Complex periodic and chaotic current oscillations related to different states of the localized corrosion of iron in chloride-containing sulfuric acid solutions

Current oscillations generated during pitting of Fe in chloride-containing 0.75 M H2SO4 solutions were monitored and analyzed under potentiostatic conditions in a wide range of chloride ion concentrations. The increase of chloride ion concentration leads to: (i) deviation of the oscillation type from monoperiodicity observed in the chloride-free system and (ii) increase of the current in the passive region up to a limiting value and simultaneously to the disappearance of the passive-to-active transition region in the I–E curve. The current oscillations for relatively low chloride ion concentrations (≤20 mM) are complex periodic or aperiodic depending on the potential. For relatively high chloride ion concentrations (>20 mM) the current oscillations become aperiodic (chaotic). Two limiting cases of the pitting growth process are distinguished on the basis of the type of oscillation observed for low and high chloride ion concentrations. First, the unstable pit growth related to complex periodic oscillations; the oxide passive film is formed but it is destabilized by Cl− ions. Then repeated breakdown and formation of the passive film occurs and the current oscillates between a maximum and a minimum value which represent the active and passive states, respectively, of the specific Fe ∣ 0.75 M H2SO4+x M Cl− system. The second case of aperiodic (chaotic) oscillations is related to a continued pit growth; the formation of the oxide passive film is prevented and a salt layer is formed resulting in a limiting current region with oscillations. The potential and the Cl− ion concentration in the vicinity of the Fe electrode control the pitting corrosion and the oscillatory phenomena associated with it. Polishing state pitting was found to proceed at more positive potentials and high Cl− ion concentrations.