CHROMIUM(VI)-FREE SLIP FOR DIFFUSION COATING

Abstract

The invention relates to a kit for preparing a chromium(VI)-free slurry suspension for diffusion coating of metal surfaces, the kit comprising, as separate components, a powder mixture and a liquid binder mixture, configured to be combined for preparing the slurry suspension, wherein the powder mixture comprises a powdered diffusion metal, wherein the binder mixture comprises an aqueous solvent and a phosphate binder, and wherein both the powder mixture and the binder mixture are free of chromium(VI) salts. The invention further relates to a method of diffusion coating a metal surface using such a kit, and to the use of such a kit in various industries.

Claims

1. A kit for preparing a chromium(VI)-free slurry suspension for the diffusion coating of metal surfaces, the kit comprising, as separate components, a powder mixture and a liquid binder mixture, configured to be combined to obtain the slurry suspension, wherein the powder mixture comprises a diffusion metal powder, wherein the binder mixture comprises an aqueous solvent and a phosphate binder, and wherein both the powder mixture and the binder mixture are free of chromium(VI) salts.

2. The kit according to claim 1, wherein the kit is a two-component system, one component being the powder mixture and the other component being the binder mixture.

3. The kit according to claim 1, wherein the powder mixture is a dry powder.

4. The kit according to claim 1, wherein the pH of the binder mixture is pH 2.3 to pH 2.9.

5. The kit according to claim 1, wherein the solvent of the binder mixture is water or an aqueous solvent having a water content of at least 80% by volume, and/or in that the solvent content of the binder mixture, relative to its total weight, is from 40 to 80% by weight.

6. The kit according to claim 1, wherein the weight fraction of the phosphate binder in the binder mixture, as expressed by the cumulative weight of phosphoric acid and phosphoric acid anions without counter-ions, is between 5 and 25%.

7. The kit according to claim 1, wherein the phosphate binder contains hydrogen or dihydrogen phosphates of at least one cation of the group aluminum, zinc or magnesium.

8. The kit according to claim 1, wherein the diffusion metal is a metal selected from the group consisting of aluminum, silicium, nickel and chromium, or a metal alloy selected from the group consisting of aluminum-nickel and aluminum-chromium alloys.

9. The kit according to claim 1, wherein the diffusion metal of the powder mixture is untreated.

10. The kit according to claim 1, wherein the powder mixture comprises a water-insoluble catalyst contained as the diffusion metal.

11. The kit according to claim 1, wherein the powder mixture comprises a water-insoluble dye.

12. A method of diffusion coating a metal surface, comprising the steps of: (i) combining the powder mixture and the binder mixture of a kit according to claim 1 to form a chromium(VI)-free slurry suspension; (ii) applying the slurry suspension to the metal surface; (iii) drying the applied slurry suspension by allowing the aqueous solvent of the binder mixture to evaporate, and to obtain a solid binder matrix on the metal surface, having particles of the diffusion metal powder dispersed therein; and (iv) heating the binder matrix coated metal surface to temperatures of greater than 500 C.

13. The method according to claim 12, wherein steps (ii) and (iii) of application and drying are repeated several times, to obtain a multilayer binder matrix on the metal surface.

14. The method according to claim 12, wherein step (iii) of drying comprises treating for 10-30 minutes at a temperature greater than 50 C.

15. Use of a kit according to claim 1 in the aerospace industry, the energy industry, the automotive industry, the oil industry, the metalworking industry or the maritime industry.

16. The kit according to claim 1, wherein the pH of the binder mixture is pH 2.5 to pH 2.7.

17. The kit according to claim 1, wherein the weight fraction of the phosphate binder in the binder mixture, as expressed by the cumulative weight of phosphoric acid and phosphoric acid anions without counter-ions, is between 10 and 20%.

18. The kit according to claim 1, wherein the powder mixture comprises a halide of a metal contained as the diffusion metal.

19. The method according to claim 12, wherein steps (ii) and (iii) of application and drying are repeated two to four times, to obtain a multilayer binder matrix on the metal surface.

20. The method according to claim 12, wherein step (iii) of drying comprises treating for 10-30 minutes at a temperature from 100 C. to 150 C.

Description

[0038] Further details and advantages of the invention will become apparent from the examples described in the following, with reference to the figures. The figures show:

[0039] FIG. 1: a schematic representation of a surface alumination using a slurry suspension prepared with a kit according to the invention; and

[0040] FIG. 2: a schematic representation of a surface alumination using a Cr(VI)-free slurry suspension based on prior art organic binders.

