I.V. Aoki

Ac-impedance and Raman spectroscopy study of the electrochemical behaviour of pure aluminium in citric acid media

Abstract The electrochemical behaviour of pure aluminium in citric acid solution was investigated with a rotating disk electrode (RDE) at the open circuit potential (OCP), in the presence and in the absence of chloride, using electrochemical impedance spectroscopy and Raman spectroscopy. The impedance diagrams are composed of four frequency domains: at very high frequency (VHF), a Warburg-like behaviour was attributed to the porosity effect of the external part of the oxide layer. A slightly depressed capacitive loop at high frequency (HF) was explained as the combination of two capacitive loops, one corresponding to the same phenomenon as in the VHF domain but insensitive to the porosity (barrier layer resistance in parallel to the oxide film capacity), the other one to the charge transfer resistance in parallel to the double layer capacity. An inductive loop at low frequencies (LF) appeared as due to the relaxation of oxygen species adsorbed at the filmxa0∣xa0solution interface, while the very low frequencies (VLF) capacitive loop was ascribed to the coverage relaxation of adsorbed citrate species. Possible modulation of the passive layer thickness was also envisaged as a phenomenon at the origin of this latter loop. The diagrams shape is also insensitive to the addition of 104 ppm chloride ions to the solution, even though a diameter decrease of the HF capacitive loop is observed together with an increase of the frequency response. Raman Spectroscopy showed that citrate ions adsorb strongly to the electrode surface and that the time scale of this process is consistent with the frequency domain observed for the VLF capacitive loop. The interaction of citrate with the oxide layer is hindered by the addition of chloride ions to the electrolytic solution.

Copper phthalocyanine as corrosion inhibitor for ASTM A606-4 steel in 16% hydrochloric acid

A study based on the corrosion inhibition properties of copper phthalocyanine is described. Coverage degrees of copper phthalocyanine (Cu-phcy) on ASTM-A606-4 steel, obtained by weight loss measurements, were fitted to Langmuir, Frumkin, Temkin and Flory–Huggins adsorption isotherms. A better fit to the Langmuir isotherm was obtained. The polarization curves showed that polarization of both the anodic and cathodic reactions were verified for concentrations higher than 10−4 M, indicating a mixed type action. Only the anodic reactions were polarized for lower concentrations. At high frequencies the Nyquist diagrams showed one capacitive loop attributable to double layer charging and a small one at intermediary frequencies attributable to the faradaic process of hydrogen evolution. The formation of an adsorbed film was characterized by increasing charge transfer resistance values in the low frequency range for increasing inhibitor concentrations. According to the techniques used in this study, copper phthalocyanines showed a high corrosion inhibiting efficiency for all concentrations.

Local electrochemical investigation of copper patina

The patination of copper is known for its complexity and heterogeneous formation. For a deeper investigation, a locally resolved surface analysis was considered. An exact determination of the accessed area and a potentiostatic control in a three-electrode configuration was reached with the use of the electrochemical microcell technique, which enables local electrochemical measurement such as local electrochemical impedance spectroscopy and cyclic voltammetry. Such a technique provides a unique way for performing the investigation of heterogeneities on electrode surfaces. The local electrochemical measurements on the artificially patinated surface have allowed distinguishing areas of different reactivity even when the analysis of the surface revealed a homogenous chemical composition of patina. Local measurements with the electrochemical microcell showed the presence of small defects on the patina layer that can be modelled by considering a hemispherical diffusion process at small active areas surrounded by larger less reactive domains.

Using the electrochemical impedance spectroscopy to characterize carbon steel in biodiesel medium

Departamento de Fisico-Quimica Instituto de Quimica UNESP - Univ. Estadual Paulista, Caixa Postal 355, 14800-060, Araraquara, SP

Corrosion of Pure Aluminium in Organic Acid Media in the Presence and in the Absence of Chloride Ions: Mass Transport Effects

The influence of mass transport on the corrosion behaviour of pure aluminium in 0.75% citric acid solution in the presence and in the absence of 104 ppm of chloride ions under anodic and cathodic polarisation was investigated with a rotating disk electrode and using electrochemical impedance spectroscopy. For anodically polarised electrodes, the impedance diagrams are composed of a high frequency (HF) capacitive loop followed by a low frequency (LF) inductive loop, whose shape and size are modified neither with immersion time nor with rotation speed. Addition of chloride ions causes a diminution of the diameter of the HF capacitive loop while the influence on the inductive loop cannot be clearly detected due to data scattering. Under conditions of cathodic polarisation, the diagrams present the shape described above, however, the diameter of both loops decreases with immersion time and electrode rotation speed.