Electrolyte Resistance Fluctuations Research Articles

Electrolyte-resistance change due to an insulating sphere in contact with a disk electrode

This work is aimed at improving the quantitative analysis of the electrolyte-resistance (ER) fluctuations generated by two-phase systems with dispersed gaseous, liquid, or solid insulating entities in a conductive electrolyte. The primary potential distribution around a disk electrode in contact with a small insulating sphere, which simulates a spherical particle, drop, or gas bubble sitting on a disk electrode used as a sensor, was calculated with a collocation method in order to derive the increment in ER caused by the sphere. For a sphere of size equal to one-tenth of the electrode size, the values of the ER increments were found to be very low and to depend on the sphere position: 0.3% close to the edge of the electrode and 0.05% at its centre. Despite the influence of variations in the electrolyte temperature and of the approximate horizontality of the electrode, these low values could be measured experimentally by scanning insulating spheres of 1 and 2 mm in diameter above or in contact with a stainless steel electrode of 10 mm in diameter, using the motorized translation stage of a scanning electrochemical microscope and a home-made electronic device measuring low-amplitude ER fluctuations.

Measurement of Electrolyte Resistance Fluctuations Generated by Oil-Brine Mixtures in a Flow-Loop Cell

Abstract Measurements of current and electrolyte-resistance fluctuations between two working electrodes in a flow-loop cell have been carried out with the objective to assess and/or monitor corrosion and characterize hydrodynamic flows in pipelines. Linear flow velocities of 1 m/s and 2 m/s were used to ensure different hydrodynamic conditions for oil-brine mixtures with a large range of volumetric oil content (VOC) from 0% to 80% (water-cut from 100% to 20%). Previous results obtained in conventional cylindrical cells, in which the oil-brine mixture was homogenized with a rotating magnet, have been confirmed. The current fluctuations present a very complex and nonmonotonic behavior with the VOC, which yields corrosion monitoring virtually impossible in these conditions. Conversely, the electrolyte-resistance fluctuations appeared to be a reliable and very sensitive tool for the real-time analysis of hydrodynamic conditions and flow composition that can be used for the indirect assessment of oil-brine cor...

Fluctuations of concentration overpotential generated at gas-evolving electrodes

The electrode overpotential is usually expressed in terms of partial components related to ohmic, activation and concentration effects. For a gas-evolving electrode, the overpotential fluctuates because of bubble evolution but only the global overpotential fluctuations and, more recently, their ohmic part are easily accessible by routine experiments. In the present paper, high-amplitude fluctuations of concentration overpotential have been experimentally revealed by an anomalous behaviour of the current fluctuations induced by hydrogen evolution on platinum electrodes under potential control in alkaline media with or without the presence of a second mass-transport-controlled reaction. Inverted current jumps, reproducibly observed owing to an artificial bubble-nucleation site created on the edge of the rotating electrode, which provoked periodic bubble eruption, were ascribed to local concentration changes drastically affecting the current-lines distribution. These experimental results validate the theoretical models that predicted those anomalies but remained unverified up to now. A quantitative estimation of each overpotential component at bubble eruption is addressed by means of a simplified analytical approach supported by the simultaneous measurement of the electrochemical current noise and electrolyte resistance fluctuations.

A new approach for monitoring corrosion and flow characteristics in oil/brine mixtures

Monitoring of corrosion processes and hydrodynamic flow pattern in oil/brine mixtures of various compositions between 0 and 80% in volume of oil has been investigated by means of electrochemical impedance and noise measurements. In such two-phase electrolytes the electrode behaviour is mostly characterized by the fact that the state of the corroding metallic surface evolves in a highly dynamic way as a result of local heterogeneities yielded by the non-uniform phase distribution near the electrode. This is why conventional electrochemical current and/or potential noise, as well as electrochemical impedance measurements, showed very poor reproducibility and weak sensitivity to electrolyte composition and flow changes. In contrast to conventional techniques, the electrolyte resistance fluctuations have shown to be a highly sensitive and reliable tool conveying real-time information about the two-phase flow pattern and electrolyte composition. Such measurements can be an indirect way to evaluate the environment aggressiveness in industrial plant and field applications.

Oxygen evolution on electrodes of different roughness: an electrochemical noise study

Rough and porous Ni layers have been obtained by cathodic deposition from a NiCl2, NH4Cl solution, at high current density. Characterisation by SEM has shown that they consisted of micro-dendrites separated by pores with a typical diameter of 1 μm. In addition, circular hollows (10–100 μm in diameter) were found on the deposit surface; their density varied with the deposition current density and deposition charge. The surface roughness of the Ni deposits, measured by EIS, was found to increase roughly linearly with the deposition charge, and to be little dependent on current density, provided a threshold value was exceeded. The oxygen evolution reaction has been studied on these electrodes by simultaneous real-time measurements of potential and electrolyte resistance fluctuations. The analysis of the electrochemical noise indicated that the dimensions of oxygen bubbles detaching from the electrodes slightly increased with the deposit surface roughness. It is not clear, however, whether or not this increase was associated with the effect of the small (1 μm) or the large (10–100 μm) features on the electrode-bubble interactions.

