Microstructural evolution and hot corrosion mechanisms of a TiAl-based alloy exposed to pure and mixed molten salt systems

Hot corrosion significantly impacts the service life of TiAl-based components in aero-engine environments. In this work, the hot corrosion behavior of Ti45Al8Nb alloy is systematically and comparatively investigated when exposed to molten Na₂SO₄ at 900 °C and molten Na₂SO₄–NaCl (75:25 wt%) at 700–900 °C. Under identical experimental conditions, the mixed-salt environment causes markedly higher material loss, severe scale spallation, and intensified microstructural damage compared with pure Na₂SO₄, and the corrosion severity exhibits a strong temperature dependence. In pure Na₂SO₄, a relatively compact multilayer corrosion scale composed of Ti-, Al-, and Nb-rich phases develops, in which Nb promotes stratified oxide formation that suppresses inward diffusion of corrosive species. In contrast, in the NaCl-containing environment, Cl⁻ ions trigger chlorine-induced self-sustaining reactions at the metal–scale interface, reducing scale adherence and accelerating degradation. Meanwhile, Nb participates in the acidic dissolution of Al₂O₃, while sulfur species contribute to internal sulfidation, primarily through TiS formation, leading to enhanced pit nucleation and deepening. These mechanisms collectively destabilize the corrosion scale, intensify intergranular attack, and result in a significantly higher degradation rate.