HOT ANTI-CORROSION COATINGS FOR TURBINE DISCS

Abstract

The present invention relates to the general field of hot corrosion protection of nickel-based alloys, particularly in the aeronautical field, specifically for turbine disks. More particularly, it relates to coating compositions designed to increase the resistance of nickel-based alloys to hot corrosion, especially type II corrosion, and their application in the aeronautical field, particularly for turbine disks. Figure for the abstract: None

Description

Title of the invention: HOT ANTI-CORROSION COATINGS FOR TURBINE DISCS Field of invention

[0001] The present invention relates to the general field of protection against hot corrosion of nickel-based alloys, in particular in the field of aeronautics, in particular for turbine discs.

[0002] It relates more particularly to coating compositions intended to increase the resistance of nickel-based alloys against hot corrosion, in particular type II, typically between 600°C and 850°C, and their application in the field of aeronautics, in particular for turbine disks. Technical background

[0003] In high and low pressure turbines, forged nickel-based alloys are used for the manufacture of discs. These materials are also used for aeronautical turbine applications, gas turbines, land-based turbines and marine turbines.

[0004] For this application, the Inconel 718 alloy (according to the AFNOR standard: NC19FeNb) serves as a reference material in terms of mechanical strength and resistance to the operating environment for temperatures up to 650°C. However, improving the performance of the turbine parts requires an increase in temperatures at the combustion chamber outlet and therefore an increase in the temperatures experienced by the turbines, and more particularly by the discs. The mechanical stresses (creep, fatigue, etc.) experienced by the parts in service are also increased. Thus, due to the microstructural instability of Inconel 718 (evolution of the hardening phases y” ô) above 650°C, forged nickel-based alloys with a y / y' structure such as AD730 are used, and preferred for the hottest disc stages. The AD730 alloy developed by Aubert & Duval appears to be a candidate for these hot turbine disc applications.This alloy, however, remains in the same range as other y / y' alloy grades already used for this type of application due to its proximity to their composition and micro structure.

[0005] These materials operate in hot environments with maximum temperatures ranging between 650 and 750°C, reaching up to 800°C at peak over short periods, in particular between 700 and 750°C, more particularly between 700 and 710°C. They are also subjected to atmospheres containing gaseous sulfur derivatives (SO2(g) / SO3(g)) resulting from the combustion of kerosene and reactions with pollutants ingested by the engine in operation. In addition, it is possible to see the accumulation of pollutant deposits (mainly sand) in the retention areas of the discs. Thus, the combined effect of temperature, atmosphere and pollutant deposits can lead to the initiation of hot corrosion phenomena.

[0006] Forged nickel-based alloy materials intended for use such as turbine discs contain chromium for the purpose of hardening the y matrix of the material so as to improve the mechanical characteristics but also to allow the development of a protective layer of chromia (Cr2O3) when exposed to heat. Although relatively loaded with chromium, nickel-based alloys of y / y' structure for discs have lower resistance compared to the Inconel 718 alloy despite relatively close chromium contents. The other alloying elements constituting the material therefore have an indirect role on the propensity of the substrate to develop a protective oxide layer via the establishment of an external oxide layer effective against hot corrosion of type IL

[0007] Depending on the operating conditions (temperature, duration of exposure, presence of pollutants), it may however be necessary to coat the turbine discs with a nickel-based alloy of y / y' or y / y' ' structure in order to provide them with a protective barrier against the environment.

[0008] These coatings have the major disadvantage of including chromates in their compositions, which are a species impacted by REACH regulations;

[0009] The work carried out by Safran aimed at studying the hot behavior of nickel-based alloys with a y / y' structure for new generation discs such as AD730, has shown that these alloys appear more sensitive to oxidation and corrosion phenomena than Inconel 718 as shown, respectively, in figures [Fig.l] and [Fig.2].

[0010] The new architectures which will incorporate these forged nickel-based alloys of type y / y', could be limited by the susceptibility of these alloys with respect to the operating environment in which they will have to endure:

[0011] - increased mechanical stresses and, consequently, the acceptable threshold corrosion indication will need to be reassessed;

[0012] - higher temperatures, leading to kinetics of phenomena increased chemical.