Comparative Analysis of Potential, Current, and Electrolyte Resistance Fluctuations in Two-Phase Oil/Water Mixtures

Abstract The fluctuations of potential, current, and electrolyte resistance generated by uniform corrosion processes on a mild-steel rotating disk electrode in a mixture of paraffin oil and brine have been measured. The mixture composition has been varied to assess the possibility of characterizing the type of hydrodynamic flow from the noise measurements. Results have shown that, contrary to electrochemical potential and current noise, electrolyte resistance fluctuations, both in the time and frequency domains, are a highly sensitive parameter for monitoring changes in the flow composition.

Electrochemical noise analysis of O 2 evolution on PbO 2 and PbO 2-matrix composites containing Co or Ru oxides

Electrochemical noise measurements have been performed on various electrode materials during oxygen evolution reaction. Fluctuations of both electrode potential and electrolyte resistance, recorded under galvanostatic polarization, have been analysed in order to separate ohmic and non-ohmic contributions. The anode materials compared in this study were PbO 2 and composite oxides consisting of a PbO 2 matrix and variable amounts of catalytic dispersed phases (Co 3O 4, RuO 2 and hydrous Co oxides); the composite oxide layers were generally obtained by anodic codeposition of fine particles (typically 0.1–1 μm in size) with a PbO 2 matrix electrochemically grown by Pb 2+ oxidation. The power spectral density (PSD) of the electrolyte resistance fluctuations is very similar for PbO 2 and composite electrodes, suggesting a similar gas evolution profile. On the contrary, the potential PSD is markedly different for PbO 2 and composites containing catalytic particles. In the latter case, the potential fluctuations are entirely due to ohmic effects, while non-ohmic components are clearly dominant in the case of O 2 evolution on pure PbO 2, probably because of the much higher activation overpotential. Furthermore, on PbO 2 electrodes a plateau was obtained at intermediate frequencies (0.1–10 Hz), which was tentatively explained by bubble coalescence phenomena.

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. I. Characterisation of hydrogen evolution on vertical unstressed electrodes

For investigating stressed carbon–steel electrode (AISI 4140) cracking induced by hydrogen embrittlement with the electrochemical noise (EN) technique, the contribution of the evolving hydrogen bubbles to the EN has to be determined. Under cathodic current control, various reasons may explain the fluctuations of the electrode potential. In the first paper of this series the electrolyte resistance (ER) fluctuations, which yield ohmic-drop fluctuations, were analysed for unstressed vertical tensile specimens. A simplified model was proposed to tentatively derive the characteristic parameters of the gas evolution, such as the bubble mean size and evolution mean rate, from the power spectral density (PSD) of the ER fluctuations. However, results in only qualitative agreement with optical observations were obtained, indicating that the complicated screening and dragging effects of rising bubbles have to be taken into account in the modelling for vertical electrodes. The electrode potential fluctuations of unstressed tensile specimens were investigated in the second paper of this series and the influence of a stress was examined in the third paper.

Electrochemical Noise Measurements of Coalescence and Gas-Oscillator Phenomena on Gas-Evolving Electrodes

This paper is aimed at identifying the different mechanisms of hydrogen bubble evolution on Pt disk electrodes of different sizes (0.5, 1.0, and 5.0 mm in diam) in a wide current density range. This was accomplished by means of time and frequency electrochemical noise analysis combining potential and electrolyte resistance fluctuations simultaneously measured to separate ohmic and nonohmic fluctuations. At weak current densities, coalescence processes were shown to be a major component of bubble growth in the case of small electrodes while diffusion-controlled bubble growth was dominant on larger surface ones. For all electrode diameters, a new mechanism was spontaneously switched on at higher current densities to enhance evacuation of dissolved gas produced at the interface. This new mechanism was a periodical gas-oscillator collective phenomenon originated in the simultaneous evolution of bubbles at the border of the electrode where the metal-insulator discontinuity acted as a continuous gas cavity, giving preferential bubble nucleation. Once present, the gas-oscillator mechanism was a strongly dominant component of the evolution process, so that most of the bubble production was shifted from the center to the edge of the electrode. © 2002 The Electrochemical Society. All rights reserved.

