Chromium-free coating for the electrical insulation of grain-oriented electrical steel strip

Abstract

The present invention relates to an aqueous composition based on phosphates, aluminum ions and other metal ions, in particular manganese ions and zinc ions, as well as silicates, siloxanes or silanes, for providing anticorrosive coatings on metal surfaces, in particular for the electrical insulation of grain-oriented electrical steel strip. For the desired functionality, the compositions according to the invention contain the constituents mentioned in a prescribed ratio to one another. The invention also relates to a method for coating the surface of a metallic workpiece by using the aqueous compositions, wherein the method produces outstanding results, in particular on grain-oriented electrical steel strip, with regard to corrosion protection and adherence.

Claims

1. A water-based chromium-free composition, comprising: a) phosphoric acid and/or the acid anions thereof; b) solvated aluminum cations; c) solvated ions containing Zn and Mn and oxidizing counter-anions thereto and optionally Fe; d) at least one silicate and/or at least one water-soluble silane or siloxane; e) solvated cations of the element Mg wherein a molar ratio of the solvated aluminum cations of component b) to solvated cations of the element Mg is no less than 0.6 and no greater than 3; with the proviso that the water-based chromium-free composition is also free of permanganates of divalent metals; wherein a molar ratio of the aluminum cations according to component b) to the phosphoric acid and/or the acid anions thereof, in each case calculated as PO.sub.4, according to component a), is no greater than 1.0 and no less than 0.1; wherein a molar ratio of a total amount of the solvated ions according to component c), calculated as the total amount of the elements Fe, Zn and Mn, to the phosphoric acid and/or the acid anions thereof according to component a), calculated as PO.sub.4, is no greater than 1.0 and no less than 0.1; and wherein a molar ratio of the silicates, silanes or siloxanes, calculated as SiO.sub.2, according to component d), to the phosphoric acid and/or the acid anions thereof, calculated as PO.sub.4, according to component a), is no greater than 3.0 and no less than 0.2.

2. The composition according to claim 1, wherein phosphoric acid and/or of the acid anions thereof, in each case calculated as PO.sub.4, is present in the composition in an amount of at least 10 g/kg and not more than 500 g/kg.

3. The composition according to according to claim 1, wherein dispersed silicates, if present, in the composition has a D90 value of no more than 500 nm.

4. The composition according to claim 1, wherein the molar ratio of solvated aluminum cations according to component b) to solvated ions of the element Zn according to component c) is no less than 3 and no greater than 20.

5. The composition according to claim 4, wherein the molar ratio of solvated aluminum cations according to component b) to solvated ions of the element manganese according to component c) is no less than 1 and no greater than 10.

6. The composition according to claim 1, wherein in total less than 1 g/kg of water-soluble compounds of the element boron is present.

7. The composition according to claim 1, wherein in total less than 100 ppm of water-soluble compounds of the elements Zr, Ti and/or Hf is present.

8. The composition according to claim 1, wherein no compounds that release halide ions are present as a source for the solvated cations of components b) and c).

9. The composition according to claim 1, wherein component d) consists of the at least one water-soluble silane.

10. The composition according to claim 9, wherein in total less than 100 ppm of water-soluble compounds of the elements Zr, Ti and/or Hf is present in the composition; wherein the at least one water-soluble silane is an organosilane selected from aminosilanes of the general chemical formula (I):
H.sub.2N[(CH.sub.2).sub.mNH].sub.y(CH.sub.2).sub.nSiX.sub.3(I) wherein the substituents .sub.nX.sup.u are each selected independently of each other from alkoxy groups having no more than four carbon atoms; m and n each independently of each other are integers in a range of 1 to 4; and y is an integer in a range of 0 to 8.

11. A method for coating a surface of a metallic material, comprising a step of contacting the surface of the metallic material with the composition according to claim 1.

12. The method according to claim 11, wherein the contacting step applies a wet film of the composition onto the surface of the metallic material, the method further comprising a step of drying said wet film.

13. The method according to claim 12, wherein the drying step comprises supplying heat such that a peak metal temperature of at least 500 C. but no more than 1200 C. is reached.

