Aqueous composition for coating corn-oriented steel

Description

TECHNICAL FIELD

[0001] The present invention relates to aqueous compositions suitable for coating grain-oriented steel ("GO"), which is used, for example, in transformers. BACKGROUND OF THE INVENTION

[0002] Numerous processes for producing grain-oriented electrical steel sheet are described in the prior art (see, among others, US 5,288,736, US 3,159,511, US 5,643,370, JP 2002-2112639, JP 56-158816, DE 1226129, DE 1252220, DE 197 45 445, DE 602 191 58, EP 0 484 904, EP 1 752 548, EP 2 022 874, EP 2 264 220). Grain-oriented electrical steel sheet is used in the production of transformers, dynamos, and high-power generators to ensure the required soft magnetic properties.

[0003] Grain-oriented steel is essentially a low-carbon steel (carbon content of approximately 0.01% to approximately 0.1%) with a high silicon content of approximately 2.5% to approximately 7.0%. Grain orientation is achieved through selected rolling, annealing, and tempering steps. Sheets of this steel are ultimately dipole-oriented in the rolling direction and magnetizable. Such steel sheets are often produced as steel strips with a thickness of approximately 0.2 to approximately 0.4 mm. To protect them from corrosion until processing (transport, punching, etc.), the sheet is usually coated with a 1 to 2 µm layer of Mg silicate ("forsterite") at the factory, i.e., immediately after production. This is achieved by coating with MgO, which reacts with surface silicon from the steel to form silicate in an annealing process ("batch annealing"). This coating is subsequently referred to as the “base coating”.

[0004] Methods for applying the "primer coating" are described, for example, in DE 198 16 200, DE 602 191 58 and DE 27 43 859 and essentially comprise the following steps: Application of an approximately 10% aqueous MgO dispersion, drying at 100°C, annealing in a hydrogen gas atmosphere at 1000-1350°C, cooling, and brushing off excess MgO.

[0005] The primer coat provides adequate preliminary corrosion protection and is essentially electrically insulating.

[0006] Due to the type of coating, irregularities, especially the finest pores, can occur in the base coat, which can lead to corrosion over time, initially unnoticed.

[0007] US Pat. No. 4,120,702 discloses a method for coating steel sheet having a protective silicate layer. The method involves first coating the surface with an aqueous solution containing phosphate ions, silicon dioxide grains, iron and / or manganese ions, and negative ions. During the coating process, the steel sheet is heated to a temperature between 400°C and 1100°C for periods of approximately 4 to 10 minutes, forming a protective phosphate layer. WO 2014 / 180610 A1 discloses an aqueous composition for coating grain-oriented steel, which contains, in addition to water, aluminum cations, manganese cations, phosphate ions, and colloidal silicon dioxide.

[0008] It is an object of the present invention to provide methods and agents that make it possible to improve the corrosion resistance of grain-oriented steel and to electrically insulate its surface. Furthermore, these agents should not contain environmentally harmful metals such as chromium, which are currently present in many coating materials for grain-oriented steel.

[0009] In order to ensure user-friendliness, it is a further object of the present invention to provide compositions for coating grain-oriented steel which can be used directly without mixing several components and which can also be stored for a longer period of time without quality restrictions. SUMMARY OF THE INVENTION

[0010] The present invention relates to an aqueous composition for coating grain-oriented steel comprising Aluminium cations, manganese cations, dihydrogen phosphate, hydrogen phosphate and / or phosphate anions, colloidal silicon dioxide and optionally iron cations, wherein the aluminum cations present in the composition, calculated as Al 2 O 3 , manganese cations, calculated as MnO, dihydrogen phosphate, hydrogen phosphate and / or phosphate anions, calculated as P 2 O 5 , colloidal silicon dioxide, calculated as SiO 2 , and optionally iron cations, calculated as FeO, the molecular formula (Al 2 O 3 ) 2 (MnO) 1.8-2.4 (FeO) 0-0.2 (P 2 O 5 ) 5-7 (SiO 2 ) ≥30 result.

