EP4051824A1

EP4051824A1 - Method for depositing a coating from an improved composition suspension

Info

Publication number: EP4051824A1
Application number: EP20789635.8A
Authority: EP
Inventors: Sébastien DOUBLET; Nicolas RAMENATTE; Stéphane MATHIEU; Michel VILASI; Léo PORTEBOIS
Original assignee: Centre National de la Recherche Scientifique CNRS; Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Universite de Lorraine
Current assignee: Centre National de la Recherche Scientifique CNRS; Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Universite de Lorraine
Priority date: 2019-10-28
Filing date: 2020-10-16
Publication date: 2022-09-07
Also published as: FR3102490A1; WO2021083694A1; US20220396885A1; FR3102490B1

Abstract

The invention concerns a method for depositing a coating on at least one part of a metal substrate, said method comprises the following successive steps: a) mixing a liquid and a powder comprising particles of a metal material to be deposited and particles of a stripping agent in order to prepare a suspension, b) applying the suspension prepared in step a) on at least one part of the surfaces of the metal substrate so as to obtain a substrate/suspension assembly, c) carrying out a heat treatment of the substrate/suspension assembly comprising heating to a first temperature of at least 500°C and heating to a second temperature greater than the first temperature, and d) recovering a substrate at least partially coated with a coating, the stripping agent being a halogenated compound and the suspension comprising 10% to 60% by weight of the stripping agent relative to the total mass of the powder.

Description

Process for depositing a coating from a suspension of improved composition The present invention relates to the production of a protective coating on a substrate, said coating being in particular suitable and intended to protect at least part of the surface of the substrate against corrosion.

The invention applies in particular to the deposition of a coating on a substrate having one or more difficult to access cavities. Thus, the substrate can be at least part of a heat exchanger-reactor, the cavities being formed by millimeter channels intended for the circulation of the various fluids within the heat exchanger-reactor.

The invention can also be applied to the deposition of a coating on parts intended for the aeronautical industry, such as parts of turbines, in particular parts comprising cooling channels of millimeter size and / or areas difficult to clean. 'access.

In a context of intensification of industrial processes used by the petroleum and chemical industries, the engineers concerned seek to improve the efficiency of the units they design. In order to increase efficiency, the approach followed during the development of new production units is to replace, when possible, exchangers and reactors by apparatus of the structured exchanger or exchanger-reactor type.

These exchangers or exchangers - reactors make it possible to have much more efficient mass and heat transfers due to their high volume to surface area ratio. In addition, they allow good thermal and structural stability and finer control of the operating conditions of the process leading to a higher production yield. These devices usually consist of assembled grooved plates, constituting channels of millimeter size.

The latest technological advances relating to the manufacture of metal parts by additive processes make it possible to design high efficiency reactor-exchangers while minimizing their dimensions. One of the additive manufacturing techniques, also called 3D printing, is powder bed fusion which gives the material larger exchange surfaces resulting from high surface roughness. The developed surface is approximately 1.4 times greater than that obtained by so-called conventional manufacturing processes, thus accentuating heat exchange. When this manufacturing technology is used, a heat post-treatment operation is often undertaken in order to permanently consolidate the part and stabilize the microstructure. This revolves around a heat treatment under pressure of inert gas at high temperature. The low oxygen pressure present in the inert gas is still sufficient to form an oxide layer of a few microns on the surface of the parts. In the processes according to the prior art, this oxide layer forms an interdiffusion barrier between the metallic material to be deposited and the material constituting the substrate to be coated, preventing the deposition of protective coatings without a prior stripping step for the cleaning processes. conventional deposits.

In the case of exchangers or reactor-exchangers used in processes for cracking or reforming of hydrocarbons, such as, for example, the process of steam methane reforming used for the industrial production of synthesis gas, the walls of the Channels of the exchangers or exchanger-reactors must be covered with a protective coating so as to block the catastrophic effects of the significant carbon activity which prevails within these processes. These conditions are at the origin of the so-called “Metal Dusting” phenomena, characterized by a disintegration of the alloys into metal and graphite particles, thus leading to the ruin of these exchangers or exchanger-reactors if they are not protected.

The protective coatings used can be based on aluminum or chromium so as to develop stable oxide layers of the alpha-AhC or Cr ₂ Û _{3 type} protective. In general, the aluminum or chromium is deposited in the gas phase, an operation which is also referred to as aluminization or chromium in the gas phase. The aluminum or chromium oxide is then obtained either by a specific oxidation step in an oven under controlled partial pressure of oxygen or not, or by directly using the exchanger or exchanger-reactor under an oxidizing atmosphere.

