Preparation and chemical stability of iron-nitride-coated iron microparticles

Iron-nitride-coated iron microparticles were prepared by nitridation of the surface of iron microparticles with ammonia gas at a temperature of 510 °C. The phases, composition, morphology, magnetic properties, and chemical stability of the particles were studied. The phases were α-Fe, ε-Fe3N, and γ-Fe4N. The composition varied from the core to the surface, with 99.8 wt% Fe in the core, and 93.8 wt% Fe and 6 wt% N in the iron-nitride coating. The thickness of the iron-nitride coating was about 0.28 μm. The chemical stability of the microparticles was greatly improved, especially the corrosion resistance in corrosive aqueous media. The saturation magnetization and the coercive force were 17.1×103 and 68 kA/m, respectively. It can be concluded that iron-nitride-coated iron microparticles will be very useful in many fields, such as water-based magnetorheological fluids and polishing fluids.