Corrosion resistance of Al2O3/Cr composite coating on sintered NdFeB magnet

To improve the corrosion resistance of sintered NdFeB magnets, a composite coating of Al₂O₃/Cr was applied to the surface of the magnet using a combination of cathodic plasma electrolytic deposition (CPED) and double glow plasma deposition techniques. The morphologies of the surface and fractured cross-section, as well as the phase composition and corrosion resistance of the composite coatings, were examined through SEM, XRD, accelerated corrosion immersion test, and electrochemical tests, respectively. The influence of the composite coating on the magnetic characteristics of sintered NdFeB magnet was also evaluated. The results show that the Al₂O₃ intermediate layer is completely covered by the Cr coating and the interface between the Al₂O₃ layer and the Cr layer forms a mechanical interlocking effect. The surface of the Al₂O₃/Cr coating exhibits a hump cellular-like structure with the preferred orientation of Cr (200) plane. Compared with uncoated NdFeB magnet, the maximum magnetic energy product of the Al₂O₃/Cr composite coated sample decreases by 11.1 %. In the sulfuric acid immersion test, the weight loss of the Al₂O₃/Cr coated sample is only 15.04 % of that of the uncoated sample. The Al₂O₃/Cr coated NdFeB sample also demonstrates superior corrosion resistance compared to both the uncoated and Cr coated samples in the electrochemical tests. The Al₂O₃/Cr composite coating provides a new strategy for improving the corrosion resistance of sintered NdFeB magnets.