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.