Molten chloride salt technology for next generation concentrating solar power plants: corrosive impurity monitoring and corrosion mitigation A thesis accepted by the Faculty of Energy-, Process- and Bio-Engineering of the University of Stuttgart to fulfill the requirements for the degree of Doctor of Engineering Sciences (Dr.-Ing.) by Gong, Qing born in Jiangsu China Main referee: Prof. Dr. André Thess Co-referee: Prof. Dr. Yongliang Li Chairman of exam: Prof. Dr. Günter Scheffknecht Co-supervisor: Dr. Wenjin Ding Date of the oral exam: 22.09.2023 Institute for Building Energetics, Thermotechnology and Energy Storage (IGTE) at the University of Stuttgart 2023 Declaration I hereby certify that this dissertation is entirely my own work except where otherwise indicated. Passages and ideas from other sources have been clearly indicated. Stuttgart, ________________ ___________________________ Gong, Qing Preface and Acknowledgement My doctoral research project started during the Covid-19 pandemic, without the support of my supervisors, colleagues, and family, it would have been challenging to carry out my research. Here, I would like to express my sincere gratitude to them. First and foremost, I would like to thank Professor André Thess for his guidance during my doctoral studies. His mentorship reminded me of the critical link between scientific research and commercialization. I would also like to express my special thanks to Professor Yongliang Li, who kindly agreed to be the co-examiner of my thesis. I would like to extend my heartfelt gratitude to my direct supervisor at the German Aerospace Center, Dr. Wenjin Ding. As a leading figure in the field of thermal energy storage based on chloride salt, he provided me with unrestricted supervision and discussion. The majority of the scientific achievements presented in this thesis were completed through detailed discussions with him, which also reflected his dedication. I am grateful to Dr. Thomas Bauer and Dr. Alexander Bonk for their leadership in the TSF group during the Covid-19 pandemic, which enabled me to devote all my energy to scientific research in the group of TSF. In the laboratory, I received tremendous support from Ms. Andrea Hanke, Mr. Markus Braun, and Mr. Ralf Hoffmann. Reliable chemical analysis is impossible without Ms. Hanke's meticulous work in the chemical laboratory. Mr. Braun and Mr. Hoffmann showed me the profound skills of German engineers in test-rig design, construction, and automatic control. My main experimental platform, HaloTES, was restarted and improved with their help. In the doctoral office, my scientific discussions with Mr. Julian Steinbrecher and Mr. Sumit Kumar were unforgettable. Although our research focuses were different, we still shared our fresh experimental results with each other and received valuable feedback. We also argued and debated due to different viewpoints, but these discussions with peers were free, open, and effective. Throughout all of my doctoral study, my family was by my side. My parents provided me with unwavering support, and my wife (who was my girlfriend), Ms. Yinxia Zhang, overcame the pandemic lockdown and traveled all the way from China to Germany to reunite with me and get married. The encouragement of my family gave me the courage to overcome difficulties, during my doctoral research. I express my sincere gratitude to them. Contents Abstract ................................................................................................................................. I Kurzzusammenfassung in deutscher Sprache ............................................................. III Nomenclature .................................................................................................................... V Introduction .................................................................................................................... 1 1.1 Molten salt thermal energy storage ...................................................................... 2 1.1.1 Commercial molten nitrate salt thermal energy storage ........................... 3 1.1.2 Next generation molten salt thermal energy storage ................................. 4 1.2 Corrosion of molten chloride salt ......................................................................... 7 1.2.1 Corrosion rate of Fe-Cr-Ni alloy in molten chloride salt ........................... 7 1.2.2 Corrosion mechanism and MgOHCl measurement ................................... 9 1.2.3 Corrosion control strategy and salt purification ....................................... 12 1.2.4 Cost estimation of chloride-TES .................................................................. 14 1.3 Breaking the trilemma of material selection for chloride-TES ........................ 16 Publications ................................................................................................................... 19 2.1 Papers and contribution report ........................................................................... 19 2.2 International conferences ..................................................................................... 20 Paper I: Monitoring of Extremely Low-Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl ........................................................................................... 21 Paper II: Compatibility of Fe-based alloys with purified molten MgCl2-KCl-NaCl salt at 700 °C ...................................................................................................................... 37 Paper III: Selection of cold tank structural material utilizing corrosion control at 500 °C ........................................................................................................................................ 53 Methods ......................................................................................................................... 65 3.1 MgOHCl measurement ........................................................................................ 65 3.2 Compatibility research of alloys with purified salt .......................................... 66 Results and Discussion ................................................................................................ 67 4.1 Results ..................................................................................................................... 67 4.1.1 Monitoring of Extremely Low-Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl ...................................................................... 67 4.1.2 Analysis of purified chloride salt ................................................................ 69 4.1.3 Corrosion of Fe-based alloy in purified chloride salt ............................... 70 4.2 Discussion .............................................................................................................. 71 4.2.1 Cost estimation of chloride-TES .................................................................. 71 4.2.2 Industrial process design of chloride-TES ................................................. 76 4.2.3 Future work .................................................................................................... 78 Summary and outlook ................................................................................................. 79 References ...................................................................................................................... 81 I Abstract In the next generation of concentrating solar power plants (Gen3 CSP), the envisaged higher thermal-to-electrical conversion efficiency requires high operating temperatures. However, the existing nitrate-based thermal storage system (nitrate- TES) is unable to meet the demand for operating temperatures above 700 °C, limiting the thermal-to-electrical conversion efficiency to around 40%. Molten chloride salts, such as the MgCl2-KCl-NaCl-system, are considered promising thermal storage media. However, despite the potential advantages of low cost, large operating range, and high abundance, corrosion has been a century-long challenge for chloride salts, hindering their further development as high-temperature thermal storage media and heat transfer fluids (HTFs). For the MgCl2-KCl-NaCl salt system, the MgOHCl species, has been identified as a main corrosive impurity, which is produced by a mix of the salts hygroscopicity nature and hydrolysis reactions. The implementation of quantitative analysis and subsequent removal of corrosive impurities is crucial for effective management of metal corrosion in molten chloride salts. In this dissertation, chemical and electrochemical methods are developed that can monitor corrosive MgOHCl-impurities at concentrations down to tens of ppm (parts per million) in molten chloride salts. At such low analytical concentrations, the acceptable impurity level of MgOHCl in molten chloride salt can be discussed for cost- effective Fe-based alloy quantitatively. In addition, purification methods to reduce the MgOHCl impurity concentration were developed to investigate their impact on the corrosion of commercially available and economically viable structural materials. A simple and effective magnesium-based corrosion mitigation strategy was developed to be effective in reducing the MgOHCl impurity concentration to allow the use of Fe-based alloys as structural materials. This corrosion mitigation strategy has been verified in long-term tests up to 2000 hours at 700 °C and has confirmed a potential service life of > 30 years for Fe-based alloys with corrosion rates of < 15 µm/year, which was not thought possible prior to this work. This conclusion is confirmed from several perspectives, including mass loss, microstructural analysis and concentration of corrosion products in salts. Considering the allowable stress of the material, this work indicates that Fe-based alloys, represented by austenitic stainless steels, can be the main structural materials for the hot part at 700 °C of the chloride-TES system. Moreover, the magnesium-based chloride salt mitigation strategy is used in an isothermal exposure experiment at 500 °C, confirming that ferritic-martensitic steel can also achieve a corrosion rate below 15 µm/year at this temperature. Thus, ferritic- II martensitic steel can be a candidate for the structural material of the cold part in the chloride-TES system. The corrosion test results indicate excellent compatibility between the low-cost Fe- based alloy and purified MgCl2-KCl-NaCl, even at temperatures as high as 700 °C. This compatibility makes the chloride-TES system economically competitive with ex- isting nitrate-TES systems, and opens up the possibility for integration into next-gen- eration power cycles, such as the supercritical CO2 Brayton power cycle. This integra- tion will potentially increase the thermal-to-electrical conversion efficiency of Gen-3 CSP from the current 40% to ~55%, thereby further reducing its levelized cost of elec- tricity (LCOE). III Kurzzusammenfassung in deutscher Sprache In der nächsten Generation von konzentrierenden Solarkraftwerken (Gen3 CSP) er- fordert die angestrebte höhere Wärme-Elektrizitäts-Umwandlungseffizienz höhere Betriebstemperaturen. Das vorhandene Nitrat-basierte Wärmespeichersystem (nit- rate-TES) kann jedoch den Bedarf an Betriebstemperaturen über 700 °C nicht erfüllen und begrenzt die Wärme-Elektrizitäts-Umwandlungseffizienz auf etwa 40 %. Ge- schmolzene Chloridsalze wie das MgCl2-KCl-NaCl-System gelten als vielverspre- chende Wärmespeichermedien. Trotz potenzieller Vorteile wie niedriger Kosten, gro- ßer Betriebsbereich und hoher Verfügbarkeit ist Korrosion ist jedoch jeher eine Her- ausforderung für Chloridsalze, die ihre weitere Entwicklung als Hochtemperatur- Wärmespeichermedium und Wärmeträgerfluid bisher behindert. Für das MgCl2-KCl- NaCl-Salzsystem ist die kritischste korrosive Verunreinigung ein Hydroxychlorid (MgOHCl), welches durch hygroskopische Eigenschaften und Hydrolyse-Reaktionen des Salzes entsteht. Eine quantitative Analyse der