Introduction
MgCl2-KCl-NaCl molten chloride salt has received much attention in recent years due to its wide working temperature range (420-800°C), low vapor pressure, low material cost, and good heat capacity (Mehos et al., 2017; Ding et al., 2018a; Turchi et al., 2018; Villada et al., 2021). It has potential use in the next-generation concentrating solar power (CSP) plants as thermal energy storage (TES) material and heat transfer fluid (HTF), allowing the operating temperature of the CSP plant to be increased from about 560°C to >700°C. With such a high operating temperature, the energy efficiency of the power cycle in the CSP plant can rise from 40% to >50% when integrated with the supercritical carbon dioxide (sCO2) Brayton power cycle, which would significantly reduce the levelized cost of electricity (LCOE) of CSP (Mehos et al., 2017). However, the molten MgCl2-KCl-NaCl mixture is strongly corrosive to commercial Fe-Cr-Ni alloys even under a protective inert atmosphere (Ding et al., 2018c; Sun et al., 2018), which greatly limits its application.
Numerous studies have shown that the corrosion in molten MgCl2-KCl-NaCl salt is caused by corrosive impurities, especially MgOHCl (Ding et al., 2018c; Choi et al., 2019; Grégoire et al., 2020; Sun et al., 2020; Zhao, 2020), not by the molten chloride salt itself (Zhang et al., 2020). As a consequence of the strong hygroscopicity of MgCl2, some moisture is inevitably absorbed in MgCl2-KCl-NaCl in practical applications. Subsequently, the main corrosive impurity MgOHCl is generated as a hydrolysis product during heating in the melting process, resulting in the strong salt corrosivity to the metallic structural materials, such as Fe-Cr-Ni alloys (Kipouros and Sadoway, 2001; Kashani-Nejad, 2005; Ding et al., 2018c). To represent the concentration of MgOHCl (C(MgOHCl))in the molten salt, the unit parts per million oxygen (ppm O) is defined as the mass fraction of oxygen (mO in MgOHCl) in the total mass of the salt sample (msample), as shown in Eq. 1 (Skar, 2001).
The estimated acceptable impurity level of MgOHCl for the different types of alloys based on the literature (Ding et al., 2018c; Ding et al., 2019a; Ding et al., 2019b; Kurley et al., 2019) and their relative cost factors (Gilardi et al., 2006) are summarized in Table 1. To allow the use of inexpensive alloys (e.g. stainless steels) for TES/HTF with molten MgCl2-KCl or MgCl2-KCl-NaCl at ≥700°C, the salt impurity needs to be controlled at the tens of ppm O level by monitoring and salt purification to control the salt corrosivity (Ding et al., 2019b; Kurley et al., 2019). As shown in Figure 1, a corrosion control system (CCS) integrated into the molten chloride TES/HTF system has been proposed in the previous work, which contains the main parts—part of online corrosion monitoring and part of corrosion mitigation (Villada et al., 2021). For CCS, reliable in situ and ex situ monitoring techniques of MgOHCl at the tens of ppm level are vital to ensure its effectiveness, efficiency, and economics.
To measure the oxygen-containing impurity concentration or redox potential of molten salts, electrochemical methods including cyclic voltammetry (CV) (Skar, 2001; Ding et al., 2017; 2018b; Choi et al., 2019; Gonzalez et al., 2020; Guo et al., 2021), square wave voltammetry (SWV) (Song et al., 2018), chronopotentiometry (CP) (Zhang et al., 2020), and open-circuit potentiometry (OCP) (Choi et al., 2019; Gonzalez et al., 2020) have been employed in molten chloride salts (Williams et al., 2021). Among them, an approach combining in situ and ex situ measurement of MgOH+Cl− was investigated, in which cyclic voltammetry (CV) was employed as the in situ measurement of MgOHCl (Skar, 2001; Ding et al., 2018b; Guo et al., 2021), while ex situ methods of titration (Skar, 2001; Ding et al., 2018b) and carbothermal reduction (Skar, 2001) were used for the ex situ measurement to calibrate the in situ CV measurement. The reduction peak in the cyclic voltammogram—peak B, shown in Figure 2, represents the reaction of MgOH+ to MgO, as shown in Eq. 2 (Skar, 2001; Ding et al., 2018b; Guo et al., 2021). Moreover, the current density of peak B in the cyclic voltammogram is linearly linked to the concentration of MgOH+.
It was pointed out in the work of Skar (2001) and Ding et al. (2018b) that the ex situ measurements based on titration and carbothermal reduction could result in an over-measurement of the MgOH+ concentration and biased calibration of CV since other impurities (e.g. MgO or H2O) cannot be quantitatively excluded in these ex situ measurements. In the MgCl2-KCl-NaCl molten salt, non-corrosive MgO and corrosive MgOHCl commonly co-exist. When the temperature is >555°C, MgOHCl can be decomposed into MgO and HCl, as shown in Eq. 3 (Kipouros and Sadoway, 2001; Kashani-Nejad et al., 2005).
To measure non-corrosive MgO and corrosive MgOHCl separately, Klammer et al. (2020) used water and methanol to extract MgOHCl/MgO based on the solubility difference (Klammer et al., 2020). This method can measure the concentration of MgOHCl down to 0.1 wt% (∼200 ppm O) by a typical ethylenediaminetetraacetic acid (EDTA) titration technique and has been used to measure the purity of MgCl2-containing chloride mixtures after pre-purification. However, due to the solubility of MgOHCl in methanol, this method cannot measure the extremely low-concentration MgOHCl.
