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

Perturbation of the passive-active transition state of the Fe \ 0.75 M H2SO 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 perturbati on 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 region s are: (i) an extension of the oscillatory region beyond the Flade potentia l of the halide-free system, towards the stable passive region; (ii) a shif t of the transpassivation potential towards lower values: (iii) an increase of the current in the passive region; (v) deviation of the maximum oscilla tory current from the kinetics of the active legion. According to theses ch anges the aggressiveness of halide ions is classified in the order Cl- > Br - > I-. The main changes observed in potentiostatic I-t curves are: (i) exi stence of an induction period of time before the oscillations start; (ii) t he oscillations deviate From the monoperiodicity observed for the halide-fr ee system and they become complex periodic and aperiodic. The dynamical res ponse of the halide-free Fe \ 0.75 M H2SO4, 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 stab ility of the oxide film and in particular the transition between an unstabl e in acid solutions oxide related to Fe3O4 and a more stable one related to Fe3O4 \ gamma-Fe2O3. Halide ions trigger local dissolution by an adsorptio n mechanism, which leads to the destabilization of the film related to gamm a-Fe2O3 and establishment of appropriate electrochemical conditions for the lifting-off of the oxide and activation of the Fe electrode. (C) 2000 Else vier Science Ltd. All rights reserved.