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

Abstract

Corrosion testing of Hastelloy N in molten fluoride salt was performed in purified molten 2 7 LiF-BeF 2 (66-34mol%) (FLiBe) salt at 700°C for 1000 hours, in pure nickel and graphite capsules. In the nickel capsule tests, the nearsurface region of the alloy exhibited an about 200 nm porous structure, an approximately 3.5 µm chromium depleted region, and MoSi 2 precipitates. In the tests performed in graphite capsules, the alloy samples gained weight due to the formation of a variety of Cr 3 C 2 , Cr 7 C 3 , Mo 2 C and Cr 23 C 6 , 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 surface or grain boundary, which induced an approximately 1.4 µm Ni 3 Fe alloy layer in this region. The carbide containing layer extended to about 7 µm underneath the Ni 3 Fe layer. The presence of graphite dramatically changes the mechanisms of corrosion attack in the Hastelloy N in molten FLiBe salt. Evaluated by in terms of the depth of attack, graphite clearly accelerates the corrosion, but the results appear to indicate that the formation of Cr 23 C 6 phase might stabilize the Cr and mitigate its dissolution in molten FLiBe salt. Moreover, a thermal diffusion controlled corrosion model that was fundamentally derived from Fick's second law was applied to predict the corrosion attack depth of 17.2 µm/year for the Hastelloy N in molten FLiBe in the pure nickel capsule at 700°C.

Corrosion-induced microstructural developments in 316 stainless steel during exposure to molten Li2BeF4(FLiBe) salt

“…This salt was re-melted and further purified by sparging with a H 2 -HF gas mixture for the present study. The details of the purification process have been described previously [15], [22], [23].…”

“…Based on previous literature, the corrosion attack distance is generally expressed in terms of the depth of Cr depletion in Cr-containing alloys in high-temperature molten fluoride salt [15], [32], [33]. Under the assumption of Cr diffusion controls corrosion rate, the effective diffusion coefficient (D eff ) of Cr through alloy matrix can be obtained from the following equation [22], [32]:…”

“…Using the calculated D eff value the Cr depletion profiles in the near-surface regions of the tested 316 stainless steel samples can be calculated by using a solution of Fick's second law, as follow [22]:…”

Study of tritium transport characteristics in a transportable fluoride-salt-cooled high-temperature reactor

“…Pi Li et al researched the permeation behaviors of tritium in the candidate structural materials of FHRs with the method of gas‐driven permeation . Guiqiu Zheng examined the corrosion behavior of 316 stainless steel and Hastelloy N before and after neutron irradiation . However, little work has been carried out on establishing a comprehensive model about tritium transport behaviors in FHRs.…”

“…24 Guiqiu Zheng examined the corrosion behavior of 316 stainless steel and Hastelloy N before and after neutron irradiation. 25,26 However, little work has been carried out on establishing a comprehensive model about tritium transport behaviors in FHRs. John Stempien developed a primary model of tritium transport with MATLAB code, 27 but it has not been applied to analyze a reactor.…”