The corrosion behavior of a nanocrystalline (NC) low carbon steel (LCS) surface film deposited on a conventional polycrystalline (PC) LCS by magnetron sputtering was studied in 0.5 M H2SO4 solution using impedance and potentiodynamic polarization techniques. The corrosion inhibiting effect of a non toxic organic compound (methionine) and synergistic KI additives was also studied. Microstructure characterization by X-ray diffraction and atomic force microscopy revealed no phase changes due to nanoprocessing and the grain size of the NC surface layer was ~ 40 nm. Electrochemical results show that both the PC and NC surfaces underwent active corrosion with no transition to passivation in the potential range studied. Surface nanocrystallization however increased the corrosion susceptibility of low carbon steel more than two-fold, leading to a decrease in interfacial impedance and an increase in the kinetics of the anodic reaction. Methionine inhibited the corrosion of both specimens with comparable inhibition efficiencies. Iodide ions synergistically increased the inhibition efficiency of methionine on both specimens. This synergistic effect was more pronounced for the bulk steel and has been discussed vis-à-vis the more positive surface charge on bulk steel surface at the corrosion potential compared to the nanocrystalline surface.