B Grégoire

C Oskay

T M Meißner

M C Galetz

Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickelbased alloy in molten chlorides. Part II: NaCl-KCl-MgCl2 ternary system

Keywords: Concentrated solar power (CSP), Molten salts, Chlorides, Corrosion, Thermodynamics, Kinetics

Molten chlorides are promising heat transfer fluids (HTF) and thermal energy storage (TES) materials for thirdgeneration concentrated solar power (CSP) plants. Despite their low cost and wide operating temperature ranges (400-800°C), structural materials experience severe corrosion in commercially available chloride salts. The understanding of the corrosion mechanisms is therefore critical for successful plant design to ensure constant power generation over the 30 years of expected lifetime. This work investigates the corrosion behavior of ferriticmartensitic P91 steel and Inconel 600 nickel-based alloy in molten NaCl-KCl-MgCl2 (24.5-20.5-55.0 wt.%) under isothermal conditions at 700°C in Ar. For both alloys, the corrosion attack is associated with the selective dissolution of Cr and the removal of Cr-rich carbides from the alloy matrix leaving subsurface voids. The initial "impurity-driven" corrosion mechanism associated with the cathodic reduction of MgOH + impurities leads to the formation of insoluble MgO on the substrates' surface. Compositional and microstructural features of the alloys, especially the distribution of Cr-rich carbides as well as the solubility and mobility of carbon within the metallic matrix, are found to significantly affect their corrosion resistance. The experimental observations indicate, that the formation of galvanic pairs and solid-state diffusion mechanisms play a major role regarding the corrosion attack of the alloys. The systematic corrosion experiments conducted in this study indicate a lower corrosiveness of NaCl-KCl-MgCl2 ternary mixture than that of NaCl-KCl binary mixture in static conditions at 700°C under Ar. This work gives significant insights into corrosion issues that may be expected in third-generation industrial CSP plants.

Introduction

Concentrated solar power (CSP) technology has received increasing interest over the past decades as one of the most competitive alternatives to fossil fuels in supplying electricity efficiently while drastically reducing the CO2 emissions on a global scale [START_REF] Baharoon | Historical development of concentrating solar power technologies to generate clean electricity efficiently -A review[END_REF][START_REF] Tasbirul Islam | A comprehensive review of state-of-the-art concentrating solar power (CSP) technologies: Current status and research trends[END_REF]3]. The principle of CSP systems is to concentrate the solar radiation using programmable mirrors (heliostats) onto a so-called receiver where the focused solar energy is converted into thermal energy and stored via heat transfer fluids (HTF) [START_REF] Baharoon | Historical development of concentrating solar power technologies to generate clean electricity efficiently -A review[END_REF][START_REF] Vignarooban | Heat transfer fluids for concentrating solar power systems -A review[END_REF]. In modern CSP plants such as solar power towers, the primary cycle containing the HTF is combined with a heat exchanger to produce steam in a secondary cycle and drive a conventional steam turbine for power generation [START_REF] Vignarooban | Heat transfer fluids for concentrating solar power systems -A review[END_REF]. The use of molten salts as HTF and for thermal energy storage (TES) considerably increased the competitiveness of CSP technology by overcoming the intermittency of solar resources. As a matter of fact, the TES system using molten nitrate salts at Gemasolar CSP plant in Spain (solar power tower operational since May 2011) extended its electricity generation autonomy for up to 15 hours without sunlight [START_REF] Burgaleta | Gemasolar, the first tower thermosolar commercial plant with molten salt storage[END_REF].

Despite an improved dispatchability for electricity generation, the operating temperature of the state-of-theart HTF "solar salt" (non-eutectic mixture of 60 wt.% NaNO3-40 wt.% KNO3) is limited to maximum operating temperatures ranging between 560-580°C due to thermal decomposition reactions [START_REF] Bonk | Advanced heat transfer fluids for direct molten salt line-focusing CSP plants[END_REF][START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF][START_REF] Bonk | Impact of solar salt aging on corrosion of martensitic and austenitic steel for concentrating solar power plants[END_REF]. Since the overall thermodynamic efficiency of a CSP plant is correlated to the maximum operating temperature of the HTF and its liquidus temperature, alternative salt mixtures with appropriate thermophysical properties have been investigated for the next generation of CSP plants [START_REF] Li | Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl2, CaCl2, ZnCl2 for heat transfer and thermal storage fluids in CSP[END_REF][START_REF] Xu | Basic properties of eutectic chloride salts NaCl-KCl-ZnCl2 and NaCl-KCl-MgCl2 as HTFs and thermal storage media measured using simultaneous DSC-TGA[END_REF][START_REF] Mohan | Thermo-economic analysis of high-temperature sensible thermal energy storage with different ternary eutectic alkali and alkaline earth metal chlorides[END_REF][START_REF] Alva | Thermal energy storage materials and systems for solar energy applications[END_REF]. Among these alternative salt mixtures, molten chlorides are particularly promising owing to their beneficial characteristics such as low costs, good heat transfer properties and thermal stability up to 800°C [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF][START_REF] Li | Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl2, CaCl2, ZnCl2 for heat transfer and thermal storage fluids in CSP[END_REF][START_REF] Xu | Basic properties of eutectic chloride salts NaCl-KCl-ZnCl2 and NaCl-KCl-MgCl2 as HTFs and thermal storage media measured using simultaneous DSC-TGA[END_REF][START_REF] Mohan | Thermo-economic analysis of high-temperature sensible thermal energy storage with different ternary eutectic alkali and alkaline earth metal chlorides[END_REF], which offer great prospects to power a supercritical carbon dioxide (s-CO2) Brayton cycle. The relatively high liquidus temperature of common binary chloride eutectics (e.g. 657°C for NaCl-KCl, 459°C for MgCl2-NaCl or 504°C for CaCl2-NaCl [START_REF] Myers | Thermal Energy storage using chloride salts and their eutectics[END_REF]) limits their utilization for industrial applications. Consequently, ternary chloride salt mixtures have been developed to further decrease the liquidus temperature without having a significant effect on the decomposition temperature [START_REF] Li | Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl2, CaCl2, ZnCl2 for heat transfer and thermal storage fluids in CSP[END_REF][START_REF] Xu | Basic properties of eutectic chloride salts NaCl-KCl-ZnCl2 and NaCl-KCl-MgCl2 as HTFs and thermal storage media measured using simultaneous DSC-TGA[END_REF][START_REF] Mohan | Thermo-economic analysis of high-temperature sensible thermal energy storage with different ternary eutectic alkali and alkaline earth metal chlorides[END_REF][START_REF] Mohan | Assessment of a novel ternary eutectic chloride salt for next generation high-temperature sensible heat storage[END_REF]. Among the different ternary systems investigated, NaCl-KCl-MgCl2 salt mixtures are particularly attractive, as such mixtures possess a wide operating temperatures range, adequate thermophysical properties and are quite inexpensive [START_REF] Mohan | Thermo-economic analysis of high-temperature sensible thermal energy storage with different ternary eutectic alkali and alkaline earth metal chlorides[END_REF][START_REF] Mohan | Assessment of a novel ternary eutectic chloride salt for next generation high-temperature sensible heat storage[END_REF][START_REF] Fernández | Corrosion evaluation of eutectic chloride molten salt for new generation of CSP plants. Part I: Thermal treatment assessment[END_REF]. However, structural materials (i.e. iron-and nickel-based alloys) experience severe corrosion in chloride salt mixtures, which renders their use as HTF very challenging [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF][START_REF] Kruizenga | Corrosion mechanisms in chloride and carbonate salts[END_REF][START_REF] Vignarooban | Vapor pressure and corrosivity of ternary metal-chloride molten-salt based heat transfer fluids for use in concentrating solar power systems[END_REF][START_REF] Liu | Corrosion behavior of Ni-based alloys in molten NaCl-CaCl2-MgCl2 eutectic salt for concentrating solar power[END_REF][START_REF] Sun | Corrosion behavior of 316SS and Ni-based alloys in a ternary NaCl-KCl-MgCl2 molten salt[END_REF][START_REF] Sun | Effect of alloying elements on the corrosion behavior of Ni-based alloys in molten NaCl-KCl-MgCl2 salt at different temperatures[END_REF][START_REF] Gomez-Vidal | Corrosion resistance of aluminaforming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests[END_REF][START_REF] Gomez-Vidal | Corrosion resistance of aluminaforming alloys against molten chlorides for energy production. II: Electrochemical impedance spectroscopy under thermal cycling conditions[END_REF]. In the event of damage, the costs of replacing the components are further increased by the unscheduled shutdown of the power plant. Therefore, an improved understanding of the prevailing corrosion mechanisms and mitigation strategies are decisive to ensure reliable and consistent power generation for the 30 years expected lifetime of a CSP plant [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Fernández | Corrosion evaluation of eutectic chloride molten salt for new generation of CSP plants. Part I: Thermal treatment assessment[END_REF][START_REF] Gomez-Vidal | Corrosion resistance of aluminaforming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests[END_REF]. It is known that the level of impurities (e.g. moisture, O2, metallic and oxide impurities or foreign ions) strongly affects the corrosion rates of metallic alloys in chloride melts [START_REF] Kruizenga | Corrosion mechanisms in chloride and carbonate salts[END_REF][START_REF] Ozeryanaya | Corrosion of metals by molten salts in heat-treatment processes[END_REF][START_REF] Guo | Corrosion in the molten fluoride and chloride salts and materials development for nuclear applications[END_REF]. Appropriate measures can therefore be taken to limit the corrosiveness of chloride melts: (i) utilization of an inert atmosphere (e.g. Ar or N2) as ullage gas, (ii) hindering the introduction of water in the system or include a drying step upon heating of the salt mixture and (iii) purifying the salt mixture before its introduction in the thermal energy storage system. Nevertheless, it is worth mentioning that the operation of a CSP plant requires very large amounts of salt as HTF (e.g. 8500 tons of nitrate salts in Gemasolar CSP plant [START_REF] Burgaleta | Gemasolar, the first tower thermosolar commercial plant with molten salt storage[END_REF]). The purification of such large amounts of salt at industrial scale with satisfactory results presents several challenges which are still to be overcome and decreases the overall economic viability of using chloride salt mixtures as HTF for the next generation of CSP plants.

Since MgCl2 is highly hygroscopic, this salt is very sensitive to moisture and special care should be taken when handling MgCl2 to limit the absorption of water molecules [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF][START_REF] Mohan | Thermo-economic analysis of high-temperature sensible thermal energy storage with different ternary eutectic alkali and alkaline earth metal chlorides[END_REF][START_REF] Vidal | Molten chloride technology pathway to meet the U.S. DOE SunShot initiative with Gen3 CSP[END_REF]. In practice, the hydration of MgCl2 at ambient temperature and its further hydrolysis reactions upon heating are unavoidable. The hydrolysis reactions of MgCl2 have been thoroughly investigated using differential thermal analysis (DTA) as well as differential scanning calorimetry (DSC) in the literature and the following reactions may occur upon heating [START_REF] Vidal | Molten chloride technology pathway to meet the U.S. DOE SunShot initiative with Gen3 CSP[END_REF][START_REF] Shoval | The effect of alkali halides on the thermal hydrolysis of magnesium chloride and magnesium bromide[END_REF][START_REF] Friedrich | Magnesium technology. Metallurgy, Design data, Applications[END_REF] with the progressive dehydration of hydrated phases: MgCl2.xH2O(s) ⇄ MgCl2.(x-y)H2O(s) + yH2O(g) T ≥ 117°C

MgCl2.2H2O(s) ⇄ MgCl2.H2O(s) + H2O(g) T ≥ 242°C

MgCl2.H2O(s) ⇄ MgCl2(s) + H2O(g) T ≥ 304°C

At about 182°C, the partial hydrolysis of MgCl2 following reaction (4) can also take place [START_REF] Vidal | Molten chloride technology pathway to meet the U.S. DOE SunShot initiative with Gen3 CSP[END_REF][START_REF] Shoval | The effect of alkali halides on the thermal hydrolysis of magnesium chloride and magnesium bromide[END_REF][START_REF] Friedrich | Magnesium technology. Metallurgy, Design data, Applications[END_REF]:

MgCl2.2H2O(s) ⇄ MgOHCl(s) + HCl(g) + H2O(g) T ≥ 182°C

Upon further heating at temperatures above 350°C and among a wide range of possible reactions the complete hydrolysis of MgCl2 into MgOHCl and its further dehydroxylation above 554°C occur [START_REF] Vidal | Molten chloride technology pathway to meet the U.S. DOE SunShot initiative with Gen3 CSP[END_REF][START_REF] Shoval | The effect of alkali halides on the thermal hydrolysis of magnesium chloride and magnesium bromide[END_REF][START_REF] Friedrich | Magnesium technology. Metallurgy, Design data, Applications[END_REF]:

MgCl2.H2O(s) ⇄ MgOHCl(s) + HCl(g) T ≥ 350°C

MgOHCl(s) ⇄ MgO(s) + HCl(g) T ≥ 554°C

Four major problems can therefore be considered upon thermal hydrolysis of MgCl2: (i) the formation of oxidizing MgOH + impurities through ionic dissociation of MgOHCl, (ii) the formation of magnesium oxide (MgO) within the melt modifying its thermophysical properties, (iii) the progressive loss of MgCl2 implying an alteration of the salt chemistry and of its liquidus temperature and (iv) the requirement to handle the acid gases (i.e.

hydrogen chloride, HCl) produced during the reactions. In addition, the acid gases released may accelerate the corrosion of metallic components as reported by Kruizenga [START_REF] Kruizenga | Corrosion mechanisms in chloride and carbonate salts[END_REF]. The systematic formation of hydrogen chloride gas during thermal hydrolysis of MgCl2 forces the Deacon reaction [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF] to proceed to the right in presence of oxygen (O2), i.e. generation of chlorine (Cl2):

4HCl(g) + O2(g) ⇄ 2Cl2(g) + 2H2O(g) (ΔrG°700°C = 14.3 kJ/mol) (7)
Reaction [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF] illustrates the influence of the ullage gas (i.e. partial pressure of O2) on the formation of Cl2. The generation of Cl2 in the system has a twofold effect: it can accelerate the corrosion of metallic alloys following the generalized reaction [START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF] and causes serious health hazards, which requires its neutralization with a strong basis (e.g. sodium hydroxide NaOH) before release in the environment.

