Corrosion Behavior of Pre-Carburized Hastelloy N, Haynes 244, Haynes 230, and Incoloy 800H in Molten FLiNaK

Austenitic alloys such as austenitic stainless steels and Ni-based alloys have been specified as container materials for molten salt reactors (MSRs). In MSR environments, these alloys are susceptible to carburization because (1) graphite components provide a source of carbon, (2) oxide films are not protective in molten halide salts, and (3) MSR operating temperatures fall within the temperature range of carbide formation. Carburization may occur simultaneously with corrosion by the selective dissolution of Cr. To study the corrosion behavior of a carburized microstructure, Hastelloy N, Haynes 244, Haynes 230, and Incoloy 800H samples were pre-carburized at 900 degrees C in a hydrocarbon atmosphere prior to exposure to molten FLiNaK at 700 degrees C. Pre-carburized samples featured a carbide-rich microstructure, particularly near the sample surface. The face-centered-cubic matrix of pre-carburized samples was found to be depleted in Cr, Mo, and to a lesser extent W, which are strong carbide-forming elements. Chromium dissolution in pre-carburized samples was suppressed compared to untreated samples due to the reduction in Cr activity. However, selective attack of the connected carbide structure along grain boundaries was observed in pre-carburized Haynes 230 and Incoloy 800H, which are alloys with high Cr content.