Mixed results were reported on the anticorrosion of graphenecoated metal surfaces while graphene serves as an effective short-term barrier against corrosion and oxidation due to its low permeability. to gases,, the galvanic cell, between graphene and the, metal substrate facilitates extensive corrosion in the long" run. Defects in the graphene layer provide pathways for the permeation of oxidizing species. We study the role of defects in graphene in the anticorrosion using first-principles theoretical modeling. Experiments in the highly reactive environment indicate that the oxidized products primarily distribute along the grain boundaries of graphene. We analyze the thermodynamics of the absorption of S and O on the grain boundaries of graphene on the basis of density functional theory. The insertion of S and O at the vacancy sites is energetically favorable. The interstitial impurities facilitate structural transformation of graphene and significantly decrease the mechanical strength of the graphene layer. Furthermore, the presence of the interstitial S and Oreduces the chemical stability of graphene by enhancing the of vacancies and promoting dispersive growth of corrosive reactants along the grain boundaries.