Vapor phase deposition techniques are known using a gaseous precursor of the coating to be produced. This precursor can be produced in direct proximity to the surface to be coated, as is the case with the case-case cementation process, also called pack-cementation or “in-pack” cementation, or be transported via a carrier gas on the tank. surface to be coated, as is the case with “out-of-pack” cementation processes which can be operated in natural convection or with forcing the passage of the carrier gas into hollow cavities to be protected. The main difficulties encountered for case cementation are linked to the filling of parts having a complex geometry or very small dimensions (a few mm) with the cement powder (precursor mixture of the coating). The main limitations of the techniques using gaseous precursors relate to the rapid depletion of the gaseous mixture of reactive species resulting in heterogeneities in chemical composition and / or in thickness of the coating. It remains very difficult to obtain a homogeneous coating over long lengths of hollow cavities or on surfaces that are difficult to access.

Liquid phase deposition techniques are also known based on the use of suspensions containing reagents mixed in a liquid phase called solvent. These techniques implement an operating protocol including the preparation of the suspension, its application and heat treatment.

In addition to difficulties in being injected into small cavities with complex geometries, certain known suspensions involve constituents incompatible with the European regulation concerning the registration, evaluation and authorization of chemical substances, as well as the applicable restrictions. to these substances (regulation known as “REACH” EC 1907/2006), such as chromates.

Other existing suspensions are complex in formulation or involve additional substrate preparation steps. Thus, document EP-A-2956565 discloses a process for depositing a coating requiring, before applying the suspension, a step of stripping the surface of the substrate to be coated and, after the heat treatment of the substrate assembly. suspension, a step of removing pulverulent residues. As for document EP-A-3049545, it discloses a process using a suspension comprising an agent for protecting the powder of the metal to be deposited, a diluent and at least one additive which promotes wetting of the surface to be coated. Following the application of the suspension, this process requires steps to remove the solvent and organic compounds used to suspend the powders.

Besides their complexity of implementation or formulation, these formulations are not entirely satisfactory, in particular on certain parts of complex geometry and / or having cavities with large aspect ratios of length / width or length / depth and / or a high surface roughness, for which the parts may occasionally exhibit absence of coatings or heterogeneity of coating thickness. The object of the present invention is to resolve all or part of the above-mentioned problems, in particular by proposing an improved process for depositing a coating on a substrate allowing the production of a coating of more homogeneous thickness with a rate of greater coverage, on surfaces with high roughness, exhibiting oxide or unfused type defects and / or difficult to access, in particular on substrates having at least one cavity of characteristic size of the order of a millimeter and length / length ratios high width and / or length / depth, and the industrial implementation of which is greatly simplified compared to the prior art.

The solution according to the invention is then a method for depositing a coating on at least part of a metal substrate, said method comprising the following successive steps: a) mixing of a liquid and a powder comprising particles of a metallic material to be deposited and particles of a stripping agent so as to prepare a suspension, b) application of the suspension prepared in step a) on at least part of the surfaces of the metallic substrate so as to obtain a substrate-suspension assembly, c) performing a heat treatment of the substrate-suspension assembly comprising heating to a first temperature of at least 500 ° C and heating to a second temperature above the first temperature, and d) recovery of a substrate coated at least in part with a coating, said stripping agent being a halogenated compound and the suspension comprising from 10% to 60% by mass of said stripping agent relative to the total mass of the powder.

The use of a suspension containing a halogenated compound in the content according to the invention makes it possible to dispense with the pickling step which usually precedes the application of the suspension on the substrate to be coated and to obtain complete coverage. or almost complete of the surface to be coated, typically a degree of coverage greater than 99%, preferably equal to 100%, and this without it being necessary to remove the residues before recovering the coated substrate.

The use of a halogenated compound as a stripping agent makes it possible to destabilize in situ the oxides and / or the organic compounds, such as fats, oils, adjuvants, liquid residues, present at the surface of the substrate. Thus, during the rise in temperature carried out during the heat treatment of the substrate assembly - suspension, the stripping agent changes from the solid state to the liquid state, which makes it possible to dissolve the surface oxides by the molten halogenated salts. When the pickling agent changes to the gaseous state, it makes it possible to degrade the surface oxides by means of transport in the gas phase. Etching is thus carried out in situ during the heat treatment of the substrate, and not prior to the latter, which allows the surfaces to be coated to be reactive regardless of their initial surface state.

In addition, in the gaseous state, the halides contained in the pickling agent react with the powder of the metal or of the alloy to be deposited and create a vapor phase which, by means of diffusion mechanisms in the gas phase, ensures a rate of higher coverage and better accessibility to areas of complex geometry and / or high aspect ratios. The stripping agent thus acts as a transport agent.

The content of the stripping agent in the powder, which can also be expressed in the form of the metallic material / stripping agent mass ratio, is also determined so that all of the compounds introduced into the suspension are used by the process, in particular via the formation of gaseous species during the heat treatment. In other words, by using the halogenated compound in the contents according to the invention, it is possible to adjust the quantity of reactants involved in relation to the stoichiometry of the chemical reaction with the oxides and / or the organic compounds initially present at the surface of the product. substrate. This formulation thus makes it possible to obtain a total transformation of all the constituents of the suspension, without any powdery residues remaining after the heat treatment of the substrate. The coated part can thus be put into service immediately after application of the coating.