[0011] It has surprisingly been found that the objects discussed above can be achieved with an aqueous composition according to the present invention. The storage-stable composition according to the invention makes it possible to provide corrosion-resistant protection and electrical insulation for grain-oriented steel without the composition comprising environmentally harmful metals such as chromium. The composition according to the invention can be applied directly to the steel or to steel primed with forsterite.

[0012] A process for preparing an aqueous composition for coating grain-oriented steel is further described, comprising the step of mixing aluminum cation-releasing compounds, manganese cation-releasing compounds, dihydrogen phosphate, hydrogen phosphate and / or phosphate anion-releasing compounds, colloidal silicon dioxide and optionally iron cation-releasing compounds as defined in the present patent application (see claim 1).

[0013] To prepare the composition according to the invention, the individual components are dissolved in water as described above. Methods for mixing such compounds with water are well described in the prior art. By mixing these components, it is possible to produce storage-stable compositions.

[0014] Another aspect of the present invention relates to a method for coating grain-oriented steel comprising applying an aqueous composition according to the present invention.

[0015] Yet another aspect of the present invention relates to grain-oriented steel, preferably grain-oriented steel sheet, obtainable by a coating process according to the present invention. DESCRIPTION OF THE EMBODIMENTS

[0016] The aqueous composition according to the invention comprises, in addition to water, aluminum cations, manganese cations, dihydrogen phosphate, hydrogen phosphate and / or phosphate anions, colloidal silicon dioxide and optionally iron cations in a specific molar ratio to one another. This ratio is expressed in the empirical formula (Al 2 O 3 ) 2 (MnO) 1.8-2.4 (FeO) 0-0.2 (P 2 O 5 ) 5-7 (SiO 2 ) ≥30, wherein the aluminum cations contained in the composition are calculated as Al 2 O 3 , manganese cations are calculated as MnO, dihydrogen phosphate, hydrogen phosphate and / or phosphate anions are calculated as P 2 O 5 , colloidal silicon dioxide is calculated as SiO 2 and optionally iron cations are calculated as FeO. The metal cations are preferably added to the aqueous composition as metal hydroxides, metal oxides or metal salts. Dihydrogen phosphate, hydrogen phosphate and / or phosphate anions can be added to the compositions either as phosphoric acid or as phosphates.

[0017] According to a preferred embodiment of the present invention, the above-mentioned components are added to the aqueous composition according to the invention in an amount such that the empirical formula (Al 2 O 3 ) 2 (MnO) 1.8-2.4 (FeO) 0-0.2 (P 2 O 5 ) 5-7 (SiO 2 ) 30-100 , preferably (Al 2 O 3 ) 2 (MnO) 1.8-2.4 (FeO) 0-0.2 (P 2 O 5 ) 5-7 (SiO 2 ) 30-80 , even more preferably (Al 2 O 3 ) 2 (MnO) 1.8-2.4 (FeO) 0-0.2 (P 2 O 5 ) 5-7 (SiO 2 ) 30-70 , results.

[0018] The aqueous composition according to the invention may comprise other metal cations (apart from aluminum and manganese cations) in addition to or instead of iron cations. The molar ratio of these metal cations, calculated as oxide, to the other components in the composition corresponds in total to that of the iron cations (see claim 1).

[0019] This aqueous composition can be used for coating grain-oriented steel, especially grain-oriented steel sheet. Grain-oriented steel sheet is susceptible to corrosion after production, so it is coated with a primer coating (usually an aqueous MgO dispersion). Since this primer coating usually only inadequately protects the steel sheet against corrosion due to micropores and macropores in the coating, it is necessary to provide the primed steel sheet with an additional coating. This (additional) coating can be achieved using the aqueous composition according to the invention.

[0020] Pores in the base coat can be detected, for example, by applying a diluted permanganate solution. Depending on the extent of the porosity, such a solution will decolorize over time and in a concentration-dependent manner, triggered by the access of Mn VII ions to the locally exposed steel surface and their oxidation products (accompanied by the reduction of Mn VII to Mn II / III). If such porosity is detected in such a test, this deficiency can also be remedied using the coating or composition according to the invention. The pores in the initial coating are closed, and at the same time, lasting corrosion protection is established, which is also characterized by excellent electrical insulation.

