EP2907894B2 - Method for production of a substrate with a chromium VI free and cobalt-free passivation - Google Patents

Description

The invention relates to a method for producing a substrate provided with a chromium(VI)-free and cobalt-free passivation.

The passivation of metallic substrates is a proven method, but components of passivation solutions have proven to be problematic from a health and environmental perspective. Chromium(VI) compounds can often no longer be used, and elements such as cobalt and nickel are also often no longer desirable.

A chromium(VI)-free and cobalt-free passivation composition is used in the US$4,578,122 Nitrate ions and chromium(III) compounds are used in an aqueous acidic solution, with additional activating metal ions, e.g., iron, aluminum, lanthanum, or cerium ions, being added. The ratio of nitrate ions to chromium(III) ions and activating metal ions should not be less than 4:1.

The addition of the US$4,578,122 The use of the activating metal ions mentioned above is either no longer desirable for reasons of health or environmental protection or the use of the metal ions is expensive.

The DE-OS 3 213 384 discloses a first acidic and a second alkaline passivation that is chromium(VI)-free and cobalt-free. However, this two-stage passivation is not yet optimized with regard to corrosion protection.

The WO 95/24517 A1 relates to a coated metal sheet pretreated with an insoluble composite layer containing a siloxane. The composite layer is obtained by rinsing the metal sheet with a basic solution containing at least 0.005 mol/L of a dissolved silicate or a dissolved aluminate, at least 0.1 vol.% of an organofunctional silane, and at least 0.02 vol.% of a crosslinking agent having 2 or 3 trialkoxysilyl groups.

The US 2006/0054248 A1 relates to a method for coloring and protecting metal parts from corrosion by applying a solution containing a trivalent chromium compound and at least one other metal salt selected from the group of iron salts, nickel salts, and cobalt salts, which, together with the chromium compound, are capable of coloring the surface of the metal part, and a phosphate, to a surface of the metal part to obtain a colored surface. A top coat is then applied to the colored surface to achieve high corrosion resistance. The cover layer can, for example, be based on an organosilane silicate coating.

Furthermore, the GB 0297024 A a method for treating metal surfaces, in particular zinc surfaces and zinc alloy surfaces, by treatment with an aqueous acidic solution containing effective amounts of A) hydrogen ions to achieve a pH of about 1.5 to about 2.2, B) an oxidizing agent, C) at least one element selected from iron, cobalt, nickel, molybdenum, manganese, aluminum, lanthanum, lanthanide mixture or cerium ions or mixtures thereof, or instead of C) iron and cobalt ions. Other solutions for treating metal surfaces further contain D) chromium ions, which are essentially in the oxidation state +3, and iron ions in combination with another metal selected from C) or cerium ions, or A), B), C) and D) and F), a soluble silicate compound compatible with the treatment solution or A), B), C) and D) and G), a mixture of 1-hydroxyethylidene-1,1-diphosphonic acid and citric acid or mixtures of A), B), C) and D) with two or more compounds selected from E), F) and G). The solution can further optionally contain halides or a wetting agent.

The US 6,478,886 B1 relates to an agent for sealing metallic substrates, in particular zinc or zinc alloy substrates, which consists of an aqueous dispersion of A) at least one silane derivative and B) at least one colloidal silicon dioxide and/or a colloidal silicate. The agent is preferably used for the direct coating of metallic substrates without prior chromating.

The EP 0 337 411 B1 discloses a process for producing an acidic chromium(III)-containing and fluoride-containing passivation bath for surfaces made of zinc, zinc alloys and cadmium.

The DE 10 2009 042 861 A1 relates to a composition for passivating zinc and its alloys, consisting of a passivation solution and an activating solution, wherein the passivation solution consists of 20 to 200 g/l of chromium(III) compounds, e.g. as chromium chloride or chromium nitrate, 20 to 600 g/l of soluble nitrate selected from sodium, potassium or ammonium nitrate, 5 to 100 g/l of fluoride selected from sodium, potassium or ammonium chloride, 5 to 20 g/l of organic acids selected from oxalic acid, malonic acid, tartaric acid, formic acid, acetic acid and citric acid, hydrochloric or nitric acid, and wherein the activating solution consists of 1 to 200 g/l of a tin(II) or tin(IV) salt and 10 to 700 g/l of a phosphonic acid or its derivatives.

Finally, the scientific publication concerns M. Strle, B. Kurbus, S. Pejovnik and I. Kadivec, "Silane treatment of silicate fillers - II", Advanced Powder Technol., Vol. 5, No. 3, 1994, pages 269 to 279 a process for the surface modification of silicate-containing fillers by silanization and the use of the modified fillers in organic polymers.

The task is therefore to provide a process for the passivation of metallic substrates that produces good corrosion protection and avoids unnecessary risks to health and the environment.

The object underlying the invention is achieved by a method according to claim 1.

The present invention therefore relates to a method for producing a metallic substrate provided with a chromium(VI)-free and cobalt-free passivation by applying
(a) a first acidic passivation, wherein to produce the first acidic passivation an aqueous acidic composition is applied to the substrate which comprises a chromium(III) compound selected from the group consisting of chromium(III) sulfate, chromium(III) hydroxide, chromium(III) dihydrogen phosphate, chromium(III) chloride, chromium(III) nitrate, sodium chromium(III) sulfate, potassium chromium(III) sulfate and chromium(III) salts of organic acids, wherein the composition comprises the chromium(III) compound in amounts of at least 0.05 g/L, based on the aqueous acidic composition, and a phosphonic acid in amounts of 0.5 to 3 wt.%, based on the aqueous acidic composition, and (b) a second alkaline passivation on the metallic substrate, wherein to produce the second alkaline passivation an aqueous alkaline composition is used which comprises silane-modified and/or siloxane-modified silicates.

The method according to the invention provides for the production of a metallic substrate provided with a chromium(VI)-free and cobalt-free passivation by applying a first acidic and a second alkaline passivation. An aqueous alkaline composition used to produce the second alkaline passivation contains silane-modified silicates. The combination of the two passivations, acidic and alkaline, provides good corrosion protection.

