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• A HTF based on quaternary nitrate salt was investigated. • Major parts of the phase diagram were assessed. • Heat capacity of selected mixtures was measured. • Thermal stability was investigated in long-term tests. • Decomposition mechanisms were identified to be cation-selective. Thermal energy storage (TES) is a vastly growing technique that allows for the generation of dispatchable electricity in modern concentrating solar power (CSP) plants. In solar tower systems the key to success is the use of a heat transfer fluid (HTF) and storage medium known as Solar Salt. This nitrate salt mixture of NaNO 3 and KNO 3 is considered thermally stable, non-toxic and environmentally friendly. In line-focusing CSP plants Solar Salt is still not being used as heat transfer fluid due to the inherent risk of freezing and related systematic failure. Accordingly, there is a need to develop nitrate salts with lower melting temperatures but yet acceptably high thermal stability. The information on molten nitrate mixtures with low melting points (⪅100 °C) is limited, especially in terms of isothermal stability tests. This work presents thermo-physical data of the complex quaternary Ca,Li,Na,K//NO 3 system. Melting temperatures of more than 100 mixtures were assessed and compositions with melting points below 100 °C were identified. The heat capacity of selected mixtures was in the range of 1.5–1.6 kJ/kg K and generally increased with increasing Li-content. Thermal stability, with Solar Salt as reference salt, indicated that the stability of all mixtures did not exceed 500 °C but that the achievable ΔT was 360 °C, about 90 °C higher than that of Solar Salt. Some compositions are therefore are potential HTF and storage media but the overall Ca-content plays a crucial role in the decomposition of the quaternary mixtures during operation at high temperatures. [ABSTRACT FROM AUTHOR]

This study investigates the impact of stabilizing molten salt chemistry on the corrosion behavior of four steel types at temperatures ranging from 570 °C to 620 °C. The research explores the influence of temperature and gas atmosphere on corrosion mechanisms through detailed analysis of molten salt, corrosion rates, and microstructures. Findings reveal that systematic changes in salt chemistry can modify mass transport mechanisms, enabling the development of corrosion mitigation strategies even at extreme temperatures. This discovery marks a significant breakthrough in the advancement of Solar Salt-based storage systems for high-temperature applications, facilitating the utilization of renewable energy sources. • Comparison of state-of-the-art (570 °C) vs. enhanced temperatures of 620 °C in Solar Salt. • Impact of Gas phase composition on Salt Stability and Corrosion. • Analysis of both attacking medium (molten salt) and metal specimens. • Corrosion Analysis by Mass Gain, Descaling, Surface- and Cross-section analysis. • Derivation of corrosion mechanisms for decomposed vs. stabilized Solar Salts. [ABSTRACT FROM AUTHOR]

• Purge flow with nitrous gases stabilizes the molten salt up to 620 °C. • Experiments demonstrate reaction equilibria with oxide ions. • Evaluation of reaction equations emphasizes the effect of N 2 formation. Thermal energy storage systems are integrated in concentrating solar power plants to improve the flexibility of the electricity generation. Commonly, the liquid storage material Solar Salt, a nitrate salt mixture, is applied to transport and store solar thermal energy. The lifetime and the temperature range of Solar Salt in the storage units are restricted by decomposition reactions of the material. Oxide ion formation is one of the fundamental issues. So far, it has not been proven if oxide ion formation can be prevented by addition of gaseous reaction products to the gas atmosphere. Also, a reliable reaction equation for the oxide ion formation is missing. In the presented experiments, molten salt at 600 and 620 °C is purged with a gas mixture of nitrogen, oxygen, and nitrous gases. Post-analysis of salt samples reveals stabilizing effects, depending on the purge gas compositions. Chemical equilibrium of the oxide ion forming reaction is demonstrated. It is proven that oxide ion formation can be controlled and suppressed. Reaction equations are evaluated and selected in order to quantify the reaction thermodynamics. The results contribute to recommendations for operating conditions and gas handling in storage systems of solar thermal power plants, which finally ensure reliable and constant material properties for extended lifetime and high temperatures. [ABSTRACT FROM AUTHOR]

