Kinetics of Redox Reactions on Passive Electrodes

Abstract

Overpotential measurements for the Fe + + +/Fe + + 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 between 10 -7 and 10 -5 amp/cm 2 and cathodic transfer coefficients of 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 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, the easy direction of electron flow being 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 demonstrate the existence of a potential drop across the passive film, the magnitude of which depends on the thickness and composition of the film.The electrochemical cell was described previously (9). Potentials were measured through a Luggin-Haber capillary probe 0.05 cm OD placed 0.10 cm

Corrosion modelling of iron based alloy in nuclear waste repository

“…The overall order of magnitude is acceptable (see Fig. 3 in [41])), but discrepancies have arisen for the Tafel slopes. The cathodic and the anodic Tafel slopes estimated graphically from the dashed CDP in Fig.…”

“…The cathodic and the anodic Tafel slopes estimated graphically from the dashed CDP in Fig. 9 were respectively −68 and +492 mV instead of −90 mV (˛c = 0.69) and +270 mV (˛a = 0.23) from [41]. The usual correction would be to change the values of the Butler-Volmer coefficients a 0 r and b 0 r in Eq.…”

“…(21)). Makrides has published results concerning the kinetics of this step on a passive iron electrode in acid solution 1 [41]. From the experimental current density at V = +0.7 V/SHE (j ≈ −700 A cm −2 ), the value for k 0 r was determined.…”

“…The target of this section is to show how 2 These computations have been performed for a solution with the same pH and redox potential than those used in [41] but with a concentration in ferric cations lowered down to 10 −13 mol L −1 . The ferric concentration used by Makrides (0.3 mol L −1 ) led to "very high" cathodic current densities (≈10 8 A cm −2 ) for V ranging from −0.6 to −0.8 V/SHE.…”

Energies of Activation of Electrode Reactions: A Revisited Problem

“…52,59,65,66,74,75,, redox reactions (Refs. 39,41,78,(180)(181)(182)(183)(184)(185)(186)(187)(188), oxygen reduction and evolution reactions (Refs. 20,30,71,(189)(190)(191)(192)(193)(194)(195)(196), and a number of other inorganic and organic electrode reactions (Refs.…”