Method for surface modification of stainless steels used in molten salt environments

Abstract

Provided herein are methods for increasing the corrosion resistance of stainless steel in molten salt environments. A bulk material with a nanolayered structure of alternating layers of CrxN and the stainless steel is formed within a surface region thereof via plasma nitridation. In the CrxN layers, x is 1, 2 or, as an average, any value in between. Also provided is the bulk material with increased corrosion resistance in a molten salt environment formed by the plasma nitridation.

Description

[0001] METHOD FOR SURFACE MODIFICATION OF STAINLESS STEELS USED IN MOLTEN SALT ENVIRONMENTS

[0002] Cross-Reference to Related Applications

[0003] This international patent application claims benefit of priority under 35 U.S.C. §119(e) of provisional patent application U.S. Serial No. 63 / 551 ,407, filed February 8, 2024, the entirety of which is hereby incorporated in its entirety.

[0004] BACKGROUND OF THE INVENTION

[0005] Field of the Invention

[0006] The present invention relates generally to the fields of molten salt reactors and metal alloy nitridation. More specifically, the present invention relates methods for nitriding a stainless steel bulk material to increase resistance to corrosion in molten salt environments.

[0007] Description of the Related Art

[0008] Molten salt reactors (MSRs) are a type of advanced nuclear reactor that use a molten salt mixture as both fuel and coolant (1 ). These reactors operate at high temperatures, allowing for efficient energy production and potentially enhanced safety features compared to traditional nuclear reactors. There are no certified structural materials yet for molten salt reactors.

[0009] Various alloys certified for light water reactor (LWR) applications cannot be used for molten salt reactors, due to poor corrosion susceptibility in molten salts (2). Corrosion in molten salts is distinctly different from traditional light water reactors (LWRs). Corrosion in LWR can be mitigated by forming an oxide barrier layer. However, a similar oxide layer cannot form in molten chloride and fluoride salts due to the presence of chlorine or fluorine ions. The molten salt reactor corrosion operates via the active dissolution of compositional atoms into the salt. Particularly, the selective dissolution of Cr into salts is the most severe issue (3). 316 SS has intergranular corrosion attack and Cr depletion in FLiBe salt, with an expected corrosion attack depth of about 16 pm for one-year service at 700°C (4). This depletion is caused by the diffusion of Cr from the bulk to the near-surface grain boundary, followed by the dissolution of Cr from the grain boundaries into the salt. Such dissolution is accompanied by vacancy exchange and the formation of cavities / cracks after the agglomeration of excessive vacancies. Furthermore, in the flowing salt with a temperature gradient, corrosion leads to mass loss from the hot region and mass gain in the cold region. Such mass transfer can lead to blockage and is another materials issue, which could be even worse than corrosion itself for a molten salt reactor system.

[0010] Many other Ni-based alloys have been developed for better corrosion resistance in molten salts, but these alloys face other issues, particularly with high- temperature strength (5). One example is the Ni-based Hastelloy N developed at Oak Ridge National Laboratory (ORNL). It exhibits superior corrosion resistance but is not suitable for high-temperature applications. Hastelloy-N may be used for operating temperatures up to 982°C. But the maximum allowable stress for Hastelloy-N decreases dramatically for temperatures above 600°C, which is a limiting factor for usage as a structural material for reactor vessels (6). Ni-based alloys are generally problematic in reactor cores due to the transmutation-induced production of helium and Co-60. Helium, generated through (n,a) reactions, can lead to embrittlement and swelling, while Co-60, a strong gamma emitter produced through neutron activation, adds significant challenges for radiation safety and waste management.

[0011] For molten salt reactors, salts consist of LiF mixed with other fluorides. Fluoride salts have very high melting temperatures. In the accident scenario, the leaked salt would freeze and enclose radioactive fission fragments, which is one important safety benefit. Two optimized salts developed over decades at ORNL are FLiBe (2LiF-BeF2) and FliNaK (46.5LiF-11.5NaF-42KF). Both have excellent thermal properties, thermochemical stability, and chemical compatibility. FLiBe has an additional advantage as a fuel solvent to dissolve uranium or thorium fissile materials and is the top choice for liquid fuel. FLiNaK is excellent as coolants due to their high heat capacity.

[0012] The most efficient way to prevent chromium dissolution is by introducing atoms that form strong bonds with chromium, thereby immobilizing it in the matrix. Nitrogen (N) is an ideal choice because it can form strong bonds with chromium. However, introducing nitrogen and forming the desired CrxN structure is not straightforward.

