Corrosion monitoring and mitigation techniques on advanced thermal energy storage materials for CSP plants

High temperature corrosion is one of the most important issues for materials selection, structure design and service life prediction of engineering parts that are exposed to high temperature environments. The prevention of high temperature corrosive attacks on materials plays a critical role in aspects such as reliability, quality, safety and profitability of any industrial sector associated with high temperature process and in the study case, concentrated solar power (CSP) market, using inorganic molten salts as storage material. The aim of this research is the development of corrosion tests through conventional gravimetric techniques focussed on thermal energy storage (TES) materials as well as electrochemical impedance spectroscopy (EIS) analysis for storage systems monitoring. To mitigate corrosion, different alumina forming austenitic (AFA) alloys were exposed to the most promising molten salts to be proposed for the new generation of CSP plants, nitrate, carbonate and chloride molten salts. Ni base alloys showed a better behaviour in the corrosive environments and electrochemical impedance tests revealed the formation of a protective layer in the first 100 h. The gravimetric results and the scanning electron microscopy (SEM) analyses that were performed, confirmed the excellent behaviour of the steels tested in nitrate salts reducing the corrosion in these promising novel molten salts compared with the solar salt currently used in commercial CSP. On the other hand, carbonate and chloride molten salts showed a higher corrosion rate in the materials proposed, especially chloride molten salts.