With suitable thermophysical properties and commercial viability, molten chlorides are promising candidates to replace nitrate salts as heat transfer fluids (HTF) and thermal energy storage (TES) materials for next-generation concentrated solar power (CSP) plants. Nevertheless, structural materials including steels and nickel-based alloys experience severe corrosion in molten chlorides. The explicit understanding of the corrosion mechanisms is therefore decisive in order to propose viable technical solutions to increase the durability of these materials. In this work, the corrosion behavior of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy is investigated in molten NaCl–KCl at 700 °C under Ar. The salt mixtures after exposure as well as the corroded samples were simultaneously characterized. The systematic metallographic study after the water-free preparation indicates that electrochemical processes play a major role in the overall corrosion mechanisms. In both P91 steel and Inconel 600, the formation of micro-galvanic pairs between Cr-rich phases (anodic sites) and the alloy matrix (cathode) leads to the selective dissolution of Cr from the alloys resulting in the formation of subsurface voids. The influence of O2 and of various alloying elements on the corrosion kinetics is discussed. Based on the experimental observations, several corrosion mechanisms combining electrochemical reactions, solid-state diffusion and chloridation-oxidation processes have to be taken into account.