Electrochemical impedance study of corrosion of pure Ni, Fe, Cr, and Fe-5Cr alloy in molten nitrides

Molten nitrates or nitrate–nitrite mixtures have been used as heat transfer fluid as well as thermal energy storage medium in concentrated solar power plants and other industrial applications. However, containment materials in contact with these molten salts are subjected to molten salt corrosion, which has been a concern over the years. In this investigation, the corrosion behavior of pure Ni, Fe, Cr, and Fe-5Cr alloy (in weight percent) has been examined at 400°C in molten nitrates using electrochemical impedance spectroscopy and gravimetric tests. It is shown that Ni, Fe, and Fe-5Cr exhibit mass gain, with the best corrosion resistance for Ni followed by Fe-5Cr and Fe, while pure Cr goes through fast dissolution, forming hexavalent chromium product K₃Na(CrO₄)₂. The increase of salt basicity from Solar Salt to Hitec could accelerate the dissolution of Cr. Nevertheless, the formation of an inner Cr-containing scale could still offer the alloy good protection because the external insoluble Ni or Fe oxides can protect it from dissolution.