Abstract Sensible heat storage in molten nitrate salts is a key technology when it comes to thermal energy storage in combination with concentrating solar power (CSP) plants. Currently, a mixture of sodium and potassium nitrate called Solar Salt is used at temperatures between 280 and 560 °C. Our research approaches comprise an improvement of Solar Salt stability at temperatures above 560 °C, which comes along with a broadening of the operating temperature range. In this way, a higher storage capacity and efficiency, as well as economic advances can be achieved. Numerous publications about the chemistry of molten nitrate salt mixtures reveal the formation of chemical species such as nitrite ions, nitrous gases and oxide ions and carbonate ions. Among these, the reversible reaction of nitrate to nitrite with release of oxygen is the predominant reaction at temperatures below 560 °C. So far, there is a lack of literature that pays particular attention on the contributions of reaction kinetics and mass transport effects on the reaction velocity of the nitrate/nitrite-reaction. However, understanding of these physico-chemical phenomena is essential, because further decomposition reactions build on the nitrate/nitrite-reaction. In this study, experiments at different scales (50 mg to 100 g) are compared with regard to the reduction reaction velocity at 500 °C. A data treatment procedure for thermogravimetric analysis experiments successfully eliminates effects of salt evaporation on the measured mass changes. The chemical equilibrium of the salt samples turned out to be reasonable, when compared to literature values. It was found, that the surface-to-volume ratio of the salt melt samples significantly affects the evaporation rates and the reaction velocity. The time period that is needed until chemical equilibrium is reached at 500 °C clearly increases when the surface-to-volume ratio decreases. The thermogravimetric analysis apparatus enables experiments, which measure reaction kinetics without mass transport limitations. It is demonstrated that a simulation model adapted from literature displays a reaction velocity that incorporates both reaction kinetics and mass transport without differentiation of the two phenomena. [ABSTRACT FROM AUTHOR]

Molten salts are widely used as thermal energy storage media in the Concentrating Solar Power (CSP) technologies. The melting point and the thermal stability of the salt determine the applicable temperature range of the storage system. The focus of this paper is to evaluate the effect of different gas atmospheres on the thermal stability of binary Solar Salt and two ternary salt mixtures containing lithium nitrate (Lithium mixture) and sodium nitrite (Hitec). The isothermal stability experiments were carried out at 550 °C during 500 h and the results show that the initial decomposition reaction of nitrate to nitrite depends strongly on the gas atmosphere. It is observed that changes in the nitrate-nitrite-ratio are the key parameters influencing the melting temperatures of the salt mixtures. For example, for experiments with oxygen in the atmosphere a large increase of the liquidus temperature of the Hitec mixture was observed. Metal oxides are formed during the irreversible decomposition of nitrite-ions but are found to affect the solidus and liquidus temperature of the salt mixtures only marginally. No carbonate formation was detected according to titration analysis of the salt mixtures in our experiments due to the absence of carbon dioxide in the atmosphere. [ABSTRACT FROM AUTHOR]

Highlights • Characterization of quartzite and basalt varieties as potential filler materials. • Isothermal test at 560 °C in Solar Salt with a maximum duration of 10000 h. • Determination of the stone microstructure by QEMScan before and after thermal test. • DCS measurements of melting temperature and enthalpy of Solar Salt after thermal test. • Ion chromotography measurements of salt composition of Solar Salt after thermal test. Abstract Solar thermal power plants with integrated thermal storage are candidates for renewable energy production concepts. For cost reduction of thermal energy storage a single tank concept, the so called thermocline storage concept, instead of the two-tank molten salt thermal storage is as promising cost reduction option. Further cost reductions in the thermocline storage are expected by replacing a significant amount of Solar Salt by a low cost filler material. Such filler materials have to be stable in molten salt at temperatures up to 560 °C. In this work degradation studies on quartzite and basalt types in molten salt are carried out after a preselection has been published elsewhere recently. The investigations are focused on the compatibility of natural stones with Solar Salt, a mixture of sodium nitrate and potassium nitrate, as common heat storage material. This work addresses changes of the molten salt properties and in the microstructure of the natural stones depending on the exposure time in molten salt at temperatures of approximately 560 °C. In the first step of the material investigation the natural stones were isothermally stored in Solar Salt at a maximum temperature of 560 °C for up to 10.000 h. After the thermal treatment the microstructure of the stones was investigated by QEMSCAN (Quantitative Evaluation of Minerals by Scanning electron microscopy). By means of this analysis method the changes in the microstructure of quartzite and basalt was detected and arising stone components are identified. The melting temperature und enthalpy of Solar Salt was measured and compared with the salt properties before the thermal treatment. Additionally, the specific heat capacities of basalt and quartzite depending on the temperature were determined. The results are essential to verify the suitability of quartzite and basalt as potential filler materials in modern thermocline storage concepts. [ABSTRACT FROM AUTHOR]

