Influence of oxide impurities on the durability of candidate alloys for molten-salt thermal energy storage systems

As the Concentrated Solar Power (CSP) industry pushes the boundaries of operational efficiency, extending working temperatures becomes essential. However, this advancement presents a challenge for thermal energy storage (TES) systems: the degradation of structural materials in contact with molten solar salt. Conventional alloys, such as austenitic stainless steels, exhibit limitations under these aggressive conditions, particularly when corrosive oxide impurities accumulate during salt aging. This study explores the corrosion behavior of two multi-principal element alloys (MPEAs)—a eutectic (EMPEA) and a six-element composition alloy (MPEA6)—alongside Haynes 230 in molten solar salt at 620°C with varying sodium oxide (Na 2 O) content. By simulating salt degradation through controlled Na 2 O additions (0.04 and 0.06 wt.%), the impact of salt chemistry on the alloy degradation was evaluated. After 120 h of exposure, all alloys exhibited net mass loss, which increased with oxide content. However, significant differences were observed among the investigated materials. The eutectic MPEA showed the lowest net mass change under the tested conditions, while Haynes 230 experienced the highest, particularly in oxide-enriched salts. Salt chemical analysis showed that Cr was the dominant dissolved element, with concentrations increasing from approximately 21 ppm to 45 ppm as Na 2 O content increased, whereas Ni, Fe, and Al remained at lower levels. The results indicate that oxide impurities significantly modify the corrosion response of alloys by promoting selective dissolution processes and altering oxide-scale stability. These findings contribute to the development of materials for TES systems and demonstrate the value of oxide additions as an accelerated approach for evaluating alloy performance.