Ex-situ and in-situ methods for quantifying moisture concentration in MgCl2-containing molten salts and their impact on materials corrosion

초록

MgCl2-based molten salts offer favorable thermal properties and cost efficiency, making them attractive for molten salt reactors and heat storage applications. However, their strong hygroscopicity introduces moisture that can accelerate corrosion of structural materials, requiring accurate quantification. This study developed two complementary methods to quantify moisture species (H₂O and MgOHCl) in NaCl-MgCl2 molten salts: thermogravimetric analysis and differential scanning calorimetry (TG-DSC) as an ex-situ technique, and electrochemical measurements as an in-situ technique. TG-DSC simultaneously quantified both H2O and MgOHCl, with minimum quantifiable amounts of 1,000 ppm and 100 ppm, respectively. The electrochemical method, using a tungsten electrode, reliably quantified MgOHCl and yielded a diffusion coefficient of MgOH+ at 500 °C as 1.34 ∙ 10−5 cm2∙s−1, aligning well with molecular dynamics (MD) simulations. Cross-validation of the two methods showed good agreement in the MgOHCl concentration range of 900–10,800 ppm, with an error of 10%. Corrosion experiments on SS316 using TG-DSC demonstrated its practical applicability and confirmed the significant influence of moisture on corrosion behavior. A clear correlation between MgOHCl consumption and corrosion depth was observed in short-term corrosion tests, with corrosion occurring predominantly along grain boundaries and accompanied by the formation of MgCr2O4. The developed analytical methods provide a robust framework for ex-situ and in-situ moisture quantification in MgCl2-containing molten salts and can facilitate studies on moisture-induced corrosion in molten salt systems.

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