14. The method according to claim 12, wherein a wet film is applied in such an amount that, after drying, a dry film layer thickness of at least 0.5 m but no more than 5 m is achieved.

15. The method according to claim 11, wherein the metallic material is a flat product.

16. The method according to claim 15, wherein the flat product is a steel strip which comprises an inorganic coating substantially composed of magnesium oxide.

17. A steel strip comprising a metallic surface, an inorganic first coating, substantially composed of magnesium oxide, on the metallic surface and, on said inorganic first coating, an inorganic second coating, wherein the inorganic second coating is substantially composed of silicates and an anion matrix of phosphates, obtained by the method according to claim 14.

18. A water-based chromium-free composition, comprising water and: a) phosphoric acid and/or acid anions thereof; b) water-soluble inorganic compounds of Al selected from the group consisting of hydroxides phosphates, pyrophosphates, and combinations thereof; c) water-soluble inorganic compounds of Mn, and optionally of Fe, selected from hydroxides, phosphates; pyrophosphates, and combinations thereof; d) water-soluble inorganic compounds of Mg selected from oxides, hydroxides, phosphates and combinations thereof; e) water-soluble inorganic compounds of Zn selected from ZnO, Zn(NO.sub.3).sub.2 and combinations thereof f) at least one silicate and/or at least one water-soluble silane or siloxane; with the proviso that the composition contains no permanganates of divalent metals and no sulphates; wherein pH value of the composition is not below 0.5, and not above 3.0; wherein a molar ratio of solvated Al cations from b) to the phosphoric acid and/or phosphoric acid anions, in each case calculated as PO.sub.4, is no greater than 1.0 and no less than 0.1; wherein a molar ratio of a total amount of solvated cations of Zn from c), solvated cations of Mn from d) and, if present solvated cations of Fe from d), calculated as the total amount of the elements Zn, Mn and, if present Fe, to the phosphoric acid and/or the phosphoric acid anions thereof, calculated as PO.sub.4, is no greater than 1.0 and no less than 0.1; wherein a molar ratio of the silicates, silanes or siloxanes, calculated as SiO.sub.2, to the phosphoric acid and/or the phosphoric acid anions thereof, calculated as PO.sub.4, is no greater than 3.0 and no less than 0.2; wherein a molar ratio of the solvated Al cations from b) to the solvated cations of Zn from c) is no less than 3, and no greater than 20; wherein a molar ratio of the solvated Al cations from b) to the solvated cations of Mn from d) is no less than 3, and no greater than 20; wherein a molar ratio of the solvated Al cations from b) to the solvated cations of Mg from e) is no less than 0.6 and no greater than 3.

19. The composition according to claim 18, wherein e) is Zn(NO.sub.3).sub.2.

20. The composition according to claim 18, wherein: a) is phosphoric acid; b) is aluminum hydroxide; c) is manganese dihydrogen phosphate; d) is magnesium oxide; e) is zinc nitrate.

Description

EXEMPLARY EMBODIMENTS

(1) Coatings according to the invention of grain-oriented steel strip were produced using compositions B1 to B4 according to the invention.

(2) Composition B1:

(3) 22.8 g phosphoric acid (75% by weight);

(4) 4.4 g aluminum hydroxide;

(5) 2.0 g manganese dihydrogen phosphate;

(6) 43.0 g silica dispersion (30% by weight);

(7) 27.4 g water

(8) Composition B2:

(9) 23.0 g phosphoric acid (75% by weight);

(10) 4.4 g aluminum hydroxide;

(11) 2.0 g manganese dihydrogen phosphate;

(12) 1.2 g zinc oxide

(13) 43.0 g silica dispersion (30% by weight);

(14) 27.6 g water

(15) Composition B3:

(16) 21.2 g phosphoric acid (75% by weight);

(17) 2.2 g aluminum hydroxide;

(18) 1.1 g magnesium oxide; 2.0 g manganese dihydrogen phosphate;

(19) 0.5 g zinc nitrate;

(20) 43.0 g silica dispersion (30% by weight);

