Method for forming a cathodic protection coating on a turbomachine part

Abstract

The invention relates to a method for forming a cathodic protection coating on a substrate (1) forming a turbomachine part, comprising at least: - the deposition, on the substrate, of particles (11) for the cathodic protection of the substrate, this deposition being effected by electrophoresis from an organic electrolyte (10) comprising at least said particles, and - the formation of an inorganic matrix in a porous area of the particle deposit thus produced, comprising at least: • impregnating said deposit with an impregnating composition, • thermally drying the deposit impregnated with the impregnating composition, and • densifying the deposit by mechanical compaction, after the thermal drying treatment, so as to make said deposit electrically conductive.

Description

Technical Field

[0001] The present invention relates to a method for forming a cathodic protective coating on a turbomachine component from an organic electrolyte. The invention is particularly relevant for the protection of compressor or turbine shafts used in aeronautical or industrial turbomachinery. Previous technique

[0002] High-strength steels, typically exceeding 1000 MPa, such as Maraging 250, ML340, or 40CDV12, can be used to form turbomachinery components, such as compressor or turbine shafts. However, these steels can be susceptible to corrosion during operation.

[0003] To protect parts from corrosion, it is common practice to coat them with anti-corrosion paints applied by spraying (manually or automatically). With this method, controlling the paint thickness can be quite tricky, especially if the part has a complex geometry. This can result in coatings that do not conform to the part's technical specifications, potentially leading to reduced characteristics (corrosion resistance in case of insufficient thickness, adhesion in case of excessive thickness).

[0004] Solutions have been proposed to address the problem of uniform deposition on complex parts. In this regard, US patent 3,787,305 proposes the electrophoretic deposition of aluminum cathodic protection particles with a resin, generally acrylic. The deposition is carried out using an aqueous electrolyte in which the resin is dissolved, and a voltage higher than that of water electrolysis is applied. This results in significant local pH variations around the electrodes, leading to the precipitation of the resin containing the aluminum particles on the surface of the working electrode. As will be detailed later, water electrolysis presents several problems that the inventors encountered during their work.Furthermore, the deposited resin is electrically insulating, which limits the achievable deposit thickness in a single electrophoretic deposition step to approximately 20 micrometers. This value may prove insufficient to completely cover surface defects in the substrate or provide adequate corrosion protection. The process continues with the calcination of the organic resin using a relatively high-temperature heat treatment, which can affect the microstructure of some substrates and lead to the development of additional porosities in the coating. Moreover, if a significant deposit thickness is desired—that is, 20 µm or greater—it is necessary to repeat the electrophoretic deposition and resin calcination sequence, one or more times, after the initial resin layer has been calcined. This considerably lengthens and complicates the process.

[0005] French patent FR1533589 discloses a process for forming a uniform thickness coating resistant to oxidation on the surface of a stainless steel panel, this process comprising the following operations: cleaning the surface; deposition by electrophoresis of a porous coating matrix on this surface from a bath comprising aluminum particles suspended in ethanol, this bath also comprising aluminum chloride AICI as a mineral electrolyte; drying of the matrix; dispersion around the matrix of a liquid phosphate glass-based binder containing chromic and phosphoric acids, in the form of finely divided droplets; air drying of the binder impregnating the matrix; and hardening of the binder at 315°C to fix the matrix particles to each other and to the surface of the object.

[0006] It is desirable to have a process for forming an anti-corrosion coating that overcomes the disadvantages of the prior art. Description of the invention

[0007] The invention relates to a method for forming a cathodic protective coating on a substrate forming a turbomachine part, comprising at least: the deposition on the substrate of cathodic protection particles of the substrate, this deposition being carried out by electrophoresis from an organic electrolyte comprising at least said particles, and the formation of an inorganic matrix in a porosity of the particle deposit thus produced, comprising at least: an impregnation by an impregnation composition of said deposit, a heat treatment of drying of the deposit impregnated by the impregnation composition, and a densification by mechanical compaction of the deposit, after the heat treatment of drying, in order to make said deposit electrically conductive.