Concentrating solar power coupled to thermal energy storage (TES) is a vastly growing industrial process allowing for the generation of dispatchable and green electricity. This paper focuses on direct molten salt line-focusing technology using linear Fresnel and parabolic trough collector systems. Direct molten salt technology utilizes molten salt as heat transfer fluid in solar field and TES medium. Nitrate salts can be applied since they cover a wide temperature range. As storage medium Solar Salt, a binary NaNO 3 − KNO 3 (60-40 wt%) mixture, is most commonly used but variations of this system have promising thermal properties in terms of a lower melting temperature to minimize the risk of undesired salt freezing events. These modified salts are typically ternary, ternary reciprocal or higher order systems formed by adding additional cations, anions or both. In this study five molten salt systems Solar Salt, HitecXL (CaKNa//NO3), LiNaK-Nitrate, Hitec (NaK//NO23) and CaLiNaK//NO23 are both investigated and critically reviewed. Their thermo-physical properties including phase diagrams, composition, melting ranges, melting temperature, minimum operation temperature, thermal stability, maximum operation temperature, density, heat capacity, thermal conductivity, viscosity and handling are evaluated and the most recommended values are discussed and highlighted. This review contributes to a better understanding of how the listed properties can be determined in terms of measurement conditions and provides temperature dependent data useful for future simulations of direct molten salt LF CSP plants. [ABSTRACT FROM AUTHOR]

Molten chlorides are promising alternative thermal energy storage (TES) materials to be applied in concentrating solar power (CSP) plants. Their high thermal stability makes them appropriate candidates to replace the commercial TES materials in CSP, nitrate salts (stable up to 550 °C), when higher operating temperatures (e.g., 700 °C) are required for higher efficiency of energy conversion. However, the application of molten chlorides at higher temperatures causes increased corrosiveness of containers and structural materials. In this work, cyclic voltammetry (CV) is applied to measure the concentration of corrosive oxide/hydroxide impurities in the molten chlorides in - situ , in order to assist the corrosion control. Before the electrochemical experiments, pre-electrolysis (PE) was performed on molten MgCl 2 /KCl/NaCl (60/20/20 mole%) salts to remove most of the impurities including oxide/hydroxide. By fitting the CV data from PE with those from the acid consumption (AC) measurements on the simultaneously collected salt samples, the parameter for the calculation of the concentration of MgOH + from the peak current density was determined under the conditions 500 °C and sweep rate of 200 mV/s. By adding NaOH pellets into the molten salts, a significant change of a peak was observed in the cyclic voltammograms due to the concentration change of the most stable hydroxide species MgOH + . Moreover, when the determined parameter was used in the calculation of the concentrations of MgOH + from the peak current densities, the calculated concentrations of MgOH + compare well with those from AC. [ABSTRACT FROM AUTHOR]

Microporous, highly charged and cation exchangeable clays are interesting carriers not only for various functional molecules such as cationic dyes, surfactants, proteins, drugs and complexes, but also for water insoluble molecules. In water, swellable nanohectorite derivatives (“laponites”) with dimensions of about 25 nm in diameter and 1 nm in height delaminate to form individual nanoparticles, which proved to be very useful for applications in aqueous dispersions, but less advantageous for the use in filter systems for water treatment, membrane technology, or as catalysts. To immobilize the particles in aqueous environments and still provide accessibility and high cation exchange rates on the clays, we used an inversion process to prepare porous nanoclay polysulfone composites with high clay contents up to 60%. Successful incorporation of the clays in the matrix required clay pre-modifications with reactive silanes such as octyltriethoxysilane or octadecyltriethoxysilane. Interestingly, unmodified laponite can also be used as a template to form a porous polysulfone network due to its leakage in the inversion process. The high accessibility and activity found in composites prepared with a mixture of unmodified and modified laponite particles pave the way for further modifications with functional molecules for various applications. In this study, we characterize the nanoclay polysulfone composites via scanning electron microscopy, nitrogen gas adsorption of the dried composite, and we determine the cation exchange capacity in the wetted state. [ABSTRACT FROM AUTHOR]

The implementation of inexpensive scalable thermal energy storage will play a crucial role in the successful establishment of dispatchable renewable energy technologies. Storage based on molten nitrate salts is one of the most relevant technologies implemented in the GWh-scale. Yet, there is significant cost reduction potential in replacing the conventional two-tank system by a single-tank thermocline system. The latter involves the use of a single-tank design where a large fraction of the costly salt is replaced by an inexpensive filler material. Industrial waste materials such as AlferRock (red-mud-derived Fe 2 O 3 -rich ceramics), have significant potential since they are readily available in the Mt-range and have adequate thermal and mechanical resistance at the intended temperature of use. This study explicitly investigates the corrosion resistance of AlferRock in Solar Salt, 60% NaNO 3 -40% KNO 3 mixture, at 560 °C. By variation of particle sizes classical exposure tests as well as accelerated test methods can be applied to understand the long-term stability of this waste material under relevant conditions. Salt chemistry and compositional changes in the filler are analyzed in terms of ion chromatography, titration, diffraction techniques (XRD) as well as electron microscopy (SEM) coupled with element mapping (EDX). • A novel filler material for thermocline storage is investigated. • Salt chemistry and cross-section analyses are performed. • The mixed Fe-Ti-Si-O phases in AlferRock show high stability in Solar Salt. • Little structural changes are observed after exposure tests. • No otherwise typical dissolution of metal species in molten salt is observed. [ABSTRACT FROM AUTHOR]