Depending on the case, the invention may include one or more of the following characteristics:

- The suspension comprises from 20% to 30% by mass of said stripping agent relative to the total mass of the powder.

- the powder contains only particles of the metallic material and particles of the stripping agent.

- the powder is free of any compound or binding agent.

- the particles of the metallic material have a first average equivalent diameter and the particles of the stripping agent have a second average equivalent diameter, the ratio between the first average equivalent diameter and the second average equivalent diameter being between 1, 2 and 10, preferably between 3.5 and 7.5. the particles of the metallic material have a first average equivalent diameter of between 7 and 15 μm and / or the particles of the stripping agent have a second average equivalent diameter of between 2 and 6 μm.

the particles of the metallic material have a first particle size distribution centered around the first average equivalent diameter and the particles of the stripping agent have a second particle size distribution centered around the second average equivalent diameter, said first particle size distribution and / or said second particle size distribution exhibiting a dispersion index of less than 1.2, preferably less than 1.13, more preferably less than 0.9.

- the particles of the suspension have an average sphericity factor of at least 0.3, preferably at least 0.5.

the metal substrate has at least one cavity of equivalent diameter e _m m <2 mm and / or a length / width ratio greater than 150, the particles of the suspension each having an equivalent diameter less than e _m m / 10.

- the coating obtained in step d) has an average thickness of between 15 and 200 μm, preferably between 30 and 60 μm, and variations in thickness of less than 10% in relative standard deviation, preferably of less than 5 %, the relative standard deviation being defined as the ratio between the standard deviation of the thickness and the average thickness.

- the mass ratio between the powder and the liquid is between 1, 5 and 3.

- the stripping agent is a halogenated compound of which the halogen is fluorine or chlorine.

- the stripping agent belongs to the system of fluorinated compounds, preferably said stripping agent is chosen from the group consisting of KxAIF _y , NFUF, MgF2, and CaF2, preferably with x ranging between 1 and 3 and including between 4 and 6 .

- the liquid includes alcohol, preferably the liquid is ethanol.

- the metallic material to be deposited comprises aluminum or an aluminum alloy.

- the powder comprises at least 97%, preferably at least 99%, preferably at least 99.5% by mass of particles of the same metal or alloy, in particular aluminum or an aluminum alloy, by relative to the total mass of the particles of metallic material.

- in step c), heating is carried out at a first temperature between 500 ° C and 700 ° C for 1 to 4 hours and heating at a second temperature between 900 and 1100 ° C for 1 to 3 hours.

- step c) is carried out under an inert, reducing atmosphere or under vacuum. - The method comprises, between steps c) and d), a step e) of carrying out under an oxidizing atmosphere an additional heat treatment with heating at a third temperature between 900 ° C and 1100 ° C for 1 to 5 hours.

- the metallic material to be deposited is aluminum or an aluminum alloy.

- the suspension is used for the development of a protective coating against corrosion.

the suspension used in the context of the invention has characteristics close to a paint.

- the substrate to be coated is a metal part having a surface roughness of at least 20 μm and / or surface defects of the oxide or unfused type.

- the substrate to be coated is a part resulting from an additive manufacturing process, preferably a process by laser fusion on a powder bed.

According to another aspect, the invention relates to a substrate coated at least in part with a coating obtained by a method according to the invention, characterized in that the coated part of said substrate has a degree of coverage of its surface greater than 99%. , preferably greater than 99.5%, more preferably equal to 100%

In particular, the substrate is or forms part of a heat exchanger or a reactor-exchanger. The substrate can also be or form part of a turbine.

Furthermore, the coating obtained in step d) may have an average thickness of between 15 and 200 μm, preferably between 30 and 60 μm, and variations in thickness of less than 10% in relative standard deviation, preferably. less than 5%, the relative standard deviation being defined as the ratio between the standard deviation of the thickness and the average thickness.

In addition, the invention relates to an exchanger-reactor or exchanger comprising a plurality of walls stacked parallel to each other and to a stacking direction so as to define between them at least a first series of stages configured for the flow. of a first fluid to be placed in an indirect heat exchange relationship with a second fluid, at least one stage of the first series comprising a plurality of channels configured for the flow of the first fluid, said channels having a coating obtained by the process according to the invention with a degree of coverage of the surface of said channels greater than 99%, preferably greater than 99.5%, more preferably equal to 100%. The present invention will now be better understood by virtue of the following description, given solely by way of non-limiting example and made with reference to the appended figures, among which:

Fig. 1 shows schematically the different steps of a process according to one embodiment of the invention.

Fig. 2 shows schematically a heat treatment carried out during a process according to one embodiment of the invention.

Fig. 3 shows in cross section a coating made with a process according to the prior art and a coating made with a process according to an embodiment of the invention.

Fig. 4 shows in cross section a coating produced on the internal surfaces of a part with a method according to an embodiment of the invention.

Fig. 5 shows in cross section a coating made on internal surfaces of another part with a method according to an embodiment of the invention.

Fig. 6 is a three-dimensional schematic view of a heat exchanger-reactor according to one embodiment of the invention.