[0008] Electrophoresis allows for a homogeneous deposit with a controlled thickness, unlike spray gun methods, even on complex or large parts. The use of an organic electrolyte eliminates the harmful effects associated with water electrolysis. Indeed, the inventors' research has shown that water electrolysis, which can occur during electrophoretic deposition with an aqueous electrolyte, can lead to hydrogen embrittlement of the part if the deposition electrode is the steel cathode (indicated by a negative sign), and to a bubbling phenomenon that affects the homogeneity of the deposit. Anodic deposition with an aqueous electrolyte, on the other hand, may require a basic pH range to obtain negatively charged particles, which can result in corrosion of the deposited particles.The invention also makes it possible to obtain a wide range of deposit thicknesses in a single electrophoretic deposition step. Such thicknesses can be more difficult to achieve in a single deposition step when the deposition is carried out using an aqueous electrolyte under a direct current voltage. The inorganic matrix formed within the deposit's porosity constitutes a binding phase that holds the cathodic protection particles to the substrate and binds these particles together to ensure the cohesion of the deposit. Mechanical compaction for densification brings the cathodic protection particles into contact with the substrate, making the coating dense and electrically conductive. Thanks to this compaction, the coating acquires effective sacrificial properties to combat corrosion.

[0009] In one embodiment, the organic electrolyte comprises an alcoholic liquid medium in which the particles are suspended.

[0010] Such a characteristic is advantageous in order to have an electrolyte with good environmental and sanitary compatibility and a larger electroactivity range.

[0011] In particular, the alcoholic liquid medium can be formed at least 50% by volume from propanol, for example from propan-2-ol.

[0012] The use of propanol is advantageous because it eliminates the need for a dispersant in the electrolyte, thus simplifying the process.

[0013] In one embodiment example, the thickness of the cathodic protection particle deposit on the substrate is greater than or equal to 40 µm.

[0014] The invention is particularly advantageous in this case because it makes it possible to achieve such thicknesses in a single electrophoretic deposition step, without having to interrupt the deposition.

[0015] In one embodiment, the cathodic protection particles are made of aluminum or aluminum alloy. However, the invention is not limited to the use of such a material, and other examples will be described later.

[0016] In one example of implementation, the substrate is made of steel.

[0017] As with the particles, the invention is not limited to a particular family of materials for the substrate, the latter being more generally metallic, for example in metallic alloy or even in composite material as long as it has sufficient electrical conductivity to allow electrophoretic deposition.

[0018] In one embodiment, the impregnation composition includes at least one silicate of an alkali metal or an alkaline earth metal.

[0019] This characteristic is advantageous because it eliminates the need for an acidic medium, which may be required during sol-gel deposition to prevent damage to certain substrates. The use of a sol-gel process to form the inorganic matrix, however, remains within the scope of the invention and will be described below.

[0020] In one example of implementation, the formation of the inorganic matrix includes a heat treatment to stabilize the deposit.

[0021] This characteristic advantageously allows for the removal of as much of the liquid medium as possible and makes the deposit insoluble in water.

[0022] It should be noted that the drying heat treatment can be carried out at a first temperature, followed by the stabilization heat treatment at a second temperature higher than the first. In this case, these two treatments are distinct and performed at different temperatures. Alternatively, a single heat treatment step can be performed in which the deposit is both dried and stabilized (the stabilization and drying heat treatments being combined). Yet another alternative involves no stabilization treatment, only a drying heat treatment.

[0023] In particular, the stabilizing heat treatment can be carried out before densification by mechanical compaction. However, it does not depart from the scope of the invention if the stabilizing heat treatment is carried out after this densification.

[0024] In particular, a temperature less than or equal to 500°C, for example less than or equal to 450°C, may be imposed during the heat treatment for drying and the possible heat treatment for stabilization.

[0025] Imposing a limited temperature during heat treatment helps to avoid any risk of damaging the substrate due to exposure to excessively high temperatures.

[0026] In one example embodiment, the substrate is a compressor shaft or a turbine shaft, for example made of high-strength steel. Brief description of the drawings

