Protective thermal spray coatings for TES applications in CSP plants

Abstract

Corrosion caused by using molten salts in thermal storage systems in Concentrated Solar Power (CSP) plants is a major problem in this field. To eliminate this problem, the use of nanofluids and the application of Inconel-625 coatings by thermal spray techniques (High Velocity Oxygen Fuel - HVOF and Cold Gas Spray - CGS) are proposed. This study focuses on evaluating the effectiveness of these coatings, deposited on AISI 316 stainless in mitigating nanofluid-induced corrosion in CSP plant Thermal Energy Storage (TES) systems. For this purpose, a total immersion test in NaNO₃ with silica nanoparticles was carried out in a furnace at 450 °C under air atmosphere for 30 days (720 h) and 90 days (2160 h). The test was also performed on SS316L and Inconel-625 bulk substrate samples for better comparison. Evaluation of corrosion behaviour relied on measuring the reduction in cross-sectional thickness of the test samples. Furthermore, detailed characterization was performed using Laser Scattering (LS), Scanning Electron Microscopy (SEM), and Field Emission Scanning Electron Microscopy (FESEM). The coating surface was also studied by X-Ray Diffraction (XRD), and the molten salt-based nanofluids were studied by Inductively Coupled Plasma (ICP). The results obtained revealed notably minimal corrosion rates per year for both deposition methods, after 3 months of testing. This demonstrates the effectiveness of both HVOF and CGS Inconel-625 coatings as a reliable solution to decrease the level of corrosion in TES units. However, future studies should be conducted over longer periods and with operating conditions closely replicating working conditions.