Chromium(VI)-free slurry 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 chro- mium(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.

Description

CHROMIUM(VI)-FREE SLURRY FOR DIFFUSION COATING FIELD OF THE INVENTION The present application relates to a storable slurry composition for diffusion coating of metals. BACKGROUND Diffusion coating of metal components serves to protect the surface of the compo- nents against environmental influences, in particular against high-temperature corro- sion. In diffusion coating, an oxide-forming diffusion metal is applied to the surface of the metal component and the surface is then heat-treated to facilitate diffusion of the diffusion metal into near-surface layers of the component material. Aluminum is the most commonly used oxide-forming diffusion metal in such applications, but depend- ing on the substrate material, also other oxide-forming metals such as nickel, chro- mium, manganese, germanium, silicon, magnesium, tin, titanium and zinc, or corre- sponding metal alloys such as chromium-aluminum alloys or mixtures may be used. When using alloys or mixtures, several oxide formers are present in the diffusion layer. There are various methods for applying the diffusion metal to the substrate surface. The most widely used are a so-called pack cementation method, in which the diffu- sion metal is deposited on the substrate surface from a gas phase, and a so-called slurry method. The slurry method, which is the one of interest herein, comprises the application of a liquid suspension of a metal powder, possibly comprising a mixture of several metals or alloys, as diffusion metal and a binder, a so-called slurry, to the substrate surface. Application can be, for example, by means of brushing, spraying, dipping or film cast- ing. The application is followed by a drying step to remove solvent from the suspen- sion, and a thermal diffusion process. The slurry can be applied several times to achieve a certain dry film thickness. The binder acts to fix the diffusion metal particles on the substrate after application of the slurry. Acidic phosphate binders containing chromium(VI) were frequently used in commercially available slurries. They form a polyphosphate network, in which the diffusion metal particles are immobilized on the substrate surface. Chromium(VI)- containing materials are advantageous because they passivate the oxidation-sensi- tive diffusion metal and possibly substrate surface and thus protect it from etching attack by the acidic phosphate binder, and because they promote formation of a ce- ramic-like and temperature-stable polyphosphate network on the substrate surface. Due to their carcinogenic properties, however, the use of chromium(VI) salts should be avoided. Using other highly oxidized heavy metal salts as an alternative is likewise undesirable for occupational safety and environmental protection reasons. Since in the absence of the chromium(VI) salts or comparable highly oxidized heavy metal salts, the acidic phosphate binder may attack the oxidation-sensitive diffusion metal and possibly substrate surface, various alternative formulations have been de- veloped in the prior art. They comprise commercially available compositions based on organic polymer binders, but these show significant disadvantages compared to phosphate binders due to their lack of heat stability. The organic binders burn when exposed to heat, exposing the metal particles and causing them to evaporate or melt and run. Organic solvent-based systems are another known alternative, which is like- wise linked to a number of problems. Specifically, in a multi-layer application they dissolve the lower layers and drying properties are suboptimal, such that they only allow for application of very thin layers and application has to be repeated often to achieve a reasonable layer thicknesses. In addition, organic solvents are undesirable for environmental protection and occupational safety reasons. SUMMARY OF EMBODIMENTS The problem to be solved by the invention is providing a slurry for diffusion coating, which fulfils health and environmental protection requirements and has excellent properties. Against this background, the invention relates to a kit for the preparation of a chro- mium(IV)-free slurry suspension for the diffusion coating of metal surfaces, the kit comprising, as separate components, a powder mixture and a binder mixture, which are configured to be combined to prepare 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. Hence, the invention solves the technical problem by providing a system comprising at least two components, one component being a binder mixture, and the other com- ponent being a powder mixture adapted for a particular application. To obtain the slurry suspension for application, the powder mixture and the binder mixture are mixed and homogenized. The resulting slurry suspension comprises the aqueous solvent and suspended diffusion metal powder and phosphate binder. It is free of chromium(VI) salts, which are classified as a Substance of Very High Concern (SVHC) under the REACH Regulation. The product is provided as a multi-component system, in particular a two-component system, and is thus stable for storage despite the inherent reactivity of the acidic phosphate binder vis-à-vis