[0027] [ Fig. 1 ] There figure 1 illustrates, schematically and partially, the electrophoretic deposition of cathodic protection particles on the substrate. Fig. 2 ] There figure 2 illustrates, schematically and partially, the substrate coated by these particles. Fig. 3 ] There figure 3 illustrates, schematically and partially, an example of the formation of an inorganic matrix in the porosity of a particle deposit that can be implemented within the framework of the invention. Fig. 4 ] There figure 4 is a cross-sectional scanning electron microscopy photograph of a cathodic protection particle deposit. Fig. 5 ] There figure 5 is a graph showing the evolution of the thickness of the cathodic protection particle deposit as a function of deposition time at a constant electric field. Fig. 6 ] There figure 6 is a graph showing the evolution of the porosity of the cathodic protection particle deposit as a function of deposition time at a constant electric field. Fig. 7 ] There figure 7 is a graph showing the evolution of the thickness of the cathodic protection particle deposit as a function of the applied electric field at a constant deposition time. Fig. 8 ] There figure 8 is a graph showing the evolution of the porosity of the cathodic protection particle deposit as a function of the applied electric field at constant deposition time. Fig. 9 ] There figure 9 represents, schematically and partially, a galvanic coupling setup used to evaluate the cathodic protection conferred by a coating obtained by implementing an example of a process according to the invention. Fig. 10 ] There figure 10 is a graph showing the evolution of the galvanic coupling potential as a function of time. Fig. 11 ] There figure 11 is a graph showing the evolution of the galvanic coupling current density over time. Fig. 12 ] There figure 12 is a photograph of a sample following the galvanic coupling test. Fig. 13 ] There figure 13 is a photograph of a sample following the galvanic coupling test. Fig. 14 ] There figure 14 is a photograph of a sample following the galvanic coupling test. Fig. 15 ] There figure 15 is a photograph of a sample following the galvanic coupling test. Fig. 16 ] There figure 16 corresponds to a comparative test result showing the influence of compaction on corrosion resistance. Description of the implementation methods

[0028] THE figures 1 et 2 represent the deposition of cathodic protection particles 11 onto substrate 1 in an example of a process according to the invention. This deposition is carried out by electrophoresis using an organic electrolyte 10 comprising the particles 11 suspended in an organic liquid medium. The particles 11 and the organic liquid medium may have various compositions, as will be described later. The particles 11 may be the only particles suspended in the organic liquid medium, but this does not depart from the scope of the invention if the electrolyte 10 also comprises additional particles, distinct from the particles 11, suspended in the organic liquid medium. The additional particles may have an average size D50 smaller than the average size D50 of the particles 11. The additional particles may be present in a smaller quantity compared to the particles 11.It is advantageous to choose additional particles of limited hardness so that they deform during mechanical compaction and do not disrupt this step. The additional particles can be metallic or ceramic. The substrate 1 to be coated is immersed in the organic electrolyte 10. The surface of the substrate 1 intended to be coated by the particles 11 may have been previously prepared conventionally by chemical and / or mechanical etching. The surface of the substrate 1 comprises an electrically conductive material. The substrate 1 can be made of a metallic material, for example, aluminum or aluminum alloy, or steel. A composite material substrate 1 can also be used as long as its electrical conductivity is sufficient to allow the deposition of the particles 11 by electrophoresis. The substrate 1 is a turbomachine component, for example, an aircraft or industrial turbomachine component.The substrate 1 can be a compressor shaft or a turbine shaft. The substrate 1 can be intended for use at a temperature less than or equal to 1000°C, for example less than or equal to 500°C, depending on the material used for the substrate. As illustrated in the diagrams. figures 1 et 2 The substrate 1 constitutes an electrode connected to the first terminal of an electrical generator G. A counter electrode 15 is positioned opposite the surface of the substrate 1 to be coated and is also immersed in the electrolyte 10. The counter electrode 15 is connected to a second terminal of the electrical generator G, different from the first terminal. A stirring device (not shown) may be present in the electrolyte 10 to ensure mixing of this bath during deposition. Due to the application of an electric field between the substrate 1 and the counter electrode 15, the electrically charged particles 11 move and are deposited onto the substrate 1, resulting in a deposit 6 of particles 11. The particles 11 can be deposited in contact with the substrate 1. In the illustrated example, the substrate 1 is negatively charged during deposition, and the particles 11 are positively charged.However, we do not depart from the scope of the invention if the substrate 1 is positively charged and the particles 11 are negatively charged.