M(s) + x/2 Cl2(g) ⇄ MClx(s,g) (8)
with M corresponding to one of the metal alloying elements [START_REF] Zahs | Chloridation and oxidation of iron, chromium, nickel and their alloys in chloridizing and oxidizing atmospheres at 400-700[END_REF].

The detrimental influence of O2 on the corrosion of metallic alloys in molten chlorides indicates that tolerable corrosion rates in molten chlorides can only be reached in environments with low partial pressure of O2 such as high-purity Ar or nitrogen (N2) atmospheres [START_REF] Kruizenga | Corrosion mechanisms in chloride and carbonate salts[END_REF][START_REF] Vignarooban | Vapor pressure and corrosivity of ternary metal-chloride molten-salt based heat transfer fluids for use in concentrating solar power systems[END_REF][START_REF] Liu | Corrosion behavior of Ni-based alloys in molten NaCl-CaCl2-MgCl2 eutectic salt for concentrating solar power[END_REF]. In a recent publication, Ding et al. investigated the influence of MgOH + impurities on the corrosion behavior of Incoloy 800H [START_REF] Ding | Molten chloride salts for next generation CSP plants: Electrolytical salt purification for reducing corrosive impurity level[END_REF]. The authors developed an electrochemical cell to purify a ternary NaCl-KCl-MgCl2 melt by forcing the reduction of MgOH + species into insoluble MgO at the cathode, following reaction [START_REF] Bonk | Impact of solar salt aging on corrosion of martensitic and austenitic steel for concentrating solar power plants[END_REF], and the oxidation of Mg into soluble Mg 2+ cations at the anode, following reaction [START_REF] Li | Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl2, CaCl2, ZnCl2 for heat transfer and thermal storage fluids in CSP[END_REF]. The overall electrochemical reaction can therefore be written as [START_REF] Xu | Basic properties of eutectic chloride salts NaCl-KCl-ZnCl2 and NaCl-KCl-MgCl2 as HTFs and thermal storage media measured using simultaneous DSC-TGA[END_REF]:

2MgOH + + 2e -→ 2MgO(s) + H2(g) (9)
Mg(s) → Mg 2+ + 2e - ( 10
) 2MgOH + + Mg(s) → 2MgO(s) + Mg 2+ + H2(g) (11)
The authors have shown that the corrosion rate of Incoloy 800H was significantly reduced by reducing the level of corrosive MgOH + impurities [START_REF] Ding | Molten chloride salts for next generation CSP plants: Electrolytical salt purification for reducing corrosive impurity level[END_REF]. Using atomic absorption spectroscopy (AAS), they analyzed the concentrations of alloying elements Cr, Fe and Ni dissolved in the melts after exposure of Incoloy 800H to molten NaCl-KCl-MgCl2 with and without purification. While Ni was detected neither in purified nor in impurified salts (AAS detection limit of 10 ppm in weight), the concentrations of Cr and Fe dissolved in the melt were higher in the impurified salt compared to that of in the purified salt. In addition, AAS analyses indicated that the concentration of Cr dissolved in the melts is always higher than the one of Fe. This indicates that the reactivity of Ni with the corrosive species is very low in comparison to Cr and Fe and that Cr is preferentially dissolved. This is in good agreement with the electromotive force (EMF) series in NaCl-KCl-MgCl2 in respect of the apparent standard potential electrodes (given in V vs. Pt 2+ /Pt at 475°C) of the couples Cr 2+ /Cr (-1.472), Fe 2+ /Fe (-1.259) and Ni 2+ /Ni (-0.868) [START_REF] Gaur | Standard electrode potentials in molten chlorides[END_REF]. Moreover, it also agrees with the standard Gibbs free energy of reaction between MgOHCl and Cr (-106.5 kJ/mol), Fe (-52.9 kJ/mol) and Ni (8.6 kJ/mol) at 700°C following reaction [START_REF] Mohan | Thermo-economic analysis of high-temperature sensible thermal energy storage with different ternary eutectic alkali and alkaline earth metal chlorides[END_REF] with thermodynamic data from Ref. [START_REF] Wagman | Selected values of chemical thermodynamic properties[END_REF]:

2MgOHCl(s) + M(s) → 2MgO(s) + MCl2(s) + H2(g) (12)
with M corresponding to Cr, Fe or Ni.

In a second study [START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF], Ding et al. concluded that the corrosion mechanisms of various alloys in molten NaCl-KCl-MgCl2 were controlled by electrochemical reactions rather than Cl2-induced corrosion under Ar atmosphere.

The corrosion attack of iron-and nickel-based materials in molten NaCl-KCl-MgCl2 was usually associated with the selective dissolution of Cr resulting in porous Cr-depleted layers of several microns depth from the substrates' surface [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF][START_REF] Sun | Corrosion behavior of 316SS and Ni-based alloys in a ternary NaCl-KCl-MgCl2 molten salt[END_REF][START_REF] Sun | Effect of alloying elements on the corrosion behavior of Ni-based alloys in molten NaCl-KCl-MgCl2 salt at different temperatures[END_REF]. The formation of MgOHCl species within NaCl-KCl-MgCl2 melts was identified as the most important factor influencing the corrosion resistance of alloys by Vidal and Klammer [START_REF] Vidal | Molten chloride technology pathway to meet the U.S. DOE SunShot initiative with Gen3 CSP[END_REF] as well as Fernández

and Cabeza [START_REF] Fernández | Corrosion evaluation of eutectic chloride molten salt for new generation of CSP plants. Part I: Thermal treatment assessment[END_REF]. This was further confirmed in a recent study where different MgOHCl concentrations in a commercial dehydrated carnallite obtained by different purification methods had a clear impact on the corrosion rates of Haynes 230 [START_REF] Zhao | Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications[END_REF].

In our study, the main objective was to elucidate the corrosion mechanisms in presence of such corrosive impurities and give corrosion rates to what can be considered as an unforeseen event (i.e. incomplete drying and purification process of the salt, rehydration of the salt or unintentional ingress of moisture or oxygen into the system). Ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy were selected for the molten chloride corrosion experiments since both alloys are TES containment material candidates for CSP applications [3].

Following our previous study focusing on the NaCl-KCl binary system [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF], the present study investigates the corrosion behavior of P91 steel and Inconel 600 in molten NaCl-KCl-MgCl2 at 700°C under Ar atmosphere for up to 300 h exposure. The influence of testing conditions on the corrosion rates of the alloys is discussed following a probabilistic approach. Related corrosion mechanisms are also presented in light of the experimental observations and thermodynamic considerations.

Experimental procedures

Materials

The nominal and tested chemical compositions of as-received ferritic-martensitic P91 steel (Vallourec) and Inconel 600 (VDM Metals) are compiled in Table 1. Optical emission spectrometry (OES) measurements were performed with a SPECTROMAXx analyzer from SPECTRO Analytical Instruments GmbH (Kleve, Germany).

Ferritic-martensitic P91 steel is widely employed in conventional and nuclear power plants and heat exchanger systems with an excellent combination of high temperature creep resistance up to 650°C, high thermal conductivity and good weldability [START_REF] Pandey | Evolution of phases in P91 steel in various heat treatment conditions and their effect on microstructure stability and mechanical properties[END_REF][START_REF] Zhang | Study on the softening mechanism of P91 steel[END_REF]. This material has also attracted interest for the development of piping components and storage tanks in CSP plants and offers a cost-efficient alternative to austenitic steels and nickelbased alloys [START_REF] Encinas-Sánchez | Electrochemical impedance spectroscopy (EIS): An efficient technique for monitoring corrosion processes in molten salt environments in CSP applications[END_REF][START_REF] Oskay | Scale formation and degradation of diffusion coatings deposited on 9% Cr steel in molten solar salt[END_REF]. Inconel 600 is an austenitic solution strengthened nickel-based alloy that has been extensively used over the past decades for steam generator tubes in the nuclear industry [START_REF] Zhang | Effect of precipitated carbides on the fretting wear behavior of Inconel 600 alloy[END_REF][START_REF] Scott | General corrosion and stress corrosion cracking of Alloy 600 in light water reactor primary coolants[END_REF]. Owing to its high nickel content and relatively low Cr content, Inconel 600 was also found to present superior corrosion resistance in chloridation-oxidation conditions (i.e. atmospheres rich in chlorine and hydrogen chloride) [START_REF] Zheng | Chloridation-oxidation of nine commercial high-temperature alloys at 800°C[END_REF][START_REF] Grabke | Role of alloying elements and carbides in the chlorine-induced corrosion of steels and alloys[END_REF] as well as in molten chlorides [START_REF] Kruizenga | Corrosion mechanisms in chloride and carbonate salts[END_REF]. The comparison of the corrosion behavior of these two reference industrial alloys is therefore expected to give valuable information for alloy selection and development for CSP applications. As-received P91 and Inconel 600 metallic plates were machined into rectangular samples of, respectively, 25 mm x 6 mm x 3 mm and 25 mm x 5 mm x 3 mm dimensions. The metallic samples were then ground with progressively finer SiC papers (P320, P800 and P1200), washed with distilled water and ultrasonically cleaned in ethanol before any other step. The as-received microstructure of both alloys was characterized before exposure following the same procedure as the one described in Ref. [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF]. In Fig. 1a, ferritic-martensitic P91 steel presents a characteristic tempered martensite obtained after normalizing and a subsequent tempering treatment [START_REF] Pandey | Evolution of phases in P91 steel in various heat treatment conditions and their effect on microstructure stability and mechanical properties[END_REF][START_REF] Zhang | Study on the softening mechanism of P91 steel[END_REF].

The electrochemical etching highlights the distribution of precipitates at the prior austenite grain boundaries (PAGBs) and at the martensite lath boundaries, corresponding to M23C6 carbides (M: Cr, Mn, Fe, Mo) and fine MX [M: V, Nb and X: C, N] carbonitrides [START_REF] Pandey | Evolution of phases in P91 steel in various heat treatment conditions and their effect on microstructure stability and mechanical properties[END_REF][START_REF] Zhang | Study on the softening mechanism of P91 steel[END_REF]. In Fig. 1b, as-received Inconel 600 shows large austenite grains with Cr-rich intergranular carbides formed after solution annealing and rapid cooling [START_REF] Zhang | Effect of precipitated carbides on the fretting wear behavior of Inconel 600 alloy[END_REF]. It should be mentioned here that no intragranular carbides were observed within the γ-solid solution matrix of Inconel 600 before exposure.

Corrosion tests in molten salt environment

NaCl, KCl and MgCl2 anhydrous salts (all > 99 wt.% purity) were purchased from Alfa Aesar, Germany. The in at.%). The theoretical and experimental eutectic temperatures are, respectively, 383 and 387°C [START_REF] Mohan | Thermo-economic analysis of high-temperature sensible thermal energy storage with different ternary eutectic alkali and alkaline earth metal chlorides[END_REF][START_REF] Mohan | Assessment of a novel ternary eutectic chloride salt for next generation high-temperature sensible heat storage[END_REF]. The plastic containers were carefully sealed with a tight moisture-barrier and stored in a desiccator between each use to prevent moisture absorption of the highly hygroscopic MgCl2 salt. However, it should be noted that the initial weighing of the salt was performed in ambient air, which most likely resulted in some water absorption before sealing of the container. This was taken into account in the interpretation of the results. Before exposure, the airtight containers with the salt mixtures were thoroughly mixed for 2 h in a turbular mixer to ensure their homogeneity in composition.

Based on the target temperature for the next generation of CSP plants, the temperature selected for the corrosion experiments was 700°C [START_REF] Sun | Corrosion behavior of 316SS and Ni-based alloys in a ternary NaCl-KCl-MgCl2 molten salt[END_REF]. All elements in contact with the salt mixture and the metallic samples (i.e.

crucibles, lids and rods, see Fig. 2a) were made of alumina to avoid galvanic couples and because of its chemical inertness in molten chlorides [START_REF] Ozeryanaya | Corrosion of metals by molten salts in heat-treatment processes[END_REF]. The alumina components were systematically baked in air at 1000°C for 24 h before the corrosion experiments following ISO 17245 [START_REF]International Standard ISO, Corrosion of Metals and Alloys -Test Method for High Temperature Corrosion Testing of Metallic Materials by Immersing in Molten Salt or Other Liquids Under Static Conditions[END_REF]. After a final grinding step with P1200 SiC paper, the metallic samples were rinsed with distilled water and ultrasonically cleaned in ethanol. The sample dimensions were then measured with a digital micrometer (mean of 5 different measurements with a maximum deviation of ± 5 µm). The samples were ultrasonically cleaned in ethanol and dried a second time right before embedding in the salt mixture.