[0013] Accelerator-based nitrogen ion implantation is expensive and requires high vacuum conditions, making it unsuitable for treating typical reactor components. Plasma-based nitridation is a low-cost, low-vacuum process, but it faces certain challenges. Traditional nitridation often results in a nitrogen-strengthened phase (commonly referred to as the S-phase), where nitrogen is trapped in interstitial sites, and the steel retains its original composition and structure (7). This means the corrosion problem remains unresolved, as chromium can still leach into the salt through grain boundary diffusion. At high temperatures and with prolonged processing times, nitridation may lead to the formation of a continuous nitride layer (8). Such a layer is highly likely to cause delamination and debonding issues due to the creation of a continuous ceramic-steel interface. Ideally, the structure should consist of CrxN embedded within the steel matrix without forming a single continuous interface, thereby avoiding the challenges associated with conventional nitridation methods.

[0014] Thus, there remain unmet needs in the art that addresses the alloy-related challenges in molten salt environments. Particularly, the art is deficient in surface nitriding methods and processes to immobilize chromium in stainless steels and prevent its diffusion into molten salts at high temperatures. The present invention fulfils this longstanding need and desire in the art.

[0015] SUMMARY OF THE INVENTION

[0016] The invention disclosed herein addresses the alloy-related challenges in the transition from current light water reactors to the next-generation molten salt reactors. The high-temperature strength alloys currently qualified for use in light water reactors suffer from corrosion issues when exposed to molten salt. Conversely, the corrosionresistant alloys developed for molten salt reactors fail to meet the stringent high- temperature strength requirements. Applying a surface nitriding technique that forms a surface layer containing nanolayered CrxN to immobilize chromium and to prevent its diffusion into molten salts. The technique ensures the compatibility of light water reactor alloys in molten salt environments.

[0017] The present invention is directed to a method for increasing corrosion resistance of a stainless steel bulk material in a molten salt environment. In this method, a nanolayered composite comprising alternating layers of CrxN and steel is formed within a surface region of the stainless steel, wherein x is 1 or 2, or, as an average, any value in between. The CrxN layers are structurally effective to immobilize chromium therein, thereby preventing its leaching into the molten salt. The present invention is further directed to a nitrided stainless steel bulk material produced by the method described herein.

[0018] The present invention is directed further to a process for increasing corrosion resistance of a stainless steel bulk material for use in a molten salt environment. In this process, a surface region on the bulk material is plasma nitrided in a nitriding chamber under conditions whereby a nanolayered structure comprising alternating layers of CrxN and steel form within the surface region of the bulk material. In the CrxN x is 1 or 2, or, as an average, any value in between. The CrxN layers are embedded into the surface region with no continuous interface between the nanolayered structure and the stainless steel. The CrxN layers immobilize the chromium therein to prevent its leaching into the molten salt.

[0019] The present invention is directed to a related process further comprising cleaning the surface region of a nitrided bulk material after nitridation via mechanical removal, a chemical etchant, or a combination thereof. The present invention is directed to another related process further comprising annealing a nitrided bulk material inside the nitridation chamber or outside the nitridation chamber at a temperature of about 200°C to about 1000°C for 1 second to about 1000 hours.

[0020] Other and further aspects, features, and advantages of the present invention will be apparent from the following description of the presently preferred embodiments of the invention. These embodiments are given for the purpose of disclosure.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] So that the matter in which the above-recited features, advantages and objects of the invention, as well as others which will become clear, are attained and can be understood in detail, more particular descriptions of the invention briefly summarized above may be had by reference to certain embodiments thereof which are illustrated in the appended drawings. These drawings form a part of the specification. It is to be noted, however, that the appended drawings illustrate preferred embodiments of the invention and therefore are not to be considered limiting in their scope.

[0023] FIGS. 1A-1 B depict a molten salt reactor (FIG. 1A), where the inner surface of the steel wall contains nanolayered CrN ceramic and nanolayered steel (FIG. 1 B).

[0024] FIG. 2 depicts a nitridation process for the inner walls of a molten salt reactor, utilizing hollow cages to enhance the uniformity of the nitridation. FIG. 3 schematically illustrates the components used during a nitridation process.

[0025] FIG. 4 illustrates a cathodic cage for a nitridation device with a sample placed within for surface nitridation.

[0026] FIGS. 5A-5D plots SEM, EDS, XRD, and indentation characterization results of nitrided 316L, obtained using a bias of 600V for 15 hours.

