Thermal gradient mass transfer of type 316L stainless steel and alloy 718 in flowing mercury

Thermal convection loops (TCLs) fabricated from 316L stainless steel (SS) and containing mercury and a variety of 316L coupons representing variable surface conditions and heat treatments have been operated continuously for periods up to 5000 h. In each case, the maximum TCL temperature was about 305 deg. C, the minimum temperature about 240 deg. C, and the Hg velocity was constant at either 1.2 m/min or 5 m/min, depending on the TCL cross-section diameter. Wetting of 316L by Hg was somewhat sporadic and inconsistent, and was generally encouraged by steam cleaning and/or gold-coating of specimens prior to testing as well as relatively high exposure temperatures. Interaction of 316L and Hg was observed to generate a porous surface layer substantially depleted of Ni and Cr which resulted in transformation to ferrite, but the maximum penetration detected for all of the test conditions corresponded to only about 60-70 μm/yr, with far less penetration for most exposures. In limited testing, alloy 718 was found more resistant to wetting/attack than 316L