[0013] There is therefore a need for new corrosion protection coatings, adapted to future developments in turbines and which are compatible with the REACH regulation with regard to substances derived from hexavalent chromium (CrVI).

[0014] It is therefore necessary to have coatings which make it possible to limit the appearance of hot corrosion in the areas likely to see this phenomenon appear and which would make it possible to increase the service life of the parts in the critical areas likely to see corrosion appear. Summary of the invention

[0015] The invention thus relates to a hot anti-corrosion coating composition for nickel-based alloys, characterized in that it comprises:

[0016] - 16 to 18.5% by mass, preferably 16.8 to 18% by mass, of chromium,

[0017] - 12 to 25% by mass, preferably 18 to 25% by mass of iron,

[0018] - 1 to 5% by mass, preferably 1 to 3.5% by mass of titanium,

[0019] - 1.5 to 5% by mass, preferably 2 to 4% by mass of aluminum,

[0020] - 0 to 5% by mass of tungsten,

[0021] - 0 to 5% by mass of tantalum,

[0022] - 0 to 1% by mass of niobium,

[0023] - 0 to 5.5% by mass, preferably 0.5 to 5.5% by mass, more preferably 3 to 5.5% by mass of molybdenum,

[0024] relative to the total mass of the composition, and in that the total quantity of tungsten, tantalum and / or niobium, when present in the composition, remains less than 5% by mass, relative to the total mass of the composition,

[0025] the complement being made up of nickel and unavoidable impurities.

[0026] The total content of unavoidable impurities in the composition is at most 0.5% by mass, relative to the total mass of the composition.

[0027] It is well known to those skilled in the art that an alloy will never be pure and will therefore contain unavoidable impurities which are by-products of extraction of the majority elements or impurities introduced during production.

[0028] The compositions of the invention are nickel-based alloy coating compositions whose characteristics are intended to increase resistance to hot corrosion, in particular type II, produced at the outlet of the combustion chamber and constituting the environment of high and low pressure turbines.

[0029] The invention also relates to the use of a nickel-based alloy coated with a composition according to the invention, for the manufacture of a part for aeronautical turbines, gas turbines, land and marine turbines.

[0030] Another subject of the invention is a part made of nickel alloy, in particular of gamma / gamma prime (Y / Y') nickel alloy, coated with a composition according to the invention.

[0031] The part of the invention is a part of a turbojet, in particular a turbine disk, in particular aeronautical turbines, gas turbines, land and marine turbines. Brief description of the figures

[0032] The invention will be better understood, and other aims, details, characteristics and advantages thereof will appear more clearly on reading the detailed explanatory description which follows, of embodiments of the invention given as of purely illustrative and non-limiting examples, with reference to the attached schematic drawings in which:

[0033] [Fig.l] compares the oxidation facies of Inconel 718 and AD730 after exposure in air at 750°C for different times. The images clearly show the difference in behavior between AD730 and Inconel 718 and in particular the greater susceptibility of AD730 to oxidation.

[0034] [Fig.2] compares the corrosion facies between Inconel 718 and AD730 during an ex position at 700°C in the presence of gaseous corrosive agents (SO2(g)) and / or solid sulfate-based pollutants (Na2SO4(s)). This figure shows the greater susceptibility of AD730 to hot corrosion phenomena.

[0035] [Fig.3] represents examples of implementation of coating compositions according to the invention for the purpose of providing protection against corrosion to nickel-based alloys. “TTH” means “heat treatment”. Detailed description of the invention

[0036] The invention thus provides a hot anti-corrosion coating composition for nickel-based alloys characterized in that it comprises:

[0037] - 16 to 18.5% by mass, preferably 16.8 to 18% by mass, of chromium,

[0038] - 12 to 25% by mass, preferably 18 to 25% by mass of iron,

[0039] - 1 to 5% by mass, preferably 1 to 3.5% by mass of titanium,

[0040] - 1.5 to 5% by mass, preferably 2 to 4% by mass of aluminum,

[0041] - 0 to 5% by mass of tungsten,

[0042] - 0 to 5% by mass of tantalum,

[0043] - 0 to 1% by mass of niobium,

[0044] - 0 to 5.5% by mass, preferably 0.5 to 5.5% by mass, more preferably 3 to 5.5% by mass of molybdenum,

[0045] relative to the total mass of the composition, and in that the total quantity of tungsten, tantalum and / or niobium, when present in the composition, remains less than 5% by mass, relative to the total mass of the composition,

[0046] the complement being made up of nickel and unavoidable impurities.