oxidation-sensitive diffusion metal powders under chro- mium(VI)-free conditions. Hydrogen gas does not form and any explosion hazard is avoided. The powder mixture is a dry mixture, i.e., is preferably in the form of a dry powder. The binder mixture is a liquid mixture. The ceramic layer formed by the phosphate binders immobilizes the diffusion metal particles on the substrate surface and is stable enough to withstand certain mechan- ical impact, for example during transport. This stands in contrast to prior art ap- proaches using organic solvents or polymer binders, for example polyurethane bind- ers, as explained further above. In the diffusion process, the glassy ceramic binder matrix causes molten metal not to run and form droplets or evaporate, thus reducing the risk of inhomogeneous film thicknesses. The glassy ceramic matrix also allows for an addition of catalysts, which, like the metals, are trapped during heating and do not evaporate immediately, but actually accelerate the reaction. The ceramic matrix ultimately becomes porous over depletion of the metal powders and can then be removed mechanically, for example by sandblasting, after the surface has cooled down. No chromium(VI)-containing dust is formed due to the absence of chro- mium(VI) salts. The slurry suspension is water-based and free of volatile organic solvents, thus elim- inating health and explosion hazards and a need for cumbersome precautions such as exhaust systems or forced-air ovens during drying. The water-based approach allows the slurry to air-dry quickly on the substrate surface without running. After drying or an initial low temperature baking, another layer can be applied to increase thickness of the coating. The ceramic structure formed by the phosphate binders is not changed or dissolved upon application of further layers and does not run. Since the drying behaviour is good, thicker coatings can be formed more quickly when com- pared to known solvent-based systems, such that layer thicknesses as required can be achieved with just two to four and, in particular, two to three iterations instead of, for example, seven iterations. This enables significant cost savings and throughput increase in coating formation. The composition and pH of the binder mixture are such that no immediate etching of the diffusion metal powders, even after combination with the powder mixture. The pH of the binder mixture is pH 2.3 to pH 2.9, in particular pH 2.5 to pH 2.7. In this range, for example, finely divided aluminum powder is stable for several hours to days. The solvent of the binder mixture pure water or an aqueous solvent, for example an aqueous alcohol mixture with a water content of at least 80% by volume. The solvent or water content of the binder mixture is 40 to 80 wt.%, measured against the total weight of the binder mixture. The solvent content does not only influence the reactivity of the binder mixture to the diffusion metal powders, but can be used to adjust the viscosity of the finished slurry. The phosphate binder of the binder mixture comprises acidic monohydrogen or dihy- drogen phosphates of at least cation selected from the group of aluminum, magne- sium or zinc. It can be prepared from phosphoric acid and aluminum, magnesium and / or zinc salts or from raw materials such as Al(H2PO4)3, Zn(H2PO4)3 and Mg(H2PO4)3. It is particularly preferred that the phosphate binder comprises propor- tions greater than 10-50%, or preferably 20-30%, of Al[H2PO4]3. In an example, two or three different ones of the cations identified above may be present in the phos- phate binder as counter-ions of the acidic phosphates. The cations stabilize the mix- ture against gel formation. The relative proportion of phosphoric acid, acidic mono- hydrogen or dihydrogen phosphates, and basic phosphate determines the pH. The proportion of phosphate binder within the binder mixture, in terms of cumulative weight fractions of phosphoric acid or phosphoric acid anions without counter-ions, is preferably between 5 and 40%, further preferably between 10 and 30%. The diffusion metal is a metal selected from the group consisting of aluminum, nickel, chromium, or silicon, or a metal alloy selected from the group consisting of aluminum- nickel-alloys or aluminum-chromium-alloys. Using untreated metal powders can be preferred. The 2-component-approach of the present invention enables a good shelf-life even without Cr(VI) and without expensive modification of the metal powders by methods like coating, for example. In the given context, untreated means that the metal powders are simply present as metal pow- ders and have not undergone any treatment to, for example, coat them. An aluminum powder will, of course, have an oxide layer on the exposed surface, but this layer will have simply formed upon handling in air, without any special treatment. In embodi- ments of the invention, mixtures of aluminum powder and silicon powder can be used, and at least one and preferably all of the powder types in the mixture may be un- treated. Alternatively, coated powders can be used to further inhibit acidic attack, for example powders coated with silicon oxide <semantics>SiOx<annotation encoding="application / x-tex">SiO_x< / annotation>< / semantics> (x=1-2). To carry out an alumination, for example (whereby the term is