| Magnesium alloys have good application prospects in the automotive,aerospace and electronics industries due to excellent properties such as light weight and high specific strength.However,the poor corrosion resistance of magnesium alloys which limits its promotion.Organic coating is an important method to effectively improve the corrosion resistance of magnesium alloys.Graphene oxide(GO),as an excellent two-dimensional carbon nanomaterial,has shown its broad prospects in the field of anticorrosion.Combined with surface modification technology,GO/epoxy resin composite coating can enhance the corrosion resistance of magnesium alloys significantly.Manipulation on the ratio of GO to n-hydroxymethyl acrylamide(NHAM),results in the formation of different NGO.And the reaction mechanism of GO and NHAM was explored and the structure of NGO could be regulated.NGO/epoxy composite coatings are prepared by screening out suitable NGO solution and added to epoxy resin coating on magnesium alloy surface.And then the structure and performance of the composite coating were analyzed.In the salt spray and immersion corrosion environment,explored the anti-corrosion resistance mechanism of the composite coating was explored through analyzing the structural changes of the coating on magnesium alloys.The chemical composition,element mapping,bonding method and micro-morphology were investigated using FTIR,TG,XPS,XRD,SEM and etc.The corrosion characterized through immersion,hydrogen evolution experiment and salt spray and electrochemical tests as well.The results showed the NGO/epoxy resin coating with a network structure and an optimal ratio of 10 mL GO(0.1 mg/mL):0.3 g NHAM.The combination of NHAM and GO mainly occured at the edge of the GO sheets(condensation reaction of-COOH and-OH)and the reaction in the middle of the GO sheets(polymerization reaction of C=C double bond in the GO plane).NHAM monomer can grow into a macromolecule long chains in and around the GO sheets through self-polymerization,lead to an increase in the distance between GO sheets and an improvement in their dispersibility in solution,and provide the possibility of good compatibility or compounding with epoxy resins.The optimal content of NGO is 0.005%-0.02%for NGO/epoxy composite coating,and the corrosion resistance of the composite coating would be significantly decreased when the content reached 0.03%.There was a microporous structure in the epoxy resin coating,NGO could reduce the cohesion in the epoxy layer which presented a positive effect on shrinkage and formed a strong crosslink with the epoxy matrix through the action of super hydrogen bonding.Therefore,the addition of NGO improved the microporous structure in the coating and effectively blocked the invasion of aggressive medium,and improved the corrosion resistance of the epoxy resin coating.This scenario is ascribed to the formation of the corrosion products Mg(OH)2 and Mg5(CO3)4(OH)2·xH2O clogged in pores which limited the invasion of corrosion ions.It is easy to form corrosion pits and difficult to deposit a large amount of corrosion products on the surface of the coatings by salt spray test.In the early stage of immersion,the corrosion profile look like a ring which caused by H2 formed bubbles on the surface of the coating.The surface tension changed at the edge of the bubble easily damaged the coating structure which first fell off in the ring of the coating.And the barrier effect of the NGO can effectively prevent the corrosion pits from deepening.The layers peeling characteristic of NGO/epoxy composite coating of magnesium alloys during the corrosion process effectively prolong the coating anti-corrosion time,indicating that NGO has critical impact on the epoxy resin coating and anti-corrosion performance on magnesium alloy. |
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