The first acidic and second alkaline passivation are applied as aqueous compositions. In the context of this invention, the term "passivation" refers to both the aqueous composition for passivating the substrate or for applying the aqueous compositions, as well as the coating applied to the surface of the metallic workpiece. Treating the surface of the metallic substrate with the acidic and alkaline aqueous compositions results in the deposition of chemical components contained therein, which form a coating on the surface of the substrate, i.e., the passivation. The coating(s) provide improved protection against corrosion.

According to the invention, the first, acidic passivation is chromium(VI)-free and cobalt-free. Preferably, it is also nickel-free.

According to the invention, the substrate or workpiece coated with a first acidic passivation is coated with a second passivation, wherein the second passivation is an alkaline passivation. Corrosion protection is significantly increased by this second alkaline passivation. Workpieces in which the acidic passivation does not contain vanadium or tungsten become significantly more resistant to corrosion by a second alkaline passivation applied to the first acidic passivation. However, corrosion protection is particularly improved when the first acidic passivation is produced using an aqueous acidic composition containing vanadium and/or tungsten or their compounds.

An essential feature of the invention is the application of a silicate-containing aqueous composition as a second, alkaline passivation to coat the first acidic passivation. In this way, a silicate compound is applied to the first acidic passivation. Typical silicate compounds are water glasses, but aqueous polysilicates or colloidal silicates are also well suited for the second, alkaline passivation. It is preferred that the second alkaline passivation comprises sodium, potassium, lithium, and/or ammonium silicate. A second alkaline passivation comprising a mixture of silicate compounds can also be applied to the workpiece. Both colloidal silicates and dissolved silicates can be used. Silane-modified or siloxane-modified silicates, in which silanes or siloxanes are bonded to the silicates, preferably polysilicates, have also proven to be well suited for the implementation of the invention. Most silicates form alkaline Solutions or suspensions. However, if necessary, alkalinity can be increased by adding alkalis, such as caustic soda.

The use of lithium polysilicate in the aqueous composition for passivating metallic substrates has proven particularly advantageous for producing the second alkaline passivation. Applying an aqueous composition of lithium polysilicate or mixing lithium polysilicate with other water glasses (sodium and/or potassium silicate) or colloidal silica sols to the first acidic passivation results in significantly improved corrosion protection. At the same time, the use of lithium polysilicate to produce the second alkaline passivation prevents the formation of gray haze on the surface of the metallic substrate passivated according to the invention, which is common for passivations made from aqueous compositions containing sodium or potassium water glasses.

According to the invention, the aqueous alkaline composition used for the second alkaline passivation comprises a silane or siloxane. The addition of the silane or siloxane serves to further enhance corrosion protection. Preferably, a vinyl and/or aminosilane is used to produce the second alkaline passivation; however, epoxysilanes and the siloxanes of the silanes mentioned above and below are also suitable. In particular, alkylalkoxysilanes, here: mono-, di-, or trialkylalkoxysilanes, are suitable, individually or in combination with silicates, for building up a corrosion-protective coating on the metallic workpiece already treated with an acidic aqueous composition. Various silane compounds can be used in mixtures with one another. Particularly suitable silane compounds are methacryloxymethyltriethoxysilane, 3-aminopropylmethyldiethoxysilane, 3-aminopropyltriethoxysilane, N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane, 3-glycidyloxypropyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, methyltrimethoxysilane and 3-mercaptopropyltrimethoxysilane.

Silane or siloxane can be used in amounts ranging from 1% to 99% by weight, based on the total amount of the aqueous composition of the second alkaline passivation. However, even aqueous compositions containing only small amounts of silane, e.g., up to 20% by weight, demonstrate significantly improved corrosion protection.

According to the invention, an aqueous alkaline composition is used to produce the second alkaline passivation, which composition comprises both silicates and silanes and/or siloxanes or mixtures of silicates and silanes or siloxanes or compounds of a silicate and a silane component, hereinafter: silane-modified silicates, or compounds of a silicate and a siloxane component, hereinafter: siloxane-modified silicates. The silanes or siloxanes are typically bound to the silicate(s) as covalent side chains, for example by hydrolysis. These silane-modified or siloxane-modified silicates form excellent corrosion protection on a first acidic passivation, far exceeding the effect of a simple acidic or alkaline passivation. The silane-modified or siloxane-modified silicates can be detected on the metallic substrate by means of NMR spectroscopy. In particular, silicon-carbon bonds (SiC bonds) can be detected. At the same time, the second alkaline passivation forms an excellent basis for further coatings, e.g., color-imparting coatings or coatings containing lubricants or other additives that further improve the usability of the coated surface. Where silane-modified silicates are mentioned or described in connection with this invention, the use of siloxane-modified silicates is always also meant and encompassed.

According to a further preferred embodiment of the invention, the second alkaline passivation can be produced from partially or—preferably—fully hydrolyzed silicate and silane or siloxane compounds. The joint hydrolysis of silicates and silanes or siloxanes in aqueous solution, on the one hand, results in the formation of the silane-modified or siloxane-modified silicates. On the other hand, the factory-based hydrolysis can remove released alcohols, so that users can be provided with aqueous alkaline compositions that are low in volatile organic compounds (low VOC) or that are free of volatile organic compounds (VOC-free). The alcohols released by hydrolysis can be removed, for example, by ultramembrane filtration or reverse osmosis, but also by distillation, e.g., vacuum distillation. Typical aqueous alkaline compositions according to the invention for passivating metallic substrates contain a maximum of 1% by weight of alcohol, preferably a maximum of 0.3% by weight of alcohol.

If desired, it is readily possible to add additives to the aqueous composition for the second alkaline passivation. These additives are usually added to the liquid composition from which the second alkaline passivation is produced. They exert their effect either during application, as in the case of defoamers or stabilizers, or after application and, if necessary, drying of the second alkaline passivation, as in the case of lubricants or dyes.

