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...

Experimental

Materials and Experimental Setup

KCl (purity >99 wt%) and NaCl (purity >99 wt%) were purchased from Alfa Aesar, Germany, while anhydrous MgCl2 (purity >99 wt%) was supplied by Magnesia, Germany. They were used to synthesize the eutectic salt mixture of MgCl2-NaCl-KCl (47.1-30.2-22.7 mo...

CV Experiments

Three groups of CV experiments at different temperatures (500°C, 600°C, and 700°C) were conducted. Once the temperature of the salt reached the target temperature, the counter electrode, reference electrode of CV, and the two graphite electrodes of el...

Titration Experiments

The acid consumption method based on titration was used for the quantitative measurement of the total amount of MgOHCl in a salt sample. A high-precision automatic titration instrument, 905 Titrando, purchased from Metrohm Germany, was employed. The ti...

Error Analysis for Titration and CV

For ex situ direct titration to measure the concentration of MgOHCl in 500 mg samples, there are two main sources of error: one caused by the co-existing MgO impurity in the salt sample and another caused by the titration experiment (i.e., titration co...

Results and Discussion

In previous work, some efforts have been made to understand the relation between the height of peak B (in Figure 2) and the concentration of MgOHCl in the MgCl2-containing chloride salts (Skar, 2001; Ding et al., 2017; 2018b; Choi et al., 2019; Gonzalez et al., 2020; Guo et al., 2021). It was found that adding NaOH can increase the height of peak B because of the reaction shown in Eq. 15. In addition, the potential difference between peak B and peak A is comparable ∼1.5 V. The reactions corresponding to peak A and peak A’ are seen as the typical peaks of Mg2+ reduction and its reverse reaction of Mg oxidation, as shown in Eq. 16 and Eq. 17, respectively, which can be seen as a marker. This evidence suggests that the peak at the potential of about 0 V in this work can be seen as the peak B corresponding to the reaction shown in Eq. 2. Different from the previous work (Ding et al., 2018b; Guo et al., 2021), the gradient of the MgOHCl concentration in this work was not obtained by adding NaOH but was obtained by electrolysis and thermo-decomposition to decrease the concentration of MgOHCl. Hence, the concentration of MgOHCl in this work was reduced to tens of ppm O since this level was interesting for corrosion control.

Concentration of MgOHCl Measured by Titration

The decrease in the MgOHCl concentration with electrolysis time at 500°C is shown in Figure 9, while the decrease in the MgOHCl concentration by thermal decomposition at 600 and 700°C with enhanced holding time is displayed in Figure 10. All the concentration da...

CV Results

As shown in Figures 11-13, the cyclic voltammograms in this work show similar features to those in the literature (Ding et al., 2018b; Choi et al., 2019; Guo et al., 2021). For example, the potential of peak B is about 1.5 V higher than that of peak A ...

Conclusion

The MgCl2-KCl-NaCl molten salt shows low corrosivity to the metallic structural materials (i.e., alloys) at ≥700°C when the concentration of the main corrosive impurity MgOHCl is as low as tens of ppm O. To allow the use of inexpensive alloys (e.g. st...

Data Availability Statement

The original contributions presented in the study are included in the article/supplementary material; further inquiries can be directed to the corresponding authors.

Author Contributions

WD, QG, and YC contributed to the conception and design of the study. QG, WD, and JS performed the statistical analysis. QG wrote the first draft of the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

This research has been performed within the DLR-DAAD fellowship program (Nr. 57540125). The authors would like to thank the colleagues M. Braun, R. Hoffmann, and A. Hanke for technical support in the different laboratories.

References