Double coating protection of Nd–Fe–B magnets: Intergranular phosphating treatment and copper plating

• We combined intergranular phosphating and copper plating to protect Nd–Fe–B.
• The phosphate conversion coating was formed preferably on the intergranular region.
• The phosphating coating can obviously improve the corrosion resistance of Nd–Fe–B.
• The corrosion resistance of Nd–Fe–B was improved by double coating protection.

In this work, a double coating protection technique of phosphating treatment and copper plating was made to improve the corrosion resistance of sintered Nd–Fe–B magnets. In other words, the intergranular region of sintered Nd–Fe–B is allowed to generate passive phosphate conversion coating through phosphating treatment, followed by the copper coating on the surface of sintered Nd–Fe–B. The morphology and corrosion resistance of the phosphated sintered Nd–Fe–B were observed using SEM and electrochemical method respectively. The phosphate conversion coating was formed more preferably on the intergranular region of sintered Nd–Fe–B than on the main crystal region; just after a short time of phosphating treatment, the intergranular region of sintered Nd–Fe–B has been covered by the phosphate conversion coating and the corrosion resistance is significantly improved. With the synergistic protection of the intergranular phosphorization and the followed copper electrodeposition, the corrosion resistance of the sintered Nd–Fe–B is significantly better than that with a single phosphate film or single plating protection.