The iron samples treated with ODA are studied by using AC impedance spectroscopy, cathodic charging curves and Auger electron spectroscopy. The results reveal that ODA penetrates into iron oxide layers and increases the impedance values of the electrodes. The sample treated with 25 mg/L ODA has the best anti-corrosion performance. The structures of surface layers of ODA-treated samples are as follows: at lower ODA concentrations: iron?iron oxide?mixed ODA-iron oxides and at higher ODA concentrations: iron?iron oxide?mixed ODA-iron oxides?ODA.