Fig. 7 shows schematically different sectional planes of the exchanger-reactor of Fig. 5.

As seen in Fig. 1, the first step of process a) is the preparation of a suspension from a liquid 1, i. e. the solvent, and a powder forming the dry matter to be incorporated into the solvent. The powder comprises particles of the metallic material to be deposited 2 and particles of the pickling agent 3.

Advantageously, the powder contains only particles of the metallic material to be deposited and particles of the stripping agent. As the stripping agent functions as both a stripping agent and a transport agent, no additives or activating agents are required. The suspension therefore consists of the liquid, the metallic material and the pickling agent. The formulation of the suspension is thus greatly simplified.

According to the invention, the suspension comprises from 10% to 60%, by mass of said stripping agent relative to the total mass of the powder. Preferably, the suspension comprises from 20 to 30%, by mass of said stripping agent relative to the total mass of the powder.

It should be noted that the mass ratio between the metallic material and the pickling agent will be adapted as a function in particular of the metallurgical quality of the powder and of the substrate, in particular the thickness of oxides present at the surface, and the thickness of coating to be deposited. In particular, the greater the initial thicknesses of oxides present at the surface of the powder or substrate, the more the quantity of pickling agent increases.

The stripping agent can be a halogenated compound, the halogen of which is fluorine or chlorine. Advantageously, fluorine is used from among the halogens given its strongest electronegativity, in particular compared to that of oxygen. It will thus associate in a privileged way with all the chemical elements present in the fluoride system. In particular, it is possible to use a halogenated compound of formulation KxAIF _y with x ranging from 1 to 3 and y ranging from 4 to 6. This formulation offers the advantage of exhibiting a high range of activity over a wide range of temperatures.

It also complies with REACH regulations (EC 1907/2006).

It is also possible to envisage the use as a stripping agent of the following compounds: NEUF, MgF2, and CaF2,

The metallic material 2 can be formed in whole or in part from a single body or from an alloy. Preferably, the metallic material is formed in whole or in part from aluminum or an aluminum alloy.

It should be noted that the term “metallic material” is understood to mean any metallic material used to form the coating. The powder can thus comprise particles of the same metal or alloy, possibly with particles of other metals or alloys. Preferably, the powder comprises at least 97%, preferably at least 99%, by mass of particles of the same metal or alloy relative to the total mass of the particles of metallic material.

Preferably, the metallic material is such that the coating obtained is a protective coating of the substrate against corrosion.

Preferably, the liquid is alcohol, in particular ethanol. There is no oxidation reaction of the metallic material in ethanol, allowing long-term stable suspensions to be prepared. In addition, ethanol can be easily processed on an industrial scale and its use is the most compatible with REACH regulations.

After mixing the liquid and the powder, the suspension is applied (step b) to at least part of the surfaces of the metal substrate so as to obtain a substrate-suspension assembly. It should be noted that the use of a stripping agent according to the invention makes it possible to apply the suspension directly after preparation to surfaces to be coated as unworked. This eliminates the tedious stripping, cleaning and drying steps which were implemented in the prior art.

Advantageously, the suspension is prepared during step a) from particles whose particle size distribution is controlled and adapted to the intended application, in particular to the location, the geometry and / or the surface condition of the substrate to be coated. The preparation of the suspension can thus comprise a grinding step during which the particles of the powder are ground in order to obtain a suitable particle size distribution of said particles.

Preferably, the particles of the metallic material to be deposited have a first average equivalent diameter and the particles of the stripping agent have a second average equivalent diameter.

By “equivalent diameter” of a non-spherical particle, is meant in the present application the diameter of the sphere of the same volume as said particle. The term “average equivalent diameter” is understood to mean the equivalent diameter given by the statistical particle size distribution at half of a population of particles, denoted D50. Thus, D50 is the diameter of the particles for which 50% (% by mass) of the particles of the population considered have a diameter less than D50. The equivalent diameters can be determined experimentally, for example by laser granulometry, in particular by means of a laser granulometer of the Malvern or Coulter type.

Advantageously, the ratio between the first average equivalent diameter and the second average equivalent diameter is between 1, 2 and 10, preferably between 3.5 and 7.5

This relative dimensioning of the particles of the metallic material and of the pickling agent promotes, by steric effect, better dispersion of these particles of different size within the same suspension. A more homogeneous distribution of the solid precursors in the suspension is thus obtained, which limits the presence of agglomerates in the suspension and therefore the risk of clogging of the cavities to be coated and / or of increased thickness of the coating deposited. The flow and wetting characteristics of the suspension are also improved, promoting a homogeneous distribution of the suspension on the surface to be coated and therefore the production of a coating of controlled thickness with a high coverage rate, even complete coverage of said surface. In this way, the stripping agent also performs a steric dispersion function, making it possible to limit the quantity and the number of reagents used in the suspension. In particular, it is therefore no longer necessary to add organic compounds (of the binder, dispersant, complexing, wetting or plasticizing type, etc.) as was often the case in the prior art.

