NICKEL-BASED ALLOY WITH STRUCTURAL HARDENING BY γ' PRECIPITATION, RESISTANT TO CORROSION BY MELTED SALTS

Abstract

The present invention relates to a nickel-based alloy characterized in that it comprises, by mass percentage: - 15 to 25% molybdenum; - 0.4 to 5% aluminium; - 0 to 2% titanium; - < 3% chromium; - < 3% iron; - < 3% tungsten; - < 0.1% impurities (Im1), the impurities (Im1) being cobalt, zirconium, niobium, tantalum, hafnium, silicon, yttrium, lanthanum, cerium, manganese, magnesium, copper, palladium, platinum, vanadium and carbon; - < 0.01% impurities (Im2), the impurities (Im2) being sulfur, phosphorus and boron; nickel representing the balance to reach 100%. An alloy according to the invention can be used as a structural material or as a coating for parts subjected to corrosion by molten salts, particularly at high temperatures, i.e., between 400 and 800°C, such as fluoride salts and, more particularly, chloride salts. Figure for the abstract: None

Description

Title of the invention: NICKEL-BASED ALLOY WITH STRUCTURAL HARDENING BY PRECIPITATION OF γ' RESISTANT TO CORROSION BY MELTED SALTS Technical field of the invention

[0001] The present invention is in the field of nickel-based metal alloys, usable for example as a structural material or as a coating for parts subjected to corrosion by molten salts at high temperature (between 400 and 800 °C), such as fluoride salts and, more particularly, chloride salts.

[0002] More particularly, the present invention relates to new nickel-based alloys, containing little or no chromium or iron, resistant to corrosion by molten salts at high temperature (between 400 and 800 °C), preferably by chloride salts. Technical background

[0003] Molten salts are used in many industrial installations, notably as heat transfer fluids for solar power plants or as fuel for new-generation nuclear reactors. A metallic alloy, usable as a structural material or coating, for parts subjected to corrosion in molten salt environments must therefore be able to withstand and not degrade in such environments.

[0004] However, under certain conditions, the corrosion resistance of nickel-based alloys may prove to be limited, particularly in salts, such as chloride salts at high temperature (between 400 and 800 °C).

[0005] It is therefore necessary to develop nickel-based alloys that are more resistant to corrosion in chloride salts at high temperatures (from 400 to 800 °C) than those already existing.

[0006] For example, two early molten salt reactors at the Oak Ridge National Laboratory (ORNL), the Aircraft Reactor Experiment (ARE) and the Molten Salt Reactor Experiment (MSRE), demonstrated the feasibility of Molten Salt Reactors (MSRs). The salt in these two reactors is a fluoride (NaF-ZrF4-UF4 for the ARE, and LiF-BeF2-ZrF4-UF4 for the MSRE). These developments enabled the collection of corrosion data in fluoride salt environments. More recently, further studies have been conducted to determine the resistance of nickel-based alloys to fluoride salts. All these studies have led to the optimization of the fluoride salt corrosion resistance of nickel-based alloys. The reference nickel-based alloy for fluoride salt corrosion studies is Hastelloy®N. The Hastelloy® N alloy was developed specifically for use in nuclear reactors with fluoride salts as the coolant. However, this alloy has poor corrosion resistance in chloride salt environments. Other commercial nickel-based metal alloys, such as Inconel 625 or Hastelloy C-276, appear more promising but still exhibit high corrosion rates in chloride salt environments. Excessive corrosion rates will reduce the material's lifespan and consequently increase costs.

[0007] Other materials such as ceramics corrode little in contact with chloride salts, but their difficult shaping and their specific mechanical properties require designs that are not yet mature.

[0008] There is therefore a need for a nickel-based alloy that provides very good resistance to corrosion from molten salts, and in particular from chloride salts at high temperatures, i.e. between 400 and 800°C. Summary of the invention

[0009] The present invention aims to meet these needs and overcome the disadvantages mentioned above.