korrosiven Verunreinigungen in Chloridsalzen ist ein entscheidender Weg zur Erkennung und Minimierung von mög- lichen Korrosionsreaktionen. In dieser Dissertation werden chemische und elektrochemische Methoden zur Bestim- mung von MgOHCl-Verunreinigungen in Konzentrationen im nahezu einstelligen ppm-Bereich entwickelt und untersucht. Bei solch niedrigen analytischen Konzentra- tionen kann das akzeptable Verunreinigungsniveau von MgOHCl in geschmolzenen Chloridsalzen quantitativ diskutiert werden, um eine kosteneffektive Fe-basierte Le- gierung zu erreichen. Darüber hinaus wird eine Methode zur Reduzierung des Verunreinigungsgrades un- tersucht, um die Konzentration von MgOHCl zu senken, und damit die Korrosivität der Salz-schmelze zu minimieren. Es wurde eine einfache und effektive Korrosions- minderungsstrategie auf Magnesiumbasis entwickelt, um die MgOHCl-Verunreini- gungskonzentration zu reduzieren und damit den Einsatz von Fe-basierten Legierun- gen als Strukturwerkstoffe zu ermöglichen. Diese Korrosionsminderungsstrategie wurde in Langzeittests bis zu 2000 Stunden bei 700 °C verifiziert und bestätigte eine potenzielle Lebensdauer von >30 Jahren für Fe-Legierungen mit Korrosionsraten von <15 µm/Jahr, was vor dieser Arbeit nicht für möglich gehalten wurde. Diese Schluss- folgerung wird aus verschiedenen Blickwinkeln bestätigt, einschließlich Massenver- lust, Mikrostrukturanalyse und Konzentration von Korrosionsprodukten in Salzen. Unter Berücksichtigung der zulässigen Materialbeanspruchung zeigt diese Arbeit, dass Fe-Legierungen, repräsentiert durch austenitische Edelstähle, die Hauptstruktur- werkstoffe für den heißen Teil bei 700 °C des Chlorid-TES-Systems sein können. IV Zusätzlich wurde die Mg-basierte Chloridsalzreinigungsmethode in einem isother- men Expositionsexperiment bei 500 °C eingesetzt und bestätigt, dass ferritisch-mar- tensitischer Stahl auch bei dieser Temperatur eine Korrosionsrate von weniger als 15 µm/Jahr erreichen kann. Ferritisch-martensitischer Stahl ist daher ein Kandidat für den Strukturwerkstoff des kalten Teils im Chlorid-TES-System. Die Ergebnisse der Korrosionstests zeigen eine ausgezeichnete Kompatibilität zwi- schen der kostengünstigen Fe-basierten Legierung und dem gereinigten MgCl2-KCl- NaCl, selbst bei Temperaturen bis zu 700 °C. Diese Kompatibilität macht das Chlorid- TES-System wirtschaftlich konkurrenzfähig zu bestehenden Nitrat-TES-Systemen und eröffnet die Möglichkeit der Integration in fortschrittlicheren Stromkreisläufen (z.B. sCO2 Brayton-Cycle). Diese Integration könnte der thermisch-zu-elektrische Wir- kungsgrad von 40% auf ~55% durch erhöht werden, was die Stromgestehungskosten (LCOE) reduziert und die Wettbewerbsfähigkeit von Solarkraftwerken signifikant er- höht. V Nomenclature A Surface area of metal samples cm2 Achloride Cost of alloy used in chloride-TES USD Anitrate Cost of alloy used in nitrate-TES USD Cchloride Unit cost of chloride salt USD/kg Cchloride Cost of the chloride salt inventory USD Cchloride tank Cost of chloride tank USD Cnitrate tank Cost of nitrate tank USD cp Heat capacity of molten salt kJ/(°C·kg) F1 Volume factor - F2 Cost factor of alloy - F3 Stress factor - K Factor to calculate corrosion rate with mass loss method: 8.76 ×107 µm·h·cm- 1·year-1 k1 Factor to calculate corrosion rate with micro-analysis method: 8760 h/year k2 Factor to convert energy unit: 2.78 × 10-7 MWh/kJ m Mass of the molten salt involved in two tanks kg Q Heat storage capacity MWh-th Schloride Maximum allow stress of alloy used in chloride-TES MPa VI Scorr Thickness of the corrosion layer of the exposed alloy samples µm Snitrate Maximum allow stress of alloy used in chloride-TES MPa t Immersion time h Vchloride Volume of chloride salt m3 Vnitrate Volume of nitrate salt m3 W Mass loss of metal samples g ΔT Temperature difference between cold and hot tank °C ρ Density of metal samples g/cm3 AAS Atomic absorption spectroscopy ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials CAPEX Capital expenditure CAS Chinese Academic of Sciences Chloride- TES Thermal energy storage based on molten chloride salt medium CR Corrosion rate CSP Concentrating solar power plants CV Cyclic voltammetry DOE The US Department of Energy EDX Energy-dispersive X-ray spectroscopy VII EN European Standard HTF Heat transfer fluid IRENA International Renewable Energy Agency LCOE Levelized Cost of Electricity MSR Molten salt reactor nitrate- TES Thermal energy storage based on molten nitrate salt medium NREL US National Renewable Energy Laboratory RSD Relative standard deviation sCO2 Supercritical carbon-dioxide SEM Scanning electron microscopy TES Thermal energy storage USD US dollar µm Micrometer GWel Gigawatt of electricity mol.% Molar percentage MWh-th Megawatt hour of heat ppm O parts per million oxygen USD/kW h-th US dollar per kilowatt hour of heat wt.% Weight percentage Al2O3 Aluminum oxide VIII Ar Argon Cl2 Chlorine Cr Chromium Cr2+ Chromous ion Cr3+ Chromic ion CrCl2 Dichlorochromium CrCl3 Chromic trichloride e- Electron Fe Iron H+ Hydrogen ion H2 Hydrogen H2O Dihydrogen oxide HCl Hydrogen chloride K2CO3 Dipotassium carbonate KCl Potassium chloride KNO3 Potassium nitrate Li2CO3 Dilithium carbonate Mg2+ Magnesium ion MgCl2 Magnesium chloride MgCl2·6H 2O Magnesium dichloride hexahydrate MgCr2O4 Dichromium magnesium tetraoxide MgO Magnesium oxide IX MgOH+ Magnesium hydroxide ion MgOHCl Magnesium chloride hydroxide Na2CO3 Sodium carbonate NaCl Sodium chloride NaNO3 Sodium nitrate Ni Nickel O2 Oxygen X Introduction 1 Introduction This dissertation will focus on the most challenging issues of corrosion in the high- temperature molten chloride salt technology for next-generation concentrating solar power (CSP) plants. The molten-salt based thermal energy storage (TES) is a significant system in the CSP plants, allowing the flexible and continuous electricity generation from intermittent solar energy [1,2]. In state-of-the-art commercial CSP plants, the TES system with NaNO3-KNO3 (60-40 wt.%) mixture (called Solar Salt) operates at temperatures of < 565 °C, which limited by the thermal decomposition of the nitrate ion at high temperatures [1,3-5]. In the next-generation CSP, the TES system with ≥ 700 °C operating temperature and the advanced power cycle, such as the supercritical carbon-dioxide (sCO2) Brayton cycle, are required to increase the thermoelectric conversion efficiency to > 50% and reduce the levelized-cost-of-electricity (LCOE) [1,2]. In the Section 1.1, an overview of commercial and next-generation molten salt TES will be summarized. The molten chloride salt is a promising TES medium and heat transfer fluid (HTF) for next-generation CSP, due to excellent thermophysical properties and low material costs [1,2,6]. In addition to the CSP, the molten chloride salt technology can be used extensively in Carnot Battery [7], industrial thermal management [3], and molten salt reactor [8-10], where high temperature TES/HTF are required. The MgCl2-NaCl-KCl mixture (47.1-30.2-22.7 mol.%, 56.5-22.2-21.3 wt.%) is a selected mixture with the advantages of low melting point (~383 °C), high thermal stability (stable at > 800 °C), low vapor pressure (~1 kPa at 800 °C), and low cost (~0.22 USD/kg) [11]. However, the corrosion issue is a primary impediment to the application of molten chloride salts, including the MgCl2-NaCl-KCl mixture [1,2,12]. Without corrosion mitigation strategies, the molten MgCl2-NaCl-KCl mixture has been reported to cause a corrosion rate (CR) of 1752 µm/year on stainless steel 310 (SS 310) at 700 °C under argon atmosphere in previous work of German Aerospace Center (DLR) [13]. This CR is far from the standard of < 15 µm/year, which is required for a 30-year life-time of the CSP/TES system [14]. Since as early as the 1950s, the corrosion issues of molten chloride salts plagued researchers developing nuclear molten salt reactors (MSRs) Introduction 2 [15,16]. In recent years, molten chloride salt corrosion has received renewed attention with the launch of the Gen-3 CSP projects by the US Department of Energy (DOE) [1,2]. To solve the problem of corrosion, the corrosion mechanism of molten chloride salt should be investigated and understood. Only with a comprehensive understanding of the corrosion mechanisms, affective anti-corrosion strategies can be proposed. Hence, a systematic overview on the corrosion in molten chloride is presented in the Section 1.2. Corrosion issues are often relegated to the issue of compatibility between molten chloride salt and structural materials. Thus, both the attacking molten salt medium with its impurity and the attacked structural material must be considered. In addition to the corrosion issues, mechanical properties and costs should also be considered when choosing structural materials. For instance, the oxide based ceramics such as alumina (Al2O3) show good compatibility with the molten chloride salts at high temperatures [17], but the poor fracture toughness limits the machining of this material as a complex component. Furthermore, the unit cost of structural materials should be considered when designing molten salt tanks that require significant amounts of material to manufacture tanks containing 10 to 100 thousand tons of molten chloride salt in a commercial CSP plant. The corrosion issues, mechanical properties, and the cost of structural materials should be comprehensively considered, leading to the trilemma of material selection for molten chloride salt TES, as summarized in the Section 1.3. In order to overcome the trilemma, a comprehensive corrosion mitigation methodology is proposed and investigated in this dissertation. With such corrosion mitigation methodology, the low-cost commercial steel with excellent mechanical properties can be selected as the structural materials of chloride-TES, thus effectively overcoming the trilemma. 1.1 Molten salt thermal energy storage According to a report published by the International Renewable Energy Agency (IRENA) at 2020 [18], the low-cost TES integrated in CSP is one of the only low-cost, long-duration, and large-scale energy storage solutions available, along with pumped hydro storage. By the end of 2019, the global power generation capacity of CSP plants with molten salt storage amounted to 21 GWhel [3]. Incorporating TES into CSP has proven to be an effective method for stabilizing the grid and reducing electricity abandonment. A previous report on the SunShot initiative by the Department of Energy (DOE) in the US [19] emphasized that CSP with 6 hours of TES could play a critical role in Introduction 3 stabilizing the grid, by generating approximately 1% of total electricity but reducing marginal electricity curtailment from 30% to ~10%. This percentage of curtailment reduction could be further reduced by incorporating higher amounts of TES [19]. Recently, the application of cost-effective molten salt TES in a hybrid power plant with CSP, PV and/or wind energy (e.g., in Mohammed bin Rashid Al Maktoum Solar Park, Dubai [18]) was reported. In 2022, about 30 CSP plants (approx. 