▎ 摘　　要

In this study, barrier properties of as-grown graphene layers (Gr) were systematically characterized on two widely used polycrystalline metal substrates, copper (Cu) and nickel (Ni), under ambient humid air as well as in aggressive steam/air environments. Four types of graphene coatings were used to distinguish the bonding effect: directly grown graphene coatings on Cu and Ni as well as transferred graphene coatings on Cu and Ni. Corrosion rates were examined through Tafel analysis in both single-cycle and multiple-cycle experiments. Our study shows that in-plane diffusion of water and oxygen into underlying Cu substrates triggers corrosion of Cu surfaces even in the presence of graphene coatings. Similar corrosion caused by in-plane diffusion of corrosive species was not observed in the Ni/Gr system, which effectively protects the Ni surface in most regions under steam/air environment over 1 month. The Ni/Gr system demonstrates ultra-low and stable corrosion rates compared with those of transferred graphene on Ni (TrGr/Ni) and Cu/Gr systems in destructive corrosion evolution tests. After three cycles cumulative Tafel tests, the corrosion rates of TrGr/Ni and Cu/Gr system dramatically increased 66.3 times and 361.6 times compared to that of Ni/Gr system, respectively. This study shows that the formation of a strong metal-graphene interfacial bond plays a critical role in enabling long-term corrosion-resistant by applying graphene coatings on polycrystalline metal substrates.