Chemical perturbation of the passive–active transition state of Fe in a sulfuric acid solution by adding halide ions. Current oscillations and stability of the iron oxide film

Abstract

Perturbation of the passive–active transition state of the Fe ∣ 0.75 M H 2SO 4 system was carried out by adding a low concentration of halide ions (Cl −, Br −, I −). The dynamical response of the system to this chemical perturbation is studied by following changes observed in the I– E and I– t curves. The changes observed in I– E curves within the passive–active and passive regions are: (i) an extension of the oscillatory region beyond the Flade potential of the halide-free system, towards the stable passive region; (ii) a shift of the transpassivation potential towards lower values; (iii) an increase of the current in the passive region; (v) deviation of the maximum oscillatory current from the kinetics of the active region. According to theses changes the aggressiveness of halide ions is classified in the order Cl −>Br −>I −. The main changes observed in potentiostatic I– t curves are: (i) existence of an induction period of time before the oscillations start; (ii) the oscillations deviate from the monoperiodicity observed for the halide-free system and they become complex periodic and aperiodic. The dynamical response of the halide-free Fe ∣ 0.75 M H 2SO 4 system is studied as a function of the applied potential, the concentration and nature of halide ions. On the basis of experimental results it seems that Cl − ions influence the stability of the oxide film and in particular the transition between an unstable in acid solutions oxide related to Fe 3O 4 and a more stable one related to Fe 3O 4 ∣ γ-Fe 2O 3. Halide ions trigger local dissolution by an adsorption mechanism, which leads to the destabilization of the film related to γ-Fe 2O 3 and establishment of appropriate electrochemical conditions for the lifting-off of the oxide and activation of the Fe electrode.