XPS STUDY OF THE PASSIVE FILMS FORMED ON NITROGEN-IMPLANTED AUSTENITIC STAINLESS-STEELS

Austenitic stainless steels (304-type) have been implanted with nitrogen ions in order to investigate the effects of implanted nitrogen on their electrochemical behaviour and on the nature of the passive film formed on the steels in acid (0.5M H2SO4). Alloys with two nitrogen doses have been prepared (2.5 x 10(16) and 2 x 10(17) N atoms/cm2). The implanted alloys have been characterized by N-15-NRA (nuclear reaction analysis) and XPS (X-ray photoelectron spectroscopy). Alloy surfaces with well-defined N concentrations were prepared, prior to the electrochemical measurements, by argon-ion sputtering of the implanted material for a fixed time in order to reach a well-defined point on the nitrogen depth profile. The samples were then transferred without exposure to air to an electrochemical cell mounted in an inert gas glove box. The implanted nitrogen modifies the electrochemical behaviour of the alloy. The anodic dissolution in the active state is enhanced, and the current density in the passive state is increased. Surface analysis of the alloys by XPS after passivation shows that implanted nitrogen is enriched on the surface during dissolution and passivation of the alloys. The process by which N is enriched on the surface is anodic segregation, which was first observed and characterized for S on Ni and Ni-Fe alloys [J. Oudar and P. Marcus, Appl. Surf. Sci. 3 (1979) 48; P. Marcus, A. Teissier and J. Oudar, Corros. Sci. 24 (1984) 259]. The passive films formed on both the unimplanted and implanted alloys have a bilayer structure with an inner oxide layer and an outer hydroxide layer, but on the nitrogen-implanted alloy, a chromium nitride phase is formed at the expense of the chromium oxide. After passivation of the implanted alloys, three chemical states of nitrogen are detected in the N 1s spectrum. The high binding energy (399.4 eV) peak corresponds to a nitrogen species located on the surface of the passive film, which is produced by reaction of the implanted nitrogen with the solution. The peak at low binding energy (396.3 eV) is assigned to nitrogen bonded essentially to chromium, under the form of a nitride, which is incorporated in the passive film. The peak at intermediate binding energy (397.0 eV) corresponds to implanted nitrogen in the alloy, and is detected because the passive film is very thin. The XPS measurements and the sputter depth profiles indicate that during the anodic segregation of N, the chromium-rich nitride which is formed is incorporated as islands in the passive film.