According to an advantageous embodiment of the invention, for producing the first acidic passivation of the metallic substrate, it is proposed to use an aqueous, acidic cobalt- and chromium(VI)-free composition which comprises a chromium(III) compound, an inorganic acid and optionally a fluorine source, and which is characterized in that the aqueous composition comprises a compound of the metals vanadium or tungsten, wherein this metal compound can be used individually or in a mixture with other vanadium or tungsten compounds.

Molybdenum, vanadium, and tungsten compounds, in combination with the aforementioned, known components of the first acid passivation, already provide excellent corrosion protection. The composition according to the invention exhibits a significantly reduced risk of health and environmental damage both during handling of the aqueous composition, e.g., during coating, and as a fully applied passivation, and can therefore be used with high operational reliability. Molybdenum, vanadium, and tungsten compounds are incorporated into the first acid passivation and provide improved corrosion protection.

The aqueous acidic and alkaline compositions used according to the invention for passivating metallic substrates are generally suitable for all metallic surfaces or substrates, but are particularly suitable for workpieces with a surface made of steel, iron, aluminum, or zinc, and in particular for workpieces whose surface is provided with an alloy of one or both of the metals aluminum and zinc with other metals. Typical examples are a zinc-aluminum alloy, an aluminum or zinc alloy with other metals such as iron or magnesium, e.g., with a zinc-iron alloy, all of which can be provided with a corrosion protection coating. The layer thickness of the applied coating of metal or alloy is between 5 µm and 100 µm. The metallic alloy applied to a substrate appears as a discrete layer. A typical application is coil coating, i.e., the passivation of strip steel.

Advantageously, metal-oxygen compounds of the metals molybdenum, vanadium, and/or tungsten are used in the aqueous acidic composition for passivation. Preferably, one or more of the following compounds are used in the aqueous acidic composition: potassium orthovanadate, potassium metavanadate, sodium orthovanadate, sodium metavanadate, sodium tungstate, sodium paratungstate, and vanadium pentoxide, as well as sodium molybdate and potassium molybdate. Advantageously, compounds of the metals molybdenum, vanadium, and/or tungsten are used, which dissociate in the aqueous acidic composition for passivation, thereby releasing molybdate, vanadate, and/or tungstate anions. Molybdate, vanadate, and tungstate anions are incorporated into the passivation layer and, even during the acidic passivation alone, ensure the development of very good corrosion protection.

According to the invention, the acidic aqueous composition contains a phosphonic acid or a mixture of phosphonic acids. Particular preference is given to using organic phosphonic acids, for example (1-hydroxyethane-1,1-diyl)bisphosphonic acid, 2-phosphonobutanol-1,2,4-tricarboxylic acid, aminotrimethylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid or diethylenetriaminepentamethylenephosphonic acid or mixtures thereof. The use of phosphonic acid salts can also prove advantageous in connection with the invention. The following phosphonates are particularly suitable, used individually or as a mixture: tetrasodium (1-hydroxyethane-1,1-diyl)bisphosphonate, trisodium (1-hydroxyethane-1,1-diyl)bisphosphonate, pentasodium ethylenediaminetetramethylenephosphonate or heptosodium diethylenetriaminepentamethylenephosphonate. These salts dissociate in the aqueous, acidic passivation composition, making the phosphonates available as complexing agents. Phosphonic acids can also be advantageously used in combination with vanadium and tungsten compounds in acidic aqueous compositions. The use of phosphonic acid as a complexing agent has proven effective in this case.

In the context of the process according to the invention, it is preferred if the acidic aqueous compositions for passivating metallic substrates contain one or more elements or compounds from the group comprising molybdenum, manganese, cerium, and lanthanum. By adding these elements or their compounds, preferably their salts and oxides, a further improvement in the anticorrosive properties of the passivation according to the invention is achieved.

In the context of the present invention, the aqueous, acidic passivation composition comprises a chromium(III) compound or a mixture of chromium(III) compounds selected from the group consisting of chromium(III) sulfate, chromium(III) hydroxide, chromium(III) dihydrogen phosphate, chromium(III) chloride, chromium(III) nitrate, sodium chromium(III) sulfate, potassium chromium(III) sulfate, and chromium(III) salts of organic acids. It has been proven that an aqueous, acidic passivation composition has good corrosion protection properties even without the use of a chromium(VI) compound. The chromium(III) compound is used in an amount of at least 0.05 g/l up to a maximum of saturation.

It has also proven advantageous to add a nitrate compound or a mixture of nitrate compounds to the acidic aqueous passivation composition. In addition to nitrogen-containing acids such as nitric acid, salts of these acids are also preferably used. Typical salts that are particularly suitable for use in the passivation composition are salts of alkali metals, ammonium salts, or salts of the metal ions contained in the passivation composition, e.g., chromium(III) nitrate. The nitrogen and chromium(III) compounds described above are present in the aqueous, acidic passivation composition essentially in dissociated form. The proportion of nitrate compounds is preferably 5% to 20% by weight, based on the total composition used for passivation.

Also advantageous is the use of an aqueous, acidic cobalt- and chromium(VI)-free composition for passivating metallic substrates, which composition comprises a chromium(III) compound, an acid, metal ions, nitrate ions, and optionally a fluorine source, as well as a phosphonic acid, and which is characterized in that nitrate ions are used to the sum of chromium and metal ions in a ratio of maximum 3:1, preferably maximum 1:3. The reduced use of nitrate proves to be advantageous when using this aqueous acidic composition for passivation because fewer nitrous gases are released.

However, for the safe application of the coating and the development of good corrosion protection, it is recommended that the acidic aqueous composition for passivation be adjusted to a pH value of < 4, preferably < 3. To ensure this, an acid or a mixture of acids is added. The use of organic and/or inorganic Acids are shown, typically one or more of the acids from the group consisting of phosphoric acid, hydrochloric acid, nitric acid, and/or sulfuric acid as inorganic acids, and formic acid, succinic acid, acetic acid, oxalic acid, peracetic acid, salicylic acid, and citric acid as organic acids. The organic acids alone do not always ensure the desired pH is achieved, but their addition proves useful because the organic acids also act as complexing agents in the acidic aqueous composition.

