MgCl₂–NaCl–KCl salts mixture shows great potential as a high-temperature (> 700 °C) thermal energy storage material in next-generation concentrated solar power plants. Adding Mg into molten MgCl₂–NaCl–KCl salt as a corrosion inhibitor is one of the most effective and cost-effective methods to mitigate the molten salt corrosion of commercial Fe–Cr–Ni alloys. However, it is found in this work that both stainless steel 310 and Incoloy 800H samples were severely corroded after 500 h immersion test at 700 °C when the alloy samples directly contacted with the over-added Mg in the liquid form. The corrosion attack is different from the classical impurity-driven corrosion in molten chloride salts found in previous work. Microscopic analysis indicates that Ni preferentially leaches out of alloy matrix due to the tendency to form MgNi₂/Mg₂Ni compounds. The Ni-depletion leads to the formation of a porous corrosion layer on both alloys, with the thickness around 204 µm (stainless steel 310) and 1300 µm (Incoloy 800H), respectively. These results suggest that direct contact of liquid Mg with Ni-containing alloys should be avoided during using Mg as a corrosion inhibitor for MgCl₂–NaCl–KCl or other chlorides for high temperature heat storage and transfer.