Corrosion Resistance In Molten Salt Research Articles

Fabrication of Co-Based Cladding Layer by Microbeam Plasma and Its Corrosion Mechanism to Molten Salt.

Corrosion of the molten salts Na2SO4 and NaCl has become one of the major factors in the failure of steel components in boilers and engines. In this study, CoNiCrAlY cobalt-based cladding layers with different NiCr-Cr3C2 ratios were prepared by microbeam plasma cladding technology. The influence of the NiCr-Cr3C2 content on the microstructure, mechanical properties, and molten salt corrosion resistance of CoNiCrAlY was investigated. The CoNiCrAlY with a 25 wt.% NiCr-Cr3C2 (NC25) cladding layer possessed the highest microhardness (348.2 HV0.3) and the smallest coefficient of friction (0.4751), exhibiting great overall mechanical properties. The generation of protective oxides Cr2O3, Al2O3, and spinel phase (Ni,Co)Cr2O4 is promoted by the addition of 25 wt.% NiCr-Cr3C2, which significantly reduces the corrosion of the cladding layer, and this effect is much more obvious at 950 °C than that at 750 °C. Furthermore, its corrosion mechanism was clarified. From the findings emerge a viable solution for the design and development of new high-temperature corrosion-resistant coatings.

Overview of corrosion behaviour of ceramic materials in molten salt environments

This study reviewed previous studies between the years 2015 and 2021 on how ceramic materials degraded in the presence of molten salt environments. The processes of corrosion resistance of various ceramic compositions subjected to various molten salt compositions and temperatures were also scrutinized. The results offer important new insights into the variables affecting ceramics' corrosion behaviour and the production of corrosion products. The reported result reveals that the ceramic material with the composition (Sm0.5Sc0.5)2Zr2O7 performed better than that of Sm2Zr2O7 in terms of hot corrosion resistance in molten salt (V2O5 + Na2SO4). It has also been reported that corrosion behaviour is influenced by particle size. Notably, zirconia (n-YSZ) with nanoscale grain sizes was more susceptible to hot corrosion, which was explained by increased specific surface areas. On the other hand, sintering and additives have been found to enhance corrosion resistance. The Y-Y2Si2O7 ceramic's resistance to corrosion in (V2O5 + Na2SO4) molten salt was enhanced by the addition of alumina. The results of these investigations help us understand how corrosion works and what influences ceramic materials' susceptibility to deterioration in molten salt media. This information can direct the creation of more corrosive-resistant ceramic materials for use in high-temperature environments or molten salt-based energy systems, among other corrosive uses.

Microstructure Evolution and Molten Salt Corrosion Resistance Dependent on Pre-Oxidation Treatment of NiAl Coatings

The chloride-infused molten salt shows the most potential as the medium for the upcoming generation of concentrated solar power system. However, the molten salt put for ward higher corrosion resistance demand on the critical components of concentrated solar power system. The application of NiAl coating proves to be a highly efficient method for preventing corrosion. In this paper, the microstructure, adhesion, and corrosion resistance of NiAl coatings on 310S stainless steel with/without pre-oxidation treatment were studied. The coating predominantly consisted of β-NiAl phase nanocrystals with an Al content of 41.0 at.%. Continuous α-Al2O3 oxide film was formed and no obvious cracks were observed after pre-oxidation. The adhesion strength of NiAl coating was increased by about 70% to 40.0 N through pre-oxidation. Importantly, the corrosion results of NiAl coatings by pre-oxidation treatment in the mixed salt of NaCl/MgCl2/KCl at 1073.15 K for 10 h showed no obvious diffusion of molten salt elements into the coatings. Compared to 310S stainless steel, the pre-oxidized coating sample exhibited a significant reduction of 50.2% in corrosion mass loss. The pre-oxidation of the NiAl coating creates an Al2O3 oxidation layer that effectively blocks corrosion, offering a new method for protecting stainless steels in concentrated solar power system plants.

