Forced electrochemical oscillations of iron in sulphuric acid solutions at the transition between active and passive states

Abstract

The effects of a periodically perturbed applied potential on the potentiostatic current oscillations observed during the transition of an iron electrode between the active and passive states in 2 M sulphuric acid solutions is studied experimentally in this work. Autonomous current oscillations appear during the electrodissolution of iron in sulphuric acid solutions when the iron electrode turns from the active to the passive state and vice versa. After the application of a potential perturbation the forced oscillations are conducted in a region with such a parameter space that the autonomous limit cycle is observed. The overall study was limited to the hysteresis region. The response of the oscillatory system Fe/2 M H 2SO 4 to an external perturbation of the potential was studied as a function of two parameters, the frequency, ω p, and the amplitude, E p, of the forcing potential and it was characterized with the aid of time-delay reconstructions. The various types of response are presented in the phase diagram E p-ω p/ω 0. For low amplitudes of the potential perturbation and for both ω p/ω 0 < 1 and ω p/ω 0 > 1 the response of the perturbed system exhibits wide zones of quasi-periodicity interrupted by entrainment bands which are harmonic or subharmonic. For higher amplitudes the entrainment bands become broad and as the forcing amplitude increases the system tends to be driven by the forcing frequency. From this experimental study on forced electrochemical oscillations of the system Fe/2 M H 2SO 4 it is seen that at very positive potentials for sustained periodic current oscillations to occur under stationary conditions, periodic or quasi-periodic current oscillations could be triggered by the local dissolution of the passive film with a potential pulse of a sufficient amplitude in order for the transition to the active state to be feasible. The resulting small uncovered part of the electrode surface allows metal electrodissolution and its propagation along the surface.