CORROSION INHIBITION OF PURE IRON IN NEUTRAL SOLUTIONS BY ELECTROCHEMICAL TECHNIQUES

Steady-state current-voltage curves for various disk rotation rates we re combined with transient measurements (frequency analysis of the ele ctrochemical and electrohydrodynamical impedance) in order to investig ate the corrosion inhibition by an organic surfactant of pure iron in an aerated 0.5 M NaCl solution. In the cathodic range, physical adsorp tion of the compound with the interfacial potential was observed leadi ng to the thickening or strengthening of the film. Thus, for short tim es of polarization, due to the adsorption relaxation, it was necessary to compensate the ohmic drop in the circuit to obtain the electrohydr odynamical (EHD) curves. The inhibitor acts by forming a porous layer covering the surface. From the analysis of the EHD diagrams the thickn ess and the porosity of the film were estimated. Nevertheless, a non-c lassical behaviour in the HF range of the EHD diagrams was observed an d interpreted by a mechanical action of the fluctuating flow on the fi lm. At the corrosion potential, the electrochemical impedance diagrams were essentially representative of the charge transfer process. Diffu sion across the inhibitor film is very slow and requires extremely low modulation frequencies (below 1 mHz).