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

Current oscillations generated during pitting of Fe in chloride-containing 0.75 M H2SO4 solutions were monitored and analyzed under potentiostatic con ditions in a wide range of chloride ion concentrations. The increase of chl oride ion concentration leads to: (i) deviation of the oscillation type fro m monoperiodicity observed in the chloride-free system and (ii) increase of the current in the passive region up to a limiting value and simultaneousl y to the disappearance of the passive-to-active transition region in the I- E curve. The current oscillations for relatively low chloride ion concentra tions (less than or equal to 20 mM) are complex periodic or aperiodic depen ding on the potential. For relatively high chloride ion concentrations (>20 mM) the current oscillations become aperiodic (chaotic). Two limiting case s 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 oxi de passive film is formed but it is destabilized by Cl- ions. Then repeated breakdown and formation of the passive film occurs and the current oscilla tes between a maximum and a minimum value which represent the active and pa ssive states, respectively, of the specific Fe \ 0.75 M H2SO4 + x M Cl- sys tem. The second case of aperiodic (chaotic) oscillations is related to a co ntinued pit growth; the formation of the oxide passive film is prevented an d a salt layer is formed resulting in a limiting current region with oscill ations. The potential and the Cl- ion concentration in the vicinity of the Fe electrode control the pitting corrosion and the oscillatory phenomena as sociated with it. Polishing state pitting was found to proceed at more posi tive potentials and high Cl- ion concentrations. (C) 2000 Elsevier Science S.A. All rights reserved.