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

Austenitic stainless steels (Fe-19Cr-10Ni (at.%)) have been implanted with molybdenum ions (Mo+, 100 keV, 2.5 x 10(16) atoms cm-2). The implanted material has been characterized by XPS and RBS. The implanted region has a thickness of approximately 1000 angstrom with a maximum molybdenum concentration of approximately 9 at.% Mo located at approximately 210 angstrom from the surface. The effects of implanted molybdenum on the passivation of the alloy in 0.5 M H2SO4 have been investigated by electrochemistry and XPS. Prior to electrochemical experiments, surfaces with well defined Mo concentrations were produced by argon ion sputtering the implanted alloy for a fixed time in the preparation chamber of the spectrometer in order to reach a well defined point on the molybdenum depth profile. After XPS analysis the samples were transferred without exposure to air into a glove-box with an inert atmosphere. The electrochemical behaviour of the alloy is significantly modified by the implanted molybdenum. The major effect is that the activation peak disappears. A bilayer structure (outer hydroxide/inner oxide) of the passive film is observed for both the implanted and non-implanted alloys and the thicknesses of the films are similar. On the implanted alloy the outer hydroxide layer contains molybdenum cations in the 6+ state and Mo4+ is also detected. The mean Mo6+ concentration in the film increases up to 7 at.% with increasing Mo concentration in the alloy. The hydroxide/oxide ratio [Cr3+(hyd)]/[Cr3+(ox)] increases from approximately 0.8 to approximately 3 for Mo concentrations in the alloy from 0 to 9 at.% respectively. A marked enrichment of Cr, Ni and Mo is observed in the metallic phase near the alloy/passive film interface.