Chloride salts are one candidate for a >700°C concentrating solar power (CSP) cycle, however, many reports from the literature suggest very high reaction rates between chloride salts and structural alloys. Historically, a specific methodology was established for evaluating halide salt compatibility based on solution kinetics. This study returned to that paradigm where the salts are purified and evaluated in sealed capsules before moving to a flowing experiment to determine a true corrosion rate in a temperature gradient for a commercial K–Mg–Na chloride salt. Isothermal testing focused on Ni‐based alloys 230 and 600 at 600°C–800°C. The results indicated there were promising combinations of salt chemistry, temperature, and alloy composition that reduce the extent of reaction. The results of the first monometallic thermal convection loop of alloy 600 run for 1,000 hr with a peak temperature of 700°C showed low attack with rates ≤9 µm/yr.
Type 316H stainless steel samples were exposed to flowing FLiNaK salt for 1000h in a thermal convection loop with a maximum temperature of 650°C and a minimum of 540°C. Samples in the hottest part of the loop lost mass, with a maximum mass loss of 1.8 mg/cm2, while samples in the coldest parts of the loop gained mass. Analysis of the samples that gained mass showed an iron-rich layer on the sample surfaces, suggesting that iron, not chromium, accounted for the majority of the mass transfer in the loop. Analysis of the salt showed major increases in the Cr, Fe, and Mn content of the salt during exposure. The loop was modeled using the TRANSFORM code. Modeled values matched the experimental temperature measurements showing that TRANSFORM is capable of accurately simulating the conditions in the loop.
This milestone was originally envisioned for completion in FY21 and was delayed due to the COVID-19 response and difficulties in fabricating/obtaining fluoride salts. A monometallic type 316H stainless steel thermal convection loop (TCL) was operated for 1000 h with flowing LiF-BeF2 (i.e. FLiBe) salt and a peak temperature of 650°C. In general, the attack was minimal. However, classic mass transfer was not clearly observed with mass losses in both the hot and cold legs. There was an issue with cleaning the loop with water and specimens may have oxidized, which was not an issue with the previous FLiNaK TCL.
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