Sensible heat storage is a cost-efficient and scalable technology for energy storage. The state-of-the-art storage systems in concentrating solar power (CSP) plants use the storage material Solar Salt, which is a nitrate salt mixture. Chemical stability of this salt material is crucial for lifetime reliance, and for development of the storage technology towards higher temperatures. High temperatures enhance the storage capacity, but also promote decomposition reactions. For instance, harmful gases can evolve, and oxide ions are produced, which aggravate corrosion. Up to now, it is unclear how to describe the salt chemistry, and how to quantitatively predict the problematic decomposition products. The experimental method in this study is chosen with regard to the exclusion of mass transport limitations. Thin films of salt are heated to 560–630 °C. The salt composition is analyzed by ion chromatography and acid-base titration. The ratio of nitrite to nitrate ions stabilizes, which indicates chemical equilibrium of the nitrite forming reaction. The oxide content increases continuously over time, and is interpreted in terms of a kinetic rate law. A consistent mathematical description of Solar Salt chemistry at high temperatures (≥560 °C) in contact with air is presented. It includes thermodynamic parameters, in particular the reaction enthalpy of 95 ± 4 kJ⋅mol−1 and entropy of 86 ± 5 J⋅mol−1⋅K−1 for the nitrate-nitrite reaction. The microkinetics of the oxide ion formation are characterized by an activation energy of 42 ± 3 kJ⋅mol−1. The work presented finally contributes to a forecast of material stability at and above 560 °C. Image 1 • Experimental investigation of two essential decomposition reactions. • First-time data for the reaction kinetics of corrosive oxide ions. • Model description in terms of thermodynamics and a kinetic rate law. [ABSTRACT FROM AUTHOR]

• Thermal stability limit of molten nitrate salts is enhanced from 560 °C to 600 °C. • Operation in a closed storage system increases thermal stability significantly. • Performance of Solar Salt is demonstrated in 100 g-scale. • Quasi-in situ sample analysis is used for proof of concept. • Formation of corrosive impurities is successfully suppressed at 600 ° C. The implementation of inexpensive and reliable energy storage technologies is crucial for the decarbonisation of energy intensive industry branches and energy supply. Sensible thermal energy storage (TES) in molten salts is a key technology for storage of heat in the scale of gigawatt hours but currently limited to operating temperatures of 560 °C. Increasing the maximum operating temperature while maintaining thermal stability of the storage medium is one of the main challenges next-Generation TES systems are facing. Extending the upper temperature limit by only 40 °C increases the storage capacity by more than 16% allowing for more compact storage designs and cost savings in the $ million-range for large scale storage units. Here we propose a novel storage technology from a materials point of view that pushes the thermal stability limit of Solar Salt up to 600 °C by simply but effectively sealing the storage unit including the gas system. The concentration of the unstable nitrite ion and of the corrosive oxide ion could be reduced by 16% and 75%, respectively at 600 °C, compared to a salt system with open atmosphere. We present clear evidence of the enhanced thermal stability in long-term, 100 g-scale test campaigns at previously unequalled temperatures. These findings constitute a major advance in the design and engineering of next generation storage systems. [ABSTRACT FROM AUTHOR]

This work addresses the influence of molten nitrate salt chemistry on the corrosion behavior of a martensitic high temperature steel and an austenitic stainless steel. It is one of the first addressing controlled degradation of Solar Salt by controlling the gas atmosphere on top of the melt, thus driving the formation of corrosive ions, mainly oxide ions but also nitrite ions. The stainless steel and high temperature steel samples are subjected to the different operating conditions to demonstrate the variations in corrosivity as a function of gas atmosphere and additionally of artificially added chloride impurities. The results indicate that the atmosphere has a direct impact on the formation of corrosive oxide and nitrite ions. The low Cr-steel is found to be more sensitive to chloride impurities, while the stainless steel corrosion is enhanced by the presence of nitrites and oxide ions. All studies are supported by profound molten salt analysis throughout the experiments including the analysis of nitrate, nitrite, oxide and chromate ions as well as cation compositions. Steel samples are analyzed by scanning electron microscopy methods and corrosion rates are extrapolated from 1200 h experiments. • The influence of Solar Salt ageing on corrosion effects is investigated. • Solar Salt stability is experimentally directed by different purge gases. • Nitrite and oxide ions are key components guiding corrosion resistance of steels. • SS316Ti exhibits higher corrosion rates in aged compared to fresh Solar Salt. • When Cl--impurities are present, T91 suffers from severe Cr depletion. [ABSTRACT FROM AUTHOR]

