Intrinsic coherence resonance in the chloride-induced temporal dynamics of the iron electrodissolution-passivation in sulfuric acid solutions

Abstract

The existence of intrinsic coherence resonance (ICR) is demonstrated experimentally for the Fe|0.75M H2SO4 electrochemical system in which the presence of chlorides induces current oscillations. Chlorides, destroying locally the passive oxide film of Fe, lead to pitting corrosion. It is shown that the intensity of the system internal noise, stemming perhaps from fluctuations of ionic concentrations, may be regulated upon changing the applied at the Fe electrode potential (E). Within a certain chloride concentration range (20≤cCl-/mM<50), the ICR occurs within two oscillatory regions. One is located at lower potentials, where the passive-active state dissolution (early stages of pitting) occurs, while the other takes over at higher potentials where the electropolishing state dissolution (late stages of pitting) of the Fe electrode arises. At sufficiently high chloride concentrations (≥50mM) only the electropolishing state dissolution of Fe emerges out of its passive state. To quantify the ICR, the coefficient of variation R and the effective diffusion coefficient Deff were calculated as a function of E for the chloride-induced spiking dynamics of the current, which is the characteristic response of the passive-active state dissolution of Fe. The ICR for the electropolishing state dissolution of Fe was quantified by calculating the coherence factor β as a function of E for the low-amplitude current oscillations established over a limiting current region. The coherence of chloride-induced current oscillations becomes maximal at a certain value of the potential applied at the Fe electrode as a result of an optimal level of the internal noise.