As described in Ref. [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF] and illustrated in Fig. 2a, the metallic samples were hanged vertically in the alumina crucibles containing approximately 19 g of the eutectic salt mixture. The partial embedding of the samples in the salt mixture enabled the investigation of different regions (e.g. initially immersed in the salt or above the salt immersion level) as shown by Sarvghad et al. [START_REF] Sarvghad | Corrosion of Inconel 601 in molten salts for thermal energy storage[END_REF] and Bell et al. [START_REF] Bell | Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions[END_REF]. For each experiment, two samples of each alloy were placed in the hot zone of a horizontal furnace equipped with a quartz tube. After the disposition of the crucibles at room temperature, the furnace was purged with Ar (5 ppm water vapour + 2 ppm O2) at 12 L/h to Fig. 1. Light optical cross-section images of as-received (a) ferritic-martensitic P91 steel and (b) Inconel 600 alloy after electrochemical etching with 10% oxalic acid [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF]. The corresponding BSE cross-section images at higher magnification are given in the insets. PAGBs: prior austenite grain boundaries, GBs: grain boundaries.

remove the atmospheric contaminants. The Ar flow rate was then decreased to 1.8 L/h before the heating procedure and the absolute pressure of the furnace was maintained slightly above the atmospheric pressure.

Following Ding et al. [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF], the furnace was heated at 5°C/min up to 200°C, held at 200°C for 1 h to remove residual water and further heated at 5°C/min up to 700°C. The temperature was controlled with a Type-S thermocouple and monitored at 700±4°C during the experiment as per ISO 17245 [START_REF]International Standard ISO, Corrosion of Metals and Alloys -Test Method for High Temperature Corrosion Testing of Metallic Materials by Immersing in Molten Salt or Other Liquids Under Static Conditions[END_REF]. The isothermal corrosion tests were then carried out at 700°C for 20, 100 and 300 h under an Ar flow of 1.8 L/h.

While a 1 h step at 200°C and a slow heating ramp until melting is acknowledged to fully dry NaCl-KCl mixtures in flowing inert gas without undergoing hydrolysis reactions [START_REF] Smith | Hydration of plutonium oxide and process salts, NaCl, KCl, CaCl2, MgCl2: effect of calcination on residual water and rehydration[END_REF], the presence of MgCl2 drives thermal hydrolysis as described in the sequence reactions (1)-( 5). It was previously reported by Ding et al. [START_REF] Ding | Cyclic voltammetry for monitoring corrosive impurities in molten chlorides for thermal energy storage[END_REF] that the subsequent heating procedure results in a concentration of the corrosive MgOHCl impurity of approximately 2000 ppm in molten NaCl-KCl-MgCl2. Zhao et al. [START_REF] Zhao | Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications[END_REF] recently demonstrated that commercially available salts such as "dehydrated" carnallite already contains water leading to high concentrations of MgOHCl by hydrolysis in nontreated salts (≥ 5 wt.%). Purification methods including thermal treatment and chemical treatment with metallic Mg can significantly reduce the MgOHCl content in the melt, which offers promising insights to control the corrosiveness of MgCl2-containing salt mixtures for CSP applications providing that these procedures are economically viable [START_REF] Kruizenga | Corrosion mechanisms in chloride and carbonate salts[END_REF][START_REF] Vidal | Molten chloride technology pathway to meet the U.S. DOE SunShot initiative with Gen3 CSP[END_REF][START_REF] Shoval | The effect of alkali halides on the thermal hydrolysis of magnesium chloride and magnesium bromide[END_REF][START_REF] Ding | Molten chloride salts for next generation CSP plants: Electrolytical salt purification for reducing corrosive impurity level[END_REF][START_REF] Zhao | Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications[END_REF]. As already mentioned in the introduction, the main objective of our study was to elucidate the corrosion mechanisms in presence of such corrosive impurities and give corrosion rates to what can be considered as an unforeseen event (i.e. incomplete drying and purification process of the salt, rehydration of the salt or unintentional ingress of moisture or oxygen in the system). This is an important concern in case of ingress of moisture or oxygen in the system through flanges and valves, which may be expected during CSP plant operation. The presence of MgOHCl corrosive impurities in the system was therefore expected to play an important role in the corrosion mechanisms and to give valuable information on the importance of adequate handling of MgCl2 for large scale industrial applications.

Metallographic preparation of the samples after the corrosion tests

Right after the corrosion experiments, the corroded samples were pulled out from the alumina crucibles with special care to preserve salt residues and corrosion products for metallographic observation (see Fig. 2b). The corroded samples were then systematically cold-embedded in epoxy resin by vacuum impregnation to prevent absorption of water molecules and allow cross-section preparation. After curing, a water-free metallographic preparation using petroleum oil as lubricant [START_REF] Rahts | Water-free methods of preparation for the analyses of chlorides in the high temperature oxidation in chloride containing atmospheres[END_REF] was conducted to prevent the dissolution of water-soluble compounds. Grinding was carried out automatically down to P2400 SiC paper followed by a final polishing with 3 µm and 1 µm diamond solutions. Cross-sectioned samples were systematically cleaned with dry acetone containing molecular sieve and dried with hot air. To prevent moisture absorption, especially of the highly hygroscopic MgCl2 compound, the mounted samples were stored in a desiccator. Metallographic observations were carried out on "fresh" cross-sections right after polishing to limit the evolution of the salt mixture associated with the formation of hydrated compounds.

Unlike conventional cleaning methods using acids [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF] or deionized water [START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF][START_REF] Sun | Corrosion behavior of 316SS and Ni-based alloys in a ternary NaCl-KCl-MgCl2 molten salt[END_REF][START_REF] Sun | Effect of alloying elements on the corrosion behavior of Ni-based alloys in molten NaCl-KCl-MgCl2 salt at different temperatures[END_REF], such a water-free preparation preserves the corrosion products and permits the simultaneous characterization of the salt to examine, for instance, oxide or metal residues. As shown in our previous study [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF], the additional information preserved by these means permitted a further understanding of the corrosion mechanisms taking place upon exposure of the metallic materials in chloride melts.

Dimensional metrology and characterization methods

The determination of the corrosion rates of metallic alloys in molten chlorides is very challenging since oxide scale formation, intergranular attack or metal dissolution may occur simultaneously. In addition, the corrosion front of structural materials in molten chlorides is usually not uniform and depends, among other parameters, on local thermodynamic conditions, on the existence of temperature gradients and on microstructural heterogeneities in the alloys (i.e. precipitates, metallurgical defects). This combination of effects greatly challenges the reliability of gravimetric methods to evaluate the corrosion behavior of structural materials in molten chlorides as reported by other research groups [START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF][START_REF] Sun | Effect of alloying elements on the corrosion behavior of Ni-based alloys in molten NaCl-KCl-MgCl2 salt at different temperatures[END_REF][START_REF] Bell | Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions[END_REF]. It also appears counter-intuitive to give average corrosion rates as a benchmark of the different alloys since local corrosion attacks, such as intergranular attack, may lead to premature failure of structural components at elevated temperatures by stress corrosion cracking (SCC) despite an acceptable average corrosion rate [START_REF] Walczak | Materials corrosion for thermal energy storage systems in concentrated solar power plants[END_REF][START_REF] Bell | Corrosion mechanisms in molten salt thermal energy storage for concentrating solar power[END_REF]. To overcome these limitations, dimensional metrology appears to give satisfactory results by measuring the metal loss of the substrate affected by corrosion with a probabilistic approach following ISO 26146 [START_REF]International Standard ISO, Corrosion of Metals and Alloys -Method for Metallographic Examination of Samples After Exposure to High-temperature Corrosive Environments[END_REF].

Dimensional metrology was initially developed to characterize the corrosion behavior of metallic samples exposed to Type-II hot corrosion experiments due to the complexity of the corrosion processes [START_REF] Nicholls | Modelling hot corrosion in industrial gas turbines[END_REF][START_REF] Sumner | Type II hot corrosion: Behavior of CMSX-4 and IN738LC as a function of corrosion environment[END_REF][START_REF] Grégoire | Correlations between the kinetics and the mechanisms of hot corrosion of pure nickel at 700[END_REF]. The aim of the dimensional metrology method is to accurately compare the wall thickness of the metallic samples before and after exposure to substantiate the extent of metal loss associated with corrosion. Following ISO 26146 [START_REF]International Standard ISO, Corrosion of Metals and Alloys -Method for Metallographic Examination of Samples After Exposure to High-temperature Corrosive Environments[END_REF], the initial sample thickness t0 was accurately measured with a digital micrometer before exposure. After exposure, the minimum remaining metal thickness trm unaffected by corrosion (i.e. intergranular attack, metal dissolution or microstructural instability) was measured at x50 magnification with a LEICA DMLA microscope using Imagic IMS software. Up to 100 individual measurements were performed for each area of investigation. The corresponding metal loss (ML) values were then obtained by Eq. ( 13):

ML = (t0 -trm)/2 (13)
The experimental data set was then plotted on probability plots, following ISO 26146 [START_REF]International Standard ISO, Corrosion of Metals and Alloys -Method for Metallographic Examination of Samples After Exposure to High-temperature Corrosive Environments[END_REF], with a total of 200 measurements for each testing condition (two individual samples per alloy and per exposure time). As shown in the first part of this study [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF], these probability plots are particularly attractive to depict if the data is fitting a Gaussian distribution (i.e. straight line whose slope corresponds to the standard deviation of the data set). Any -9-deviation from a straight line indicates a second distribution in the data set and can be interpreted, for example, as pitting, intergranular corrosion or passivation. This statistical approach overcomes the limitations of gravimetric methods and gives reliable corrosion data with a high degree of confidence.

After the molten salt experiments, the residual salt mixtures left in the alumina crucibles were systematically analyzed by X-ray diffraction (XRD) to identify the crystal structures of the compounds formed. To this end, the salt residues were first milled in an agate mortar with a pestle and placed in the diffractometer at ambient temperature. The analyses were performed with a BRUKER D8 Advance diffractometer with LYNXEYE 1D mode in the Bragg-Brentano configuration using Cu Kα1 radiation (0.15406 nm). Scanning electron microscopy (SEM)

and energy-dispersive X-ray spectroscopy (EDX) analyses were carried out at 20.0 kV with a PHILIPS XL40 microscope equipped with an EDAX detector. For observations in high vacuum, the cross-sectioned samples were coated with a thin conductive layer of graphite. To further assess the composition of oxide compounds, a RENISHAW inVia micro-spectrometer equipped with Nd:YAG (λexc = 532.0 nm) and He-Ne (λexc = 632.8 nm) laser sources was used for Raman and fluorescence analyses.

Results

Characterization of the residual salt mixtures after exposure

To identify the different compounds present in the salt mixtures after exposure, a fraction of the salt residues left in each alumina crucible was prepared for XRD analyses. The corresponding XRD patterns obtained after 20, 100 and 300 h exposure to molten NaCl-KCl-MgCl2 at 700°C are given in Fig. 3a (P91) and Fig. 3b (Inconel 600).

Regardless of the substrate composition and of the exposure time, similar diffraction patterns were obtained. In addition to the crystal structures of NaCl (Halite, cubic), MgCl2 (trigonal) and KMgCl3 (Perovskite, orthorhombic), hydrated compounds (i.e. Bischofite MgCl2.6H2O and Carnallite KMgCl3.6H2O) were identified. The partial transformation of the salt mixtures into Bischofite and Carnallite, as a consequence of the high hygroscopicity of MgCl2 [START_REF] Smith | Hydration of plutonium oxide and process salts, NaCl, KCl, CaCl2, MgCl2: effect of calcination on residual water and rehydration[END_REF], occurred during the XRD analyses in air after milling of the salt residues. Milling of the salt mixtures and XRD measurement should have been performed in an inert atmosphere to prevent moisture absorption.

However, the chosen conditions were more in line with our objective to preserve the salt residues for microstructural characterization since the transfer of the samples between a desiccator and the characterization methods will always result in some water absorption by MgCl2. This highlights the precautions needed at industrial scale to handle the very large amount of salts required for CSP plant operation and the pressure on adequately performing the drying and/or purification process before introduction of the salt mixture in the system (i.e. storage tanks). In addition to salt compounds, the cubic crystal structure of MgO was systematically identified in the diffraction patterns (Fig. 3a andb on the outer region (Fig. 4a andc). This experimental observation is of utmost importance and will serve as a basis to explain the mechanisms of formation of MgCr2O4 spinel by synergistic dissolution in section 4.3. Whereas quite pure NaCl crystals were randomly formed after cooling (Fig. 4d), the remaining salt mixture was rich in Cl, Mg, K and O suggesting the formation of KMgCl3 and of hydrated compounds (i.e. Bischofite and Carnallite). These observations are in good agreement with the existence of the KMgCl3-NaCl eutectic transformation at about 405°C [START_REF] Fink | Quaternary compounds in the system NaCl-KCl-MgCl2[END_REF] upon cooling of a ternary NaCl-KCl-MgCl2 salt mixture. In Fig. 4c andd MgO, i.e. for the cathodic reduction of oxidizing MgOH + species following reaction [START_REF] Bonk | Impact of solar salt aging on corrosion of martensitic and austenitic steel for concentrating solar power plants[END_REF] as previously reported by Ding et al. on a tungsten cathode [START_REF] Ding | Molten chloride salts for next generation CSP plants: Electrolytical salt purification for reducing corrosive impurity level[END_REF].