To ensure good adhesion of the passivation, the aqueous, acidic composition preferably contains a fluorine source. Such a fluorine source is preferably a compound or a mixture of compounds selected from the group comprising hydrofluoric acid, hexafluorotitanic acid, hexafluorozirconic acid, sodium fluoride (NaF), potassium fluoride (KF), ammonium fluoride (NH4F), sodium bifluoride (NaHF2), potassium bifluoride (KHF2), and ammonium bifluoride (NH4HF2). The fluorine compounds used as fluorine sources are used in an amount of 0.1% to 5% by weight based on the aqueous composition. The fluorine compounds are preferably used as technically pure, soluble powders.

It is expressly noted here that the above-described use of elements or compounds containing vanadium, tungsten, molybdenum, manganese, cerium, or lanthanum, as well as phosphonic acid, can be used individually or in any combination. Aqueous acidic compositions containing one or more of these elements or compounds provide good corrosion protection even as acidic passivation alone.

The preferred aqueous, acidic composition for passivating metallic substrates is essentially composed of substances that are largely harmless to health and have little or no environmental impact. It is free of cobalt, nickel, and chromium(VI) compounds. It is also preferably free of peroxide compounds and can be produced without the use of carboxylic acids. Furthermore, in preferred embodiments, the use of nitrate compounds is minimized, thus significantly reducing the emission of nitrous gases.

The application of the aqueous acidic passivation composition takes place at room temperature, at maximum temperatures up to 80 °C. In most cases, the metallic substrate is immersed in a bath of the aqueous acidic and then the aqueous alkaline passivation composition, but the passivation compositions can also be applied by other common and The passivating compositions can be applied to the metallic substrate using known application methods (spraying, dipping, dip-spin coating, doctor blade coating, rolling). The aqueous passivating compositions are usually applied for a treatment time of between 1 second and 180 seconds, preferably approximately 30 seconds to 120 seconds. The application of the passivating composition can be followed by drying, which can be carried out at temperatures between room temperature and approximately 250°C. Drying is aimed solely at removing excess liquid; complete reaction, e.g., hydrolysis or condensation of the components that form the passivating coating on the metallic substrate, is not necessary.

If necessary, the metallic substrate can be cleaned, especially degreased, before applying the passivation composition. Aqueous solutions for cleaning and degreasing are known in the art.

The first acidic passivation layer is applied in a layer thickness of 10 nm to 1 µm, preferably in a layer thickness of 20 nm to 500 nm. The second alkaline passivation layer is applied in a layer thickness of 10 nm to 1 µm, preferably in a layer thickness of 10 nm to 500 nm. These thin layers result from the adhesion of the aqueous solutions to the substrate or to previous passivation layers; subsequent curing is not required. The layer thickness does not change after application and drying.

The aqueous acidic and alkaline compositions required to carry out the process according to the invention are preferably supplied as a concentrate that is diluted with water for use in a concentrate:water ratio of 1:5 to 1:20, frequently 1:10. The respective aqueous acidic or alkaline compositions are each offered as one-component products.

According to the invention, the excellent corrosion protection is achieved by first applying an acidic passivation followed by an alkaline passivation to the metallic substrate. Accordingly, an analysis of the finished coated substrate shows that, starting from the substrate, a first passivation layer is detected, which contains chromium and nitrogen and possibly fluorine, vanadium and/or tungsten, or alternatively other metallic or rare earth elements. However, this first passivation layer usually does not contain silicon or the elements sodium, potassium, or lithium. A second passivation layer is applied onto this first passivation layer. The second passivation layer is therefore not applied directly to the metallic substrate. Typically, silicon, as well as sodium, potassium, and/or lithium, can be detected in the second passivation layer. However, this second passivation layer generally does not contain chromium, fluorine, tungsten, vanadium, or other metallic or rare earth elements. Non-metallic elements such as carbon, phosphorus, or nitrogen may be detected in both passivation layers.

Details of the invention are explained using the following embodiments:
For all of the following examples, the quantities given are in weight percent based on the total composition of the respective aqueous composition used to produce the passivation. Unless otherwise stated, pure substances (100%) were used. The aqueous acidic or alkaline composition for passivation is prepared by mixing or dissolving the individual components.

In the acidic aqueous composition, the water is introduced into the liquid composition for passivation, for example, through the aqueous chromium(III) salt solution, in this case a sulfate or nitrate. Smaller amounts are added at the end. These compositions have a pH value of 1.5 to 1.8. They can easily be stored for more than six months.

The aqueous alkaline composition is typically produced by adjusting the solids content or proportion of aqueous silicates by adding appropriate amounts of water and, where appropriate, by mixing in silanes. If silicates and fully or partially hydrolyzed silanes or siloxanes are used, the hydrolysis is carried out in the factory so that the ready-to-use products have a lower alcohol content than the non-hydrolyzed products or release less alcohol during processing.

By applying the aqueous acidic passivation composition by rolling at room temperature to steel sheets with a surface consisting, for example, of a zinc-iron alloy, a passivation layer is created on the metallic substrate. The application is carried out by a roller arrangement through which the steel sheet passes. Rinsing is then carried out to remove excess acidic composition. Subsequent drying is carried out in a drying oven at 150 °C, which the steel sheet provided with the initial passivation passes through. within a maximum of 10 minutes. The second alkaline coating is created in the same way.

Examples of implementation in Tables 1 and 2

Tables 1 and 2 predominantly show compositions of an aqueous acidic composition for a first acid passivation containing vanadium and tungsten compounds.

Chromium(III) sulfate and chromium(III) nitrate are, individually or together, the main components of the passivation composition, as in Experiment 11. When used as a 20% solution, the proportion of the chromium(III) compound in the passivation composition is between 64.0% and 77.2% by weight.

Although a nitrate compound can also be added in the form of chromium(III) nitrate, a nitrate salt, in this case sodium nitrate, is preferably added, as shown in Tables 1 and 2. The proportion of the nitrate compound is preferably between 13% and 16% by weight, but can also be between 5% and 10% by weight.

A fluorine salt is preferably used as an optional fluorine source. The examples shown in Tables 1 and 2 are sodium hydrogen difluoride; however, other fluorine compounds listed above are also suitable.

