Study On Preparation Technology And Protective Properties Of Protective Coating On Sintered NdFeB Surface

Sintered NdFeB permanent magnet has super magnetic force, it has unique advantages in the fields of motors and the Small-scale devices. The corrosive feature limited its range of applications. So the surface protection coating has important significance to improve its corrosion resistance.Environmental-friendly and chromium-free Zn-Al coating on sintered NdFeB magnet was prepared by dipping method with silane as adhesive and flake-like Zn and Al powder as raw materials. Al film deposited on sintered NdFeB matrix by physical vapor deposition. The phase structure and morphology of the Zn-Al coating were characterized by X-ray diffraction (XRD) and scan electron microscopy (SEM). The corrosion resistance and mechanism were analyzed by neutral salt spray (NSS)、High temperature and pressure accelerated aging test(PCT)、Thermal Shock Test and electrochemical test. The coating adhesion was measured by tension test. The results indicate that the NdFeB magnet was coated with a uniform dense Zn-Al layer. The coating thickness is about 20-40μm and coating adhesion can reach up to 10.95 MPa; Zn-Al coating XRD pattern tests show that:Different proportions of Zn-Al coating have similar diffraction peaks within slightly different peak height. The most intense diffraction peaks of Zn and Al are very sharp at 20=43.22° and 20=44.72°, which mean that Zn/Al have high consistency degree of orientation; The NSS test result is up to 600 h and far better than the traditional electroplated zinc layer. Zn-Al coating has the function of anodic protection for NdFeB matrix. The flake-like structure could extend corrosion channel of the corrosion media.Al film deposited on sintered NdFeB matrix by physical vapor deposition to improve its corrosion resistance, and the influence of pre-treatment、evaporation current、the vacuum chamber temperature on the Al film surface morphology and corrosion resistance were tested. The results indicate that:Acid pickling pre-treatment has high consistency with better corrosion resistance than sand blasting pre-treatment. With the increase of evaporation current, aluminum film with dense smooth surface morphology gradually becomes rough and uneven. Potentiodynamic polarization test results show that with the increase of evaporation current, the corrosion resistance decreased. The smallest corrosion current is 1.584×10-6 A·cm-2 at 2100A evaporation current. This result is consistent with the salt spray test. With the increase of Vacuum chamber temperature from 50℃ to 200℃, the corrosion current increased then decreased, The smallest corrosion current is 3.286×10-6 A·cm-2 at the temperature of 508℃.Tension test indicates that Acid pickling pre-treatment has high consistency with better coating adhesion than sand blasting pre-treatment. Thermal Shock Test show that all the Al film can withstand thermal shock up to 168 h.