RESOURCE-SAVING METHOD FOR ZINC PHOSPHATING OF A METAL SURFACE

Abstract

The present invention relates to a method for zinc phosphating of metal surfaces using a colloidal aqueous solution as activation stage, a zinc phosphate layer having a layer weight of less than 2.0 g/m2 being deposited on surfaces of zinc in the method step following activation. The activation stage is based on a colloidal aqueous solution containing a dispersed particulate constituent, the particulate constituent containing, in addition to dispersed inorganic compounds of phosphates of polyvalent metal cations, a polymeric organic compound as a dispersing agent that is composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms, the polymeric organic compound additionally comprising units of maleic acid, their anhydride and/or their imide and the polymeric organic compound additionally comprising polyoxyalkylene units.

Description

The present invention relates to a method for zinc phosphation of metal surfaces in a layer-forming manner using a colloidal aqueous solution as activation stage, a zinc phosphate layer having a layer weight of less than 2.0 g/m² being deposited on the surfaces of zinc in the method step following activation. The activation stage is based on a colloidal aqueous solution containing a dispersed particulate constituent, the particulate constituent containing, in addition to dispersed inorganic compounds of phosphates of polyvalent metal cations, a polymeric organic compound as a dispersing agent that is composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms, the polymeric organic compound additionally comprising units of maleic acid, their anhydride and/or their imide and the polymeric organic compound additionally comprising polyoxyalkylene units. For the formation of closed zinc phosphate coatings that protect sufficiently against corrosion and that can be fed to subsequent electrocoating, it is necessary for the content of the particulate constituents of the colloidal aqueous solution to be at least 4 g/kg based on the colloidal aqueous solution.

Layer-forming phosphation is a process for applying crystalline anti-corrosion coatings to metal surfaces, in particular to materials of the metals iron, zinc and aluminum, which has been used for decades and has been studied in depth. Zinc phosphation, which is particularly well established for corrosion protection, is carried out in a layer thickness of a few micrometers and is based on corrosive pickling of the metal material in an acidic aqueous composition containing zinc ions and phosphates. In the course of the pickling, an alkaline diffusion layer forms on the metal surface, which layer extends into the interior of the solution and within which sparingly soluble crystallites form, which crystallites precipitate directly at the interface with the metal material and continue to grow there. To support the pickling reaction on materials of the metal aluminum and to mask the bath poison aluminum, which in dissolved form disturbs the layer formation on materials of the metal, water-soluble compounds that are a source of fluoride ions are often added. By default, the zinc phosphation is set such that closed, crystalline coatings are realized at a layer weight of at least 2 g/m², especially on the zinc surfaces of the components, for good corrosion protection and coating primer. Depending on the metal surface to be phosphated, the pickling described above and the concentration of the active components in the zinc phosphation stage have to be adjusted accordingly in order to ensure correspondingly high layer weights on the surfaces of the metals iron or steel, zinc and aluminum. Zinc phosphation is always initiated by activation of the metal surfaces of the component to be phosphated. Wet-chemical activation is carried out conventionally by means of contact with colloidal aqueous solutions of phosphates (“activation stage”), which, insofar as they are immobilized on the metal surface, are used in the subsequent phosphating as a growth nucleus for the formation of the crystalline coating within the alkaline diffusion layer. Suitable dispersions in this case are colloidal, mostly neutral to alkaline aqueous compositions based on phosphate crystallites, which have only small crystallographic deviations in their crystal structure from the type of zinc phosphate layer to be deposited. In this context, WO 98/39498 A1 teaches in particular bivalent and trivalent phosphates of the metals Zn, Fe, Mn, Ni, Co, Ca and Al, it being technically preferred for phosphates of the metal zinc to be used for activation for subsequent zinc phosphation.

An activation stage based on dispersions of bivalent and trivalent phosphates requires a high level of process control in order to keep the activation performance constantly at an optimal level, in particular when treating a series of metal components. To ensure the method is sufficiently robust, foreign ions carried over from previous treatment baths or aging processes in the colloidal aqueous solution must not lead to the activation performance deteriorating. A deterioration is initially noticeable in increasing layer weights in the subsequent phosphating and ultimately leads to the formation of defective or inhomogeneous phosphate layers. Overall, the layer-forming zinc phosphation is therefore a multi-stage process that is technically complex to control and has hitherto been carried out in a resource-intensive manner, both with regard to the process chemicals and the energy to be expended. This applies in particular to the process step of zinc phosphation, which first requires a high material requirement due to layer formation, especially on components that have zinc surfaces, but also brings about a high amount of material removal into the phosphating bath in a manner linked to the required pickling rate. The high pickling rate has the effect that measures for preventing or preparing and disposing of phosphating sludges have to be taken, which in turn require the use of further process chemicals.

There is therefore a need to optimize the pretreatment line for zinc phosphation, including the activation stage and phosphation stage, such that the overall process can be carried out in a less resource-intensive manner, especially in the pretreatment of components having zinc surfaces. However, a resource-saving overall process must not be at the expense of the properties of the zinc phosphation, which must be provided as a homogeneous, closed coating with high electrical charge transfer resistance in order to make good protection against corrosion and correspondingly good coverage of the coating in a subsequent electrocoating possible.

This complex task profile can surprisingly be achieved by the deposition of relatively thin phosphate coatings, activation using a specific polymeric dispersing agent for stabilizing the colloidal constituent of an activation stage based on particulate phosphates being necessary, provided that it is ensured that a minimum amount of particulate constituents is not undershot in the activation stage. Owing to the extremely efficient stabilization of the particulate constituent that brings about activation, the specific dispersing agent ensures that high proportions of colloids can also be set in a quasi-continuous process of the zinc phosphation, which surprisingly leads to improved activation of the metal surfaces and the formation of particularly thin but homogeneous closed phosphate coatings having a high electrical penetration resistance.

