Why NdFeB magnet need to be coated?

Samarium Cobalt, Alnico, and Ceramic materials are corrosion resistant, and do not require to be coated against corrosion. Alnico is easily plated for cosmetic qualities, and Ceramics may be coated to seal the surface, which will otherwise be covered by a thin film of ferrite powder (although not a problem for most applications).

Neodymium Iron Boron magnets are susceptible to corrosion and consideration should be given to the operating environment to determine wether to be coated. Nickel or tin plating may be necessary for Neodymium Iron Boron magnets, however, the material must be properly prepared and the plating process properly controlled for successful plating. Aluminum chromate or cadmium chromate vacuum deposition has been successfully tested, with coating thickness as low as 5um. Teflon and other organic coatings are relatively inexpensive and have also been successfully tested. A further option for critical applications is to apply two types of protective coatings or to encase the magnet in a stainless steel or other housing to reduce the chances of corrosion.