Metal loss measurements and corrosion rates

The metal loss measurements of P91 ferritic-martensitic steel and Inconel 600 nickel-based alloy after exposure in molten NaCl-KCl-MgCl2 at 700°C are reported as probability plots in Fig. 5a and b following ISO 26146 [START_REF]International Standard ISO, Corrosion of Metals and Alloys -Method for Metallographic Examination of Samples After Exposure to High-temperature Corrosive Environments[END_REF]. Regardless of the substrate and of the exposure time, the metal loss values are systematically greater on the top side (i.e. above the salt immersion level) than on the immersed side (i.e. below the salt immersion level) of the samples. For each data set, the fraction of the surface enduring pitting corrosion can be estimated from the probability plots as a deviation from a Gaussian distribution (i.e. straight line with a slope corresponding to the standard deviation of the data set) towards higher metal loss values [START_REF]International Standard ISO, Corrosion of Metals and Alloys -Method for Metallographic Examination of Samples After Exposure to High-temperature Corrosive Environments[END_REF]. Whereas Inconel 600 was not prone to pitting corrosion in all testing conditions, up to 4 % of the surface of P91 steel showed pitting corrosion after 20 h exposure (Fig. 5a). This value decreases to about 2-3 % for P91 after 100 and 300 h exposure on the top side. For P91 steel, a maximum corrosion attack of 130 µm was measured after 300 h exposure (Fig. 5a). Despite the fact that Inconel 600 is not prone to pitting corrosion, the probability plots indicate that this substrate is, overall, more sensitive to corrosion in molten NaCl-KCl-MgCl2 than P91 steel. A maximum corrosion attack of 145 µm was measured for Inconel 600 after 300 h exposure (Fig. 5b).

To illustrate the corrosion behavior of the two alloys, cross-section light optical micrographs were taken at different magnifications and are given in Fig. 5c and d after 300 h exposure to molten NaCl-KCl-MgCl2 at 700°C.

Extreme care was taken during the water-free preparation of the samples to preserve the corrosion scales and the residual salt mixture attached to the samples. As shown in the low magnification micrographs in Fig. 5c andd, the salt mixture covers the entire surface of the metallic samples after exposure. This most likely indicates that the salt melt wetted all surfaces of the partially-embedded samples. As detected by XRD (cf. Fig. 3), oxide scales are observed within the salt and at the interface between the salt and the metallic substrate. The micrographs at higher magnification were taken in representative areas of the samples to highlight the corroded regions of the substrates associated with void formation (Fig. 5c andd). As observed on the probability plots, the extent of the corrosion attack was, overall, greater on Inconel 600 than on P91 steel, which seems to contradict recent literature data on molten chloride corrosion of iron-and nickel-based alloys. The reasons for the lower corrosion resistance of Inconel 600 compared to P91 steel in static conditions will be discussed in section 4.4. As described in ISO 26146 [START_REF]International Standard ISO, Corrosion of Metals and Alloys -Method for Metallographic Examination of Samples After Exposure to High-temperature Corrosive Environments[END_REF], the median metal loss and corresponding standard deviation values were determined from the probability plots presented in Fig. 5a andb. The results are summarized in Fig. 6a andb (solid columns) and compared with the metal loss data obtained after exposure to molten NaCl-KCl in the first part of this study (hatched columns). As aforementioned, both P91 steel and Inconel 600 show higher metal loss values on the top side than on the immersed side after exposure to the ternary mixture. This was also observed after exposure of P91 to the NaCl-KCl binary mixture (Fig. 6a). On the contrary, the opposite feature was noticed for Inconel 600 when exposed to the NaCl-KCl binary mixture since the largest metal loss values were recorded on the immersed side (Fig. 6b). The standard deviation values are particularly large for P91 samples exposed to molten NaCl-KCl after 100 and 300 h exposure. As shown in Ref. [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF], the large spread in metal loss values can be correlated to pitting corrosion in the form of large pits (up to 500 µm deep) after 300 h of exposure. Nevertheless, Inconel 600 is overall more sensitive to corrosion than P91, in particular in the ternary salt mixture. Regardless of the metallic substrate, the metal loss values were significantly lower after exposure in the ternary mixture than in the binary one. This counterintuitive observation indicates that the presence of the hygroscopic compound MgCl2 in the salt mixture, subjected to hydrolysis reactions, does not necessarily result in higher corrosion rates than mixtures which do not contain this compound. The reasons behind the lower corrosiveness of NaCl-KCl-MgCl2 melt than NaCl-KCl melt in static conditions will be one of the subjects of the discussion. However, it should be remembered that these experiments were performed in static conditions and that such effects may not be necessarily transferred to thermal cycling conditions or in a dynamic salt flow by combined erosion effects.

To compare the corrosion behavior of alloys in different testing conditions, corrosion rates are commonly expressed in µm/year. Therefore, the corrosion rates of both P91 and Inconel 600 alloys exposed to molten NaCl-KCl-MgCl2 at 700°C were calculated on the immersed side and on the top side from the iso-probability curves of metal loss values as a function of exposure time (Fig. 7). For each iso-probability curve, the points indicate the experimental data and the lines the best fit curves following the power-law model [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF][START_REF] Melchers | Predicting long-term corrosion of metal alloys in physical infrastructure[END_REF]: where n is the exponential coefficient and c an empirical constant (in this study c = 0). In this study, the n value was adjusted empirically to fit the median values with a precision of ± 0.05 and then kept constant to fit the other iso-probability curves. Despite the empirical nature of this model, the probabilistic approach followed in this study enables sound estimates of variability with regard to corrosion of the two investigated alloys. In addition, the n value gives a good approximation of the physical observations where n < 0.5 corresponds to a protective behavior, n = 0.5 to a purely Fickian diffusion process (i.e. parabolic-law), 0.5 < n < 1 to a non-protective behavior, n = 1 to a linear corrosion process and finally to breakaway corrosion when n > 1.

ML = k.t n + c (14)
For all testing conditions and for both alloys (Fig. 7), n values are all < 0.5, which indicates a relatively protective behavior. Nevertheless, the corrosion rates calculated from the 5%, median and 95% iso-probability curves are not acceptable regarding the requirements for industrial applications (≈ 15-30 µm/year based on a components' lifetime target of 30 years [3,[START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF][START_REF] Gomez-Vidal | Corrosion resistance of aluminaforming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests[END_REF]). As aforementioned, P91 steel is more resistant than Inconel 600 to molten NaCl-KCl-MgCl2 at 700°C with both smaller n values and lower corrosion rates (see Fig. 7). Notwithstanding the alloy composition and the initial boundary conditions (top side or immersed side), the iso-probability curves depict initially high corrosion rates in the first 20 h of exposure and, later, steady-state corrosion rates, which is characteristic of impurity-driven corrosion mechanisms [START_REF] Bell | Corrosion mechanisms in molten salt thermal energy storage for concentrating solar power[END_REF][START_REF] Sridharan | Molten salts chemistry[END_REF]. As reported by Ding et al. [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF] and other research groups [START_REF] Fernández | Corrosion evaluation of eutectic chloride molten salt for new generation of CSP plants. Part I: Thermal treatment assessment[END_REF][START_REF] Vidal | Molten chloride technology pathway to meet the U.S. DOE SunShot initiative with Gen3 CSP[END_REF][START_REF] Zhao | Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications[END_REF], this can be associated with the formation of corrosive MgOH + impurities upon thermal hydrolysis of MgCl2. It should also be noted, that the calculated corrosion rates are systematically higher on the observed at the substrate/salt interface. To a lower extent, porous MgCr2O4 scales, with an intermediate contrast in BSE mode, are also observed on the outer region of the samples (Fig. 8a, c ande). It is worth mentioning, that traces of Si, V, Mn and Fe (total amount < 2 wt.%) were also detected by EDX analyses within MgCr2O4 scales.

Raman micro-spectrometry was used to ascertain the composition of the oxide scales and Fig. 9 presents the representative spectra obtained when analyzing MgO and MgCr2O4 scales. In MgO scales, the characteristic fluorescence signal of Cr 3+ ions was systematically detected with a total of ten emission bands including the R and N lines, three band-pairs (αr, αb; βr, βb; γr, γb) and the weak band 2βr corresponding to the double of the frequency separation between the sharp R line and βr [START_REF] Deutschbein | Die lienenhafte emission und absorption der chromosphore[END_REF]. Further information is given in Appendix with the expected position of the fluorescence bands calculated from the work of Deutschbein [START_REF] Deutschbein | Die lienenhafte emission und absorption der chromosphore[END_REF] and their experimental observation for both He-Ne laser (632.8 nm) and Nd:YAG laser (532.0 nm). When analyzing MgCr2O4 scales, the three main vibrational bands of magnesiochromite spinel at 542, 605 and 684 cm -1 were successfully identified [START_REF] Wang | High pressure Raman spectroscopic study of spinel MgCr2O4[END_REF][START_REF] Ippolito | Raman fingerprint of chromate, aluminate and ferrite spinels[END_REF]. However, the R and N lines associated with the fluorescence of MgO:Cr 3+ are also observed in such locations indicating that the porous scales identified as the magnesiochromite spinel are, in fact, a mixture of MgO and MgCr2O4.

Despite the relatively protective behavior of the MgO scales formed at the substrate/salt interface, grain boundary attack is unambiguously observed on the BSE cross-section images of Fig. 8. As observed in molten NaCl-KCl [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF], the corrosion propagates at the prior-austenite grain boundaries (i.e. PAGBs) formed during the heat treatment of P91 steel carried out to confer its mechanical strength [START_REF] Pandey | Evolution of phases in P91 steel in various heat treatment conditions and their effect on microstructure stability and mechanical properties[END_REF][START_REF] Zhang | Study on the softening mechanism of P91 steel[END_REF]. This results in the selective removal of Cr-rich carbides from the tempered martensitic microstructure. Furthermore, Fe-rich metallic particles are embedded in the oxide scales on Fig. 8b andc. The low Cr content of such particles, < 1 wt.% Cr according to EDX spot measurements, suggests that some Cr-depleted grains of the substrate became loose upon exposure. After 300 h of exposure (Fig. 8e andf), the metallographic observation of the oxide scales was challenged by the high hygroscopicity of the salt mixture and, most likely, of MgCl2 leading to the formation of hydrated compounds such as bischofite and carnallite as observed by XRD (Fig. 3). The absorption of water molecules from the samples after metallographic preparation and their subsequent desorption while vacuuming the SEM chamber induce volume changes, which may be responsible for the difficulties of observation. Nevertheless, it is also possible that the formed MgO scale loses its relatively protective behavior over time, for instance with the build-up of stresses within the growing scale, which might lead to crack formation and/or spallation and thus provide easy access of the electrolyte towards the metallic substrate.

Additional cross-section images of P91 samples after 20 h exposure to molten NaCl-KCl-MgCl2 at 700°C are presented in Fig. 10. At higher magnification, it appears that the MgO scales were undoubtedly in contact with the underlying substrate at temperature. Lateral decohesion observed at the substrate/scale interface (gap filled with either resin (Fig. 10a, b andd) or salt (Fig. 10c)), most likely resulted from mechanical stresses developed upon cooling due to the mismatch of thermal expansion coefficients and due to volume changes during solidification of the salt mixture. Alternatively, such decohesion can also be attributed to the metallographic preparation of the samples including cold-embedding by vacuum impregnation and cross-section polishing. On the outer region of the samples (Fig. 10a andc), MgCr2O4 spinel appearing with an intermediate contrast in BSE was observed within the MgO scales. Interestingly, longitudinal cracks can be observed at the interface between MgO and MgCr2O4 spinel. As aforementioned, these cracks may be a consequence of the high CTE mismatch between MgO (≈10.4x10 - 6 K -1 at 25°C [START_REF] Madhusudhan Rao | Studies on thermophysical properties of CaO and MgO by γ-ray attenuation[END_REF]), P91 steel (≈10.5x10 -6 K -1 at 25°C [START_REF] Haarmann | The T91/P91 book[END_REF]) and MgCr2O4 spinel (≈5.7x10 -6 K -1 at 25°C [START_REF] Wang | In situ high-temperature powder X-ray diffraction study on the spinel solid solutions (Mg1-xMnx)Cr2O4[END_REF]) inducing high compressive stresses on MgCr2O4 spinel upon cooling of the samples. As observed in Fig. 8, intergranular attack located at the prior-austenite grain boundaries (PAGBs) is visible to a depth of several microns. In addition, fine Mo-rich particles, appearing bright in BSE, are distributed within the MgO scales in Fig. 10a, b and c.

