A new type of oscillatory phenomena induced by halides during the electrodissolution of iron in sulfuric acid solution

A new type of current oscillations was observed under potentiostatic conditions during the electrodissolution-passivation of iron in chloride-containing sulfuric acid solutions (0.75 M H2SO4). The solution was under stagnant conditions and the concentration of chlorides was in the range between 5 and 25 mM. Under the present experimental conditions diffusion and migration are the only transport mechanisms that carry the halides to the anode surface. In the absence of chlorides only simple periodic relaxation oscillations are observed. For c(Cl)-greater than or equal to 7.5 mM and as the applied potential increases period doubling (P-2) and quadrupling (P-4) occur. Further period doubling and other complex patterns are observed for higher chloride concentrations (c(Cl)-greater than or equal to 15 mM). This period doubling sequence might lead to a chaotic behavior. These oscillatory phenomena occur after certain induction period which is attributed to the time needed for the chlorides to start their specific action for breakdown of passivity. The induction period, the duration and the period of the oscillations increase on increasing the applied potential and on decreasing the chloride concentration. During oscillation film breakdown and repassivation seems to occur indicating probably unstable pitting corrosion. These results might be explained with an activator (Cl-)-inhibitor (oxide film) model. The activator accelerates electrodissolution by creating local surface-active centers through an autocatalytic action of Cl- while the inhibitor decreases the effect of the activator causing repassivation through oxide film formation.