[0047] The total content of unavoidable impurities in the composition is at most 0.5% by mass, relative to the total mass of the composition.

[0048] "Hot corrosion" as used in the present invention is defined as the attack of molten sulfate salts (Na2SO4) directly on metallic components in the presence of gaseous sulfur oxides (SO2(g) / SO3(g)), specifically on nickel-based alloys. Hot corrosion is characterized by pitting attack accompanied by subsurface attack beneath the corrosion products.

[0049] There are two main types of hot corrosion by sodium sulfate distinguished by the temperature range. Type I hot corrosion, called in English "High Temperature Hot Corrosion" (HTHC), which is the phenomenon that mainly occurs between 800 and 950 °C for nickel-based alloys. The degradation of nickel-based alloys by type I corrosion is very often characterized by the dissolution of the protective oxide layer in reaction with the salts, the formation of a non-protective, thick and porous oxide layer, by the development of cracks in the oxide layer, by a depletion of protective element as well as by internal sulfurization. Type II hot corrosion, called in English "Low Temperature Hot Corrosion" (LTHC), which is the phenomenon that mainly occurs between 650 and 800 °C for nickel-based alloys.At these temperatures, the attack is less severe than Type I corrosion in terms of thickness affected. The material has a slight internal sulfidation front and less profound depletion of protective element. However, it may have corrosion pits and a layer of corrosion products consisting of a mixture of non-protective oxide and / or sulfates on its surface.

[0050] The coating compositions according to the invention make it possible to increase the resistance of nickel-based alloys against type IL hot corrosion.

[0051] Nickel-based alloys, also called "superalloys", may be chosen, for example, from WASPALOY®, Alloy X-750, Alloy 36, Alloy 80A, Alloy 201, Alloy 625, Alloy 625 LCF® I HP®, Alloy 718, Alloy 738, Alloy L 605, VDM® Alloy 75, VDM® Alloy 188, VDM® Alloy 7188, VDM® Alloy 780, René65, René 77, AD®730, Udimet® 720. This list is not exhaustive.

[0052] Concerning the composition, nickel is the major constituent of a coating composition according to the invention which is added with different addition elements.

[0053] The elements iron (Fe), tungsten (W), chromium (Cr), and tantalum (Ta) mainly participate in the hardening of the austenitic matrix of face-centered cubic (fcc) crystal structure called y. The quantities of W and Mo must however be limited to avoid the formation of fragile phases called TCP (q, o) and reduce the corrosion resistance offered by the coating.

[0054] The elements aluminum (Al), titanium (Ti) and tantalum (Ta) promote the precipitation of the hardening phase Ni3(Al, Ti, Ta) called the y' phase, with an ordered cubic structure Ll2.

[0055] From the point of view of chemical resistance to the environment, the addition of iron to a composition according to the invention in significant proportions has proven very favorable for increasing resistance to hot corrosion of type IL.

[0056] Since iron cannot be increased indefinitely, other elements can promote chromium activity without it being necessary to significantly increase its proportion in the alloy. This concerns the cases of elements such as tungsten (W), molybdenum (Mo) or tantalum (Ta) added within the limits of the specified compositions. Tungsten (W), molybdenum (Mo) and / or tantalum (Ta) may be added individually or in combination. The total quantity of these elements must remain less than 5% by mass, relative to the total mass of the composition, to avoid the direct formation of associated oxides.

[0057] In addition, the y' phase content in the coating provides increased mechanical and chemical compatibility of the coating with y / y' type alloys but is not intended to provide mechanical properties as high as the underlying alloy.

