The effect of thiourea (TU) on electrochemical corrosion behavior of bulk nanocrystallized ingot iron (BNII) and coarse polycrystalline ingot iron (CPII) was investigated in 0.1 M H2SO4 + 0.25 M Na2SO4 solution at room temperature by electrochemical impedance spectra (EIS) and potentiodynamic polarization method. The results of Nyquist plots show that the corrosion behavior of BNII and CPII are obviously different. For CPII, an inductive loop appears in the Nyquist plots after 5 min immersion, which indicates that the interaction of inhibitor molecule and metal atom is unstable and an equilibrium cannot be obtained at such a short period. With the increase of immersion time, the Nyquist plot of CPII is not a perfect semicircle, generally attributed to the frequency dispersion. For BNII, a simple capacitive loop appears in the Nyquist plots after 5 min immersion. With the increase of immersion time, the Nyquist plots of BNII contain two capacitive loops with two time constants. The inhibition of thiourea to BNII is very limited. However, thiourea promoted the dissolution of BNII The polarization curves for both samples show the presence of a desorption potential. When the potential is higher than the desorption potential, the anodic dissolution current increase very abruptly. (C) 2007 Elsevier B.V. All rights reserved.