Effect of silicon and germanium alloying additions on the passivation characteristics of Fe3Al intermetallic in sulphuric acid solution

The effect of Si and Ge on the passivation properties of Fe75Al25 intermetallic compound has been evaluated in 0.25 M H2SO4 solution by means of potentiodynamic polarization, potentiostatic transients and electrochemical impedance spectroscopy (EIS) techniques. Fe75Al25 and Fe70Al25M5 (M = Si, Ge) intermetallic compounds were prepared by arc melting the stoichiometric amounts of pure elements in argon atmosphere. All the investigated iron aluminides exhibit a single-phase microstructure. Electrochemical studies reveal an improvement in the protective properties of passive films formed on Fe70Al25Si5 and Fe70Al25Ge5 intermetallics as compared to Fe75Al25. XPS analysis of the surface layers anodically grown on ternary iron aluminides indicate that the passive films are composed of Al2O3, Fe2O3 and SiO2 or GeO2, thus explaining the better passivation characteristics of Fe70Al25Si5 and Fe70Al25Ge5 intermetallics with respect to Fe75Al25.