Corrosion Behavior of Carbon Steel and Iron-Chromium Nickel Alloys in Molten Eutectic Sodium Chloride and Magnesium Chloride as Phase Change Materials

As a phase change material for solar energy storage at moderate-high temperatures, the molten eutectic chloride salt of sodium chloride and magnesium chloride (NaCl-52 wt% MgCl2) is strongly corrosive to metal containers and pipes. The corrosion behavior of three kinds of iron-chromium-nickel (Fe-Cr-Ni) alloys and carbon steels in the molten eutectic salt at 520 degrees C was studied. Results show that the carbon steel is corroded along grain boundaries, and iron atoms become ferrous ions (Fe2+) and ferric ions (Fe3+). A thick and compact magnesium oxide (MgO) shell is formed on the surface of carbon steel, which protects the specimen from corrosion. The MgO shell is also formed on the surface of three kinds of iron-chromium-nickel alloys, but the magnesium oxide shell cannot protect them from corrosion in the molten salt. A loose skeletal Ni-rich microstructure is formed on the specimen surface because the chromium element is preferentially corroded. The magnesium oxide (MgO) shell or particles peel off easily. In addition, the higher the content of the chromium, the more severe the corrosion. As a vessel or pipe material for solar energy storage by molten chloride salt, cheap iron-based alloys with alloying element nickel have greater development prospects.