[0021] The aqueous composition of the present invention forms a highly effective corrosion protection based on a dense layer of silicates and phosphates. This coating also exhibits the following properties: hydrolysis resistance, annealing resistance up to 1000°C, electrical insulation, good adhesion to the base coat (forsterite layer) or directly to a steel surface, non-stickiness under processing conditions, and attenuation of the sound waves caused by magnetostriction oscillation during subsequent use (in transformers, "transformer hum"). The coating compositions described in the prior art, which are usually mixed shortly before application and are not available as a ready-to-use composition, are characterized by comparable properties, even if they provide significantly inferior quality with regard to the above-mentioned properties compared to the coating according to the invention.An example in this context is DE 2247269, which discloses such coating agents. A special feature of the compositions described therein is that they contain chromate to ensure the desired corrosion protection properties of the silicate / phosphate matrix used. However, Cr VI compounds are increasingly undesirable, even legally, due to their harmful effects on human health and the environment.

[0022] There is therefore a need to provide chromium-free compositions without negatively impacting the aforementioned beneficial properties. However, obvious alternatives such as replacing chromium with tin, vanadium, titanates, or zirconium complexes fail because such compounds are either also toxic, result in insufficient composition stability, or are not available cost-effectively in large quantities. The lack of stability of such compositions is particularly disadvantageous, as the individual components must be stored separately and can only be mixed together shortly before use.

[0023] The aqueous compositions according to the invention are characterized in that they are chromium-free, storage-stable (at least three months at a room temperature of 22°C), single-component, and the coatings that can be produced therewith have the necessary physical properties described above.

[0024] It has been shown that when the Al 2 O 3 :MnO ratio is reduced to below 1:0.9, the stability of the composition already decreases significantly, and is practically lost at 1:0.75. In contrast, a ratio above 1:1.2 increasingly leads to stability problems (turbidity, precipitation) in the liquid preparation and would subsequently lead to inclusions, cloudiness, undesirable color effects, and pores in the baked, finished state of the coating. According to a particularly preferred embodiment of the present invention, the Al 2 O 3 :MnO ratio is 1:1 to 1:1.2, even more preferably 1:1.1 to 1:1.2.

[0025] The SiO 2 :P 2 O 5 ratio should preferably be more than 4.3. According to a preferred embodiment of the present invention, this ratio is more than 4.3 and less than 16.7, even more preferably more than 4.3 and less than 13.3. If the SiO 2 :P 2 O 5 ratio is less than 4.3, there could be problems with the hydrolysis and / or corrosion resistance of the coating producible with the composition according to the invention.

[0026] The ratio of Al 2 O 3 :P 2 O 5 is preferably greater than 1:2.5 to ensure sufficient SiO 2 colloid resistance. Depending on the concentration of the other cations, especially manganese, the proportion of P 2 O 5 must be adjusted stoichiometrically.

[0027] In a particular embodiment of the present invention, upon detection of pores in the base coating (forsterite, see above), part of the manganese content in a secondary coating (preparable by means of the composition according to the invention) can be replaced or supplemented by iron oxide.

[0028] According to the state of the art, Mn-Fe mixed phosphates are particularly poorly soluble and thus contribute positively to the homogeneity of the base coat (pore closure) and the stability of the secondary coating (hydrolysis resistance). Surprisingly, this can be optimally achieved through the use of iron-II oxalate, which, in a known manner, thermally decomposes reductively at temperatures above approximately 600°C (in addition to the desired metal oxide, into gases (CO and CO 2 )). Thus, not only does it fill imperfections in the base coat with iron oxide or iron phosphate, but it also reduces already partially oxidized steel surfaces.

[0029] According to a preferred embodiment of the present invention, the number of SiO 2 in the molecular formula according to claim 1 is 30 to 100, preferably 30 to 80, even more preferably 30 to 70.

[0030] According to a further preferred embodiment of the present invention, the number of P 2 O 5 in the molecular formula is 5.4 to 6.8, preferably 5.6 to 6.6, more preferably 5.8 to 6.4.