In this work, electrolysis with a Mg anode is presented to purify the molten chloride salt (MgCl 2 /KCl/NaCl 60/20/20 mol.%) for reducing its corrosivity. Using a Mg anode, the production of toxic gases like Cl 2 on an inert anode (e.g., tungsten) can be avoided. Moreover, compared to an inert anode, a lower over-potential is required to remove the corrosive impurity MgOH+ in the molten salt due to the high reactivity of Mg. In order to evaluate the effect of the salt purification, the cyclic voltammetry (CV) method developed in our previous work is used to in-situ measure the concentration of the corrosive MgOH+ impurity in the molten salt. The CV measurements indicate that the corrosive impurity is efficiently removed by electrolysis. For decreasing the cathode inactivation due to produced MgO on the surface, a pulsed potential applied on the tungsten cathode during electrolysis shows to be promising. This electrochemical salt purification method has shown to be promising by efficiently controlling the corrosivity of the molten chloride salt. The potentiodynamic polarization (PDP) measurements on a commercial high-temperature alloy (Incoloy 800 H) immersed in the molten salt indicate that the corrosion rate of the alloy is significantly reduced due to the salt purification. It could also lead to a reduction of the cost of the conventional salt purification step, structural container materials, and piping in next generation concentrated solar power (Gen3 CSP) plants. Image 1 • Electrolysis with a Mg anode purifies a molten chloride salt to reduce its corrosivity. • Cathode passivation due to produced MgO at the surface is significantly reduced by applying pulsed potential. • CV shows that concentration of corrosive impurity in purified salt has a reduction of ~93% compared to unpurified salt. • PDP of Incoloy 800 H in purified salt shows ~80% corrosion rate reduction compared to that in unpurified salt. [ABSTRACT FROM AUTHOR]

Conventional thermal energy storage (TES) media and heat transfer fluids (HTFs) currently used in commercial concentrated solar power (CSP) plants are nitrate-based molten salts with working temperature up to about 565 °C. Current interest in raising the working temperature in next-generation CSP for higher energy conversion efficiency leads to chloride-based molten salt technology (>700 °C). MgCl 2 –KCl–NaCl salt is selected to be such a TES/HTF material due to its excellent salt properties, abundance and low costs. In this review, survey and evaluation of its exact eutectic composition by evaluation of the phase diagram as well as most relevant salt properties for design, construction and operation of a test loop and projection of a real TES/HTF system are presented. They include minimum melting temperature, vapor pressure, heat capacity, density, thermal conductivity and dynamic viscosity. The working temperature range 420–800 °C is suggested for the TES/HTF system using this salt. Moreover, the recommended values of the heat capacity, density, thermal conductivity and dynamic viscosity in the suggested working temperature range are given. In addition, corrosion control strategies for this salt including corrosion monitoring and mitigation techniques are briefly reviewed, since controlling the salt corrosivity is one main concern of the engineering aspects using the molten chlorides at high temperatures. Overall, this review is expected to supply reliable values of the most relevant salt properties and recommend the most promising corrosion control strategies for further material studies, as well as modeling, simulation, detailed design and construction of molten MgCl 2 –KCl–NaCl salt TES/HTF systems. • Most relevant salt properties of MgCl 2 –KCl–NaCl to engineer this salt are reviewed. • A full set of their recommended temperature-dependent values are given. • Working temperature range of MgCl 2 –KCl–NaCl for TES/HTF is suggested. • Corrosion control strategies including corrosion monitoring and mitigation techniques are briefly reviewed. • Concept of corrosion control system integrated into TES/HTF system is introduced. [ABSTRACT FROM AUTHOR]

