Kinetics of Redox Reactions on Passive Electrodes
TYCO LABS INC WALTHAM MA
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Polarization measurements for the FeFe couple on passive Ni, Fe, and Ti were carried out in solutions of fixed ionic strength but of varying pH. Tafel lines were generally obtained with exchange currents 10 to the -7th power to 10 to the -5th power ampsq cm and cathodic transfer coefficients about 0.45. The anodic transfer coefficients were less, particularly with Ti and Fe electrodes. A limiting anodic current, which was unrelated to diffusion of Fe ion in solution, was observed under certain conditions on passive Fe and Ti. The contribution of ionic current to the total current through the film is negligible in most cases. In general, the passive film has rectifying properties, i.e., the easy direction of electron flow is from metal to solution. This rectification is additional to the usual Faradaic rectification observed with most electrochemical reactions. The apparent transfer coefficients, calculated from the anodic and cathodic polarization curves, yield sums significantly less than unity. The results suggest that a potential drop exists across the surface film, that it depends on the thickness and composition of the film, and that it has a substantial effect on the electrode kinetics of Fe oxidation on passive electrodes.
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