MICROSTRUCTURE AND CORROSION OF NITROGEN IMPLANTED AISI-304 STAINLESS-STEEL

Selected implantation conditions were used to produce three very different nitrogen-containing near-surface phases in AISI 304 stainless steel (SS), each with unique pin-on-disk wear resistance characteristics. Such phases in order of increasing wear resistance are hexagonal (epsilon) (Fe,Cr,Ni)2+xN, a high-nitrogen f.c.c. (gamma(N)) solid solution and a two-phase f.c.c. (gamma)-CrN + b.c.c. (alpha)-FeNi mixture. Corrosion properties of such surfaces in an H2O-1% NaCl solution at 37-degrees-C were investigated by methods of simple immersion and standard potentiodynamic polarization. Near-surface microstructures before and after corrosion tests were characterized by X-ray diffraction, Mossbauer spectroscopy, and scanning electron microscopy. Changes in microstructure and evidence of pitting and general corrosion were not detected in immersion tests for the unimplanted and implanted samples containing the epsilon-nitride and gamma(N) phases, whereas the implanted sample containing gamma-CrN + alpha-FeNi exhibited extensive pitting and general corrosion with the formation of iron oxides. The removal of alpha-FeNi by corrosion permitted the observation of a relatively thick nitrogen solid-solution subsurface layer. Samples containing near-surface epsilon-nitride and gamma(N) exhibited pitting potentials and corrosion currents similar to those of unimplanted 304 SS while the pitting potential of the gamma-CrN + alpha-FeNi sample was considerably lower than that of unimplanted 304 SS. It is concluded that nitrogen implantation conditions that produce the gamma(N) near-surface microstructure yield both optimal corrosion and wear resistance.