Hydrogen Absorption Estimation on Pd Electrodes from Electrochemical Noise Measurements in Single-Compartment Cells

This paper is aimed at investigating the possibility of detecting and estimating hydrogen absorption on standard cylindrical electrodes in single-compartment electrochemical cells, to avoid classical measurements with membrane-shaped samples in double-compartment cells that cannot be carried out in many practical situations. This was achieved by comparing the hydrogen bubble evolution on a strongly hydrogen-absorbing metal (Pd) and a nonabsorbing one (Pt) from electrochemical noise measurements at high charging current densities between −25 and −125 mA/cm2. The bubble evolution characteristic parameters (detachment mean radius and mean rate) were derived from the power spectral density of the electrolyte resistance fluctuations. The difference in the mean volume of gas evolved per time unit on Pt and Pd was used to derive the hydrogen absorption flux in Pd. A good agreement was obtained with values reported in literature for high cathodic current densities, that is, in the presence of important phase concentration. © 2001 The Electrochemical Society. All rights reserved.

Dynamic behaviour of an electrolyser with a two phase solid-liquid electrolyte Part II: Investigation of elementary phenomena and electrode modelling

The dynamic behaviour of an electrolyser with a two phase solid-liquid electrolyte was investigated. In a previous paper some results were obtained with high particle concentrations by means of spectral analysis of the electrolyte resistance fluctuations and of the potential fluctuations. Here low bead concentrations were used. This allows potential and electrolyte resistance transients to be well separated and to be studied in the time domain. This analysis gives a detailed view of the approach and residence of the beads near the collector, and a quantitative estimation of the ohmic drop effect for insulating and conductive beads. Ohmic drop fluctuations account for the potential fluctuations for insulating glass beads. Zinc beads behave as insulating particles in the low frequency range and generate a similar ohmic drop effect. They behave as conducting particles in the high frequency range and the fast charge exchange induces depolarizing pulses which favour the formation of compact zinc deposits on the current collector. From the analysis of both transients and PSDs of the electrolyte resistance and potential fluctuations, the mean percentage of the particles colliding with the collector has been estimated even for high bead concentrations.

Fluctuation analysis in electrochemical engineering processes with two phase flows

Fluctuation analysis in electrochemical systems appears to be a suitable method for obtaining information on the system dynamic behaviour, especially when the voltage or current fluctuations are due to elementary events on the electrode at a semi-macroscopic scale, for example growth and detachment of bubbles on a gas-evolving electrode or contacts between charged particles in a fluidized or circulating bed reactor, or pits in localized corrosion. Therefore this situation is largely encountered in electrochemical engineering processes with two phase flows. By analysing the current (voltage) fluctuations at constant potential (current) and/or the electrolyte resistance fluctuations, this technique provides quantitative parameters which are often inaccessible by traditional deterministic techniques, steady state or not, which deal with time-averaged signals. The technique also leads to a better understanding of the elementary processes on the electrode. Two examples are given: the first concerning a gas-evolving electrode and the second a circulating bed electrode.

Real‐Time Measurement of Electrolyte Resistance Fluctuations

An electronic device has been devised in order to measure simultaneously the potential fluctuations and the electrolyte resistance fluctuations of an electrode. The separation of the ohmic and faradaic components of the potential fluctuations allows a more precise identification of the elementary events generating this voltage electrochemical noise (bubble detachment on a gas‐evolving electrode, contact between particles in a fluidized electrode, etc.). Transients of the electrolyte resistance as fast as 1 ms can be measured by this device with a precision better than 0.1%.

Investigation of bubble evolution with a quartz crystal microbalance

Numerous electrochemical processes of practical interest involve gas production at an electrode surface. The subsequent bubble evolution leads to a random behaviour of the observed potential when the electrode is current controlled. It has been shown that these potential fluctuations are, at least partially, related to a screening effect of the surface by the gas bubbles [l] which gives rise to an additional electrolyte resistance and hence to ohmic drop fluctuations. At low current densities it was shown that the ohmic drop and the overvoltage increase linearly when the bubbles grow and decrease suddenly when a bubble detaches from the surface [2]. The sizes and times of occurrence of the potential jumps are random and the observed potential is a random signal which was analyzed by spectral analysis [3]. For high values of the electrolysis current the measurement of the power spectral density (p.s.d.) of the potential fluctuations leads to information on the dynamics of the bubble evolution regime, i.e. the mean number of bubbles evolved per time unit, the average bubble detachment radius and the gas evolution efficiency [3]. For low values of the electrolysis current an estimate of the detachment size of the bubbles can be obtained only from the measurement of the power spectral density of the electrolyte resistance fluctuations [4]. However, information concern- ing the physico-chemical details of the bubble detachment is still lacking. Recently, it has been proposed that an electrochemical quartz crystal microbal- ante be used in order to detect electrolytically generated bubbles [5]. The response of the quartz resonance frequency to polarisation current steps has been observed. But probably due to the narrow bandwidth and the low amplification of the electronic equipment the individual transients related to the bubble growth and