Preparation Of Waterborne Epoxy Electrophoretic Coating Doped With Graphene Oxide On The Surface Of Magnesium Alloy And Its Corrosion Behavior

As the lightest metal structural material,magnesium alloy is known as "the 21st century green engineering material”.However,magnesium alloy has poor corrosion resistance and exhibits high chemical and electrochemical activity.The protective coating can greatly improve the corrosion resistance of magnesium alloy,in which cathodic electrophoretic coating is a very environmentally-friendly magnesium alloy protection method commonly used in industry.In recent years,graphene has been widely used in the coating field due to its unique two-dimensional structure and excellent physical and chemical properties.Graphene has a large specific surface area,which can form a good physical barrier in the polymer coating and improve the protection of the coating on the metal matrix.In this work,ZK60 magnesium alloy is used as the metal matrix material,amine modified waterborne epoxy resin(EP)as electrophoretic paint,and graphene oxide(GO)as filler.Micro-arc oxidation(MAO)is used as pretreatment.The "constant pressure-approximate constant current" electrophoresis method is employed to prepare MAO-GO/EP composite coating.The effects of micro-arc oxidation pretreatment,micro-arc oxidation voltage and electrophoresis voltage on the corrosion resistance of electrophoretic coating were investgated by the scan-ning slectron microscope(SEM),electrochemical impedance spectroscopy(EIS),surface roughness,coating thickness,full immersion test,ect.It’s found that the EP coating thickness is inversely proportional to the micro-arc oxidation voltage and proportional to the electrophoresis final voltage.Through macroscopic immersion and EIS characterization,the final optimum process parameters were as follows:MAO voltage 330V,electrophoresis final voltage 120V.The MAO/EP coating prepared under the opyimum condition has less uniform thickness defects,and no macroscopic corrosion point appears on the surface when was immersed in 3.5%NaCl solution for 150 d.The impedance at the frequency of 0.01 Hz is still 109 Ω·cm2.The SEM cross-section of the MAO-GO/EP coating indicates that the dispersion of graphene oxide in the EP coating can reduce the number of defects such as pores and cracks inside the coating.Macroscopic immersion and EIS show that when the mass fraction of GO is 0.1%,a relatively continuous network-like physical shielding structure can be formed,which effectively prolongs the diffusion path of the corrosive medium and reduces the corrosion reaction rate at the interface between the coating and the magnesium alloy substrate.This layer has the best corrosion resistance.The surface of this coating was soaked in a 3.5%NaCl solution for 180 days without macroscopic corrosion and the impedance at a frequency of 0.01 Hz is still 1010 Ω·cm2.