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 Scope of the invention

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

[0002] It is aimed more particularly at 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 manufacturing discs. These materials are also used for aeronautical turbine applications, gas turbines, land-based and marine turbines.

[0004] For this application, Inconel 718 alloy (according to AFNOR standard: NC19FeNb) serves as the reference material in terms of mechanical strength and resistance to the operating environment at temperatures up to 650°C. However, improving the performance of the turbine components requires an increase in combustion chamber outlet temperatures and therefore an increase in the temperatures experienced by the turbines, and more specifically by the discs. The mechanical stresses (creep, fatigue, etc.) experienced by the components 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 disk stages. The AD730 alloy developed by Aubert & Duval appears to be a candidate for these hot turbine disk applications.However, this alloy remains in the same range as other y / y' alloy grades already used for this type of application due to its similarity in composition and microstructure.

[0005] These materials operate in hot environments with maximum temperatures ranging from 650 to 750°C, reaching up to 800°C for short periods, particularly between 700 and 750°C, and more specifically between 700 and 710°C. They are also subjected to atmospheres containing gaseous sulfur compounds (SO2(g) / SO3(g)) resulting from kerosene combustion and reactions with pollutants ingested by the operating engine. Furthermore, it is possible to see The accumulation of pollutant deposits (mainly sand) in the disc retention areas. 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 disks contain chromium to harden the y-matrix of the material in order to improve mechanical properties and also to allow the development of a protective chromium (Cr2O3) layer when exposed to heat. Although relatively chromium-rich, y / y' structure nickel-based disk alloys exhibit lower strength compared to Inconel 718 alloy, despite relatively similar chromium contents. The other alloying elements constituting the material therefore have an indirect role in the substrate's propensity to develop a protective oxide layer by establishing an external oxide layer effective against IL-type hot corrosion

[0007] Depending on the operating conditions (temperature, duration of exposure, presence of pollutants), it may however be necessary to coat the nickel-based alloy turbine discs with 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 in their compositions chromates which are a species impacted by the REACH regulation;

[0009] The work carried out by Safran aimed at studying the hot behavior of nickel-based alloys of 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.1] and [Fig.2].

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

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

[0012] - higher temperatures, leading to altered kinetics of the phenomena increased chemicals.

[0013] There is therefore a need for new corrosion protection coatings, adapted to the future developments of 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 that limit the occurrence of hot corrosion in areas likely to see this phenomenon and that would increase the lifespan of parts in critical areas likely to see corrosion. Summary of the invention

[0015] The invention thus relates to a hot anti-corrosion coating composition of 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 aluminium,

[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% molybdenum by mass,

[0024] in relation 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, in relation 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 inevitable impurities which are by-products of the extraction of the major elements or impurities introduced during the manufacturing process.

[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 object of the invention is a nickel alloy part, in particular a 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 engine, in particular a turbine disc, especially aeronautical turbines, gas turbines, land and marine turbines. Brief description of the figures

[0032] The invention will be better understood, and other objects, details, features and advantages thereof will become more apparent upon reading the following detailed explanatory description of given embodiments of the invention. by way of purely illustrative and non-limiting examples, with reference to the attached schematic drawings in which:

[0033] [Fig. 1] compares the oxidation patterns of Inconel 718 and AD730 after exposure in air at 750°C for different durations. The photographs 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 patterns between Inconel 718 and AD730 during a Exposure to 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] shows examples of the implementation of coating compositions according to the invention for the purpose of providing corrosion protection to nickel-based alloys. "TTH" means "heat treatment". Detailed description of the invention

[0036] The invention thus proposes a hot anti-corrosion coating composition of 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 aluminium,

[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% molybdenum by mass,

[0045] in relation 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, in relation 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 context of 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 corrosion accompanied by subsurface corrosion beneath the corrosion products.