Feasibility Study on Aluminum Under Laser Ablation for Corrosion Resistance in Molten Salt

Feasibility Study on Aluminum Under Laser Ablation for Corrosion Resistance in Molten Salt

Phase evolution and hot corrosion behavior of Yb2O3 and CeO2 co-doping YSZ ceramics under high temperature

The phase instability of 6–8 wt% Y2O3 partially stabilized ZrO2 (YSZ) in high temperature and molten salt corrosion environment seriously limits its application above 1200 °C. In this study, the chemical co-precipitation method prepared Yb2O3 and CeO2 co-doping YSZ (YbCeYSZ) ceramic materials to improve the above issues. It was proposed to improve the defect of high-temperature phase instability of CeO2-doped YSZ materials by adding Yb2O3. The phase stability, thermal insulation properties, and Na2SO4 + V2O5 molten salt corrosion behavior of YbCeYSZ ceramic materials were studied. The results showed that 2.5 mol% Yb2O3-4 mol% CeO2-YSZ material (2.5Yb4CeYSZ) had no monoclinic phase (m-ZrO2) after holding at 1500 °C for 120 h due to forming more defect clusters in the system. Meanwhile, the 2.5Yb4CeYSZ had excellent thermal insulation performance and molten salt corrosion resistance, which was attributed to the strong phonon scattering in the 2.5Yb4CeYSZ material system and the low reactivity between the stabilizer and the corrosive salt.

Development of Self-Passivating, High-Strength Ferritic Alloys for Concentrating Solar Power (CSP) and Thermal Energy Storage (TES) Applications

Concentrating solar power (CSP) and thermal energy storage (TES) based on molten salts still lacks economic feasibility, with the material investment costs being a major drawback. Ferritic stainless steels are a comparatively cheap class of materials that could significantly contribute to cost reductions. The addition of aluminum to ferritic steel can result in self-passivation by forming a compact Al2O3 top layer, which exhibits significantly higher corrosion resistance to solar salt compared to the Cr2O3 surface layers typically formed on expensive structural alloys for CSP and TES, such as austenitic stainless steels and Ni-base super alloys. However, to date, no ferritic stainless steel combining Al2O3 formation and sufficient structural strength is available. For this reason, cyclic salt corrosion tests under flowing synthetic air were carried out on seven Laves phase-forming, ferritic model alloys (17Cr2-14Al0.6-1Nb2.6-4W0.25Si), using “solar salt” (60 wt. % NaNO3 and 40 wt. % KNO3). The Al content was varied to investigate the influence on the precipitation of the mechanically strengthening Laves phase, as well as the impact on the formation of the Al-oxide top layer. The W and Nb contents of the alloys were increased to examine their influence on the precipitation of the Laves phase. The salt corrosion experiments demonstrated that simultaneous self-passivation against a molten salt attack and mechanical strengthening by precipitation of fine Laves phase particles is possible in novel ferritic HiperFerSCR (salt corrosion-resistant) steel. Microstructural examination unveiled the formation of a compact, continuous Al2O3 layer on the surface of the model alloys with Al contents of 5 wt. % and higher. Furthermore, a stable distribution of fine, strengthening Laves phase precipitates was achieved in the metal matrix, resulting in a combination of molten salt corrosion resistance and potentially high mechanical strength by a combination of solid solution and precipitation strengthening. These results show that high-strength ferritic alloys are suitable for use in CSP applications.

Study of Microstructure Regulation and In Situ Tensile Performance of Ni-Al Films

In this paper, Ni-Al films were prepared using magnetron sputtering technology. The microstructure of the films and the relationship between the residual stress and the adhesion strength were studied. More importantly, the mechanical strength–ductility properties of Ni-Al films were evaluated by in situ tensile testing. The results showed that the film mainly consisted of Ni3Al phase and Ni-based solid solution at the low power of Al target. The phase transition occurred, and the NiAl phase appeared when the Al sputtering power was increased. The complex structure, with the coexistence of Ni-based solid solution, Ni3Al, and NiAl phases, possessed increased residual stress and reduced adhesion strength. Meanwhile, the crack was easily penetrated through the inside and outside of the film, falling off during in situ tensile testing. While the small residual stress and large adhesion strength were obtained, only Ni3Al and NiAl coexisted, and the film fell off with difficulty. When the Al sputtering power was 400 W, the film showed the largest adhesion strength and the smallest residual stress. The best comprehensive performance was achieved with a tensile strength of 854 MPa and a yield strength of 90 MPa. The Al content of the film was up to 23.03 at.%, which was beneficial to the application and performance improvement of the film in molten salt corrosion resistance.

Feasibility Study on Aluminum Under Laser Ablation for Corrosion Resistance in Molten Salt

Molten salt is a prevalent medium in nuclear technology, especially in both reactors and waste separation (pyroprocessing) routines. Thus, the application dealing with the molten salt requires compatible materials that can withstand its aggressive environment. Research studies have shown that the purity of the molten salt environment can influence corrosion rates. Fission products, moisture, and oxide corrosion contents all distribute into the salt’s impurity and how it reacts within the system. The naturally occurring corrosion creates some protection via incomplete passivation layers; however, these same impurities start instability in the film through localized attacks. Laser ablation treatments in other research fields have proven useful in the improvement of corrosion resistance in various materials. Yet, this approach has not been tested in molten salt towards nuclear engineering applications. Thus, this becomes a motivation for our study to determine the feasibility of using the laser ablation technique on aluminum samples, creating an oxide layer and testing them in a eutectic LiCl-KCl salt at 773 K simulating a pyroprocessing environment. Over the course of the corrosion test, data sets were collected to allow electrochemical analysis – 24 hour rounds of open circuit voltage, electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV). Detailed results will be presented and discussed.