The present invention therefore relates to a method for anti-corrosion pretreatment of a metal material that at least partially has surfaces of zinc or of a component which is composed at least partially of such a metal material, in which method the metal material or the component undergoes firstly activation (i) and then zinc phosphation (ii), in consecutive method steps, the activation in method step (i) being carried out by bringing the metal material or the component into contact with a colloidal aqueous solution containing, in the dispersed particulate constituent (a) of the solution,

(a1) at least one particulate inorganic compound that is composed of phosphates of polyvalent metal cations at least partially selected from hopeite, phosphophyllite, scholzite and/or hureaulite, and
(a2) at least one polymeric organic compound that is composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms, the polymeric organic compound additionally comprising units of maleic acid, their anhydride and/or their imide and the polymeric organic compound additionally comprising polyoxyalkylene units,
the content of the particulate constituents of the colloidal aqueous solution being at least 4 g/kg based on the colloidal aqueous solution; and
a zinc phosphate layer having a layer weight of less than 2.0 g/m² being deposited on the surfaces of zinc in method step (ii).

In the context of the method according to the invention, a metal material has at least one surface made of zinc if more than 50 at. % of the metal structure on this surface, up to a material penetration depth of at least one micrometer, is composed of zinc. This regularly applies to metal materials, more than 50 at. % of which, as homogeneous materials, are composed of zinc, but also applies to materials that are provided with metal coatings of zinc, such as electrolytically galvanized or hot-dip galvanized strip steel, which can also be alloyed with iron (ZF), aluminum (ZA) and/or magnesium (ZM).

The components treated according to the present invention can be three-dimensional structures of any shape and design that originate from a manufacturing process, in particular also including semi-finished products such as strips, sheets, rods, pipes, etc., and composite structures assembled from said semi-finished products, the semi-finished products preferably being interconnected by means of adhesion, welding and/or flanging to form a composite structure.

The dispersed particulate constituent (a) of the colloidal aqueous solution in the activation (i) of the method according to the invention is the solids content that remains after drying the retentate of an ultrafiltration of a defined partial volume of the aqueous dispersion having a nominal cutoff limit of 10 kD (NMWC: nominal molecular weight cutoff). The ultrafiltration is carried out by adding deionized water (κ<1 μScm⁻¹) until a conductivity of below 10 μScm⁻¹ is measured in the filtrate.

In the context of the present invention, an organic compound is polymeric if its weight-average molar mass is greater than 500 g/mol. The molar mass is determined using the molar mass distribution curve of a sample of the relevant reference value, which curve is established experimentally at 30° C. by means of size-exclusion chromatography using a concentration-dependent refractive index detector and calibrated against polyethylene glycol standards. The average molar masses are evaluated with the aid of a computer according to the strip method using a third-order calibration curve. Hydroxylated polymethacrylate is suitable as a column material, and an aqueous solution of 0.2 mol/L sodium chloride, 0.02 mol/L sodium hydroxide, and 6.5 mmol/L ammonium hydroxide is suitable as an eluent.

The method according to the invention is characterized in that, above an amount of particulate constituents of 4 g/kg in the colloidal aqueous solution, homogeneous, closed zinc phosphate coatings already grow on the surfaces of the metal materials in the case of unusually lower layer weights, which surfaces represent a coating primer equivalent to zinc phosphate coatings of the prior art for subsequent electrocoating, while simultaneously providing excellent coverage. Notwithstanding this, the present invention aims to deposit homogeneous, closed zinc phosphate coatings on the surfaces of zinc, such that it is assumed according to the invention that, in method step (ii), a zinc phosphate coating having a layer weight of more than 0.5 g/m², preferably more than 1.0 g/m² is provided on the surfaces of zinc.

The reduction of the layer weight also allows a shorter wet-chemical exposure time or contact time with the acidic aqueous composition of the zinc phosphation, which in turn correlates both with lower pickling removal and an overall shorter pretreatment duration. Further reduction of the layer weight or the contact time in the zinc phosphation can be achieved if the content of particulate constituents in the colloidal aqueous solution is further increased such that methods according to the invention are preferred in which the content of the particulate constituents of the colloidal aqueous solution is at least 6 g/kg, particularly preferably at least 8 g/kg, more particularly preferably at least 10 g/kg. However, the achievable layer weight reduction in the zinc phosphation with a further increase in the colloid content in the activation stage is only marginal and increasingly competes with disadvantages in method control and lower colloid stability in the activation stage. It is therefore preferred to limit the content of the particulate constituents of the colloidal aqueous dispersion to 20 g/kg, particularly preferably to 15 g/kg, based in each case on the colloidal aqueous solution.

In the method according to the invention, the layer coating of zinc phosphate on the surfaces of zinc should be limited to 2.0 g/m2 for resource-saving operation of the pretreatment line. According to the invention, due to the minimum amount of particulate constituents in the colloidal aqueous dispersion of the activation stage, a sufficiently homogeneous and closed zinc phosphate coating having excellent coverage behavior in subsequent electrocoating is provided. The activation of the surfaces of zinc is effective in method step (i) of the method according to the invention in such a way that a further reduction in the layer coating and thus a further reduction in the material consumption in the zinc phosphation stage in method step (ii) is possible. It is accordingly preferred in method step (ii) to limit the layer weight of zinc phosphate on the surfaces of zinc to less than 1.8 g/m², particularly preferably less than 1.6 g/m², more particularly preferably less than 1.5 g/m². The limitation of the layer weight can be undertaken in the method control either via a reduced contact time with the acidic aqueous composition for zinc phosphation in method step (ii) and/or by increasing the content of the particulate constituents of the colloidal aqueous dispersion in method step (i). The layer weight of zinc phosphate is determined within the scope of the present invention by removing the zinc phosphate layer using aqueous 5 wt. % CrO₃ as a pickling solution that is brought into contact with a defined area of the phosphated material or component at 25° C. for 5 min immediately following the zinc phosphation and washing with deionized water (κ<1 μScm⁻¹), and subsequently determining the phosphorus content in the same pickling solution by means of ICP-OES. The layer weight of zinc phosphate can be found by multiplying the amount of phosphorus relative to surface area by a factor of 6.23.