[0049] There are two main types of hot corrosion by sodium sulfate, distinguished by their temperature range. Type I hot corrosion, also known as High Temperature Hot Corrosion (HTHC), is the phenomenon that occurs predominantly 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 thick, porous, non-protective oxide layer, the development of cracks in the oxide layer, depletion of the protective element, and internal sulfidation. Type II hot corrosion, also known as Low Temperature Hot Corrosion (LTHC), is the phenomenon that occurs predominantly between 650 and 800°C for nickel-based alloys.At these temperatures, the attack is less severe than type I corrosion in terms of the affected thickness. The material exhibits a slight internal sulfidation front and less deep depletion of protective elements. However, it may show pitting corrosion and a layer of corrosion products consisting of a mixture of non-protective oxides 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 hot-fired IL corrosion

[0051] Nickel-based alloys, also known as "superalloys", can be selected, 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] Regarding the composition, nickel is the major constituent of a coating composition according to the invention which is supplemented with various addition elements.

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

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

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

[0056] Since iron cannot be increased indefinitely, other elements can promote chromium activity without the need for a significant increase its proportion in the alloy. This applies to elements such as tungsten (W), molybdenum (Mo), or tantalum (Ta) added within the specified composition limits. Tungsten (W), molybdenum (Mo), and / or tantalum (Ta) may be added individually or in combination. The total quantity of these elements must remain below 5% by mass of the total composition to prevent the direct formation of associated oxides.

[0057] Furthermore, the content of phase y' in the coating allows for 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 compositions tested are most likely to offer increased resistance to corrosion while maintaining microstructural characteristics that guarantee the coatings good compatibility properties with y / y' type nickel alloys and allow for high Cr activity, higher than that of the substrate over the temperature range of interest, promoting resistance to the hot corrosion phenomenon.

[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 aluminium, relative to the total mass of the composition.

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

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

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

[0066] According to one embodiment, the composition does not include 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 includes 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 aluminium,

[0074] - 5% by mass of molybdenum,

[0075] in relation 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 includes:

[0078] - 18% by mass of chromium,

[0079] - 25% by mass of iron,

[0080] - 3.5% by mass of titanium,

[0081] - 2% by mass of aluminium,

[0082] - 5% by mass of molybdenum,

[0083] in relation 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 aluminium,

[0090] - 5% by mass of molybdenum,

[0091] in relation 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 produce powder(s) intended to be projected alone or mixed with a binder, or used as filler metal (target, wire, etc.).

[0094] The coating compositions of the invention can be deposited in various ways. A possible stabilizing heat treatment can be carried out to improve the compatibility between the alloy and the coating deposited after the coating deposition process.

[0095] In the long term, this allows us to consider:

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

[0097] - use of materials / parts at higher temperatures or in environments more likely to initiate hot corrosion phenomena (heavily 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 processes listed below can be used 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 cold gas spraying or Cold Spray) —> Coating alloy provided by the sprayed powder;

[0101] ii) Physical vapor deposition or PVD (cathode arc, magnetron sputtering, high power impulse magnetron sputtering or HiPIMS) —> Coating alloy brought by the targets or filler metal;

[0102] iii) Slurry (spraying, immersion (dipping / removal), painting (brush)) —> Coating alloy imparted by the slurry deposition. The metallic particles can, in certain configurations, be associated with inorganic binders. In the latter case, coating consolidation requires a heat treatment that polymerizes / consolidates and binds the particles together, ensuring the coating's adhesion to the substrate surface.

[0103] In i), it refers to 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 the coating has been applied to promote adhesion and compatibility between the coating and the substrate. Depending on the temperature of this heat treatment, interdiffusion between the coating and the substrate may occur, ensuring chemical bonding between the two.

[0105] In ii), it 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 the coating has been applied to promote adhesion and coating / substrate compatibility. Depending on the temperature of this During heat treatment, there may be interdiffusion between the coating and the substrate, which will ensure chemical bonding between the two.

[0107] In iii), the alloy compositions are deposited either alone by spraying from a slurry followed by a consolidation / diffusion treatment which would be determined according to the coating / alloy pair.