used herein shall also encompass alonization and alitization), the powder mixture contains metallic alumi- num powder, which can either be untreated or stabilized by a, for example, SiO2 coating. The powder mixture may additionally contain metallic silicon powder. Mix- tures of aluminum and silicon powders may be preferred in one embodiment. To carry out a nickel alumination, compared to the mixture just described, a part of the aluminum powder can be replaced by nickel powder, or a powder of a nickel- aluminum alloy such as NiAl3 or Ni-Al 95-5 can be used instead of or in addition to the aluminum powder. To carry out a chromium alumination, compared to the mixture just described, a part of the aluminum powder can be replaced by chromium powder, or a powder of a chromium-aluminum alloy can be used instead of or in addition to the aluminum pow- der. To carry out a chromation, the powder mixture contains metallic chromium powder, which can either be untreated or stabilized by, for example, an SiO2 coating. Preferably, the powder mixture comprises a catalyst, which can be water-insoluble and finely distributed in order to be enclosed in the binder matrix during heating like the metal powders, such as to not evaporate but actually accelerate the diffusion process. Preferably, the catalyst is a water-insoluble halogen salt of a metal that is used as a diffusion metal. Examples include AIF3, AIF3x3H2O and CrCl3, with AIF3 or AlF3x3H2O for use in alumination or modified alumination, i.e. if at least some of the diffusion metal powder is aluminum or an aluminum-containing alloy, and with CrCl3 for use in chromation or modified chromation, i.e. of at least some of the diffusion metal powder is chromium or a chromium-containing alloy. In one embodiment, the powder mixture further comprises a preferably water-insolu- ble dye or pigment for coloring. Examples comprise blue cobalt aluminate spinel. To increase corrosion resistance, the powder mixture can further comprise inert pow- ders such as Al2O3 The binder mixture or powder mixture can further comprise an anti-settling agent, a thixotropic agent, a thickener, or mixtures thereof. In use, a first step may comprise combining the powder mixture and the binder mix- ture and homogenizing the resulting mixture to obtain a slurry suspension. The slurry suspension can then be applied to the substrate surface, with precautions to avoid running. After air drying, another layer can be applied and again dried, to iteratively achieve sufficient green body thicknesses to obtain thick diffusion coatings. Prior to the actual diffusion process, the coating can be treated for about 5-60 minutes, especially for 10-30 minutes at a temperature greater than 50°C, preferably 100°C to 150°C. For example, a temperature of about 120°C can be applied. Depending on the substrate material and diffusion metal, the actual diffusion process is typically carried out at temperatures greater than 500°C, preferably between 880°C to 1150°C, for a holding time of several hours. Process gases such as argon or hy- drogen can advantageously be used at a low purge rate. To accelerate the diffusion reaction, catalysts such as those mentioned above, for example NH4F, NH4Cl, AlF3 or AIF3X3H2O can be added to the reactor. Optionally, an endpoint for the reaction can be determined, for example by optical evaluation of transverse edges or EDX analysis in a scanning electron microscope. In accordance with an aspect of at least one embodiment, there is provided 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 is a dry powder and comprises a diffusion metal powder, 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 alumi- num-nickel and aluminum-chromium alloys; wherein the binder mixture comprises an aqueous solvent and a phosphate binder, wherein the phosphate binder contains hydrogen or di-hydrogen phosphates of at least one cation of the group aluminum, zinc or magnesium, wherein the solvent of the binder mixture is water or an aque- ous solvent having a water content of at least 80% by volume, wherein the solvent content of the binder mixture, relative to its total weight, is from 40 to 80% by weight, wherein the weight fraction of the phosphate binder in the binder mixture, as ex- pressed by the cumulative weight of phosphoric acid and phosphoric acid anions without counter-ions, is between 5 and 40%, and wherein the pH of the binder mixture is pH 2.3 to pH 2.9; and wherein both the powder mixture and the binder mixture are free of chromium(VI) salts. In accordance with an aspect of at least one embodiment, there is provided a method of diffusion coating a metal surface, comprising the steps of: (i) combining the powder mixture and the binder mixture of a kit as described in the preceding paragraph 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 sol- vent 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. Preferred applications of the kit according to the invention or the slurry suspensions generated therefrom include an application to metallic components for the aerospace industry, the energy industry, the automotive industry, the oil industry, the metal pro- cessing industry and the maritime industry, for increasing their corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS Further details and advantages of the invention will