The effect of allylthiourea (ATU) on the corrosion behaviors of bulk nanocrystalline ingot iron (BNII) fabficated from conventional polycrystalline ingot iron (CPII) by severe rolling technique was studied. In the corrosive solution-0.1 mol L-1 H2SO4 + 0.25 mol L-1 Na2SO4 inhibited by ATU at ambient temperature, for CPII, when immersed for as short as 5 min, an inductive loop appears at different concentrations at the Nyquist complex planes; for BNII only a simple capacitance loop appears in the Nyqusit plot at the same period. This difference on the time effect is attributed to the higher volume fractions of defects, which are caused by severe rolling technique compared with its coarse counterpart-CPII. With the elongation of the immersion time, for BNII, two time constants appear in the Nyquist plot; for CPII, the impedance spectrum shows a slightly depressed semicircular shape, and a critical concentration is observed. The corrosion effect of ATU for BNII is really negative in comparison with CPII. (c) 2006 Elsevier Ltd. All rights reserved.