[0010] Thus, the present invention relates to a nickel-based alloy characterized in that it comprises, by mass percentage:

[0011] - 15 to 25% molybdenum;

[0012] - 0.4 to 5% aluminium;

[0013] - 0 to 2% titanium;

[0014] - < 3% chromium;

[0015] - < 3% iron;

[0016] - <3% tungsten;

[0017] - < 0.1% impurities (Iml), the impurities (Iml) being cobalt, zirconium, niobium, tantalum, hafnium, silicon, yttrium, lanthanum, cerium, manganese, magnesium, copper, palladium, platinum, vanadium and carbon;

[0018] - < 0.01% of impurities (Im2), the impurities (Im2) being sulfur, phosphorus and boron;

[0019] nickel representing the balance to reach 100%.

[0020] An alloy according to the invention must simultaneously satisfy the following equations:

[0021] 15 <WMo+Ww<25

[0022] and 0 < WCr+ WFe< 3

[0023] with WMo, Ww, WCr and WFer representing the mass percentages of molybdenum, tungsten, chromium and iron.

[0024] The alloy of the invention exhibits very good resistance to corrosion from molten salts, and in particular from chloride salts at high temperature, i.e. between 400 and 800°C.

[0025] Furthermore, it contains little or no chromium and / or iron.

[0026] An alloy according to the invention can be obtained by conventional processes such as casting, powder metallurgy, but also by laser additive manufacturing, welding-inspired additive manufacturing, or dynamic gas spraying (or cold spray). These processes are well known to those skilled in the art, who will be able to adapt the operating conditions to the type of alloy to be obtained.

[0027] An alloy according to the invention can be used as a structural material or as a coating for parts subjected to corrosion by molten salts at high temperature, i.e. between 400 and 800°C, such as fluoride salts and, more particularly, chloride salts.

[0028] Another object of the invention relates to the use of an alloy according to the invention as:

[0029] - structural material in contact with molten salts in concentration plants solar;

[0030] - structural material for • molten salt nuclear reactors, regardless of the neutron spectrum (fast or thermal); • the processes for reprocessing spent nuclear fuel; • Zr / Hf separation processes;

[0031] - material for the superheating tubes of biomass power plants.

[0032] Another object of the invention is a part made of or coated with a nickel-based alloy according to the invention.

[0033] Another object of the invention is a part according to the invention intended to be used in a molten salt reactor, in particular a high-temperature molten salt reactor between 400 and 800°C, such as chloride salts.

[0034] The part may be a component of the reactor such as the vessel, tank, piping, pumps, heat exchangers and neutron reflectors. Brief description of the figures

[0035] Other features and advantages of the invention will become apparent from the detailed description that follows with reference to the attached drawings.

[0036] [Fig-1] is a diagram of the experimental setup used for tests in salt media melted.

[0037] [Fig.2] represents a ZEISS EVO Scanning Electron Micrograph (SEM) in backscattered electrons (BSE) of a cross-section of the NiMolloy 20 HIP alloy.

[0038] [Fig.3] represents a ZEISS EVO Scanning Electron Micrograph (SEM) in backscattered electrons (BSE) of a cross-section of the NiMolloy 20 AM (Additive Manufacturing) sample.

[0039] [Fig.4a] represents a Scanning Electron Micrograph (SEM) ZEISS EVO in backscattered electrons (BSE) of a cross-section of the NiMolloy 20 HIP alloy.

[0040] [Fig.4b] represents the EDS maps of the BRUKER X-Flash 6 detector on Mi ZEISS EVO Scanning Electron Microscope (SEM) of a cross-section of the NiMolloy 20 HIP alloy. The light area corresponds to the presence of the chemical element indicated in the lower left corner.

[0041] [Fig.5a] represents a Scanning Electron Micrograph (SEM) ZEISS EVO in backscattered electrons (BSE) of a cross-section of NiMolloy 20 AM alloy (Additive Manufacturing).

[0042] [Fig. 5b] represents the EDS maps of the BRUKER X-Flash 6 detector on Mi ZEISS EVO Scanning Electron Microscope (SEM) of a cross-section of the NiMolloy 20 AM alloy (Additive Manufacturing). The light area corresponds to the presence of the chemical element indicated in the lower left corner.