become apparent from the ex- amples described in the following, with reference to the figures. The figures show: Figure 1: a schematic representation of a surface alumination using a slurry sus- pension prepared with a kit according to the invention; and Figure 2: a schematic representation of a surface alumination using a Cr(VI)-free slurry suspension based on prior art organic binders. DETAILED DESCRIPTION OF THE DRAWINGS Example 1: Preparation of a low-viscosity binder mixture In one embodiment, a low-viscosity binder mixture is obtained by mixing the following components. [Image disponible dans le document PDF, Image available in the PDF document] Example 2: Preparation of a viscous binder mixture In one embodiment, a viscous binder mixture is obtained by mixing the following components. [Image disponible dans le document PDF, Image available in the PDF document] Example 3: Preparation of a powder mixture for alumination In one embodiment, a powder mixture for alumination is obtained by mixing the fol- lowing components [Image disponible dans le document PDF, Image available in the PDF document] Example 4: Preparation of a powder mixture for chromation In one embodiment, a powder mixture for chromation is obtained by mixing the fol- lowing components. [Image disponible dans le document PDF, Image available in the PDF document] Example 5a: Preparation of a powder mixture for chromium alumination In one embodiment, a powder mixture for chromium alumination is obtained by mix- ing the following components. [Image disponible dans le document PDF, Image available in the PDF document] Example 5b: Preparation of a powder mixture for chromium alumination In one embodiment, an alternative powder mixture for chromium alumination is ob- tained by mixing the following components. [Image disponible dans le document PDF, Image available in the PDF document] Example 6a: Preparation of a powder mixture for nickel alumination In one embodiment, a powder mixture for nickel alumination is obtained by mixing the following components. [Image disponible dans le document PDF, Image available in the PDF document] Example 6b: Preparation of a powder mixture for nickel alumination In one embodiment, an alternative powder mixture for nickel alumination is obtained by mixing the following components. [Image disponible dans le document PDF, Image available in the PDF document] Example 7: Slurry suspension for alumination In one embodiment, a slurry suspension for alumination is obtained by mixing the following two components of a kit. [Image disponible dans le document PDF, Image available in the PDF document] Example 8: Slurry suspension for chromation In one embodiment, a slurry suspension for chromation is obtained by mixing the following two components of a kit. [Image disponible dans le document PDF, Image available in the PDF document] Example 9: Slurry suspension for chromium alumination In one embodiment, a slurry suspension for chromium alumination is obtained by mixing the following two components of a kit. [Image disponible dans le document PDF, Image available in the PDF document] Example 10: Slurry suspension for nickel alumination In one embodiment, a slurry suspension for nickel alumination is obtained by mixing the following two components of a kit. [Image disponible dans le document PDF, Image available in the PDF document] Application example 11: Formation of a diffusion layer (alumination) The product of Example 7 is formed by preparing and then mixing the binder mixture and the powder mixture, by shaking or stirring. 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 diam- eter 0.8-1.0 mm; 1.5-2.0 bar) and dried. The process is repeated 1-2 times, depend- ing on the intended layer thickness of the green body, and then treated at 120°C for 30 min. Green layer thicknesses of 100 - 200 µm yield diffusion layer thicknesses of 30 - 80 μm. 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 µm were ob- tained from green layers with 150 µm thickness. The result is shown schematically in Figure 1, where reference sign 100 denotes the base material, reference sign 210 denotes the green body and reference sign 220 denotes the diffusion layer. 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) 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. Figure 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 refer- ence sign 320 denoting the droplets. Application example 13: Formation of a diffusion layer (chromation) The product of Example 8 is formed by preparing and then mixing the binder mixture and the powder mixture, by shaking or stirring. 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 diam- eter 0.8-1.0 mm; 1.5-2.0 bar) and dried. The process is repeated 1-2 times, depend- ing on the intended layer thickness of the green body, and then treated at 120°C for 30 min. Green layer thicknesses of 100 - 150 µm yield diffusion layer thicknesses of 30 - 70 μm. 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 NH4Cl, NH4F or AlF3. After cool down, an ash layer can be removed by blasting with glass beads. Homogeneous diffusion layers with a thickness of 25 µm were obtained from green layers with 50 µm thickness. Metallurgical analysis of a cross section by EDX in SEM showed an increase of chro- mium content in the diffusion layer up to 60-70%.