[0058] The experimental results confirm that the tested compositions are most likely to offer increased resistance to corrosion while maintaining microstructural characteristics making it possible to guarantee the coatings good compatibility properties with nickel-based alloys of type y / y' and making it possible to achieve a high Cr activity, higher than that of the substrate over the temperature range of interest, promoting resistance to the phenomenon of hot corrosion.

[0059] According to one embodiment, the coating composition of the invention comprises 16.8 to 18% by mass of chromium, relative to the total mass of the composition.

[0060] According to another embodiment, the coating composition of the invention comprises 18 to 25% by mass of iron, relative to the total mass of the composition.

[0061] According to another embodiment, the coating composition of the invention comprises 1 to 3.5% by mass of titanium, relative to the total mass of the composition.

[0062] According to another embodiment, the coating composition of the invention comprises 2 to 4% by mass of aluminum, relative to the total mass of the composition.

[0063] According to another embodiment, the coating composition of the invention does not comprise tungsten.

[0064] According to another embodiment, the coating composition of the invention does not comprise tantalum.

[0065] According to another embodiment, the coating composition of the invention does not comprise niobium.

[0066] According to one embodiment, the composition does not comprise molybdenum.

[0067] According to one embodiment, the composition comprises 0.5 to 3% by mass of molybdenum, relative to the total mass of the composition.

[0068] When the composition comprises tungsten and / or tantalum and / or niobium and / or molybdenum, the total quantity of these elements must remain less than 5% by mass, relative to the total mass of the composition.

[0069] According to one embodiment of the invention, the coating composition of the invention comprises:

[0070] - 16.8% by mass of chromium,

[0071] - 18% by mass of iron,

[0072] - 3.5% by mass of titanium,

[0073] - 2.1% by mass of aluminum,

[0074] - 5% by mass of molybdenum,

[0075] relative to the total mass of the composition,

[0076] the complement being made up of nickel and unavoidable impurities.

[0077] According to another embodiment of the invention, the coating composition of the invention comprises:

[0078] - 18% by mass of chromium,

[0079] - 25% by mass of iron,

[0080] - 3.5% by mass of titanium,

[0081] - 2% by mass of aluminum,

[0082] - 5% by mass of molybdenum,

[0083] relative to the total mass of the composition,

[0084] the complement being made up of nickel and unavoidable impurities.

[0085] According to yet another embodiment of the invention, the coating composition of the invention comprises:

[0086] - 18% by mass of chromium,

[0087] - 25% by mass of iron,

[0088] - 1% by mass of titanium,

[0089] - 4% by mass of aluminum,

[0090] - 5% by mass of molybdenum,

[0091] relative to the total mass of the composition,

[0092] the complement being made up of nickel and unavoidable impurities.

[0093] The compositions can be manufactured by any technique known to those skilled in the art (casting, foundry, powder metallurgy depending on the type of implementation intended). The ingots produced can be atomized to manufacture powder(s) intended to be projected alone or mixed with a binder or even as a filler metal (target, wires, etc.).

[0094] The coating compositions of the invention can be deposited in different ways. An optional stabilization heat treatment can be carried out to improve the compatibility between the alloy and the deposited coating at the end of the coating removal treatment.

[0095] Ultimately this allows us to consider:

[0096] - prolonged use of parts in service or to propose extensions of service lives for areas affected by possible hot corrosion phenomena;

[0097] - use of materials / parts at higher temperatures or in environments operations more likely to initiate hot corrosion phenomena (highly polluted environments, rich in SO2(g));

[0098] - the possibility of increasing the mechanical stresses experienced by the parts in service at higher temperatures by having the mechanical characteristics carried by the substrate while minimizing the need for environmental resistance which is carried by the coating.

