Influence of atmosphere and austenitic stainless steel on the solar salt corrosivity

Kurzfassung

Efficient energy storage solutions are essential for harnessing solar energy effectively. Thermal energy storage (TES) within Concentrated Solar Power (CSP) plants has emerged as a mature and promising solution for efficiently harnessing intermittent solar energy. Solar Salt (60% NaNO3/40% KNO3) serves as an economical, low-toxicity and none pressurized heat transfer fluid (HTF) and heat storage medium in the CSP systems. However, there is currently a lack of comprehensive information on the corrosion of metallic components by Solar Salt under controlled salt decomposition. Solar Salt undergoes decomposition at elevated temperatures, generating multiple types of soluble ionic species nitrite (NO2-), and oxide (O2-) ions. While earlier studies have primarily focused on the corrosion rate of various metallic alloys in Solar Salt, understanding the role of Solar Salt chemistry on the corrosion process is a vital aspect to mitigate corrosion. This study manipulates salt decomposition by altering the purge gas (nitrogen and synthetic air) over Solar Salt and investigates the evolution of salt chemistry with and without the presence of steel. Furthermore, gold sputtering was performed on the steel samples before exposure to trace the evolution of the corrosion layer. The changes in Solar Salt chemistry were investigated using ion chromatography (IC) for nitrate and nitrite ions and titration for oxide ions, while the corrosion process is concurrently examined through mass loss measurements and SEM-EDX analysis. The aim is to evaluate the influence of varying gas atmospheres and the presence of steel on Solar Salt chemistry and its corrosivity. The results reveal that nitrogen gas purging substantially increases the decomposition of salt into nitrite and oxide ions, intensifying its corrosiveness over time. The presence of steel also influences salt decomposition depending on the purged gas atmosphere. Interestingly, the presence of gold particles within the middle of the corrosion layer in the air purged atmosphere visually illustrates a counter diffusion involving various cations and anions across the corrosion layer. By controlling the Solar Salt decomposition, the corrosivity of the Solar Salt can be decreased to a large extent and the CSP-TES system life span can be improved.