A particular further advantage of the method according to the invention is that, in addition to the surfaces of zinc, the surfaces of iron and aluminum are also very effectively activated in method step (i) and, in method step (ii), likewise very homogeneous, closed zinc phosphate coatings having a comparatively low layer weight are accessible on said surfaces. Accordingly, according to the invention, components are preferably treated that, in addition to surfaces of zinc, also have surfaces of iron and/or aluminum, a zinc phosphate layer having a layer weight of less than 2.0 g/m², particularly preferably less than 1.8 g/m², more particularly preferably less than 1.6 g/m², and very particularly preferably less than 1.5 g/m², but preferably at least 0.5 g/m², particularly preferably at least 1.0 g/m² preferably being deposited on surfaces of iron and aluminum in method step (ii). Analogously, a metal material has at least one surface made of iron or aluminum if more than 50 at. % of the metal structure on this surface, up to a material penetration depth of at least one micrometer, is composed of iron or aluminum.

In addition, it has been found in the context of the present invention that the condensed phosphates, which are dissolved in water and often added for colloid stabilization in the activation stages in the prior art methods, are only of secondary importance for maintaining consistently good phosphate coatings in the context of the present invention. It is remarkable and surprising to a person skilled in the art that, in methods according to the invention that are based on an activation stage based on the particulate constituent (a), which is present in an amount of at least 4 g/kg based on the colloidal aqueous solution, the additivation of condensed phosphates can largely be dispensed with or completely dispensed with. In a preferred embodiment of the method according to the invention, the content of condensed phosphates dissolved in water in the activation stage is less than 0.25, particularly preferably less than 0.20, more particularly preferably less than 0.15, and very particularly preferably less than 0.10, based on the phosphate content of the at least one particulate compound in the colloidal aqueous solution, in each case based on the element P.

Furthermore, it is preferred in this context for the content of condensed phosphates dissolved in water in the colloidal aqueous solution of the method according to the invention, calculated as P, to be less than 100 mg/kg, particularly preferably less than 20 mg/kg, more particularly preferably less than 15 mg/kg, and very particularly preferably less than 10 mg/kg, based on the colloidal aqueous solution. Overall, in the context of the present invention, the addition of condensed phosphates can thus be entirely dispensed with, and therefore the activation only involves small amounts of condensed phosphates that make it to the activation stage from previous cleaning stages involving the component to be pretreated, in particular when treating a large number of components in series.

In the context of the present invention, condensed phosphates are metaphosphates and polyphosphates, preferably polyphosphates, particularly preferably pyrophosphate. The condensed phosphates are preferably in the form of compounds of monovalent cations, preferably selected from Li, Na and/or K, particularly preferably Na and/or K.

The content of condensed phosphates can be determined analytically from the difference in the total phosphate content in the non-particulate constituent of the colloidal aqueous solution with and without oxidative digestion, for example by means of peroxodisulfate, the dissolved orthophosphate content being quantified by means of photometry. Alternatively, if polyphosphates are used as condensed phosphates, enzymatic digestion with a pyrophosphatase can take place instead of oxidative digestion. The non-particulate constituent of the colloidal aqueous solution is the solids content of the colloidal aqueous solution in the permeate of the above-described ultrafiltration after it has been dried to constant mass at 105° C.—that is, the solids content after the particulate constituent (a) has been separated by means of ultrafiltration.

The high tolerance of the method according to the invention with respect to carried-over foreign ions also makes it possible for the cleaning stages and rinsing stages carried out before the activation stage and also the activation stage itself to be carried out with service water instead of deionized water. In this way, the method according to the invention is carried out in a particularly resource-saving manner. It is therefore preferred according to the invention for the colloidal aqueous solution in the activation to contain at least 0.5 mmol/L, particularly preferably at least 1.0 mmol/L, more particularly preferably at least 1.5 mmol/L, but preferably no more than 10 mmol/L, of alkaline-earth metal ions dissolved in water.

Should the tolerance of the method according to the invention reach the system-specific limits in each case with exceptionally high ionic strengths, e.g., high permanent water hardness and at the same time a high content of carried-over foreign ions from previous cleaning stages, organic complexing agents can be added to mask the foreign ions in order to maintain a long bath life. In this case, it must be assessed whether the economic advantage of it being possible to carry out the activation stage and, where necessary, preceding cleaning stages and rinsing with service water is not impeded by the addition of organic complexing agents and their technical monitoring in the system tank of the activation stage. Suitable organic complexing agents which are preferred in this context are selected from α-hydroxycarboxylic acids, which in turn are preferably selected from gluconic acid, tartronic acid, glycolic acid, citric acid, tartaric acid, lactic acid, very particularly preferably gluconic acid, and/or organophosphonic acids, which in turn are preferably selected from etidronic acid, aminotris(methylenephosphonic acid), aminotris(methylenephosphonic acid), phosphonobutane-1,2,4-tricarboxylic acid, diethylenetriaminepenta(methylenephosphonic acid), hexamethylenediamine tetra(methylenephosphonic acid) and/or hydroxyphosphonoacetic acid, particularly preferably from etidronic acid.

To maintain a stable activation performance, organic complexing agents should only be added to such an extent that the amount thereof in the colloidal aqueous solution is preferably no more than twice, particularly preferably no more than 1.5 times, the amount of alkaline-earth metal ions, and is very particularly preferably no more than equimolar to the amount of alkaline-earth metal ions.

The colloidal aqueous solution in the activation (i) of the method according to the invention preferably has an alkaline pH, particularly preferably a pH above 8.0, more particularly preferably above 9.0, but preferably below 11.0, it being possible to use compounds which influence the pH, such as phosphoric acid, sodium hydroxide solution, ammonium hydroxide or ammonia, to adjust the pH. The “pH,” as used in the context of the present invention, corresponds to the negative decadic logarithm of the hydronium ion activity at 20° C. and can be determined by means of pH-sensitive glass electrodes.

