ELECTROCHEMICAL, ELLIPSOMETRICAL AND SURFACE ANALYTICAL INVESTIGATIONS OF PASSIVE LAYERS ON BINARY FE AL ALLOYS/

X-ray photoelectron spectroscopy, ISS, ellipsometry, electrochemical i mpedance spectroscopy and electrochemical methods have been used to in vestigate the passive layers (PAL) on iron/aluminium alloys with low c ontents of Al (2 up to 12 wt.%). Ellipsometric measurements were perfo rmed during the galvanostatic reduction of PAL formed on Fe/8Al in 0.5 M Na2SO4/H2SO4 (pH 3.85) at various potentials. Combining the results of ellipsometric single-layer model evaluations and of the electroche mical reduction curves it was found that the outer part of the PAL mai nly consists of the relatively stable gamma-Fe2O3. Its fraction to the overall thickness of the PAL gives a maximum value when the alloy is passivated at +1 V(SHE). At higher potentials, porous films with both decreasing capacities and polarization resistances, measured by impeda nce spectroscopy, are formed. The XPS and ISS sputter depth profiles o f PAL on Fe4/12Al formed in phthalate buffer (pH 5) at + 1 V(SHE) show ed an Al enrichment in the centre of the layers. The reductive dissolu tion of the passive layer in close combination with XPS analysis at di stinct states of this process yields similar depth profiles. These res ults suggest the formation of mixed Fe/Al oxides or oxide compounds.