[0027] FIGS. 6A-6D plots STEM and EDS characterization results of nitrided 316L, obtained using a bias of 600V for 15 hours.

[0028] FIGS. 7A-7H compare STEM images of nitrided 316L steels (FIGS. 7A-7D) and untreated 316L steels (FIGS. 7E-7H) after corrosion testing.

[0029] DETAILED DESCRIPTION OF THE INVENTION

[0030] As used herein, the articles "a" and "an" when used in conjunction with the term “comprising” in the claims and / or the specification, may refer to “one”, but it is also consistent with the meaning of “one or more”, “at least one”, and “one or more than one”. Some embodiments of the invention may consist of or consist essentially of one or more elements, components, method steps, and / or methods of the invention. It is contemplated that any composition, component or method described herein can be implemented with respect to any other composition, component or method described herein.

[0031] As used herein, the term “or” in the claims refers to “and / or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and / or”.

[0032] As used herein, the terms "comprise" and "comprising" are used in the inclusive, open sense, meaning that additional elements may be included.

[0033] As used herein, the terms "consist of and "consisting of are used in the exclusive, closed sense, meaning that additional elements may not be included.

[0034] As used herein, the term “about” refers to a numeric value, including, for example, whole numbers, fractions, and percentages, whether or not explicitly indicated. The term “about” generally refers to a range of numerical values (e.g., ± 5- 10% of the recited value) that one of ordinary skill in the art would consider equivalent to the recited value (e.g., having the same function or result). In some instances, the term “about” may include numerical values that are rounded to the nearest significant figure.

[0035] In one embodiment of the present invention, there is provided a method for increasing corrosion resistance of a stainless steel bulk material in a molten salt environment, comprising forming a nanolayered composite comprising alternating layers of CrxN and steel within a surface region of the stainless steel, wherein x is 1 or 2, or, as an average, any value in between, where the CrxN layers are structurally effective to immobilize chromium therein, thereby preventing its leaching into the molten salt.

[0036] In this embodiment, the stainless steel may be a bulk material used as a component of a molten salt reactor. Particularly, the component of the molten salt reactor is a vessel, vessel wall thereof a pipe therein, or a pipe extending therefrom. Also in this embodiment, the molten salt may be FLiBe (2LiF-BeF2) or FLiNaK (46.5LIF-11 ,5NaF-42KF). In addition, the molten salt may comprise chloride salts or fluoride salts or a mixture thereof.

[0037] In another embodiment of the present invention, there is provided a nitrided stainless steel bulk material produced by the method as described supra.

[0038] In this embodiment, the nitrided stainless steel bulk material may comprise a surface region with a nanolayered CrxN-steel composite structure; where a plurality of discrete CrxN nanolayers are interspersed within a matrix of the stainless steel without formation of a continuous CrxN or an S-phase. In an aspect, the CrxN nanolayers in the surface region may have an interlayer distance of about 1 nanometer to about 1 micrometer. In another aspect, the surface region may have a thickness of about 1 micrometer to 1 millimeter. In this embodiment, and aspects thereof, the stainless steel may be selected from the group consisting of a 304 stainless steel, a 304L stainless steel, a 304H stainless steel, a 316 stainless steel, a 316L stainless steel, and a 316H stainless steel.

[0039] In another embodiment of the present invention, there is provided a process for increasing corrosion resistance of a stainless steel bulk material for use in a molten salt environment, comprising plasma nitriding in a nitriding chamber a surface region on the bulk material under conditions whereby a nanolayered structure comprising alternating layers of CrxN and steel form within the surface region of the bulk material; wherein x is 1 or 2, or, as an average, any value in between; wherein the CrxN layers are embedded into the surface region with no continuous interface between the nanolayered structure and the stainless steel; whereby the CrxN layers immobilize the chromium therein to prevent its leaching into the molten salt.

[0040] Further to this embodiment, the process comprises cleaning the surface region of a nitrided bulk material after nitridation via mechanical removal, a chemical etchant, or a combination thereof. In another further embodiment, the process comprises annealing a nitrided bulk material inside the nitridation chamber or outside the nitridation chamber at a temperature of about 200°C to about 1000°C for 1 second to about 1000 hours.