[0108] In another embodiment, the powder made up of the alloy compositions of the invention is associated with an inorganic binder allowing a network to be formed which holds the particles together and links them to the substrate.

[0109] The assembly is then heat-treated to polymerize the inorganic binder.

[0110] These are all processes well known to a person skilled in the art who will be able to adapt the operating conditions according to the type of alloy and coating.

[0111] 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 of the turbine disk type.

[0112] 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.

[0113] 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 time during which the alloy is not degraded by the operating environment.

[0114] Another object of the invention is a nickel alloy part, in particular a gamma / gamma prime (y / y') nickel alloy, coated with a composition according to the invention.

[0115] The composition of the coating is advantageously close to that of the base alloy constituting the part it covers, allowing for good compatibility between the two parts of the system (mechanical properties, differences in thermal coefficient of expansion, etc.).

[0116] The coating is both more resistant to the environment than the base alloy and allows for extended operating times before the base alloy of the part is affected, the effective lifespan being determined by the coating thickness. Depending on the application method, this thickness can vary from 5 to 500 µm. In the case of a specific application, the thickness can be between 100 and 500 µm for thermal spraying, between 5 and 150 µm for PVD, and between 30 and 100 µm for slurry spraying.

[0117] The part of the invention is a part of a turbojet engine, in particular a turbine disc, especially 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 bal. 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 Ex3 have chromium activities at 650°C and 700°C that are higher than the intrinsic activities of the commercial René65 and AD730 alloys. Thus, a René65 or AD730 alloy coated with a coating composition according to the invention would exhibit improved oxidation / corrosion resistance compared to the same uncoated alloy.

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

[0122] The CALPHAD method was used to calculate the gamma prime phase mole fraction (y') in compositions at two temperatures, 650°C and 700°C. The coating compositions of the Exl-Ex 3 examples according to the invention exhibit gamma prime phase mole fractions greater than 23 mol% at both temperatures. These gamma prime phase mole fractions are lower than those of the commercial alloys René65 and AD730, but they are sufficiently close to ensure microstructural compatibility of the coating composition when it is deposited as a coating on a commercial alloy René65 or AD730. 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 after application. Depending on the process implemented, the filler materials (powder, wire, target, slurry, etc.) may have a starting composition slightly different from the target compositions to compensate for losses of certain elements related to the implementation of the process.

Claims

Demands

1. A nickel alloy part, in particular a gamma / gamma prime (y / y') nickel alloy, coated with a nickel-based alloy hot-dip anti-corrosion coating composition, characterized in that said composition 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 amount 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 composed of nickel and impurities unavoidable, the total content of which in the composition is at most 0.5% by mass, relative to the total mass of the composition.

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

3. Part according to claim 1 or 2, characterized in that the composition comprises 18 to 25% by mass of iron, relative to the total mass of the composition.

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

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

6. Composition of a hot-applied, nickel-based alloy anti-corrosion coating, 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 made up of nickel and unavoidable impurities whose total content in the composition is at most 0.5% by mass, relative to the total mass of the composition.

7. A nickel-based alloy hot-applied anti-corrosion coating composition, 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 nickel and unavoidable impurities, the total content of which in the composition is at most 0.5% by mass, relative to the total mass of the composition.

8. A nickel-based alloy hot-applied anti-corrosion coating composition, 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 nickel and unavoidable impurities, the total content of which in the composition is at most 0.5% by mass, relative to the total mass of the composition.

9. Use of a nickel-based alloy coated with a hot-applied nickel-based alloy anti-corrosion coating composition according to any one of claims 6 to 8, for the manufacture of a part for aeronautical turbines, gas turbines, land-based and marine turbines.

10. Nickel alloy part, in particular gamma / gamma prime (y / y') nickel alloy, coated with a coating composition hot anti-corrosion of nickel-based alloy according to any one of claims 6 to 8.

11. Part according to any one of claims 1 to 5 and 10, characterized in that it is a part of a turbojet engine, in particular a disc of aeronautical turbines, gas turbines, land and marine turbines.

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