Corrosion resistance of N-, Cr- or Cr+N-implanted AISI 420 stainless steel

AISI 420 stainless steel samples with a diameter of 25 mm were implanted with nitrogen, with chromium or with chromium and nitrogen. The implantation energy and nominal fluence for Cr were 230 keV and 2 x 10(17) ions cm(-2), and those for N were 50 keV and 2.5 x 10(17) ions cm(-2) respectively. In nitrogen implantations, N-15 isotopes were used in order to perform a depth profiling utilizing a resonance of the N-15( p,alpha gamma)C-12 reaction. For all three samples and an unimplanted reference sample, anodic polarization diagrams were determined in 1 N H2SO4 solution. The unimplanted sample showed an active peak and a passivation behavior. Chromium implantation decreased the peak current density, but both in the case of the chromium implanted sample and the nitrogen implanted sample, passivation characteristics were unchanged. In the case of the sample implanted with chromium and nitrogen the measured anodic polarization curve exhibited a totally different behavior. The active peak had almost completely disappeared ( a reduction of four orders of the magnitude in current density compared with that of the unimplanted sample) and the passivation current density was also greatly decreased. Secondary-ion mass spectroscopy was utilized in analyzing surface films, which revealed a complex structure of a passive layer in the case of the sample implanted with both chromium and nitrogen.