Corrosion of Commercial Alloys in Thermal Energy Storage Material of Molten MgCl2/KCl/NaCl under Inert Atmosphere

Abstract

Molten chloride salt mixtures are promising thermal energy storage (TES) materials to be applied in concentrating solar power (CSP) plants, due to their high thermal stability, good thermal conductivity, high heat capacity but low price. Their high thermal stability makes them appropriate candidates to replace the commercial TES materials in CSP - nitrate salt mixtures (stable up to 550 °C), when higher operating temperatures are required (up to 700 °C) for higher efficiency of thermal to electrical energy conversion. However, the application of molten chloride salts at higher temperatures causes additional challenges such as increased corrosiveness of containers and structural materials. It is well known that the corrosion rates significantly depend on the concentration of corrosive hydroxide impurities in the salts, which arise mainly from water and oxygen existing in the salts and/or the atmosphere above the salts, since they can produce very corrosive H+ ions. However, to our knowledge, few publications quantitatively show the effect of the corrosive hydroxide impurities on the corrosion rates. In our previous work, an electrochemical method based on cyclic voltammetry (CV) has been developed to fast in-situ measure the concentration of the hydroxide impurities in molten MgCl2/KCl/NaCl (60/20/20 mole%) salts at 500-700 °C. In this work, we use an electrochemical method based on open-circuit voltage (OCV) and potentiodynamic polarization (PP) to fast in-situ measure the corrosion rates of studied alloys in molten chlorides, when the concentration of the corrosive hydroxide impurities varies in a wide range. By analyzing the data from CV and PP, the effects of the hydroxide impurities and the temperature on the corrosion rates are investigated quantitatively. This helps to develop an advanced technology, which could effectively and affordably control the concentration of the impurities in operating molten chloride salts, and thus the corrosion of structural metallic components in contact with the molten salts.