Graphene has been shown to be a promising barrier for thermal corrosion due to its low gas and liquid permeability; however, there have been contradictory reports in the literature regarding the mechanisms of oxidation of graphene-coated Cu. This work systematically investigates the effect of chemical vapor deposited graphene grain size, point defect density, and underlying Cu orientation on the thermal oxidation of the underlying Cu in air. For graphene with either small grain size (similar to 0.04-0.4 mu m(2)) or large point defect density (similar to 800 mu m(-2)), oxidizers have relatively unhindered access through these defects to corrode the underlying Cu, and the corrosion is relatively independent of Cu orientation. For graphene with low defect density, the rate of Cu oxidation depends on the Cu crystal orientation on which the graphene is synthesized. Graphene-coated Cu (110) is completely corroded in just 2 min of thermal oxidation at 250 degrees C, while no surface oxidation for graphene-coated Cu (111) is observed for the same conditions. Thus, corrosion of graphene that is synthesized to achieve large grain size (> similar to 170 mu m(2)) is limited by the weakest graphene/Cu orientation interaction, not the size of the graphene grains, as was previously assumed.