Grain boundary- and time-dependent corrosion mechanism of GH3625 superalloy in a molten solar salt

Abstract

This work systematically examined the corrosion behavior of GH3625 superalloy in molten solar salt at 565–600 °C, unveiling the corrosion resistance mechanism using TEM and APT, etc. technologies. A decreasing corrosion rate with time was observed in both fine- and coarse-grained alloys. The coarse-grained alloys demonstrated a significantly lower corrosion rate than fine-grained alloys at 500 h corrosion, while the corrosion rate difference diminished when corrosion time was extended to 3000 h. It could be attributed to the corrosion resistance mechanism transformation from the grain-boundary control to the oxygen penetration through the thickened corrosion layers. Two depletion zones with different element concentrations were initially discovered and successfully explained through thermodynamics and kinetics. A corrosion multi-layer was confirmed, and NiO and Cr₂O₃ were found as the principal oxides. Furthermore, the Cr₂O₃ rather than NiO, providing the corrosion resistance in the molten solar salt, was confirmed. Meanwhile, the NiO is also significant as it protects the Cr₂O₃ from being contacted with molten solar salt. This study systematically elucidates the effects of grain boundary and corrosion time on the corrosion resistance of alloys while solving the long-standing controversy regarding which of the two plays a central protective role, providing a theoretical basis for alloy design.