What is stripping operation?
For the manufacture process of NdFeB magnet, each step will produce a certain proportion of defective products. The coating process is also existing defective products, without exception. In order to recycle these defective magnets, the magnet manufacturers will strip the coating of defective magnets, then replating them. The simple stripping operation of NdFeB magnet can be divided into chemical stripping and physical stripping, and different types of coating have its own stripping operation solution.
To some degree, the stripping operation will affect the dimension and surface state of the magnet, and later influence its magnetic properties and uniformity.
Physical stripping of NdFeB magnet
The physical stripping generally means that stripping the coating from the surface of the magnet by the physical method.
For the softer coating, the manufacturer will utilize sandblasting operation to remove it. Compressed air will drive the abrasive particle to impact the surface of the magnet, and the surface will have enough cleanliness and roughness depend on the impact and cutting action of the abrasive particle.
Both mechanical cutting and grinding process can be applied in the physical stripping of simple geometry shape NdFeB magnet. It should be noted that the direct machining will affect the dimensions of the magnet, and the feed rate must be closely controlled to avoid the embrittlement of magnet.
The difference of expansion character between coating and magnet is also used by magnet manufacturers to strip the coating. The repeated thermal shock treatment will cause the fatigue cracking. The abrasive paper is usually used for the stripping operation of complex shape magnet and halfway stripping magnet.
Chemical stripping of NdFeB magnet
Chemical stripping will utilize the chemical reaction to corrode or dissolve the coating of magnet.
The chemical stripping is suitable for small size magnet, and different coating also uses different stripping solution system. The most common coating methods for NdFeB magnet are Nickel-Copper-Nickel coating and Zinc coating. The chemical character of rare earth permanent magnet is more active than coating, hence the chemical reagent and complexing agent need to be introduced to stripping solution system.
Zinc is a kind of amphoteric metal, and it can be reacted under both acid and lye. The strong acid stripping operation is usually using the nitric acid solution to remove the coating at room temperature. The alkaline stripping solution includes sodium hydroxide/potassium hydroxide and sodium nitride, and the temperature of the solution is around 100 deg c.
The stripping process for Nickel-Copper-Nickel coating is much more complex than Zinc coating, especially the primer Nickel layer. The simple formulation of Nickel-Copper-Nickel coating stripping solution is cyanide solution.
