Molecular Insights into the Stability of Titanium in Electrolytes Containing Chlorine and Fluorine Ions

Titanium and its alloys are protected by a compact and stable passive film, which confers resistance to corrosion by the primary halogen chloride (Cl^-) while being less effective against fluoride (F^-). Although researchers have recognized different macroscopic corrosion effects of these halide ions on titanium, the underlying mechanisms remain largely unexplored. In this work, the bonding of Cl^-/F^- with stable passive films was studied in neutral and acidic (pH = 2.3) conditions. The synergistic effect between the interfacial hydrogen bond (HB) structure and halogens on titanium corrosion was first revealed using first-principles calculation and Raman spectroscopy. F^- forms more stable halogen-Ti bonds than Cl^-, resulting in titanium degradation. The proton combined with F^- exhibits a specific synergistic effect, causing corrosion of the passive film. The water hydrogen bond transformation index (HBTI) at the titanium/aqueous interface was 1.88 in an acidic solution containing F^-, significantly higher than that in neutral/acid solutions containing Cl^- (1.80/1.81) and a neutral solution containing F^- (1.81). This work clarifies the structure-activity relationship between HBTI and the destruction of titanium passive films. We propose that the microstructure of the interfacial HB is an undeniable factor in the corrosion of titanium.