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

Abstract

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 terms of the depth of attack, graphite clearly accelerates the corrosion, but the results appear to indicate that the formation of the Cr23C6 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/y for UNS N10003 in molten FLiBe in the pure nickel capsule at 700°C.