The corrosion behavior of CVD SiC coatings on SiCf/SiC composites in a simulated molten salt reactors environment

The corrosion behavior of SiC coatings deposited at 1000, 1150 and 1200 ℃ on SiCf/SiC composites was investigated. CVD SiC coatings deposited at higher temperatures provided better corrosion resistance in molten salt. SiC coating at 1200 ℃ showed the best resistance due to the high crystallinity. In addition, multi-scale characterization revealed that the dissolution of Si into molten salt resulted in the formation of a C-rich amorphous layer on SiC coatings. The fluorine was observed to diffuse through corrosion layer. According to DFT calculation, the diffusion of molten salt elements was assisted with Si vacancies and fluorine diffused fastest.

A potential candidate structural material for molten salt reactor: ODS nickel-based alloy

The commercialisation of molten salts reactors (MSRs) is hindered by the lack of structural materials capable of withstanding the corrosive environment therein. To address this problem, we herein prepared 1 wt% Y2O3 dispersion–strengthened NiMo-based alloys using powder metallurgy and evaluated their potential as structural materials for MSRs based on their mechanical properties, He swelling behaviour, and molten salt corrosion resistance. In view of the strengthening provided by homogenously dispersed Y2O3 particles, all NiMo-Y2O3 samples exhibited ultimate tensile strengths and yield strengths exceeding those of the Hastelloy N alloy, a state-of-the-art structural material for MSRs. Moreover, the volume fraction of He bubbles in the NiMo-Y2O3 samples (∼0.3%) was lower than that in the Hastelloy N alloy (0.58%), which showed that the introduction of Y2O3 nanoparticles effectively inhibited He swelling. All NiMo-Y2O3 samples showed excellent resistance to molten salt corrosion (as reflected by the absence of obvious holes therein), thus holding great promise for the development of irradiation- and molten salt corrosion–resistant structural materials for high-temperature MSRs.

Integrated high-throughput research in extreme environments targeted toward nuclear structural materials discovery

Arguably one of the most important factors in the fast deployment of advanced nuclear reactors, with major improvements in safety, is the development and qualification of radiation and corrosion tolerant materials, that serve as the structural components in reactor cores. However, the discovery, improvement, and assessment of materials resistant to radiation and corrosion in the advanced reactors’ extreme environments is quite demanding, time-consuming, and costly, which represents a significant barrier to materials innovation and qualification for nuclear energy. This short review highlights a novel, integrated, high-throughput (HTP) research framework to develop understanding and predictive models for irradiation at high doses and molten salt corrosion responses of structural Compositionally Complex Alloys (CCAs), with the objective to accelerate materials discovery for high-temperature nuclear structural applications. Using a novel in situ alloying technique, arrays of additively manufactured bulk CCAs are processed, heat-treated, and characterized, while still attached to the build plate. Leveraging recent development in automation of heavy-ion irradiation experiments at the University of Wisconsin Ion Beam Laboratory, arrays of CCAs can be rapidly irradiated to hundreds of dpa up to 800 °C. An innovative droplet corrosion method is also used to test molten salt corrosion behavior of CCAs arrays. Automated and rapid characterization methods are used to assess the irradiation and molten salt corrosion resistance of CCAs. Finally, a brief discussion of the results is presented considering future use of machine-learning-based methods to develop useful trends and highlight features of importance. Using this novel HTP approach, a robust and reliable database containing literally hundreds of data points for irradiation and corrosion responses of CCAs can be established within a year, which is considered a significant increase in the pace of nuclear structural materials research and discovery.

Corrosion performance of Ni-based structural alloys for applications in molten-salt based energy systems: Experiment & numerical validation

The molten salt corrosion performance of a Y2O3-strengthened Ni-Cr alloy (MA754®) designed for high temperature applications (> 750 °C) was compared to purpose-designed Ni-Mo-Cr molten-salt resistant alloys (GH3535, HASTELLOY-N®). The significant material mass loss of MA754® alloy is attributed to its higher Cr-content. However, Y2O3 dispersoids are shown to play only a minor role in the corrosion performance of this oxide-dispersion-strengthened (ODS) alloy. The current result, thus, points to the possibility for the development of low Cr-content ODS alloys that combines the high-temperature properties of ODS MA754® alloy with good molten salt corrosion resistance of well-established GH3535 and HASTELLOY-N® alloys.

