Effect of microstructural defects on passive layer properties of interstitial free (IF) ferritic steels in alkaline environment

The role of microstructural defects (dislocation density and grain boundary areas) on the passive film properties formed on cold- and hot-rolled interstitial free (IF) steels is investigated in 0.1 M NaOH solution. Electron backscattered diffraction (EBSD) shows higher microstructural defect density on cold-rolled samples. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) results exhibit the lower barrier properties of passive films with the increase in microstructural defects. This is attributed to the increase in donor density measured with Mott-Schottky analysis and the lower relative quantity of protective γ-Fe2O3 in passive films (composed of Fe3O4,γ-Fe2O3 and FeO(OH)) with the increase in microstructural defect density.