Molten salt corrosion-resistant nickel-based alloy structurally hardened by γ' precipitation

Abstract

The present invention relates to a nickel-based alloy characterized in that it comprises, in weight percent: - 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 sulphur, phosphorus and boron; nickel representing the remainder to 100%. An alloy according to the invention can be used as a structural or coating material for parts subjected to corrosion by molten salts, in particular at high temperature, i.e., between 400 and 800°C, such as fluoride salts and, more particularly, chloride salts.

Description

γ' PRECIPITATION HARDENED NICKEL-BASED ALLOY RESISTANT TO MOLTEN SALT CORROSION Technical field of the invention

[0001] The present invention lies 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 to chloride salts. Technical background

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

[0004] However, under certain conditions, the corrosion resistance of nickel-based alloys can be limited, particularly in salts, such as chloride salts at high temperatures (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 (400 to 800°C) than those already existing.

[0006] For example, two early molten salt reactors at 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 both reactors is a fluoride (NaF-ZrF4-UF4 for the ARE, and LiF-BeF2-ZrF4-UF4 for the MSRE). Thanks to these developments, corrosion data in fluoride salt environments could be collected. More recently, other studies have been conducted to determine the performance of nickel-based alloys in fluoride salts. All these studies have enabled the optimization of the corrosion resistance of nickel-based alloys to fluoride salts. The reference nickel-based alloy for fluoride salt corrosion studies is Hastelloy ® N. Hastelloy alloy ®N was developed specifically for the development of a nuclear reactor with a fluoride salt as the heat transfer fluid. 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 to be more attractive but have corrosion rates in chloride salt environments that remain high. Too high a corrosion rate will reduce the material's lifespan and therefore increase its cost.

[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 providing very good resistance to corrosion of molten salts, and in particular to chloride salts at high temperatures, i.e. between 400 and 800°C.

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

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

[0011] - 15 to 25% molybdenum;

[0012] - 0.4 to 5% aluminum;

[0013] - 0 to 2% titanium;

[0014] - < 3% chromium;

[0015] - < 3% iron;

[0016] - < 3% tungsten;

[0017] - < 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;

[0018] - < 0.01% 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 < W Mo + W W < 25

[0022] and 0 <W Cr + W Fe < 3

[0023] with W Mo , W W , W Cr and W Fe representing the mass percentages of molybdenum, tungsten, chromium and iron.

[0024] The alloy of the invention has very good resistance to corrosion of molten salts, and in particular to 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 foundry, powder metallurgy but also by laser additive manufacturing, additive manufacturing inspired by welding or by dynamic gas projection or cold spray in English. 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 concentrated solar power plants;

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

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

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

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

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

[0035] Other characteristics and advantages of the invention will appear on reading the detailed description which follows with reference to the attached drawings.

[0036] is a diagram of the experimental device used for tests in molten salt environments.

[0037] represents a ZEISS EVO Scanning Electron Microscope (SEM) backscattered electron (BSE) micrograph of a cross-section of the NiMolloy 20 HIP alloy.

[0038] represents a ZEISS EVO Scanning Electron Microscope (SEM) backscattered electron (BSE) micrograph of a cross-section of the NiMolloy 20 AM sample (Additive Manufacturing).

[0039] represents a ZEISS EVO Scanning Electron Microscope (SEM) backscattered electron (BSE) micrograph of a cross-section of the NiMolloy 20 HIP alloy.

[0040] represents the EDS maps of the BRUKER X-Flash 6 detector on a ZEISS EVO Scanning Electron Microscope (SEM) of a cross-section of the NiMolloy 20 HIP alloy. The light part corresponds to the presence of the chemical element indicated at the bottom left.

[0041] represents a ZEISS EVO Scanning Electron Microscope (SEM) backscattered electron (BSE) micrograph of a cross-section of the NiMolloy 20 AM alloy (Additive Manufacturing).

[0042] represents the EDS maps of the BRUKER X-Flash 6 detector on a ZEISS EVO Scanning Electron Microscope (SEM) of a cross-section of the NiMolloy 20 AM alloy (Additive Manufacturing). The light part corresponds to the presence of the chemical element indicated at the bottom left.