[0043] [Fig.6] represents a) a ZEISS EVO SEM micrograph in BSE and a line on which was carried out the EDS chemical analysis with the BRUKER X-Flash 6 detector of the NiMolloy 20 HIP sample; b) the dosage curves of the chemical elements of the sample as a function of depth (along the line). Detailed description of the invention

[0044] The present invention relates to a nickel-based alloy characterized in that it comprises, by mass percentage:

[0045] - 15 to 25% molybdenum;

[0046] - 0.4 to 5% aluminium;

[0047] - 0 to 2% titanium;

[0048] - < 3% chromium;

[0049] - < 3% iron;

[0050] - <3% tungsten;

[0051] - < 0.1% impurities (Iml), the impurities (Iml) being cobalt, zirconium, niobium, tantalum, hafnium, silicon, yttrium, lanthanum, cerium, manganese, magnesium, copper, palladium, platinum, vanadium and carbon;

[0052] - < 0.01% of impurities (Im2), the impurities (Im2) being sulfur, phosphorus and boron;

[0053] nickel representing the balance to reach 100%.

[0054] According to one embodiment of the invention, the nickel-based alloy comprises, by mass percentage, from 0 to less than 1% of chromium.

[0055] According to another preferred embodiment of the invention, the nickel-based alloy comprises, by mass percentage, from 0 to less than 1% of iron.

[0056] According to another preferred embodiment of the invention, the nickel-based alloy comprises, in mass percentage, from 0 to less than 1% of tungsten.

[0057] An alloy according to the invention must simultaneously satisfy the following equations:

[0058] 15 <WMo+Ww<25

[0059] and 0 < WCr+ WFe< 3

[0060] with WMo, Ww, WCr and WFer representing the mass percentages of molybdenum, tungsten, chromium and iron.

[0061] Nickel is the basic element of the alloy due to its superior mechanical properties and its resistance to corrosion at high temperatures. High temperature is defined as a temperature between 400 and 800°C.

[0062] Molybdenum being a refractory element that is not easily oxidized in chloride environments, it allows for increased resistance to corrosion.

[0063] According to one embodiment of the invention, the nickel-based alloy comprises, by mass percentage, 18 to 20% molybdenum.

[0064] Besides the fact that aluminium can allow the formation of a protective alumina layer on the surface against corrosion, it will also increase the mechanical properties by allowing the precipitation of intermetallics of the hardening phase gamma prime or y' (Ni3Al).

[0065] Thus, according to one embodiment of the invention, the nickel-based alloy can comprise, by mass percentage, 1.5 to 2.5% of aluminium.

[0066] Titanium is primarily present to enable the precipitation of the gamma prime phase (y') by substituting for aluminum. The presence of titanium in the alloy also allows for better temperature stability of the y' phases. Since titanium has a larger diameter than aluminum, it increases the difference in lattice parameters between the y' phase and the y matrix. This increase allows for better temperature stability of the y' phases.

[0067] The nickel-based alloy may comprise, by mass percentage, 0.5 to 1% titanium.

[0068] According to one embodiment of the invention, the nickel-based alloy may comprise, by mass percentage, less than 1% of chromium and iron (WCr+ WFe), i.e. from 0 to < 1% of chromium and iron (0 < WCr+ WFe< 1%).

[0069] In another embodiment, the alloy comprises, by mass percentage, less than 0.5% of chromium and iron (WCr+ WFe), i.e. from 0 to < 0.5% of chromium and iron (0 < WCr+ WFe< 0.5%).

[0070] In one embodiment of the invention, the alloy does not contain chromium and iron. Even if these metals are present, they cannot be detected by known analytical methods.

[0071] As for the impurities (Iml) cobalt, zirconium, niobium, tantalum, hafnium, silicon, yttrium, lanthanum, cerium, manganese, magnesium, copper, palladium, platinum, vanadium and carbon, according to one embodiment of the invention, the nickel-based alloy comprises, by mass percentage, less than 0.01%, i.e., from 0 to 0.01%.

[0072] As for the impurities (Im2) sulfur, phosphorus and boron, according to one embodiment of the invention, the nickel-based alloy comprises, by mass percentage, less than 0.005%, i.e. from 0 to 0.005%.

