Dissociative Adsorption of Hydrogen Molecules at Al 2 O 3 Inclusions in Steels and Its Implications for Gaseous Hydrogen Embrittlement of Pipelines.

Hydrogen embrittlement (HE) of steel pipelines in high-pressure gaseous environments is a potential threat to the pipeline integrity. The occurrence of gaseous HE is subjected to associative adsorption of hydrogen molecules (H₂) at specific "active sites", such as grain boundaries and dislocations on the steel surface, to generate hydrogen atoms (H). Non-metallic inclusions are another type of metallurgical defect potentially serving as "active sites" to cause the dissociative adsorption of H₂. Al₂O₃ is a common inclusion contained in pipeline steels. In this work, the dissociative adsorption of hydrogen at the α - A l 2 O 3 (0001) / α - F e (111) interface on the F e 01 1 ¯ plane was studied by density functional theory calculations. The impact of gas components of O₂ and CH₄ on the dissociative adsorption of hydrogen was determined. The occurrence of dissociative adsorption of hydrogen at the Al₂O₃ inclusion/Fe interface is favored under conditions relevant to pipeline operation. Thermodynamic feasibility was observed for Fe and O atoms, but not for Al atoms. H atoms can form more stable adsorption configurations on the Fe side of the interface, while it is less likely for H atoms to adsorb on the Al₂O₃ side. There is a greater tendency for the occurrence of dissociative adsorption of O₂ and CH₄ than of H₂, due to the more favorable energetics of the former. In particular, the dissociative adsorption of O₂ is preferential over that of CH₄. The Al-terminated interface exhibits a higher H binding energy compared to the O-terminated interface, indicating a preference for hydrogen accumulation at the Al-terminated interface. [ABSTRACT FROM AUTHOR]