Furthermore, powders with an average diameter 10 times smaller than the average equivalent diameter of the cavity or cavities to be coated are preferably used.

The invention can be applied in particular to cavities to be coated which may have equivalent diameters of less than 2 mm, or even less than 0.5 mm.

More particularly, the particles of the metallic material to be deposited may have a first average equivalent diameter of between 7 and 15 μm and / or the particles of the stripping agent may have a second average equivalent diameter of between 2 and 6 μm. These particle sizes are advantageous because they make it possible to obtain the best possible coverage rate, that is to say a rate of at least 99%, preferably at least 99.9%, more preferably equal. at 100%, in particular when the substrate has cavities of small dimensions or of high size ratio or when the substrate has a high surface roughness.

Preferably, the particles of the metallic material to be deposited have a first particle size distribution centered around the first average equivalent diameter and the particles of the stripping agent have a second particle size distribution centered around the second average equivalent diameter. By "particle size distribution" is meant the statistical distribution of the particle diameter of a population of particles, that is to say of a set of particle diameter values. In the context of the invention, the powder comprises a population of particles of metallic material and a population of particles of stripping agent, each of these populations being centered on a predetermined mean equivalent diameter.

Preferably, said first particle size distribution and / or said second particle size distribution exhibiting a dispersion index of less than 0.9, preferably less than 1.13. This limits the possible size range of the particles around the mean diameter in order to avoid having too much size difference between these particles as well as particles that are too coarse or too fine.

It should be noted that the term “dispersion index” of a population of particles is understood to mean, within the meaning of the present application, the ratio A as defined below: where D90 is the diameter of the particles for which 90% (% by mass) of the particles have a diameter less than D90 and D10 is the diameter of the particles for which 10% (% by mass) of the particles have a diameter less than D10.

Note that the particles of the suspension are generally of non-spherical shape but advantageously have an overall shape relatively close to that of a sphere, in particular the particles of the suspension may be of substantially spherical or quasi-spherical shape.

The degree of non-sphericity of a particle can be measured by a parameter called "sphericity factor" corresponding, in the present application, to the ratio between the smallest dimension and the largest dimension of the particle in question. Advantageously, the particles of the powder have a sphericity factor greater than 0.3, preferably greater than 0.5. Indeed, it has been demonstrated by rheological measurements of the flow characteristics of the suspension that a particle morphology too far from the sphericity leads to the appearance of flow threshold stresses of the suspension on the substrate, which can cause coating thicknesses.

Preferably, the mass ratio between the powder and the liquid of the suspension is between 1, 5 and 3. These values make it possible to obtain a suspension of relatively low viscosity, which promotes total or almost total coverage of the surface. to be coated and facilitates the application of the suspension, in particular in the case of an application by immersion or by spraying. Preferably, the suspension prepared in the process of the invention has a viscosity of less than 1 Pa.s, preferably between 0.1 and 1 Pa.s.

Note that step a) can optionally comprise a step of stirring the liquid-powder mixture by exposing the liquid-powder mixture to ultrasound with a power of between 100 and 200 W, preferably for a period of between a few seconds. and several tens of minutes.

In step b), the suspension can be applied by different techniques, resulting in the deposition of a homogeneous film on the exposed surfaces. The suspension can be applied by immersing the substrate in the suspension, which is a simple technique, but in this case the deposition is carried out inside and outside the substrate. A or several successive quenchings can be operated. The suspension can also be applied by forced injection, a technique particularly suitable for coating internal cavities, or by application with a brush or gun.

Determining the thickness of the suspension deposited before heat diffusion treatment is important because it conditions the amount of metallic material brought to the surface of the substrate.

For an application by dipping or by injection, this can be determined via an abacus making it possible to correlate the thicknesses deposited as a function of the number of quenchings, of the drawing speed for an optimized stirring time, for example 12 s for a 200 ml suspension.

In the case of application by spraying, the thickness of the deposit depends on various parameters: the number of passes, the rheology of the suspension or the speed of passage. By way of example, a suspension having a powder / liquid mass ratio equal to 3 applied to the gun with a passage speed of 0.5 ms ¹ leads to mass gains of the order of 20 mg.crrr ² for a adjusted flow.

Regardless of the technique used, the areas not to be covered can either be protected by a mask which will be removed before or during the coating deposition heat treatment, or preserved by filling to a desired level, for example by continuous volume monitoring via a positive displacement pump.

It should be noted that, unlike certain methods of the prior art, it is not necessary to carry out a step of removing the solvent at low temperature (150 ° C. maximum) or of removing the organic compounds after the application of the suspension.

A so-called diffusion heat treatment (step c)) is carried out directly, which makes it possible to react the deposit obtained previously with the surfaces of the substrate to be coated, resulting in the diffusion of the metallic material that it contains in the substrate and thus in the growth at the surface of the substrate of a coating enriched with this metallic material.

