Effect of pretreatment on anticorrosive phosphating conversion coating of sintered NdFeB magnets

Different pretreatment procedures were investigated, and alkaline cleaning and acid picking pretreatment techniques were developed, for the first time, to achieve a phosphating conversion coating on a sintered NdFeB magnet. The effects of different additives on the pretreatment procedures are illustrated by several visual schematics. Corrosion immersion tests showed that NdFeB substrates with optimum pretreatment and phosphating conversion coating procedures can endure at least 17 h before pitting occurs in 5 wt-% neutral NaCl solution. Potentiodynamic polarisation tests in 3.5 wt-% NaCl solution show that corrosion potential E-corr increases nearly 60 mV by combination of pretreatment and phosphating conversion coating, and the corresponding polarisation impedance value R is increased by 50%. The corrosion weight loss tests also confirm that compact and anticorrosive phosphating conversion coating could be formed on the NdFeB substrate with the optimum pretreatment.