Effects of nitrogen ion implantation on the surface characteristics of iron aluminides

Iron aluminides are of considerable interest for low to intermediate temperature structural applications in which low cost, low density and good corrosion or oxidation resistance are required. However, their application is currently limited by room temperature brittleness and low corrosion resistance. In this study, in order to improve the wear and corrosion resistance of iron aluminides in acidic solutions, steel containing Mo and Cr were fabricated and nitrogen ion implanted on the surface of samples with doses of 3.0×1017 ions/cm2 at an energy of 150 keV. The effects of nitrogen ion implantation on the electrochemical characteristics of samples were investigated using various electrochemical methods in H2SO4+KSCN and HCl solutions. AlN, CrN and Fe4N were formed in the surface by the nitrogen ion implantion. The grain boundary activation (DOS, degree of sensitization), the active current density (Ia) and reactivation current density (Ir) were significantly decreased by the nitrogen ion implantation in H2SO4+KSCN solution. Nitrogen implantation promoted the corrosion potential, pitting potential, repassivation potential and Ehys value for iron aluminides containing Mo. Whereas, the pitting and repassivation potential for iron aluminides containing B was reduced in comparison with iron aluminides containing Cr and Mo in HCl solution. From current and potential–time curves, pitting resistance and passive film stability of the nitrogen implanted iron aluminides containing Mo was higher than that of the unimplanted iron aluminides. The nitrogen ion implanted iron aluminides containing B and Mo showed rounded pit shapes in comparison with irregular pit shape of ion implanted iron aluminides without B and Mo additions. In conclusion, it is considered that the corrosion problem of iron aluminides could be solved, to some extent, by the addition of Mo and Cr to iron aluminides and nitrogen implantation on the surface of iron aluminides.