Fluoride Salt Corrosion and Mass Transfer in High Temperature Dynamic Systems

Corrosion tests ranging from 3000 to 20,000 hours were conducted on nickel and iron based alloys in fused fluoride salt at temperatures ranging from 450 to 704 C (842 to 1299 F). The objective of the experiment was to find suitable fused salt container material for application in the nuclear molten salt reactor. The materials tested were Hastelloy N, titanium modified Hastelloy N, Type 304L stainless steel, and a maraging steel. The two types of salts were (1) LiF-BeF₂ with varying amounts of UF₄, ThF₄, and ZrF₄, (fuel salt), and (2) a mixture of NaBF₄ and NaF (coolant salt). The compatibilities of these materials with the salts were determined from weight change data and chemical, metallographic, and electron microprobe analyses of the test specimens along with chemical analyses of the salts. The corrosion was manifested as temperature gradient mass transfer. The rate of corrosion of Hastelloy N in LiF-BeF₂ base salts was controlled by diffusion of chromium in the alloy and was very small (<0.1 mpy at 704 C), but the attack of stainless steel by the fuel salt was larger [1.1 mpy at 688 C (1270 F)]. Hastelloy N in NaBF₄-NaF showed a uniform attack [0.6 mpy at 607 C (1125 F)] controlled by impurity reactions. A titanium modified Hastelloy N with no iron showed smaller weight changes than standard Hastelloy N with 5% Fe under similar conditions in a fuel salt. A maraging steel with less chromium than stainless steel had a corrosion rate half that of the stainless steel under similar conditions in a fuel salt.