For good activation performance, it is necessary to use polyvalent metal cations in the form of phosphates which should be contained in the dispersed particulate constituent (a) for activation in a correspondingly high proportion. Accordingly, the content of phosphates contained in the at least one particulate inorganic compound (a1), based on the dispersed particulate constituent (a) of the colloidal aqueous solution, is preferably at least 25 wt. %, particularly preferably at least 35 wt. %, more particularly preferably at least 40 wt. %, very particularly preferably at least 45 wt. %. The inorganic particulate constituent of the colloidal aqueous solution is, in turn, that which remains when the particulate constituent (a) obtained from the drying of the retentate of the ultrafiltration is pyrolyzed in a reaction furnace by supplying a CO₂-free oxygen flow at 900° C. without admixture of catalysts or other additives until an infrared sensor provides a signal identical to the CO₂-free carrier gas (blank value) in the outlet of the reaction furnace. The phosphates contained in the inorganic particulate constituent are determined as phosphorus content by means of atomic emission spectrometry (ICP-OES) after acid digestion of the constituent with aqueous 10 wt. % HNO₃ solution at 25° C. for 15 min, directly from the acid digestion.

The active components of the colloidal aqueous dispersion which effectively promote the formation of a closed phosphate coating on the metal surfaces and in this sense activate the metal surfaces are, as already mentioned, composed primarily of phosphates, which in turn result in the formation of finely crystalline coatings, and are therefore at least partially selected from hopeite, phosphophyllite, scholzite and/or hureaulite, preferably at least partially selected from hopeite, phosphophyllite and/or scholzite, particularly preferably at least partially selected from hopeite and/or phosphophyllite and very particularly preferably at least partially selected from hopeite. An activation within the meaning of the present invention is thus substantially based on the phosphates in particulate form contained in the activation stage. Without taking into account water of crystallization, hopeites stoichiometrically comprise Zn₃(PO₄)₂ and the nickel-containing and manganese-containing variants Zn₂Mn(PO₄)₃, Zn₂Ni(PO₄)₃, whereas phosphophyllite consists of Zn₂Fe(PO₄)₃, scholzite consists of Zn₂Ca(PO₄)₃ and hureaulite consists of Mn₃(PO₄)₂. The existence of the crystalline phases hopeite, phosphophyllite, scholzite and/or hureaulite in the aqueous dispersion according to the invention can be demonstrated by means of X-ray diffractometric methods (XRD) after separation of the particulate constituent (a) by means of ultrafiltration with a nominal cutoff limit of 10 kD (NMWC: nominal molecular weight cutoff) as described above and drying of the retentate to constant mass at 105° C.

Due to the preference for the presence of phosphates comprising zinc ions and having a certain crystallinity, it is preferred for the formation of firmly adherent crystalline zinc phosphate coatings after successful activation, in the method according to the invention, for the colloidal aqueous dispersion to contain at least 20 wt. %, particularly preferably at least 30 wt. %, more particularly preferably at least 40 wt. %, of zinc in the inorganic particulate constituent of the colloidal aqueous solution, based on the phosphate content of the inorganic particulate constituent, calculated as PO₄.

However, activation within the meaning of the present invention is preferably not achieved by means of colloidal solutions of titanium phosphates, since otherwise layer-forming zinc phosphating on iron, in particular steel, is not reliably achieved. In a preferred embodiment of the method according to the invention, the content of titanium in the inorganic particulate constituent of the colloidal aqueous solution is therefore less than 0.01 wt. %, particularly preferably less than 0.001 wt. %, based on the colloidal aqueous solution. In a particularly preferred embodiment, the colloidal aqueous solution of the activation stage (i) contains a total of less than 10 mg/kg, particularly preferably less than 1 mg/kg, of titanium.

The activation stage in the method according to the invention can additionally be characterized by its D50 value, above which the activation performance decreases significantly. The D50 value of the colloidal aqueous solution is preferably below 1 μm, particularly preferably below 0.4 μm. In the context of the present invention, the D50 value denotes the particle diameter which is not exceeded by 50 vol. % of the particulate constituents contained in the colloidal aqueous solution. According to ISO 13320:2009, the D50 value can be determined at 20° C. from volume-weighted cumulative particle size distributions by means of scattered light analysis according to Mie theory immediately after the sample has been taken from the activation stage, using spherical particles and a refractive index of the scattering particles of n_D=1.52−i·0.1.

Within the meaning of the present invention, the polymeric organic compounds (a2) that are used as dispersing agents are composed partially of styrene and/or an α-olefin having no more than 5 carbon atoms and of maleic acid, its anhydride and/or its imide, and additionally comprise polyoxyalkylene units. These polymeric organic compounds (a2) cause the extremely high stability of the colloidal aqueous solution in the activation stage of the method according to the invention.

The α-olefin in this case is preferably selected from ethene, 1-propene, 1-butene, isobutylene, 1-pentene, 2-methyl-but-1-ene and/or 3-methyl-but-1-ene and particularly preferably selected from isobutylene. It is clear to a person skilled in the art that the polymeric organic compounds (a2) contain these monomers as structural units in unsaturated form covalently linked to one another or to other structural units. Suitable commercially available representatives are, for example, Dispex® CX 4320 (BASF SE), a maleic acid-isobutylene copolymer modified with polypropylene glycol, Tego® Dispers 752 W (Evonik Industries AG), a maleic acid-styrene copolymer modified with polyethylene glycol, or Edaplan® 490 (Munzing Chemie GmbH), a maleic acid-styrene copolymer modified with EO/PO and imidazole units. In the context of the present invention, polymeric organic compounds (a2) that are composed partly of styrene are preferred.

