Studies On New Pretreatment Processes, Properties Of Ni-W-P Electrodeposits And Stripping Technology Of Unqualified Ni-W-P Coatings On NdFeB Magnets

NdFeB permanent magnetic materials are known as the “magnetic king” becauseof their excellent magnetic properties, they have broad market prospect in the modernindustry and electronic technology due to sky-high energy product, coercive force andthe advantages of high energy density. But NdFeB magnets exhibit drawbacks, such assurface loose, lots hole and easy oxidation corrosion, which limits their use areas.Although the researches on improving the corrosion resistance of sintered NdFeBmagnets had made certain progress, but from the perspective of widely used magnets,the existing technologies still can not fundamentally solve the problem of poorcorrosion resistance of magnet.This paper reviews the cause of corrosion and corrosion mechanism of NdFeBmagnets, pretreatment processes before plating, the magnet surface protectiontreatment and research progress of the stripping technology of unqualified deposits onNdFeB magnets. Considering the shortcomings of NdFeB magnet during pretreatment,it is necessary to develop one-step of pickling-activation pretreatment for NdFeBmagnets before plating, in order to replace the commonly used two-step method withpickling and activation containing strong acid and fluoride. In this thesis, we studythe process of pickling and activation, analyze the mechanism of one steppickling-activation for NdFeB magnet, and compare with the commonly used two stepmethod with pickling and activation processes. Due to its high hardness, high thermalstability, high wear resistance, excellent corrosion resistance and other characteristics,NiWP alloys have attracted more and more interests. So far, however, the reportsabout directly electroplating NiWP alloy coatings on NdFeB magnets can be hardlyfound. The NiWP electrodeposits on NdFeB magnets with good adhesion andcorrosion resistance were obtained by the developed pretreatment, meanwhile, thestripping technology of nonconforming NiWP deposits on NdFeB magnets was alsostudied. The research results are as follows:1. Study on the pretreatment processes of NdFeB magnets before platingOne-step H2C2O4pickling-activation can replace the commonly used two-stepprocess of pickling and activation containing strong acid and fluorine. Activation filmcomposed of neodymium and iron oxalate formed by H2C2O4pickling-activationeffectively improves the corrosion resistance of the substrate in the subsequentprocesses. Pretreatment processes optimization was carried out by investigating the influence of various conditions containing alkaline degreasing and one-step H2C2O4pickling-activation on the binding force between the coatings and the magnets.Through the analysis of experimental results, the optimum pretreatment processes ofNdFeB magnets are shown as follows:(1) Alkaline degreasing, the concentration ofNaOH is6g/L, the concentration of Na2CO3is50g/L, the concentration of Na3PO4is70g/L, the concentration of twelve sodium dodecyl sulfate is0.5g/L, formic acid isused to adjust pH9~9.5, temperature65-70℃, ultrasonic degreasing;(2) H2C2O4pickling-activation process, the concentration of H2C2O4is0.5mol/L, roomtemperature, activation time2-4min. Meanwhile, compared to the commonpretreatment process, the electrochemically homogeneous surface state of NdFeBmagnets via the H2C2O4activation process was more propitious to improve theadhesion between the NdFeB magnets and electrodeposits.2. Study on the properties of NiWP electrodeposits on NdFeB magnets directlyA new H2C2O4pickling-activation pretreatment method was firstly successfullyused in electroplating NiWP alloys on NdFeB magnets. The performance of NiWPcoatings on NdFeB magnets was systematically studied. The NiWP electrodeposits onNdFeB magnets with good adhesion and corrosion resistance were obtained by theH2C2O4pickling-activation pretreatment. The NiWP coated NdFeB have excellentanticorrosion property in acidic, neutral and alkaline corrosion medium. It could passtrough ca.1024h neutral salt spray test without any pitting corrosion. Meanwhile, theNiWP alloy deposits showed little influence on the magnetic properties of NdFeBmagnets.3. Study on the stripping technology of unqualified NiWP deposits on NdFeBIn view of the particularity of NdFeB magnets and NiWP electrodeposits withexcellent corrosion resistance, it has the important research value to develop thenondestructive stripping technology of nonconforming NiWP deposits on NdFeBmagnets. Experiments show that the developed stripping technology not only canensure high stripping rate, but also to protect NdFeB magnets from corrosion and geta good apparent properties under the condition of the concentration of H2C2O4is0.5mol/L, the concentration of phosphoric acid is50ml/L, the current density is20A/dm2, the temperature35-45℃.