Superalloys for the Energy Sector, Part 1 - Molten Salt Energy Storage and alloy 230

The development of nuclear and renewable energy goes hand in hand with advancements in creep resistant superalloys capable of operating under extreme conditions.

In this series of articles, we will examine renewable energy solutions, the engineering difficulties those technologies pose and how some alloys meet the challenge.

Molten Salt Thermal Storage Systems (MSES)

Thermal storage systems are designed to store thermal energy for better management of heat and energy supplies. They come in various forms, from simple water tanks to more advanced technologies.

Molten Salt Energy Storage (MSES) systems store temperature energy in the medium range of 200–500°C and feature high charging/discharging power. They are used mainly to supplement Concentrated Solar Power (CSP) plants, which concentrate large amounts of thermal energy from the Sun to produce electricity. Molten salts capture heat generated during the day and release it after sunset, enabling low-emission electricity generation during peak demand hours.

Fun Fact

The “salt” in this technology’s name doesn’t refer to the table spice we know; instead, it means “salt” in the sense of a chemical compound. Commonly used salt in MSES devices is the eutectic mixture of potassium nitrate and sodium nitrate (niter), known as “solar salt”.

Challenges for Metal Alloys

Challenge: The heat receiver tubes must a) withstand frequent heating and cooling cycles in a molten mixture of nitrate and nitrite salts at 200–500°C and b) be fatigue resistant.

Corrosion: Molten salts are a good fluxing agent, effectively removing oxide scale from a metal surface. Corrosion occurs primarily through oxidation, followed by metal dissolution in the melt. The presence of water vapor in the salt accelerates corrosion. Molten salts can cause uniform, pitting, and intergranular corrosion. The molten mixture of sodium nitrate and potassium nitrite quickly corrodes pure nickel, carbon steels, and chromium-molybdenum stainless steels.

Fatigue: Tubes need to endure 30,000 cycles of expansion and contraction, corresponding to heating and cooling.

Solution: alloy 230

Alloy 230 is a nickel-chromium-tungsten-molybdenum alloy with excellent high-temperature strength and outstanding corrosion resistance in nitrogen environments, making it highly effective in molten salt thermal storage systems.

Nitrogen Compound Resistance: Nitrogen absorption results over a 168-hour test in flowing ammonia at 650°C (as reported by Haynes® International):

• Alloy 230: 0.7 mg/cm³
• Alloy 600: 0.8 mg/cm³
• Alloy 625: 0.8 mg/cm³
• Alloy X: 1.7 mg/cm³
• Alloy 800H: 4.3 mg/cm³
• Alloy 316 SS: 6.9 mg/cm³

Fatigue Resistance and Thermal Stability: alloy 230® maintains high ductility even after prolonged exposure to high temperatures. This is a key advantage of this alloy over Alloy 625 and Alloy X. Data based on Haynes® International:

 

Testimonials Supporting alloy 230®’s Value for Solar Salt Thermal Storage:

"Nickel alloy tubes, especially alloy 230, effectively withstand fatigue loads and creep."
~ Bruce Kelly, Nitrate Salt Receivers Presentation, Solar Dynamics LLC

"The higher strength of Haynes® 230® alloy allows for the use of design section thicknesses as much as 75 percent thinner than lesser alloys with no loss in load-bearing capability."
~ Haynes International

Other Alloys Worth Considering

Alloys such as Alloy 625, Alloy 600, and Alloy X are also highly resistant to corrosion in nitrate environments, but they are less fatigue-resistant. Nevertheless, they provide alternatives to alloy 230®.