[0031] The aluminum cations, manganese cations, dihydrogen phosphate, hydrogen phosphate, and / or phosphate anions, and optional iron cations present in the composition according to the invention can be introduced into it by mixing various salts, hydroxides, oxides, and / or acids with water. According to a preferred embodiment of the present invention, the composition according to the invention therefore comprises aluminum hydroxide and / or aluminum phosphate.

[0032] Manganese cations are preferably added to the aqueous composition according to the invention as manganese(II) oxide, manganese(II) oxalate and / or manganese(II) hydroxide.

[0033] According to a preferred embodiment of the present invention, iron cations are added to the aqueous composition according to the invention as iron(II) oxide and / or iron(II) oxalate, with iron(II) oxalate being particularly preferred.

[0034] Instead of or in addition to iron cations, the composition according to the invention may comprise other or additional metal cations capable of forming sparingly soluble phosphates or pyrophosphates. Preferably, the composition according to the invention contains metal cations, calculated as metal oxides, less aluminum and manganese cations, in the same stoichiometric ratio to one another as indicated in the empirical formula according to claim 1 for iron cations, calculated as iron oxide.

[0035] It has been shown according to the invention that it is particularly advantageous for the colloidal silicon dioxide contained in the aqueous composition to be free of charges. This means that colloidal silicon dioxide comprising charged metal ions or the like is less preferred or undesirable. Therefore, the colloidal silicon dioxide in the aqueous composition according to the invention is essentially free of surface charges.

[0036] According to a preferred embodiment of the present invention, the colloidal silica comprises silica particles, preferably spherical silica particles, in size between 5 and 80 nm, preferably between 5 and 60 nm, more preferably between 5 and 40 nm.

[0037] The silicon dioxide particles in the composition according to the invention preferably have a specific surface area of ​​400 to 450 m 2 / g for a size of 5 nm, a specific surface area of ​​180 to 200 m 2 / g for a size of 15 nm, a specific surface area of ​​130 to 150 m 2 / g for a size of 20 nm, a specific surface area of ​​100 to 120 m 2 / g for a size of 25 nm, a specific surface area of ​​90 to 110 m 2 / g for a size of 30 nm, a specific surface area of ​​60 to 70 m 2 / g for a size of 35 nm, and a specific surface area of ​​40 to 50 m 2 / g for a size of 40 nm.

[0038] Since only the hydroxyl groups of the colloidal silicon dioxide located on the surface and thus freely accessible for reaction and condensation are available for the tightness of the matrix to be formed, both the size of the spheres, their specific surface area, and the free availability of the hydroxyl groups (not blocked by "stabilization" of, for example, sodium ions) are important both for the stability of the liquid preparation and for the required quality of the finished coating that can be produced with the composition.

[0039] According to a preferred embodiment of the present invention, the ratio of the sum of the specific surface area of ​​the particles of colloidal silicon dioxide to the total number of moles of all metal oxides is 1:10,000 to 1:200,000, preferably 1:20,000 to 1:150,000, even more preferably 1:25,000 to 1:100,000, even more preferably 1:30,000 to 1:80,000.

[0040] According to a further preferred embodiment of the present invention, the molar ratio of the sum of the metal ions, calculated as their oxides, in particular the sum of the aluminum cations, calculated as Al 2 O 3 , and manganese cations, calculated as MnO, to silicon dioxide in the composition is 1:6.5 to 1:26.5, preferably 1:6.8 to 1:20, even more preferably 1:7.5 to 1:18, even more preferably 1:8 to 1:16.

[0041] According to a particularly preferred embodiment of the present invention, the molar ratio of the sum of the metal ions, calculated as their oxides, in particular the sum of the aluminum cations, calculated as Al 2 O 3 , and manganese cations, calculated as MnO, to silicon dioxide in the composition is preferably 1:9 to 1:13, even more preferably 1:10 to 1:12, when a surface is coated with the aqueous composition with a layer thickness of less than 1.5 pm, preferably with less than 1 µm.

