Corrosion of High Temperature Materials in Fluoride and Chloride Molten Salts

Concentrating solar power (CSP) systems that can operate at higher temperatures than 800°C can achieve higher efficiencies and power production using high temperature power cycles.  Operating at these temperatures requires identifying heat transfer fluids and materials for construction of heat transfer systems that will provide good operating characteristics and long system lifetimes. Advanced power cycles such as the supercritical CO₂ Brayton cycle can operate above 800°C and molten salts are one of the best options to transfer heat at these conditions due to their very high boiling points and good heat transfer properties.  However, a potential drawback with using molten salts at high temperatures is materials corrosion.  Therefore, it is important to identify pairs of heat transfer materials and molten salts that will have the mechanical properties and chemical stability to provide long system lifetimes. Several alloys that can be used for high temperature operation includes alloys with high nickel and chromium content, such as Haynes 230, and high cobalt and chromium content, such as Haynes NS-163. However, weight loss by dealloying of chromium from high temperature alloys containing chromium was observed when exposed to FLiNaK at 850°C for 500 hrs¹. In this work, several high temperature alloys such Haynes 230 and NS-163, TZM and a SiC/SiC composite were exposed to molten KCl-MgCl₂ and FLiNaK at temperature 750, 850 and 950°C for a long term period.  The alloys were examined via SEM/EDS post corrosion at the surface and cross-section to account for the corrosion attack and element depletion from bulk. The salts were analyzed via ICP-OES to quantify the elements that were depleted and diffused into the salt.

Reference:

1. L.C. Olson, J. W. Ambrosek, K. Sridharan, M. H. Anderson and T. R. Allen, Journal of Fluorine Chemistry, 130(1), 67-73 (2009).