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.

2205 duplex stainless steel (DSS) has good corrosion resistance due to its typical duplex organization, but the increasingly harsh CO₂-containing oil and gas environment leads to different degrees of corrosion, especially pitting corrosion, which seriously threatens the safety and reliability of oil and gas development. In this paper, the effect of temperature on the corrosion behavior of 2205 DSS in a simulated solution containing 100 g/L Cl⁻and saturated CO₂was investigated with immersion tests and electrochemical tests and combined with characterization techniques such as laser confocal microscopy and X-ray photoelectron spectroscopy. The results show that the average critical pitting temperature of 2205 DSS was 66.9 °C. When the temperature was higher than 66.9 °C, the pitting breakdown potential, passivation interval, and self-corrosion potential decreased, while the dimensional passivation current density increased, and the pitting sensitivity was enhanced. With a further increase in temperature, the capacitive arc radius of 2205 DSS decreased, the film resistance and charge transfer resistance gradually decreased, the carrier density of the donor and acceptor in the product film layer with n + p bipolar characteristics also increased and the inner layer of the film with Cr oxide content decreased, while the outer layer with Fe oxide content increased, the dissolution of the film layer increased, the stability decreased, and the number and pore size of pits increased.

The poor surface performance of titanium alloys substantially limits their application in many fields, such as the petrochemical industry. To overcome this weakness, the Cu and Ni double layers were deposited on the surface of TC4 alloy by the electroplating method, and the isothermal diffusion process was performed at 700 °C to enhance the binding ability between Cu and Ni layers. The isothermal diffusion behavior and microstructure of the coating were systematically analyzed, and tribological property and corrosion resistance of the coating were also evaluated to reveal the influence of isothermal diffusion on the surface performance. It was shown that multiple diffusion layers appeared on the Cu/Ni and Ni/Ti interface, and that Ni_xTi_y and Cu_xTi_y phases were formed in the coating with the increase of diffusion time. More importantly, Kirkendall diffusion occurred when the diffusion time increased, which led to the formation of continuous microvoids and cracks in the diffusion layer, weakening the surface performance of the Cu/Ni coatings. This paper unveils the relationship between the microstructure of the Cu/Ni coatings and isothermal diffusion behavior, providing guidelines in preparing high performance surface coatings.

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.