Inconel 600 nickel-based alloy

Whereas relatively continuous and adherent MgO scales formed on P91 (Fig. 8), MgO crystals surrounded with MgCr2O4 are found to be dispersed in the salt mixture for Inconel 600 (Fig. 11). The SEM observations indicate that the volume fraction of MgCr2O4 in the salt is larger on the outer region (Fig. 11a, c ande) than on the immersed region (Fig. 11b, d andf). As commonly observed after exposure of nickel-based alloys to molten chlorides [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Liu | Corrosion behavior of Ni-based alloys in molten NaCl-CaCl2-MgCl2 eutectic salt for concentrating solar power[END_REF][START_REF] Sun | Corrosion behavior of 316SS and Ni-based alloys in a ternary NaCl-KCl-MgCl2 molten salt[END_REF][START_REF] Sun | Effect of alloying elements on the corrosion behavior of Ni-based alloys in molten NaCl-KCl-MgCl2 salt at different temperatures[END_REF][START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF][START_REF] Abramov | Corrosive resistance of nickel Hastelloy G-35 superalloy in various aggressive media[END_REF], the corrosion attack resulted in a porous microstructure depleted in Cr (i.e. average concentration of Cr < 4 wt.% according to EDX spot analyses in the alloy matrix). The increasing depth of the corrosion attack over time, associated with a porous morphology, exceeds 80 µm after 300 h exposure (Fig. 11e and f), which indicates the poorly protective behavior of the oxide scales that are unable to inhibit the corrosion attack. Whereas the corrosion attack propagated through the grains of the Inconel 600, grain boundary attack is also observed, for instance, in Fig. 11d. Since Cr-rich carbides were only present at the grain boundaries before exposure (see Fig. 1b), these observations imply that microstructural changes occurred within the γ-(Ni) solid solution matrix of Inconel 600 over time and that secondary phases formed during isothermal exposure at 700°C. BSE cross-section images at higher magnification are given in Fig. 12 after 300 h exposure. As aforementioned, the corrosion attack resulted in a porous morphology where corrosion products precipitated in the subsurface pores to a depth of about 20 µm in the outer region of the sample (Fig. 12a). EDX spot analyses were performed in such pores and the mean concentration of the main elements was 58.8O-1.8Si-18.6Mg-2.1Ti-15.1Cr-0.5Fe-1.9Ni in at.%. This suggests the presence of both MgO and MgCr2O4 within the pores as detected by XRD by Ding et al. after 500 h exposure of In800H at 700°C in molten NaCl-KCl-MgCl2 [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF]. The detection of higher-valence elements than Cr (i.e. Si and Ti) may indicate the formation of MgSiO3, as observed after 500 h exposure of stainless steel 310 at 700°C in molten NaCl-KCl-MgCl2 in the same study [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF], or MgTiO3 perovskites. Nevertheless, complementary characterization techniques should be employed to ascertain these presumptions. Whereas both MgO and MgCr2O4 compounds were present in the corrosion products on the substrate surfaces, the fraction of MgCr2O4 was significantly higher in the outer region (Fig. 12a) than on the immersed region (Fig. 12b). This most likely highlights the influence of O2 availability (i.e. the partial pressure of oxygen) on the mechanisms of formation of MgCr2O4. These mechanisms will be presented in section 4.3.

Despite the differences in composition of the corrosion products, similar observations can be made within the alloy at the corrosion front (Fig. 12c andd). In fact, the microstructural instability of Inconel 600 at 700°C led to the precipitation of semi-continuous Cr-rich carbides at the grain boundaries and, most importantly, within the grains (i.e. intragranular carbides). The precipitation of Cr-rich carbides within the γ-(Ni) solid solution matrix of Inconel 600 was systematically observed below the corrosion front, as reported in the first part of this study [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF] and by other research groups on various nickel-based alloys [START_REF] Bell | Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions[END_REF][START_REF] Abramov | Corrosion of corrosion-resistant and high-temperature nickel-based alloys in chloroaluminate melts[END_REF]. The propagation of the corrosion attack resulted in the progressive loss of the austenitic grain microstructure of Inconel 600 together with void formation as observed in Fig. 11. The metallographic observations strongly support the hypothesis, that the Cr-rich carbides were selectively dissolved from the γ-(Ni) solid solution matrix, leading to the formation of voids. It is worth mentioning, that salt particles are randomly distributed within the voids/pores (Fig. 12c andd). Since no water was employed during the metallographic preparation of the samples to prevent dissolution of the salt, this indicates that the chloride melt was penetrating through the scales and entering the pores in the metal through capillary effects as proposed by Bell et al. [START_REF] Bell | Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions[END_REF].

Discussion

Impurity-driven corrosion mechanisms: role of corrosive MgOH + impurities

The iso-probability corrosion curves presented in Fig. 7 follow a particular trend associated with "impuritydriven" corrosion [START_REF] Bell | Corrosion mechanisms in molten salt thermal energy storage for concentrating solar power[END_REF][START_REF] Sridharan | Molten salts chemistry[END_REF]. As presented in Fig. 13, this is characterized by a transition between initially high corrosion rates within the first hours of exposure to steady-state corrosion rates when the concentration of impurities decreased to negligible amounts and/or when the corrosion becomes driven by a different mechanism [START_REF] Bell | Corrosion mechanisms in molten salt thermal energy storage for concentrating solar power[END_REF]. Considering the NaCl-KCl-MgCl2 ternary mixture, the oxidizing MgOH + impurities resulting from the hydrolysis reactions of MgCl2 upon heating (see reactions ( 4) and ( 5)) were identified as the most important oxidant by Ding et al. [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF] and other groups [START_REF] Fernández | Corrosion evaluation of eutectic chloride molten salt for new generation of CSP plants. Part I: Thermal treatment assessment[END_REF][START_REF] Vidal | Molten chloride technology pathway to meet the U.S. DOE SunShot initiative with Gen3 CSP[END_REF][START_REF] Zhao | Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications[END_REF]. The cathodic reaction of oxidizing MgOH + impurities into MgO on metallic alloys, following reaction [START_REF] Mohan | Assessment of a novel ternary eutectic chloride salt for next generation high-temperature sensible heat storage[END_REF], implies an opposite anodic reaction to maintain electric charge neutrality. The latter is therefore associated with metal dissolution from the substrate (see Fig. 13), with the potential electrode series in molten chlorides of the main constituting elements being in the order Cr 2+ /Cr < Fe 2+ /Fe < Ni 2+ /Ni [START_REF] Gaur | Standard electrode potentials in molten chlorides[END_REF]. Cr is then selectively dissolved from the substrate whereas Ni dissolution will be very limited and can thus be disregarded. This results in the partial anodic reactions ( 16) and ( 17) for Cr and Fe, respectively. Therefore, the impurity-driven corrosion mechanisms represented in Fig. 13 can be summarized with the overall electrochemical reactions ( 18) and [START_REF] Liu | Corrosion behavior of Ni-based alloys in molten NaCl-CaCl2-MgCl2 eutectic salt for concentrating solar power[END_REF].

Partial cathodic reaction:

2MgOH + + 2e -→ 2MgO(s) + H2(g) (15)
Partial anodic reactions:

Cr → Cr 2+ + 2e - (16)
Fe → Fe 2+ + 2e - Even though all reactions ( 22)- [START_REF] Guo | Corrosion in the molten fluoride and chloride salts and materials development for nuclear applications[END_REF] are thermodynamically favored at 700°C (i.e. ΔrG° < 0), the stability of gaseous chlorides will depend on the local O2 partial pressure as depicted in Fig. 14. Despite the evidence for both Cr and Fe dissolution from immersed P91 samples (Fig. 8 and Fig. 10), only a small fraction of Fe (< 0.5 at.% according to EDX spot measurements) was detected within MgCr2O4 scales. In addition, the investigation of the salt mixture did not reveal the presence of iron oxides (see XRD analyses in Fig. 3 and SEM observations in Fig. 4).

This suggests that the O2 partial pressure within the melt was too low to significantly oxidize FeCl2 and that FeCl2 was continuously evaporating from the scale to the melt while MgO was progressively formed on the surface.

However, a partial pressure of about 10 -32 atm. is sufficient to form Cr2O3 at 700°C and, as observed within the corrosion products and the salt mixture, this led to the formation of MgCr2O4 spinel together with MgO. The formation of magnesiochromite is thermodynamically favored at 700°C:

MgO(s) +Cr2O3(s) ⇄ MgCr2O4(s) (ΔrG°700°C = -31.7 kJ/mol) (26)
The fraction of MgCr2O4 spinel within the corrosion products was significantly larger on the outer region of the samples, i.e. initially above the immersion level of the salt (see Fig. 2a), and more typically for Inconel 600 (Fig. 11a, c ande). Since O2 is introduced in the system by the Ar flow (2 ppm impurities in the Ar bottle according to the furnisher), its dissolution in the melt and diffusion towards the metal is expected to be faster when a thin layer of salt is formed (creeping of the salt above the immersion level of the salt) than when a large reservoir of salt is present (bottom edge of the sample deep in the salt). It can therefore be concluded that the mechanisms of formation of MgCr2O4 are intrinsically linked with the availability of O2 and will be further discussed in section 4.3.

In addition to reacting with gaseous metal chlorides, O2 is also expected to increase the cathodic activity following reaction [START_REF] Shoval | The effect of alkali halides on the thermal hydrolysis of magnesium chloride and magnesium bromide[END_REF], thereby increasing the electrochemical dissolution rate of Cr from the alloy. The production of O 2-in the melt will in turn change the local acidic/basic character of the melt and potentially lead to fluxing mechanisms of the oxide scales as predicted and validated electrochemically by Rapp and coworkers in molten sulfates [START_REF] Lux | Saüren» und «Basen» im schmelzfluss: die Bestimmung der sauerstoffionen-konzentration[END_REF][START_REF] Flood | The acidic and basic properties of oxides[END_REF]. As already mentioned, a low concentration of Fe was measured in the scales indicating that a significant fraction of FeCl2 did not react with dissolved oxygen. Although gaseous FeCl2 molecules may have been swept away by the Ar flow, it is also possible that a fraction of FeCl2 remained in the salt and formed ternary compounds such as Rinneite (i.e. 3KCl.NaCl.FeCl2) after cooling of the samples as shown in [START_REF] Fink | Quaternary compounds in the system NaCl-KCl-MgCl2[END_REF] and in our previous study [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF]. Fig. 4b is particularly interesting since Ni-rich particles, coming from Inconel 600 substrate, were embedded in MgO. First, this indicates that Ni is thermodynamically stable in the electrolyte and that the Cl2 partial pressure does not exceed 10 -8 atm. within the melt (see Fig. 14). This also suggests that these Ni-rich particles may have acted as cathodic sites to foster the reduction of MgOH + impurities following reaction [START_REF] Mohan | Assessment of a novel ternary eutectic chloride salt for next generation high-temperature sensible heat storage[END_REF]. This hypothesis is in good agreement with the results of Ding et al. showing a deposited MgO-lump on a tungsten cathode after purifying a mixture of NaCl-KCl-MgCl2 [START_REF] Ding | Molten chloride salts for next generation CSP plants: Electrolytical salt purification for reducing corrosive impurity level[END_REF]. The estimated partial pressures of O2 and Cl2 in different locations of the samples are reported on Fig. 14 based on the experimental observations.

Whereas "chlorine-induced" corrosion mechanisms usually consider the direct reaction of Cl2 molecules with metallic alloys [START_REF] Zahs | Chloridation and oxidation of iron, chromium, nickel and their alloys in chloridizing and oxidizing atmospheres at 400-700[END_REF][START_REF] Grabke | Role of alloying elements and carbides in the chlorine-induced corrosion of steels and alloys[END_REF][START_REF] Bender | The role of alloying elements in commercial alloys for corrosion resistance in oxidizingchloridizing atmospheres. Part I: Literature evaluation and thermodynamic calculations on phase stabilities[END_REF] following adsorption and dissociation of the diatomic molecule, the formation of chloride compounds by ionic counter-migration is more likely to happen in molten chlorides since the Cl -anion has a smaller size and higher mobility than Cl2 [START_REF] Zhang | Chloride-and sulphate-induce hot corrosion mechanism of super austenitic stainless steel S13254 under dry gas environment[END_REF]. In addition, the Deacon reaction ( 7) has a positive Gibbs free energy of formation at 700°C and is not spontaneous at the testing temperature. Nevertheless, this reaction is spontaneous at temperatures lower than 600°C and it is possible that it takes place in a different region of the furnace (colder parts of the furnace instead of the hot zone). These considerations may be of great interest for industrial CSP plants during night/day cycles since the molten salt storage tanks are alternatingly filled and emptied. One could expect chlorine-induced corrosion in regions not immersed in the salt (where the adsorption of Cl2 molecules is favored), which could significantly impair the corrosion resistance of the structural materials.

Fluxing mechanisms and formation of MgCr2O4 spinel

Even though the formation of MgCr2O4 spinel has been reported nearly systematically during exposure of ironand nickel-based superalloys in molten chlorides [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Fernández | Corrosion evaluation of eutectic chloride molten salt for new generation of CSP plants. Part I: Thermal treatment assessment[END_REF][START_REF] Liu | Corrosion behavior of Ni-based alloys in molten NaCl-CaCl2-MgCl2 eutectic salt for concentrating solar power[END_REF][START_REF] Gomez-Vidal | Corrosion resistance of aluminaforming alloys against molten chlorides for energy production. II: Electrochemical impedance spectroscopy under thermal cycling conditions[END_REF], its mechanisms of formation are not fully explained and we therefore propose to bridge the gap in this study. As mentioned in section 4.2, the formation of MgCr2O4 was intrinsically linked to the availability of O2. For instance, the experimental observations indicate that the periphery of MgO crystals was in general converted into MgCr2O4 in the upper part of the salt (Fig. 4a andc) whereas such a conversion was marginally observed deep in the melt (Fig. 4b) where the amount of dissolved O2 has to be lower because of diffusion limitations from the cover gas. Another important observation is that unlike in molten NaCl-KCl [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF], Cr2O3 was not observed on all samples' surfaces and Cr was systematically present in the corrosion products as MgCr2O4 and in a small amount in MgO as confirmed by the fluorescence signals of Cr 3+ cations in MgO obtained by Raman micro-spectrometry (Fig. 9). This therefore indicates that an interaction between MgO and Cr2O3 systematically occurred and was accelerated by O2.

Since MgO is a basic oxide and Cr2O3 is a comparatively acidic oxide (according to Lux-Flood concept of acid/base reactions [START_REF] Lux | Saüren» und «Basen» im schmelzfluss: die Bestimmung der sauerstoffionen-konzentration[END_REF][START_REF] Flood | The acidic and basic properties of oxides[END_REF]), a synergistic dissolution of both oxides by exchange of O 2-is expected in molten salts following the works of Rapp and co-workers [START_REF] Hwang | Synergistic dissolution of oxides in molten sodium sulfate[END_REF][START_REF] Rapp | Hot corrosion of materials: fundamental studies[END_REF] (its occurrence depends on the local activity of O 2-anions).