Study on Molten Salt Corrosion Behavior of Several Stainless Steel Alloys

Molten salt corrosion tests were performed on several stainless steels, which were 622,316, A59,625, 686,2#825,3#825 and M400. After corrosion at 680°C for 120 h, the sequence of alloys’ mass loss was 625<3#825<A59<622< 2#825<316<686<M400. SEM/EDS analysis was used to observe the surface and cross-section morphologies of the corroded samples. The alloys with high molten salt corrosion resistance formed Cr2O3 oxide layers on the surfaces of the samples, the formation of TiO2 could also enhance the corrosion resistance. However, Mn element was harmful to the alloys used in chlorine induced corrosion environment.

Corrosion Behaviour of AISI409 Stainless Steel with Al Slurry Coating in Molten Salt

A concentrated solar power plant produces electricity by collecting thermal energy from sunlight. Then the thermal energy is stored in the heat transfer fluid (HTF). The most widely used and studied HTF is solar salt (60wt%NaNO3–40wt%KNO3) that achieves the most requirements of HTF. However, at high temperatures, the corrosion is still violent for most materials, thus the material selection is important. This research aims to investigate the feasibility for application of AISI 409 ferritic stainless steel as containers of the molten salt. AISI 409 is suitable for high temperature usage, and more economical than other grades which have higher chromium. To investigate the corrosion resistance in molten salt, AISI 409 samples were coated by dipping in Al slurry. Then they were heated up to 400°C to remove binders. The coatings with Fe-Al intermetallic and alumina layers were formed after binder removal and subsequently annealing at 700°C for 3 h in the air atmosphere. This research reported the corrosion behaviour of Al coated and uncoated samples which had been treated by immersion in molten salt at 500°C for 100 h with analysis results. The results demonstrated that weight and thickness of uncoated samples were increased due to formation of the oxide layers such as Fe2O3 and Fe3O4 on the surfaces. For the Al coated samples, the weight and thickness of the coat layers were massively decreased after being immersed in molten salt at the first 1 h due to removal of Al-O, slag or impurity in the coating. Nevertheless, the weight increased gradually until 100 h. The oxidation of the Fe-Al intermetallic layer exhibited about 35% slower oxidation than that of uncoated samples, thus the coating technique has potential to be applied against molten salt.

Effect of shot peening on the high temperature molten salt corrosion resistance of P91 boiler steel

This article is an attempt to investigate the effect of the shot peening on hot corrosion behaviour of alloy P9 when exposed to a molten salt mixture of 40%Na 2 SO 4 + 60%V 2 O 5 . The specimens were operated at 750℃. The shot peening process creates short diffusion paths, which helps to enhance the rapid fusion of the elements. It helps in the formation of protective oxides, which leads to better corrosion resistance against molten salt attack. Thermogravimetric analysis was carried out to observe the impact of molten salts on the corrosion rate of the different specimens. The shot-peened specimens possess better corrosion resistance properties as compared to the non-shot peened specimens.

Revealing 3D Morphological and Chemical Evolution Mechanisms of Metals in Molten Salt by Multimodal Microscopy.

Growing interest in molten salts as effective high-temperature heat-transfer fluids for sustainable energy systems drives a critical need to fundamentally understand the interactions between metals and molten salts. This work utilizes the multimodal microscopy methods of synchrotron X-ray nanotomography and electron microscopy to investigate the 3D morphological and chemical evolution of two-model systems, pure nickel metal and Ni-20Cr binary alloy, in a representative molten salt (KCl-MgCl2 50-50 mol %, 800 °C). In both systems, unexpected shell-like structures formed because of the presence of more noble tungsten, suggesting a potential route of using Ni-W alloys for enhanced molten-salt corrosion resistance. The binary alloy Ni-20Cr developed a bicontinuous porous structure, reassembling functional porous metals manufactured by dealloying. This work elucidates better mechanistic understanding of corrosion in molten salts, which can contribute to the design of more reliable alloys for molten salt applications including next-generation nuclear and solar power plants and opens the possibility of using molten salts to fabricate functional porous materials.