The polymeric organic compounds (a2) used as dispersing agents have polyoxyalkylene units that are preferably composed of 1,2-ethanediol and/or 1,2-propanediol, particularly preferably of both 1,2-ethanediol and 1,2-propanediol, the content of 1,2-propanediols in the entirety of the polyoxyalkylene units being preferably at least 15 wt. %, but particularly preferably not exceeding 40 wt. %, based on the entirety of the polyoxyalkylene units. Furthermore, the polyoxyalkylene units are preferably contained in the side chains of the polymeric organic compounds (a2). A content of the polyoxyalkylene units in the entirety of the polymeric organic compounds (a2) of preferably at least 40 wt. %, particularly preferably at least 50 wt. %, but preferably no more than 70 wt. %, is advantageous for the dispersibility.

For anchoring the dispersing agent with the inorganic particulate constituent of the colloidal, aqueous solution, which is at least partially formed of polyvalent metal cations in the form of phosphates selected from hopeite, phosphophyllite, scholzite and/or hureaulite, the organic polymeric compounds (a2) also have imidazole units, preferably such that the polyoxyalkylene units of the polymeric organic compounds (a2) are at least partially end-capped with an imidazole group, and therefore in the preferred embodiment terminal imidazole groups are present in the polyoxyalkylene side chain, the covalent linkage of the polyoxyalkylene units with the imidazole group preferably being carried out via a nitrogen atom of the heterocycle.

In a preferred embodiment, the amine value of the organic polymeric compounds (a2) is at least 25 mg KOH/g, particularly preferably at least 40 mg KOH/g, but preferably less than 125 mg KOH/g, particularly preferably less than 80 mg KOH/g, and therefore, in a preferred embodiment, the entirety of the polymeric organic compounds in the particulate constituent (a) also have these preferred amine values. The amine value is determined in each case by weighing out approximately 1 g of the relevant reference value—organic polymeric compounds (a2) or the entirety of the polymeric organic compounds in the particulate constituent—in 100 mL of ethanol, titration being carried out using 0.1 N HCl titrant solution against the indicator bromophenol blue until the color changes to yellow at a temperature of the ethanolic solution of 20° C. The amount of HCl titrant solution used in milliliters multiplied by the factor 5.61 divided by the exact mass of the weight in grams corresponds to the amine value in milligrams KOH per gram of the relevant reference value.

The presence of maleic acid, insofar as it is a constituent of the organic polymeric compound (a2) as a free acid and not in the form of the anhydride or imide, can impart increased water solubility of the dispersing agent, in particular in the alkaline range. It is therefore preferred for the polymeric organic compounds (a2), preferably also the entirety of the polymeric organic compounds in the particulate constituent (a), to have an acid number according to DGF CV 2 (06) (as of April 2018) of at least 25 mg KOH/g, but preferably of less than 100 mg KOH/g, particularly preferably of less than 70 mg KOH/g to ensure a sufficient number of polyoxyalkylene units. It is also preferred for the polymeric organic compounds (a2), preferably also the entirety of the polymeric organic compounds in the particulate constituent (a), to have a hydroxyl number of less than 15 mg KOH/g, particularly preferably of less than 12 mg KOH/g, more particularly preferably of less than 10 mg KOH/g, determined according to method A of 01/2008:20503 from European Pharmacopoeia 9.0 in each case.

For sufficient dispersion of the inorganic particulate constituents in the colloidal aqueous dispersion, it is sufficient for the content of the polymeric organic compounds (a2), preferably the entirety of the polymeric organic compounds in the particulate constituent (a), based on the particulate constituent (a), to be at least 3 wt. %, particularly preferably at least 6 wt. %, but preferably not exceeding 15 wt. %.

In a preferred embodiment, the present invention relates to a method for anti-corrosion pretreatment involving an aqueous dispersion. In such a preferred method according to the invention, the colloidal aqueous solution in method step (i) is obtainable as an aqueous dispersion diluted by a factor of 20 to 100,000, comprising:

based on the aqueous dispersion, at least 5 wt. % of a dispersed particulate constituent (A), which in turn comprises

(A1) at least one particulate inorganic compound which is composed of phosphates of polyvalent metal cations at least partially selected from hopeite, phosphophyllite, scholzite and/or hureaulite,
(A2) at least one polymeric organic compound that is composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms, the polymeric organic compound additionally comprising units of maleic acid, their anhydride and/or their imide and the polymeric organic compound additionally comprising polyoxyalkylene units, and

optionally at least one thickener (B) preferably selected from urea urethane resins, particularly preferably from urea urethane resins that have an amine value of less than 8 mg KOH/g, preferably of less than 5 mg KOH/g, particularly preferably of less than 2 mg KOH/g.

For the dispersed particulate constituent (A) as well as the at least one particulate inorganic compound (A1) and polymeric organic compound (A2), the same definitions and preferred specifications apply as those given above for the colloidal aqueous solution.

Due to the excellent colloid stability of the particulate constituent (A) by means of the polymeric organic compound (A2) as a dispersing agent, the dilution is preferably carried out with deionized water (κ<1 μScm⁻¹), particularly preferably with service water, in order to make the method according to the invention as resource-saving as possible. In the light of the underlying technical application, service water contains at least 0.5 mmol/L of alkaline-earth metal ions.

The presence of a thickener according to component (B) gives the aqueous dispersion, in combination with its particulate constituent, thixotropic flow behavior and thereby contributes to preventing the irreversible formation of agglomerates in the particulate constituent of the dispersion, from which primary particles cannot be detached. The addition of the thickener is preferably to be controlled such that the aqueous dispersion has a maximum dynamic viscosity of at least 1000 Pa·s, but preferably of below 5000 P·s, at a temperature of 25° C. in the shear rate range of from 0001 to 0.25 reciprocal seconds, and preferably exhibits shear thinning behavior at 25° C. at shear rates above that which is present at the maximum dynamic viscosity, i.e., a decrease in viscosity as the shear rate increases, such that the aqueous dispersion as a whole has thixotropic flow behavior. The viscosity over the specified shear rate range can in this case be determined by means of a cone and plate viscometer having a cone diameter of 35 mm and a gap width of 0.047 mm.

