in thatin the alkaline aqueous composition (II) of the conditioning in process step ii) a total of less than 10 mg / kg, preferably less than 5 mg / kg, particularly preferably less than 1 mg / kg of polymeric organic compounds with a molecular weight above 500 g / mol is contained.

9. Method according to one or more of the preceding claims, characterized in that in the alkaline aqueous composition (II) of the conditioning in process step ii) a total of less than 10 mg / kg, preferably less than 5 mg / kg, particularly preferably less than 1 mg / kg of organosilanes and / or siloxanes, calculated as Si(OCH2CH3)4, is contained.

10. Method according to one or more of the preceding claims, characterized in thatin the alkaline aqueous composition (II) of the conditioning in process step ii) in each case less than 50 mg / kg, preferably in total less than 100 mg / kg, particularly preferably in total less than 50 mg / kg of compounds of metal elements dissolved in water, whose standard reduction potential (Me 0 / Me n+ ) is greater than that of iron (Fe 0 / Fe 2+ ) calculated as the amount of the respective element.

11. Method according to one or more of the preceding claims, characterized in that in the alkaline aqueous composition (II) of the conditioning in process step ii) less than 50 µmol / kg, preferably less than 20 µmol / kg, particularly preferably less than 10 µmol / kg of compounds of the elements Cu, Ni, Co, Bi or Ag dissolved in water, calculated as the amount of the respective element, are contained.

12. Method according to one or more of the preceding claims, characterized in that the acidic aqueous composition (I) in process step i) of the conversion treatment stage contains at least 0.10 mmol / kg, preferably at least 0.30 mmol / kg, particularly preferably at least 0.40 mmol / kg, but preferably not more than 5.0 mmol / kg, particularly preferably not more than 3.0 mmol / kg, very particularly preferably not more than 2.0 mmol / kg of fluorocomplexes of the elements Zr and / or Ti, calculated as the amount of the elements Zr and / or Ti.

13. Method according to one or more of the preceding claims, characterized in that the acidic aqueous composition (I) in process step i) of the conversion treatment stage additionally contains copper ions dissolved in water, preferably in an amount of at least 0.05 mmol / kg, but preferably less than 4.0 mmol / kg, particularly preferably less than 2.0 mmol / kg of copper ions dissolved in water.

14. Method according to one or more of the preceding claims, characterized in thatthe conversion treatment stage (I) in process step i) is carried out for a duration sufficient to form a layer of Zr and / or Ti of at least 20 mg / m 2 , preferably at least 40 mg / m 2 , but preferably not more than 200 mg / m 2 on the surfaces of steel of the series components.

15. Method according to one or more of the preceding claims 2 to 14, characterized in that the components of the series additionally have surfaces made of zinc, preferably additional surfaces made of zinc and aluminum.

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