In this paper, nanocrystalline Co coatings were prepared using pulse reverse electrodeposition method. The electrochemical corrosion behavior of nanocrystalline (NC) Co compared with coarse-grained Co (CG) coatings in different corrosion media were characterized using potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). Results showed that in the NaOH or NaCl solutions, the NC Co exhibited improved corrosion resistance when compared with CG Co coatings, which is due to the higher grain boundary density in NC materials to quickly form a stable and protective passive film. In the case of NC Co coatings in HCl or H2SO4 solutions, since no obviously passive process can be observed, high grain boundary density in NC Co will accelerate corrosion by providing high-density of active sites for preferential attack. The controversial experimental results on NC Co coatings in different corrosion media can be reasonably explained by the positive or negative effect of high-density network of grain boundaries in NC materials. (c) 2006 Elsevier Ltd. All rights reserved.