Why zinc is a good barrier for hydrogen

Abstract

Understanding the origins of the excellent H-barrier performance of Zn coatings is essential for optimizing their use as coatings on high strength steels and performance in various H-containing environments. We compare H-trapping energies associated with point defects in the metals Al, Cu, Ni, Pd, Cr, Fe, V, Mg, Ti, and Zn. Zn shows exceptionally weak interaction with H, with trapping energies ranging from 0.19 to 1.20 eV/at, far from the threshold of −0.6216 eV/at for irreversible trapping. Al and Mg also have low H-affinities, while Ti shows the strongest H-affinity. Band-structure projections reveal that all metals except Zn and Al form a localized Hmetal hybrid band, indicating that an available d-band is critical for strong H-bonding. In Zn, the fully filled d-band lies too low in energy to contribute to H bonding, explaining its low H-trapping energies. The d-band center position correlates with the trapping energies in different defect configurations. ZnO provides a more favorable site for H than metallic Zn, suggesting enhanced diffusion along partially oxidized grain boundaries. Stronger trapping at Zn/Al₂O₃, Zn/Fe, and Zn/ZnO interfaces also has implications for electroplating, where H generated during deposition may become immobilized at such interfaces, reducing the effectiveness of post-deposition bake-out treatments.