Corrosion Mechanisms of Commercial Superalloys in Binary and Ternary Chloride Molten Salts

In concentrated solar power (CSP) systems, structural materials face severe corrosion challenges induced by molten chlorides, with the corrosion severity being highly dependent on the salt composition. This study systematically compares the corrosion behavior of two representative superalloys, Inconel 625 and SS321, in binary NaCl–KCl and ternary MgCl₂–NaCl–KCl molten salts at 700 °C. The corrosion products and microstructural features were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD), in combination with static exposure tests to elucidate the underlying mechanisms. The results show that in NaCl–KCl molten salts, both alloys primarily form Cr₂O₃ as the protective product. However, the corrosion scale of SS321 is porous, whereas Inconel 625 develops a dense NiCr₂O₄ inner layer, exhibiting superior corrosion resistance. In the MgCl₂–NaCl-KCl molten salt system, Cr₂O₃ is replaced by a dense MgO layer forms on Inconel 625, coupled with Mo surface enrichment, which significantly inhibits Cr depletion and leads to a notably reduced corrosion rate relative to the binary salt. In contrast, the transformation of Cr₂O₃ on SS321 into porous MgCr₂O₄ exacerbates intergranular corrosion, resulting in a substantial degradation of corrosion resistance. This study elucidates the distinct corrosion pathways and mechanisms of different alloys in binary and ternary chloride salts, providing important guidance for the selection of molten salt compositions and corrosion-resistant structural materials in CSP applications.