Current oscillations induced by chlorides during the passive-active transition of iron in a sulfuric acid solution

Current oscillations observed under potentiostatic conditions within the passive-active transition region have been studied for an iron electrode anodically polarized in a chloride-containing 0.75 M H2SO4 solution. The perturbation of the Fe\0.75 M H2SO4 system by adding a low concentration of chlorides (c(Cl-)) induces complex periodic current oscillations across the passive-active potential region. In this region only mono-periodic oscillations are observed in the absence of chlorides. On increasing the c(Cl-) gradually the oscillatory potential region shifts steadily towards positive potentials and is extended to the passive region. On increasing the applied potential, period doubling (P-2) and quadrupling (P-4) occur. Further period doubling and other complex oscillatory patterns are observed for higher chloride concentrations (c(Cl-) greater than or equal to 15 mM). The bifurcations, which are observed from one dynamical regime to another in response to a smooth variation of the applied potential and the chloride concentration, were studied. A two-parameter bifurcation diagram (E-c(Cl-)) was constructed, There is experimental evidence that current oscillations induced by Cl- are associated with localized corrosion. The oscillatory phenomena induced by Cl- occur after a certain induction time. The induction time is attributed to the time needed for chlorides to start their action at the passive surface. Both the induction time (tau(ind)) and the oscillation period (T) increase by increasing the applied potential and by decreasing the c(Cl-) Quantities such as tau(ind) and T provide a measure of the susceptibility of the oxide film to the chloride attack. The Cl- ions accelerate electrodissolution by creating local active centers at the surface through an autocatalytic action of Cl- while the oxide film formation retards the effect of Cl- causing repassivation. A qualitative explanation of the experimental results is discussed. (C) 1999 Elsevier Science S.A. All rights reserved.