The descriptions of the composition according to the invention in Tables 1 and 2 show that organic acids can be used individually or in combination. These acids act as complexing agents but also support a low pH. However, the addition of an inorganic acid, preferably nitric acid, is essential for adjusting the pH. Table 1 Compositions for acid passivation (chromium(III) sulfate) Experiment No.: 1 2 3 4 5 no ¹ no ¹ no ¹ no ¹ required ² Chromium(III) compound Chromium(III) sulfate (solution, 20%) 77.2% 72.7% 72.2% 64.0% 70.0% Nitrate compound Sodium nitrate 9.7% 15.8% 15.8% 13.5% 13.5% Fluorine compound Sodium hydrogen difluoride 3.5% 2.8% 2.8% 2.8% 1.2% Organic acid citric acid 2.5% 2.5% 2.5% Oxalic acid 2.5% 2.5% Inorganic acid Nitric acid HNO ₃ 3.1% 2.2% 2.2% 2.2% 2.2% Vanadium or tungsten compound Sodium vanadate 1.5% 1.5% Potassium vanadate Vanadyl sulfate 15.0% Sodium tungstate Sodium olybdate 0.5%. Potassium molybdate Phosphonic acid 1-Hydroxyethane-1,1-diphosphonic acid 1.5% Water 4.0% ^. 2.5% 2.5% 9.1% sum 100.0% 100.0% 100.0% 100.0% 100.0% ¹ ne = not in accordance with the invention
² req. = according to the invention

However, the use of nitric acid is only preferred because it is considered an additional source of nitrate ions. The pH value, preferably below 4, can also be easily adjusted using sulfuric acid, hydrochloric acid, or phosphoric acid, for example, as well as mixtures of inorganic and/or organic acids; see Experiments 11 and 12 in Table 2. Small amounts of up to 5% by weight of inorganic acid are generally sufficient to achieve a pH value below 4.

Vanadates and tungstates are added in amounts between 0.1% and 5% by weight, preferably in amounts between 0.5% and 3% by weight. The results in Tables 1 and 2 show that even small amounts of vanadates or tungstates, or mixtures of vanadates and tungstates, significantly increase the corrosion protection effect of a passivation composition.

The anticorrosive effect of this initial, acidic passivation solution is further enhanced when molybdates or manganates, or mixtures of molybdates and manganates, are used. Amounts of 0.05% to 3% by weight per molybdenum compound are sufficient to achieve a significant synergistic effect in corrosion protection. Preferably, up to 1.5% by weight of molybdates and up to 0.5% by weight of manganates are used.

Furthermore, the addition of phosphonic acids has proven beneficial. They act as complexing agents. The addition of individual phosphonic acids is already effective. However, the addition of mixtures of different phosphonic acids also shows good results. Phosphonic acids are added in amounts of 0.01% to 5% by weight, preferably in amounts of 0.5% to 3% by weight. It is expressly noted here again that the use of elements or compounds containing vanadium, tungsten, molybdenum, manganese, cerium, or lanthanum, as well as phosphonic acid, individually or in any combination, ensures good corrosion protection properties even with an initial acid passivation. Table 2 Composition for acid passivation (chromium(III) nitrate and sulfate) Experiment No.: 6 7 8 9 10 11 12 required ² no ¹ required ² no ¹ required ² no ¹ no ¹ Chromium(III) compound Portion Portion Portion Portion Portion Portion Portion Chromium(III) sulfate solution 20% 5.0% 70.0% Chromium(III) nitrate solution 20% 65.7% 65.7% 65.7% 65.7% 65.7% 65.7% Nitrate compound Sodium nitrate 15.8% 15.8% 15.8% 15.8% 15.8% 15.8% 15.8% Fluorine compound Sodium hydrogen difluoride 0.2% 0.2% 0.2% 1.7% 1.7% 1.7% 1.7% Organic acid citric acid 1.0% 1.0% 2.5% 2.5% 0.1% 0.1% Oxalic acid 2.5% Salicylic acid 2.5% succinic acid 1.0% 1.0% 1.0% 1.0% 1.0% 1.0% Inorganic acid Nitric acid HNO3 2.2% 2.2% 2.2% 2.2% 2.2% 1.2% 1.2% sulfuric acid 1.5% 11.5% Vanadium or tungsten compound Sodium vanadate Potassium vanadate 1.5% Vanadyl sulfate 1.3% 1.3% Sodium tungstate 1.0% 1.0% 1.0% Sodium olybdate 1.0% 1.0% 1.0% Potassium molybdate 0.5% 0.5% 0.5% Phosphonic acid 1-Hydroxyethane-1,1-diphosphonic acid 1.0% 1.0% 1.0% Amino-tris(methylenephosphonic acid) Ethylenediamine-tetra (methylenephosphonic acid) 1.0% 0.7% Diethylenetriamine penta (methylenephosphonic acid) 1.5% 1.0% 1.0% Hexamethylenediamine-tetra (methylenephosphonic acid) Hydroxyethyl-amino-di (methylenephosphonic acid) 0.5% 0.5% 0.5% 2-Phosphonobutane-1,2,4-tricarboxylic acid 0.5% 0.5% Bis(hexamethylenetriaminepent a (methylenephosphonic acid)) 0.2% 0.2% 0.2% Sodium permanganate 0.1% 0.1% 0.1% Water 11.1% 11.1% 11.1% 7.3% 7.3% 3.2% 3.9% sum 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% ¹ ne = not in accordance with the invention
² req. = according to the invention

The first, acidic passivation is applied to steel sheets with a zinc-iron alloy surface, which may have been pretreated in a known manner, in particular, for example, cleaned or degreased.

Examples of implementation in Tables 3 and 4

According to the invention, a second alkaline passivation is applied to the dried first acidic passivation, which was applied from an aqueous acidic composition to the steel sheet with an iron-zinc alloy surface. In the present case, the aqueous alkaline compositions explained in more detail below are applied to the first acidic passivation according to embodiments 1 and 2 to produce a second alkaline passivation.