[0042] According to a particularly preferred embodiment of the present invention, the molar ratio of the sum of the metal ions, calculated as their oxides, in particular the sum of the aluminum cations, calculated as Al 2 O 3 , and manganese cations, calculated as MnO, to silicon dioxide in the composition is preferably 1:10 to 1:14, even more preferably 1:11 to 1:13, when a surface is coated with the aqueous composition with a layer thickness of 2 to 10 pm, preferably with a layer thickness of 2 to 5 µm.

[0043] According to another preferred embodiment of the present invention, the aqueous composition according to the invention has a solids content of between 10% and 70%, preferably from 20% to 60%, even more preferably from 25% to 40%.

[0044] A process for preparing an aqueous composition for coating grain-oriented steel is further described, comprising the step of mixing aluminum cation-releasing compounds, manganese cation-releasing compounds, dihydrogen phosphate, hydrogen phosphate and / or phosphate anion-releasing compounds, colloidal silicon dioxide and optionally iron cation-releasing compounds as defined above.

[0045] Ion-releasing compounds are compounds capable of releasing ions into water (e.g., metal ions such as aluminum). Ion-releasing compounds can be salts, oxides, oxalates, or hydroxides.

[0046] Another aspect of the present invention relates to a method for coating grain-oriented steel comprising applying an aqueous composition according to the present invention.

[0047] According to another preferred embodiment of the present invention, the grain-oriented steel is primed with forsterite.

[0048] As already mentioned at the beginning, the grain-oriented steel to be coated may include a primer coating to protect it from rapid corrosion after its manufacture. The primer coating preferably comprises forsterite.

[0049] According to another preferred embodiment of the present invention, the grain-oriented steel is in the form of a sheet. Such sheets can be used, for example, for the manufacture of transformers.

[0050] According to a particularly preferred embodiment of the present invention, the aqueous composition is applied to the grain-oriented steel in an amount of 1 to 50 g / m 2< , preferably 2 to 40 g / m 2< , more preferably 3 to 30 g / m 2< , even more preferably 4 to 20 g / m 2< .

[0051] The aqueous composition is preferably applied to grain-oriented steel by a dipping process, rolling process or spraying process.

[0052] According to a preferred embodiment of the present invention, the grain-oriented steel coated with the aqueous composition is treated at a temperature of 500°C to 900°C, preferably 600°C to 850°C.

[0053] According to another preferred embodiment of the present invention, the aqueous composition is applied to the grain-oriented steel in a layer thickness of 100 nm to 20 pm, preferably 200 nm to 10 pm.

[0054] A further aspect of the present invention relates to grain-oriented steel, preferably grain-oriented steel sheet, obtainable by a process according to the present invention. EXAMPLES Example 1: Preparation of aqueous compositions for coating grain-oriented steel

[0055] In a mixture of 400g of 75% phosphoric acid and 135ml of water, 78g of aluminum trihydroxide, followed by 40g of manganese(II) oxide, and 7g of iron(II) oxalate were dissolved to form a clear, viscous phosphate-containing solution. The resulting solution had a total weight of 660g. 800g of charge-free silica sol (colloidal silicon dioxide) with a 30% solids content (SiO2 spheres with an average diameter of 35nm) were added to 200g of the phosphate-containing solution to form a clear, homogeneous preparation. The calculated composition was (Al2O3)2(MnO)2.2(FeO)0.2(SiO2)53(P2O5)6.3(Composition 1).

[0056] After application to a primer-coated GO sheet (ie a grain-oriented steel sheet coated with forsterite) in a quantity of 5g / m 2 , the coating was briefly dried in air and baked for 60 seconds at 820°C.

[0057] By adjusting the stoichiometric ratios of the components listed above, the following additional compositions (compositions 2 to 9) could be produced: Composition 2 (Al 2 O 3 ) 2 (MnO) 2.1 (FeO) 0.18 (SiO 2 ) 75 (P 2 O 5 ) 6.2 Composition 3 (Al 2 O 3 ) 2 (MnO) 2.2 (FeO) 0.18 (SiO 2 ) 49 (P 2 O 5 ) 6.3 Composition 4 (Al 2 O 3 ) 2 (MnO) 2.0 (FeO) 0.2 (SiO 2 ) 32 (P 2 O 5 ) 6.5 Composition 5 (without iron oxide) (Al 2 O 3 ) 2 (MnO) 2.2 (SiO 2 ) 55 (P 2 O 5 ) 6.1 Composition 6 (not according to the invention) (Al 2 O 3 ) 2 (MnO) 1.75 (FeO) 0.15 (SiO 2 ) 55 (P 2 O 5 ) 6.2 composition 7 (Iron oxide instead of iron oxalate in the phosphate-containing solution) (Al 2 O 3 ) 2 (MnO) 2.2 (FeO) 0.2 (SiO 2 ) 53 (P 2 O 5 ) 7

