The role of oxide layer formation during corrosion inhibition of mild steel in neutral aqueous media

The influence of phosphonic acid with Ca cation on iron surfaces in neutral aerated media (0.5 mol dm(-3) NaClO4) has been studied by electrochemical and surface analytical methods. X-ray induced photoelectron (XPS) and Auger electron spectroscopy (XAES) provided the possibility for identification of different chemical states of the constituents of the corrosion protective layers. The presence of certain cations led to a synergistic effect. It was found that calcium ions were incorporated into the film that developed on the mild steel surface, containing a mixed oxide and hydroxide complex. Electrochemical Impedance Spectroscopy (EIS) was used to determine the inhibitor efficiency of phosphonic acid at different concentrations. The data obtained were fitted and modelled by an equivalent circuit model. (C) 1997 Elsevier Science Ltd.