Project Profile: Fundamental Corrosion Studies in High-Temperature Molten Salt Systems for Next-Generation CSP Systems

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-- This project is inactive --

The Savannah River National Laboratory (SRNL), under the National Laboratory R&D competitive funding opportunity, worked with United Technology Research Center and the University of Alabama to understand corrosion when operating concentrating solar power (CSP) systems at high temperatures with advanced power cycles and to develop corrosion mitigation strategies to lengthen system lifetimes. By improving high-temperature operation, CSP systems can achieve greater efficiencies and thereby reduce the overall system cost.

Approach

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Molten chlorides have many properties that have made them one of the leading heat transfer fluid candidates for operation of CSP systems at temperatures above 550°C. However, one of the potential drawbacks to using molten chlorides is that they may cause corrosion in iron-nickel-chromium alloys if the chemical environment is uncontrolled. This project characterized the corrosion of Fe-Ni-Cr alloys in molten chloride salts and identified corrosion mechanisms for temperatures between 700°C to 850°C where the solar receiver and the hot side of the heat transfer system operates. Corrosion mitigation methods were identified and tested for effectiveness. 

Innovation

Corrosion can be mitigated using a variety of methods such as changes in major salt constituents he or alloy composition, coatings, corrosion inhibitors, and cathodic protection. This project will investigate the corrosion rate of iron-rich, nickel-rich, and cobalt-rich alloys with a low-cost potassium chloride – magnesium chloride eutectic salt to characterize the corrosion of each and will attempt to use methods such as adjustment of the redox potential in the salt to mitigate the corrosion mechanisms. Redox adjustment can be performed by the addition of active metals to the system such as Mg or Zr. The corrosion and corrosion mechanisms will be modeled to provide tools that system designers can use in predicting corrosion in their systems.

Conclusions

The main corrosion mechanism identified for the alloys was the selective oxidation of chromium and manganese, elements with the lowest reduction potentials in the alloys. Corrosion was determined to be mass transfer limited based on electrochemical studies together with corrosion rate analysis at different temperatures. Ni-based alloys such as Haynes 230 had the lowest corrosion rates and the rate limiting step for corrosion of Haynes

230 was grain boundary diffusion. SRNL was able to demonstrate that redox control using Mg and Zr was able to inhibit corrosion for representative alloys from the iron-, nickel-, and cobalt- families. It was demonstrated that the corrosion could be mitigated at these high temperatures to levels below the 15 mm/yr goal by using Mg and Zr. Electrochemical experiments showed that the addition of Mg to the system reduced the electrochemical potential below the point where Cr selective oxidation can occur. This demonstrates the ability mitigate the corrosion mechanisms that were identified.

Publications, Patents, and Awards

  • H. S. Cho, J.W. Van Zee, S. Shimpalee, B. A. Tavakoli, J. W. Weidner, B. L. Garcia-Diaz, M. J. Martinez-Rodriguez, L. Olson, J. Gray, “Dimensionless Analysis for Predicting Fe-Ni-Cr Alloy Corrosion in Molten Salt Systems for Concentrated Solar Power Systems,” CORROSION, 2016;72(6):742-760. doi: http://dx.doi.org/10.5006/1865
  • B. L. Garcia-Diaz,; L. Olson; M. J. Martinez-Rodriguez; R. Fuentes; H. Colon-Mercado; and J. Gray, "High Temperature Electrochemical Engineering and Clean Energy Systems," Journal of the South Carolina Academy of Science:2016, 14(1), Article 4.   Available at: http://scholarcommons.sc.edu/jscas/vol14/iss1/4
  • L. Olson, R. Fuentes, M. Martinez-Rodriguez, J. Ambrosek, K. Sridharan, M. Anderson, B. Garcia-Diaz, J. Gray, T. Allen, “Impact of Corrosion Test Container Material in Molten Fluorides”; Journal of Solar Energy Engineering: Including Wind Energy and Building Energy Conservation.  Vol. 137, Dec. 2015. 

 

Learn about other DOE competitive awards for concentrating solar power research that are in progress.