Microstructure And Corrosion Resistance Of The Microarc Oxidated Ceramic/epoxy Resin Composite Coatings On Sintered NdFeB Permanent Magnets

In order to improve the corrosion resistance of the sintered Nd Fe B magnets, ceramic coating were prepared on sintered Nd Fe B magnets by the technique of plasma electrolytic deposition, and composite films were also prepared by composite technique of plasma electrolytic deposition and electrophoretic deposition technology. The microstructure of the coatings were systematically observed by SEM、EDS and XRD, the corrosion resistance of the ceramic coating and composite films, which were prepared under different electrical parameters was also analyzed.The results showed that the coatings, obtained in aluminate system, were composed of γ-Al2O3. the thickness of the coatings, obtained in aluminate system, increased with applied voltage and time increased, and decreased with frequency increased; While the coatings obtained in silicate system may be composed of amorphous phase of Si O2,with the increase of the applied voltage, the thickness of the coatings have increased. Applied voltage has a great effect on the surface appearance and roughness when the electrolyte concentration is fixed in the systems. The surface roughness decreased firstly and increased afterwards with the increasing of applied voltage in aluminate system, which was opposite in silicate system. however with the rising of the processing time, the roughness first increased and then decreased in aluminate system; The thickness、roughness and aperture of the coatings in aluminate system were both lower than those in silicate system.The coatings, formed on sintered Nd Fe B magnets in aluminate system showed a low corrosion current density of 2.17×10-6A/cm2 and corrosion potential of-0.833 V, coatings obtained in silicate system showed corrosion current density of 2.15×10-6A/cm2 and corrosion potential of-0.932 V, The corrosion current densities of the coatings were decreased by an order magnitudes compared to the substrates. In a word, coatings obtained in aluminate exhibited the best corrosion resistance.After dealing with the electrophoresis, The thickness of the epoxy and composite film decreased with the increasing of plasma electrolytic deposition voltage and processing time, while increased with increasing of electrophoresis time.Composite membrane layer of the silicate system, and corrosion potential changes little, but it is about 0.4V higher than the corrosion potential of matrix. Compared with the corrosion current density of matrix, corrosion current density decreased three to four orders of magnitude; With the increasing of micro-arc oxidation voltage, corrosion current density fluctuated, corrosion current density of the composite film samples with 380 V micro-arc oxidation voltage is the most lowest, about 4.49×10-9 A/cm2。The results of salt spray test showed that, composite membrane layer improve the corrosion resistance of magnets significantly, the corrosion resistant performance of composite membrane layer in aluminate system is better than the silicate system’s. In aluminate system, composite film samples handled under the micro-arc oxidation of 20 V voltage, with 60~90min processing time, 1000 Hz frequency, under 60 V electrophoresis voltage for 45 s, have more than 500 h salt spray test failure time.