The adsorption characteristics of corrosive anions (Cl⁻, HS⁻, S²⁻, HCO₃⁻ and CO₃²⁻) on TiO₂ of TC4 titanium alloy in a NaCl solution containing H₂S and CO₂ were studied by density functional theory (DFT). The stable adsorption configuration of each corrosive species on the TiO₂ (110) surface was obtained by geometric optimization, and the electronic structure and interface binding energy were calculated and analyzed. The results showed that the optimal adsorption positions of Cl⁻, HS⁻, S²⁻, HCO₃⁻ and CO₃²⁻ on TiO₂ (110) were all bridge positions. There was a strong charge interaction between the negatively charged Cl, S and O atoms in Cl⁻, HS⁻, S²⁻, HCO₃⁻ and CO₃²⁻ and the positively charged Ti atoms of TiO₂. The interface bonding was mainly caused by charge movement from around Ti atoms to around Cl, O, S atoms. The energy levels were mainly caused by the electron orbital hybridization of Cl-3p⁵, S-3p⁴, O-2p⁴ and Ti-3d². All adsorption configurations were chemical adsorption. The order of influence of the five ions on the stability of TiO₂ was S²⁻ > CO₃²⁻ > Cl⁻ > HS⁻ > HCO₃⁻. Finally, a novel corrosion mechanism was proposed to illustrate the dynamic evolution processes of pits.

The service environment of OCTG (Oil Country Tubular Goods) in oil and gas fields is becoming more and more severe due to the strong affinity between ions or atoms of corrosive species coming from solutions and metal ions or atoms on metals. While it is difficult for traditional technologies to accurately analyze the corrosion characteristics of OCTG in CO₂-H₂S-Cl⁻ systems, it is necessary to study the corrosion-resistant behavior of TC4 (Ti-6Al-4V) alloys based on an atomic or molecular scale. In this paper, the thermodynamic characteristics of the TiO₂(100) surface of TC4 alloys in the CO₂-H₂S-Cl⁻ system were simulated and analyzed by first principles, and the corrosion electrochemical technologies were used to verify the simulation results. The results indicated that all of the best adsorption positions of corrosive ions (Cl⁻, HS⁻, S²⁻, HCO₃⁻, and CO₃²⁻) on TiO₂(100) surfaces were bridge sites. A forceful charge interaction existed between Cl, S, and O atoms in Cl⁻, HS⁻, S²⁻, HCO₃⁻, CO₃²⁻, and Ti atoms in TiO₂(100) surfaces after adsorption in a stable state. The charge was transferred from near Ti atoms in TiO₂ to near Cl, S, and O atoms in Cl⁻, HS⁻, S²⁻, HCO₃⁻, and CO₃²⁻. Electronic orbital hybridization occurred between 3p⁵ of Cl, 3p⁴ of S, 2p⁴ of O, and 3d² of Ti, which was chemical adsorption. The effect strength of five corrosive ions on the stability of TiO₂ passivation film was S²⁻ > CO₃²⁻ > Cl⁻ > HS⁻ > HCO₃⁻. In addition, the corrosion current density of TC4 alloy in different solutions containing saturated CO₂ was as follows: NaCl + Na₂S + Na₂CO₃ > NaCl + Na₂S > NaCl + Na₂CO₃ > NaCl. At the same time, the trends of R_s (solution transfer resistance), R_ct (charge transfer resistance), and R_c (ion adsorption double layer resistance) were opposite to the corrosion current density. The corrosion resistance of TiO₂ passivation film to corrosive species was weakened owing to the synergistic effect of corrosive species. Severe corrosion resulted, especially pitting corrosion, which further proved the simulation results mentioned above. Thus, this outcome provides the theoretical support to reveal the corrosion resistance mechanism of OCTG and to develop novel corrosion inhibitors in CO₂-H₂S-Cl⁻ environments.