XPS INVESTIGATION OF PASSIVE FILMS ON AMORPHOUS FE-CR ALLOYS

The passivation of the amorphous alloys Fe70Cr10X13C7(X = P, B) was studied combining X-ray photoelectron spectroscopy (XPS) surface analysis and electrochemical measurements. After removing the native (iron oxide) films by mechanical polishing, the alloys form a thin (2 nm), non-protective oxide film with a composition close to the bulk alloy; the corrosion potentials are in the active range. A subsequent anodic passivation in 1 N H2SO4 results in passive films enriched in chromium oxyhydroxide (70-80% of the cations in the film), and phosphates and borates are incorporated into the passive film. Only phosphorus, not boron, is enriched underneath the passive film. The presence of an intermediate phosphorus species (identified as elemental P) at the interface film/substrate is revealed after passivation. The role of film composition, especially the presence of phosphates, and of the P enrichment underneath the film are discussed with respect to the stability of the passive films. It is concluded that the outstanding corrosion resistance and the high repassivation rate of the amorphous alloy with 13% P may be mainly attributed to the enrichment of phosphorus underneath the passive film, reducing to a great extent the ionic conductivity of the interface, and to the incorporation of chromium oxyhydroxide and phosphates in the passive film. A new method that allows simultaneous evaluation of the thickness and composition of surface films on a multicomponent alloy, taking into account the carbon contamination, is presented.