[0099] In a non-exhaustive manner, the methods listed below can make it possible to implement the compositions listed:

[0100] i) Thermal spraying (atmospheric plasma spraying or APS, vacuum plasma spraying or VPS, hypersonic thermal spraying in English High Velocity Oxy Fuel or HVOF, dynamic projection by cold gas or Cold Spray) —> Alloying of the coating provided by the projected powder;

[0101] ii) Physical vapor deposition or PVD (cathodic arc, magnetron sputtering, high power impulse magnetron sputtering or HiPIMS) —> Alloying of the coating provided by the targets or filler metal;

[0102] iii) Slurry (spraying, immersion (dipping / removing), painting (brushing)) —> Alloying of the coating provided by the deposit of the slip. The metallic particles can, in certain configurations, be associated with inorganic binders. In the latter case, the consolidation of the coating requires a heat treatment which allows the particles to be polymerized / consolidated and held together and ensures the adhesion of the coating to the surface of the substrate.

[0103] In i), this involves the projection of molten powder by a heat source of the alloy coating compositions of the invention onto the surface of the part to be protected.

[0104] A heat treatment can be added after coating removal to promote adhesion and coating / substrate compatibility. Depending on the temperature of this heat treatment there may be coating / substrate interdiffusion which will ensure chemical anchoring between the two.

[0105] In ii), this is a method of evaporating the targets of alloy coating compositions of the invention on the surface of the part to be protected.

[0106] A heat treatment may be added after coating deposition to promote coating / substrate adhesion and compatibility. Depending on the temperature of this heat treatment, there may be coating / substrate interdiffusion which will ensure chemical anchoring between the two.

[0107] In iii), the alloy compositions are deposited either alone by spraying from a slurry followed by a consolidation treatment /

[0108]

[0109]

[0110] [YES]

[0112]

[0113]

[0114]

[0115]

[0116]

[0117]

[0118] diffusion which would be determined according to the coating / alloy pair. In another embodiment, the powder consisting of the compositions of the alloys of the invention is associated with an inorganic binder making it possible to have a network which holds the particles together and makes the connection with the substrate. The assembly is then heat treated to polymerize the inorganic binder. These are all processes well known to those skilled in the art who will know how to adapt the operating conditions according to the type of alloy and coating. The coating compositions of the invention make it possible to effectively protect materials and parts based on nickel alloy, for applications in the field of aeronautics, in particular turbine disks. Thus, the invention also relates to the use of a nickel-based alloy coated with a composition according to the invention, for the manufacture of a part for aeronautical turbines, land and marine turbines, and gas turbines. The coating compositions of the invention make it possible to delay the appearance of corrosion on the surface of the substrate and thus extend the period during which the alloy is not degraded by the operating environment. Another subject of the invention is a part made of nickel alloy, in particular of gamma / gamma prime (Y / Y') nickel alloy, coated with a composition according to the invention. The composition of the coating is advantageously close to that of the basic alloy constituting the part it covers, making it possible to envisage good compatibility between the two parts of the system (mechanical properties, differences in thermal expansion coefficient, etc.). The coating is both more environmentally resistant than the base alloy and allows for longer operating times before the base alloy constituting the part is reached, the duration of effectiveness being conditioned by the thickness of the coating. Depending on the implementation process, the latter can vary from 5 to 500 pm. In the case of a specific application, the thickness can be between 100 and 500 pm for an application by thermal spraying, between 5 and 150 pm for an application by PVD and between 30 and 100 pm for an application by slip spraying. The part of the invention is a part of a turbojet, in particular a turbine disk, in particular aeronautical turbines, gas turbines, land and marine turbines. Examples Alloy Ni Cr Fe Co Nb Ti Mo Al W EX1 bal. 16.8 18 3.5 5 2.1 EX2 bal. 18 25 3.5 5 2 EX3 ball. 18 25 1 5 4 René65 ball. 16 0.9 13 0.7 3.7 4 2.1 4 AD730 16 4.3 9 1.1 3.6 3 2 2.5

[0119] Three coating compositions Exl to Ex3 were studied and compared to the commercial alloys René65 and AD730. Chrome Activity:

[0120] The CALPHAD method was used to calculate the chromium activity in the y phase. The coating compositions of Ex 1 to Ex 3 have chromium activities at 650°C and 700°C higher than the intrinsic ones of the commercial alloys René65 and AD730. Thus, a René65 or AD730 alloy coated with a coating composition according to the invention would exhibit improved behavior with respect to oxidation / corrosion resistance than the same alloy uncoated.