[0041] In an aspect of all embodiments, the plasma nitriding step may comprise maintaining a temperature of the bulk material to at least 450°C during nitridation; maintaining a vacuum within the nitriding chamber of about 0.0001 torr to about 10 torr; setting a chamber wall of the nitriding chamber as ground; applying a negative voltage to the bulk material of about -1V to about -1000V; and nitriding the bulk material for about 1 minute to about 1000 hours.

[0042] In another aspect of all embodiments, the plasma nitriding step may comprise positioning a hollow cage within the nitriding chamber; setting the hollow cage to a negative voltage of about -1V to about -1000V; placing the bulk material inside the hollow cage; and applying the same voltage as applied to the hollow cage or applying a bias to the bulk material different from that applied to the hollow cage or floating the bulk material without an applied voltage.

[0043] In yet another aspect of all embodiments the bulk material may be a chamber wall of the nitridation chamber, the plasma nitriding step comprising bombarding the chamber wall with nitrogen atoms whereby a nanolayered CrxN-metal composite forms on an inner surface of the chamber wall to produce a nitrided chamber wall.

[0044] In this embodiment and aspects thereof, the bulk material may be a component used inside a molten salt reactor, where the plasma nitriding step is applied to a surface region of the component to increase corrosion resistance thereof in the molten salt environment. Particularly, the component may be a nitrided chamber wall or a pipe of the molten salt reactor. In addition, during the plasma nitriding step a direct current (DC) bias, an alternating current (AC) bias, or a combination thereof is applied for different components inside the nitridation chamber. In addition in this embodiment and aspect thereof, the molten salt may be FLiBe (2LiF-BeF2) or FLiNaK (46.5LIF-11 ,5NaF-42KF). Furthermore, the molten salt may comprise chloride salts or fluoride salts or a mixture thereof.

[0045] Provided herein is a method for plasma nitridation that enables the formation of a unique structure for applications in molten salt reactor-like environments. The nitrided alloys comprise a bulk material, such as high-temperature stainless steel 316H, and a surface layer containing a nanolayered metal-ceramic composite. The nanolayered composite consists of a plurality of interwoven discrete nanolayers of steel and CrxN. No continuous CrxN layer or S-phase is present in the nitrided bulk material. The CrxN nanolayers immobilize chromium, preventing it from leaching into the salt. Unlike traditional coatings, the nanolayered steel-CrxN composites form within the original steel, avoiding the introduction of a separate layer or interface. This eliminates the risk of debonding at the interface, which is critical for ensuring durability in harsh reactor environments. The nitridation process can be applied to modify steels for use as in-core components of molten salt reactors. Furthermore, the modified nitridation technique can be used to treat the inner surface of a vessel or a nitridation chamber wall, enabling its application as a reactor vessel in molten salt reactors.

[0046] One advantage of plasma nitriding lies in the appropriate thickness of the resulting nitride layer in the surface region of the bulk material, typically ranging from 10 to 100 micrometers. The thickness may be adjusted by varying the nitriding parameters such as temperature and time, for example, but not limited to, about 1 micrometer to 1 millimeter. Considering the observation that corrosion-attacked layer thickness in the 300 series of steels is typically in the range of 50 micrometers to 100 micrometers, the layer thickness of the plasma-treated region or surface region is sufficient for corrosion protection. On the other hand, the treated region is not too thick for its purpose. The interlayer distance between the discrete CrxN nanolayers interspersed within a matrix of the stainless steel is about 1 nm to about 1000 nm.

[0047] Another significant advantage of plasma nitriding is that it introduces a corrosion-resistant surface layer without altering the entire bulk material. This expedites the acceptance of these alloys by NRC, especially for those alloys that are already qualified by the ASME. Examples of certified alloys include 316 SS, 316H SS, 304 SS, and 304H SS. Among the 300 series steels, 316H is a high-temperature strength alloy, suitable for working temperature up to 816°C. “H” stands for high carbon content (ranging from 0.04% to 0.10%). It can be used as reactor vessel, guard vessel, piping and heat exchangers in MSR, if corrosion issues can be resolved.

[0048] 304H and 316H SS are two alloys of particular interest. Both are qualified for high-temperature nuclear reactor applications by the ASME. Both are allowed for design and construction of Class A nuclear pressure boundary components and in- core components for service at temperatures up to 816°C for about 300,000 hours. Both have been identified for incorporation into the new Class B Code cases. If the corrosion issues of both alloys in molten salt reactor-like environments are resolved, this immediately impacts the deployment of molten salt reactors by providing an accelerated pathway to licensing. Table 1 summarizes the composition of both alloys, their proposed MSR applications, and their current codification under ASME.