3 GWel in total) have been announced to be built in China together with photovoltaic and wind energy [20]. During the operation of GW-level energy storage, the molten salt TES has demonstrated the following major advantages, comparing with other running or potential long-duration and large-scale energy storage solutions. • Low cost and long service life. Molten salt TES has a capital cost as low as 20- 33 USD/kWh-th with a designed service life of 30 years, making it a cost- effective and long-term solution for energy storage that is easily scalable into the GWh-capacity range [1,21]. • Safety. Molten salt TES systems are considered safe due to the low toxicity and non-flammable nature of the salt [3]. In the event of a catastrophic accident, there is no risk of the accident escalating to a larger scale, such as flaming and explosion. • Low system complexity. In the CSP, the molten salt system is used as both TES and HTF medium. This advantage reduces the number of heat exchanges, increasing the overall efficiency of the system [22]. In the following sections, the commercial and next generation molten salt TES will be introduced, providing a comprehensive understanding of commercial molten salt TES and insights into the motivation and directions of next generation molten salt TES. 1.1.1 Commercial molten nitrate salt thermal energy storage The parameters of commercial molten nitrate TES are significant references for next- generation CSP, including the main structural materials of tanks, operating temperature, cost breakdown, and properties of molten salt. In the report published by the US National Renewable Energy Laboratory (NREL) [1], there is a case study of commercial TES with nitrate salt (nitrate-TES) based on the Abengoa project for a tower CSP plant, where the costs breakdown and some key parameters are available, as summarized in the Table 1. For a nitrate-TES with 2703 MWh-th, the total capital expenditure (CAPEX) is about 53 million US dollar (USD), i.e., 20 USD/kWh-th. In this report [1], as can be known from the Table 1, three key parts account for 84% of total cost: hot tank (19%), cold tank (8%), and nitrate salt (57%). Introduction 4 The salt mixture used in commercial TES systems is Solar Salt (NaNO3-KNO3 60-40 wt.%) with a maximum operating temperature of 565 °C. The unit material cost of such nitrate salt mixture is about 1.1 USD/kg, according to the cost available in ref. [1] published in 2017. For a 100 MW CSP plant with 10-hour TES storage, the mass of salt required is about 27,000 tons. The main structural material of the hot tank is SS 347, while that of the cold tank is ASTM A 516 Grade 70 carbon steel. The structural materials have a maximum allowable stress of about 115 MPa at their serving temperatures, which is available in the ASME Boiler and Pressure Vessel Code 2010 [23]. Table 1. A case study on commercial nitrate salt TES, the data are summarized from Ref. [1]. Material Price USD thousands TES cost USD/kWh-th Percentag e % Hot tank SS 347 10 016 3.71 19 Cold tank ASTM A 516 70 4361 1.61 8 Salt inventory NaNO3-KNO3 (60- 40 wt.%) 30 122 11.14 57 Structural steel - 666 0.25 1 Tank insulation - 3 724 1.38 7 Electrical - 481 0.18 1 Foundatio ns - 3 050 1.13 6 Site work - 347 0.13 1 Total 52 767 20 100 1.1.2 Next generation molten salt thermal energy storage As shown in Fig. 1 [24], the thermal-to-electrical efficiency in next-generation CSP can be increased to over 50% by operating at temperatures of ≥ 700 °C and combining with e.g., the sCO2 power cycle. Molten chloride and molten carbonate have been selected by DOE as TES/HTF candidates for next-generation CSP due to their higher thermal stability compared to commercial nitrate salts [1]. Introduction 5 Fig. 1. The relationship between thermal conversion efficiency and operating temperature with different power cycles. The plot is adapted from Ref [24]. In addition to the thermal stability, some properties (melting point, max. working temperature, density, heat capacity, and viscosity) of molten nitrate, chloride, and carbonate are summarized in Table 2 based on the literature [1,2,5,11,25]. Besides the properties of molten salt, the price is a significant parameter for the selection of next generation molten salt for TES. As summarized in Table 2, the MgCl2-KCl-NaCl has the lowest price which is about one-tenth that of Li2CO3-Na2CO3-K2CO3 and one-third of NaNO3-KNO3, with a cost of around 0.22 USD/kg [25]. Table 2. Selected concerned properties of typically considered molten nitrate, chloride and carbonate salt mixture [1,2,5,11,25]. Molten salt (wt.%) NaNO3-KNO3 (60-40) MgCl2-NaCl- KCl (56.5-22.2- 21.3) Li2CO3- Na2CO3-K2CO3 (32.1-33.4-34.5) Melting point (°C) 240 383 397 Maximum working temperature (°C) 565 >800 800 Density (g/cm3) 1.8 (400 °C) ~1.7 (600 °C) 2 (600 °C) Heat capacity (kJ/(kg·K)) 1.5 (400 °C) 1.14 (600 °C) 1.8 (600 °C) Unit price (USD/kg) 0.8-1.1 0.22-0.35 2.5-3 Introduction 6 Viscosity (mPa s) 1.03 ~3.2 (600 °C) 10.7 (600 °C) The cost breakdown of each component of TES systems reported in [1] is presented in Fig. 2. For carbonate-TES, the cost of K2CO3-Li2CO3-Na2CO3 salt is over 30 USD/kWh, which is higher than the cost of the entire nitrate-TES system. This is primarily due to the high cost of Li2CO3. In contrast, the cost of the MgCl2-KCl salt in chloride-TES is lower than that of nitrate and carbonate TES. The ternary system of MgCl2-KCl-NaCl demonstrates even lower cost than the binary MgCl2-KCl mixture since NaCl is the cheapest salt in the MgCl2-KCl-NaCl system [11]. However, the cost of the hot and cold tanks in chloride-TES is significantly higher than that in nitrate-TES, mainly due to the selection of nickel-based alloy Haynes 230 and stainless steel 347 as the primary structural materials for the hot and cold tanks, respectively [1]. This material selection strategy is primarily driven by the high corrosiveness of molten chloride salt. In general, higher Ni content in Fe-Cr-Ni alloys translates to better corrosion resistance but also a higher unit cost. Alloys with >60 wt.% Ni content can be up to ten times more expensive than stainless steel with 10-30 wt.% Ni [26], while stainless steel is typically more expensive than low alloy steel and carbon steel. Therefore, if the compatibility issues between molten chloride salt and structural materials, especially corrosion, can be addressed, the cost of chloride-TES can be reduced to a similar or even lower level than nitrate-TES. Fig. 2. Breakdown of TES costs with different molten salts [1]. Once corrosion issues with molten chloride salt are resolved, the next-generation CSP plant could benefit from using an advanced power cycle (such as sCO2 Brayton) at higher operating temperatures, resulting in significantly higher energy conversion Introduction 7 efficiency and lower Levelized Cost of Electricity (LCOE) than current commercial CSP plants. In the next section 1.2, the critical corrosion issues related to molten chloride salt and structural materials will be systematically introduced. 1.2 Corrosion of molten chloride salt For the application in the next-generation CSP with thousand-ton scale, the MgCl2- NaCl-KCl is recommended as one of the most promising compositions, with the advantages of cost-effective (~0.22 USD/kg [1,25]), low-melting-point (~383 °C [11,27]) high thermal stability (stable above 800 °C [6,11]) and low vapor pressure (~1kPa at 800 °C [11]) [6,11,28]. Some of the concerned properties can be found in the Table 2. In this dissertation, the corrosion investigation is based on the mixtures of MgCl2-KCl- NaCl (47.1–30.2–22.7 mol.%, 56.5–22.2–21.3 wt.%) for a focused exploration, according to the previous study [11,27]. Since the corrosion of molten chloride salt has been a long-standing problem, there are numerous references available, including summarizing of corrosion rate at different temperature [6,10,12,13,16,29-36], exploration of corrosion mechanism[8,13,16,37-43], and proposing of corrosion strategies [8,16,31,44-46]. However, there are clear research gaps for large-scale (ten-thousand scale) and long- lifetime (up to 30 years) application of molten chloride salt technology due to the corrosivity. The following sections provide an overview of the corrosion issues and identify the research gaps around which the research covered in this thesis revolves. 1.2.1 Corrosion rate of Fe-Cr-Ni alloy in molten chloride salt The corrosion rate (CR) of an alloy in a corrosive environment can be quantitatively evaluated in micrometers per year (µm/year). Two primary methods can be used to measure the CR of steel in molten salt. The first approach for measuring CR involves comparing the mass of alloy samples before and after exposure tests, as shown in Eq. 1 [47]. 𝐶𝑅 (µ𝑚 𝑦𝑒𝑎𝑟⁄ ) = 𝐾 × 𝑊𝐴 × 𝑡 × 𝜌 Eq. 1 where, K is a constant 8.76 ×107 in µm·h·cm-1·year-1; W is the mass loss of samples in g; Introduction 8 A is the sample’s surface area in cm2, t is immersion time in hour; ρ is the density of samples in g/cm3. Another approach for measuring the CR is through microanalysis. After the immersion test of an alloy in the molten salt, the cross-section of the alloy samples can be analyzed using scanning electron microscopy (SEM), typically coupled with energy-dispersive X-ray spectroscopy (EDX) methods. By considering the immersion time and thickness of the corrosion layer, the CR can be calculated using Eq. 2. 𝐶𝑅 (µ𝑚 𝑦𝑒𝑎𝑟⁄ ) = 𝑆𝑐𝑜𝑟𝑟 × 𝑘1𝑡 Eq. 2 where, Scorr is the thickness of the corrosion layer of the exposed alloy samples in µm; t is the immersion time in hour (h). k1 is a constant indication 8760 hours per year (h/year). The targeted corrosion rate for structural materials used in CSP plants is to be < 15 µm/year for a 30-year lifespan [14]. Achieving this target is considered extremely ambitious. Some representative CRs of typical materials in chloride salt are summarized in the Table 3. According to available literature [6,35,48,49], Fe-Cr-Ni alloys in molten chloride exhibit CRs > 1000 µm/year at temperatures ≥ 700 °C, even in inert atmospheres. For example, Ding et al. reported that at 700 °C, SS 310, In 800H, and C- 276 had CRs of approximately 1752, 876, and 526 µm/year, respectively, in molten MgCl2-KCl-NaCl salt, as determined by microanalysis [13]. Zhao et al. found that the CR of the nickel-based alloy Haynes 230 was > 3200 µm/year in molten MgCl2-KCl salt at 800 °C, based on mass-loss analysis [6,48]. In this experiment, the furnace vessel made of Ni was damaged by the untreated molten chloride salt [48]. In other words, even expensive nickel-based alloys cannot achieve a CR target of < 15 µm/year at ≥ 700 °C without corrosion mitigation strategies. Table 3. Representative data from molten chloride salt corrosion experiments. Introduction 9 Molten salt (mol. %) Temper- ature °C Material (Ni wt.%) Atmos- phere Test time h CRs µm/year Ref. MgCl2-KCl- NaCl (45.5-21.5- 33) 500 SS 347 (9-12) Vacuum 1000 ~120 [12,35] MgCl2-KCl- NaCl (45.5- 21.5-33) 500 SS 316 (10-14) Vacuum 1000 ~10 [12,35] MgCl2-KCl- NaCl (45.5- 21.5-33) 500 Ha N (~71) Vacuum 1000 50 [12,35] MgCl2-KCl- NaCl (60- 20-20) 700 SS 310 (19-22) Argon 500 1752 [13] MgCl2-KCl- NaCl (60- 20-20) 700 In 800H (30.52) Argon 500 876 [13] MgCl2-KCl- NaCl (60- 20-20) 700 C 276 (~60) Argon 500 526 [13] MgCl2-KCl (50-50) 800 Ha 230 (~60) Nitrogen 100 >3200 [6,48] To address the issue of strong corrosivity, it is necessary to systematically understand the corrosion mechanism of molten chloride salt (see Section 1.2.2). Based on this understanding, effective corrosion mitigation strategies can be developed (see Section 1.2.3). 