CORROSION INHIBITION OF PURE IRON IN NEUTRAL SOLUTIONS BY ELECTROCHEMICAL TECHNIQUES

Steady-state current-voltage curves for various disk rotation rates were combined with transient measurements (frequency analysis of the electrochemical and electrohydrodynamical impedance) in order to investigate the corrosion inhibition by an organic surfactant of pure iron in an aerated 0.5 M NaCl solution. In the cathodic range, physical adsorption of the compound with the interfacial potential was observed leading to the thickening or strengthening of the film. Thus, for short times of polarization, due to the adsorption relaxation, it was necessary to compensate the ohmic drop in the circuit to obtain the electrohydrodynamical (EHD) curves. The inhibitor acts by forming a porous layer covering the surface. From the analysis of the EHD diagrams the thickness and the porosity of the film were estimated. Nevertheless, a non-classical behaviour in the HF range of the EHD diagrams was observed and interpreted by a mechanical action of the fluctuating flow on the film. At the corrosion potential, the electrochemical impedance diagrams were essentially representative of the charge transfer process. Diffusion across the inhibitor film is very slow and requires extremely low modulation frequencies (below 1 mHz).