A thickener according to component (B) is a polymeric chemical compound or a defined mixture of chemical compounds that, as a 0.5 wt. % constituent in deionized water (κ<1 μScm⁻¹) at a temperature of 25° C., has a Brookfield viscosity of at least 100 mPa·s at a shear rate of 60 rpm (=rounds per minute) using a size 2 spindle. When determining this thickener property, the mixture should be mixed with water in such a way that the corresponding amount of the polymeric chemical compound is added to the water phase at 25° C. while stirring and the homogenized mixture is then freed of air bubbles in an ultrasonic bath and left to stand for 24 hours. The measurement value of the viscosity is then read within 5 seconds immediately after application of a shear rate of 60 rpm by the number 2 spindle.

An aqueous dispersion according to the invention preferably contains a total of at least 0.5 wt. %, but preferably no more than 4 wt. %, particularly preferably no more than 3 wt. %, of one or more thickeners according to component (B), the total content of polymeric organic compounds in the non-particulate constituent of the aqueous dispersion preferably also not exceeding 4 wt. % (based on the dispersion). The non-particulate constituent is the solids content of the aqueous dispersion in the permeate of the above-described ultrafiltration after it has been dried to constant mass at 105° C.—that is, the solids content after the particulate constituent has been separated by means of ultrafiltration.

Certain classes of polymeric compounds are particularly suitable thickeners according to component (B) and are also readily commercially available. In this connection, the thickener according to component (B) is above all preferably selected from polymeric organic compounds, which in turn are preferably selected from polysaccharides, cellulose derivatives, aminoplasts, polyvinyl alcohols, polyvinylpyrrolidones, polyurethanes and/or urea urethane resins, and particularly preferably from urea urethane resin.

A urea urethane resin as a thickener according to component (B) of the preferred method according to the invention for providing a colloidal aqueous solution starting from the aqueous dispersion is a mixture of polymeric compounds which results from the reaction of a polyvalent isocyanate with a polyol and a mono-and/or diamine. In a preferred embodiment, the urea urethane resin results from a polyvalent isocyanate, preferably selected from 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2(4),4-trimethyl-1,6-hexamethylene diisocyanate, 1,10-decamethylene diisocyanate, 1,4,-cyclohexylene diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, 2,6-toluene diisocyanate, 2,4-toluene diisocyanate and mixtures thereof, p- and m-xylylene diisocyanate, and 4-4′-diisocyanatodicyclohexylmethane, particularly preferably selected from 2,4-toluene diisocyanate and/or m-xylylene diisocyanate. In a particularly preferred embodiment, the urea urethane resin results from a polyol selected from polyoxyalkylene diols, particularly preferably from polyoxyethylene glycols, which in turn are preferably composed of at least 6, particularly preferably at least 8, more particularly preferably at least 10, but preferably less than 26, particularly preferably less than 23, oxyalkylene units.

Urea urethane resins that are particularly suitable and therefore preferred according to the invention can be obtained by first reacting a diisocyanate, for example toluene-2,4-diisocyanate, with a polyol, for example a polyethylene glycol, to form NCO-terminated urethane prepolymers, followed by further reaction with a primary monoamine and/or with a primary diamine, for example m-xylylenediamine. Urea urethane resins that have neither free nor blocked isocyanate groups are particularly preferred. Such urea urethane resins, as a constituent of the aqueous dispersion from which the colloidal aqueous solution of the method according to the invention can be obtained by dilution, promote the formation of loose agglomerates of primary particles, which, however, are stabilized in the aqueous phase and protected against further agglomeration to such an extent that the sedimentation of the particulate constituent in the aqueous dispersion is largely prevented. To further promote this property profile, urea urethane resins which have neither free or blocked isocyanate groups nor terminal amine groups are preferably used as component (B). In a preferred embodiment, the thickener according to component (B), which is a urea urethane resin, therefore has an amine value of less than 8 mg KOH/g, particularly preferably of less than 5 mg KOH/g, more particularly preferably of less than 2 mg KOH/g, determined according to the method as previously described for the organic polymeric compound (A2) in each case. Since the thickener is substantially dissolved in the aqueous phase and can thus be assigned to the non-particulate constituent of the aqueous dispersion, while component (A2) is substantially bound in the particulate constituent (A), an aqueous dispersion for providing the colloidal aqueous solution of the activation in which the entirety of the polymeric organic compounds in the non-particulate constituent preferably has an amine value of less than 16 mg KOH/g, particularly preferably of less than 10 mg KOH/g, more particularly preferably of less than 4 mg KOH/g, is preferred. It is further preferred for the urea urethane resin to have a hydroxyl number in the range of from 10 to 100 mg KOH/g, particularly preferably in the range of from 20 to 60 mg KOH/g, determined according to method A of 01/2008:20503 from European Pharmacopoeia 9.0. With regard to the molecular weight, a weight-average molar mass of the urea urethane resin in the range of from 1000 to 10000 g/mol, preferably in the range of from 2000 to 6000 g/mol, is advantageous according to the invention and therefore preferred, in each case determined experimentally, as previously described in connection with the definition according to the invention of a polymeric compound.

Without the addition of auxiliaries, the pH of the dispersion for providing the colloidal aqueous solution of the activation of the method according to the invention is usually in the range of 6.0-9.0, and such a pH range is therefore preferred according to the invention. For compatibility with the actual and regularly alkaline colloidal aqueous solution in the activation stage, however, it is advantageous for the pH of the aqueous dispersion to be above 7.2, particularly preferably above 8.0, if necessary as a result of adding compounds that react in an alkaline manner. Since some polyvalent metal cations have an amphoteric character and can therefore be detached from the particulate constituent at higher pH values, the alkalinity of the aqueous dispersion according to the invention is ideally limited, such that the pH of the aqueous dispersion is preferably below 10 and particularly preferably below 9.0.