[0049] TABLE 1

[0050] Materials selection

[0051] It is contemplated that applications of the nitriding methods and CrxN nanostructures formed thereby extend beyond nuclear technology to diverse industry processes involving molten salt and high temperatures. The development of alloys with high-temperature strength and resistance to corrosion in molten salt environments yields a range of significant advantages across diverse industry applications including the following.

[0052] Solar Thermal Energy: Concentrated solar power systems harness mirrors or lenses to concentrate sunlight onto a receiver, generating elevated temperatures for electricity production. Molten salts function as effective heat transfer mediums and thermal energy storage agents within these systems.

[0053] Chemical Processing: Specific chemical processes capitalize on molten salts for their unique characteristics as reaction media or catalysts, making corrosionresistant alloys invaluable for maintaining equipment integrity. The most recent example includes using molten salt as a catalyst for the synthesis of high-energy 2D material, and using molten salt as a shielding medium to synthesize oxidationresistant proton materials.

[0054] Metallurgy: Molten salts are used in metallurgical procesing for metal refining and alloying. One example is the reduction of titanium dioxide to titanium.

[0055] Aerospace: Aerospace applications, such as hypersonic flight and reentry vehicles, often encounter extreme temperatures. Molten salts are employed as heat- resistant working solutions in these scenarios.

[0056] Thermal Processing of Biomass: Molten salts have unique catalytic properties and can be utilized in the thermal processing of biomass. The processing decomposes biomasses into a mixture of liquids, non-condensable gases, and solid chars. Molten salt is simultaneously used as the heat carrier, catalyst, and solvent.

[0057] Thermal Energy Storage: Molten salt has a superior capability to store heat at low cost. It is expected to play a key role in a low-carbon society. It can be used to store energy from a reactor and stabilize the electric grid by integrating solar and wind energies, which have electricity production fluctuations. Molten salt can store energy more than 30 times cheaper than lithium-ion batteries.

[0058] The present embodiments are best described by reference to those figures illustrating the same, but are not meant to limit the present invention in any fashion.

[0059] FIGS. 1A-1 B illustrate one embodiment of the corrosion-resistant molten salt reactor. In FIG. 1A the reactor vessel 2 comprises the vessel wall 6, molten salt 4, a top exit pipe 8 for processing, a side exit pipe 10 for circulation, a molten salt pump 12, a heat exchanger 14, and a main bottom exit 16. In FIG. 1 B the dashed box highlights a small cross-sectional area of the vessel wall 6. The cross-section shows the stainless steel 18 as the bulk material and a surface steel-ceramic composite layer, which includes nanolayered CrxN 20 and nanolayered steel 22.

[0060] FIG. 2 illustrates a vessel 30 used as a molten salt reactor for creating the nanolayered CrxN-steel composite on the inner surface of an MSR vessel. The vessel 30 contains a vessel wall 32 made of LWR-certified steels. The vessel further includes an inner hollow cage 34, which is used to produce nitrogen plasma. On the wall of the hollow cage, there are holes 36 designed to generate nitrogen plasma. These holes help ensure uniform plasma distribution inside the vessel. Inside the hollow cage is an inner pipe 38, which is insulated from the hollow cage by insulating materials 40. On the surface of the inner pipe, it is optional to have small holes 44 that are used to release gas atoms 46 into the nitriding chamber.

[0061] In one embodiment the inner pipe is biased with a positive voltage, serving as the anode, while the hollow cage is biased negatively, acting as the cathode. The vessel wall remains unbiased / floated or negatively biased. When nitrogen plasma is generated near the holes in the hollow cage, nitrogen atoms bombard the vessel wall due to its proximity to the hollow cage even when the vessel wall is floated, resulting in nitridation. The vessel wall may be negatively biased to further attract nitrogen atoms if the bias voltage is appropriately selected.

[0062] In another embodiment, the hollow cage is removed, and the inner pipe continues to serve as the anode, while the vessel wall becomes the cathode. In this configuration, nitrogen atoms directly bombard the vessel wall without the use of the hollow cage.

[0063] In both embodiments, the anode is a pipe. However, the pipe may be replaced with a plate of arbitrary shape. Additionally, in the described setups, nitrogen gas may be introduced into the chamber via the inner pipe. However, nitrogen gas also may be injected into the vessel through other portals or windows. It should be noted that the vacuum pumping portal is not shown. Pumping may be conducted through portal 42 or any other suitable openings. When nitridation is required at specific temperatures, the vessel may need supplemental heating provided by a heater.