[0029] The particles 11 can have an average size D50 less than or equal to 30 µm, for example, between 10 nm and 30 µm. Particles of various shapes can be used. For example, the particles 11 may have a form factor approximately equal to 1, having, for example, a substantially spherical geometry. The particles 11 may be in solid form. The particles 11 may be metallic. The material of the particles 11 is chosen according to the material of the substrate 1 to cathodically protect the latter. Thus, the particles 11 constitute a sacrificial material that corrodes preferentially compared to the underlying substrate 1 in order to preserve it. The cathodic protection coating, obtained after mechanical compaction, is electrically conductive so as to allow electrical conduction between the coating and the substrate and to achieve this preferential corrosion of the cathodic protection particles 11.The material of the particles 11 is chosen so as to present an oxidizing / reducing couple with a standard potential strictly lower than that formed by the material of the substrate 1. For example, for a steel substrate 1, cathodic protection particles 11 made of aluminum or aluminum alloy can be used, but for the same substrate 1, particles 11 made of zinc or zinc alloy, or of magnesium or magnesium alloy, could also be used, for example.

[0030] The mass content of particles 11 in the electrolyte 10, before the start of the electrophoretic deposition, can be greater than or equal to 0.1%, for example between 0.1% and 20%.

[0031] The mass content of the organic liquid medium in the electrolyte 10, before the start of the electrophoretic deposition, can be greater than or equal to 75%, for example between 75% and 99.9%.

[0032] The organic liquid medium may consist of at least 50% by volume of an organic compound or a mixture of organic compounds. This organic compound or mixture of organic compounds may be present in the organic liquid medium at a volume concentration greater than or equal to 75%, for example, greater than or equal to 95%. The organic liquid medium may be substantially free of water or contain water in a limited amount that does not significantly affect the deposition process through the phenomenon of water electrolysis. The volume content of water in the organic liquid medium may typically be less than or equal to 5%.

[0033] In one example, the organic liquid medium is composed of at least 50% by volume of an alcohol or a mixture of alcohols. The alcohol or mixture of alcohols may be present in the organic liquid medium at a volume concentration greater than or equal to 75%, for example, greater than or equal to 95%. The alcohol(s) used may be C2 or C3, for example, chosen from ethanol and propan-2-ol. It should be noted that the organic liquid medium is not necessarily alcoholic; it may, for example, contain acetone or be composed exclusively of acetone.

[0034] The organic liquid medium may contain a dispersant. The dispersant can be steric, ionic, or electrosteric. Among ionic dispersants, metallic salts can be used, for example, chlorides and nitrates, such as: AlCl₃·6(H₂O), MgCl₂·6(H₂O), Mg(NO₃)₂·6(H₂O), and Al(NO₃)₃·9(H₂O). The dispersant can be present in the organic liquid medium at a concentration greater than or equal to 0.1 mmol / L, for example, between 0.1 mmol / L and 2.5 mmol / L. Other salts can be considered, such as sulfates or phosphates. Other types of ionic dispersants are also possible, such as diiodine, a mixture of diiodine and acetone (see publication Journal of the European Ceramic Society (2011), vol. 31, pp. 1075-1086), and triethylenamine (TEA). Electrosteric dispersants such as polyelectrolytes, like polyethyleneimine or polyacrylic acid, can also be used.As mentioned above, the use of propanol in electrolyte 10 is advantageous by making the use of a dispersant unnecessary.

[0035] The deposition of particles 11 can be achieved by applying a continuous or pulsed voltage. An electric field greater than or equal to 5 V.cm⁻¹, for example between 5 V.cm⁻¹ and 200 V.cm⁻¹, or even between 5 V.cm⁻¹ and 60 V.cm⁻¹, can be applied during deposition.

[0036] According to one variant, a continuous or pulsed current can be applied during the deposition of the particles 11. A surface current density greater than or equal to 10 nA.cm -2< , for example between 10 nA.cm -2< and 10 mA.cm -2< , can be imposed during the deposition.

[0037] The deposition of particles 11 on substrate 1 can be carried out for a duration greater than or equal to 10 seconds, for example between 10 seconds and 1 hour.

[0038] The thickness e of the deposit 6 of particles 11 on the substrate 1 can be greater than or equal to 1 µm, for example, greater than or equal to 40 µm. This thickness e can typically be between 1 µm and 300 µm, for example, between 40 µm and 300 µm. The volumetric porosity of the deposit 6 of particles 11 on the substrate 1 can be greater than or equal to 50%, for example, between 50% and 60%. The thickness e and the porosity of the deposit 6 are controlled by the time and the voltage or current applied during the electrophoretic deposition. The electrical parameters to be used are determined by a person skilled in the art based on the electrolyte 10 used.

[0039] Details relating to the electrophoretic deposition step of particles 11 have just been described. The following section describes, in connection with the figure 3 , details relating to the formation of the inorganic matrix in the porosity of the deposit 6.

