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

Iron aluminides are of considerable interest for low to intermediate temper ature structural applications in which low cost, low density and good corro sion or oxidation resistance are required. However, their application is cu rrently limited by room temperature brittleness and low corrosion resistanc e. In this study, in order to improve the wear and corrosion resistance of iron aluminides in acidic solutions, steel containing Mo and Cr were fabric ated and nitrogen ion implanted on the surface of samples with doses of 3.0 x 10(17) ions/cm(2) at an energy of 150 keV. The effects of nitrogen ion i mplantation on the electrochemical characteristics of samples were investig ated using various electrochemical methods in H2SO4 + KSCN and HCl solution s. AlN, CrN and Fe4N were formed in the surface by the nitrogen ion implant ion. The grain boundary activation (DOS, degree of sensitization), the acti ve current density (I-a) and reactivation current density (I-r) were signif icantly decreased by the nitrogen ion implantation in H2SO4 + KSCN solution . Nitrogen implantation promoted the corrosion potential, pitting potential , repassivation potential and E-hys value for iron aluminides containing Mo . Whereas, the pitting and repassivation. potential for iron aluminides con taining B was reduced in comparison with iron aluminides containing Cr and Mo in HCl solution. From current and potential-time curves, pitting resista nce and passive film stability of the nitrogen implanted iron aluminides co ntaining Mo was higher than that of the unimplanted iron aluminides. The ni trogen ion implanted iron aluminides containing B and Mo showed rounded pit shapes in comparison with irregular pit shape of ion implanted iron alumin ides without B and Mo additions. In conclusion, it is considered that the c orrosion 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. (C) 2001 Elsevier Science B.V. All rights reser ved.