[0064] Traditional nitridation techniques may or may not involve the use of a hollow cage. The method that employs a hollow cage is referred to in the literature as hollow cathode cage plasma nitriding, or CCPN. Unlike conventional DC plasma nitriding, where intense plasma is generated close to the sample surface, CCPN utilizes a hollow cage structure with periodic holes to distribute the plasma more evenly across the chamber. This results in a relatively uniform distribution of plasma throughout the space, ensuring consistent bombardment of the sample without intense localized effects. The cathode cage geometry (shapes, sizes, cage thickness, and dimensions of hollow holes) needs to be designed and optimized to create a high-density plasma between inner wall of the vessel, if the inner wall is to be nitrided.

[0065] Traditional nitridation, whether with or without a hollow cage, nitrides a component positioned inside a plasma chamber or inside a hollow cage. However, herein, nitridation is extended to either the inner components inside the cage or, alternatively, to the chamber walls outside the cage. This dual nitridation capability is particularly important when the chamber itself is intended to function as a vessel for molten salt reactors. Nitriding the chamber wall is achieved by controlling the voltage applied to the chamber wall, which either must be floated or negatively biased at an appropriate bias voltage.

[0066] FIG. 3 illustrates the components used in a nitriding process for a pipeline wall. The pipeline wall 50 is nitrided by bombarding with N atoms. Disposed at a distance substantially close to the pipeline wall, the hollow cage 52 functions as a cathode and is biased negatively. Holes 54 on the hollow cage are utilized for producing a uniform N plasma. Nitrogen gas is injected into the inner pipe, which is located inside the hollow cage. The inner pipe 56 functions as an anode and is biased positively. Holes 58 on the inner tube surface are utilized for releasing N atoms, which are injected into the chamber through the inner tube. The inner tube is insulated from the hollow cage by insulating materials 60. The assembly of the hollow cage and the inner tube rotates during the nitridation process, which further improves the uniformity of nitridation on the pipeline wall.

[0067] The following example is given for the purpose of illustrating various embodiments of the invention as examples and are not meant to limit the present invention in any fashion.

[0068] EXAMPLE

[0069] Nitridation of 316L stainless steel (316L SS)

[0070] A hollow cathode cage plasma nitriding (CCPN) device is used for nitriding a sample positioned inside a hollow cathode cage (FIG. 4). In this setup, the hollow cage is significantly smaller and situated far from the chamber wall. The sample, placed on top of an insulating disk, is close enough to the hollow cage to ensure effective bombardment by the nitrogen plasma. In this example, the chamber wall is grounded and is not intended to be nitrided. FIG. 5A shows the cross-sectional scanning electron microscopy (SEM) images of the nitrided 316L SS along with the corresponding Energy-Dispersive X-ray Spectroscopy (EDS) mapping. Nitriding modifies the surface up to a depth of 60 microns, evident from the image contrast (FIG. 5A). Within this region, N is enriched, Fe is reduced, while chromium is unchanged. No oxides or oxide precipitates are observed. EDS line scan suggests the formation of CrxN (FIG. 5B). X-ray diffraction analysis (XRD) analysis observes the characteristic peaks of CrxN (FIG. 5C). Cross sectional indentation mapping shows that the nitride layer exhibits significantly enhanced hardness, increased by a factor of about three (FIG. 5D).

[0071] The nitrided region displays distinctive nanometer-scale patterns, with alternating arrangements of Fe-enriched phase and CrxN phases in a lamellar structure (as shown in FIG. 6A). The spacing between adjacent Fe layers measures approximately 50 nm. SEM mapping reveals a correlated spatial distribution of Cr and N. Conversely, Fe and Ni exhibit enrichment between adjacent CrxN layers (as shown in FIG. 6B). The EDS line scan shows the periodic formation of nanolayers with a Cr- to-N ratio close to 1 :1 (FIG. 6C). In FIG. 6D, an FFT (Fast Fourier Transform) filtered STEM image of the CrxN phase is presented. The interplanar spacing of the (200) plane of the nitride is 2.11 A, which corresponds to a lattice parameter of 4.22 A, aligning well with the literature-reported value for CrxN.

[0072] The formation of Fe-enriched phase and CrxN lamellar structures is believed to be the decomposition product of metastable nitrogen-expanded austenite. Early studies show that low-temperature nitriding (<450°C) of 316 SS ends with nitrogen- expanded austenite, the so-called S phase in literature, while high-temperature nitridation (> ~450°C) ends with a CrxN-containing compound layer. A temperature higher than 500°C may result in y-Fe, based on the Cr-N phase diagram.