[0073] According to one embodiment of the invention, the nickel-based alloy is characterized in that it comprises, by mass percentage:

[0074] - 77.38% nickel;

[0075] - 20% molybdenum;

[0076] - 2.1% aluminium;

[0077] - 0.52% titanium.

[0078] As mentioned above, an alloy according to the invention can be obtained by conventional processes such as, for example, Hot Isostatic Pressing, casting, powder metallurgy, but also by laser additive manufacturing, welding-inspired additive manufacturing, or dynamic gas spraying (or cold spray). These are well-known processes in the prior art, and those skilled in the art are able to adapt the operating conditions to obtain the desired alloy.

[0079] For example, Hot Isostatic Pressing (HIP) is a process for compressing metal powders using an inert gas in a HIP furnace under pressure (100-200 MPa) and high temperature (900 to 1400°C). The gas pressure acts uniformly in all directions to achieve isostatic properties and 100% densification. This technology can be used in a process for manufacturing an alloy according to the invention, comprising the following steps:

[0080] 1) Gas atomization of metal powders

[0081] 2) Design and manufacture of containers

[0082] 3) Filling and sealing of containers

[0083] 4) Hot Isostatic Compression

[0084] 5) Disposal of the container by machining or by chemical means.

[0085] 6) High-quality heat treatment and final machining.

[0086] Another method for manufacturing an alloy according to the invention is additive manufacturing:

[0087] - by laser powder bed fusion (L-PBF, for Laser-Powder Bed) Fusion), which consists of first spreading a thin layer of metal powder onto a build platform. Then, a beam is used to locally melt areas of the powder bed corresponding to sections of the object to be manufactured, previously modeled in 3D on a computer. The process is then repeated on a new layer, and so on, until the desired part is obtained, after clearing away the excess unassembled material.

[0088] - by powder projection (in English LMD, for Laser Metal Deposition) where the laser It is used to create a local melt pool on the part being manufactured, into which a nozzle will project material, which will fuse under the effect of heat. Then, the platform on which the part is placed and / or the projection head moves, following the predefined 3D model, and repeats the operation until the object is obtained.

[0089] The manufacturing process of an alloy according to the invention may substantially modify the mechanical behavior of the alloy but has little influence on the corrosion properties of the alloy obtained.

[0090] An alloy according to the invention resists corrosion in chloride salt environments at high temperatures (400 to 800 °C) preferably, but also in fluoride salt environments. Examples of chloride salts include NaCl, LiCl, CaCl2, and MgCl2. Examples of fluoride salts include NaF, ZrF4, LiF, and BeF2.

[0091] Several research fields are interested in corrosion by molten salts and chlorides, in particular:

[0092] - Structural materials in contact with molten salts in power plants Solar concentration. Chloride salts can be used as a heat transfer fluid and for energy storage.

[0093] - Structural materials for molten salt nuclear reactors with all the Types of neutron spectra (fast or thermal), technology belonging to the fourth generation of nuclear reactors. Chlorides are studied as a heat transfer fluid in which the fuel (U, Pu, Th) is dissolved.

[0094] - Structural materials for fuel reprocessing processes Spent nuclear fuel. The fuel can be dissolved in chloride salts to separate the actinides and fission products, which are recovered by electrodeposition.

[0095] - Structural materials for Zr / Hf separation processes. For manufacturing the In the fuel cladding used in pressurized water reactors, the zinc (Zr) used must be depleted in hydrogen (Hf), which is naturally extracted along with the zinc, to limit neutron capture of this element. Chloride salts can be used to solubilize these elements and then separate them by distillation.

[0096] - The materials of the superheating tubes of biomass power plants. The vapors The fluids circulating in these tubes contain chloride-rich deposits that are liquid at operating temperatures. This poses problems of chloride corrosion.

[0097] Another object of the invention therefore relates to the use of an alloy according to the invention as

[0098] - structural material in contact with molten salts in concentration plants solar;

[0099] - structural material for • Molten salt nuclear reactors • the processes for reprocessing spent nuclear fuel; • Zr / Hf separation processes;

[0100] - material for the superheating tubes of biomass power plants.

