Abstract:

Corrosion of Ni- and Fe-based alloys in molten halide salts at temperatures between 550 °C and 800 °C poses a critical challenge for next-generation nuclear power systems that employ molten salts as heat-transfer media or fuels. In such environments, corrosion is generally governed by selective dissolution of the more reactive alloying elements (dealloying) at intermediate to high homologous temperatures. However, the coupled effects of high-temperature microstructural evolution, material property changes and corrosion remain poorly understood, particularly for alloys rich in more noble elements, limiting their industrial application. Notably, molten salt corrosion observed in these alloys at intermediate homologous temperatures is not adequately explained by conventional dealloying theory.

In this work, advanced three-dimensional electron microscopy techniques are used to directly visualize the chemical and microstructural evolution of Ni–Cr alloys following corrosion in molten fluoride salts. Three-dimensional reconstruction of the dealloyed region reveals the critical role of grain boundaries in governing bulk dealloying and subsequent recrystallization via a diffusion-induced recrystallization (DIR) mechanism. Furthermore, grain boundaries are shown to promote molten salt infiltration, leading to the formation of morphological features similar to discontinuous precipitation (DP). This multi-modal analysis demonstrates that the development of porosity and tunnel-like features along and across grain boundaries directly influences recrystallization within the corroded volume. Chromium dissolution during dealloying establishes a compositional gradient at the reaction front, which provides a thermodynamic driving force that, together with enhanced diffusion kinetics and grain boundary migration, enables DIR.

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Mehdi Mosayebi