EXAMPLE 1: PREPARATION OF A LOW-VISCOSITY BINDER MIXTURE

[0041] In one embodiment, a low-viscosity binder mixture is obtained by mixing the following components.

TABLE-US-00001 Component Preferred range General range Water 60-80 wt. % 50-90 wt. % Phosphoric acid 15-30 wt. % 10-40 wt. % Aluminum hydroxide 3-9 wt. % 1-10 wt. % Magnesium hydroxide 1-3 wt. % 1-5 wt. % Zinc oxide 1-4 wt. % 1-5 wt. % Additionally: LiOH, NaOH, KOH, <1 wt. % <2 wt. % Ca(OH).sub.2, Ba(OH).sub.2 pH 2.3-2.6 2.3-2.9

EXAMPLE 2: PREPARATION OF A VISCOUS BINDER MIXTURE

[0042] In one embodiment, a viscous binder mixture is obtained by mixing the following components.

TABLE-US-00002 Component Preferred range General range Water 40-60 wt. % 30-70 wt. % Phosphoric acid 30-40 wt. % 20-50 wt. % Aluminum hydroxide 5-15 wt. % 1-20 wt. % Magnesium hydroxide 1-3 wt. % 1-5 wt. % Zinc oxide 2-5 wt. % 1-5 wt. % Additionally: LiOH, NaOH, KOH, <1 wt. % <2 wt. % Ca(OH).sub.2, Ba(OH).sub.2 pH 2.3-2.6 2.3-2.9

EXAMPLE 3: PREPARATION OF A POWDER MIXTURE FOR ALUMINATION

[0043] In one embodiment, a powder mixture for alumination is obtained by mixing the following components

TABLE-US-00003 Aluminum powder with/without SiO.sub.2 80-100 wt. % Silicon powder 0-15 wt. % AlF.sub.3x3H.sub.2O (as required)/AlF.sub.3 0-5 wt. %

EXAMPLE 4: PREPARATION OF A POWDER MIXTURE FOR CHROMATION

[0044] In one embodiment, a powder mixture for chromation is obtained by mixing the following components.

TABLE-US-00004 Chromium powder 95-100 wt. % CrCl.sub.3 0-5 wt. %

EXAMPLE 5A: PREPARATION OF A POWDER MIXTURE FOR CHROMIUM ALUMINATION

[0045] In one embodiment, a powder mixture for chromium alumination is obtained by mixing the following components.

TABLE-US-00005 CrAl alloy e.g. CrAl 50/50 95-100 wt. % CrCl.sub.3/AlF.sub.3 x3H.sub.2O/AlF.sub.3 0-5 wt. %

EXAMPLE 5B: PREPARATION OF A POWDER MIXTURE FOR CHROMIUM ALUMINATION

[0046] In one embodiment, an alternative powder mixture for chromium alumination is obtained by mixing the following components.

TABLE-US-00006 Chromium powder 35-60 wt. % Aluminum powder 35-60 wt. % CrCl.sub.3/AlF.sub.3 x3H.sub.2O/AlF.sub.3 0-5 wt. %

EXAMPLE 6A: PREPARATION OF A POWDER MIXTURE FOR NICKEL ALUMINATION

[0047] In one embodiment, a powder mixture for nickel alumination is obtained by mixing the following components.

TABLE-US-00007 NiAl alloy e.g. NiAl 50/50 95-100 wt. % NiCl.sub.3/AlF.sub.3 x3H.sub.2O/AlF.sub.3 0-5 wt. %

EXAMPLE 6B: PREPARATION OF A POWDER MIXTURE FOR NICKEL ALUMINATION

[0048] In one embodiment, an alternative powder mixture for nickel alumination is obtained by mixing the following components.

TABLE-US-00008 Nickel powder 35-60 wt. % Aluminum powder 35-60 wt. % NiCl.sub.3/AlF.sub.3 x3H.sub.2O/AlF.sub.3 0-5 wt. %

EXAMPLE 7: SLURRY SUSPENSION FOR ALUMINATION

[0049] In one embodiment, a slurry suspension for alumination is obtained by mixing the following two components of a kit.

