Topographical Feature Regulation Of Graphene And Its Effect On Anticorrosion Property Of Coatings

Graphene materials have been intensively studied in metal protection field due to its ultra-thin structure and perfect impermeability to aggressive species.However,the improved corrosion protection properties brought by graphene are severely restricted due to the inevitable crumpling and folding of graphene nannosheets.It also has the"corrosion promoting activity",which can easily induce micro-galvanic corrosion between graphene and the metal at the damage of the coating.In this paper,we start from the two aspects of realizing the structure spreading of graphene nanosheets and inhibiting its"corrosion promotion activity"and propose to control the topographical feature of graphene through surface modification.The effects of rigid or flexible self-unfold graphene on the anti-corrosion performance of composite coatings were studied separately.And the the protective performance of the coatings was urther improved by the directional arrangement of the fillers.The results indicated that,unlike other graphene nanosheets with irregular crumpling and folding morphology,modified graphene samples with high diameter-to-thickness ratio and special unfold flake morphology can lengthen the diffusion path of aggressive species in coating matrix and enhance the corrosion protection performance of epoxy coatings effectively.The main contents of this paper include the following aspects:(1)Preparation of rigid and unfolded SiO₂/graphene nanosheets(SiO₂-rGOs)and its anti-corrosivity.Graphene was sandwiched into mesoporous silica via a soft-template method achieving rigid and self-unfolded SiO₂-rGOs.The surface morphology of SiO₂-rGOs were adjusted by controlling the degree of modification.Research indicates that SiO₂-rGOs-10sample has high aspect ratio and ideal unfold structure.The barrier properties of the nanosheets-reinforced coatings(SiO₂-rGO/WEPc)are evaluated by measuring their corrosion protection performance.Electrochemical test results reveal that the SiO₂-rGO-10/WEPc exhibits an enhanced protection efficiency of 99.5%higher than that of blank epoxy coating(WEPc)and 99.4%higher than that of rGOc.It can still protect the metal matrix from corrosion after 35 days immersion in 3.5 wt.%Na Cl solution.The scratch test shows that the composite material also has a high insulativity which effectively inhibit the undesired corrosion-promotion activity of graphene.Numerical results further demonstrate that,comparing with conventional graphene nanosheets with irregular crumpling and folding morphology,rigid and self-unfolded SiO₂-rGOs perform better in prolonging and twisting the diffusion pathway of aggressive species in the coating matrix.(2)Preparation of rigid and unfolded LDH/graphene nanosheets(LDH-rGOs)and its anti-corrosivity.The Mg Al-layered double hydroxides(LDHs)were prepared by hydrothermal method and then LDHs and graphene were tunable assembled by electrostatic adsorption to achieve the rigid LDH/graphene nanosheets(LDH-rGOs).The electrostatic force between material interfaces induces the unfolding of flexible graphene nanosheets.The morphology of the modified material can be adjusted by controlling the size of graphite powder.The results indicate that LDH-rGOs sample prepared with small size graphene nanosheets(LDH-rGO-Ss)has the ideal flake structure.The LDH-rGOs reinforced coatings(LDH-rGO/WEPc)were prepared by adding different LDH-rGOs to waterborne epoxy coatings.The electrochemical experiments reveal that the LDH-rGO-S/WEPc sample exhibits an enhanced protection efficiency of 99.7%higher than that of WEPc and 99.5%higher than that of graphene/epoxy coating(rGO/WEPc).Unfortunately,the scratch test shows that LDH-rGOs have the similar corrosion-promotion activity to graphene,which can easily accelerate the galvanic corrosion of the metal substrate at the coating defects.(3)Preparation of flexible and unfolded sulfonated polyaniline/graphene nanosheets(SPANi-rGOs)and its anti-corrosivity.Due to the inflexibility,rigid fillers near the air/coating interface are easy to pierce the coating surface and cause rapid coating failure.Therefore,surface modification of graphene with SPANi was made to achieve flexible and unfold SPANi-rGOs.The self-unfolding behaviour can be ascribed to strong inner-layer and inter-layer graphene electrostatic repulsion.These nanosheets were mixed into waterborne epoxy coatings with different addition to prepare the composite coatings(SPANi-rGO/WEPc).The results of electrochemical experiments indicate that the SPANi-rGO-0.25/WEPc sample(SPANi-rGOs addition amount is 0.25 wt.%)exhibits an enhanced protection efficiency of99.9%higher than that of WEPc and 81.2%higher than that of rGOc,showing a high anticorrosive performance.The scratch tests show that the SPANi-rGOs sample has no significant corrosion-promotion activity under the addition of 0.25 wt.%.