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 pa ssive-active transition region have been studied for an iron electrode anod ically polarized in a chloride-containing 0.75 M H2SO4 solution. The pertur bation of the Fe\0.75 M H2SO4 system by adding a low concentration of chlor ides (c(Cl-)) induces complex periodic current oscillations across the pass ive-active potential region. In this region only mono-periodic oscillations are observed in the absence of chlorides. On increasing the c(Cl-) gradual ly the oscillatory potential region shifts steadily towards positive potent ials and is extended to the passive region. On increasing the applied poten tial, period doubling (P-2) and quadrupling (P-4) occur. Further period dou bling and other complex oscillatory patterns are observed for higher chlori de 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, w ere studied. A two-parameter bifurcation diagram (E-c(Cl-)) was constructed , There is experimental evidence that current oscillations induced by Cl- a re associated with localized corrosion. The oscillatory phenomena induced b y Cl- occur after a certain induction time. The induction time is attribute d to the time needed for chlorides to start their action at the passive sur face. Both the induction time (tau(ind)) and the oscillation period (T) inc rease by increasing the applied potential and by decreasing the c(Cl-) Quan tities such as tau(ind) and T provide a measure of the susceptibility of th e oxide film to the chloride attack. The Cl- ions accelerate electrodissolu tion by creating local active centers at the surface through an autocatalyt ic action of Cl- while the oxide film formation retards the effect of Cl- c ausing repassivation. A qualitative explanation of the experimental results is discussed. (C) 1999 Elsevier Science S.A. All rights reserved.