Characterization of Sintered NdFeB Magnets after Phosphating in Alkaline and Acidic Environments

摘要

Despite their excellent magnetic properties NdFeB permanent magnets are highly susceptible to corrosion. Conversion coatings, such as phosphating, have been evaluated as promising methods for corrosion control of these types of magnets. In the present work, alkaline and acid solutions have been used for phosphating a NdFeB commercial magnet. Characterization of the phosphate layer formed in alkaline solution (pH>8) of Na_2HPO_4, was carried out by Auger Electron Spectroscopy (AES), Energy Dispersive X-ray analysis (EDX), and Electrochemical Impedance Spectroscopy (EIS). AES and EIS results showed that the layer formed in the alkaline solution was not protective. On the other hand, the phosphate layer formed in the acid solution (3,8H<4,6) of NaH_2PO_4 produced a significant increase in the magnet corrosion resistance. It is proposed that improvement in the corrosion resistance of the magnet phosphated in the acid solution was due to a more uniform attack of the whole surface leading to the formation of a more continuous and protective conversion layer, compared to that formed in alkaline solution. The resistance of the conversion coating layer formed in acidic medium, estimated from EIS data, is in-between 5000 and 20000 Ω cm~2, while that of the layer formed in the alkaline environment is less than 1000 Ω cm~2. It is believed that precipitation of phosphates at high pH's do not produce a homogeneous coverage of the various phases of sintered NdFeB magnets, since the precipitation is favored at the more active areas of the magnet.