Potential step measurements in an aniline-oxalic acid medium shows passivation of steel occurred prior to polyaniline deposition. The steel first dissolved as Fe2+ followed by the formation of a FeC2O4 complex on the surface of the steel. A decrease in potential from 0 to -0.5 V results in a smaller anodic current density and requires more time to form the oxalate complex. Potential steps more negative than -0.6 V do not completely passivate the steel surface. X-ray photoelectron spectroscopy measurements of the passive layer suggested that it is a mixture of Fe2+ and Fe3+ oxalate layers. Atomic force microscopy imaging shows the oxalate layer is composed of large polygonal crystals whereas the polyaniline coating exhibits very small rounded-mound structures. (C) 2003 Published by Elsevier Science B.V.