The next-generation of solar power plants will require high-temperature fluids, such as molten salts in the range of 550 to 750°C, to store heat and generate electricity. At those temperatures, however, molten salts eat away at common alloys used in heat exchangers, piping, and storage vessels of so-called concentrating solar power (CSP) plants. Engineers at the U.S. Department of Energy’s National Renewable Energy Laboratory are looking at reducing corrosion in CSP plants with nickel-based coatings.
CSP plants with low-cost thermal storage let facilities deliver electricity whenever it is needed, helping to support grid reliability. Molten salts are commonly used both for the heat-transfer fluid and thermal-energy storage because they withstand high temperatures and retain the collected solar heat for many hours.
To commercially use molten salts containing sodium chloride, potassium chloride, and magnesium chloride, the corrosion rate in the storage tanks must be less than 20 micrometers per year so that the CSP plant can have a 30-year life.
Bare stainless steel alloys tested in a molten chloride corrode as quickly as 4,500 micrometers per year, so NREL researchers are looking at different types of nickel-based coatings, which are commonly used for reducing oxidation and corrosion. One such coating, NiCoCrAlYTa, has shown the best performance so far. It limited the corrosion rate to 190 micrometers per year—not yet at the goal but a large improvement, reducing corrosion by more than 96% compared to uncoated steel. That particular coating was pre-oxidized over a 24-hour period before coming in contact with the salts, during which a uniform and dense layer of aluminum oxide was formed and served to further protect the stainless steel from corrosion.
This shows that surface protection is critical in reducing corrosion in surfaces exposed to chlorine-containing vapor. However, the corrosion rates are still considerably high for CSP. This effort highlights the relevance of testing materials durability in solar power applications. More R&D is needed to achieve the target corrosion level needed, which could include combining surface protection with chemical control of the molten salt and the surrounding atmosphere.
Additional tests will evaluate coatings under thermal cycling and the introduction of oxygen-containing atmospheres that increase the oxidation potential of the storage tanks. Adding oxygen ensures formation of protective scales that could reform in the presence of oxygen if cracks appear during operation. This is based on work that showed aluminum oxide layers were able to grow and remained stuck to the surface in the presence of air during thermal cycling.