Study of material compatibility for a thermal energy storage system with phase change material

Abstract

The suitability of stainless steel 316L and Inconel 625 for use in a latent heat thermal energy storage (TES) system was investigated. A NaCl-NaF eutectic mixture with a melting temperature of 680◦C was used as the phase change material (PCM). Containers were filled with the PCM prior to heating to 750◦C, then examined after 100 and 2500 h of high-temperature exposure by analyzing the material surface and cross-section areas. A small amount of corrosion was present in both samples after 100 h. Neither sample suffered significant damage after 2500 h. The undesirable inter-granular grain boundary attack found in SS316L samples was in the order of 1-2 µm in depth. On Inconel 625 sample surface, an oxide complex formed, resisting material dissolution into the PCM. The surface morphology of tested samples remained largely unchanged after 2500 h, but the corrosion pattern changed from an initially localized corrosion penetration to a more uniform type. After 2500 h, the corrosion depth of Inconel 625 remained at roughly 1-2 µm, indicating that the corrosion rate decelerated. Both materials demonstrated good compatibility with the chosen NaF-NaCl eutectic salt, but the low corrosion activity in Inconel 625 samples shows a performance advantage for long term operation.