[0121] The alloy compositions according to the invention demonstrate their interest as a protective coating against oxidation / corrosion for nickel-based alloys. Molar fraction of phase y':

[0122] The CALPHAD method was used to calculate the gamma prime phase mole fraction (y') in the compositions at two temperatures 650°C 700°C. The coating compositions of examples Exl-Ex 3 according to the invention have gamma prime phase mole fractions greater than 23 mol% regardless of the temperature. These gamma prime phase mole fractions are lower than those of the commercial René65 and AD730 alloys, but they are close enough to ensure the microstructural compatibility of the coating composition when it is deposited as a coating on a commercial René65 or AD730 alloy. Alloy a(Cr)650°C a(Cr)700°C y' 650 °C (mol.%) y' 700 °C (mol.%) Exl 0.79 0.53 31.6 30.2 Ex2 0.76 0.52 24.2 32.9 Ex3 0.79 0.55 28.7 23.5 René65 0.67 0.46 38.7 38.0 AD730 0.73 0.49 37.1 36.0

[0123] In terms of implementation, the compositions listed in the table above are defined as the coating compositions to be achieved at the end of the application. Depending on the process implemented, the filler materials (powder, wire, target, slip, etc.) may have a starting composition slightly different from the compositions targets to compensate for losses on certain elements linked to the implementation of the process.

Claims

Claims

1. Nickel-based alloy hot-dip anti-corrosion coating composition characterized in that it comprises: - 16 to 18.5% by mass of chromium, - 12 to 25% by mass of iron, - 1 to 5% by mass of titanium, - 1.5 to 5% by mass of aluminum, - 0 to 5% by mass of tungsten, - 0 to 5% by mass of tantalum, - 0 to 1% by mass of niobium, - 0 to 5.5% by mass of molybdenum, relative to the total mass of the composition, and in that the total quantity of tungsten, tantalum and / or niobium, when present in the composition, remains less than 5% by mass, relative to the total mass of the composition, the remainder being constituted by nickel and unavoidable impurities.

2. Composition according to claim 1, characterized in that it comprises 16.8 to 18% by mass of chromium, relative to the total mass of the composition.

3. Composition according to one of claims 1 or 2, characterized in that it comprises 18 to 25% by mass of iron, relative to the total mass of the composition.

4. Composition according to any one of claims 1 to 3, characterized in that it comprises 1 to 3.5% by mass of titanium, relative to the total mass of the composition.

5. Composition according to any one of claims 1 to 4, characterized in that it comprises 2 to 4% by mass of aluminum, relative to the total mass of the composition.

6. Composition according to any one of claims 1 to 5, characterized in that it comprises: - 16.8% by mass of chromium, - 18% by mass of iron, - 3.5% by mass of titanium, - 2.1% by mass of aluminum, - 5% by mass of molybdenum, relative to the total mass of the composition, the remainder being constituted by nickel and unavoidable impurities.

7. Composition according to any one of claims 1 to 5, characterized in that it comprises: - 18% by mass of chromium, - 25% by mass of iron, - 3.5% by mass of titanium, - 2% by mass of aluminum, - 5% by mass of molybdenum, relative to the total mass of the composition, the remainder being constituted by nickel and unavoidable impurities.

8. Composition according to any one of claims 1 to 5, characterized in that it comprises: - 18% by mass of chromium, - 25% by mass of iron, - 1% by mass of titanium, - 4% by mass of aluminum, - 5% by mass of molybdenum, relative to the total mass of the composition, the remainder being constituted by nickel and unavoidable impurities.

9. Use of a nickel-based alloy coated with a composition according to any one of claims 1 to 8, for the manufacture of a part for aeronautical turbines, gas turbines, land and marine turbines.

10. Part made of nickel alloy, in particular of gamma / gamma prime (Y / Y') nickel alloy, coated with a composition according to any one of claims 1 to 8.

11. Part according to claim 10, characterized in that it is a part of a turbojet, in particular a disk of aeronautical turbines, gas turbines, land and marine turbines.

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