XPS study of passive films formed on molybdenum‐implanted austenitic stainless steels

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Abstract
Austenitic stainless steels (Fe–19Cr–10Ni (at.%)) have been implanted with molybdenum ions (Mo+, 100 keV, 2.5 × 1016 atoms cm−2). The implanted material has been characterized by XPS and RBS. The implanted region has a thickness of ∼1000 Å with a maximum molybdenum concentration of ∼9 at.% Mo located at ∼210 Å 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 transfered 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 ∼0.8 to ∼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.