This coupling is therefore expected to accelerate simultaneously the acidic fluxing of MgO [START_REF] Friedrich | Magnesium technology. Metallurgy, Design data, Applications[END_REF] and the basic fluxing of Cr2O3 (29) resulting in the formation of MgCr2O4 as a reaction product (30):

MgO(s) → Mg 2+ + O 2- (acidic fluxing) (28) Cr2O3(s) + O 2-→ 2CrO2 - (basic fluxing) (29) MgO(s) + Cr2O3(s) → Mg 2+ + 2CrO2 -→ MgCr2O4(s) (synergistic dissolution) (30)
The relative basicity of NaCl-KCl-MgCl2 ternary melt precludes the stability of Cr2O3 that systematically undergoes basic fluxing and transforms into MgCr2O4. However, the mechanisms must also take into consideration the initial nucleation of Cr2O3 on MgO to enable their synergistic dissolution. This may be because the nucleation of Cr2O3 is energetically favored on MgO serving as a support for reaction [START_REF] Gomez-Vidal | Corrosion resistance of aluminaforming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests[END_REF] in a region where the partial pressure of oxygen is sufficient to transform CrCl2 into Cr2O3. A striking observation to support this hypothesis is the growth of thick MgCr2O4 scales on the Al2O3 rods used to hang the samples vertically in the melt (see Fig. 2a) in the form of very large crystals (Fig. 15). It is very likely that the corundum structure of the Al2O3 rods acted as nucleation site for Cr2O3, which also possesses the corundum structure. Since the rod was in a region with a higher availability of oxygen (a thin film of salt was most likely wetting the rod upon exposure as observed in Fig. 15a The deviation of the magnesiochromite from the ideal MgCr2O4 stoichiometry probably explains its high electrical conductivity and its poisoning effect regarding the ohmic drop provided by MgO on the alloys' surfaces. For the sake of clarity, it was decided to note magnesiochromite as MgCr2O4 even though a more appropriate way of writing is (Mg,Fe 2+ )1+x(Cr,Fe 3+ )2-xO4.

In fact, the higher corrosion rates on the outer region (Fig. 7a andb) than on the immersed region of the samples (Fig. 7c andd) suggest that the formation of MgCr2O4 at the expense of MgO decreases the corrosion resistance of the alloys. The detrimental influence of MgCr2O4 may be a consequence of its high electrical conductivity (σ ≈ 10 -2 Ω -1 .cm -1 at 800°C [START_REF] Petric | Electrical conductivity and thermal expansion of spinels at elevated temperatures[END_REF]) in comparison to MgO (≈ 10 -11 Ω -1 .cm -1 at 700°C [START_REF] Lewis | The electrical conductivity of magnesium oxide at low temperatures[END_REF]). The formation of MgCr2O4 spinel would therefore inhibit the beneficial effect of MgO by increasing the charge-transfer rates between alloys and the electrolyte. Nevertheless, it should be mentioned that the presence of MgCr2O4 is alternatively a cause and a consequence of enhanced Cr dissolution from the substrate and that it is rather the availability of oxygen that plays a significant role in the overall corrosion mechanisms; (i) by reacting with gaseous metal chlorides releasing chlorine and (ii) by feeding the cathodic reaction.

Compositional and microstructural effects

The driving force for electrochemical corrosion is further increased with a larger difference of potentials between cathodic and anodic areas, an increased mobility of charges in the electrolyte and in the metal and a larger cathode/anode surface ratio. In the particular case of molten chlorides, the corrosion of metallic alloys is mainly attributed to the selective dissolution of Cr [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF][START_REF] Sun | Corrosion behavior of 316SS and Ni-based alloys in a ternary NaCl-KCl-MgCl2 molten salt[END_REF][START_REF] Sun | Effect of alloying elements on the corrosion behavior of Ni-based alloys in molten NaCl-KCl-MgCl2 salt at different temperatures[END_REF][START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF]. Therefore, compositional and microstructural features of the alloys, especially its Cr distribution, will greatly influence their corrosion behavior in molten chlorides. Since the corrosion experiments were carried out at 700°C, solid-state diffusion mechanisms within the alloys must be taken into account to further understand the corrosion mechanisms. The microstructural instability of metallic alloys was indeed reckoned to influence the propagation of the corrosion in molten chlorides [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF][START_REF] Abramov | Corrosive resistance of nickel Hastelloy G-35 superalloy in various aggressive media[END_REF][START_REF] Abramov | Corrosion of corrosion-resistant and high-temperature nickel-based alloys in chloroaluminate melts[END_REF] as well as in chloride-carbonate mixtures [START_REF] Bell | Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions[END_REF]. Bell et al. concluded that the vulnerability of nickel-based alloy 601 resulted from the dissolution of carbide forming elements Cr and Mn [START_REF] Bell | Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions[END_REF]. However, the same authors indicated that it is unclear if the carbide phases are being removed or if there is a link between the carbide formation and their proneness for dissolution.

In this study, metal loss measurements and metallographic observations clearly indicate that the ferriticmartensitic P91 steel is more resistant than Inconel 600 when exposed to molten NaCl-KCl-MgCl2 at 700°C. As revealed by electrochemical etching in Fig. 1a and elsewhere [START_REF] Pandey | Evolution of phases in P91 steel in various heat treatment conditions and their effect on microstructure stability and mechanical properties[END_REF][START_REF] Zhang | Study on the softening mechanism of P91 steel[END_REF], the tempered ferritic-martensitic microstructure of P91 shows carbides (mostly Cr-rich M23C6 carbides) at the prior austenite grain boundaries and between lath boundaries. While BSE cross-section images of P91 after exposure unambiguously revealed the selective removal of these Cr-rich carbides leaving voids mostly at the prior austenite grain boundaries after 100 and 300 h exposure (Fig. 8c-f), the Fe-rich metallic matrix also suffered corrosion in the early stage of exposure (Fig. 8a-b and Fig. 10). The systematic presence of Mo-rich particles within MgO scales (Fig. 10a-c), most likely corresponding to Mo-rich laves phases [START_REF] Peng | The effect of phase parameter variation on hardness of P91 components after service exposures at 530-550°C[END_REF] although further characterization techniques (e.g. TEM) should be employed, may indicate that they supported the cathodic reaction (reduction of corrosive MgOH + species). This is a very important finding, since several studies use Mo crucibles for molten chloride exposures. With a small cathode/anode surface ratio, the driving force for corrosion of P91 steel is relatively low, which could explain its comparably good behavior in molten NaCl-KCl-MgCl2 at 700°C. This results in the formation of relatively pure MgO scales on the surface of P91 although MgCr2O4 spinel, indicating the outward transport of Cr species from the alloy to the scales, is also observed. P91 steel also has a lower nominal concentration of Cr than Inconel 600 (see Table 1), which may also contribute to the fact that less Cr was available to form MgCr2O4 spinel on P91 surfaces.

As commonly reported in the literature [START_REF] Kowaka | Effect of heat treatment on the susceptibility to stress corrosion cracking of Alloy 600[END_REF][START_REF] Kai | The effects of heat treatment on the sensitization and SCC behavior of Inconel 600 alloy[END_REF], Cr-rich M23C6 and M7C3 carbides are confined at the grain boundaries in the pristine solution-annealed microstructure of Inconel 600 (Fig. 1b). However, heat treating Inconel 600 in the vicinity of 700°C will foster the precipitation of intragranular Cr-rich carbides within the γ-(Ni) solid solution matrix and limit sensitization of the grain boundaries [START_REF] Kowaka | Effect of heat treatment on the susceptibility to stress corrosion cracking of Alloy 600[END_REF][START_REF] Kai | The effects of heat treatment on the sensitization and SCC behavior of Inconel 600 alloy[END_REF][START_REF] Hall | The microstructural response of mill-annealed and solution-annealed Inconel 600 to heat treatment[END_REF]. This was indeed observed in Fig. 12c and d with both intergranular and intragranular carbides below the corrosion front. In molten NaCl-KCl-MgCl2, the electromotive forces are in the order Cr 2+ /Cr < Fe 2+ /Fe < Ni 2+ /Ni [START_REF] Gaur | Standard electrode potentials in molten chlorides[END_REF]. Consequently, the difference in potentials between the Cr-rich phases (e.g. Cr-rich M23C6 or M7C3 carbides) and Ni-rich matrix results in the formation of electrochemical cells. With a lower electrode potential, Cr is then selectively dissolved from the alloy leaving voids within the microstructure (see Fig. 11). This hypothesis is in good agreement with the experimental observations of Abramov and co-workers [START_REF] Abramov | Corrosive resistance of nickel Hastelloy G-35 superalloy in various aggressive media[END_REF][START_REF] Abramov | Corrosion of corrosion-resistant and high-temperature nickel-based alloys in chloroaluminate melts[END_REF] as well as with our previous study [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF] indicating the formation of micro-galvanic pairs between the γ-(Ni) solid solution matrix and secondary phases when exposing nickelbased alloys to molten chlorides. The driving force of the corrosion attack is further increased by the large cathode/anode surface ratio. As a result, Cr-rich carbides are continuously dissolved as the electrolyte permeates through the porous oxide scales and the porous region of the substrate by capillary effects as proposed by Bell et al. [START_REF] Bell | Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions[END_REF]. The outward transport of Cr as gaseous CrCl2 and its reaction with oxygen above the surface of the alloy results in the formation of magnesiochromite as discussed in sections 4.2 and 4.3. The continuous dissolution of Cr progressively enriches the corrosion scales in magnesiochromite, especially in the outer region of the samples where there is a higher O2 availability (compare Fig. 12a andb). As aforementioned, MgCr2O4 has a higher electrical conductivity (σ ≈ 10 -2 Ω -1 .cm -1 at 800°C [START_REF] Petric | Electrical conductivity and thermal expansion of spinels at elevated temperatures[END_REF]) than MgO (≈ 10 -11 Ω -1 .cm -1 at 700°C [START_REF] Lewis | The electrical conductivity of magnesium oxide at low temperatures[END_REF]), which increases the chargetransfer reaction rates. The enrichment of the scales in MgCr2O4 at the expense of MgO results in a positive feedback loop regarding further corrosion attack of Inconel 600. This effect is likely to explain the higher corrosion rates noted on the outer region than on the immersed region of the samples (compare Fig. 7b andd).

  investigated salt mixture was prepared in hermetic plastic containers by weighing the desired amounts of salts according to the eutectic composition (24.5 NaCl-20.5 KCl-55.0 MgCl2 in wt.%, i.e. 33.0 NaCl-21.6 KCl-45.4 MgCl2

Fig. 2 .

 2 Fig. 2. (a) Schematic representation of the experimental set up used for the molten salt exposure [34] and (b) corresponding macrograph of one Inconel 600 sample after partial embedding in molten NaCl-KCl-MgCl2 for 100 h at 700°C showing the residual salt mixture attached to the sample. The locations 1 to 4 correspond to the areas of investigation with, respectively, 1: top edge, 2: outer region, 3: immersed region and 4: bottom edge.

Fig. 3 .

 3 Fig. 3. X-ray diffraction patterns of the salt residues obtained after exposure of (a) P91 and (b) Inconel 600 to molten NaCl-KCl-MgCl2 at 700°C in flowing Ar for 20, 100 and 300 h. Bischofite: MgCl2.6H2O and Carnallite: KMgCl3.6H2O.

Fig. 4 .

 4 Fig. 4. BSE cross-section images taken in representative areas of the residual salt mixtures after (a), (b) 100 h exposure of Inconel 600 and (c), (d) 300 h exposure of P91 to molten NaCl-KCl-MgCl2 at 700°C in flowing Ar. Refer to Fig. 2a for the selected regions.

Fig. 5 .

 5 Fig. 5. Probability plots of the metal loss values of (a) P91 and (b) Inconel 600 substrates after exposure to molten NaCl-KCl-MgCl2 at 700°C in flowing Ar and (c), (d) corresponding cross-section light optical micrographs after 300 h exposure. Please note the varying magnifications in cross-section images. The readers are referred to Fig. 2a for the selected locations.

Fig. 6 .

 6 Fig. 6. Summary of median metal loss measurements with standard deviation for (a) P91 and (b) Inconel 600 materials after 20, 100 and 300 h exposure to molten NaCl-KCl (data from Ref. [34]) or to molten NaCl-KCl-MgCl2 at 700°C in flowing Ar.

Fig. 7 .

 7 Fig. 7. Iso-probability curves of metal loss values as a function of exposure time for (a), (c) P91 and (b), (d) Inconel 600 alloys exposed to molten NaCl-KCl-MgCl2 at 700°C in flowing Ar. (a), (b) top side corresponds to location 2 in Fig.2a and (c), (d) immersed side corresponds to location 3 in Fig.2a. Points indicate experimental data and lines the best fit curves. For each testing condition, the n values as well as the estimated propagation rates corresponding to 5%, median and 95% iso-probability curves are reported on the graphs.

Fig. 8 ,Fig. 8 .

 88 Fig. 8, BSE cross-section images taken in representative areas of the samples are presented after 20, 100 and 300 h of exposure. For all investigated areas, relatively continuous MgO scales appearing dark in BSE mode are

Fig. 9 .

 9 Fig. 9. Raman signal of magnesiochromite (MgCr2O4) and fluorescence of Cr 3+ in MgO (λexc = 632.8 nm) in the oxide scales formed on P91 after 20 h exposure to molten NaCl-KCl-MgCl2 at 700°C in flowing Ar. See Appendix for the identification of the fluorescence bands of the MgO:Cr 3+ system.

Fig. 10 .

 10 Fig. 10. BSE cross-section images of P91 after 20 h exposure to molten NaCl-KCl-MgCl2 at 700°C in flowing Ar taken in representative regions of the samples. (a), (c) outer region initially above the salt (location 2) and (b), (d) immersed region initially embedded in the salt mixture (location 3).