The above-described aqueous dispersion for providing the colloidal aqueous solution is for its part preferably obtainable by

i) providing a pigment paste by triturating 10 parts by mass of an inorganic particulate compound (A1) with 0.5 to 2 parts by mass of the polymeric organic compound (A2) in the presence of 4 to 7 parts by mass of water and grinding until a D50 value of less than 1 pm has been reached, as determined by means of dynamic light scattering after dilution with water by a factor of 1000, for example by means of Zetasizer® Nano ZS from Malvern Panalytical GmbH;
ii) diluting the pigment paste with such an amount of water, preferably deionized water (κ<1 μScm⁻¹) or service water and a thickener (B) that a dispersed particulate constituent (A) of at least 5 wt. % and a maximum dynamic viscosity of at least 1000 Pa·s at a temperature of 25° C. in the shear rate range of from 0.001 to 0.25 reciprocal seconds is set; and
iii) setting a pH in the range of from 7.2 to 10.0 using a compound that reacts in an alkaline manner, preferred embodiments of the dispersion being obtained similarly by selecting corresponding components (A1), (A2) and (A) in the provided or required amount as necessary in each case, as described in connection with the colloidal aqueous solution.

The aqueous dispersion can also contain auxiliaries, for example selected from preservatives, wetting agents and defoamers, which are contained in the amount necessary for the relevant function. The content of auxiliaries, particularly preferably of other compounds in the non-particulate constituent that are not thickeners and not compounds that react in an alkaline manner, is preferably less than 1 wt. %. In the context of the present invention, a compound that reacts in an alkaline manner is water-soluble (water solubility: at least 10 g per kg of water with κ<1 μScm⁻¹) and has a pK_B value of above 8.0 for the first protonation step.

The resource-saving method control according to the present invention is particularly effective in the case of the zinc phosphation of components in series, i.e., during operation of a pretreatment line for zinc phosphation. In a preferred embodiment of the method according to the invention, a large number of specific components, which at least partially consist of a metal material having at least one surface of zinc, are therefore treated in series. A pretreatment in series is when the components of the series are each first activated and then zinc-phosphated according to the invention and for this purpose are brought into contact with baths for activation and zinc phosphation that are provided in system tanks, the individual components being brought into contact one after the other and thus at different times. In this case, the system tank is the container in which the colloidal aqueous solution is located for the purpose of activation or in which the acidic, aqueous composition is located for the purpose of phosphation.

There may be a rinsing step between the activation (i) and the zinc phosphation (ii) in order to reduce the carryover of alkaline constituents into the acidic aqueous composition for zinc phosphation, but a rinsing step is preferably dispensed with in order to fully maintain the activation of the metal surfaces. A rinsing step is used exclusively for the complete or partial removal of soluble residues, particles and active components that are carried over by adhering to the component from a previous wet-chemical treatment step, from the component to be treated, without metal-element-based or semi-metal-element-based active components, which are already consumed merely by bringing the metal surfaces of the component into contact with the rinsing liquid, being contained in the rinsing liquid itself. For example, the rinsing liquid can simply be city water or deionized water or, if necessary, can also be a rinsing liquid that contains surface-active compounds to improve the wettability by means of the rinsing liquid.

For layer-forming zinc phosphation and the formation of semi-crystalline coatings, which are the aim of the activation of the metal materials, it is preferred for the phosphation in method step (ii) by bringing the surfaces into contact with an acidic aqueous composition containing 5-50 g/L of phosphate ions, 0.3-3 g/L of zinc ions and an amount of free fluoride. According to the invention, the amount of phosphate ions includes orthophosphoric acid and the anions, dissolved in water, of the salts of orthophosphoric acid, calculated as PO₄.

An amount of free fluoride or source of free fluoride ions is essential for the process of layer-forming zinc phosphation, insofar as components comprising not only surfaces of zinc but also surfaces of iron or aluminum are to be zinc-phosphated in a layer-forming manner, as required, for example, for zinc phosphation of automobile bodies, which are at least also partially made of aluminum. In this context, it is advantageous if the amount of free fluoride in the acidic aqueous composition is at least 0.5 mmol/kg, particularly preferably at least 2 mmol/kg. The concentration of free fluoride should not exceed values above which the phosphate coatings predominantly have adhesions that can be easily wiped off, since these adhesions cannot be avoided even by a disproportionately increased amount of particulate constituents in the colloidal aqueous solution of the activation. Therefore, it is also economically advantageous, and therefore preferred, for the concentration of free fluoride in the acidic aqueous composition of the zinc phosphating to be below 15 mmol/kg, particularly preferably below 10 mmol/kg and more particularly preferably below 8 mmol/kg, in the method according to the invention based on activation (i) followed by zinc phosphating (ii).

The amount of free fluoride can be determined potentiometrically by means of a fluoride-sensitive measuring electrode at 20° C. in the relevant acidic aqueous composition after calibration with fluoride-containing buffer solutions without pH buffering. Suitable sources of free fluoride ions are hydrofluoric acid and the water-soluble salts thereof, such as ammonium bifluoride and sodium fluoride, as well as complex fluorides of the elements Zr, Ti and/or Si, in particular complex fluorides of the element Si. In a phosphation process according to the present invention, the source of free fluoride is therefore preferably selected from hydrofluoric acid and the water-soluble salts thereof and/or complex fluorides of the elements Zr, Ti and/or Si. Salts of hydrofluoric acid are water-soluble within the meaning of the present invention if their solubility in deionized water (κ<1 μScm⁻¹) at 60° C. is at least 1 g/L, calculated as F.

In order to suppress what is known as “pin-holing” on the surfaces of the metal materials which are made of zinc, it is preferred, in such methods according to the invention in which zinc phosphating is carried out in step (ii), for the source of free fluoride to be at least partially selected from complex fluorides of the element Si, in particular from hexafluorosilicic acid and the salts thereof. The term pin-holing is understood by a person skilled in the art of phosphation to mean the phenomenon of local deposition of amorphous, white zinc phosphate in an otherwise crystalline phosphate layer on the treated zinc surfaces or on the treated galvanized or alloy-galvanized steel surfaces.

