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

The utilization of the Solar Salt (60 wt%NaNO₃/40 wt%KNO₃) mixture as a Thermal Energy Storage (TES) medium is gaining importance due to its scalability and cost-effectiveness. However, the corrosion of metallic components presents a significant challenge. This study explores the intricate interplay between salt chemistry and its corrosivity, particularly at elevated temperatures exceeding the state-of-the-art bulk temperature 565 °C. The study manipulates salt decomposition by adjusting the oxygen partial pressure in the purge gas over Solar Salt and investigates the evolution of salt chemistry with and without the presence of steel. It analyzes the corrosion behavior of two types of stainless steel, AISI 316L and AISI 310, under different gas purging atmospheres. Furthermore, it employs a gold particle tracing technique to identify and monitor the formation and growth of the corrosion layer on the steel surface. The results reveal that nitrogen gas purging significantly enhances salt decomposition and its corrosivity over time. The presence of steel also influences salt decomposition depending on the purged gas atmosphere. In a nitrogen atmosphere, the presence of steel can increase the nitrite levels, while an air atmosphere results in an elevated concentration of oxide ions. In air, the AISI 310 alloy shows slightly better performance than AISI 316L. Both alloys experience substantial mass loss in the nitrogen-purged 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.