TABLE-US-00009 Component Preferred range General range Binder mixture Example 1 50-60 wt. % 40-70 wt. % Powder mixture Example 3 40-50 wt. % 30-60 wt. %

EXAMPLE 8: SLURRY SUSPENSION FOR CHROMATION

[0050] In one embodiment, a slurry suspension for chromation is obtained by mixing the following two components of a kit.

TABLE-US-00010 Component Preferred range General range Binder mixture Example 2 40-50 wt. % 30-60 wt. % Powder mixture Example 4 50-60 wt. % 40-70 wt. %

EXAMPLE 9: SLURRY SUSPENSION FOR CHROMIUM ALUMINATION

[0051] In one embodiment, a slurry suspension for chromium alumination is obtained by mixing the following two components of a kit.

TABLE-US-00011 Binder mixture Example 2 40-50 wt. % Powder mixture Example 5a or 5b 50-60 wt. %

EXAMPLE 10: SLURRY SUSPENSION FOR NICKEL ALUMINATION

[0052] In one embodiment, a slurry suspension for nickel alumination is obtained by mixing the following two components of a kit.

TABLE-US-00012 Binder mixture Example 2 40-50 wt. % Powder mixture Example 6a or 6b 50-60 wt. %

APPLICATION EXAMPLE 11: FORMATION OF A DIFFUSION LAYER (ALUMINATION)

[0053] The product of Example 7 is formed by preparing and then mixing the binder mixture and the powder mixture, by shaking or stirring.

[0054] The slurry suspension thus obtained is sprayed onto a degreased and corundum blasted (120 to 220 mesh) base material MAR M247 with a spray gun (nozzle diameter 0.8-1.0 mm; 1.5-2.0 bar) and dried. The process is repeated 1-2 times, depending on the intended layer thickness of the green body, and then treated at 120 C. for 30 min. Green layer thicknesses of 100-200 m yield diffusion layer thicknesses of 30-80 m.

[0055] Diffusion treatment is carried out in an oven under argon or hydrogen atmosphere at 880 C. and 4h holding time. After cool down, an ash layer can be removed by blasting with glass beads. Homogeneous diffusion layers with a thickness of 70 m were obtained from green layers with 150 m thickness.

[0056] The result is shown schematically in FIG. 1, where reference sign 100 denotes the base material, reference sign 210 denotes the green body and reference sign 220 denotes the diffusion layer.

[0057] Metallurgical analysis of a cross section by EDX in SEM showed a mass ratio of 25%/7% aluminum to silicium in the diffusion layer.

COMPARATIVE EXAMPLE 12: FORMATION OF A DIFFUSION LAYER (ALUMINATION)

[0058] Prior art Cr(VI)-free slurry suspensions based on an organic binder do not typically yield homogeneous diffusion layers, but interrupted layers with defects like holes. In addition, depletion and the formation of droplets (aluminum beads) occurs. These effects are attributable to the absence of a phosphate matrix in the organic system.

[0059] FIG. 2 is a schematic illustration of such diffusion layers, with reference sign 100 denoting the base material, reference sign 310 denoting the green body and reference sign 320 denoting the droplets.

APPLICATION EXAMPLE 13: FORMATION OF A DIFFUSION LAYER (CHROMATION)

[0060] The product of Example 8 is formed by preparing and then mixing the binder mixture and the powder mixture, by shaking or stirring.

[0061] The slurry suspension thus obtained is sprayed onto a degreased and corundum blasted (120 to 220 mesh) base material MAR M247 with a spray gun (nozzle diameter 0.8-1.0 mm; 1.5-2.0 bar) and dried. The process is repeated 1-2 times, depending on the intended layer thickness of the green body, and then treated at 120 C. for 30 min. Green layer thicknesses of 100-150 m yield diffusion layer thicknesses of 30-70 m.

[0062] Diffusion treatment is carried out in an oven under argon or hydrogen atmosphere at 1000-1150 C. and 4-10 hours holding time. The reaction rate and the final thickness of the diffusion layer can be significantly increased by adding catalysts such as NH.sub.4Cl, NH.sub.4F or AlF.sub.3.

[0063] After cool down, an ash layer can be removed by blasting with glass beads.

[0064] Homogeneous diffusion layers with a thickness of 25 m were obtained from green layers with 50 m thickness.

[0065] Metallurgical analysis of a cross section by EDX in SEM showed an increase of chromium content in the diffusion layer up to 60-70%.