[0043] represents a) a ZEISS EVO MEB micrograph in BSE and a line on which the EDS chemical analysis was carried out 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, in mass percentage:

[0045] - 15 to 25% molybdenum;

[0046] - 0.4 to 5% aluminum;

[0047] - 0 to 2% titanium;

[0048] - < 3% chromium;

[0049] - < 3% iron;

[0050] - < 3% tungsten;

[0051] - < 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;

[0052] - < 0.01% 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, in mass percentage, from 0 to less than 1% of chromium.

[0055] According to another preferred embodiment of the invention, the nickel-based alloy comprises, in 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 < W Mo + W W < 25

[0059] and 0 <W Cr + W Fe < 3

[0060] with W Mo , W W , W Cr and W Fe representing the mass percentages of molybdenum, tungsten, chromium and iron.

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

[0062] Molybdenum being a refractory element that is not very oxidizable in chloride environments, it helps increase resistance to corrosion.

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

[0064] In addition to the fact that aluminum can allow the formation of a layer of alumina on the surface to protect against corrosion, it will also increase the mechanical properties by allowing the precipitation of intermetallics from the hardening phase gamma prime or γ' (Ni3Al).

[0065] Thus, according to one embodiment of the invention, the nickel-based alloy may comprise, in mass percentage, 1.5 to 2.5% of aluminum.

[0066] Titanium is mainly present to allow the precipitation of the gamma prime (γ') phase by substituting aluminum. The presence of titanium in the alloy also allows for better temperature stability of the γ' phases. Titanium, having a larger diameter than aluminum, will increase the difference in lattice parameters between the γ' phase and the γ matrix. This increase allows for better temperature stability of the γ' phases.

[0067] The nickel-based alloy may contain, in mass percentage, 0.5 to 1% of titanium.

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

[0069] In another embodiment, the alloy comprises, in mass percentage, less than 0.5% of chromium and iron (W Cr + W Fe ), or from 0 to < 0.5% of chromium and iron (0 <W Cr + W Fe < 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 (Im1) 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, in 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, in mass percentage, less than 0.005%, or from 0 to 0.005%.

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

[0074] - 77.38% nickel;

[0075] - 20% molybdenum;

[0076] - 2.1% aluminum;

[0077] - 0.52% titanium.

[0078] As indicated above, an alloy according to the invention can be obtained by conventional processes such as, for example, Hot Isostatic Compression or Compaction, foundry, powder metallurgy but also by laser additive manufacturing, additive manufacturing inspired by welding or by dynamic gas projection or cold spray in English. These are processes well known in the state of the art and the person skilled in the art is able to adapt the operating conditions to arrive at the desired alloy.

[0079] For example, Hot Isostatic Compression (HIP) is a process for compressing metal powders using a neutral gas in a HIP furnace under pressure (100-200 MPa) and at 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 chemical means.

[0085] 6) Quality heat treatment and final machining.

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

[0087] - by laser powder bed fusion (L-PBF), which consists of first spreading a thin layer of metal powder on a manufacturing plate. 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 is used to locally create a molten pool on the part being manufactured, into which a nozzle will project material, which will assemble under the effect of heat. Then, the plate on which the part is placed and / or the projection head move, following the predefined 3D model, and repeat the operation, until the object is obtained.

[0089] The method of manufacturing an alloy according to the invention can significantly 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 temperature (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, for example, NaF, ZrF4, LiF and BeF2.

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

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

[0093] - Structural materials for molten salt nuclear reactors with all 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 reprocessing 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. To manufacture the fuel cladding used in pressurized water reactors, the Zr used must be depleted in Hf, which is naturally extracted with Zr, 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 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 concentrated solar power plants;

[0099] - structural material for molten salt nuclear reactors, spent nuclear fuel reprocessing processes; Zr / Hf separation processes;

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

[0101] Another subject 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 of the order of millimeters or even centimeters.

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

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

[0105] For example, the inventors developed a nickel-based alloy containing, in mass percentage:

[0106] - 77.38% nickel

[0107] - 20% molybdenum,

[0108] - 2.1% aluminum,

[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 were homogenized at 1050°C for two hours. They were then quenched with helium and cut into wafers (31.6x6.4x1.1 mm 3 ) to be tested for corrosion.

