Under the same conditions, the corrosion rate of 316SS at 500 ☌ is less than 1/20 of that at 700 ☌. However, the higher the temperature is, the faster the corrosion of the alloy. For CSP equipment, a high working temperature can realize the more efficient conversion of thermal energy to electrical energy and faster corrosion.
High temperature damages the MgO protective layer, which will accelerate the corrosion. The dense MgO film formed on the alloy surface can slow down the alloy corrosion, and the molten salt temperature has a great influence on the stability of the MgO protective layer. Similar to other molten MgCl 2-based salts, the oxygen in the molten salts will react with MgCl 2 to form MgO at high temperatures. The corrosivity of molten KCl-MgCl 2-based salts is reduced because Mg treatment removes corrosive impurities. The corrosion rate of 316 L and N alloys in 700 ☌ molten KCl-MgCl 2 salts was reduced using carbon tetrachloride as the chlorination reagent to reduce dissolved oxide. The corrosion performances of Hastelloy C-276 and Haynes 230 corroded in 800 ☌ molten KCl-MgCl 2 salts were weak after pretreatment at 500 ☌. Purification of molten chlorides usually uses one of three methods: high-temperature drying, chemical purification, and metal reduction. Purification of molten chlorides helped to remove dissolved oxide and reduced the corrosion of salt.
The impurities in molten salts were the main reason for the alloy corrosion. Similar to the NaCl-KCl-MgCl 2, NaCl-MgCl 2, NaCl-KCl-CaCl 2, NaCl-KCl-ZnCl 2, and nitrate salts, the corrosion control of the alloys in molten KCl-MgCl 2 salts is a challenging task.
In order to explore the interaction between molten KCl-MgCl 2 salts and alloys, the corrosion behavior and mechanism have been investigated in the literature. KCl-MgCl 2 molten salts should also have good compatibility with heat storage containers, pipelines, and valves to realize their application as heat transfer and storage medium. In addition to its excellent heat transfer performance, the price of KCl-MgCl 2 salts is not high, so it is considered one of the best heat transfer and storage mediums in next-generation concentrated solar power (CSP) and high-temperature reactors. The vapor pressure of molten KCl-MgCl 2 salt is lower than 13 kPa even at 800 ☌. Its melting point is approximately 426 ☌, and its thermal conductivity is about 0.4 W KCl-MgCl 2 eutectic salt has excellent thermophysical properties.
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