Study On Aluminum-Manganese Amorphous Coating Electrodeposited From Ionic Liquid On NdFeB Magnet

NdFeB permanent magnet materials are the third generation of rare earth permanent magnetic materials developed in the 1980s.Because of its superior magnetic properties and rich reserves of raw materials,NdFeB is widely used in modern industrial and electronic technology fields.However,the presence of rare earth neodymium and a multiphase structure of NdFeB make it easily corrode in humid,high temperature and hydrogen environment,which limits its wider application.The use of surface protective coating can effectively improve the corrosion resistance of NdFeB.In this paper,the Al-Mn amorphous coating was electrodeposited on the surface of NdFeB by room-temperature ionic liquids.The specific problems of the NdFeB pretreatment process,the relationship between Al-Mn coating thickness and corrosion resistance,and Al-Mn coating post-treatment were Investigated.The SEM,EDS,XRD,electrochemical workstation,pull-out tester and neutral salt spray tester were used to analyze and characterize the morphology,composition and properties of the coatings.The effects of pickling and anode activation pretreatment on the morphology,composition and adhesion strength of NdFeB were studied.The results showed that the pickling time of 30 seconds can well remove the oxide of NdFeB and obtain the coating-substrate adhesion strength of up to 6.2 MPa.The prolongation of pickling time,however,resulted in the decrease of the adhesion strength between Al-Mn coating and NdFeB;After the anodic activation in ionic liquid,the NdFeB matrix and Al-Mn coating showed a more excellent combination,and the adhesion strength exceeded 13.4 MPa.The morphology,composition and corrosion resistance of amorphous Al-Mn coatings with different thicknesses were studied.The results show that in the initial stage of electrodeposition of Al-Mn alloy,there is a phenomenon of preferential deposition of Mn.With the progress of electrodeposition,the content ratio of A1 and Mn in the coating tends to be constant,and the two are uniformly distributed in the coating.When the thickness of the Al-Mn coating is 4 ?m or less,the protection of the coating on the NdFeB substrate is not good,and the corrosion resistance of the coating significantly increases with the increase of thickness.When the thickness of the coating is 4?m or more,the coating is dense and uniform and the corrosion resistance improves slowly.The existence of surface passivation film makes the electrode potential of AI-Mn coating shifted positively,which caused the protective behavior of the coating on NdFeB changed.In the scratch immersion test,the preferential corrosion position was changed from the Al-Mn coating to the NdFeB matrix;After 500 h salt spray test,the minimum thickness of Al-Mn coating that no visible NdFeB corrosion occurred was 6?m?The effect of post-treatment on the corrosion resistance of AI-Mn amorphous coatings was investigated.The results showed that the film formed after the treatment of Alodine 5200 passivation solution had a lower resistance than that of the natural passivation film on Al-Mn coating.Although hot water and steam treatment are environmentally friendly and convenient,short-term treatment can not effectively improve the corrosion resistance of the Al-Mn coating,and long-term treatment will destroy the compactness of the Al-Mn coating,resulting in the formation of nano-holes on the surface of the coating,which in turn reduces the corrosion resistance of Al-Mn coating.