Surface Protection of Sintered NdFeB Magnet: Electroplating

The standard electrode potential E0(Nd3+/Nd)=-2.431V for sintered NdFeB magnet. The electrochemical potentials of various phases in sintered NdFeB permanent magnets are different, with Nd-rich and B-rich phases acting as anodes relative to the main Nd2Fe14B phase, undergoing preferential corrosion. Since the Nd-rich and B-rich phases are distributed at the grain boundaries of the main phase, the difference in area between the anode and cathode is significant during electrochemical corrosion. A small amount of the grain boundary phase acting as the anode bears a large corrosion current density, accelerating grain boundary corrosion. After grain boundary phase corrosion, the main phase grains lose their bonding with the surrounding grains and fall off.

Sintered NdFeB Magnet Electroplating Process

Before electroplating sintered NdFeB permanent magnet materials, it is necessary to undergo processes such as chamfering, degreasing, pickling, and ultrasonic cleaning to clean the surface, so that subsequent electroplating can achieve a well-bonded, low-porosity, and durable corrosion-resistant coating.

Common Electroplating Layers for Sintered NdFeB Magnet

Currently, the dominant coatings include Zn electroplating, Ni electroplating, NiCuNi electroplating, NiCu+chemical Ni electroplating, and NiCuNi+electrophoretic epoxy coating.

The standard electrode potential of nickel is E0=-0.25V, which is positive compared to NdFeB, acting as a cathodic protection coating. Therefore, the requirement for the density of the nickel coating is very high. Typically, a multi-layer system such as NiCuNi is used to reduce the porosity of the coating. Relatively speaking, NiCuNi electroplating is costlier, but due to its excellent high-temperature resistance, oxidation resistance, corrosion resistance, decorative performance, as well as good mechanical properties such as compression resistance, bending resistance, and impact resistance, it remains the mainstream technology for protecting sintered NdFeB magnet in the industry.

The standard electrode potential of zinc is E0=-0.762V (the corrosion potential of electroplated zinc is more negative compared to NdFeB). Zn coating can provide sacrificial anode protection for NdFeB magnets. Since zinc itself is non-magnetic and relatively inexpensive, it can be widely used. However, the corrosion of the zinc coating deteriorates sharply with increasing temperature, so careful consideration should be given when using zinc plating for high-temperature protection.