[0101] Another object of the invention is a part made of or coated with a nickel-based alloy according to the invention.

[0102] As a coating, the thickness of the alloy layer is on the order of a millimeter or even a centimeter.

[0103] Another object of the invention is a part according to the invention intended to be used in a molten salt reactor, in particular a high-temperature molten salt reactor between 400 and 800°C, such as chloride salts.

[0104] The part may be a component of the reactor such as the vessel, tank, piping, pumps, heat exchangers and neutron reflectors EXAMPLES

[0105] By way of example, the inventors have developed a nickel-based alloy containing, by mass percentage:

[0106] - 77.38% nickel

[0107] - 20% molybdenum,

[0108] - 2.1% aluminium,

[0109] - 0.52% by mass of titanium.

[0110] It was obtained by atomizing a powder which was then consolidated by Hot Isostatic Pressing (HIP). The powders used for compaction have an average diameter of 47 µm.

[0111] After hot isostatic compaction, the samples are homogenized at 1050°C for two hours. They were then helium quenched and cut into platelets (31.6x6.4x1.1 mm3) for corrosion testing.

[0112] The setup used for the molten salt corrosion tests is shown in [Fig. 1] and consists of:

[0113] - of a heating oven / collar for melting the salt and keeping it warm pérature,

[0114] - of a quartz crucible containing the molten salt and the samples,

[0115] - a thermocouple for regulating temperature,

[0116] - several locations for electrodes (Ag / AgCl reference electrodes, tungsten or graphite / glassy carbon electrodes).

[0117] The fractions of salts used are as follows:

[0118] - 55% (wt) of MgCl2.

[0119] - 45% (wt) of NaCl.

[0120] The salts used are highly hygroscopic, and the presence of water accelerates the corrosion of metallic materials. It is therefore important to minimize the presence of oxygen in the salt; it must therefore be purified.

[0121] This purification process consists of two main steps. First, the salt is heated above 100°C for several tens of hours to remove as much moisture as possible. Then, electrolysis is performed, and the oxygen content is monitored using a cyclic voltamogram (scan speed = 100 mV / s). The working electrode is made of vitreous carbon or graphite, and the counter electrode is tungsten.

[0122] Once the salt has been purified, the samples are fully immersed at 600 °C for 168 hours. The samples are then hand-polished using SiC paper according to the following range: P320 - P500 - P1200. Polishing ensures a good surface finish and removes any oxide layers.

[0123] The results presented in the remainder of this section were obtained for an alloy according to the invention with the following composition, expressed as mass percentages:

[0124] - 77.38% nickel,

[0125] - 20% molybdenum,

[0126] - 2.1% aluminum, and

[0127] - 0.52% by mass of titanium.

[0128] This alloy is obtained by atomization followed by Hot Isostatic Pressing or HIP, as described above.

[0129] It is also prepared by additive manufacturing by laser powder bed fusion.

[0130] The following references will be used to distinguish the two samples:

[0131] - NiMolloy 20 HIP: sample of the composition described above obtained by Hot Isostatic Compaction (HIP).

[0132] - NiMolloy 20 AM: sample of the composition described above obtained in Additive manufacturing (AM) by laser powder bed fusion, on a TRUMPH TRUEPRINT 1000 machine (55 pm beam).

[0133] Table 1 summarizes the specific mass variations of the two samples described above. To do this, the sample is weighed before and after testing; this variation is then expressed as a percentage of its surface area, resulting in the specific mass variation. These values ​​are very low and 10 times lower than those of commercial alloys such as Inconel 625 or Haynes 230 (Table 1) under the same conditions (fully immersed at 600°C for 168 hours). The manufacturing process has little influence, as the results obtained on samples from laser additive manufacturing or the Hot Isostatic Compaction are very similar.

[0134] [Table 1]

[0135] Specific mass variations for samples immersed in salt Reference sample Specific gravity variation (mg / cm2) NiMolloy 20 HIP (according to the invention) -0.144 NiMolloy 20 AM (according to the invention) 0.062 Inconel 625 -2.120 Haynes 230 -2.350

[0136] As shown in Table 1, the specific gravity variations of the two samples described above were measured using a precision balance with an accuracy of 106 g. These values ​​are very low and 10 times lower than those of commercial alloys such as Inconel 625 or Haynes 230 (Table 1) under the same conditions (fully immersed at 600°C for 168 hours). Based on these results, the manufacturing process has little influence, as the results obtained for samples produced by laser additive manufacturing or Hot Isostatic Pressing are very similar.