This treatment comprises heating to a first temperature of at least 500 ° C in order to activate the stripping agent and heating to a second temperature above the first temperature in order to allow the interdiffusion of the elements of the substrate with those of the suspension deposited on the substrate, which has the effect of consolidating the coating. Preferably, the treatment comprises a first heating at a first temperature between 500 ° C and 700 ° C for 1 to 4 hours and a second heating to a second temperature between 900 ° C and 1100 ° C for 1 to 3 hours.

Step c) can be carried out under an inert, reducing or vacuum atmosphere, in a static or dynamic condition, preferably it is carried out under an atmosphere under very low total pressure so as to activate the passage into the vapor phase of the halogenated compound.

Optionally, the method may comprise, after step c) a step e) carried out under an oxidizing atmosphere, for example in air, and comprising an additional heat treatment with heating to a third temperature between 900 ° C and 1100 ° C for 1 at 5 o'clock. This additional treatment makes it possible to form an oxide layer consisting of the deposited metallic material (for example the stable alpha-AhC type protective oxide layers) continuous and a few microns thick, before commissioning of the coated part. This is useful when the actual operating conditions of the coated part do not allow these protective oxide layers to be formed in-situ.

Fig. 2 schematically shows a heat treatment according to one embodiment of the invention. Stage 24 corresponds to heating to the first temperature, stage 25 to heating to the second temperature and stage 26 to heating to the third temperature.

At the end of step c) (or of step e) in the case where the method comprises such an additional heat treatment step, a coated substrate (step d)) is obtained which can be used directly in the targeted application.

Preferably, the final coating has a high degree of coverage of the surface to be covered, that is to say a rate of at least 99%, preferably at least 99.5%, more preferably of at least 99.9% and more preferably equal to 100%.

Preferably, the final coating has an average thickness of between 20 and 200 µm, preferably between 30 and 60 µm. In addition, its thickness is preferably of low variability, i.e. the coating exhibits variations in thickness not exceeding 10%, preferably not exceeding 5% in relative standard deviation, the relative standard deviation being defined as the ratio between the standard deviation of the thickness and the average thickness. In the present application, the term “average thickness” is understood to mean the arithmetic mean of the thickness.

In the context of the invention, the substrate may be a metal part having one or more of the following characteristics: a high surface roughness, that is that is to say a surface roughness of at least 20 μm, surface defects of the oxide or unmelted type, internal regions which are difficult to access, at least one cavity of characteristic size less than or equal to a millimeter or of an equivalent diameter less or equal to 1 mm, a cavity or a channel having a length / width ratio and / or a length / depth ratio high, that is to say greater than or equal to 150, preferably greater than or equal to 300.

In particular, the part may be the result of an additive manufacturing process, that is to say obtained by 3D printing, preferably a part resulting from a process by laser fusion on a powder bed.

The substrate is chosen from metallic substrates, in particular substrates formed wholly or partly of iron or nickel, substrates made of an alloy or superalloy, in particular an iron-based or nickel-based alloy or superalloy, composite substrates comprising one or more several metals and / or alloy (s) and / or superalloy (s) containing nickel. In particular, the substrate may comprise at least 90%, preferably at least 95%, by mass of nickel relative to the total mass of the particles of metallic material.

The substrate to be coated can in particular be at least part of a heat exchanger or a reactor-exchanger. Said exchanger or exchanger-reactor can be used for the production of synthesis gas, that is to say of a mixture of hydrogen and carbon monoxide. Advantageously, the exchanger-reactor according to the invention is of the millistructured, or even microstructured, type. In other words, the fluid exchange or distribution channels of the millistructured exchanger are of millimeter dimensions, for example with a height and / or a width of the order of 0.2 mm to 2 mm. The structuring and reduction of the fluid passage section in the channels making it possible to intensify the heat and mass transfers during the reaction and the method according to the invention makes it possible to protect the channel surfaces from corrosion phenomena due to the strong carbon activity within these channels. The channels may in particular have a length / width ratio and / or a high length / depth ratio greater than or equal to 150, or even greater than or equal to 300.

The substrate can also be chosen from the interior of tubes, turbine blades, storage capacities.

Fig. 6 is a three-dimensional view of an example of an exchanger-reactor which can be used for the implementation of a process for producing hydrogen by steam reforming which can have at least part of a surface coating deposited according to the process. of the invention. In operation, a first fluid F1, for example a mixture of methane and water vapor, enters the exchanger via an inlet connector 2 and leaves it via an outlet manifold 6. These collectors (or heads) are generally fitted with input and output connectors 1 and 7 providing fluid connections with other equipment in the production installation.

The exchanger has three dimensions: length, measured in the longitudinal direction x, width, measured in the lateral direction y, and height, measured in the stacking direction z of the walls 12 of the exchanger (not visible in FIG. 1). ). The exchanger comprises at least a first series of stages 10 and a second series of stages 11 superimposed on each other in the stacking direction z. Preferably, the floors extend substantially parallel to each other and to a plane defined by the x and y directions. Each stage is delimited between two adjacent walls 12, a pair of side edges arranged parallel to the lateral direction y and a pair of longitudinal edges arranged parallel to the longitudinal direction x. Preferably, the walls 12 extend generally parallel to each other and parallel to the longitudinal direction x and to the lateral direction y, said longitudinal and lateral directions being orthogonal to each other.

