Correlating the microstructure, growth mechanism and magnetic properties of FeSiAl soft magnetic composites fabricated via HNO3 oxidation

Abstract

Nitric acid oxidation has been used to fabricate insulation coatings of the FeSiAl soft magnetic composites (SMCs). Growth mechanism of the coatings obtained with varied HNO3 concentration has been systematically studied based on careful analysis of the coating thickness, microstructure and composition. The oxidation process for the powders reacted with 10 wt% HNO3 corresponds to the steady-state passivation, while pitting corrosion occurs with raised HNO3 concentration of 30 wt%. Origin of the pitting corrosion has been analyzed based on the point defect model. Evolution of the coatings has been revealed under different growth conditions and correlated to the magnetic performance of the SMCs. The samples oxidized with 10 wt% HNO3 mainly form a thin and compact oxidation layer with dominating Al2O3, AlO(OH) and a small amount of Fe3O4, while increased HNO3 concentration (30 wt%) gives rise to mixed Al2O3, AlO(OH), Fe2O3 and Fe3O4 in the thicker coating layers. Enhanced magnetic performance of the FeSiAl SMCs can be achieved by optimized thickness and composition of the coatings (μe = 180.4, Pcv = 327.2 mW/cm3 for the SMCs oxidized by 10 wt% HNO3 and μe = 157.3, Pcv = 241.2 mW/cm3 via oxidization by 30 wt% HNO3 for 5 min).