This paper proposes a new blackening technology for growing magnetite (Fe3O4) coating on surface of carbon steel. Dense black coating composed of Fe3O4 ultrafine particles could be successfully prepared by hydrothermal treatment of the carbon steel substrate in the N2H4 center dot H2O-FeSO4-NaOH solution at 150 degrees 8C. Electrochemical analysis, including Tafel and electrochemical impedance spectroscopy, indicated that the anodic dissolution reaction was effectively limited and the corrosion resistance increased by the Fe3O4 coating. The key factors and growth mechanism for the hydrothermal formation of the Fe3O4 coating are also discussed. (c) 2008 Elsevier B.V. All rights reserved.