The collectors 6, 7 are configured so as to evacuate all of the stages 10 of the first series in the first fluid F1 and to recover said first fluid F1 from these stages. The manifolds 2, 6 can be welded to the parallelepipedal body of the heat exchanger or of the heat exchanger-reactor or else, advantageously, be directly manufactured with the body of the heat exchanger to obtain a part without an assembly interface, preferably by an additive manufacturing process.

A second fluid F2 circulates in stages 11 of the second series. The second fluid F2 is typically a heat transfer fluid. It allows, in the case of a hydrocarbon reforming process such as methane steam reforming, to provide the heat required for the chemical reaction that occurs in stages 10 where the first reactive fluid F1 circulates.

In addition, the exchanger-reactor according to the invention can comprise a third series of stages 15 interposed between stages 10, 11 of the first and second series. These stages 15 may be intended in particular for the circulation of a third fluid F3 which is preferably formed by the product of the chemical reaction carried out in stages 10, preferably synthesis gas rich in hydrogen. Preferably, the reactor exchanger comprises fluidic connection means connecting at least one stage 10 of the first series with a stage 15 of the third series. The stages 10, 11 of the first and second series can be positioned alternately but not necessarily.

The fluid passes through the exchange zone 4 and is collected in a distribution (outlet) zone 5 arranged downstream of said exchange zone 4. The outlet distribution zone 5 operates in conjunction with the distribution zone of inlet 3 and is designed to ensure homogeneous recovery of the fluid from the exchange channels 41 of the exchange zone 4. The fluid leaves the body of the exchanger or of the exchanger-reactor via an outlet head 6 via the output connector 7.

Fig. 7 is a partial exploded view of three types of stages 10, 11, 15 intended to be superimposed in a heat exchanger-reactor according to FIG. 6. Preferably, each stage 10, 15 of the third series is also divided, along the longitudinal direction x, into a circulation zone comprising circulation channels 41, 151, preferably rectilinear, and at least one distribution zone comprising distribution channels 31, 155. The inlet distribution zones 3 and are designed to ensure homogeneous recovery of the fluid from the exchange channels 41 of the exchange zone 4. Preferably, each stage 11 of the second series comprises also rectilinear circulation channels 111 which extend parallel to the longitudinal direction x.

According to another possibility, the deposition process is intended to coat at least part of a turbine.

Example 1:

A suspension with a volume of 200 ml was produced comprising 67% of powder relative to the total mass of the suspension, that is to say having a powder / liquid mass ratio of about 2, from 24 g of ethanol, 37.8 g of aluminum particles and 10.1 g of powder of halogen compound of type K _y AIF _x as stripping agent, i.e. a metallic material / stripping agent mass ratio of 3.74 . The stripping agent particles had an average equivalent diameter of 4 μm and a purity of the order of 99.99% relative to the total mass of the stripping agent particles. The aluminum particles had an average equivalent diameter of 11.4 µm, a sphericity factor of 0.55 and a purity of the order of 99.95%.

The suspension was synthesized in a single step by means of a sonotrode with a power of 160 W. The two types of particles were introduced into a container and then the ethanol added. The suspension was then stirred using the sonotrode with an amplitude of 100% for a period of 12 s. The suspension was kept under stirring.

The suspension was deposited in 2 successive stages: (1) immersion of the parts to be coated in the suspension at ambient temperature, then withdrawal with a constant speed of 8 mm / s, (2) continuous drawing at a constant speed of 8 mm / s. The thickness of the deposit obtained was about 80 µm. The latter was checked by optical microscopy on several control samples.

Then, the coated parts were placed in a vacuum oven where the heat treatment corresponding to step c) and Fig. 2 has been achieved.

Fig. 4 shows the sectional view of the final coating obtained on the internal surface of a part of complex geometry. The average coating thickness is on the order of 50 µm and no uncoated areas are detectable.

Example 2:

A suspension with a volume of 200 ml was produced comprising 50% powder relative to the total mass of the suspension, that is to say having a powder / liquid mass ratio equal to 1 from the following ingredients: 48 g of ethanol, 37.8 g of aluminum powder, 10.1 g of powdered halogen compound of type K _y AIF _x as stripping agent, i.e. a metallic material / stripping agent mass ratio of 3.74 .

The stripping agent particles had an average equivalent diameter of 4 µm and a purity of about 99.99% based on the total mass of the stripping agent particles. The aluminum particles had an average equivalent diameter of 11.4 µm and a purity of around 99.95%.

The suspension was synthesized in a single step using a sonotrode with a power of 160 W. The two types of particles were introduced into a container and then the ethanol added. The suspension was then stirred using the sonotrode with an amplitude of 100% for a period of 12 s. The suspension was kept under stirring.

