Current oscillations and mass-transport control during electrodissolution of iron in phosphoric acid solutions

This work presents a characterisation of the global electrochemical and non-linear dynamical behaviour across the active-passive transition region of the Fe/H3PO4 system. The Fe/H3PO4 system is a new electrochemical oscillator displaying a rich dynamical response on varying several controllable parameters such as the phosphoric acid concentration (C-H3PO4), the Fe-disc rotation speed (omega) and the applied potential (E). The current-potential (I-E) curves show two transition points. The first transition occurs from the active to the limiting current plateau. On increasing C-H3PO4, the limiting current (I-L) decreases. The I-L is governed by full mass-transport conditions and is associated with the electropolishing process of iron in concentrated phosphoric acid solutions as well as with current oscillations. The second transition occurs from the limiting current to passivity and is accompanied by current oscillations for moderate and high C-H3PO4 (4 M less than or equal to C-H3PO4 < 14.8 M). The oscillatory region is located within a loop that is formed in the I-E curve when a forward and a subsequent backward potential scans were applied, because reactivation occurs at lesser positive potentials than the potential at which the system leaves the limiting current region. The observed current oscillations can be classified into two main classes: The complex oscillations (small amplitude, aperiodic, mixed-mode and periodic) that occur at the least positive potentials (end of the plateau) and the periodic relaxation oscillations that occur at the most positive potentials (on the site of the passive state). It is concluded that the ir(u)-potential drop (where R(u) is the solution uncompensated resistance) must be taken into account along with the electrode and surface processes for understanding the origin of the current oscillations in the Fe/H3PO4 system. In order to explain the rich dynamical response of the Fe/H3PO4 system, the effect of the electrolyte composition on the kinetic of the electrode and surface processes should be considered. Copyright (C) 1996 Elsevier Science Ltd