[0058] Compositions 2 to 7 were also applied to a primer-coated GO sheet in an amount of 5g / m 2<, briefly dried in air and baked for 60 seconds at 820°C. Composition No. SiO 2 : P 2 O 5 Al 2 O 3 : MnO Al 2 O 3 : P 2 O 5 m 2< SiO 2 per Me x O y *) Me x O y : - SiO 2 molecular formula 1 8,41 1:1,1 1:3,15 47.000 1:12,05 (Al 2 O 3 ) 2 (MnO) 2.2 (FeO) 0.2 (SiO 2 ) 53 (P 2 O 5 ) 6.3 2 12,10 1: 1,05 1:3,10 68.000 1:17,52 (Al 2 O 3 ) 2 (MnO) 2.1 (FeO) 0.18 (SiO 2 ) 75 (P 2 O 5 ) 6.2 3 7,78 1:1,1 1:3,15 44.000 1:11,19 (Al 2 O 3 ) 2 (MnO) 2.2 (FeO) 0.18 (SiO 2 ) 49 (P 2 O 5 ) 6.3 4 4,92 1: 1 1:3,25 28.000 1:7,62 (Al 2 O 3 ) 2 (MnO) 2.0 (FeO) 0.2 (SiO 2 ) 32 (P 2 O 5 ) 6.5 5 9,02 1:1,1 1:3,05 51.000 1:13,10 (Al 2 O 3 ) 2 (MnO) 2.2 (SiO 2 ) 55 (P 2 O 5 ) 6.1 6 8,87 1 : 0,875 1:3,10 55.000 1:14,10 (Al 2 O 3 ) 2 (MnO) 1.75 (FeO) 0.15 (SiO 2 ) 55 (P 2 O 5 ) 6.2 7 7,57 1:1,1 1:3,50 47.000 1:12,05 (Al 2 O 3 ) 2 (MnO) 2.2 (FeO) 0.2 (SiO 2 ) 53 (P 2 O 5 ) 7 *) calculated with SiO 2 colloid 35 nm / 65 m 2< / g; higher m 2< number adjustable with 20 nm / 140 m 2< / g. Me x O y denotes the sum of all metal ions calculated as their oxides Example 2: Comparative compositions:

[0059] In order to demonstrate the advantages of the composition according to the invention over other compositions from the prior art, corresponding tests were carried out with comparative compositions. Comparative composition 1 (Example B1 from WO 2014 / 180610 (Al, Mn)) (Al 2 O 3 ) 3 (MnO) 2 (SiO 2 ) 20 (P 2 O 5 ) 27 Comparative composition 2 (Example 1 from EP 2 264 220 Al, (KMnO 4 )) (Al 2 O 3 ) 5 (MnO 2 )(K 2 O) 0.5 (SiO 2 ) 29 (P 2 O 5 ) 5.5 Comparative composition 3 (Example 2 from DE 2247269 (Al, Cr)) (Al 2 O 3 ) 2 (CrO 3 ) 2,4 (SiO 2 ) 12 (P 2 O 5 ) 6 Comparative composition4 (Example B3 from WO 2014 / 180610 (Al, Mn, Zn, Mg)) (Al 2 O 3 ) 1.6 (MnO) 0.6 (ZnO) 0.2 (MgO) 2 (SiO 2 ) 16 (P 2 O 5 ) 5

