The corrosion properties of micro- and nanocrystalline Co and Co-1.1 wt% P alloys were studied in deaerated 0.1 M H2SO4 solution using open-circuit potential measurement, polarization tests, ac impedance measurements and XPS. The potentiodynamic polarization tests revealed that all samples exhibited active anodic dissolution without a distinct transition to passivation up to -0.1 V-SCE. While the anodic polarization curve of the nanocrystalline Co was almost identical to that of its microcrystalline Co, the cathodic kinetics of the nanocrystalline Co increased. For both micro- and nano Co, an inductive loop was observed in the low frequency and this could be attributed to the adsorption behaviour. Compared to the nanocrystalline Co, impedance measurement showed that addition of P resulted in an increase of the interfacial impedance at the open-circuit potential. This enhanced corrosion resistance was closely related to an enrichment of elemental P on the electrode surface and was confirmed by X-ray photoelectron spectroscopy (XPS). However, superior corrosion resistance did not last due to the formation of a non-protective surface film at higher anodic potentials. (c) 2005 Elsevier Ltd. All rights reserved.