Microscopic mechanism of hydrogen trapping by carbides in Cr-containing oilwell tubes: A first-principles study

Based on density functional theory (DFT), this study focuses on hydrogen-induced damage in Cr-containing tubing steels under H ₂ S–CO ₂ corrosive environments. Four typical carbides, namely Fe ₃ C, Cr ₃ C ₂ , Cr ₇ C ₃ , and Cr ₂₃ C ₆ , were selected as unified research objects. An atomic-scale analytical framework integrating surface activity, hydrogen adsorption, charge transfer, and energy evolution along linear interpolation paths was established to systematically compare the hydrogen-trapping capability and migration tendency of different carbides. Particular emphasis was placed on revealing the influence mechanism of the Cr atomic coordination environment in Cr-rich carbides on hydrogen adsorption and stable trapping. The results show that the Cr ₃ C ₂ (001) surface exhibits the highest surface energy of 3.850 J/m ² , indicating the strongest surface activity and initial hydrogen adsorption tendency. Adsorption energy calculations demonstrate that the Cr-atom-constituted hollow sites on Cr ₃ C ₂ and Cr ₇ C ₃ surfaces possess hydrogen adsorption energies as low as −1.204 eV and −1.195 eV, respectively, which are significantly stronger than that of the Fe-hollow site on Fe ₃ C (−0.631 eV). In addition, hydrogen atoms tend to occupy multi-atom coordinated concave sites, where stable trapping is achieved through multiple bonding interactions. The linear-interpolation-based path energy scanning results indicate that Cr ₃ C ₂ can effectively immobilise hydrogen atoms and suppress their penetration into the steel matrix, whereas Cr ₇ C ₃ can act as a deep trap, driving hydrogen atoms to diffuse into its bulk phase and enabling long-term immobilisation. These findings suggest that Cr ₃ C ₂ and Cr ₇ C ₃ play different types of hydrogen-blocking roles in Cr-containing tubing steels, serving as surface-retarding and bulk deep-trap precipitates, respectively. This study provides a theoretical basis for precipitate design in tubing steels resistant to stress corrosion cracking.