1.2.2 Corrosion mechanism and MgOHCl measurement One positive aspect is that pure MgCl2-KCl-NaCl mixtures are theoretically non- corrosive to Fe-Cr-Ni alloys at high temperatures. Zhang et al. determined through redox potential research that pure MgCl2-KCl-NaCl mixture do not cause corrosion to Ni and Cr metals [38]. This conclusion can be extended to most Fe-Cr-Ni alloys since Fe metal activity is lower than Cr. It is well accepted that the corrosion of Fe-Cr-Ni alloys in molten MgCl2-KCl-NaCl is primarily driven by the presence of corrosive impurities with high redox potentials [13,32,33,48]. The high hygroscopicity of molten chloride salt leads to the adsorption of moisture from the air and the formation of crystalline hydrates, such as MgCl2·xH2O Introduction 10 (where x = 1 to 6), which can subsequently generate the corrosive impurity MgOHCl through hydrolysis [50-53]. The reactions of MgCl2·xH2O during heating, including dehydration and hydrolysis, have been studied intensively in the magnesium electrolytic production process [50,51,53,54]. It has been shown, as indicated by Eq. 3 and Eq. 4 [50,51], that at temperatures ranging from 240 to 400 °C, the hydrolysis reaction occurs simultaneously with dehydration, producing the corrosive MgOHCl and HCl in the molten chloride salt. In addition to the heating process, the presence of H2O impurities in the inert gas phase during the operation of chloride-TES at 420 °C to 800 °C can also increase the concentrations of MgOHCl and HCl, leading to the high corrosiveness of molten MgCl2-KCl-NaCl. MgCl2 · 6H2O (s/l) → MgCl2 (s/l) + 6H₂O (g) (Dehydration at temeprature from 117 to 400 °C) Eq. 3 MgCl2 · H2O (s/l) → MgOHCl (s/l) + HCl(g) (Hydrolysis at temperature of > 240 °C) Eq. 4 Ding et al. constructed the corrosion mechanism of MgCl2-KCl-NaCl to Fe-Cr-Ni alloy and pointed out that the MgOHCl is the main corrosive impurities in this molten salt system, due to its high solubility in molten chloride salt, high redox potential to Fe- Cr-Ni alloy, and relatively high thermal stability [13,31,45,52]. As shown in Fig. 3, the dissolved corrosive MgOHCl impurity in molten chloride salt can react with Cr or other metals in structural materials, resulting in the continuous corrosion of Fe-Cr-Ni alloy. This corrosion mechanism is widely accepted by molten chloride salt community and has been validated in other laboratories [6,8,33,37,40,46,55]. Introduction 11 Fig. 3. Schematic diagram of the corrosion mechanism of Fe-Ni-Cr based alloys in molten MgCl2-KCl-NaCl under inert atmosphere [13,31]. It is believed that there are acceptable concentrations of MgOHCl for Fe-Cr-Ni alloys with different nickel contents at a constant temperature condition (e.g., 700 °C) to achieve a CR target of 15 µm/year [6,13,31]. However, the quantitative measurement of the concentration of MgOHCl in molten chloride samples is a challenging task that involves two main aspects. Firstly, Fe-based alloys, such as stainless steel, are less expensive than nickel-based alloys but have a lower acceptable concentration of MgOHCl at high temperatures of ≥ 700 °C [31,32]. To establish a quantitative relationship between MgOHCl concentration and the CR of Fe-Cr-Ni alloys, the concentration of MgOHCl needs to be measured at the level of tens of ppm [31,32]. Secondly, the non-corrosive MgO and the corrosive MgOHCl are often coexist in the molten chloride salt at high temperatures, as shown in Eq. 5 [50,56]. Due to their similar physicochemical properties and significantly low concentrations, it is difficult to obtain accurate concentration of the corrosive impurity MgOHCl. MgOHCl (s/l) → MgO (s) + HCl (g) ( > 555 °C) Eq. 5 Klammer et al. used water and methanol to extract MgOHCl and MgO based on the solubility difference, which can separately measure the concentration of MgOHCl Introduction 12 down to 0.1 wt.% (~1000 ppm) by a typical ethylenediaminetetraacetic acid (EDTA) titration [55]. However, this method showed a large standard deviation of ~40% at the measurement limit of 0.1 wt.% in the following MgOHCl measurement study [46]. In addition to the measurement based on chemical analysis, MgOHCl can be measured in-situ using electrochemical method, such as cyclic voltammetry (CV) [45,52,53,57]. As illustrated in Fig. 4, Ding et al. reported that the current density of peak B (as shown in Fig. 4 (a)) is linearly related to the concentration of MgOHCl at a fixed temperature [45,52]. Therefore, the slope of the proportional function between the concentration of MgOHCl measured by titration and the current density of peak B can be calculated, as shown in Fig. 4 (b). In practice, the electrical signal obtained from CV is sufficient to determine the concentration of MgOHCl, enabling in-situ measurement of this impurity. However, measurements based on electrochemical methods should be calibrated and validated by the results of chemical analysis such as titration. In other words, a more accurate chemical and ex-situ measurement of MgOHCl is the prerequisite of the electrochemical and in-situ measurement. Fig. 4. a) Cyclic voltammogram in the MgCl2-KCl-NaCl at 500 °C obtained by tungsten working electrode. Peak B is the feature peak for the concentration of MgOHCl in molten chloride salt b) The linear relationship between concentration of MgOHCl and current density of peak B. 1.2.3 Corrosion control strategy and salt purification Typically, the mitigation of high-temperature corrosion in molten chloride salts involves two fundamental aspects: optimization of the structural materials and reduction of molten salt corrosivity. Optimization of structural materials encompasses a diverse range of methods, including metallic coatings [58,59] and pre-oxidation techniques [31,60] as well as the use of non-metallic materials with excellent corrosion resistance such as oxide ceramics [17], composite materials [61], and insulating firebrick [49,62]. However, thermal shock concerns may arise with coating and pre- oxidation techniques. Furthermore, non-metallic structural materials, such as oxide b) a) Introduction 13 ceramics, often exhibit inferior fracture toughness and/or intricate shape limitations when compared to metallic materials. Hence, based on the impurity-driven corrosion mechanism of molten chloride salt to Fe-Cr-Ni alloys, the corrosive impurity MgOHCl should be strictly monitored and suppressed. Several different methods have been investigated to reduce the corrosive impurity in molten chloride salts. These methods include electrochemical techniques [44,63,64], thermal approaches[6,13,46,48], the addition of active metals (such as Mg) [10,16,31,36,46,48,49,65-67], carbochlorination [10,32,68], and chlorination processes [10,69]. Kashani-Nejad et al. investigated the thermal decomposition of MgOHCl by thermal gravimetric analysis (TGA) [50,56]. It shows that the corrosive MgOHCl decomposes to MgO and HCl at the temperature of > 555 °C, as shown in Eq. 5. The MgO is non- corrosive to the Fe-Cr-Ni alloys while the HCl has limited solubility in the molten chloride salt [12,70]. However, the thermal decomposition of MgOHCl is relatively slow. According to the investigation by Zhao et al. in the National Renewable Energy Laboratory (NREL) [48], the Ha 230 nickel-based alloy shows a CR of about 953 µm/year at 800 °C in the MgCl2-KCl treated by thermal purification. Compared to the CR of > 3200 µm/year in untreated chloride salt [48], the CR of 953 µm/year is significantly reduced, but it is still far from the target of 15 µm/year. Some chemical methods have been developed to purify the molten chloride salt in depth. Kurley et al. from the Oak Ridge National Laboratory (ORNL) reported a purification method of carbochlorination [32,68]. Approximately600 g MgCl2-KCl salt was purified by sparging with CCl4 gas at 850 °C for over 100 hours. Samples of SS 316L and Alloy N (a type of nickel-based alloy) samples were then exposed to the purified salt at 700 °C for 100 hours. The exposure results show that the stainless steel and the nickel-based alloy have comparable CRs of ~ 18 µm/year in the purified molten MgCl2-KCl salt at 700 °C. However, due to its high cost and toxicity, the CCl4-sparging purification method may be a challenge for large-scale purification. Another method of chemical purification is Mg-additive, which is regarded as a low- cost, low-complexity, and effective method of treating large-scale molten chloride salt [10,46,49]. The reaction between Mg and MgOHCl is shown in Eq. 6. The Mg metal can reduce the MgOHCl to non-corrosive MgO and H2, while itself is oxidized to MgCl2. Mg(s/l) + 2MgOHCl (l) → MgCl2 (l) + 2MgO (s) + H2(g) Eq. 6 Introduction 14 Zhao et al. added 0.75 wt.% Mg in molten MgCl2-KCl-NaCl at 650 °C for 3 hours, reducing the concentration of MgOHCl from ~0.9 wt.% to ~0.3 wt.% [6,46,48]. The following exposure experiment of Ha 230 samples in purified salt shows that this nickel-based alloy has ~27 µm/year CR at 800 °C, which is much better than that in non-treated salt (> 3200 µm/year) and thermally treated salt (953 µm/year). Some studies have also been carried out on the compatibility of Mg-purified chloride salts with Fe-based alloys (e.g., stainless steel) [31,66]. Ding et al. from DLR mixed 1 wt.% Mg, MgCl2-KCl-NaCl, and metal samples in a crucible and heated it to 700 °C. After 500 hours, the immersed samples of SS 310, In 800H, and C-276 were obtained analysis by mass loss and SEM-EDX. According to the SEM-EDX analysis, the CRs of SS 310, In 800H, and C-276 are 298, 262, and 29.8 µm/year, respectively. Although the CR of the nickel-based alloy C-276 is close to the target of 15 µm/year, the CRs of the Fe-based alloys (SS 310 and In 800H) are still far from the target. Ren et al. in the Chinese Academic of Sciences (CAS) purified the molten MgCl2-KCl- NaCl with a Mg rod at 600 °C for 24 hours [66]. After the pre-treatment, standard SS 316 and carburized SS 316 samples were immersed in purified salt containing 1 wt.% Mg at 700 °C for 400 hours. The mass loss results indicate that the CRs of samples is ~ 51 µm/year, but the cross-section analyzed by SEM shows that the corrosion depths of the samples are 20 – 40 µm, i.e., the CRs based on microstructural analysis are 350 – 700 µm/year. The above examples show that chemical purification methods, such as the addition of Mg, can significantly reduce the CRs of Fe-Cr-Ni alloys in molten chloride salts. However, there is no literature that experimentally demonstrates that a Fe-based alloy in Mg-purified MgCl2-KCl-NaCl at 700 °C achieves the target of 15 µm/year. 1.2.4 Cost estimation of chloride-TES As a next-generation TES system, the chloride-TES is expected to be superior or at least not inferior to the nitrate-TES in most key indicators, including operating temperature, lifetime, system stability, and cost. In other words, nitrate-TES is a natural reference for chloride-TES. In the roadmap of next generation CSP published by NREL [1], the cost of TES is clearly in the consideration of roadmap developers, as shown in Fig. 2. The costs of each component of nitrate-TES are available in Table 1, which plays a role of baseline and reference. Extra attention is given on three variables in calculation of chloride-TES: costs of salt, hot tank, and cold tank. These three items account for 84 % of the total cost of nitrate-TES, while the costs of these parts in chloride-TES vary Introduction 15 significantly from those in nitrate-TES due to the different molten salt systems and operating temperatures. Cost of the chloride salt can be estimated by multiplying mass of total chloride to unit cost, as shown in Eq. 7. The mass of chloride salt used in TES can be calculated to achieve the same heat storage capacity of nitrate-TES combining with heat capacity of chloride salt and the temperature difference between hot and cold salt, as shown in Eq. 8. 𝐶𝑐ℎ𝑙𝑜𝑟𝑖𝑑𝑒 = 𝑐𝑐ℎ𝑙𝑜𝑟𝑖𝑑𝑒 · 𝑚 Eq. 7 𝑚 = 𝑄𝑘2 · 𝑐𝑝 · 𝛥𝑇 Eq. 8 where, Cchloride is the cost of the chloride salt inventory in USD. cchloride is unit cost of chloride salt in USD/kg. Q is the heat storage capacity of a molten salt TES in MWh-th, m is the mass of the molten salt involved in two tanks in kg, cp is the heat capacity of molten salt in kJ/(°C·kg), ΔT is the temperature difference between cold and hot tank in °C, k2 is a constant of 2.78 × 10-7 MWh/kJ, The cold/hot tank costs of chloride-TES can be estimated by three factors: F1 (volume factor), F2 (cost factor of alloy) and F3 (stress factor), as shown in Eq. 9, Eq. 10, Eq. 11, and Eq. 12 [1]. 𝐶𝑐ℎ𝑙𝑜𝑟𝑖𝑑𝑒 𝑡𝑎𝑛𝑘 = 𝐶𝑛𝑖𝑡𝑟𝑎𝑡𝑒 𝑡𝑎𝑛𝑘 · 𝐹1 · 𝐹2 · 𝐹3 Eq. 9 𝐹1 = (𝑉𝑐ℎ𝑙𝑜𝑟𝑖𝑑𝑒 𝑉𝑛𝑖𝑡𝑟𝑎𝑡𝑒⁄ )0.8 Eq. 10 𝐹2 = (𝐴𝑐ℎ𝑙𝑜𝑟𝑖𝑑𝑒/𝐴𝑛𝑖𝑡𝑟𝑎𝑡𝑒) Eq. 11 𝐹3 = (𝑆𝑛𝑖𝑡𝑟𝑎𝑡𝑒/𝑆𝑐ℎ𝑙𝑜𝑟𝑖𝑑𝑒) Eq. 12 where, Introduction 16 Cchloride tank is cost of chloride tank in USD, Cnitrate tank is cost of nitrate tank in USD, F1 is equal to the ratio between volume of chloride salt (Vchloride) and nitrate salt (Vnitrate) with 0.8 scaling exponent, F2 is equal to the ratio between unit cost of alloy used in chloride-TES (Achloride) and nitrate-TES (Anitrate), F3 is equal to the ratio between maximum allow stress of alloy used in chloride-TES (Schloride) and nitrate-TES (Snitrate). The maximum allowable stress is available in the standard of ASME Boiler & Pressure Vessel code [23]. As shown in Fig. 2, based on such estimation model, the NREL has estimated the cost of chloride-TES with Ha 230 hot tank and SS 347 cold tank. In this case, the cost of chloride-TES is 58 USD/kWh-th, which is much expensive than nitrate-TES (20-33 USD/kWh-th). Currently, there is no estimation of chloride-TES cost with stainless steel or other in-expensive Fe-based alloy tanks, due to the doubts of researchers about the compatibility of Fe-based alloys with highly corrosive chloride salts. Hence, it is meaningful to establish cost estimation of chloride-TES with Fe-based alloys as main structural materials based on the results of corrosion experiments. 