“We are very excited about the potential implications to provide corrosion-resistant coatings for CSP applications that could improve the economic viability,” says Johney Green, the NREL’s associate laboratory director for mechanical and thermal engineering sciences.

Problems with Molten Salts

To function, CSP plants require high-temperature fluids such as molten salts in the range of 550 to 750 °C to store heat and generate electricity. Molten salts mixtures containing sodium chloride (NaCl), potassium chloride (KCl), and magnesium chloride (MgCl₂) are commonly used for both heat transfer fluid and thermal energy storage because they can withstand high temperatures and retain the collected solar heat for many hours.

At those high temperatures, however, the salts can eat away at common iron-nickel (FeNi) alloys such as Incoloy 800H (UNS N08810) and AISI 310 (UNS S31000) stainless steel (SS) used in the heat exchangers, piping, and storage vessels of CSP systems, the NREL explains.

To commercially use the molten salt mixtures, the corrosion rate must be slow—less than 20 μm/y, the NREL says—so that a CSP plant can achieve the projected 30-year service life for its containment materials.

By comparison, bare SS alloys tested in a molten chloride corroded as fast as 4,500 μm/y, the NREL says.

Nickel-based Solutions

To address this problem, NREL engineer and researcher Judith Gomez-Vidal began applying different types of nickel-based coatings, commonly used to reduce oxidation and corrosion, to the SS alloys. For her experiments, the 800H and 310 alloys were polished using abrasive paper until the surface was flat, and then machined to 8 mm in diameter and 12 mm in height. The nickel-based coating deposition was set up to have thicknesses of ~1 mm for electrochemical corrosion tests.

One such nickel coating, NiCoCrAlYTa, comprised of 23.0 wt% Co, 20.0 wt% Cr, 8.5 wt% Al, 0.6 wt% Y, and 4.5 wt% Ta, with the balance Ni, showed very strong performance. It limited the corrosion rate to 190 μm/y—not yet at the goal, but an enormous improvement compared to uncoated SS in the form of a 96% reduction in the corrosion rate.

That particular coating was preoxidized in air at 900 °C for 24 h with a heating/cooling rate of 0.5 °C/min. From there, metallographic characterization of the corroded surfaces using electron microscopy and imaging showed a uniform and dense layer of aluminum oxide (Al₂O₃) was formed before exposure to the molten chloride system. This, in turn, considerably reduced the alloy’s corrosion.

“The use of surface protection is very promising to mitigate corrosion in molten salts, in particular to those surfaces exposed to chlorine-containing vapor,” Gomez-Vidal says.

The corrosion evaluations were performed at 700 °C in a nitrogen atmosphere using a potentiostat. A Type-K thermocouple in an alumina well was used near electrodes to record temperatures, and the electrochemical cells were sealed and purged with nitrogen for about 24 h before corrosion testing.

Potentials were continuously recorded after the alloy’s immersion in the molten chloride, and polarization studies were conducted immediately after by applying cathodic and anodic external potentials. More than three coupons per test were performed under the same conditions to evaluate the consistency of the results.

“The chromium and aluminum in the coating were preferentially oxidized during preoxidation,” Gomez-Vidal says. “This oxide layer could help increase the corrosion resistance of the coatings. Alumina is a protective oxide with few defects in its structure, which minimizes or avoids the diffusion of elements. Thus, corrosion is controlled or mitigated.”

Further Rate Reductions Needed

Even with the 96% improvement in efficiency, the corrosion rate of 190 μm/y is still significantly more than the target of 20 μm/y or less needed for CSP plants to achieve a 30-year service life. As a result, more research is planned.

“The rates of corrosion are still considerably high for CSP,” Gomez-Vidal says. “This effort highlights the relevance of testing materials durability in solar power applications. More R&D [research and development] is needed to achieve the target corrosion level needed, which could include the synergy of combining surface protection with chemical control of the molten salt and the surrounding atmosphere.”

Further tests will require evaluation of the coatings under thermal cycling and the introduction of oxygen-containing atmospheres to increase the oxidation potential of the systems. The NREL notes that the addition of oxygen ensures the formation of protective scales that could reform in the presence of oxygen if cracks appear during operation.

Similarly, Gomez-Vidal says she has found other projects in which Al₂O₃ layers are able to form and remain adhered to the surface in the presence of air during thermal cycling of samples.

The research is funded by the DOE’s SunShot Initiative, which is a national effort to drive down the cost of solar electricity and support solar adoption. The program is aimed to make solar energy a low-cost electricity source through R&D efforts with both public and private partners.

The NREL is the DOE’s primary national laboratory for research and development on renewable energy and energy efficiency. The NREL is operated for the DOE by the Alliance for Sustainable Energy, LLC (Lakewood, Colorado) nonprofit group.

Source: NREL, www.nrel.gov. Contact Judith Gomez-Vidal, NREL—email:judith.vidal@nrel.gov.

References

1 “NREL Investigates Coatings Needed for Concentrating Solar Power,” NREL News Releases, Sept. 18, 2017, https://www.nrel.gov/news/press/2017/nrel-investigates-coatings-needed-for-concentrating-solar-power.html (Oct. 11, 2017).