Molten chloride salt is a candidate heat transfer medium (HTM) for the next generation high temperature concentrating solar power (CSP). A corrosion product MgO will be formed on metal surface in the molten chloride containing MgCl₂. Based on the corrosion behavior comparison of a carbon steel and three kinds of Fe-Cr-Ni alloys in solid (345 °C) and molten state NaCl-MgCl₂ (445 and 545 °C), the effects of MgO on corrosion resistance of the four kinds of samples were discussed. Results show that in solid eutectic NaCl-MgCl₂ at 345 °C, the MgO layer is dense and complete on the carbon steel surface, which can protect the specimen from corrosion. In the molten salt, a dense MgO layer is also formed on the four kinds of sample surface, but the molten salt penetrates the MgO/matrix interface along cracks of the oxide layer due to the peeling off of MgO layer under thermal stress, which cannot protect specimen from corrosion. The corrosion mechanism is the chemical-electrochemistry reactions.