[0073] In normal corrosive environments (non-MSR like), forming CrxN was to be avoided since chromium loss in the matrix increases corrosion susceptibility in water / steam environments. In molten salts, the opposite is targeted by intentionally forming CrxN and avoiding S phase, in order to immobilize chromium and thus increase corrosion resistance in MSR-like environments. Hence, plasma nitriding needs to operate at a relatively higher temperature (>450°C) than previous studies.

[0074] After 100 hours of corrosion testing in FLiNaK at 700°C, untreated 316L shows evident granular corrosion and chromium depletion. An SEM image of the cross- section of untreated sample reveals small cracks (FIG. 7A). The chromium depletion is much more noticeable in the cross-sectional EDS mapping (FIG. 7B), even in regions without obvious microcracks. Cr depletion follows the grain boundary and occurs at a depth ranging from 0 to about 10 microns. The planar SEM image illustrates intergranular cracking on the corroded surface (FIG. 7C). EDS mapping confirms intergranular chromium depletion (FIG. 7D).

[0075] The nitrided sample exhibits significantly enhanced corrosion resistance in FLiNaK. No microcracks were observed from either cross-sectional or planar SEM images (FIGS. 7E, 7G). Chromium distribution is homogeneous in both the cross- sectional and planar EDS mapping (FIGS. 7F, 7H). Homogeneous elemental distributions are also observed for other elements such as Fe and N. There is no evidence that N is lost during the 700°C corrosion testing, as judged by the yield before and after corrosion testing, which is expected from the strong CrxN bonding.

[0076] For the nitriding process, in one example, the ambient atmosphere consists of 90% nitrogen and 10% hydrogen (H). The addition of hydrogen serves to mitigate oxygen effects arising from chamber degassing and moisture. The choice of cage materials in the CCPN setup matches those of the substrates to be nitrided. In a standard CCPN treatment, the substrate temperature is determined by the plasma flux and the bias applied between the anode and cathode. The bombarding energies of N atoms on the substrate greatly influence the thermal energy transferred to the substrate. Consequently, in a typical CCPN process, the substrate temperature is primarily influenced by the voltage bias.

[0077] While sputtering of the cage materials is inevitable in CCPN, its impact on the substrate is counterbalanced by plasma sputtering and self-cleaning on the surface, a phenomenon sensitive to the voltage bias. The interplay among these parameters underscores the importance of optimizing the processing conditions. The nitridation process requires optimizing a range of biases and chamber pressures to facilitate nitriding without causing deposition of cage materials to the substrate.

[0078] There are several methods to remove or to avoid the surface contaminant layer or deposited layer formed from material removed from the hollow cage. A first method, as explained above, is to avoid the use of a hollow cage.

[0079] A second method is to apply plasma sputtering after the nitridation process. This can easily be achieved by switching from nitrogen plasma nitridation to argon plasma sputtering. As a heavy atom, argon is highly efficient for surface sputtering. Careful control of the argon plasma treatment time can effectively remove any surface contaminant layer that may form during the nitridation step.

[0080] A third method involves surface polishing or etching. This is accomplished by using mechanical polishing with materials such as SiC paper or alumina oxide paper. The surface contaminant layer is relatively easy to remove. However, once etching reaches the nanostructured layer beneath the contaminant layer, the etching rate drops significantly due to the high hardness of the nanolayered region.

[0081] The following references are cited herein.

[0082] 1 . Serp et al., Progress in Nuclear Energy.77:308, 2014.

[0083] 2. Raiman et al., J. Nucl. Mat.. 561 :153551 , 2022.

[0084] 3. Guo et al., Progress in Materials Science. 97:448, 2018.

[0085] 4. Zheng et al., J. Nucl. Mat.. 482:147, 2016.

[0086] 5. MacPherson et al., CF-57-4-27 (Rev.). Oak Ridge National Laboratory, 1957.

[0087] 6. Serp et al., Prog. Nucl. Energy, 77:308, 2014.

[0088] 7. Adachi, et al., Metals, 11 (10), 1538 2021 .

[0089] 8. Lakhtin, et al. Met Sci Heat Treat, 27:3-7, 1985.

