Chemical surface modification can be used as a method for corrosion protection of sensitive powders based on intermetallic alloys between rare earth and transition metals. Surface coating is used for preventing fine powders, based on Nd-Fe-B, or Sm2Fe17−xTaxN3−δ prepared by HDDR processing and mechanically alloying, from hydrolysis. Powders coated by chemisorbed organic substance, after exposing to a humid atmosphere, do not show any chemical or physical change.
Different coating agents were used and the sufficient amount of various materials was optimised with the emphasis on minimising their quantity. Simple experiment shows that the surfactant is successfully chemisorbed on the powder surface and that the coated powders are hydrophobic indefinitely. Magnetic properties were measured on samples after they were exposed to the same corrosion tests. Measurements on coated and bonded samples were compared with the measurements of non-coated and bonded samples. By using Auger electron spectroscopy the thickness of the coating was controlled. In order to distinguish the nature of the bonding between the powder surface and the surface-active substance FT-IR spectroscopy in absorbency and diffuse reflection modes was used. The protection of the fine particles is based on the formation of a covalent bond between the hydroxyl groups at the particle surface and the surface-active substance. The monomolecular layer of organic substance does not damage the magnetic properties of the powder, but successfully protect the powder against humidity.