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John O | September 2017

Researchers investigate coatings needed for concentrating solar power


scientists at the u.s. department of energy national renewable energy laboratory (nrel) are studying how nickel-based coatings can reduce corrosion levels in heat exchangers and other components of next-generation concentrating solar power (csp) plants that require high-temperature fluids in the range of 550-750°c to store heat and generate electricity.

 


researcher judith gomez-vidal determined the coatings needed to allow concentrating solar power plants to store and use molten salts. (dennis schroeder/nrel)

 

molten salts are common fluids used in csp for heat transfer and thermal energy storage because of their ability to withstand high temperatures and retain solar energy for hours.

 

“to commercially use molten salt mixtures containing sodium chloride, potassium chloride, and magnesium chloride,” a report from the nrel explained, “the corrosion rate in the storage tanks must be slow—less than 20 micrometers per year—so that a concentrating solar power plant can achieve a 30-year life.”

 

applying different types of nickel-based coatings to stainless steel, researchers found that nicocralyta limited the corrosion rate to 190 micrometers per year. while short of the stated goal, it is a 96% reduction in steel’s corrosion rate.

 

“that particular coating was pre-oxidized over a 24-hour period, during which a uniform and dense layer of aluminum oxide was formed and served to further protect the stainless steel from corrosion,” the report added.

 

more tests will be conducted to evaluate the coatings under thermal cycling and introduce oxygen to the environment to increase the oxidation potential of the system. “the addition of oxygen ensures the formation of protective scales that could reform in the presence of oxygen if cracks appear during operation,” the report noted.

 

this research was funded by the department of energy’s sunshot initiative to lower the cost of solar power and make it more accessible.

 

the research was recently published in materials degradation. the abstract stated:

 

“corrosion evaluations of incoloy 800 h (in800h) and stainless steel aisi 310 (310ss), in bare and coated conditions, were performed in 34.42 wt% nacl – 55.47 wt% kcl at 700 °c in a nitrogen atmosphere. this nacl–kcl composition has a melting point of 657 °c, which makes it suitable for latent-heat thermal energy storage in concentrating solar power applications. several nickel-based mcralx coatings were tested, where m = ni and/or co and x = y, ta, hf, and/or si.

 

“electrochemical testing was carried out to determine corrosion rates. the bare in800h and 310ss alloys corroded rapidly (~2500 and 4500 µm/yr, respectively, assuming uniform corrosion). concentrating solar power plants need containment materials with a lifetime of at least 30 years; thus, these corrosion rates are excessive. corrosion mitigation approaches are being investigated to obtain degradation on the order of 20 µm/yr or lower.

 

“the lowest corrosion rate of 190 µm/yr was obtained for atmospheric plasma spray nicocraly coatings pre-oxidized in air at 900 °c for 24 h with a heating/cooling rate of 0.5 °c/min. metallographic characterization of the corroded surfaces showed that the formation of a uniform thin alumina scale before exposure to the molten chloride system considerably reduced the corrosion of the alloy.

 

“however, the rates of corrosion determined herein are considerable, highlighting the relevance of testing materials durability in solar power applications.”

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