Magnetic strength and corrosion of rare earth magnets

Introduction: Rare earth magnets have been used in orthodontics, but their corrosion tendency in the oral cavity limits long-term clinical application. The aim of this project was to evaluate several; magnet coatings and their effects on magnetic flux density. Methods: A total of 60 neodymium-iron-boron magnets divided into 6 equal groups-polytetrafluoroethylene-coated (PTFE), parylene-coated, and noncoated-were subjected to 4 weeks of aging in saline solution, ball milling, and corrosion testing. Results: A significant decrease in magnet flux density was recorded after applying a protective layer of parylene, whereas a slight decrease was found after applying a protective layer of PTFE. After 4 weeks of aging, the coated magnets were superior to the noncoated magnets in retaining magnetism. The corrosion-behavior test showed no significant difference between the 2 types of coated magnets, and considerable amounts of iron-leached ions were seen in all groups. Conclusions: Throughout the processes of coating, soaking, ball milling, and corrosion testing, PTFE was a better coating material than parylene for preserving magnet flux density. However, corrosion testing showed significant metal leaching in all groups.