[0040] An impregnation composition 20 is impregnated into the porosity of the particle deposit 6 11 in order to form the inorganic matrix 40. The example illustrated in the figure 3 This illustrates impregnation using a dip-shrink technique. In this technique, the substrate 1, coated with a deposit of particles 11 (deposited) 6, is connected to a movable device 30, allowing its immersion in a bath of impregnation composition 20 to perform the impregnation. Generally, the impregnation composition 20 can be in liquid form. For example, the shrinkage rate of the substrate 1 by the device 30 can be between 1 and 1000 mm.min⁻¹. The viscosity of the impregnation composition 20 at 20°C can be between 1 mPa.s and 500 mPa.s, for example, between 1 mPa.s and 200 mPa.s. The viscosity is measured using a rheometer with a shear rate of 644 s⁻¹. A person skilled in the art will readily recognize that other impregnation methods are possible, such as manual or automated spray impregnation.The impregnation composition 20 can fill at least 50% of the volume of the porosity of the deposit 6, for example at least 75% of this volume, or even substantially all of this volume.

[0041] The impregnation composition 20 may include at least one sol-gel precursor or an alkali or alkaline earth silicate. For example, the impregnation composition 20 may include sodium silicate (Na₂SiO₃) or a calcium or magnesium silicate. The sol-gel precursor may be selected from: silicon alkoxides such as TEOS (tetraethoxysilane) and TMOS (tetramethoxysilane), aluminum-containing sol-gel precursors such as aluminum trisec-butanoate and aluminum triisopropylate, or zirconium-containing sol-gel precursors such as zirconium tetrapropoxide. The impregnation composition 20, like the cathodic protection coating 50 to be obtained, may be phosphate-free, particularly aluminum phosphate-free. The impregnation composition 20, as well as the cathodic protection coating 50 to be obtained, may be free of chromium-based compounds in oxidation state +VI.The impregnation composition 20, as well as the cathodic protection coating 50 to be obtained, can be free of chromate compounds or lead. These characteristics allow compliance with environmental regulations.

[0042] Once impregnation with the impregnation composition 20 is complete, the deposit 6 impregnated with the impregnation composition 20 can be heat-treated to consolidate and stabilize the cathodic protection particle deposit. This heat treatment includes at least one drying heat treatment to consolidate the deposit by removing most of the liquid medium present in the impregnation composition while retaining a solid portion that binds the particles 11. A temperature of 70°C or higher, for example, between 70°C and 100°C, can be applied during the drying heat treatment. The duration of the drying heat treatment can be one hour or more, for example, between one and three hours.

[0043] If desired, a stabilizing heat treatment can be performed to completely eliminate the liquid medium and obtain a water-insoluble protective coating. As mentioned above, drying and stabilization can be carried out during a single heat treatment, or a first drying heat treatment can be performed followed by a second stabilizing heat treatment at a higher temperature. In the latter case, a temperature of 250°C or higher, or even between 250°C and 500°C, can be applied during the stabilizing heat treatment.

[0044] Heat treatment (including drying and any stabilization treatment) can lead to a chemical transformation of the impregnation composition, for example, its polymerization in the case of a sol-gel precursor. The chemical transformation of the impregnation composition undergone during heat treatment may be distinct from pyrolysis.

[0045] A temperature of 500°C or lower, for example, 450°C or lower, can generally be imposed during heat treatment. A temperature of 200°C or higher, for example, between 200°C and 500°C or between 200°C and 450°C, can generally be imposed during heat treatment. The heat treatment can be carried out for at least one hour, for example, for at least ten hours. The heat treatment can be carried out in air.

[0046] After the heat treatment for drying, the impregnated impregnation composition 20 can be compacted to further densify the particle deposit 11. This compaction can be carried out by spraying organic or inorganic particles, for example, corundum, glass, or sodium bicarbonate. Water-soluble particles, such as sodium bicarbonate, can also be sprayed for compaction. Document FR 3 102 694 describes a suitable compaction technique.

