Stainless steel 316, duplex steel 2205 and carbon steel 1008 were examined for compatibility with the eutectic mixture of NaCl + Na2SO4 at 700 °C in air for thermal energy storage. Electrochemical measurements combined with advanced microscopy techniques were employed. Results confirmed oxidation to be the primary attack to all metals. Ferritic alloy 1008 was found the most vulnerable alloy with severe oxidative attack. Alloys 316 and 2205 showed close performance while grain boundary oxidative and sulfur attack threatened these materials. Alloy 2205 showed chromium depletion followed by selective oxidation of ferrite in the presence of a semi-protective film on the surface.