Electrochemical Current Oscillations During Localized Corrosion of Iron

In the present work the onset of current oscillations at late stages of iron pitting corrosion in chloride containing sulfuric acid solutions is further studied. By increasing the chloride concentration and potential, repassivation of iron becomes gradually unlikely giving rise to the formation of salt layers in front of the iron electrode and electropolishing processes. Occurrence of qualitative changes in electrochemical current oscillations as the chloride concentration and potential are varied is shown and discussed. Analysis of the transient behavior of current vs. time curves occurring before the establishment of a specific oscillatory pattern, reflect changes in the composition of salt layers in front of the electrode. At a critical potential value (bifurcation potential, E bif ) the system exhibits a transition from one type of oscillations (beating, aperiodic bursting) to another (chaotic oscillations of small amplitude). The temporal behavior of the system at potentials E < E bif is associated with a characteristic average time between bursts indicative of crisis-induced intermittency. In terms of physicochemical processes, the E bif coincides with the repassivation potential used in steady-state pitting corrosion studies to indicate different states of dissolution due to changes in the pit chemistry. Since the present system exhibits a non-steady passive-active transition state we suggest that E bif implies a transition from the electropolishing state dissolution to a passive-active state dissolution due to changes in the concentration of chloride and hydrogen ions inside pits.