Preparation and molten salt corrosion behaviors of TiO2/Al2O3/Incone1625 composite coatings on the surface of Q235 alloy at 900 °C

Sol-gel method combined with plasma surfacing was used to prepare TiO2/Al2O3/Incone1625 composite coatings on the surface of Q235 alloys. To investigate the corrosion resistance of this coatings, samples with Incone1625 single coating and TiO2/Al2O3/Incone1625 composite coating were corroded with NaCl and Na2SO4 salts with the weight ration of 1:1at 900 °C in air for 24 h. Molten salt corrosion thermodynamic results show the composite coatings caused lower oxidation weight gain than the sample with single coating. Combination of XRD and SEM results illustrates that the surface and cross-sectional morphology were composed of an outermost loose and cracked NiO and iron oxides layer for the Incone1625 coated sample. For the composite coating sample, the oxide film was composed of more compact NiO and Cr2O3 as the main layer. There were inner oxidation regions for both samples, but region for sample with composite coating was more thinner than that for sample with single coating. This work shows that TiO2/Al2O3/Incone1625 composite coating improves the molten salt corrosion resistance properties of the Q235 alloy at 900 °C by promoting the formation of the protective Cr2O3 film on the sample surface.

Corrosion resistance of TaSiCN coatings in NaCl-KCl molten salt

TaSiCN composite coatings were deposited on Ta alloy substrates by radio frequency sputtering with different N2 gas flow rates. The composition, microstructure, and NaCl-KCl molten salt corrosion resistance of the coatings were investigated. The results showed that all TaSiCN coatings consist of TaN, TaC and Ta(C,N) nano-crystalline, amorphous SiCx, SiNx, CNx and sp2 amorphous carbon. With increasing N2 gas flow rate, the N content of the coatings increased from 20 at% to 35 at%, the C content decreased from 48 at% to 34 at%, and the content of Ta and Si remains slight change. In comparison with Ta alloy substrate, the samples with TaSiCN coatings exhibited better corrosion resistance against NaCl-KCl molten salt at 750 °C, where the coating with the N content of 28 at% displayed the best corrosion resistance. The corrosion products of the coatings mainly included Ta2O5, Na2Ta4O11, and SiOx, etc. The corrosion mechanism of TaSiCN coatings could originate from a cyclic process of passivation, penetration, stripping, and repassivation. Our study could provide a potential corrosion resistant coating for the electrolytic crucible of pyrochemical processes.

Effects of laser surface remelting on the molten salt corrosion resistance of yttria-stabilized zirconia coatings

In the present study, atmospheric plasma-sprayed yttria-stabilized zirconia coatings were post-remelted by a continuous diode laser to improve the hot corrosion resistance for as-sprayed coatings. The coating surfaces were covered with a salt mixture (V2O5 and Na2SO4) and then subjected to a hot corrosion test at 1100 °C in air. The influence of laser parameters including power and scanning rate on the coating microstructure and corrosion resistance was investigated. Results showed that the hot corrosion resistance can be improved by producing a dense and smooth surface and reducing the coating permeability to the molten salt. The transformation of the hot corrosion mechanisms was clarified on the basis of the observed corrosion behaviors. A laser power of 1500 W and scanning rate of 9 mm/s can produce minimal surface roughness with few segmented cracks, which can provide improved performance of the hot corrosion resistance.

Corrosion Resistance of Boronized, Aluminized, and Chromized Thermal Diffusion-Coated Steels in Simulated High-Temperature Recovery Boiler Conditions

In this study, the high-temperature molten salt corrosion resistance of bare steels and steels with protective coatings, fabricated by thermal diffusion processes (boronizing, aluminizing and chromizing), were investigated and compared. Surface engineering through thermal diffusion can be used to fabricate protective coatings against corrosion, while alleviating issues around possible cracking and spallation that is typical for conventional thermal-sprayed coatings. In this regard, samples of low carbon steel and 316 stainless steel substrates were boronized, chromized, and aluminized through a proprietary thermal diffusion process, while some of the samples were further coated with additional thin oxide and non-oxide layers to create new surface architectures. In order to simulate the actual corrosion conditions in recovery boilers (e.g., from black liquor combustion), the surfaces of the samples sprayed with a modeling salt solution, were exposed to low-temperature (220 ∘C) and high-temperature (600 ∘C) environments. According to microstructural and X-ray diffraction (XRD) studies and results of hardness determination, the coatings with multilayered architectures, with and without additional oxide layers, showed successful resistance to corrosive attack over bare steels. In particular, the samples with boronized and chromized coatings successfully withstood low-temperature corrosive attack, and the samples with aluminized coatings successfully resisted both low- and high-temperature molten salt corrosive attacks. The results of this study conducted for the first time for the thermal diffusion coatings suggest that these coatings with the obtained architectures may be suitable for surface engineering of large-sized steel components and tubing required for recovery boilers and other production units for pulp and paper processing and power generation.