The suspension is deposited in 2 successive steps: (1) injection of the suspension into the internal cavities of a part to be coated at a flow rate of 60 mL.min ^{· 1} , (2) withdrawal of the suspension at a flow rate 60 mL · min ^{· 1.} The thickness of the deposit obtained is about 40 µm. The latter was checked by optical microscopy on several control samples.

Then, the coated parts were placed in a vacuum oven where the heat treatment corresponding to step c) and to Fig. 2 has been achieved. Fig. 5 shows the sectional view of the final coating obtained on the internal surface of a part of complex geometry. The average coating thickness is on the order of 25 µm and no uncoated area is detectable.

Claims

1. A method of depositing a coating on at least part of a metallic substrate, said method comprises the following successive steps: a) mixing a liquid and a powder comprising particles of a metallic material to be deposited and particles of a stripping agent so as to prepare a suspension, b) application of the suspension prepared in step a) on at least part of the surfaces of the metal substrate so as to obtain a substrate-suspension assembly, c) performing a heat treatment of the substrate-suspension assembly comprising heating to a first temperature of at least 500 ° C and heating to a second temperature above the first temperature, and d) recovering a substrate coated with less in part of a coating, said pickling agent being a halogenated compound and the suspension comprising from 10% to 60% by mass of said pickling agent relative to the total mass of the powder, characterized in that the particles of the material mixed The metal have a first average equivalent diameter and the particles of the stripping agent have a second average equivalent diameter, the ratio between the first average equivalent diameter and the second average equivalent diameter being between 1, 2 and 10.

2. Method according to claim 1, characterized in that the suspension comprises from 20% to 30% by mass of said stripping agent relative to the total mass of the powder.

3. Method according to one of claims 1 or 2, characterized in that the powder contains only particles of the metallic material and particles of the stripping agent.

4. Method according to one of the preceding claims, characterized in that the particles of the metallic material have a first average equivalent diameter and the particles of the stripping agent have a second average equivalent diameter, the ratio between the first average equivalent diameter and the second mean equivalent diameter being between 3.5 and 7.5.

5. Method according to one of the preceding claims, characterized in that the particles of the metallic material have a first average equivalent diameter of between 7 and 15 μm and / or the particles of the stripping agent have a second average equivalent diameter of between 2 and 6 pm.

6. Method according to one of claims 4 or 5, characterized in that the particles of the metallic material have a first particle size distribution centered around the first mean equivalent diameter and the particles of the stripping agent have a second particle size distribution centered around the second mean equivalent diameter, said first particle size distribution and / or said second particle size distribution having a dispersion index of less than 1, 2, preferably less than 0.9.

7. Method according to one of the preceding claims, characterized in that the particles of the suspension have an average sphericity factor of at least 0.3, preferably at least 0.5.

8. Method according to one of the preceding claims, characterized in that the metal substrate has at least one cavity of equivalent diameter e _m m <2 mm and / or a length / width ratio greater than 150, the particles of the suspension having each an equivalent diameter less than e _m m / 10.

9. Method according to one of the preceding claims, characterized in that the mass ratio between the powder and the liquid is between 1, 5 and 3.

10. Method according to one of the preceding claims, characterized in that the stripping agent is a halogenated compound of which the halogen is fluorine or chlorine.

11. Process according to one of the preceding claims, characterized in that the stripping agent belongs to the system of fluorinated compounds, preferably said stripping agent is chosen from the group consisting of KxAIFy, NFUF, MgF2, and CaF2, preferably with x included between 1 and 3 and including between 4 and 6.

12. Method according to one of the preceding claims, characterized in that the liquid comprises an alcohol, preferably the liquid is ethanol.

13. Method according to one of the preceding claims, characterized in that the metallic material comprises aluminum or an aluminum alloy.

14. Method according to one of the preceding claims, characterized in that, in step c), heating is carried out at a first temperature of between 500 ° C and 700 ° C for 1 to 4 hours and heating at a second temperature between 900 and 1100 ° C for 1 to 3 hours.

15. Method according to one of the preceding claims, characterized in that step c) is carried out under an inert, reducing or vacuum atmosphere.

16. Method according to one of the preceding claims, characterized in that it comprises, between steps c) and d), a step e) of carrying out under an oxidizing atmosphere an additional heat treatment with heating to a third temperature of between between 900 ° C and 1100 ° C for 1 to 5 hours.

17. Method according to one of the preceding claims, characterized in that the substrate to be coated is a metal part having a surface roughness of at least 20 μm and / or surface defects of the oxide or unfused type.

18. Method according to one of the preceding claims, characterized in that the substrate to be coated is a part resulting from an additive manufacturing process, preferably a process by laser fusion on a powder bed.

19. Substrate coated at least in part with a coating obtained by a process according to one of claims 1 to 16, characterized in that the coating obtained in step d) has an average thickness of between 15 and 200 μm, preferably between 30 and 60 μm, and thickness variations of less than 10% in relative standard deviation, preferably less than 5%, the relative standard deviation being defined as the ratio between the standard deviation of l 'thickness and average thickness.