Molten salt corrosion in the receiver's piping system and storage tanks is still one of the major drawbacks of concentrated solar power (CSP) plants and is currently covered by using expensive high-alloyed steels or Ni-base alloys. The employment of cheaper structural materials combined with protective coatings is an attractive alternative to increase cost-efficiency. The present study investigates the corrosion resistance of three different coatings (a pure Ni, a Cr and a combined Ni + Cr coating) deposited on ferritic-martensitic X20CrMoV12-1 steel during isothermal immersion in molten solar salt in comparison to uncoated Ni-base alloy Haynes 230. Exposure tests were conducted at 600 °C for up to 1000 h. To identify the individual role of the main elements, exposure tests of pure Fe, Ni and Cr were undertaken as well. In addition to the cross-sectional investigations via light-optical microscopy and EPMA, X-ray diffraction measurements were conducted to identify the corrosion products. These were complemented by weight change measurements and chemical analysis of the salts after the exposure tests. The study reveals differences between the corrosion behaviour of the different coating approaches, which is discussed as a function of coating composition and degradation mechanisms. Whereas pure Cr or pure Ni coatings offer hardly any improved protection of the uncoated X20CrMoV12-1 substrate, the combined Ni + Cr coating performs very well and improves the scaling behaviour of X20CrMoV12-1 significantly. The outcome highlights the correlation between the solubility of the pure metals Fe, Ni and Cr in molten solar salt and the corrosion resistance of the investigated coatings. • Scaling behavior of X20 steel significantly improved by a Ni + Cr diffusion coating. • Dissolution of Cr-containing phases accelerated thermal decomposition of HTF. • Pure Ni and Fe grew thicker scales compared to the investigated Ni–Fe–Cr alloys. • Unlike Ni and Fe, pure Cr dissolved rapidly in the HTF forming soluble chromates. • Ambivalent role of Cr: improved scaling behavior but was susceptible to dissolution. [ABSTRACT FROM AUTHOR]

Molten chloride salts such as MgCl 2 /KCl/NaCl are promising thermal energy storage (TES) materials and heat transfer fluids (HTF) in next generation concentrated solar power (CSP) plants with elevated operation temperatures (>700 °C) due to their high thermal stability and low material costs. However, they have strong corrosivity against metallic structural materials at high temperatures which can be related to the presence of hydrolysis products such as MgOHCl. In an electrolytic purification method reported in previous work, a W-cathode and Mg-anode were used to reduce the concentration of these impurities, thereby effectively purifying the molten MgCl 2 /KCl/NaCl salt. However, the W-cathode passivation due to production of MgO on the surface limited its cathode effectiveness. In this work, an improved electrolytic purification method is presented to avoid the electrode passivation by using two identical Mg-electrodes and alternating voltage (AV, i.e., switching the voltage direction applied periodically). A continuous electrolytic salt purification in the AV-mode is successfully performed on the molten MgCl 2 /KCl/NaCl salt in the 100 g-scale. Cyclic voltammetry (CV) on the molten salt shows that the purification can effectively reduce the concentration of the main corrosive impurity MgOH+Cl−. Potentiodynamic polarization (PDP) measurements on a commercial alloy (Incoloy 800 H) immersed in the molten salt indicate significantly reduced corrosion rates (i.e., reduced salt corrosivity) compared to a non-purified salt. The continuity of the improved method allows for a long-term effective purification without the risk of passivation and deactivation of electrodes, thereby showing great potential in corrosion control of molten chloride salt TES system. Image 1 • Improved electrolytic purification method purifies molten chloride salt to reduce corrosivity. • Electrode passivation is reduced by using two identical Mg electrodes and alternating voltage. • Mechanism of the improved electrolytic purification is proposed. • A continuous electrolytic purification on molten MgCl 2 /KCl/NaCl salt is successfully performed. • CV and PDP measurements show significantly reduced impurity concentrations and salt corrosivity. [ABSTRACT FROM AUTHOR]

Using low-melting-point electrolytes could overcome various key challenges of low-cost sodium-based liquid metal batteries (Na-LMBs), e.g. high rates of self-discharge and degradation of structural materials, by lowering their operating temperatures. Molten halide salts are considered promising electrolyte candidates for Na-LMBs due to their high stability and electrical conductivity. In this work, thermodynamic simulation via FactSageTM and thermal analysis via e.g. Differential Scanning Calorimeter (DSC) were carried out to explore the NaI-LiI-KI system, since it could be a promising electrolyte for Na-LMBs due to its low melting point and Na solubility. The results show that the eutectic NaI-LiI-KI performs as a pseudo-binary salt with a melting point of ~290 °C. In this pseudo-binary salt, the solubility of NaI in the eutectic LiI-KI is ~7 mol%. Using the eutectic NaI-LiI-KI electrolyte, Na-LMBs could be operated at < 350 °C. Moreover, the Na solubility and Na+ conductivity of the eutectic NaI-LiI-KI electrolyte, which are vital to the battery performance, were estimated by calculation based on the literature data. Additionally, its applicability and economy were also discussed based on cost pre-analysis of salt materials, salt treatment and structural materials regarding salt corrosivity. Image 1 • Eutectic NaI-LiI-KI is a promising low-melting-point electrolyte for Na-LMBs. • Eutectic NaI-LiI-KI performs as pseudo-binary salt with melting point of ~290 °C. • Solubility of NaI in the eutectic LiI-KI is ~7 mol% at ~290 °C. • Na solubility and conductivity of Na+ in NaI-LiI-KI are estimated by calculation. • Applicability and economic of NaI-LiI-KI are discussed based on cost pre-analysis. [ABSTRACT FROM AUTHOR]