Claims

1. A kit for preparing a chromium(VI)-free slurry suspension for the diffusion coat- ing 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 is a dry powder and comprises a diffusion metal powder, 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; wherein the binder mixture comprises an aqueous solvent and a phos- phate binder, wherein the phosphate binder contains hydrogen or dihydrogen phosphates of at least one cation of the group aluminum, zinc or magnesium, wherein the solvent of the binder mixture is water or an aqueous solvent hav- ing a water content of at least 80% by volume, wherein the solvent content of the binder mixture, relative to its total weight, is from 40 to 80% by weight, 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 40%, and wherein the pH of the binder mixture is pH 2.3 to pH 2.9; and wherein both the powder mixture and the binder mixture are free of chro- mium(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 2 or 3, wherein the pH of the binder mixture is pH4. The kit according to any one of claims 1 to 3, 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 30%.

5. The kit according to any one of claims 1 to 4, wherein the diffusion metal of the powder mixture is untreated.

6. The kit according to any one of claims 1 to 5, wherein the powder mixture comprises a water-insoluble catalyst7. The kit according to claim 6, wherein the water-insoluble catalyst is a halide of a metal contained as diffusion metal.

8. The kit according to any one of claims 1 to 7, wherein the powder mixture comprises a water-insoluble dye.

9. 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 any one of claims 1 to 8 to form a chromium(VI)-free slurry suspen- sion; (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 dis- persed therein; and (iv) heating the binder matrix coated metal surface to temperatures of greater than 500°C.

10. The method according to claim 9, wherein steps (ii) and (iii) of application and drying are repeated a plurality of times, to obtain a multilayer binder matrix on the metal surface.

11. The method according to claim 9, 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.

12. The method according to claim 9, wherein steps (ii) and (iii) of application and drying are repeated two to three times, to obtain a multilayer binder matrix on the metal surface.

13. The method according to any one of claims 9 to 12, wherein step (iii) of drying comprises treating for 10-30 minutes at a temperature greater than 50°C14. The method according to any one of claims 9 to 12, wherein step (iii) of drying comprises treating for 10-30 minutes at a temperature between 100°C and 150°C.

15. Use of a kit according to any one of claims 1 to 8 in the aerospace industry, the energy industry, the automotive industry, the oil industry, the metalworking industry or the maritime industry.