the salt eats the tank

Problem statement

Concentrated solar power (CSP) and next-generation nuclear plants use molten nitrate salts for thermal energy storage at roughly 300–565°C. Raising storage temperature toward the 700°C+ regime the DOE Gen3 CSP roadmap targets would significantly improve thermodynamic efficiency and enable industrial process heat applications. But today's nitrate salts are stability-limited to about 565°C — above that they progressively decompose, and the decomposition products aggressively corrode containment alloys and heat exchangers. Candidate replacements (chloride and carbonate salt mixtures) introduce new corrosion and handling challenges that static lab tests miss.

Why this matters

Long-duration thermal energy storage is critical for grid flexibility and industrial decarbonization. Higher operating temperatures improve round-trip efficiency and enable coupling with industrial processes that need 600–900°C heat (cement, steel, chemicals). The temperature ceiling at 565°C limits CSP and advanced nuclear to electricity generation only, excluding the ~30% of industrial energy demand that requires high-temperature process heat.

What’s been tried and why it hasn’t worked

Chloride and carbonate salt mixtures can operate at 700–800°C. Nickel-based superalloys resist corrosion better than stainless steels. Ceramic and oxide coatings have been tested as barriers. Containment is already the weak point at today's nitrate temperatures: the Crescent Dunes CSP plant (Nevada) lost roughly eight months of operation after its hot-salt tank leaked in October 2016, and a second leak in 2019 idled the plant for over two years. However, chloride salts are hygroscopic, complicating handling and introducing moisture and oxygen impurities that sharply accelerate corrosion — Sandia's mechanism review found chlorides do not readily form protective oxide layers, so salt purity control, not alloy choice alone, governs corrosion rates. Corrosion under flowing salt with thermal cycling substantially exceeds what static lab immersion tests predict, and coatings that survive static immersion fail under cycling and flow. Nickel superalloys work but are prohibitively expensive for the large tank volumes required (thousands of tonnes of salt per installation).

What would unlock progress

Containment materials or coatings validated at 700°C+ under flowing salt with thermal cycling, for durations long enough to support the ~30-year plant life the Gen3 roadmap assumes — not just short static coupon tests. Salt purification methods that maintain low impurity levels during sustained operation (not just at initial fill). Cost-effective alloy alternatives to nickel superalloys for large-volume containment.

Entry points for student teams

A team could design — not build — a corrosion test loop that circulates chloride or carbonate salt across material coupons at >600°C under thermal cycling: flow path and pump selection, coupon holder geometry, impurity control and monitoring, sampling cadence, and a pre-specified analysis plan for comparing dynamic rates against the static-immersion literature, handed to a group that already operates a salt loop. A second door needs no salt at all: assemble the corrosion rates already published in the open DOE literature (the NREL and Sandia reports on OSTI, and the loop studies they cite) into one comparison of static-crucible vs. flowing results at matched temperature and alloy, then use free thermochemical data (NIST-JANAF Thermochemical Tables, NIST SRD 13) to estimate how much of the spread is explained by moisture and oxygen impurity rather than by alloy choice. Screening protective coatings (oxide, nitride, or ceramic) under realistic thermal cycling is the strongest hands-on version, and it genuinely requires a molten-salt-capable furnace with inert-atmosphere salt handling — university high-temperature materials labs and the DOE national labs working this problem (Sandia, ORNL, NREL) are who own that setup. Materials science, corrosion engineering, thermodynamics, and thermal systems skills apply.