This study investigates the evolution process of hot corrosion in bi-phase Ti-6.5Al-1.5Zr-3.5Mo-0.3Si (TC11) titanium alloy induced by sodium chloride (NaCl). Through morphological progression and elemental distribution analyses, the integrated processes involved in the onset of internal corrosion during hot corrosion are elucidated, revealing intriguing phenomena that deviate from conventional mechanisms. The development of corrosion can be divided into three stages: First there is the rapid growth of the external corrosion layer induced by Cl; then a Cl and O induced internal corrosion is initiated when the external corrosion layer reaches a critical thickness, at the same time the external corrosion decreases; finally, the chlorine pressure is too low to act as the corrosion depth increases, O becomes a major contributor to deep internal corrosion, leading to O induced internal corrosion. Significant interphase selective corrosion was observed in this internal corrosion zone: the β phase was corroded while the α phase remained uncorroded, challenging the prevailing notion that the β phase is more corrosion-resistant in titanium alloys. This distinctive phenomenon results from the differences in oxygen solubility, element diffusion rates and the reactivity between the α and β phases.
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