AN INVESTIGATION OF THE CORROSION OF PURE IRON BY ELECTROCHEMICAL TECHNIQUES AND INSITU OBSERVATIONS

The electrochemical behaviour of pure iron in an aerated 0.5 M NaCl solution was studied by steady-state determination of current-voltage curves for various disc rotation rates and transient measurements of frequency analysis of the electrochemical and electrohydrodynamical impedance. During the experiments, in situ photographs were taken of the electrode surface. Whatever the rotation rate. iron corrosion is initiated by pits and spreads out in the form of corrosion product layers, one of which is poorly adherent. In the cathodic range and for short immersion times, the electrode surface appears to be almost uniformly active from the viewpoint of mass transport whereas for long immersion times oxygen reduction occurs through the porous layers of the corrosion product. Concerning these immersion times, it was shown that the role of the occluded pits is negligible by comparison with the porous film effect. It is emphasized that at the corrosion potential the electrochemical impedance diagrams show two diffusion processes: the very low frequency loop corresponds to the diffusion across the layer and the medium frequency loop is essentially representative of the mass transport in the liquid phase. The degree of coupling between the two processes is dependent on the immersion time and the rotation rate.