Abstract Molten chloride salts are promising advanced high-temperature (400–800 °C) thermal energy storage (TES) and heat transfer fluid (HTF) materials in next generation concentrated solar power (CSP) plants for higher energy conversion efficiencies. However, severe corrosion of structural materials in contact with molten chloride salts is one of the most critical challenges limiting their applications at elevated temperatures. In this work, two corrosion mitigation strategies are investigated to alleviate the hot corrosion of structural materials in molten chloride salts: (1) adding corrosion inhibitor and (2) using a Fe-Cr-Al alloy with a protective alumina layer on the surface after pre-oxidation. Three commercial high temperature Fe-Cr-Ni alloys (SS 310, Incoloy® 800 H and Hastelloy® C-276) were exposed to molten MgCl 2 -NaCl-KCl (60–20–20 mol%) mixed salts with 1 wt% Mg as corrosion inhibitor, for 500 h at 700 °C under inert atmosphere. By addition of the Mg inhibitor, the corrosion rates of the studied alloys were found to be significantly reduced, more precisely by ~ 83% for SS 310, ~ 70% for In 800 H and ~ 94% for Ha C-276 compared with the exposure tests without Mg addition. The corrosion mitigation mechanism of Fe-Cr-Ni based alloys in molten chloride salts by adding Mg is discussed based on corrosion thermodynamics. To assess the second mitigation strategy two pre-oxidized alumina forming Fe-Cr-Al alloys were exposed to the same molten chloride salts without Mg corrosion inhibitor under the same conditions. It is observed that the adherent alumina scales can effectively inhibit the dissolution of Cr and Fe and the bulk penetration of corrosive impurities. Overall, both strategies offer enormous potential for enhancing the expected lifetime of commercial alloys in molten chloride salts. Graphical abstract fx1 Highlights • By Mg-addition, the corrosion rates of alloy SS 310, In 800H and Ha C-276 are significantly reduced. • Adding Mg inhibitor not only mitigates the corrosion, but also promotes the formation of MgO layer on metal surfaces. • Impurity-driven corrosions of Fe-Cr-Ni based alloys in molten chloride salts in absence and presence of Mg are discussed. • Pre-oxidized alumina scales show corrosion resistance and stability after 500h exposure in chloride salts at 700°C. [ABSTRACT FROM AUTHOR]

Hot corrosion behavior of three commercial alloys (stainless steel SS 310, Incoloy 800 H, Hastelloy C-276) in molten MgCl 2 /NaCl/KCl (60/20/20 mol%) under inert atmosphere was investigated by immersion tests at 700 °C for 500 h. SS 310 exhibited the highest corrosion rate, while Hastelloy C-276 showed the best corrosion resistance. All the studied alloys could not meet the requirements for commercial application (i.e., corrosion rate < 10 µm/year for 30 year's lifetime). Microstructural analysis on the exposed alloy specimens using SEM, EDX and XRD shows that Cr was dissolved preferentially than Fe and Ni to form a corrosion layer with a porous structure during the corrosion. Moreover, the corrosion products (e.g., MgO, MgCr 2 O 4 , etc.) precipitated on the surface of the exposed specimens, as well as in the pores of the Cr-depleted corrosion layer. For SS 310 containing 2 wt% Si, Si was also dissolved and corrosion products containing Si were observed in the pores of the corrosion layer. Based on these findings, an impurity-driven corrosion mechanism is proposed to describe the hot corrosion behavior of the studied alloys in molten MgCl 2 /NaCl/KCl under inert atmosphere, which could assist the development of corrosion mitigation technologies in future work. [ABSTRACT FROM AUTHOR]

