| Magnesium alloy is the lightest metal structural material in engineering applications,its density is only 2/3 of aluminum alloy and 1/4 of steel.Moreover,magnesium alloy is known as a"green engineering material in the 21st century"because of its high specific strength,specific stiffness,excellent damping and electromagnetic shielding properties.However,due to the low potential of the standard electrode and the porous oxide film on the surface,magnesium alloys are prone to hydrogen evolution corrosion in Cl-or CO2environments.In order to solve the problem of poor corrosion resistance of magnesium alloys,researchers mainly focus on the surface treatment of magnesium alloys,such as chemical conversion,anodic oxidation and micro-arc oxidation.But the traditional method has some problems,such as low efficiency,toxicity to the human body and environment,high energy consumption and high cost.Therefore,it is necessary to develop a magnesium alloy coating with low cost,environmental protection,high bonding strength and excellent anti-corrosion properties.Organic coating is a widely used surface protection technology,which has the characteristics of low cost and easy operation and can effectively solve key problems such as poor corrosion resistance of magnesium alloys.However,due to the evaporation of the solvent,the coating inevitably produces defects and becomes the diffusion channel of the corrosive medium.Due to its impermeability,lubricity and excellent mechanical properties,graphene has been widely used in the field of surface protection.The addition of graphene fills in the tiny defects inside the coating,but its own chemical inertness results in poor dispersion in solvents and polymers.As a derivative of graphene,graphene oxide（GO）is the key to solving the problem of graphene agglomeration.Therefore,in this paper,Mg-3.0Zn-0.5Ca alloy is taken as the research object.By constructing graphene-based integrated coatings on magnesium alloy surface,the effects of GO modification,intermediate layer,the internal functional gradient of coating and layer structure on corrosion and wear resistance are systematically studied.The main research contents are summarized as follows:（1）In view of the poor dispersion and galvanic corrosion of GO in the coating,through the ring-opening reaction between polyethyleneimine（PEI）and the epoxy group of GO,the-NH2 functional group was grafted in GO to obtain functional graphene（FG）to provide compatibility with water-based epoxy resin（EP）.In addition,a rough insulating interlayer is used to isolate the electron transfer and improve the binding force.Cerium/water-based epoxy/functional graphene（Ce/EP/FG）coatings were prepared by chemical conversion and spin-coated assisted self-assembly.The coating has a double-layer structure,with the outer layer providing protection against external damage and the inner layer increasing the density and binding force of the outer layer.The corrosion rate of Ce/EP/FG composite coating is 2131 times lower than that of magnesium alloy,and the wear rate is reduced by 90%.（2）Because of the Ce/EP/FG wear rate higher than Ce/EP,the coating hardness can be effectively improved by increasing the content of FG.However,when graphene is added too much,aggregation occurs and the coating becomes a new diffusion channel for the corrosive medium,reducing the corrosion resistance.Ce/functional gradient coating（Ce/FGC）with concentration gradient structure were obtained by control of coating concentration assisted self-assembly technology,and different concentrations of coatings were compared.The results showed that FG was evenly distributed in the transition area,which enhanced the"maze"effect and improved the corrosion resistance,and had low content at the bottom,which weakened the galvanic corrosion.In addition,the Ce/FGC has high surface hardness and provides wear protection.The Ce/FGC has the highest corrosion rate of 2.97×10-6 mm/y and the lowest wear rate of 0.04mm3N-1m-1,which provides the integrated protection of corrosion and wear resistance for magnesium alloys.（3）In order to further improve the long-term corrosion protection ability of EP/FG coating,Ce/EP/FG-layer by layer（Ce/EP/FG-L）coating with a layer-by-layer assembly method was prepared by using a spray pen.The layer structure gives play to the barrier property of FG,making the coating show excellent corrosion resistance and long-term service performance.With the arrangement of the surface of the EP layer,the-NH2functional groups on the surface of FG enhance the binding strength between the layers.The results showed that the Ce/EP/FG-L coating maintained a high impedance after 7days of immersion in 3.5wt%Na Cl solution.In addition,the COF curve value of Ce/EP/FG-L coating is lower than that of Ce/EP/FG-M coating,and the coating has a certain lubrication effect to reduce the wear rate. |
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