(21) 24.0 g water

(22) Composition B4:

(23) 20.9 g phosphoric acid (75% by weight);

(24) 2.0 g aluminum hydroxide;

(25) 1.5 g magnesium oxide;

(26) 1.0 g manganese dihydrogen phosphate;

(27) 0.3 g zinc nitrate;

(28) 43.0 g silica dispersion (30% by weight);

(29) 23.0 g water

(30) Comparison Composition V1 (Contains Chromium):

(31) 22.4 g phosphoric acid (75% by weight);

(32) 4.2 g aluminum hydroxide;

(33) 6.9 g chromium oxide solution (45% by weight);

(34) 43.0 g silica dispersion (30% by weight);

(35) 36.0 g water

(36) Comparison Composition V2 (Chromium-Free):

(37) 22.2 g phosphoric acid (75% by weight);

(38) 4.4 g aluminum hydroxide;

(39) 43.0 g silica dispersion (30% by weight);

(40) 23.2 g water

(41) Comparison Composition V3 (without Aluminum):

(42) 25.9 g phosphoric acid (75% by weight);

(43) 3.5 g magnesium oxide;

(44) 2.0 g manganese dihydrogen phosphate;

(45) 0.5 g zinc nitrate;

(46) 43.0 g silica dispersion (30% by weight);

(47) 25.2 g water

(48) The concentrations of the respective elements documented in Table 1 were thus present in compositions B1 to B4 and V2 and V3.

(49) TABLE-US-00001 TABLE 1 Concentrations of components a) to e) in 10.sup.4 mol/g in the respective compositions Al Zn Mn Mg PO.sub.4 SiO.sub.2 B1 5.65 0.80 19.3 21.6 B2 5.61 1.45 0.80 19.2 21.2 B3 3.00 0.32 0.85 3.00 19.1 23.0 B4 2.77 0.15 0.44 3.95 18.4 23.4 V2 6.12 18.3 23.2 V3 0.28 0.80 8.59 21.6 21.5

(50) All compositions were evenly applied to the surface of grain-oriented electrical steel strip using a pinch coater, the metal sheets were annealed at 850 C. for 90 seconds. The layer thickness of the dry film was 2 to 3 m. Subsequently, the properties of the metal sheets thus coated were analyzed. The results are shown in Table 2.

(51) TABLE-US-00002 TABLE 2 Concentrations of components a) to e) in 10.sup.4 mol/g in the respective compositions Elements Oxidizing.sup.1 Odor.sup.2 Appearance.sup.3 Alkali resistance.sup.4 B1 Al, Mn 3 3 B2 Al, Mn, Zn + 3 4 B3 Al, Mn, Zn, + + 1 1 Mg B4 Al, Mn, Zn, + + 2 1 Mg V1 Al, Cr + 1 1 V2 Al 4 5 V3 Mn, Zn, Mg + + 5 3 .sup.1oxidizing anions are present (+/yes; /no) .sup.2hydrogen sulfide odor detectable (+/no; /yes) .sup.3The metal sheets should be colorless and have a metallic sheen after coating and drying in the furnace. The evaluation was made based on the following scale: (1) clear and shiny (2) clear, but dull (3) slightly milky and dull (4) milky and dull (5) white coating .sup.4The alkali resistance of the coating was tested by way of abrasion using cotton wool saturated with a 20% by weight KOH solution. The evaluation was made after 50 double strokes based on the following scale: (1) no change in the coating (2) minimal visible change without change in the hue (3) visible change with slight change in the hue (4) clearly visible change with clear change in the hue (5) considerable change in the coating and hue

(52) This demonstrates that compositions B1 to B4 according to the invention always supply coatings that have a sufficiently good appearance. In addition, the presence of zinc in compositions B2 to B4 is able to suppress the development of hydrogen sulfide in the coating process so much that odor is no longer noticed. If moreover the element magnesium is added, as was in the case in compositions B3 and B3 according to the invention, coatings having a flawless appearance are achieved, which furthermore exhibit alkali resistance as can otherwise only be achieved by chromium-containing compositions (V1) known from the prior art.