1.3 Breaking the trilemma of material selection for chloride-TES The following paragraph discusses the aim and scope of the presented work in a broader context. The current trilemma of structural material selection for chloride-TES can be summarized in a triangle as shown in Fig. 5 (a). Three key parameters for structural metals are placed at the vertex of a triangle as corrosion resistance in molten chloride salt (compatibility), cost-effective compared with main structural materials of nitrate-TES, and mechanical properties. On each of the three sides of the triangle are three different types of material, one material can only satisfy the requirements of the two attached vertexes but not the opposite vertex. As can be seen, nickel-based alloy is too expensive to be selected as the main structural material. Some non-metallic materials (e.g., ceramics) show acceptable corrosion resistance in molten chloride salt, but their poor mechanical properties (e.g., low ductility) limit their use in complex components such as tanks, pipes, valves, pumps, etc. Fe-based alloy is widely used in nitrate-TES, as shown in Table 1. However, the compatibility issues, especially corrosion issue, of Fe-based alloy with MgCl2-KCl-NaCl at high temperature is still a challenging problem, hindering the further development of chloride-TES with Fe- based alloy. Introduction 17 Fig. 5. (a) the trilemma of material selection for chloride-TES. (b) with proper corrosion mitigation strategies, the Fe-based alloy could meet all requirements of compatibility, cost-effectiveness, and mechanical properties. Breaking the trilemma of molten chloride salt TES is a key step in advancing molten chloride salt technology. One promising approach to break this trilemma could involve achieving a CR of < 15 µm/year for Fe-based alloys through proper corrosion mitigation strategies, as shown in Fig. 5 (b). However, there is no experimental evidence that the Fe-based alloy can achieve such a target in a cost-effective way (e.g., (a) (b) Introduction 18 Mg-additive). One possible reason is that the existing measurements of MgOHCl have the limits above the acceptable level for stainless steel in molten chloride salt. Another possible reason is that the salt purification process needs to be optimized to meet the acceptable concentration of MgOHCl. A potential route to achieve the CR target of Fe-based alloys in molten chloride salt is to develop an optimized purification method for molten chloride salt, based on a lower limit MgOHCl monitoring method. Then the compatibility of such purified salt with Fe-based alloys should be investigated. The CRs of alloy samples in molten chloride salt should be studied by both mass loss and microstructural analysis, with the expectation of achieving a CR < 15 µm/year at ≥ 700 °C. During this study, some research gaps in this direction were filled and summarized in three journal papers, which are listed in Chapter 2. In the work presented in Paper I, a direct titration technique was developed to measure the concentration of MgOHCl at the level of < 0.01 wt.% (< 100 ppm). The reliable MgOHCl concentration data obtained by titration were used to calibrate the electrochemical data of the CV to develop an in-situ monitoring technique for MgOHCl at the tens of ppm level. In the work presented in Paper II, the exposure test procedure was optimized by immersing the steel samples in the molten salt, which had been purified with liquid Mg. The SS 310 (EN 1.4845) and In 800H (EN 1.4876) were exposed in MgCl2-KCl-NaCl for up to 2000 h (~2.8 months) at 700 °C with 2.8 wt.% Mg under an inert atmosphere. After the exposure test, the corrosion behavior of the metal samples was characterized via microscopic and mass loss methods. In parallel, corrosion impurity (MgOHCl) and corrosion products (e.g., Crx+, Fex+, and Nix+) in the salt samples were also quantitatively analysed separately with the direct titration presented in Paper I and atomic absorption spectroscopy (AAS). In the work presented in Paper III, the P91 (EN 1.4903) and SS 304 (EN 1.4301) were selected as the candidates for the cold tank structural materials. The MgCl2-KCl-NaCl were purified with liquid Mg at 700 °C, according to the procedure described in Paper II. The P91 and SS 304 samples were then exposed to the MgCl2-KCl-NaCl melts at 500 °C for up to 1400 h. Then, the extracted salt samples of this series of experiments were analysed by direct titration and AAS, as in Paper II. The CR of the alloy samples was determined by microscopic and mass loss methods. Finally, the cost of the P91 cold tank for chloride-TES was estimated based on the corrosion test and data available in the literature [1,23]. Publications 19 Publications 2.1 Papers and contribution report I. Monitoring of Extremely Low-Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl Gong Q, Ding W, Chai Y, Bonk A, Steinbrecher J, Bauer T. Chemical Analysis and Electrochemical Monitoring of Extremely Low-Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl. Frontiers in Energy Research. 2022;10.https://doi.org/10.3389/fenrg.2022.811832 Qing Gong contributed to the conception and design of the study, performed the statistical analysis, wrote the first draft of the manuscript. II. Compatibility of Fe-based alloys with purified molten MgCl2-KCl-NaCl salt at 700 °C Gong Q, Shi H, Chai Y, Yu R, Weisenburger A, Wang D, Bonk A, Bauer T, Ding W. Molten chloride salt technology for next-generation CSP plants: Compatibility of Fe- based alloys with purified molten MgCl2-KCl-NaCl salt at 700 °C. Applied Energy. 2022;324:119708.https://doi.org/10.1016/j.apenergy.2022.119708 Qing Gong contributed to the conceptualization, methodology, investigation, writing – original draft, and visualization. III. Selection of cold tank structural material utilizing corrosion control at 500 °C Gong Q, Hanke A, Kessel F, Bonk A, Bauer T, Ding W. Molten chloride salt technology for next-generation CSP plants: Selection of cold tank structural material utilizing corrosion control at 500 °C. Solar Energy Materials and Solar Cells. 2023;253. https://doi.org/10.1016/j.solmat.2023.112233 https://doi.org/10.3389/fenrg.2022.811832 https://doi.org/10.1016/j.apenergy.2022.119708 https://doi.org/10.1016/j.solmat.2023.112233 Publications 20 Qing Gong contributed to the writing – original draft, visualization, methodology, investigation, and conceptualization. 2.2 International conferences Some of the research results in this thesis have been published in the following international conferences: ⚫ Gong Q, Ding W, Chai Y, Shi H, Bonk A, Bauer T: Molten Chloride Salts for Thermal Energy Storage in Next Generation CSP: Mg Inhibitory Effect in Corrosion Control System (CCS) for MgCl2-KCl-NaCl, IRES 2021, Oral presentation, 2021.03, online ⚫ Gong Q, Ding W, Shi H, Chai Y, Yu R, Weisenburger A, Wang D, Bonk A, Bauer T: Molten chloride salt technology for next-generation CSP plants: Compatibility of cost-effective Fe-based alloys with Mg-purified molten MgCl2-KCl-NaCl at 700°C, ICAE 2021, Oral presentation, 2021.11, online. ⚫ Gong Q, Ding W, Bonk A, Bauer T: In- and ex-situ monitoring of extremely low MgOHCl in molten chloride salt for next-generation CSP, EUCHEMSIL 2022, Oral presentation, 2022.06, Patras, Greece. ⚫ Gong Q, Ding W, Bonk A, Bauer T: Molten Chloride Salt Technology for Next- generation CSP Plants: An Economically Competitive Future Scenario Using Fe- based Alloys with Corrosion Control, SolarPaces 2022, Oral presentation, 2022.09, Albuquerque, NM, USA. Publications 21 Paper I: Monitoring of Extremely Low-Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl Gong Q, Ding W, Chai Y, Bonk A, Steinbrecher J, Bauer T. Chemical Analysis and Electrochemical Monitoring of Extremely Low-Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl. Frontiers in Energy Research. 2022;10.https://doi.org/10.3389/fenrg.2022.811832 https://doi.org/10.3389/fenrg.2022.811832 Publications 22 Chemical Analysis and Electrochemical Monitoring of Extremely Low-Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl Qing Gong 1*, Wenjin Ding 1*, Yan Chai 1, Alexander Bonk1, Julian Steinbrecher 1 and Thomas Bauer 2 1Institute of Engineering Thermodynamics, German Aerospace Center (DLR), Stuttgart, Germany, 2Institute of Engineering Thermodynamics, German Aerospace Center (DLR), Cologne, Germany MgCl2–KCl–NaCl is a promising thermal energy storage (TES) material and heat transfer fluid (HTF) with high operating temperatures of >700°C for next-generation concentrating solar power (CSP) plants. One major challenge for future implementation of the molten chloride TES/HTF technology arises from the presence of some corrosive impurities, especially MgOHCl, a hydrolysis product of hydrated MgCl2. Even extremely low- concentration MgOHCl (tens of ppm O in weight) can cause unneglectable corrosion of commercial Fe–Cr–Ni alloys, which limits their service time as the structural materials in the molten chloride TES/HTF system. Thus, the chemical analysis and monitoring techniques of MgOHCl at the tens of ppm O level are vital for corrosion control. In this work, a chemical analysis technique based on direct titration and a high-precision automatic titrator was developed for an exact measurement of MgOHCl at the tens of ppm O level. It shows a standard deviation below 5 ppm O and an average error below 7 ppm O when the concentration of MgOHCl is 36 ppm O. Moreover, compared to other methods available in some literature reports, it can exclude the influence of co-existing MgO on the MgOHCl concentration measurement. This chemical analysis technique was used to calibrate the previously developed electrochemical method based on cyclic voltammetry (CV) to achieve reliable in situ monitoring of MgOHCl in the MgCl2–KCl–NaCl molten salt at a concentration as low as the tens of ppm O level. The in situ monitoring technique shows a monitoring limitation of <39 ppm O. The two techniques for MgOHCl measurement developed in this work could be used to develop an in situ corrosion control system to ensure the long service time of the molten chloride TES/ HTF system in next-generation CSP plants. Keywords: MgOHCl concentration measurement, direct titration, cyclic voltammetry, corrosion control, next- generation concentrating solar power Edited by: Xiaohui She, University of Birmingham, United Kingdom Reviewed by: Yafei Wang, University of Wisconsin-Madison, United States Brenda Garcia-Diaz, Savannah River National Laboratory (DOE), United States Zhu Jiang, Southeast University, China Zhongfeng Tang, Shanghai Institute of Applied Physics (CAS), China *Correspondence: Qing Gong qing.gong@dlr.de Wenjin Ding wenjin.ding@dlr.de Specialty section: This article was submitted to Process and Energy Systems Engineering, a section of the journal Frontiers in Energy Research Received: 09 November 2021 Accepted: 10 May 2022 Published: 22 June 2022 Citation: Gong Q, Ding W, Chai Y, Bonk A, Steinbrecher J and Bauer T (2022) Chemical Analysis and Electrochemical Monitoring of Extremely Low- Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl. Front. Energy Res. 10:811832. doi: 10.3389/fenrg.2022.811832 Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 8118321 ORIGINAL RESEARCH published: 22 June 2022 doi: 10.3389/fenrg.2022.811832 INTRODUCTION MgCl2–KCl–NaCl molten chloride salt has received much attention in recent years due to its wide working temperature range (420–800°C), low vapor pressure, low material cost, and good heat capacity (Mehos et al., 2017; Ding et al., 2018a; Turchi et al., 2018; Villada et al., 2021). It has potential use in the next- generation