Fig. 11 .

 11 Fig. 11. BSE cross-section images of Inconel 600 after (a), (b) 20 h, (c), (d) 100 h and (e), (f) 300 h exposure to molten NaCl-KCl-MgCl2 at 700°C in flowing Ar taken in representative regions of the samples. The readers are referred to Fig. 2a for the selected regions.

Fig. 12 .

 12 Fig. 12. BSE cross-section images of Inconel 600 after 300 h exposure to molten NaCl-KCl-MgCl2 at 700°C in flowing Ar taken in representative regions of the samples. (a) outer region initially above the salt (location 2), (b), (d) immersed region initially embedded in the salt mixture (location 3) and (c) top edge of the sample (location 1). Arrows in (c), (d) indicate the propagation direction of the corrosion attack.

Fig. 13 .

 13 Fig. 13. Schematic representation of the correlations between the concentration of corrosive MgOH + impurities in the melt and the corrosion rates of metallic alloys as a function of time following the impurity-driven corrosion mechanism. This illustration is based on the electrochemical measurements carried out by Ding et al. in Ref. [30]. C i : Initial concentration of MgOH + after heating and C t : Concentration of MgOH + as a function of time. The dashed lines represent the case with a lower initial concentration of MgOH + within the melt.

O2Fig. 14 .

 14 Fig. 14. Superimposed thermodynamic stability diagram of the Cr-Fe-Ni-Cl-O system at 700°C. The solid arrow from mark 0 to mark 1 illustrates the increase in Cl2 partial pressure associated with the Deacon reaction (7) upon heating of the salt mixture and the active oxidation reactions (22)-(25). The semi-continuous arrow from marks 1 to 4 illustrates the evolution of the partial pressures of O2 and Cl2 from the gas phase towards the corroded region of the alloys upon exposure to the NaCl-KCl-MgCl2 ternary mixture.

  ), the reaction of volatile CrCl2 with O2 was favored. The nucleation and growth of Cr2O3 on the Al2O3 template therefore occurred. Because the rod was in electrical contact with the alloy substrate and since Cr2O3 possesses semi-conducting properties at elevated temperatures[START_REF] Parsa | Effect of oxygen partial pressure on the semiconducting properties of thermally grown chromia on pure chromium[END_REF], the reduction of MgOH + into MgO on Cr2O3 was kinetically favored. Then the simultaneous formation of MgO and Cr2O3 provoked the synergistic dissolution described in reaction[START_REF] Ding | Molten chloride salts for next generation CSP plants: Electrolytical salt purification for reducing corrosive impurity level[END_REF] leading to the formation of MgCr2O4. These mechanisms are summarized in Fig.16. It should be noted here that despite being an efficient electrical insulator, the formation of MgCr2O4 on Al2O3 poisoned its insulating behavior. A similar effect can therefore be expected within the MgO scales since their progressive conversion into MgCr2O4 occurred with increasing exposure time (compare Fig.11a, c and e).

Fig. 15 .

 15 Fig. 15. (a) Light optical cross-section image and (b) BSE-cross section image at higher magnification highlighting the formation of MgCr2O4 scales on the Al2O3 rod used to hang the samples vertically (see Fig.2a). EDX spot measurements shown in (b) are summarized in the Table in (c) (at.%). The deviation of the magnesiochromite from the ideal MgCr2O4 stoichiometry probably explains its high electrical conductivity and its poisoning effect regarding the ohmic drop provided by MgO on the alloys' surfaces. For the sake of clarity, it was decided to note magnesiochromite as MgCr2O4 even though a more appropriate way of writing is (Mg,Fe 2+ )1+x(Cr,Fe 3+ )2-xO4.

Fig. 16 .

 16 Fig. 16. Proposed mechanisms of formation of magnesiochromite on Al2O3. The same mechanisms are expected to take place on growing MgO scales such that their periphery is always converted into MgCr2O4. Because the synergistic dissolution of MgO and Cr2O3 readily occurs, step 4 is only an intermediate of reaction (the interface MgO-Cr2O3 is metastable).

Table 1

 1 Nominal and tested chemical compositions of the two commercial alloys investigated in this study (wt.%). OES: optical emission spectrometry.

	Alloy	Composition	Fe	Cr	Ni	Mo	Mn	V	Ti	Si	C	Other
	P91	Nominal	Bal.	8.0-9.5	≤ 0.40	0.85-1.05	0.30-0.60	0.18-0.25	-	0.20-0.50	0.08-0.12	Nb: 0.06-0.10
	(1.4903)	OES	89.0	8.7	0.20	0.84	0.43	0.20	-	0.24	0.08	Nb: 0.08
	Inconel 600	Nominal	6.0-10.0	14.0-17.0	Bal.	-	≤ 1.00	-	≤ 0.30 ≤ 0.50 ≤ 0.15	Cu: ≤ 0.50
	(2.4816)	OES	8.3	17.1	73.2	-	0.25	-	0.21	0.40	0.04	Cu: 0.07

  ) with diffraction peaks at 2θ values of 37.2° (111), 43.2° (200), 62.6° (220), 75.0° (311) and78.9° (222). Additional diffraction peaks were associated to the cubic structure of magnesiochromite (i.e. MgCr2O4) with its main diffraction peaks at 35.9° (311), 43.5° (400), 53.6° (422), 57.3° (333) and 63.0° (440). The formation of MgO and of MgCr2O4 within the salt mixtures by confirmed with SEM observations (Fig.4). It is worth mentioning, that MgCr2O4 appearing brighter than MgO in BSE mode was systematically enveloping MgO crystals

top side (Fig.7a and b) than on the immersed side (Fig.7c and d) for both alloys, which may indicate the effect of oxygen availability on the corrosion mechanisms. These effects will be discussed in section 4.2.

- 24-Overall corrosion reactions:

Acknowledgements

The European Union's Horizon 2020 research and innovation program is gratefully acknowledged for funding this research (RAISELIFE, grant agreement No. 686008). The authors would like to thank Daniela Hasenpflug and Ellen Berghof-Hasselbächer for helping with the metallographic preparation of the samples. Susann Rudolphi is gratefully acknowledged for performing the optical emission spectrometry (OES) measurements of the alloys.

2MgOH + + Cr(s) → 2MgO(s) + Cr 2+ + H2(g) [START_REF] Vignarooban | Vapor pressure and corrosivity of ternary metal-chloride molten-salt based heat transfer fluids for use in concentrating solar power systems[END_REF] 2MgOH + + Fe(s) → 2MgO(s) + Fe 2+ + H2(g) [START_REF] Liu | Corrosion behavior of Ni-based alloys in molten NaCl-CaCl2-MgCl2 eutectic salt for concentrating solar power[END_REF] Following reaction [START_REF] Mohan | Assessment of a novel ternary eutectic chloride salt for next generation high-temperature sensible heat storage[END_REF], the reduction of MgOH + impurities on the substrates' surface should lead to the formation of MgO and H2. As a matter of fact, MgO was systematically observed on the surface of P91 and Inconel 600 after exposure to molten NaCl-KCl-MgCl2 (see Fig. 8, Fig. 10 and Fig. 11), as previously shown by Ding et al. [START_REF] Ding | Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere[END_REF][START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF] and other groups [START_REF] Liu | Corrosion behavior of Ni-based alloys in molten NaCl-CaCl2-MgCl2 eutectic salt for concentrating solar power[END_REF][START_REF] Sun | Effect of alloying elements on the corrosion behavior of Ni-based alloys in molten NaCl-KCl-MgCl2 salt at different temperatures[END_REF] on various iron-and nickel-based alloys. As a result of the progressive formation of MgO on the surface of the metallic substrates and because of the low solubility of MgO in molten chlorides [START_REF] Bell | Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions[END_REF],

the latter results in the formation of a physical barrier between the molten salt and the metallic substrate, as observed in Fig. 10. After initially high corrosion rates, the reduction of MgOH + impurities into quite dense and continuous MgO scales decreases the diffusion rates of the corrosive species from the melt towards the substrate.

Because MgO possesses a low electrical conductivity at 700°C (σ ≈ 10 -11 Ω -1 .cm -1 [START_REF] Lewis | The electrical conductivity of magnesium oxide at low temperatures[END_REF]), this results in a relatively protective behavior of the MgO scales in static conditions by decreasing the charge-transfer rates between the melt and the metallic alloys. Similar observations were reported by Ding et al. when exposing stainless steel 310, Incoloy 800H and Hastelloy C-276 to NaCl-KCl-MgCl2 with 1 wt.% Mg addition [START_REF] Ding | Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials[END_REF]. The addition of 1 wt.% Mg was indeed promoting the formation of dense and continuous MgO scales on top of the substrates thereby increasing the corrosion resistance of the alloys (corrosion rates decreasing by a factor of 3 for Incoloy 800H and of 16 for

Hastelloy C-276 with addition of Mg). Because of a great concentration gradient, H2 released by reaction ( 15) is less likely to accumulate within the scales and is continuously swept away by the carrier gas (i.e. Ar flow).

Influence of O2 availability on the corrosion mechanisms

To maintain charge neutrality, the formation of metallic cations at the metal/electrolyte interface resulting from the partial anodic reactions ( 16) and [START_REF] Kruizenga | Corrosion mechanisms in chloride and carbonate salts[END_REF] will provoke the electromigration of Cl -anions towards the metal surface and their reaction following reactions [START_REF] Sun | Corrosion behavior of 316SS and Ni-based alloys in a ternary NaCl-KCl-MgCl2 molten salt[END_REF] and [START_REF] Sun | Effect of alloying elements on the corrosion behavior of Ni-based alloys in molten NaCl-KCl-MgCl2 salt at different temperatures[END_REF]:

Even though CrCl2 and FeCl2 compounds are less stable than NaCl, KCl and MgCl2 at 700°C, they possess high vapor pressures and their continuous evaporation takes place at the corrosion front [START_REF] Zahs | Chloridation and oxidation of iron, chromium, nickel and their alloys in chloridizing and oxidizing atmospheres at 400-700[END_REF], leaving voids within the material (see Fig. 10 and Fig. 12). The volatile chloride compounds then diffuse from the metal surface towards the electrolyte through cracks and pores of the oxide scales and other diffusion paths. As described in the widely accepted active-oxidation process [START_REF] Zahs | Chloridation and oxidation of iron, chromium, nickel and their alloys in chloridizing and oxidizing atmospheres at 400-700[END_REF][START_REF] Grabke | Role of alloying elements and carbides in the chlorine-induced corrosion of steels and alloys[END_REF][START_REF] Bender | The role of alloying elements in commercial alloys for corrosion resistance in oxidizingchloridizing atmospheres. Part I: Literature evaluation and thermodynamic calculations on phase stabilities[END_REF][START_REF] Zhang | Chloride-and sulphate-induce hot corrosion mechanism of super austenitic stainless steel S13254 under dry gas environment[END_REF], the oxygen partial pressure increases gradually within the scale towards the scale/electrolyte interface and, depending on local thermodynamic conditions (see Fig. 14), the gaseous chlorides can react with dissolved oxygen forming porous oxide scales and Cl2 following reactions ( 22)- [START_REF] Guo | Corrosion in the molten fluoride and chloride salts and materials development for nuclear applications[END_REF]:

FeCl2(g) +

It should be noticed that similar microstructural observations were given after exposure of iron-and nickelbased alloys in Cl2-contanining environments with a porous "sponge-like" microstructure of the alloys' subsurface region and chains of pores along the grain boundaries [START_REF] Grabke | Role of alloying elements and carbides in the chlorine-induced corrosion of steels and alloys[END_REF][START_REF] Bender | The role of alloying elements in commercial alloys for corrosion resistance in oxidizingchloridizing atmospheres. Part I: Literature evaluation and thermodynamic calculations on phase stabilities[END_REF]. Grabke et al. indicated that the carbides were preferentially attacked by Cl2 leaving holes in the metal matrix through the formation of gaseous metal chlorides following reaction [START_REF] Gaur | Standard electrode potentials in molten chlorides[END_REF] in the case of Cr23C6 carbides [START_REF] Grabke | Role of alloying elements and carbides in the chlorine-induced corrosion of steels and alloys[END_REF]:

Similarly, Bender and Schütze proposed reaction [START_REF] Wagman | Selected values of chemical thermodynamic properties[END_REF] for the preferential attack of Cr7C3 carbides releasing gaseous CO2 and volatile chloride compounds [START_REF] Bender | The role of alloying elements in commercial alloys for corrosion resistance in oxidizingchloridizing atmospheres. Part I: Literature evaluation and thermodynamic calculations on phase stabilities[END_REF]:

Considering the low oxygen partial pressure (< 10 -32 atm.) prevailing at the reaction front in the melt in the present study (see Fig. 12c andd for an illustration), the formation of gaseous CO and CO2 is not thermodynamically favored with equilibrium oxygen partial pressures of about 3.1x10 -22 and 5.5x10 -22 atm. at 700°C, respectively. In addition, considerations in section 4.1 suggest that the attack is dominated by electrochemical corrosion rather than chlorine-induced corrosion. Despite negative Gibbs free energies, reactions [START_REF] Gaur | Standard electrode potentials in molten chlorides[END_REF] and ( 32) cannot take place within the alloy and the general reaction associated with carbide dissolution can therefore be written:

where CrxCy corresponds to Cr-rich carbides (e.g. Cr7C3 or Cr23C6).