[0137] The SEM used for the analyses is a ZEISS EVO SEM equipped with a BRUKER XFLASH 6 X-ray spectroscopy detector (Energy-Dispersive X-Ray Spectroscopy or EDS). Observations were made to assess the condition of the alloy using cross-sections after corrosion of NiMolloy 20 HIP [Fig. 2] and NiMolloy 20 AM [Fig. 3] samples. These images are electron backscattered to highlight contrasts in chemical composition on the surface of the samples. For these two samples, no intergranular attack or pitting corrosion was observed, indicating good corrosion resistance.

[0138] The X-rays emitted by the atoms ionized by the SEM electron beam will be analyzed using an energy-dispersive spectrometer (EDS). EDS maps are shown in [Fig. 4] for the NiMolloy 20 HIP sample and in [Fig. 5] for the NiMolloy 20 AM sample. The elements studied during these EDS analyses are: nickel, molybdenum, aluminum, and titanium present in the alloy, as well as magnesium, sodium, and chlorine present in the salt.

[0139] The mappings carried out do not reveal any depletion of alloying element and in particular of molybdenum as is commonly seen in commercial alloys.

[0140] [Fig. 6a] and [Fig. 6b] relating to the NiMolloy 20 HIP sample do not allow highlight the depletion of chemical elements.

Claims

Demands

1. Nickel-based alloy characterized in that it comprises, by mass percentage: - 15 to 25% molybdenum; - 0.4 to 5% aluminium; - 0 to 2% titanium; - < 3% chromium; - < 3% iron; - < 3% tungsten; - < 0.1% impurities (Iml), the impurities (Iml) being cobalt, zirconium, niobium, tantalum, hafnium, silicon, yttrium, lanthanum, cerium, manganese, magnesium, copper, palladium, platinum, vanadium and carbon; - < 0.01% impurities (Im2), the impurities (Im2) being sulfur, phosphorus and boron; nickel representing the balance to reach 100%.

2. Alloy according to claim 1, characterized in that it simultaneously satisfies the following equations: 15 < WMo + Ww < 25 and 0 < WCr + WFe < 3 with WMo, Ww, WCr and WFe representing the mass percentages of molybdenum, tungsten, chromium and iron.

3. Alloy according to any one of claims 1 or 2, characterized in that it comprises, by mass percentage, 18 to 20% molybdenum.

4. Alloy according to any one of claims 1 to 3, characterized in that it comprises, by mass percentage, 1.5 to 2.5% aluminium.

5. Alloy according to any one of claims 1 to 4, characterized in that it comprises, by mass percentage, 0.5 to 1% titanium.

6. Alloy according to any one of claims 1 to 5, characterized in that it comprises, by mass percentage, less than 1% of chromium and iron.

7. Alloy according to any one of claims 1 to 6, characterized in that it comprises, by mass percentage, from 0 to <1% of tungsten.

8. Alloy according to any one of claims 1 to 7, characterized in that it comprises, by mass percentage, less than 0.01% of impurities (Iml).

9. Alloy according to any one of claims 1 to 8, characterized in which it includes, by mass percentage, less than 0.005% of impurities (Im2).

10. Alloy according to any one of claims 1 to 9, characterized in that it comprises, by mass percentage, - 77.38% nickel; - 20% molybdenum; - 2.1% aluminium; - 0.52% titanium.

11. Use of an alloy according to any one of claims 1 to 10, as - structural material in contact with molten salts in concentrated solar power plants; - structural material for • molten salt nuclear reactors • spent nuclear fuel reprocessing processes; • Zr / Hf separation processes; - superheat tube material for biomass power plants.

12. 12. Part made of or coated with a nickel-based alloy according to any one of claims 1 to 10.

13. Part according to claim 12, intended for use in a molten salt reactor.

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