concentrating solar power (CSP) plants as thermal energy storage (TES) material and heat transfer fluid (HTF), allowing the operating temperature of the CSP plant to be increased from about 560°C to >700°C. With such a high operating temperature, the energy efficiency of the power cycle in the CSP plant can rise from 40% to >50% when integrated with the supercritical carbon dioxide (sCO2) Brayton power cycle, which would significantly reduce the levelized cost of electricity (LCOE) of CSP (Mehos et al., 2017). However, the molten MgCl2–KCl–NaCl mixture is strongly corrosive to commercial Fe-Cr-Ni alloys even under a protective inert atmosphere (Ding et al., 2018c; Sun et al., 2018), which greatly limits its application. Numerous studies have shown that the corrosion in molten MgCl2–KCl–NaCl salt is caused by corrosive impurities, especially MgOHCl (Ding et al., 2018c; Choi et al., 2019; Grégoire et al., 2020; Sun et al., 2020; Zhao, 2020), not by the molten chloride salt itself (Zhang et al., 2020). As a consequence of the strong hygroscopicity of MgCl2, some moisture is inevitably absorbed in MgCl2–KCl–NaCl in practical applications. Subsequently, the main corrosive impurity MgOHCl is generated as a hydrolysis product during heating in the melting process, resulting in the strong salt corrosivity to the metallic structural materials, such as Fe–Cr–Ni alloys (Kipouros and Sadoway, 2001; Kashani-Nejad, 2005; Ding et al., 2018c). To represent the concentration of MgOHCl (C(MgOHCl))in the molten salt, the unit parts per million oxygen (ppm O) is defined as the mass fraction of oxygen (mO in MgOHCl) in the total mass of the salt sample (msample), as shown in Eq. 1 (Skar, 2001). C(MgOHCl)[ppmO] � mO in MgOHCl/msample ×10 6 (1) The estimated acceptable impurity level of MgOHCl for the different types of alloys based on the literature (Ding et al., 2018c; Ding et al., 2019a; Ding et al., 2019b; Kurley et al., 2019) and their relative cost factors (Gilardi et al., 2006) are summarized in Table 1. To allow the use of inexpensive alloys (e.g. stainless steels) for TES/HTF with molten MgCl2–KCl or MgCl2–KCl–NaCl at ≥700°C, the salt impurity needs to be controlled at the tens of ppm O level by monitoring and salt purification to control the salt corrosivity (Ding et al., 2019b; Kurley et al., 2019). As shown in Figure 1, a corrosion control system (CCS) integrated into the molten chloride TES/HTF system has been proposed in the previous work, which contains the main parts—part of online corrosion monitoring and part of corrosion mitigation (Villada et al., 2021). For CCS, reliable in situ and ex situ monitoring techniques of MgOHCl at the tens of ppm level are vital to ensure its effectiveness, efficiency, and economics. To measure the oxygen-containing impurity concentration or redox potential of molten salts, electrochemical methods including cyclic voltammetry (CV) (Skar, 2001; Ding et al., 2017; 2018b; Choi et al., 2019; Gonzalez et al., 2020; Guo et al., 2021), square wave voltammetry (SWV) (Song et al., 2018), chronopotentiometry (CP) (Zhang et al., 2020), and open-circuit potentiometry (OCP) (Choi et al., 2019; Gonzalez et al., 2020) have been employed in molten chloride salts (Williams et al., 2021). Among them, an approach combining in situ and ex situ measurement of MgOH+Cl− was investigated, in which cyclic voltammetry (CV) was employed as the in situ measurement of MgOHCl (Skar, 2001; Ding et al., 2018b; Guo et al., 2021), while ex situ methods of titration (Skar, 2001; Ding et al., 2018b) and carbothermal reduction (Skar, 2001) were used for the ex situ measurement to calibrate the in situ CV measurement. The reduction peak in the cyclic voltammogram—peak B, shown in Figure 2, represents the reaction of MgOH+ to MgO, as shown in Eq. 2 (Skar, 2001; Ding et al., 2018b; Guo et al., 2021). Moreover, the current density of peak B in the cyclic voltammogram is linearly linked to the concentration of MgOH+. MgOH+ + e− → MgO(s) + 1 2 H2(g) (2) It was pointed out in the work of Skar (2001) and Ding et al. (2018b) that the ex situ measurements based on titration and carbothermal reduction could result in an over-measurement of the MgOH+ concentration and biased calibration of CV since TABLE 1 | Comparison of the Ni content, cost of the alloys, and their estimated acceptable impurity level with a target corrosion rate of 15 μm/year at ≥700°C for molten MgCl2-KCl or MgCl2-KCl-NaCl under inert atmosphere. Alloy Example Ni (wt%) Relative cost factor (compared to 3-series stainless steel) Acceptable MgOHCl level Data source 3-series stainless steel SS 310 and SS 316 <30 1 Tens of ppm O Ding et al. (2019b) Kurley et al. (2019) Incoloy In 800 H 30–50 ~2.5 Tens of ppm O Ding et al. (2019a) Ding et al. (2019b) Hastelloy Ha 276, Ha 230, and Ha N >50 ~10 Hundreds of ppm O Ding et al. (2018c) Ding et al. (2019b) Kurley et al. (2019) Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 8118322 Gong et al. Corrosive Impurity Monitoring and Controlling other impurities (e.g. MgO or H2O) cannot be quantitatively excluded in these ex situ measurements. In the MgCl2-KCl-NaCl molten salt, non-corrosive MgO and corrosive MgOHCl commonly co-exist. When the temperature is >555°C, MgOHCl can be decomposed into MgO and HCl, as shown in Eq. 3 (Kipouros and Sadoway, 2001; Kashani-Nejad et al., 2005). MgOHCl → MgO(s)+HCl(g) (3) To measure non-corrosive MgO and corrosive MgOHCl separately, Klammer et al. (2020) used water and methanol to extract MgOHCl/MgO based on the solubility difference (Klammer et al., 2020). This method can measure the concentration of MgOHCl down to 0.1 wt% (~200 ppm O) by a typical ethylenediaminetetraacetic acid (EDTA) titration technique and has been used to measure the purity of MgCl2- containing chloride mixtures after pre-purification. However, due to the solubility of MgOHCl in methanol, this method cannot measure the extremely low-concentration MgOHCl. In general, it was proposed that the in situ electrochemical CV measurement of MgOHCl calibrated by a reliable ex situ chemical analysis method is a promising approach to monitor MgOHCl (i.e., salt corrosivity) in molten MgCl2–KCl–NaCl. However, existing electrochemical and chemical measurement methods of MgOHCl have minimum measurement limits above the acceptable MgOHCl level. For corrosion control, the MgOHCl concentration should be controlled at tens of ppm O to allow inexpensive alloys (e.g., stainless steel) to withstand the corrosion of molten MgCl2–KCl–NaCl at ≥700°C. In this experiment, aiming to develop reliable chemical analysis and in situ monitoring techniques for MgOHCl at the tens of ppm O level, the following experiments were designed and carried out: • Different concentrations of MgOHCl from thousands to tens of ppm O level were obtained by electrolysis at 500°C and thermal decomposition at 600 and 700°C. • A chemical analysis technique based on direct titration and a high-precision automatic titrator was developed to measure MgOHCl concentration at the tens of ppm O level. • The reliable concentration data on MgOHCl obtained by titration were used to calibrate the CV data to develop an in situ monitoring technique for MgOHCl at tens of ppm O. EXPERIMENTAL Materials and Experimental Setup KCl (purity >99 wt%) and NaCl (purity >99 wt%) were purchased from Alfa Aesar, Germany, while anhydrous MgCl2 (purity >99 wt%) was supplied byMagnesia, Germany. They were used to synthesize the eutectic salt mixture of MgCl2-NaCl-KCl (47.1-30.2-22.7 mol%) for the experiments. This eutectic salt composition is suggested by the previous work (Villada et al., 2021). Figure 3 shows the setup of electrochemical experiments. A chemically stable glassy carbon crucible purchased from HTW Germany (Sigradur® G) was used in this work to prevent the reaction of the strongly corrosive molten chloride salt with the crucible. The 250 g chloride salts were heated in an argon atmosphere (purity ≥99.999%, H2O ≤ 0.5 ppm, 10 nL/h, and pressure above atmospheric pressure of about 0.1 bar) to 500°C, 600°C, or 700°C. As shown in Figure 3, an alumina plate was used under the glassy carbon crucible to electrically insulate the glassy carbon and the autoclave system made of steel from each other. As shown in Figure 3, five electrodes were used for electrolysis and cyclic voltammetry (CV). All the electrochemical experiments were conducted using a ZENNIUM electrochemical workstation from Zahner GmbH (Germany). Table 2 summarizes the electrodes used in different electrochemical experiments and the material composition of these electrodes. Three tungsten electrodes (1 mm diameter, purity >99.5%, purchased from Alfa Aesar) were used as working (~32 mm2), counter (~100 mm2), and quasi reference electrodes (~100 mm2) for CV, while two graphite electrodes FIGURE 2 | Cyclic voltammogram in eutectic MgCl2/KCl/NaCl at 500°C obtained by a tungsten working electrode with a sweep rate of 200 mV/s (Ding et al., 2017; 2018b). FIGURE 1 | Corrosion control system (CCS) integrated into the molten chloride TES/HTF system, which contains the online corrosion monitoring and mitigation parts, and has been proposed in the previous work (Villada et al., 2021). Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 8118323 Gong et al. Corrosive Impurity Monitoring and Controlling (size: 40 mm × 10 mm x 2 mm) were used for electrochemical purification. Due to their larger surface area compared with tungsten electrodes used in our previous work (Ding et al., 2017; 2018b), graphite electrodes with a 4 cm2 area were used to have a fast purification rate and be able to purify more salt before electrode passivation. CV Experiments Three groups of CV experiments at different temperatures (500°C, 600°C, and 700°C) were conducted. Once the temperature of the salt reached the target temperature, the counter electrode, reference electrode of CV, and the two graphite electrodes of electrolysis were inserted into the molten salts but without touching the crucible bottom, while the immersion depth of the working electrode of CV was fixed to 10 mm (i.e., the contact area of the tungsten electrode with the melt is about 31.4 mm2). A sweep rate of 200 mV/s was used in all CV experiments as in the previous work (Ding et al., 2017; 2018b), while the potential voltage of CV was from 0.5 V to −1.7 V vs. reference. Figure 4 is an example cyclic voltammogram of the eutectic MgCl2-NaCl-KCl salt before electrolysis treatment at 500°C, in which peak B represents the reduction reaction in Eq. 2 and is in line with the previous work (Ding et al., 2018b). Skar (2001) and Guo et al. (2021) discovered that the peak current density of the peak B (ip(B)) is proportional to the concentration of MgOHCl (C(MgOHCl)) in molten MgCl2- NaCl or (-KCl) salts(Skar, 2001; Guo et al., 2021), which is in FIGURE 3 | Schematic representation of the experimental setup for electrochemical salt purification and CV experiments. TABLE 2 | Electrodes used for electrochemical purification via electrolysis and monitoring the impurity concentration via CV. Experiment Application Electrodes shown in Figure 3 Working Counter Reference Electrolysis Electrochemical purification 4 5 — Cyclic voltammetry (CV) Monitoring impurity concentration 2 3 1 1: tungsten quasi-reference electrode (~100 mm2). 2: tungsten working electrode (~32 mm2). 3: tungsten counter electrode (~100 mm2). 4: graphite anode (10 mm× 40 mm) + tungsten wire for salt purification. 5: graphite cathode (10 mm × 40 mm) + tungsten wire for salt purification FIGURE 4 | Example of a cyclic voltammogram for molten MgCl2-KCl- NaCl salt with impurity ion MgOH+ at 500°C. The current density of peak B in this case was 520 mA/cm2. The working, reference, and counter electrodes of CV were tungsten. Sweep rate: 200 mV/s. Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 8118324 Gong et al. Corrosive Impurity Monitoring and Controlling accordance with the Randles–Sevcik equation (Randles, 1948; Ševčík, 1948): ip(B)� 0.4463 (nF) 3 2 (RT) 1 2 C∞(MgOHCl)D1 2v 1 2 (4) where ip(B) represents the current density of peak B in A/m2, n is the number of electrons transferred in the reaction (n = 1 for the reduction reaction in Eq. 2), F is the Faraday’s constant (F = 96,485.3 C/mol), R is the universal gas constant (R = 8.314 J/ (Kmol)), T is the temperature of the molten salt in K, C∞ (MgOHCl) is the bulk concentration of MgOHCl in mol/m3, D is the diffusion coefficient of the reacting species (hereMgOH+) in m2/s, which is a function of temperature, and v is the potential sweep rate in V/s. In this study, sweep rate v is a fixed value of 0.2 V/s (200 mV/s). However, considering the limit of MgO solubility in molten MgCl2-containing chloride salts (Boghosian et al., 1991), MgO was habitually saturated in this work. Hence, the equation is amended to the soluble-insoluble model of the Berzins–Delahay equation (Berzins and Delahay, 1953), as shown in Eq. 5. ip(B)� 0.6105 (nF) 3 2 (RT) 1 2 C∞(MgOHCl)D1 2v 1 2 (5) Eq. 5 can be simplified to Eq. 6, where the concentration of C(MgOHCl) [ppm O] is proportional to ip(B). Thus, the current density of peak B obtained from CV can be used to monitor the concentration of MgOH+ impurities in situ in the molten chloride salts (Skar, 2001). The k(T,D) in Eq. 5 is related to the temperature, sweep rate, and diffusion coefficient of the CV measurement, as shown in Eq. 7, where c2 = 3.3 × 10−2 (ppm Ocm3 ·mV ½)/(mAK½ ·s) is calculated with a fixed sweep rate of 200 mV/s, R, F, and n in Eq. 5. C(MgOHCl) [ppmO]� k(T,D).ip(B) (6) k(T,D) � c2( T Dv ) 1 2 (7) In the CV experiments, all current densities of peak B were read with a tangent at the starting point of peak B as the baseline (see Figure 4). The current densities of peak B would then be calibrated using an advanced titration process (see Titration Experiments section) to improve the measurement accuracy and precision of CV. In order to study the relationship between the current density of peak B and the concentration of MgOHCl in the molten salt (i.e., calculating the k(T,D) in Eq. 6), the different concentrations of MgOHCl were achieved by electrolysis and thermal decomposition. In this work, different concentrations of MgOHCl at 600 and 700°C were obtained through thermal decomposition during a certain waiting time since MgOHCl decomposes to MgO and HCl at temperature >555°C, as shown in Eq. 3 (Kipouros and Sadoway, 2001; Kashani-Nejad et al., 2005). As the heating time increased, the concentration of MgOHCl in the molten salt gradually decreased. At 500°C, different MgOHCl concentrations were obtained through electrolysis. For electrolysis, the voltage between the working electrode and counter electrode was 1.7 V, preventing the formation of Mg at a voltage higher than 1.7 V (see Peak A in Figure 4). The corrosive MgOHCl impurity was decomposed by electrolysis: Cathode(Reduction): 2MgOH++2e− → 2MgO(s) +H2(g) (8) Anode (Oxidation): Cl− → Cl2(g)+2e− (9) Figures 5A,B show the flowchart of CV experiments at 500, 600, and 700°C. Cyclic voltammograms were obtained at different temperatures and different MgOHCl concentrations. After each CV experiment, 1−2 g chloride salts, as shown in Figure 5C, were taken out from the crucible using a sample pipet (see Figure 3) for further titration experiments. During the short sample extraction, the system remained under an Ar (purity: 99.999%) atmosphere all the time to avoid air leakage into the autoclave. Combining the data on titration and CV experiments, k(T,D) is determined. Titration Experiments The acid consumptionmethod based on titration was used for the quantitative measurement of the total amount of MgOHCl in a salt sample. A high-precision automatic titration instrument, 905 Titrando, purchased from Metrohm Germany, was employed. The titration method is as follows: first, about 500 mg sample was weighed by using an analytical balance. Then, the solid samples were dissolved in a beaker with 150 ml of ultrapure water (HiPerSolv, VWR, Germany). After installation of the beaker with samples, the standard titrant (0.01 M HCl) was charged into a 20 ml cylinder from the reagent bottle and then dripped into the salt solution through the rotation of the gear at an average rate of 0.2 ml/min (slower at pH values near 7). Meanwhile, a stirrer homogenized the solution. The titration was performed under a nitrogen purge to exclude any interference from carbon dioxide/ carbonic acid. Hydrochloric acid solution (0.01 M) purchased from Merck KGaA was used to prepare the standard titrant. The titer of the HCl solution was calibrated with sodium carbonate (Na2CO3). The pH value and the amount of the consumed titrant were plotted by the computer, as shown in Figure 6. The equivalence point (EP) is marked out, where moles of acid (HCl) and moles of base (MgOHCl) neutralize each other, as shown in Eq. 10. MgOH+(aq.) +H+(aq.)→ Mg2+(aq.) +H2O(l) (10) C(MgOHCl)[ppmO] � ti× CHCl × VHCl/msample × MO ×1000 (11) The concentration of MgOHCl in samples in ppm O is calculated according to Eq. 11, where ti is the titer of HCl, i.e., the ratio of actual concentration to theoretical concentration (0.01 M); VHCl is the volume of HCl at EP in mL; CHCl is 0.01 mol/L;msample is the mass of the salt sample in g, MO is the molar mass of oxygen, i.e., 16 g/mol. Error Analysis for Titration and CV For ex situ direct titration to measure the concentration of MgOHCl in 500 mg samples, there are two main sources of error: one caused by the co-existing MgO impurity in the salt Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 8118325 Gong et al. Corrosive Impurity Monitoring and Controlling sample and another caused by the titration experiment (i.e., titration conditions and equipment). For the first error, once the MgCl2-KCl-NaCl-(MgO- MgOHCl) sample is placed in water, two types of impurities will dissolve in the water or react with water, as shown in Eq. 12 and Eq. 13 (Chen et al., 2018; Klammer et al., 2020). MgOHCl → Mg2+ +OH−+Cl− (12) MgO + H2O→ Mg2++ 2OH− (13) Empirically, the dissolution of MgOHCl in water (see Eq. 12) occurs rapidly, when the aqueous solution is not saturated with Mg(OH)2 (i.e., pH < 10.4) (Dong et al., 2010). In this work, the pH value of the salt solution for titration was smaller than 10 because of the low MgOHCl concentration (<4000 ppm O in salt samples), as shown in Figure 6. Unlike the rapid dissolution of MgOHCl shown in Eq. 12, the reaction between MgO and H2O shown in Eq. 13 has been experimentally proved slow in an alkali or a neutral environment, as shown in Figure 7 (Fruhwirth et al., 1985). As can be seen, in the pH range of 6–9, the dissolution rate of MgO is less than 10−11 mol cm−2 s−1. Thus, when we know the diameter and concentration of MgO particles in the system, the amount of HCl consumed by MgO can be estimated. Taking the highest concentration of MgOHCl (4000 ppm O) as a reference, the maximum mass fraction of MgO was 1 wt% (assuming that all MgOHCl was converted to MgO) (Zhao, 2020). We measured the diameter of MgO in MgCl2-KCl-NaCl and found that the broad peak of the laser- based particle-size analyzer was 10–30 µm. Using the parameters mentioned earlier and the density of MgO (3.58 g/cm3), the total surface area (A) in 250 g of chloride salt can be calculated, as shown in Table 3. Then, the maximum dissolution rate of MgO in this direct titration experiment can be calculated according to Eq. 14. E � k.A.Mo ms ×60×2×106 (14) where E is the error caused by MgO in ppm O/min; k is the MgO dissolution rate, adopting 10−11mol cm−2 s−1; A is total MgO surface area in cm2, listed in Table 3; MO is the molar mass of oxygen, i.e., 16 g/mol; ms is the total salt mass, 250 g; 60 is the factor of second to minute; 2 means one MgO equivalent to two MgOHCl in titration; and 106 is the factor of ppm. FIGURE 5 | Flowchart of CV experiments at (A) 500°C, (B) 600, and 700°C; (C) salt specimen taken from the glassy carbon crucible for titration. FIGURE 6 | Typical pH change curve of a direct titration of a salt sample containing MgOHCl with the 0.01 M HCl titrant recorded by using the high- precision titrator. FIGURE 7 | pH vs. the rate of dissolution of MgO powder (°) and a (100) MgO crystal (■) at 25°C in aqueous solution (Fruhwirth et al., 1985). The dissolution rate is smaller than 10−11 mol cm−2 s−1 in the pH interval of [5.5, 10]. Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 8118326 Gong et al. Corrosive Impurity Monitoring and Controlling The calculated error caused by MgO in ppm O/min is listed in Table 3. In the direct titration, the time required for the titration to reach EP was not exceeded by 5 min for neutralization of 50 ppm O MgOHCl by 0.01 M HCl, which means that the titration error due to co-existing MgO is less than 1.6 ppm O (0.32 × 5) for the MgOHCl concentration measurement. The second error source of the measurement based on direct titration in this work was the experimental error, which could be caused by the dissolved carbon dioxide (i.e. carbonic acid) and the fluctuation of the HCl amount injected by gear etc. An experiment was carried out to determine the relative standard deviation (RSD) and percentage error of direct titration. An amount of 1.308 mg of Mg(OH)2 was weighed and dissolved in 2 L distilled water as the 1.13 × 10−5mol/L Mg(OH)2 standard solution. Then, about 50 ml of standard solution was extracted and measured by direct titration, whose HCl consumption is equivalent to 36 ppm O MgOHCl in the 500 mg salt sample. The titration of the standard solution was repeated 13 times to obtain sufficient data for statistical analysis (see Table 4). It was found that the average standard deviation was 4.35 ppm O (RSD = 10.2%), and the average error was +6.62 ppm O (average relative error = +18%) for measuring the equivalent of 36 ppm O MgOHCl in 500 mg samples by direct titration using the titrator. Compared to this error, the titration error due to MgO is much smaller and thus neglectable. For in situ measurement by CV, the repeatability of cyclic voltammograms is significant to determine the current density of peak B and link it to the concentration of MgOHCl. Figure 8 TABLE 3 | Error caused by the dissolution of MgO with different particle sizes in an aqueous solution with pH 5.5–10.4. Salt mass in g Maximum MgO fraction in wt% MgO particle diameter (D) in µm Total MgO surface area (A) in cm2 Dissolution rate (k) in mol/(Cm2 s) Maximum error (E) in ppm O/min 250 1 10 4190 10−11 0.32 250 1 20 2095 10−11 0.16 250 1 30 1397 10−11 0.10 TABLE 4 | Thirteen times direct titration experiments with the standard Mg(OH)2 solution to determine RSD and relative error at a concentration equivalent 36 ppmO in 500- mg salt sample. C(MgOH2) in mol/L Volume in ml V (0.01 M HCl) in ml (theoretical) V (0.01 M HCl) in ml (titration) Equivalent MgOHCl ppm O in 500 mg salt sample (theoretical) Equivalent MgOHCl ppm O in 500 mg salt sample (titration) Relative error % 1.13 × 10−5 50 0.11 0.1381 36 44 23.15 1.13 × 10−5 50 0.11 0.1363 36 44 21.54 1.13 × 10−5 50 0.11 0.1522 36 49 35.72 1.13 × 10−5 50 0.11 0.1291 36 42 15.12 1.13 × 10−5 50 0.11 0.1418 36 46 26.45 1.13 × 10−5 50 0.11 0.1244 36 40 10.93 1.13 × 10−5 50 0.11 0.1285 36 41 14.59 1.13 × 10−5 50 0.11 0.1583 36 51 41.16 1.13 × 10−5 50 0.11 0.1246 36 40 11.11 1.13 × 10−5 50 0.11 0.1384 36 45 23.42 1.13 × 10−5 50 0.11 0.1090 36 35 −2.80 1.13 × 10−5 50 0.11 0.12 36 38 4.06 1.13 × 10−5 50 0.11 0.1280 36 41 14.14 FIGURE 8 | Three cyclic voltammograms with the same molten salt batch were carried out directly and immediately after each other. The current densities of peak B were 43, 39, and 44 mA/cm2 in three measurements, respectively. Temperature: 500°C, after 30 min electrolysis. Working, reference, and counter electrodes of CV: tungsten. Sweep rate: 200 mV/s. Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 8118327 Gong et al. Corrosive Impurity Monitoring and Controlling displays three times the CV curves in molten chloride salt at 500°C after 30 min of electrolysis at 1.7 V. The difference of ip(b) in the same situation is the deviation of the current density. The average height of peak B is 42 mA/cm2, while the relative standard deviation (RSD) is 6.3%, showing good stability at the low current density of ip(b). Generally, the closer to the detection limit, the greater will be the relative standard deviation. The 42 mA/cm2 height of peak B is the smallest current density obtained at 500°C. Due to the thermal decomposition of MgOHCl (Eq. 3), the CV voltammograms at 600 and 700°C are not suitable for error analysis. The MgOHCl concentration changed during several CV measurements at 600 and 700°C. Hence, for all the CV measurements in this work, the relative standard errors are set at 6.3%. Both precision and accuracy data in this work are summarized in Table 5. To determine the MgOHCl concentrations in salt samples and their errors, three titrations were carried out for each sample. When the standard deviation of one batch of samples was higher than 6.62 ppm O (the average error), the standard deviation of three titrations was adopted as the main error source. When the standard deviation was smaller than 6.62 ppm O, the average error (6.62 ppm O) was adopted as the domination error for the direct titration. RESULTS AND DISCUSSION In previous work, some efforts have been made to understand the relation between the height of peak B (in Figure 2) and the concentration of MgOHCl in the MgCl2-containing chloride salts (Skar, 2001; Ding et al., 2017; 2018b; Choi et al., 2019; Gonzalez et al., 2020; Guo et al., 2021). It was found that adding NaOH can increase the height of peak B because of the reaction shown in Eq. 15. In addition, the potential difference between peak B and peak A is comparable ~1.5 V. The reactions corresponding to peak A and peak A’ are seen as the typical peaks of Mg2+ reduction and its reverse reaction of Mg oxidation, as shown in Eq. 16 and Eq. 17, respectively, which can be seen as a marker. This evidence suggests that the peak at the potential of about 0 V in this work can be seen as the peak B corresponding to the reaction shown in Eq. 2. Different from the previous work (Ding et al., 2018b; Guo et al., 2021), the gradient of the MgOHCl concentration in this work was not obtained by adding NaOH but was obtained by electrolysis and thermo-decomposition to decrease the concentration of MgOHCl. Hence, the concentration of MgOHCl in this work was reduced to tens of ppm O since this level was interesting for corrosion control. Mg2++OH− → MgOH+ (15) Peak A : Mg2++2e− → Mg (16) Peak A′: Mg → Mg2++2e− (17) TABLE 5 | Summary of the precision and accuracy for direct titration and CV. Data on direct titration come from repeated titration of 36 ppm O equivalent MgOHCl; data on CV come from repeated scanning of 500°C salts after 30-min electrolysis. Measurement method Absolute error Relative error (accuracy) % Standard deviation Relative standard deviation (RSD, precision) % Direct titration +6.62 ppm O +18 (at 36 ppm O) 4.35 ppm O 10.2 (at 36 ppm O) CV Calibrated by direct titration Calibrated by direct titration 2.65 mA/cm2 6.3 (at 42 mA/cm2) FIGURE 9 | Concentration of MgOHCl at 500°C decreases with the increasing electrolysis time. The black line using the left y-axis shows the concentration of MgOHCl in ppm O; the red line using the right y-axis shows the electrolysis current in A. FIGURE 10 | Concentration of MgOHCl decreases with increasing holding time at 600 and 700°C. Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 8118328 Gong et al. Corrosive Impurity Monitoring and Controlling Concentration of MgOHCl Measured by Titration The decrease in the MgOHCl concentration with electrolysis time at 500°C is shown in Figure 9, while the decrease in the MgOHCl concentration by thermal decomposition at 600 and 700°C with enhanced holding time is displayed in Figure 10. All the concentration data in Figures 9, 10 were obtained by the direct titration method. Figure 9 shows the relationship between the concentration of MgOHCl and electrolysis time. Generally, the concentration of MgOHCl decreased with the increasing electrolysis time at 500°C. However, as can be seen from the red line in Figure 9, the first two electrolysis experiments were carried out for 5 min, with the decrease in current from ~700 to ~400 mA, while the last two electrolysis experiments were conducted for 10 min with the decrease in current from ~400 to ~100 mA because of the MgO passivation. During the running of electrolysis, the poor electrical conductivity of MgO was generated, as shown in Eq. 8, which covered the surface of the graphite electrode and hindered electronic transmission, leading to the low efficiency of MgOHCl removal (Ding et al., 2019a). Two methods were carried out in order to mitigate the problem of the passive MgO film on the electrode and increase the efficiency of electrolysis. First, the positive and negative electrodes of electrolysis were reversed after each electrolysis. Once reversing the cathode and anode, an instantaneous recovery was displayed on the current curve, which could be attributed to the disruption of the MgO film by the generated chlorine gas (Ding et al., 2021). The reversing electrodes did not completely solve the problem of electrode passivation. Hence, second, after the first two times of 5-min electrolysis, the duration of electrolysis time was expanded to 10 min to obtain a measurable reduction of MgOHCl at lower currents. Compared to our previous work (Ding et al., 2017), the electrolysis electrodes were improved. The tungsten wire electrodes were replaced by 10 mm × 40 mm graphite foils. After 30 min of electrolysis, the MgOHCl concentration decreased from 3603 ppm O to 415 ppm O, i.e., the concentration of MgOHCl was reduced to about 17% of the original concentration. In our previous work (Ding et al., 2017), after 25 min of electrolysis with a tungsten wire electrode, the concentration of MgOH+ was reduced to only 69% of the original MgOH+ concentration (from 10,400 ppm O to 7200 ppm O). At 600 and 700°C, the reason for the decrease of MgOHCl with increasing heating time is the thermal decomposition ofMgOHCl (Kipouros and Sadoway, 2001). Furthermore, the impurity level at 600°C was significantly higher than at 700°C because MgOHCl decomposes faster at 700°C than at 600°C, which compares well with that reported in some literature reports (Kashani-Nejad, 2005; Kashani-Nejad et al., 2005). In addition, before molten chloride salt reached 700°C, thermal decomposition had already occurred at the temperature range between 600 and 700°C during heating with a 5 K/min rate. For comparison, Table 6 lists different measurement methods of MgOHCl in molten MgCl2-containing chlorides (Skar, 2001; Ding et al., 2017, 2018b; Kurley et al., 2019; Klammer et al., 2020). Kurley et al. (2019) carried out the method of direct titration to measure oxygen-containing impurities in the salt samples, which were purified by CCl4 bubbling with as low as 1.6 ppm O impurity. This indicates that direct titration is suitable for the low concentration impurity determination of the molten chloride salt. However, the authors did not discuss the error based on their results nor did they explain the principle of direct titration to measure MgOHCl in MgCl2-containing salt. In this study, it was confirmed that direct titration can measure the tens of ppm O level MgOHCl with acceptable errors. In addition, this method can exclude the interference of MgO with a relatively uncomplicated method, compared with the method published by Ding et al. (2018b) and Klammer et al. (2020). Therefore, it has relatively higher precision than the back-titration employed in our previous work (Ding et al., 2017; 2018b). CV Results As shown in Figures 11–13, the cyclic voltammograms in this work show similar features to those in the literature (Ding et al., 2018b; Choi et al., 2019; Guo et al., 2021). For example, the potential of peak B is about 1.5 V higher than that of peak A at 500–700°C. Although the height of peak A′ is not relevant to the MgOHCl concentration, it is still noticeable that the heights of oxidation peaks are different from each other. Peak A′ corresponds to the oxidation of Mg to Mg 2+, as shown in Eq. 17. This can be explained by the fact that the amount of deposition Mg on the tungsten electrodes increased as the experiment proceeded, resulting in reaction enhancement at peak A’. When the MgOHCl concentration in melts was relatively high, the deposition Mg can react with MgOHCl rapidly, as shown in Eq. 18. After electrolysis or decomposition, the MgOHCl concentration decreased significantly, resulting in a slower reaction of Eq. 18 and more Mg deposition on the electrodes. This deposition of Mg could cause the high peak A’. MgOHCl(l)+Mg(s/l)→ MgO(s)+MgCl2(l) +H2(g) (18) Comparing the concentrations shown in Figure 9 and the heights of peak B shown in Figure 11, it is clearly visible that the height of peak B decreases from 453 to 42 mA/cm2, with the decreasing concentration of MgOHCl from 3640 to 615 ppmO at 500°C with 30-min electrolysis at 1.7 V. Similarly, comparing the heights of peak B in Figures 12, 13, with the decomposition of MgOHCl, the heights of peak B decreased significantly at 600 and 700°C as well. After 180 min of holding in the furnace under an argon atmosphere at 600°C, the current density of peak B decreased from 507 to 69 mA/cm2, corresponding to the concentration of MgOHCl decreasing from 4211 to 894 ppm O as seen from the black line in Figure 10. At 700°C, after 210min of holding, the current density of peak B decreased from 166 to 24 mA/cm2, which corresponds to 363 and 39 ppm O MgOHCl (the red line in Figure 10). In general, the height of peaks B in this work always decreases with the decline in the MgOHCl concentration measured by titration. It is a promising method to measure the MgOHCl concentration in situ with the current density of peak B on Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 8118329 Gong et al. Corrosive Impurity Monitoring and Controlling TABLE 6 | Comparison with measurement methods of MgOHCl in molten chlorides. Measurement method Separate measurement of MgOHCl and MgO Minimum detection limit Measurement accuracy ppm O Data source Carbothermal reduction–acid consumption–iodometric titration Yes >70 ppm O ±20 Skar, (2001) Extraction-EDTA titration Yes >200 ppm O ±40 (0.02 wt% MgOHCl) Klammer et al. (2020) Direct titration Yes ~1.6 ppm O (not analyzed) Kurley et al. (2019) Back titration No >200 ppm O ~200 Ding et al. (2017) Ding et al. (2018b) Direct titration using a high-precision automatic titrator Yes ~36 ppm ~6.62 This work FIGURE 11 | Cyclic voltammograms of chloride molten salt before electrolysis (i.e., 0 min), after 5, 10, 20, and 30 min of electrolysis at 500°C. The working, counter, and reference electrodes of CV with a sweep rate of 200 mV/s were tungsten. The working and counter electrodes of electrolysis were graphite. The voltage on the working and counter electrodes of electrolysis was kept at −1.7 V. FIGURE 12 | Cyclic voltammograms of chloride molten salt, holding at 600°C for 25, 60, 90, 150, and 180 min. The working, reference, and counter electrodes of CV were tungsten. Sweep rate: 200 mV/s. FIGURE 13 | Cyclic voltammograms of chloride molten salt, holding at 700°C for 40, 70, 160, and 210 min. The working, reference, and counter electrodes of CV were tungsten. Sweep rate: 200 mV/s. There is a correlation between the current density and impurity concentration. FIGURE 14 | Peak B current densities vs. concentrations of MgOHCl in molten MgCl2-NaCl-KCl (47.1-30.2-22.7 mol%) at 500°C ( ), 600°C ( ) and 700°C ( ). Error bars in this plot are from section (Error analysis for titration and CV). The solid lines are the linear regressions. Frontiers in Energy Research | www.frontiersin.org June 2022 | Volume 10 | Article 81183210 Gong et al. Corrosive Impurity Monitoring and Controlling CV (an electrochemical signal) when it can be calibrated by the ex situ measurement. In previous work, it has been repeatedly demonstrated that the density of peak B is linearly related to the concentration of MgOHCl in MgCl2