[0060] Comparative compositions 1 to 4 were applied to a primer-coated GO sheet in an amount of 5g / m 2 as described in Example 1, briefly dried in air and baked for 60 seconds at 820°C. Assembly No. SiO 2 : P 2 O 5 Al 2 O 3 : MnO Al 2 O 3 : P 2 O 5 m 2< SiO 2 per Me x O y *) Me x O y : - SiO 2 molecular formula 1 0,74 1 : 0,25 1:3,38 7.800 1:2 (Al 2 O 3 ) 8 (MnO) 2 (SiO 2 ) 20 (P 2 O 5 ) 27 2 5,27 / 1:1,10 17.400 1:4,46 (Al 2 O 3 ) 5 (MnO 2 ) (K 2 O) 0.5 (SiO 2 ) 29 (P 2 O 5 ) 5.5 3 2,0 / 1:3,0 10.600 1:2,73 (Al 2 O 3 ) 2 (CrO 3 ) 2.4 (SiO 2 ) 12 (P 2 O 5 ) 6 4 3,2 1 : 0,37 1:3,13 14.200 1:3,64 (Al 2 O 3 ) 1.6 (MnO) 0.6 (ZnO) 0.2 (MgO) 2 (SiO 2 ) 16 (P 2 O 5 ) 5 *) calculated with SiO 2 colloid 35 nm / 65 m 2< / g; higher m 2< number adjustable with 20 nm / 140 m 2< / g. Me x O y denotes the sum of all metal ions calculated as their oxides Example 3: Testing of the compositions and coatings from Examples 1 and 2

[0061] In order to determine and evaluate the quality of the compositions according to Examples 1 and 2 and their suitability for coating grain-oriented steel, several tests were carried out. Stability of the compositions

[0062] One aim of the invention is to provide storage-stable aqueous compositions to ensure sufficient user-friendliness. For this reason, the stability of the aqueous composition was assessed. Over an extended period of time, it was observed whether the aqueous composition remained stirrable and whether particles settled. Both properties are important for the storage stability of the compositions. Appearance of the steel surface (corrosion phenomena) / Hydrolysis resistance

[0063] A crucial quality criterion for coating compositions used to coat grain-oriented steel is their ability to protect the coated steel from corrosion. To determine this, a stack of coated, water-wetted sheet samples, whose Mg silicate (forsterite) base coats were coated with the compositions according to Examples 1 and 2, were tightly wrapped in a water- and vapor-impermeable film and stored in a heating cabinet at 90°C for 8 hours. The surface of the coated sheets was then visually assessed. Color of the baked coating

[0064] After applying the compositions to the GO sheet and subsequent heating (see above), the color was visually assessed. Coating inclusions (solid)

[0065] Inclusions in the finished coating can also be a relevant criterion for the quality of the composition according to the invention. Any inclusions were visually detected and assessed. Pores and blistering

[0066] Blistering in the finished coating on the sheet metal is generally undesirable, as blisters are precursors to later corrosion phenomena. Blistering can be assessed visually. Results

[0067] The results of the above tests are shown in the following table: Composition No. stability Appearance of the steel surface Color of the coating Solid inclusions Pores / Bubbles Hydrolysis resistance * 1 > 3 months Evenly Light grey, glossy - - 1 2 > 3 months Slight spot corrosion Light grey glossy Occasional light streaks Occasionally 1 3 > 3 months Evenly Light grey, matt even - - 2 4 > 3 months Evenly Light grey, matt even Bright surface streaks - 2 5 > 3 months Slight spot corrosion Light grey matt - - 1 6 < 1 month Corrosion in peripheral zones Light grey light streaks Irregular surface - 2 7 > 3 months Evenly Light grey, matt even - - 1-2 Comparative composition no. stability Appearance of the steel surface Color of the coating Solid inclusions Pores / Bubbles Hydrolysis resistance * 1 4 days Evenly milky cloudy Black dots (MnO 2 ) Evenly distributed 3 2 8 hours Significant spot corrosion Light gray Dark streaks Occasionally 3-4 3 Multi-component material (< 5 hours) Evenly Gray-yellowish Green streaks Occasionally 1 4 1 day Evenly light gray Light grain - 1-2 * 1 = optimal, 2 = acceptable for practice, 3 = satisfactory, needs improvement, 4 = not suitable

[0068] The results impressively demonstrate that the compositions according to the invention (compositions 1 to 5 and 7) exhibit a high storage stability of more than three months, and the coatings produced therewith exhibit high hydrolysis resistance and extremely low susceptibility to corrosion. The comparative compositions from the prior art exhibit low storage stability in a ready-to-use mixture. The hydrolysis stability of the coatings produced therewith is also not optimal. Composition 6 also shows that a lower molar ratio between Al 2 O 3 and MnO (2:1.75) in the composition leads to lower storage stability.