This second passivation is applied as an aqueous composition. The aqueous coating composition is alkaline, although a pH value > 9, preferably between pH 10 and pH 12, can also be achieved through the use of alkalis. However, an alkaline pH is usually already established through the use of silicates, e.g. alkali silicates. Polysilicates are used to carry out embodiments 3 and 4. The solids content (solids based on the total amount of the aqueous solution) is 20% for the preferably used lithium polysilicates, 40% for sodium and potassium silicate solutions (water glasses), and 20% for colloidal silicates, although colloidal silicates with a solids content of up to 30% are also available and suitable. The molecular weight of the lithium polysilicate is between 200 and 300 g/mol and is thus lower than the molecular weight of the water glasses used. Silane is used in 100% solids form. Table 3 Second alkaline passivation (aqueous composition polysilicate and silane) Silane component Experiment No. Lithium polysilicate Experiment No. Soda water glass Experiment No. colloidal silicate Methacryloxymethyltriethoxysilane 1 0 14 0 27 0 Methacryloxymethyltriethoxysilane 2 15.2 15 1.3 28 27.8 3-Aminopropylmethyldiethoxysilane 3 4.1 16 41.5 29 17.3 3-Aminopropyltriethoxysilane 4 7.5 17 33.2 30 47.5 N -(2-Aminoethyl)-3-aminopropylmethyl-dimethoxysilane 5 70.8 18 1.5 31 13.9 3-Glycidyloxypropyltrimethoxysilane 6 19.3 19 2.9 32 28.9 Vinyltrimethoxysilane 7 5.9 20 70.2 33 55.5 Vinyltriethoxysilane 8 30.1 21 23.2 34 5.2 Methyltrimethoxysilane 9 100.0 22 100.0 35 100.0 Methyltrimethoxysilane 10 20.3 23 7.9 36 48.7 3-Mercaptopropyltrimethoxysilane 11 7.5 24 21.3 37 2.3 CoatOSil MP 200* 12 27.5 25 3.9 38 45.0 N -[3-(Trimethyloxysilyl)propyl]ethylenediamine 13 25.6 26 9.2 39 44.1 *CoatOSil MP 200 is an oligomer of gammaglycidyloxypropyltrimethoxysilane

Table 3 shows compositions for a second alkaline passivation, each of which, with the exception of two reference tests with lithium polysilicate (test no. 1) and methyltrimethoxysilane (test no. 9), consists of a silane-modified silicate. The numerical values indicate the amount of silane used in weight percent based on the total composition of silane and silicate. It is supplemented with silicate to 100 weight percent. For example, an aqueous alkaline composition for producing a second alkaline passivation from vinyltrimethoxysilane and lithium polysilicate (test no. 7) consists of 5.9 weight percent silane and 94.1 weight percent lithium polysilicate (solids content 20%). In this case, the aqueous alkaline composition therefore contains an amino-functional, silane-modified lithium polysilicate. An alternative second alkaline passivation is prepared from an aqueous alkaline composition comprising vinyltrimethoxysilane and sodium silicate (Test No. 20). This aqueous alkaline composition consists of 70.2% by weight silane and 29.8% by weight silicate (solids content 40%). In this case, the aqueous alkaline composition contains a vinyl-functional, silane-modified silicate. Colloidal silicate, sodium silicate (sodium polysilicate), and lithium polysilicate are used, with the latter being preferred. A fully hydrolyzed product is used, allowing for a substantially VOC-free application of the second alkaline passivation.

The steel sheet treated with the first acidic passivation according to embodiments 1 and 2 is immersed in the aqueous composition or coating liquid of a silane-modified silicate and then dried using the same conditions as described for the production of the first acid passivation.

Also well-suited as a second alkaline passivation are alkaline, aqueous compositions containing a silicate modified with various silanes. Table 4 shows such compositions, in which up to eight different silanes are used, each to modify a silicate.

The experiments in Tables 3 and 4 show that the proportions of silane and silicate for the silane-modified silicate can be varied within a wide range. The silicate content can vary between 1% and 99% by weight; preferably, it is between 20% and 90% by weight. The silane can be used in the same amounts as the silicate; both are used in complementary proportions, so that they add up to 100% by weight in the formulations specified here. Preferably, up to 20% by weight of silane is used. Based on the solids content, according to a particularly advantageous embodiment, lithium polysilicate and silane are used in a ratio of approximately 1:1.

With the first, acidic passivation, very thin layers of up to 300 nm are applied, usually up to 150 nm, preferably up to 100 nm. Despite the thin layer thickness, the first passivation according to the invention provides good corrosion protection. The second alkaline passivation is applied in a layer thickness of up to 1 µm, advantageously from 10 nm to 500 nm. The thickness of the second layer is preferably 300 nm. Table 4 Second alkaline passivation (aqueous composition polysilicate and several silanes) Silane component / Experiment No. 40 41 42 43 44 Methacryloxymethyltriethoxysilane 1.0% 15.0% 0.0% 5.0% 2.0% 3-Aminopropylmethyldiethoxysilane 5.0% 1.0% 3-Aminopropyltriethoxysilane 1.3% N -(2-Aminoethyl)-3-aminopropylmethyldimethoxysilane 15.0% 3-Glycidyloxypropyltrimethoxysilane 4.0% 20.0% 21.0% 5.0% Vinyltrimethoxysilane 5.0% 2.0% 2.7% 5.0% Vinyltriethoxysilane 1.5% 3-Mercaptopropyltrimethoxysilane 5.0% 3.0% CoatOSil MP 200* 5.0% 25.0% 2.0% 5.0% 45.0% N -[3-(Trimethyloxysilyl)propyl]ethylenediamine 15.0% 6.0% 5.0% 7.5% Silicate component, each used as polysilicate Lithium polysilicate 90.0% Soda water glass 70.0% 55.0% colloidal silicate 20.0% 30.0% *CoatOSil MP 200 is an oligomer of gammaglycidyloxypropyltrimethoxysilane

The aqueous composition for the second alkaline passivation was prepared by joint hydrolysis of the silanes or siloxanes and the silicates, here polysilicates, and subsequent removal of the released alcohols by vacuum distillation.