MgCl 2 -KCl-NaCl molten chloride salt is a promising candidate for thermal energy storage medium and heat transfer fluid for next-generation Concentrating Solar Power (CSP) plants (Gen-3 CSP). The main challenge has yet been the selection of economical yet corrosion-resistant structural materials to be used. Previous work by the authors has demonstrated that simple corrosion control strategies can mitigate corrosion effects, thereby allowing the use of classical stainless steels as structural materials in the hot part (e.g., ≥700 °C) of the CSP system. This study addresses the selection of cold tank materials, which have to withstand corrosion effects up to temperatures of 500 °C. Two cost-effective commercial types of steels, P91 and SS 304, were examined as candidates for structural materials, and controlled corrosion experiments were performed in molten MgCl 2 -KCl-NaCl salt at 500 °C for 1400 h. Before the exposure tests, the chloride salt was purified using a simple yet effective Mg-doping technique. The results show that the corrosion rates (CRs) of P91 samples are consistently low (<15 μm/year) for both macroscopic and microscopic analysis. The cheaper P91 steel outperforms the more expensive SS 304 in terms of corrosion resistance and also may prove to be beneficial in terms of mechanical properties and economics. Overall, the use of P91 as a cold tank structural material allows for significant cost reduction of the cold tank for chloride-TES system and enhances its competitiveness compared with commercial nitrate-TES. • P91 and SS304 were exposed in Mg-purified MgCl 2 -KCl-NaCl for 1400 h at 500 °C. • Corrosion rate of P91 was <15 μm/year in the purified MgCl 2 -KCl-NaCl at 500 °C. • The using of P91 can significantly reduce material cost of cold tank of chloride-TES. [ABSTRACT FROM AUTHOR]

• The molten MgCl 2 -KCl-NaCl salt was purified with an optimized Mg-additive method. • The corrosion rates of Fe-based alloys were controlled under 15 µm/year in molten chloride salt at 700 °C. • The corrosion development of Fe-based alloy was displayed during the 2000 h exposure test. Molten chlorides, such as MgCl 2 -KCl-NaCl, are promising advanced high-temperature (up to 800 °C) thermal energy storage (TES) materials in next-generation concentrating solar power (CSP) plants. However, their high corrosivity to commercial Fe-Cr-Ni alloys impedes the commercial applications of chloride-TES. In this work, we investigated the corrosion of two selected commercial Fe-based alloys (SS 310 and In 800H) in molten MgCl 2 -KCl-NaCl salt, aiming to study the feasibility of affordable Fe-based alloys instead of expensive Ni-based alloys in the chloride-TES system. The alloy samples were immersed in the liquid-Mg-purified molten salt at 700 °C for 2000 h under a protective inert gas atmosphere. After the corrosion test, SEM-EDX microstructural analysis and mass loss analysis showed that corrosion rates of the immersed alloy samples were lower than 15 µm/year, and the corrosion rates had a decreasing tendency with increasing immersion time during the 2000-hour test. To our best knowledge, this is the first experimental demonstration that corrosion rates of the Fe-based alloys in molten MgCl 2 -KCl-NaCl salt at 700 °C can be controlled below the target (15 µm/year) proposed by the US Department of Energy (DOE). Using affordable Fe-based alloys as main structural materials, the cost of chloride-TES (27 USD/kWh) could be comparable to that of commercial nitrate-TES (20–33 USD/kWh). Taking advantage of chloride-TES with higher operating temperature, the next-generation CSP plant could use an advanced power cycle (e.g., sCO 2 Brayton) to have a much higher energy conversion efficiency, leading to a significantly lower Levelized Cost of Electricity (LCOE) than the current commercial CSP plant. [ABSTRACT FROM AUTHOR]

Potassium nitrate and sodium nitrate in mixing proportion of KNO 3 –NaNO 3 40-60 wt% (also called solar salt) has been successfully used for over a decade as heat storage medium for concentrated solar power parabolic-trough collector plants at temperatures up to 400 °C. At temperatures of 560 °C, reached in state-of-the-art solar tower systems, corrosion of metallic components in contact with solar salt can become an issue and has caused leaks and plant shut-downs in recently built tower projects. While the corrosion rates of several materials have been determined for different temperatures in static molten salt immersion experiments, there is a lack of corrosion data for dynamic in-service conditions. In this work, a dynamic corrosion test has been conducted on 19 different material types including protective coatings, mimicking flow-rate, temperature gradient and draining of in-service operation of a receiver in a concentrated solar power tower. The measured corrosion rates are presented and compared to static corrosion tests reported in literature. • A dynamic molten salt test mimicking operation conditions has been conducted. • The corrosion rate and mechanism of 19 material types has been investigated. • Ni-based alloys showed the lowest corrosion rates, followed by austenitic steels. • The dynamic conditions are less aggressive for Ni-based alloys than static tests. • Coatings improve the corrosion resistance of ferritic steels. [ABSTRACT FROM AUTHOR]