[0112] The setup used for molten salt corrosion tests is presented and consists of:

[0113] - a heating oven / collar to melt the salt and maintain its temperature,

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

[0115] - a thermocouple to regulate the temperature,

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

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

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

[0119] - 45% (wt) NaCl.

[0120] The salts used are very 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, so it must be purified.

[0121] This purification is composed of two main stages. First, the salt is heated above 100°C for several dozen hours to remove as much moisture as possible from the salt. Then, electrolysis is carried out and the oxygen content is monitored using a cyclic voltammogram (scanning speed = 100 mV / s). The working electrode is made of glassy 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 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, in mass percentage:

[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 then Hot Isostatic Compaction 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 in English or CIC in French).

[0132] - NiMolloy 20 AM: sample of the composition described above obtained in additive manufacturing (AM) by laser fusion on a powder bed, on a TRUMPH TRUEPRINT 1000 machine (55 µm 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 reported to its surface area, which gives 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 (totally immersed at 600°C for 168 hours). The manufacturing process has little influence because the results obtained on samples from laser additive manufacturing or Hot Isostatic Compaction are very similar.

[0134] [Table 1]

[0135] Specific mass variations for samples immersed in saltReference sampleSpecific mass variation (mg / cm 2 )NiMolloy 20 HIP (according to the invention)-0.144NiMolloy 20 AM (according to the invention)0.062Inconel 625-2.120Haynes 230-2.350

[0136] As shown in Table 1, the specific mass variations of the two samples described above using a precision balance with an accuracy of 10 -6 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 (totally immersed at 600°C for 168 hours). According to these results, the manufacturing process has little influence because the results obtained on samples from laser additive manufacturing or Hot Isostatic Compaction 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 in English). Observations were made to see the state of the alloy using cross-sections after corrosion of the NiMolloy 20 HIP and NiMolloy 20 AM samples. These images are in backscattered electrons to highlight contrasts in the chemical composition on the surface of the samples. For these two samples, no intergranular attack or corrosion pitting is observed, which is synonymous with good corrosion resistance.

[0138] The X-rays emitted by the atoms ionized by the electron beam of the SEM will be analyzed using an energy dispersive spectrometer or EDS (Energy Dispersive Spectroscopy in English). EDS maps are presented 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 maps carried out do not highlight any depletion of alloying elements 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 us to highlight any depletion of chemical elements.

Claims

Nickel-based alloy characterized in that it comprises, in mass percentage: - 15 to 25% molybdenum; - 0.4 to 5% aluminum; - 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%. [Rectified according to rule 91, 02.12.2024]Alloy according to claim 1, characterized in that it simultaneously satisfies the following equations:15 < W Mo + W W < 25 and 0 <W Cr + W Fe < 3 with W Mo , W W , W Cr and W Fe representing the mass percentages of molybdenum, tungsten, chromium and iron. Alloy according to one of claims 1 or 2, characterized in that it comprises, in mass percentage, 18 to 20% of molybdenum. Alloy according to any one of claims 1 to 3, characterized in that it comprises, in mass percentage, 1.5 to 2.5% of aluminum. Alloy according to any one of claims 1 to 4, characterized in that it comprises, in mass percentage, 0.5 to 1% of titanium. Alloy according to any one of claims 1 to 5, characterized in that it comprises, in mass percentage, less than 1% of chromium and iron. Alloy according to any one of claims 1 to 6, characterized in that it comprises, in mass percentage, from 0 to <1% of tungsten. Alloy according to any one of claims 1 to 7, characterized in that it comprises, in mass percentage, less than 0.01% of impurities (Im1). Alloy according to any one of claims 1 to 8, characterized in that it comprises, in mass percentage, less than 0.005% of impurities (Im2). Alloy according to any one of claims 1 to 9, characterized in that it comprises, in mass percentage, - 77.38% nickel; - 20% molybdenum; - 2.1% aluminum; - 0.52% titanium. 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; processes for reprocessing spent nuclear fuel; Zr / Hf separation processes;- material for superheating tubes in biomass power plants.

12. Part made of or coated with a nickel-based alloy according to any one of claims 1 to 10. Part according to claim 12, intended to be used in a molten salt reactor.

Images