Inhibition of iron corrosion by imidazole: An electrochemical and surface science study

The difference in corrosion inhibition behavior of imidazole deposited on iron by two different methods has been exam- ined using electrochemical impedance spectroscopy (EIS), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Adding imidazole directly into the aqueous corrosive medium has been observed to provide greater protec- tion against corrosion, in comparison to the direct deposition of the inhibitor onto the metal surface prior to exposure. Analysis of the alternating current (AC) impedance spectra as a func- tion of time revealed that at steady state, the value of the polarization resistance (R _p) in the case of imidazole addition to the corrosive medium was 20 times larger than the baseline, and in the predeposited case it was only three times larger. Results of XPS investigations suggested that in the former case, the imidazole reacts through the "pyrrole-type" nitrogen with the aromatic ring nearly parallel to the metal surface. In the latter situation, a "pyridine-type" nitrogen-iron interaction is also present, with some imidazole molecules oriented nor- mal to the surface. SEM studies show that the oxide formed after exposure to aqueous corrosive medium is much thicker than the native oxide on iron and is porous and less protective in nature.