Claims

1. Aqueous composition for coating grain orientated steel comprising - aluminium cations, - manganese cations, - dihydrogen phosphate, hydrogen phosphate and / or phosphate anions, - colloidal silicon dioxide and - optionally iron cations, wherein the aluminium cations present in the composition, calculated as Al2O3, manganese cations, calculated as MnO, dihydrogen phosphate, hydrogen phosphate and / or phosphate anions, calculated as P2O5, colloidal silicon dioxide, calculated as SiO2, and optionally iron cations, calculated as FeO, give the chemical formula (Al2O3)2(MnO)1,8-2,4(FeO)0-0,2(P2O5) 5-7(SiO2)≥30.

2. Aqueous composition according to claim 1, wherein the number of SiO2 in the chemical formula is 30 to 100, preferably 30 to 80, even more preferably 30 to 70.

3. Aqueous composition according to claim 1 or 2, wherein the number of P2O5 in the chemical formula is 5.4 to 6.8, preferably 5.6 to 6.6, even more preferably 5.8 to 6.4.

4. Aqueous composition according to any one of claims 1 to 3, wherein it comprises aluminium hydroxide and / or aluminium phosphate.

5. Aqueous composition according to any one of claims 1 to 4, wherein it comprises manganese(II) oxide, manganese(II) oxalate and / or manganese(II) hydroxide.

6. Aqueous composition according to any one of claims 1 to 5, wherein it comprises iron oxide iron(II) oxide and / or iron(II) oxalate.

7. Aqueous composition according to any one of claims 1 to 6, wherein the colloidal silicon dioxide is free of surface charges.

8. Aqueous composition according to any one of claims 1 to 7, wherein the colloidal silicon dioxide comprises silicon dioxide particles, preferably spherical silicon dioxide particles, in the size between 5 and 80 nm, preferably between 5 and 60 nm, more preferably between 5 and 40 nm.

9. Aqueous composition according to any one of claims 1 to 8, wherein the ratio of the sum of the specific surface area of the particles of colloidal silicon dioxide to the total number of moles of all metal oxides is 1:10000 to 1:200000, preferably 1:20000 to 1:150000, more preferably 1:25000 to 1:100000, still more preferably 1:30000 to 1:80000.

10. Aqueous composition according to any one of claims 1 to 9, wherein the molar ratio of the sum of the metal ions, calculated as their oxides, to silicon dioxide in the composition is 1:6.5 to 1:26.5, preferably 1:6.8 to 1:20, more preferably 1:7.5 to 1:18, still more preferably 1:8 to 1:16.

11. Aqueous composition according to any one of claims 1 to 10, wherein the molar ratio of the sum of the metal ions, calculated as their oxides, to silicon dioxide in the composition is preferably 1:9 to 1:13, more preferably 1:10 to 1:12, when a surface is coated with the aqueous composition with less than 1.5 pm, preferably less than 1 pm, layer thickness.

12. Aqueous composition according to any one of claims 1 to 11, wherein the molar ratio of the sum of the metal ions, calculated as their oxides, to silicon dioxide in the composition is preferably 1:10 to 1:14, more preferably 1:11 to 1:13, when a surface is coated with the aqueous composition having a layer thickness of 2 to 10 pm, preferably 2 to 5 pm.

13. A method of coating grain orientated steel comprising applying an aqueous composition according to any one of claims 1 to 12.

14. Process according to claim 13, wherein the grain-oriented steel is primer-coated with forsterite.

15. Grain orientated steel, preferably grain orientated steel sheet, obtainable by a process according to any one of claims 13 or 14.