Study On Ni-based Composite Coating For Corrosion And Wear Behavior Via Electrodeposition On AZ91D Mg Alloy

Magnesium alloy has the advantages of high specific strength and low density,excellent casting properties,excellent electromagnetic shielding properties and biodegradability,so it is widely used in aviation,communication and medical equipment and other fields.However,the high wear and high corrosion defects of magnesium alloys lead to short service life.Therefore,improving the use defects of magnesium alloys by means of surface modification has become the focus of researchers.Among them,the use of electrodeposition method to construct metal coating on the surface of magnesium alloy is considered to be an effective solution to this problem.In this project,a series of pure nickel coatings,graphene/nickel composite coatings and double-layer graphene/nickel composite coatings were prepared on magnesium alloys by electrodeposition technology,and three kinds of coatings under different preparation parameters were systematically studied.Improvements in layer microstructure,hydrophobicity,mechanical properties,tribological behavior,and corrosion properties.Research indicates:1.With the increase of current density,the surface hardness of pure nickel coating first increased and then decreased.The hardness of the pure nickel coating prepared under the current density of 4 A/dm²is the highest（551 HV）,which is about8 times that of the magnesium alloy（76 HV）,which is mainly because the pure nickel coating has the smallest grain size at this current density,Has the best grain refinement effect.In particular,the pure nickel coating at this current density greatly improves the corrosion resistance of magnesium alloys,and its polarization potential is increased from-2.37 V of magnesium alloys to-0.5 V.2.With the increase of graphene doping concentration,the mechanical properties and corrosion and wear resistance of graphene/nickel composite coatings（Gr/Ni）were significantly improved.The microhardness of the composite coating prepared by doping with 0.2 g/L graphene is 983 HV,which is more than half higher than that of the pure nickel coating.Secondly,the incorporation of graphene greatly improves the bonding properties of the composite coating and the substrate,and its critical load（52.9 N）is 6-7 times that of pure nickel.Most importantly,the composite coating prepared by doping with 0.2 g/L graphene has almost no corrosion and wear products on the surface under the wear test conditions of the marine environment,and the wear rate is the lowest.The wear rate of the coating was reduced to 3.5x10^-5mm³/（N m）and 2.5x10^-5mm³/（N m）in atmospheric and marine environments,respectively.Compared with the corrosion and wear of pure nickel coating in marine environment,graphene doping improves the surface wettability of nickel-based coating,which is beneficial to prevent the intrusion and penetration of corrosive medium.At the same time,the micro-nano electrode effect of graphene reduces the grain size of the coating and the excellent self-lubricating effect of graphene itself,further improving the wear resistance of the coating.3.The novel bilayer Gr/Ni composite coating constructed by graphene and multi-interface synergies achieves excellent long-term corrosion resistance and wear resistance.This is mainly because the excellent interface effect brought by the multi-layer design greatly delays the intrusion time of the corrosive medium to the coating and reduces its destructive effect on the substrate.This provides design ideas for magnesium alloys serving in marine and aviation environments.