A novel waterborne epoxy coating with anti-corrosion performance under harsh oxygen environment

• The graphene modified by eco-friendly N-CQDs could stably disperse in water for more than 90 days. • A facile spin coating method was designed to fabricate orientated graphene-based waterborne epoxy coating. • The coating exhibited excellent corrosion resistance under harsh oxygen environment. • N-CQDs reinforced interface bonding force and induced the formation of protective film on steel plate surface. • The orientation of graphene maximized the barrier effect of graphene and prevented the formation of conductive network. Waterborne coating is eco-friendly, but exhibits limited anti-corrosion performance. Graphene is an ultrahigh anti-corrosion material for its perfect impermeability. Unfortunately, graphene tends to aggregate and randomly align in waterborne resin to form conductive network to accelerate corrosion. Herein, N-doped carbon quantum dots (N-CQDs) modified graphene (N-CQDs@Gr) was prepared via π-π interactions between N-CQDs and graphene, stably dispersing in water over 90 days and showing good dispersibility and compatibility in waterborne resin. Meanwhile, we designed a spin coating method and successfully fabricated orientated N-CQDs@Gr/waterborne epoxy (N-CQDs@Gr/WEP) coating, which exhibited long-term anti-corrosion performance. The impedance modulus of N-CQDs@Gr/WEP coating was about 200 times higher than that of waterborne epoxy (WEP) coating after 90 days of immersion in 3.5 wt% NaCl solution and still remained above 10 9 ohm cm 2 after 260 days of immersion. The coating also exhibited outstanding corrosion resistance under harsh oxygen environment. After 10 days of immersion in harsh oxygen environment, the coating still protected the metal well, and the impedance modulus of the coating was still maintained above 10 10 ohm cm 2 which was about three orders of magnitude higher than that of WEP coating. The excellent anti-corrosion performance of N-CQDs@Gr/WEP coating was interpreted as the reinforced interface bonding force, the orientation of graphene and the protective film covering metal surface. The orientation of graphene maximized the barrier effect of graphene and avoided the formation of graphene conductive network. The protective film formed in normal environment was mainly N-CQD@Fe 3+ complex film, whereas in harsh oxygen environment was mainly passive film.