[0047] A cathodic protective coating 50 is obtained which is electrically conductive and comprises an inorganic matrix 40 derived from the impregnation composition 20 which holds the particles 11 together and ensures their adhesion to the substrate 1. As indicated above, it is not outside the scope of the invention if the compaction is carried out after drying but before stabilization. Examples Example 1: Obtaining a covering electrophoretic deposit of significant thickness

[0048] A deposit of 6 aluminum particles was made by electrophoresis on the surface of a steel substrate. The coated substrate is visible at the figure 4 The deposition was carried out using an electrolyte 10 formed by a suspension of aluminum particles 11 in pure propan-2-ol without the addition of any additives. The deposition was carried out for a duration of 10 minutes under an electric field of 30 V.cm⁻¹. The deposit is opaque, homogeneous, and of significant thickness, in this case 100 µm. The resin indicated on the figure 4 corresponds to an epoxy resin used for coating the sample in order to be able to observe the sample under a scanning electron microscope.

[0049] Example 2: Influence of time on the deposition of cathodic protection particles Several deposits of aluminium particles (10 g / L) in a pure propan-2-ol electrolyte were carried out by imposing a constant electric field of 10 V.cm -1< and varying the deposition times between 3 minutes and 20 minutes.

[0050] The thickness of the deposit obtained varies from 15 µm to 90 µm depending on the deposition time ( figure 5 No decrease in deposition rate is observed over this range of deposition times, meaning that greater thicknesses could be achieved with longer deposition times. The only theoretical limitation on coating thickness would come from particle depletion in the suspension. No flaking of the deposit was observed up to thicknesses of approximately 200 µm. The measured porosities of these deposits are similar across the entire range of deposition times and are in the range of 50% to 60%, as shown in the figure 6 Porosity is independent of deposition time. Example 3: Influence of the electric field on the deposition of cathodic protection particles

[0051] Several aluminum particle depositions in a pure propan-2-ol electrolyte were performed by imposing a deposition time of 10 minutes and varying the applied electric field between 5 V.cm⁻¹ and 60 V.cm⁻¹. The results obtained are provided to figures 7 et 8 It is possible to achieve an aluminum powder deposition by electrophoresis with a controlled thickness over a wide range from 5 µm to 220 µm ( figure 7 ), while maintaining a similar microstructure, with a porosity varying from 50% to 60% ( figure 8 Applying higher electric field values ​​could allow for higher thickness values, up to 500 µm. Example 4: Demonstration of the cathodic protection conferred by the coating obtained by implementing the invention

[0052] A deposit of aluminum particles on a steel substrate was obtained by applying an electric field of 10 V.cm⁻¹ for 10 minutes. The deposit was then impregnated with sodium silicate by dip-shrinkage at a shrinkage rate of 300 mm.min⁻¹, subjected to heat treatment at 400°C for 3 hours, and then densified by mechanical compaction using sodium bicarbonate particles with a particle size between 100 µm and 300 µm at a relative pressure of 3 bars.

[0053] A galvanic coupling experiment was performed for 5 minutes to demonstrate that the resulting coating provided cathodic protection against the underlying steel substrate. The setup 100 used is illustrated in the figure 9 The tested coating 50 was electrically connected to a bare steel substrate 110 using zero-resistance ammeter mode ("ZRA"). The current flowing between the two electrodes was measured and corresponds to the galvanic coupling current. The common potential of the electrodes was also measured and plotted over time. As shown in the figure 9 The working electrode WE is connected to the bare 15CDV6 steel substrate. The counter electrode CE is either bare steel 1 (serving as a reference) or coated 50 (symbol 1 / 50 on the figure 9 designating one of the bare substrate 1 or the coating 50). The reference electrode 120 is an Ag / AgCl type electrode. All electrodes were immersed in a 0.05M sodium chloride bath 130. The figure 9 also shows the "COM" and "A" terminals of the ammeter and their connection to the WE and CE electrodes.

[0054] As shown by figure 10 The common potential of the bare substrate connected to coating 50 (curve "A") exhibits a higher cathodic potential than that of the substrate connected to the same bare substrate 1 (curve "B"). The more negative galvanic coupling potential indicates that cathodic protection is indeed effective. According to the figure 11 , the measured galvanic coupling current density stabilizes at a value of - 30 µA.cm -2< (curve « A ») which shows that the coating 50 provides, by its oxidation, electrons to the bare substrate 110 so that it is cathodically polarized and therefore protected.

