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Electrochemical Impedance Spectroscopic technique (EIS) and Local Electrochemical Impedance Spectroscopic technique (LEIS) have been used to investigate the corrosion behavior of zinc–nickel + chromate conversion coating on carbon steel. The measurements were conducted in chloride media, 0.6 M and 10 mM NaCl solutions with coating with and without artificial defect. An electrical equivalent circuit was proposed to analyze the EIS spectra in 0.6 M NaCl solution. LEIS was also performed in the vicinity of a scratch exposing the carbon steel to the corrosive solution. It was shown that the local impedance measurement was very useful to characterize in situ the electrochemical reactivity around a defect.

International audience; This paper presents experimental evidences of an intrinsic coupling between the alpha and Q parameters of constant phase elements (CPE) used in equivalent electrical circuits for EIS data fitting. Clear correlations have been found for two different experimental conditions, anodic dissolution and scale deposit, for which CPE behavior appears as the result of a time constant distribution originated from surface inhomogeneity. Results are in agreement with a coupling function that relates CPE parameters with interfacial quantities such as the ohmic and charge transfer resistances, and also with a characteristic interfacial capacitance whose meaning is addressed here. Although this relationship was initially derived from a theoretical model in which perfect CPE behavior was caused by a double-layer capacity distribution along the interface, it has been extensively used in the literature to estimate interface capacitance from CPE parameters in very different complex systems without any definitive evidence of it. However, its exportability to real systems can be explained from the fact that it can be deduced from simple and general arguments. In this paper we argue on these ideas in the basis of experimental evidences.

An impinging jet cell was used to investigate the influence of the convection on the kinetics of scale deposition in a carbonically pure water. CaCO 3 crystallized due to the increase of the local pH obtained by the reduction of dissolved oxygen. The working electrode was the transparent electrode of an electrochemical quartz crystal microbalance which records versus time the mass of the deposit and the microscope image of the electrode/solution interface. The chronoelectrogravimetric curves showed that the rate of the CaCO 3 deposition increased with the convection until a limiting value of the Reynolds number Re while the current of oxygen reduction continues to increase following a Re 1/2 law. This result was confirmed by the measurement of the growth rate of the two CaCO 3 crystal forms obtained by using the optical method. On the contrary, the nucleation rate was not influenced by the flow rate. It was shown that the kinetics of scale deposition was directly linked to the interfacial pH; a theoretical approach gives the interfacial pH with respect to the Re number.