Sintered NdFeB magnets possess excellent magnetic properties. However, the corrosion resistance property of NdFeB is very poor due to its multiphase microstructure consisting of matrix phase Nd 2 Fe 14 B, Nd-rich phase, and B-rich phase. The corrosion behavior of NdFeB magnets in sodium hydroxide (NaOH), sodium chloride (NaCl), nitric acid (HNO 3 ), and oxalic acid (H 2 C 2 O 4 ) solutions was investigated by immersion and electrochemical tests. HNO 3 is the strongest corrosive electrolyte compared with the other three solutions. The increase in HNO 3 concentration can accelerate the corrosion of NdFeB magnets. NaCl belongs to medium corrosion electrolyte. A NaCl concentration of 0.5 M shows the severest corrosive feature in comparison with other concentrations of NaCl solution. NdFeB hardly suffers corrosion in NaOH and H 2 C 2 O 4 solutions owing to the formation of passivation films on the surface of magnets. Based on the corrosion behavior of NdFeB in different electrolytes, the possible corrosion mechanisms are discussed.
The absorption of hydrogen by NdFeB magnet has been investigated by using the electrochemical charging technique at constant cathodic current density Ic ranging from 0 to 4 mA/cm2. Open circuit potential measurements (OCP) and polarization curves were carried out to study the corrosion behavior of the charged NdFeB magnet in 0.01 mol/L NaCl solution. The results showed that hydrogen had a strong influence on the corrosion of NdFeB magnet. The open circuit potential became gradually negative due to the hydrogen incorporation into the NdFeB magnet. The corrosion resistance was reduced gradually with the increasing cathodic current density Ic. The surface structure and the morphology of the charged NdFeB magnet were examined by XRD and SEM. The results revealed that the effect of the absorbed hydrogen focused mostly on accelerating the exfoliation corrosion of Nd2Fe14B matrix grain.
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