Oxide formation and conversion on carbon steel in mildly basic solutions

Surface oxide film growth and conversion processes on carbon steel were studied using a range of electrochemical techniques and ex situ surface analyses. The electrochemical study included (i) cyclic voltammetry as a function of various scan conditions and (ii) 7-day potentiostatic oxidation at a range of potentials while periodically performing Electrochemical Impedance Spectroscopy. Carbon steel surfaces at various stages of electrochemical oxidation were examined by SEM, Raman and X-ray photoelectron spectroscopy (XPS). These studies yield a consistent picture of film formation/conversion processes on carbon steel at pH 10.6, which is different to that reported for basic solutions (pH > 13). Oxide film formation/conversion mechanisms for three potential regions are proposed. In region I (<=-0.6V vs SCE), the main oxide formed is Fe3O4 which grows via a solid-state process; in region II (-0.5V <= E (vs SCE)<= -0.2V), continuous growth of the Fe3O4 layer is accompanied by its anodic conversion to a more maghemite (gamma-Fe2O3)-like phase near, or at, the oxide/solution interface by a similar solid-state mechanism to that described for region I; in region III (0.0V < E (vs SCE) < 0.4 V), the anodic conversion of this Fe3O4/gamma-Fe2O3 oxide to gamma-FeOOH leads to a significant structural change, which can lead to film fracture and the introduction of enhanced transport pathways in the film. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.