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Figure 5: Self-healing property of r-GO/oil coating. ((a)-(d)) Snapshots showing that a scratch about 0.5 mm wide fully heals in seconds (scale bar = 0.5 mm). ((e)-(h)) The corresponding optical microscopy images reveal that oil can rapidly flow to the scratched area, followed by reorganization of particles to reestablish the network (scale bar = 50 μm). ((i)-(l)) Self-healing of r-GO/oil coating when scratched in (i) water, (j) 5% HCl, (k) 10% HCl, and (l) 20% HCl. The position of the scratch is indicated by the red arrows. The healing time in more concentrated HCl solution increases since the flow of oil to the exposed area is increasingly hindered by H2 evolution at faster etching rate. In 20% HCl, the rate of H2 evolution is too fast to allow self-healing to proceed. (m) Open-circuit current of an Al wire coated with r-GO/oil immersed in 1 M (3%) HCl during a scratch test. When the coating is scratched, a small area of the metal is exposed. The resulting local corrosion triggers a spike in the current that quickly dissipates within a few seconds, indicating that the coating has self-healed to restore its protective barrier property.