The compositions described in Tables 1-4 for the first, acidic passivation (Tables 1, 2) and the second, alkaline passivation (Tables 3, 4) were applied sequentially to steel sheets as explained above in connection with the application of the first, acidic passivation.

For comparison, untreated steel sheets and steel sheets treated with only a first acidic passivation or only a second passivation were also tested. These comparison objects and the steel sheets treated with both acidic and alkaline passivations according to the invention were then subjected to the neutral salt spray test according to DIN EN ISO 9227. The results of this test are summarized in Table 5. All steel sheets used to carry out Examples 1 to 4 have a zinc alloy surface.

Row 1 of Table 5 shows the corrosion protection results for steel sheets tested with the first acid passivation but without the second alkaline passivation. Column 1 of Table 5 shows steel sheets tested without the first acid passivation but with the second alkaline passivation. The test results in column 1 and row 1 show the test results for a steel sheet without passivation.

Columns 1-12 of Table 5 show the corrosion protection results for the aqueous compositions of the first, acidic passivation listed in Tables 1 and 2. Rows 1-44 in Table 5 show the second alkaline passivations applied to these acidic passivations.

The compositions of the first acid passivation of experiments 1, 5 and 7 were carried out without vanadium or tungsten compounds.

• -- kein Korrosionsschutz: Standzeit im Salzsprühtest < 24 Stunden
• - mäßiger Korrosionsschutz: Standzeit im Salzsprühtest >24 Stunden
• ○ durchschnittlicher Korrosionschutz: Standzeit im Salzsprühtest >48 Stunden + guter Korrosionsschutz: Standzeit im Salzsprühtest >150 Stunden
• ++ hervorragender Korrosionsschutz: Standzeit im Salzsprühtest >360 Stunden (Weissrost), Standzeit im Salzsprühtest >720 Stunden (Rotrost)

The results presented in Table 5 were evaluated as follows:

• -- no corrosion protection: service life in salt spray test < 24 hours
• - moderate corrosion protection: service life in salt spray test >24 hours
• ○ Average corrosion protection: Service life in salt spray test >48 hours + Good corrosion protection: Service life in salt spray test >150 hours
• ++ Excellent corrosion protection: Service life in salt spray test >360 hours (white rust), service life in salt spray test >720 hours (red rust)

In detail, a comparison of the first column in Table 5 with the subsequent columns shows that, without a first acid passivation, even with an otherwise highly effective second alkaline passivation, only average corrosion protection can be achieved at best. On the other hand, it is clear that, while it is important to apply an acid passivation beforehand to achieve good or excellent corrosion protection, the quality of the measured corrosion protection for the two-layer passivation according to the invention depends more on the composition of the second alkaline passivation. This can be seen from the fact that the results in a row (except for the "without" column) are all classified in the same way.

Furthermore, it has been shown that compositions for the second alkaline passivation containing a silane-modified lithium polysilicate predominantly provide excellent corrosion protection when applied over an acidic passivation (tests 1-13 of the second passivation). Acidic and alkaline passivations according to the invention provide particularly good results when the acidic passivation contains vanadium, tungsten, or their compounds. However, the aqueous alkaline compositions consisting of a silicate modified with multiple silanes also predominantly provide excellent corrosion protection on the substrate of an acidic passivation.

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• Zeichnungen siehe Ansicht "Originaldokument"
  
  

An alkaline passivation, in which colloidal silicate or water glass is used in combination with silane, i.e. silane-modified, and which is applied to an acidic passivation, leads to good, sometimes even excellent results in the neutral salt spray test.

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Steel sheets with no or only acid or alkaline passivation and a surface coated with a zinc-iron alloy coated, and the steel sheets having a zinc-iron alloy surface provided according to the invention with a first acid passivation and a second alkaline passivation were also tested for their corrosion resistance in the neutral salt spray test as explained above. A steel sheet with a zinc-iron alloy surface, but without any coating, shows a corrosion resistance of less than 24 hours (test column 1, line 1: --). Zinc-iron alloy-coated steel sheets that had at least received acid passivation (tests in line 1) or that had only received alkaline passivation (tests in column 1) show low to average corrosion resistance in the salt spray test.

Steel sheets with a zinc-iron alloy surface to which both a first acid passivation and a second, alkaline passivation comprising silane-modified silicates have been applied generally show at least good, but often excellent, corrosion protection.

Particularly noteworthy are the results for the acid-passivated substrates produced using vanadium and tungsten compounds (tests 2-4, 6, 8-12), which were then treated with a second alkaline passivation according to the invention.

When evaluating the effect that the second alkaline passivation contributes to corrosion protection, it is shown that alkaline passivations with lithium polysilicate predominantly (tests lines 1-13) provide excellent corrosion protection, especially when lithium polysilicate is modified with one or more silanes or siloxanes.

Colloidal silicates or silica sols also provide good corrosion protection, especially when the colloidal silicates are modified in combination with silanes or siloxanes (tests lines 28-39; 41, 44). The same applies to silicates modified in a mixture with several silanes or siloxanes simultaneously. In these cases, excellent results are predominantly achieved in the salt spray test.

But water glasses are also very well suited for the production of aqueous, alkaline passivation solutions; passivations produced with such solutions show good corrosion protection results, especially when the silicates used have been modified with silanes or siloxanes (tests 14-26; 42, 43).

It is worth emphasizing that these passivations, which offer good to excellent corrosion protection, do not contain cobalt or chromium(VI) compounds. It is also worth noting that these acidic and alkaline passivations can be applied and dried essentially VOC-free, not least because fully hydrolyzed silane-modified silicates, especially polysilicates, are preferred.

Furthermore, it is shown that the effect of the second alkaline passivation does not depend on the composition of the first acidic passivation. Rather, it is shown that the combination of an acidic and an alkaline passivation can achieve good to very good corrosion protection even if, for example, the acidic passivation contains few or no vanadium or tungsten compounds or phosphonic acid.

