Electrochemical and surface characterization of the passive film on Fe-Cr-Pd alloys

Ferritic Fe-40Cr-0.1Pd, Fe-40Cr-0.2Pd and palladium-ion-implanted Fe-40Cr alloys were exposed to 0.5 M HCl solution at various temperatures. Electrochemical measurements show that the Fe-40Cr alloy containing a low concentration of palladium (0.13 wt.%) exhibits spontaneous passivation in 0.5 M HCl solution from 25 to 60 °C, and unstable passivation at 85 °C. However, the Fe-40Cr alloy with 0.23 wt.% Pd content undergoes spontaneous passivation in 0.5 M HCl solution from 25 to 85 °C. Surface implantation of palladium ions at a dose of 2.1 × 10¹⁵ ions cm^-2 leads to unstable passivation at 85 °C. Thus palladium doses higher than 2.1 × 10¹⁵ ions cm^-2 are needed for the maintenance of spontaneous passivation. Auger electron spectroscopy depth profiling and X-ray photoelectron spectroscopy (XPS) have been used for the characterization of the surface film formed on the Fe-40Cr-0.2Pd alloy in 0.5 M HCl solution at 25 °C. The AES depth profile shows that the surface film formed spontaneously on the Fe-40Cr-0.2Pd alloy in 0.5 M HCl solution is enriched in palladium relative to chromium. Furthermore, XPS measurements indicate that the palladium is incorporated in the spontaneous passive film as the Pd²⁺ species. The enrichment of palladium in the spontaneous passive film permits a low overvoltage for hydrogen evolution and thereby promotes passivation of the Fe-40Cr-0.2Pd alloy. However, the AES depth profile reveals that there is an enrichment of chromium relative to iron in the passive film formed anodically on Fe-40Cr-0.2Pd alloy at 220 mV. The average chromium content [Cr]/ ([Cr] + [Fe]) reaches a value of about 0.765 within the passive film and decreases towards the film-substrate interface. © 1989.