Corrosion resistance of NdFeB magnets coated by Al

Hot steam corrosion tests were performed on heat-treated Al coated NdFeB magnets. The results suggest that the corrosion process can be divided into different stages. Initially, Nd which diffuses across the Al coating from the Nd-rich grain boundaries of the magnet during annealing, reacts with the moisture forming either Nd oxide or hydroxide at the surface. Upon further corrosion, the brittle Al-Fe-Nd-O-B compounds are formed, and they appear as lumps of oxides or hydroxides of Nd and Fe on the coating layer. And finally, the whole coating is consumed due to corrosion, and the corrosion process proceeds into the magnet material forming mostly Fe oxide or hydroxides. Among various annealed coated magnets, the one which is heat treated at 500/spl deg/C for 10 mins has the highest corrosion resistance against the moist and salty environment.