Brian C. Kelleher

CORROSION OF 316L STAINLESS STEEL ALLOY AND HASTELLOY-N SUPERALLOY IN MOLTEN EUTECTIC LiF-NaF-KF SALT AND INTERACTION WITH GRAPHITE

Abstract Molten FLiNaK salt [46.5%LiF-11.5%NaF-42%KF (mol%)] has been proposed for use as a secondary reactor coolant and medium for transfer of high-temperature process heat from nuclear reactors to chemical plants. Two alloys—Hastelloy-N superalloy (Hastelloy-N) and Type 316L stainless steel alloy (316L steel)—were exposed to molten FLiNaK salt in a 316L steel crucible under argon cover gas at 850·C for 1000 h. Graphite was also introduced into the test with the goal of studying the corrosion behavior of relevant reactor material combinations. The results show that corrosion of 316L steel occurred primarily through surface depletion of Cr. Contrarily, Hastelloy-N experienced weight gain due to the electrochemical plating of corrosion products, Fe and Cr, derived from the 316L steel crucible. The graphite sample enhanced the corrosion of the 316L steel sample and crucible, which induced the formation of (Cr,Fe)7C3 and (Mo,Cr,Fe)2C carbides on the surface of graphite. These carbide formations were attributed to the nonelectric transfer between 316L steel and graphite. Besides reducing the availability of chromium to plate, the presence of graphite did not change the basic corrosion of the 316L steel and plating process of Hastelloy-N.

High-Temperature Corrosion of UNS N10003 in Molten Li2BeF4 (FLiBe) Salt

Corrosion testing of UNS N10003 in molten fluoride salt was performed in purified molten 27LiF-BeF2 (66–34 mol%) (FLiBe) salt at 700°C for 1,000 h, in pure nickel and graphite capsules. In the nickel capsule tests, the near-surface region of the alloy exhibited an approximately 200 nm porous structure, an approximately 3.5 μm chromium-depleted region, and MoSi2 precipitates. In the tests performed in graphite capsules, the alloy samples gained weight because of the formation of a variety of Cr3C2, Cr7C3, Mo2C, and Cr23C6 carbide phases on the surface and in the subsurface regions of the alloy. A Cr-depleted region was observed in the near-surface region where Mo thermally diffused toward either the surface or the grain boundary, which induced an approximately 1.4 μm Ni3Fe alloy layer in this region. The carbide-containing layer extended to approximately 7 μm underneath the Ni3Fe layer. The presence of graphite dramatically changes the mechanisms of corrosion attack in UNS N10003 in molten FLiBe salt. In t...

Observed Redox Potential Range of Li 2 BeF 4 Using a Dynamic Reference Electrode

Abstract A compact electrochemical probe has been used to measure the redox potential ranges of molten Li2BeF4, a candidate nuclear reactor coolant commonly referred to as flibe, via a dynamic beryllium reference electrode. This probe is capable of operating on a loop, but was used on a static system in salt at temperatures up to 600°C. The probe has been used to measure Li2BeF4 salt with observed redox potentials ranging from –1.792 ± 0.002 V to –0.465 ± 0.134 V, yielding individual errors as low as ± 4 mV, and weighted groupings with errors as low as ± 1 mV. The most reducing measurement taken with acceptable error was –0.962 ± 0.011 V. This probe can be adapted for use in many laboratory experiments using flibe and should be considered for any corrosion experiment supporting the development of a next-generation molten salt reactor.

Electrochemistry in Molten LiF-BeF 2 Salt for Fluoride Salt-Cooled High Temperature Reactor Applications

A dynamic beryllium reference electrode has been developed for in situ electrochemistry studies of molten LiF-BeF2 (66-34 mol%) (FLiBe) salt which is being considered as the primary coolant for the fluoride salt-cooled high temperature reactor (FHR). Measurement and control of the salt redox potential can mitigate corrosion which is an important challenge in high temperature molten fluoride salts. This three electrode probe has demonstrated good reproducibility in the measurement of the salt’s redox potential with an error of ±4 mV at 500 °C. The response of the probe to changes in salt chemistry was investigated by the intentional additions of fluorides of nickel, iron, and chromium, which are corrosion products of common structural materials. The equilibrium potentials of these metal impurity fluorides were measured in solutions containing approximately 250 ppm of each impurity using cyclic voltammetry. This approach is being extended to study electrochemical purification of molten FLiBe.