Claims (18)

1. A method for the manufacture of a metallic substrate provided with a passivation which is free of chromium VI and cobalt, by applying

   (a) a first acidic passivation, wherein for the manufacture of the first acidic passivation an aqueous acidic composition is applied to the substrate, the composition containing a chromium (III) compound which is selected from the group consisting of chromium (III) sulphate, chromium (III) hydroxide, chromium (III) dihydrogen phosphate, chromium (III) chloride, chromium (III) nitrate, sodium chromium (III) sulphate, potassium chromium (III) sulphate and chromium (III) salts of organic acids, wherein the composition contains the chromium (III) compound in quantities of at least 0.05 g/l, based on the aqueous acidic composition, and a phosphorous acid in quantities of 0.5 % by weight to 3 % by weight, based on the aqueous acidic composition, and

   (b) a second alkaline passivation to the metallic substrate, wherein for the manufacture of the second alkaline passivation an aqueous alkaline composition is used, which contains silane-modified and/or siloxane-modified silicates.
2. The method according to claim 1, characterised in that with the second alkaline passivation, an aqueous alkaline composition is applied to the substrate, the composition containing a silane-modified and/or a siloxane-modified silicate with a 1 % by weight to a 99 % by weight proportion of silane.
3. The method according to claim 1 or claim 2, characterised in that for the manufacture of the second alkaline passivation an aqueous alkaline composition is applied to the metallic substrate, the composition containing one or more silicates from the group comprising colloidal silica sols, sodium silicate, potassium silicate, lithium silicate and ammonium silicate, with all silicates in this case also comprising polysilicates.
4. The method according to claim 3, characterised in that for the manufacture of the second alkaline passivation an aqueous alkaline composition is applied which contains lithium polysilicate or a mixture of lithium polysilicate with colloidal silica sols or sodium silicate, potassium silicate and/or ammonium silicate.
5. The method according to at least one of claims 1 to 4, characterised in that the aqueous alkaline composition applied for the manufacture of the second alkaline passivation contains a vinyl silane, amino silane or epoxy-functional silane and/or a siloxane or a mixture of these silanes or siloxanes.
6. The method according to claim 5, characterised in that the aqueous solution applied for the manufacture of the second alkaline passivation contains one or more silanes from the group comprising methacryloxy methyltriethoxy silane, methacryloxy methyltriethoxy silane, 3-aminopropyl methyldiethoxy silane, 3-aminopropyl triethoxy silane, N-(2-aminoethyl) 3-aminopropyl methyldimethoxy silane, 3-glycidyloxy propyltrimethoxy silane, vinyl trimethoxy silane, vinyl triethoxy silane, methyl trimethoxy silane, as well as 3-mercaptopropyl trimethoxy silane and siloxanes.
7. The method according to one of claims 1 to 6, characterised in that the aqueous alkaline composition used for the manufacture of the second alkaline passivation contains silicates, silanes, siloxanes and silane-modified and/or siloxane-modified silicates which are used in a partly or completely hydrolysed form.
8. The method according to at least one of the preceding claims 1 to 7, characterised in that a substrate is coated which has a metallic surface from the group comprising a surface of zinc, aluminium, a zinc-aluminium alloy, a zinc-iron alloy or an alloy of zinc or aluminium with one or more other metals.
9. The method according to at least one of the preceding claims 1 to 9, characterised in that the second alkaline passivation has a layer thickness of 10 nm to 1 µm, preferably of 20 nm to 500 nm.
10. The method according to at least one of claims 1 to 10, characterised in that a first acidic passivation is applied, which is optionally subsequently dried, and in that a second alkaline passivation is applied to the dried first passivation.
11. The method according to at least one of claims 1 to 10, characterised in that for the manufacture of the first acidic passivation an aqueous acidic composition is applied to the substrate, the composition containing a nitrate compound.
12. The method according to at least one of claims 1 to 11, characterised in that for the manufacture of the first acidic passivation an aqueous acidic composition is applied to the substrate, the composition containing a source of fluorine, wherein as the source of fluorine a compound is selected from the group comprising hydrofluoric acid, hexafluorotitanic acid, hexafluorozirconic acid, sodium fluoride, potassium fluoride, ammonium fluoride, sodium bifluoride, potassium bifluoride and ammonium bifluoride.
13. The method according to at least one of claims 1 to 12, characterised in that for the manufacture of the first acidic passivation an aqueous acidic composition is applied to the substrate, the composition containing one or more compounds of the metals molybdenum, vanadium or tungsten.
14. The method according to at least one of claims 1 to 13, characterised in that for the manufacture of the first acidic passivation an aqueous acidic composition is applied to the substrate, the composition containing one or more of the compounds from the group comprising potassium molybdate, sodium molybdate, potassium orthovanadate, potassium metavanadate, sodium orthovanadate, sodium metavanadate, sodium tungstate, sodium paratungstate and vanadium pentoxide.
15. The method according to one of claims 1 to 14, characterised in that for the manufacture of the first acidic passivation an aqueous acidic composition is applied to the substrate, in which one or more acids are used from the group comprising (1-hydroxyethane-1,1-diyl) biphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, aminotrimethylene phosphonic acid, ethylenediamine tetramethylene phosphonic acid or diethylenetriamine pentamethylene phosphonic acid.
16. The method according to one of claims 1 to 14, characterised in that for the manufacture of the first acidic passivation an aqueous acidic composition is applied to the substrate, the composition containing phosphonates individually or in a mixture, from the group comprising tetrasodium (1-hydroxyethane-1,1-diyl) biphosphonate, trisodium (1-hydroxyethane-1,1-diyl) biphosphonate, pentasodium ethylenediamine tetramethylene phosphonate or heptasodium diethylenetriamine pentamethylene phosphonate.
17. The method according to at least one of claims 1 to 16, characterised in that for the manufacture of the first acidic passivation an aqueous acidic composition is applied to the substrate, the composition containing one or more of the elements or compounds thereof from the group comprising molybdenum, manganese, cerium and lanthanum.
18. The method according to at least one of the preceding claims 1 to 17, characterised in that the first acidic passivation has a layer thickness of 10 nm up to 1 µm, preferably up to 500 nm.