Reaction [START_REF] Zhao | Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications[END_REF] can therefore be interpreted as the partial anodic reaction. Because carbon is stable at such low oxygen partial pressures, it diffuses back to the γ-(Ni) solid solution matrix. Since carbon has a low solubility in γ-(Ni) and its diffusivity in Ni (≈ 10 -9 cm²/s at 700°C [START_REF] Berry | Diffusion of carbon in nickel[END_REF]) is larger than the diffusivity of Cr in Ni (≈ 10 -11 cm²/s at 700°C [START_REF] Murarka | Diffusion of chromium in nickel[END_REF]), further precipitation of Cr-rich carbides occurs within the alloy matrix, as observed in Fig. 12c andd and in Refs. [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF][START_REF] Bell | Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions[END_REF]. This results in a self-sustaining attack driven by solid-state diffusion mechanisms within the alloy following alternative precipitation/dissolution of Cr-rich carbides and formation of void channels. In addition, this will drastically decrease the mechanical strength of solution-strengthened alloys such as Inconel 600 in which carbide precipitation increases the tensile strength and creep resistance. While the formation of void channels is mainly assigned to the selective dissolution of Cr-rich carbides, it is likely that the defective structure of the alloy in the vicinity of the carbides is also prone to dissolution and the accumulation of defects such as vacancies may further aggravate the formation of a porous microstructure. Fine-scale characterization (e.g. TEM)

should be conducted to verify the validity of these hypotheses. Nevertheless, our findings clearly demonstrate that considering a nickel-based alloy to be, de facto, more corrosion resistant than any given steel in chloride melts might not be necessarily true and that microstructural instability (e.g. distribution of Cr-rich phases, diffusion flux of Cr from the alloy, C solubility and mobility) may shorten the lifetime of an otherwise stable metallic matrix.

These insights should also be put in perspective with mechanical considerations and thermal cycling conditions that will also influence the overall lifetime of the components.

Influence of testing conditions on the experimental corrosion rates: focus on MgCl2

The metal loss data presented in Fig. 6 clearly indicates that the corrosion attack of P91 and Inconel 600 is more pronounced in the binary NaCl-KCl compared to the ternary NaCl-KCl-MgCl2 melt. For instance, the empirical median corrosion rate calculated from the metal loss data is about 30 times greater for Inconel 600 immersed in molten NaCl-KCl (7194 µm/year [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF]) than in molten NaCl-KCl-MgCl2 (212 µm/year, Fig. 7d). Similar observations are made after exposure of P91 in molten NaCl-KCl-MgCl2 with median corrosion rates about 6 to 30 times smaller than after exposure to the binary NaCl-KCl salt mixture. In addition, whereas P91 is sensitive to pitting corrosion in molten NaCl-KCl, as presented in our previous study [START_REF] Grégoire | Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl-KCl binary system[END_REF], the presence of MgCl2 within the melt inhibits this phenomenon due to the formation of MgO. This considerably affected the maximum corrosion rates recorded on P91 locally exceeding 10 mm/year in NaCl-KCl melt to only about 200 µm/year in the ternary salt mixture.

At first view, these results seem to be in contradiction with the findings of Zhu et al. in Ref. [START_REF] Zhu | Effects of cations on corrosion of Inconel 625 in molten chloride salts[END_REF]. In their work, the authors investigated the corrosion behavior of Inconel 625 in molten NaCl, KCl, CaCl2 and MgCl2 as well as in different chloride mixtures (e.g. NaCl-MgCl2, KCl-MgCl2, NaCl-CaCl2 and KCl-CaCl2). After 20 h of exposure at 900°C, Inconel 625 was found to suffer more severe corrosion in molten MgCl2 and in molten chloride mixtures with high MgCl2 contents. The authors attributed this to the higher hygroscopicity of MgCl2 resulting in a higher basicity (i.e.

higher activity of O 2-anions) of the melt [START_REF] Zhu | Effects of cations on corrosion of Inconel 625 in molten chloride salts[END_REF]. In fact, the higher the basicity of the melt, the easier the dissolution of Cr2O3 into chromate anions (CrO4 2-) following basic fluxing mechanisms [START_REF] Rapp | Hot corrosion of materials: fundamental studies[END_REF]. In addition, Zhu et al. carried out the experiments in air, which further increased the basicity of the molten salts. It should also be pointed out that while the authors used anhydrous NaCl and KCl salts, MgCl2 was hydrous resulting in a very high level of corrosive MgOH + impurities within the melt after heating (see the reaction pathways (1)-( 5)). These testing conditions intensified the negative influence of MgCl2 on the corrosion behavior of Inconel 625 at 900°C [START_REF] Zhu | Effects of cations on corrosion of Inconel 625 in molten chloride salts[END_REF]. Overall, the salt mixtures with MgCl2 were then more corrosive than the ones without MgCl2.

Unlike Zhu et al. [START_REF] Zhu | Effects of cations on corrosion of Inconel 625 in molten chloride salts[END_REF], our experiments were carried out at 700°C in Ar and with anhydrous MgCl2 (although partially hydrated by the initial weighing and the transfer of the samples to the reaction chamber). In such testing conditions, the activity of O 2-anions is expected to be much lower such that the basic fluxing of Cr2O3 does not form chromate but chromite anions as discussed in section 4.3. Moreover, the use of anhydrous MgCl2 considerably limited the introduction of water molecules in the furnace and the consequential hydrolysis reactions (see the reaction pathways (1)-( 5)). This resulted in a lower concentration of corrosive MgOH + impurities after heating of the salt mixture and thus led to a lower extent of metal dissolution as presented in Fig. 13. As discussed in section 4.1, the reduction of MgOH + on the metallic substrates forms relatively continuous MgO scales presenting a low solubility in the melt. Once the concentration of MgOH + impurities reaches negligible amounts, these MgO scales form a physical barrier against further diffusion of other oxidants such as O2. With a low electrical conductivity, MgO decreases the intensity of the cathodic reactions and, as a result, slows down the metal dissolution rate. In fact, the most important degradation mechanism of such MgO scales was caused by their synergistic dissolution with Cr2O3 forming MgCr2O4 spinel characterized by a significantly higher electrical conductivity than MgO.

To conclude, it appears that depending on the testing conditions (e.g. gas composition, temperature, use of hydrous or anhydrous salts), MgCl2 can be classified as detrimental or relatively beneficial in comparison to molten chlorides not containing MgCl2 when considering the corrosion behavior of structural materials. The fact that the experimental testing conditions can lead to different conclusions while investigating the effect of a single parameter should be regarded as of the utmost importance. The development of new standard test methods for molten salt corrosion, especially when handling hygroscopic salts such as MgCl2, should therefore be considered to design corrosion experiments at laboratory scale. This would provide significant insight into corrosion issues under in-service conditions, which is critical for successful plant design.

Conclusions

The corrosion behavior of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten NaCl-KCl-MgCl2 (24.5-20.5-55.0 wt.%) has been investigated at 700°C under Ar. Based on the experimental observations, the following conclusions can be drawn:

(1) Both alloys showed impurity-driven corrosion mechanisms associated with the formation of corrosive MgOH + impurities within the melt upon thermal hydrolysis of MgCl2. This electrochemical process combines the reduction of MgOH + into insoluble MgO on the surface and metal dissolution from the substrates. The precipitation of MgO at the metal/electrolyte interface resulted in a relatively protective behavior under isothermal and static conditions, especially for P91 steel.

(2) The corrosion attack was associated with the selective dissolution of Cr and the removal of Cr-rich carbides from the alloy matrix. The microstructural instability of Inconel 600 at 700°C with excessive formation of intragranular Cr-rich carbides within the γ-(Ni) solid solution matrix led to the formation of galvanic pairs. The alternative precipitation and dissolution of Cr-rich carbides resulted in the formation of chains of pores within the alloy, which was further aggravated by solid-state diffusion mechanisms.

(3) The O2 availability had a clear effect on the corrosion kinetics by shifting the equilibrium reactions towards oxidation of gaseous CrCl2 species into Cr2O3 as well as by participating in the cathodic activity once the concentration of MgOH + impurities reached negligible levels.

(4) The ferritic-martensitic P91 steel showed a better corrosion resistance than Inconel 600 in molten NaCl-KCl-MgCl2. This was attributed to a lower potential difference between Cr-rich phases and the Fe-rich matrix than between Cr-rich phases and the Ni-rich matrix, respectively. In addition, the higher dissolution rate of Cr and outward transport of Cr-containing species in the case of Inconel 600 resulted in a larger fraction of MgCr2O4 spinel at the expense of MgO within the corrosion products.

(

Appendix

The fluorescence of Cr 3+ ions in MgO has long been used to characterize the MgO cubic structure with a sharp emission line (R-line) centered at 698.1 nm at 77 K in a purely cubic field [START_REF] Deutschbein | Die lienenhafte emission und absorption der chromosphore[END_REF][START_REF] Imbusch | Fluorescence of MgO:Cr 3+ ions in noncubic sites[END_REF][START_REF] O'neill | Site-selective laser spectroscopy of energy transfer in MgO:Cr 3+[END_REF][START_REF] Feofilov | Fluorescence spectroscopy of Cr 3+ ions in MgO nanofilms on Al2O3 and GaN substrates[END_REF]. In 1932, Deutschbein has shown that additional vibrational bands are associated to the fluorescence of Cr 3+ ions in MgO and gave their corresponding emission wavelengths and width in frequency (cm -1 ) at both 20°C and -195°C [START_REF] Deutschbein | Die lienenhafte emission und absorption der chromosphore[END_REF]. The ionoluminescence of the MgO:Cr 3+ system is very sensitive even at very low concentration of Cr 3+ , e.g. below the detection limit of ICP-OES (inductively coupled plasma optical emission spectrometry) and in the detection limit of PIXE (proton-induced X-ray emission) [START_REF] Perea | Ion-irradiation-induced damage in nuclear materials: Case study of a-SiO2 and MgO[END_REF]. With the appropriate laser source, Raman spectroscopy therefore appears very promising to characterize the physical properties of MgO by Cr 3+ fluorescence. The Raman shift values associated with the fluorescence bands of the MgO:Cr 3+ system were calculated from the emission wavelengths given in the work of Deutschbein (see Table 2) for both Nd:YAG (λexc = 532.0 nm) and He-Ne (λexc = 632.8 nm) lasers following equation [START_REF] Ding | Molten chloride salts for next generation CSP plants: Electrolytical salt purification for reducing corrosive impurity level[END_REF]:

where λexc corresponds to the wavelength of the laser source in nm, λemission corresponds to the emission wavelength of the fluorescence band in nm and Δω its corresponding Raman shift in cm -1 .

Table 2

Summary of the fluorescence emission bands identified in the MgO:Cr 3+ system at 20°C by Deutschbein [START_REF] Deutschbein | Die lienenhafte emission und absorption der chromosphore[END_REF] and their corresponding Raman shift values calculated for the Nd:YAG (λexc = 532.0 nm) and He-Ne (λexc = 632.8 nm) lasers. The experimental data obtained in this study for a reference MgO powder (see Fig. 17) are given in the last two columns and fairly agree with the theoretical calculations.

Fluorescence emission bands

Raman shift values (cm -1 ) Band λemission (nm)

Relative intensity Width (cm -1 )

Calculated from [START_REF] Deutschbein | Die lienenhafte emission und absorption der chromosphore[END_REF] This study (MgO powder) λexc = 532.0 nm λexc = 632.8 nm λexc = 532.0 nm λexc = 632. The experimental Raman shift values associated with the fluorescence bands of Cr 3+ in the reference MgO powder are in good agreement with the theoretical calculations for both laser sources (see Table 2). These fluorescence bands were labelled accordingly in Fig. 9 and Fig. 17. Unlike the spectra obtained for the MgO scales in Fig. 9, the N-line corresponding to the electric dipole of locally compensated Cr 3+ tetragonal centers [START_REF] Feofilov | Fluorescence spectroscopy of Cr 3+ ions in MgO nanofilms on Al2O3 and GaN substrates[END_REF] was not observed for the reference MgO powder (Fig. 17a andb). Since there is a dependence of the N-line intensity on the Cr 3+ concentration in MgO [START_REF] O'neill | Site-selective laser spectroscopy of energy transfer in MgO:Cr 3+[END_REF], this may indicate different levels of Cr impurities within the MgO crystals. The Cr 3+ concentration is indeed expected to be higher in the MgO scales formed during the molten salt exposures than in the reference MgO powder (< 5 ppm according to the furnisher).

Whereas three additional Raman bands centered at 275, 441 and 1076 cm -1 are observed in Fig. 17a, only one

Raman mode centered at 441 cm -1 is observed in Fig. 17b. The fact that the other two Raman modes (275 and 1076 cm -1 ) were not observed with the He-Ne laser may be related to the higher fluorescence background, which probably masked the low intensity vibrational modes in the Raman range. The two bands centered at 275 and 441 cm -1 correspond to MgO [START_REF] Awwad | A novel method to improve the anticancer activity of natural-based hydroxyapatite against the liver cancer cell line HepG2 using mesoporous magnesia as a microcarrier[END_REF]. To the best of the authors' knowledge, the additional Raman mode at 1076 cm -1 does not correspond with a known Raman active mode of MgO. Since magnesium carbonate is the main impurity in the MgO powder (≤ 1.5% according to the furnisher), this vibrational mode may be attributed to the C-O symmetrystretch of magnesite (i.e. MgCO3) following the work of Jiang et al. [START_REF] Jiang | First-principles analysis of vibrational modes of calcite, magnesite and dolomite[END_REF]. The presence of MgCO3 residues in the MgO powder may have resulted from an incomplete calcination process since MgO powder is obtained by calcination of magnesite in air [START_REF] Sasaki | Effect of calcination temperature for magnesite on interaction of MgO-rich phases with boric acid[END_REF].