In method step (ii) of the method according to the invention, the preferred pH of the acidic aqueous composition is above 2.5, particularly preferably above 2.7, but preferably below 3.5, particularly preferably below 3.3. The content of the free acid in points in the acidic aqueous composition of the zinc phosphating in method step (ii) is preferably at least 0.4, but preferably no more than 3.0, particularly preferably no more than 2.0. The proportion of free acid in points is determined by diluting 10 mL sample volume of the acidic aqueous composition to 50 mL and titrating with 0.1 N sodium hydroxide solution to a pH of 3.6. The consumption of mL of sodium hydroxide solution indicates the point number of the free acid.

The conventional addition of additives for zinc phosphating can also be carried out similarly in the context of the present invention, such that the acidic aqueous composition in method step (ii) can contain the conventional accelerators such as hydrogen peroxide, nitrite, hydroxylamine, nitroguanidine and/or N-methylmorpholine-N-oxide and additionally cations of the metals manganese, calcium and/or iron in the form of water-soluble salts, which have a positive influence on layer formation. An embodiment in which a total of less than 10 ppm of nickel and/or cobalt ions are contained in the acidic aqueous composition for zinc phosphation in method step (ii) is particularly preferred from an ecological point of view.

In the method according to the invention, a good coating primer for a subsequent dip coating, in the course of which a substantially organic cover layer is applied, is produced. Accordingly, in a preferred embodiment of the method according to the invention, the zinc phosphation, with or without an intermediate rinsing and/or drying step, but preferably with a rinsing step and without a drying step, is followed by dip coating, particularly preferably electrocoating, more particularly preferably cathodic electrocoating, which preferably contains water-soluble or water-dispersible salts of yttrium and/or bismuth in addition to the dispersed resin, which preferably comprises an amine-modified polyepoxide.

Claims

1. A method for anti-corrosion pretreatment of a metal material that at least partially has surfaces of zinc or of a component that is composed at least partially of such a metal material, in which method the metal material or the component undergoes firstly (i) an activation step and then (ii) a zinc phosphation step, in consecutive method steps,

wherein the activation step (i) is carried out by contacting the metal material or the component with a colloidal aqueous solution containing (a) a dispersed particulate constituent of the solution comprising:

(a1) at least one particulate inorganic compound that is composed of phosphates of polyvalent metal cations at least partially selected from hopeite, phosphophyllite, scholzite and/or hureaulite, and

(a2) at least one polymeric organic compound that is composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms, the at least one polymeric organic compound additionally comprising units of maleic acid, maleic anhydride and/or the imide thereof and the polymeric organic compound additionally comprising polyoxyalkylene units,

the content of the dispersed particulate constituents of the colloidal aqueous solution being at least 4 g/kg based on the colloidal aqueous solution; and

a zinc phosphate layer having a layer weight of less than 2.0 g/m2 being deposited on the surfaces of zinc in method step (ii).

2. The method according to claim 1, wherein the colloidal aqueous solution contains condensed phosphates dissolved in water, calculated as the element P, in an amount less than 100 mg/kg.

3. The method according to claim 1 wherein the colloidal aqueous solution in the activation step (i) has an alkaline pH.

4. The method according to claim 1, wherein phosphates, calculated as PO4, contained in the at least one particulate inorganic compound (a1), based on the dispersed inorganic particulate constituent of the colloidal aqueous solution, are present in an amount of at least 25 wt. %.

5. The method according to claim 1, wherein the polymeric organic compounds (a2) of the colloidal aqueous solution contain the polyoxyalkylene units in their side chains, wherein content of polyoxyalkylene units in in total of the polymeric organic compounds (a2) is at least 40 wt. %.

6. The method according to claim 1, wherein the organic polymeric compounds (a2) of the colloidal aqueous solution also have imidazole units, optionally at least some of the polyoxyalkylene units of the polymeric organic compounds (a2) are end-capped with an imidazole group.

7. The method according to claim 1, wherein the colloidal aqueous solution contains at least one thickener as a further component b), which is selected from urea urethane resins, having an amine value of less than 8 mg KOH/g.

8. The method according to claim 1, wherein a total of polymeric organic compounds in and based on the particulate constituent of the colloidal aqueous solution is at least 3 wt. % and does not exceed 15 wt. %.

9. The method according to claim 1, wherein the colloidal aqueous solution has a D50 value<1 μm.

10. The method according to claim 1, wherein the content of the particulate constituents of the colloidal aqueous solution is at least 6 g/kg but no more than 20 g/kg, in each case based on the colloidal aqueous solution.

11. The method according to claim 1, wherein the colloidal aqueous solution is obtainable as an aqueous dispersion diluted by a factor of 20 to 100,000, comprising

based on the aqueous dispersion, at least 5 wt. % of a dispersed particulate constituent (A), which in turn contains

(A1) at least one particulate inorganic compound which is composed of phosphates of polyvalent metal cations at least partially selected from hopeite, phosphophyllite, scholzite and/or hureaulite,

(A2) at least one polymeric organic compound that is composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms, wherein the polymeric organic compound additionally has units of maleic acid, its anhydride and/or its imide and the polymeric organic compound additionally comprises polyoxyalkylene units, and optionally at least one thickener selected from urea urethane resins having an amine value of less than 5 mg KOH/g.

12. The method according to claim 1 wherein a zinc phosphate layer having a layer weight of less than 1.8 g/m2 is deposited on the surfaces of zinc in method step (ii).

13. The method according to claim 1, wherein the zinc phosphation in method step (ii) is carried out by contact with an acidic aqueous composition containing free fluoride, 5-50 g/kg of phosphates dissolved in water, calculated as PO4, 0.3-3 g/kg of zinc ions and contains a total of less than 0.1 g/kg of ions of the elements nickel and cobalt.

14. The method according to claim 1, wherein components that are treated, comprise surfaces of iron and/or aluminum, in addition to the surfaces of zinc; and a zinc phosphate layer having a layer weight of less than 2.0 g/m2 is deposited on all surfaces of zinc, iron and aluminum in method step (ii).