Claims

WHAT IS CLAIMED IS:1 . A method for increasing corrosion resistance of a stainless steel bulk material in a molten salt environment, comprising: forming a nanolayered composite comprising alternating layers of CrxN and steel within a surface region of the stainless steel, wherein x is 1 or 2, or, as an average, any value in between, said CrxN layers structurally effective to immobilize chromium therein, thereby preventing its leaching into the molten salt.

2. The method of claim 1 , wherein the stainless steel is a bulk material used as a component of a molten salt reactor.

3. The method of claim 2, wherein the component of the molten salt reactor is a vessel, vessel wall thereof a pipe therein, or a pipe extending therefrom.

4. The method of claim 1 , wherein the molten salt is FLiBe (2LiF-BeF2) or FLiNaK (46.5UF-11 ,5NaF-42KF).

5. The method of claim 1 , wherein the molten salt comprises chloride salts or fluoride salts or a mixture thereof.

6. A nitrided stainless steel bulk material produced by the method of claim 1.

7. The nitrided stainless steel bulk material of claim 6, comprising a surface region with a nanolayered CrxN-steel composite structure; wherein a plurality of discrete CrxN nanolayers are interspersed within a matrix of the stainless steel without formation of a continuous CrxN or an S-phase.

8. The nitrided stainless steel bulk material of claim 7, wherein the CrxN nanolayers in the surface region have an interlayer distance of about 1 nanometer to about 1 micrometer.

9. The nitrided stainless steel bulk material of claim 7, wherein the surface region has a thickness of about 1 micrometer to 1 miillimeter.

10. The nitrided stainless steel bulk material of claim 6, comprising a stainless steel selected from the group consisting of a 304 stainless steel, a 304L stainless steel, a 304H stainless steel, a 316 stainless steel, a 316L stainless steel, and a 316H stainless steel.

11. A process for increasing corrosion resistance of a stainless steel bulk material for use in a molten salt environment, comprising: plasma nitriding in a nitriding chamber a surface region on the bulk material under conditions whereby a nanolayered structure comprising alternating layers of CrxN and steel form within the surface region of the bulk material; wherein x is 1 or 2, or, as an average, any value in between; wherein the CrxN layers are embedded into the surface region with no continuous interface between the nanolayered structure and the stainless steel; whereby the CrxN layers immobilize the chromium therein to prevent its leaching into the molten salt.

12. The process of claim 11 , further comprising: cleaning the surface region of a nitrided bulk material after nitridation via mechanical removal, a chemical etchant, or a combination thereof.

13. The process of claim 11 , further comprising: annealing a nitrided bulk material inside the nitridation chamber or outside the nitridation chamber at a temperature of about 200°C to about 1000°C for 1 second to about 1000 hours.

14. The process of claim 11 , wherein the plasma nitriding step comprises: maintaining a temperature of the bulk material to at least 450°C during nitridation; maintaining a vacuum within the nitriding chamber of about 0.0001 torr to about 10 torr; setting a chamber wall of the nitriding chamber as ground;applying a negative voltage to the bulk material of about -1V to about -1000V; and nitriding the bulk material for about 1 minute to about 1000 hours.

15. The process of claim 11 , wherein the plasma nitriding step comprises: positioning a hollow cage within the nitriding chamber; setting the hollow cage to a negative voltage of about -1V to about -1000V; placing the bulk material inside the hollow cage; and applying the same voltage as applied to the hollow cage or applying a bias to the bulk material different from that applied to the hollow cage or floating the bulk material without an applied voltage.

16. The process of claim 11 , wherein the bulk material is a chamber wall of the nitridation chamber, the plasma nitriding step comprising: bombarding the chamber wall with nitrogen atoms whereby a nanolayered CrxN-metal composite forms on an inner surface of the chamber wall to produce a nitrided chamber wall.

17. The process of claim 11 , wherein the bulk material is a component used inside a molten salt reactor, said plasma nitriding step applied to a surface region of the component to increase corrosion resistance thereof in the molten salt environment.

18. The process of claim 17, wherein the component is a nitrided chamber wall or a pipe of the molten salt reactor.

19. The process of claim 17, wherein during the plasma nitriding step a direct current (DC) bias, an alternating current (AC) bias, or a combination thereof is applied for different components inside the nitridation chamber.

20. The process of claim 11 , wherein the molten salt is FLiBe (2LiF-BeF2) or FLiNaK (46.5UF-11 ,5NaF-42KF).

21. The process of claim 11 , wherein the molten salt comprises chloride salts or fluoride salts or a mixture thereof.