[0055] The images of the different electrodes were taken following the galvanic coupling experiment. When two identical bare steel substrates are connected to each other, no significant current is observable and both substrates exhibit marked corrosion pitting following the test ( figures 12 And 13). In the case where the bare substrate is bonded to the coating being tested, no pitting corrosion is visible on the steel following the galvanic coupling test ( figure 14 The tested coating also shows no marked degradation ( figure 15 ). Example 5: Effect of mechanical compaction

[0056] Aluminum particles were deposited onto a 15CDV6 steel substrate by electrophoresis using an electrolyte consisting of a suspension of aluminum particles dispersed in propan-2-ol without the addition of any additives. The deposition was achieved by applying a series of pulsed voltage cycles alternating between zero potential difference and +10V with a duty cycle of 1 / 6. The pulse application frequency was 1 Hz, and the total treatment time was 30 minutes.

[0057] The deposit was then impregnated with sodium silicate by dipping and shrinking at a shrinkage rate of 300 mm / min. The assembly was then subjected to heat treatment at 400°C for 3 hours and subsequently densified by mechanical compaction using sodium bicarbonate particles with a particle size between 100 µm and 300 µm at a relative pressure of 3 bar. A reference deposit, identical in every respect to the previously described deposit except that compaction was omitted, was also prepared using the same procedure. The uncompacted deposit had a thickness of 22 µm and the compacted deposit a thickness of 18 µm.

[0058] A corrosion test was conducted under the following conditions: immersion in a solution of water + 0.05 M NaCl or 3 g / L NaCl. The specimens were continuously monitored with photographs taken at regular intervals.

[0059] The results obtained from this trial are provided to the figure 16 in which photographs of the reference coating, without compaction, are provided in the left column and those of the compacted coating in the right column. Without compaction, the coating acts as a barrier; however, the inherent defects in the deposit allow corrosive products to reach the substrate and initiate corrosion of the steel substrate. Compaction densifies the deposit by bringing the aluminum particles within it into contact. This contact of the aluminum particles ensures the formation of a dense, conductive film, and the aluminum acts as a sacrificial anode, thus improving corrosion resistance and retarding corrosion, as can be seen in the figure 16 , the appearance of corrosion pitting.

Claims

1. A method for forming a cathodic protection coating (50) on a substrate (1) forming a turbomachine part, comprising at least: - deposition, on the substrate, of particles (11) for cathodic protection of the substrate, this deposition being performed by electrophoresis from an organic electrolyte (10) comprising at least said particles in suspension in an organic liquid medium, the electrically charged particles moving and being deposited on the substrate in order to obtain the deposition of particles due to the application of an electric field between the substrate and a counter-electrode (15), and - forming an inorganic matrix (40) in pores of the deposit (6) of particles thus produced, comprising at least: • impregnating (20) said deposit with an impregnation composition, • drying heat treatment of the deposit impregnated by the impregnation composition, and • densifying the deposit by mechanical compacting, after the drying heat treatment, in order to make the deposit electrically conductive.

2. The method according to claim 1, wherein the organic electrolyte (10) comprises an alcoholic liquid medium in which the particles (11) are in suspension.

3. The method according to claim 2, wherein the alcoholic liquid medium is formed at least 50% by volume by propanol.

4. The method according to any one of claims 1 to 3, wherein a thickness (e) of the deposit (6) of cathodic protection particles (11) on the substrate (1) is greater than or equal to 40 µm.

5. The method according to any one of claims 1 to 4, wherein the cathodic protection particles (11) are made of aluminium or aluminium alloy.

6. The method according to any one of claims 1 to 5, wherein the substrate (1) is made of steel.

7. The method according to any one of claims 1 to 6, wherein the impregnation composition comprises at least one alkali metal silicate or alkaline earth metal silicate.

8. The method according to any one of claims 1 to 7, wherein the formation of the inorganic matrix comprises a stabilisation heat treatment of the deposit, the drying heat treatment being performed at a first temperature then the stabilisation heat treatment being performed at a second temperature greater than the first temperature, or a single and same treatment step in which the deposit is both dried and stabilised being performed, the stabilising and drying heat treatments being combined in this case.

9. The method according to claim 8, wherein the stabilisation heat treatment is performed before the densifying by mechanical compacting.

10. The method according to any one of claims 1 to 9, wherein a temperature less than or equal to 500°C is imposed during the drying heat treatment and the optional stabilisation heat treatment.

11. The method according to any one of claims 1 to 10, wherein the densifying by mechanical compacting of the deposit is carried out by projecting particles.

12. The method according to any one of claims 1 to 11, wherein the substrate (1) is a compressor shaft or a turbine shaft.

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