Electrochemical Microcell Research Articles

Engineering a continuous flow electrochemical micro-cell for biosensor applications: new achievements

The functional tests of a previously obtained continuous flow micro-cell revealed a rather low value of the sensitivity of the chronoamperometric measurements as well as a poor reliability of both fluidic and electric connections. The engineering of the micro-cell regards the improvement of the following characteristics: (1) integrating the reference electrode on the chip, (2) designing a specific pattern of the electrodes and (3) serially distributing them along the pipeline reservoir. Thus, a plug-flow device is obtained and the preliminary tests confirm its functionality together with an improved sensitivity. The use of photosensitive glass (Foturan®) for obtaining the reservoir and its vertical openings allowed an improvement of the transversal shape and thus of the fluid flow. Better connections are achieved by the development of more reliable and rigid glass fluidic connections as well as of flexible elastomer-based solderless electric connections combined with both a standard DIL40 socket and standard flat cable connectors.

Functional testing and characterisation of (bio-)chemical sensors on wafer level

An automated computer-assisted system for the functional testing and characterisation of (bio-)chemical sensors on wafer level is developed and integrated into a commercial prober station. The system enables the identification and selection of “good” sensors on wafer level and thus, allows to avoid further expensive bonding, encapsulation and packaging processes for defective or non-functioning sensor structures. Moreover, a specifically designed flow-through electrochemical microcell offers the possibility of wafer-level characterisation of (bio-)chemical sensors in terms of sensitivity, drift, hysteresis and response time at an early process stage. The system has been exemplarily tested using wafers combining pH-sensitive capacitive electrolyte–insulator–semiconductor structures as well as ion-sensitive field-effect transistors with different geometrical sizes and gate layouts.

SAE 1045 steel/WC–Co/Ni–Cu–Ni/SAE 1045 steel joints prepared by dynamic diffusion bonding: Microelectrochemical studies in 0.6 M NaCl solution

Corrosion of SAE 1045 steel/WC–Co/Ni–Cu–Ni/SAE 1045 steel interfaces was investigated in 0.6 M NaCl solution using an electrochemical microcell, which enables local electrochemical characterization at the micrometer scale. Two pieces of steel, one with a WC–Co coating covered with Ni (12 μm) and Cu (5 μm) layers, and the other with a Ni (15 μm) layer, were welded by dynamic diffusion bonding. A WC–Co coating was applied to the steel by the high velocity oxygen-fuel process, and Ni–Cu and Ni layers by electroplating. Polarization curves were recorded using an electrochemical microcell. Different regions of welded samples were investigated, including steel, cermet coating, and steel/cermet and steel/Ni–Cu–Ni/cermet interfaces. Optical and electronic microscopes were employed to study the corroded regions. Potentiodynamic polarization curves obtained using the microcell revealed that the base metal was more susceptible to corrosion than the cermet. In addition, cermet steel/cermet and steel/Ni–Cu–Ni/cermet joints exhibited different breakdown potentials. Steel was strongly corroded in the regions adjacent to the interfaces, while the cermet was less corroded. Iron oxides/hydroxides and chloride salts were the main corrosion products of steel. After removal of the superficial layer of corrosion products, iron oxides were mainly observed. Chloride ions were detected mainly on a copper-enriched layer placed between two Ni-enriched layers.

Functional testing and characterisation of (bio-)chemical sensors on wafer level

An automated computer-assisted system for the testing and characterisation of (bio-)chemical sensors on wafer level is developed and integrated into a commercial prober station. The system enables the identification and selection of “good” sensors on wafer level and thus, allows avoiding expensive bonding, encapsulation and packaging processes for defective or non-functioning sensor structures. Moreover, a specifically designed flow-through electrochemical microcell offers the possibility of wafer-level characterisation of (bio-)chemical sensors in terms of sensitivity, hysteresis and response time at an early process stage. The system has been exemplarily tested using wafers combining pH-sensitive field-effect transistors with different geometrical sizes and gate layouts.

Modelling of anodic dissolution of pure aluminium in sodium chloride

A mathematical model for simulating a passive aluminium (Al) surface with a pit in which active electrochemical metal dissolution occurs has been developed. The model includes hydrolysis products of Al and the species obtained as a result of homogeneous reactions between chloride and Al 3+ ions and Al hydrolysis products. The model does not assume the equilibrium state in solution: all terms in homogeneous reactions are treated explicitly using kinetic constants taken from the literature. The validity of assuming reaction equilibrium has been addressed. Solution potential values and species concentrations are predicted for different dissolution current densities. The acidity in the pit is explained by the hydrolysis of Al 3+; an analytical expression for the pH values at the pit bottom for a given dissolution current density is presented. The model is applied to a real capillary geometry used in electrochemical microcell experiments. It was found that for r cap/ r pit < 100, where r cap and r pit are the capillary end and pit radii, respectively, the insulating capillary wall affects the species concentrations and the solution potential. Moreover, for r cap/ r pit < 20, the shape of the capillary, which might not be cylindrical, should be taken into account.

Local electrochemical studies of the microstructural corrosion of AlCu4Mg1 as-cast aluminium alloy and influence of applied strain

The microstructure of AlCu4Mg1 as-cast aluminium alloy was first determined by means of field emission-scanning electron microscope with an integrated electron dispersion spectrometer, secondary ion mass spectroscopy and atomic force microscopy. Large precipitates (Al2Cu, Al–Si–Mn–Fe–Cu, oxides) were located at grain boundaries, whereas small particles (aluminium, magnesium and copper) were present in grains. The electrochemical response and pitting susceptibility of sites containing precipitates were then investigated after polishing using the electrochemical microcell technique. After straining, big scattering was observed in the electrochemical response. The most active places corresponded to the sites containing wide microcracks and severe damages in the matrix. In this case, the corrosion potential was around −1,000 mV vs. Ag/AgCl, and the current in the passive domain was five times higher than on the strained matrix. In the absence of severe damage in the matrix or wide microcracks, the corrosion potential was more anodic and the current density in the passive range was around 0.5 mA cm−2. Local polarisation curves carried out in sites containing large precipitates and no defects induced by straining were very close to those obtained in grains far from precipitates.

Relationships between microstructure and pitting corrosion of ADI in sodium chloride solution

Austempered ductile iron (ADI) has complex microstructure containing a multiphase matrix (called ‘ausferrite’), graphite spheres and oxide inclusions. The corrosion resistance of ADI is related to its microstructure which is determined by heat treatment parameters (like austempering temperature, austempering time, austenitising temperature and austenitising time). In the present paper, the electrochemical behaviour and corrosion resistance of ADI have been investigated by means of the electrochemical microcell technique and classical electrochemical measurements in sodium chloride solution. Particular attention has been paid to the influence of austempering temperature on the microstructure and pitting corrosion. It has been shown that ADI austempered at 430 °C has upper ausferritic microstructure and reveals a better corrosion resistance in sodium chloride solution than ADI austempered at 280 °C. Moreover, the corrosion resistance increases as the volume fracture of ferrite increases and the carbon content of austenite decreases. The good corrosion behaviour of ADI austempered at 430 °C was also related to the good coarsening of the austenite grains and broad ferrite needles (less ferrite/austenite interfaces). It has been demonstrated that silicon is the alloying element hindering the anodic dissolution of the alloy.

Electrochemically Induced Modulation of the Catalytic Activity of a Reversible Redoxsensitive Riboswitch

AbstractOver the past decade, RNA conformation has been shown to respond to external stimuli. Thus, dependent on the presence of a high affinity ligand, specifically designed ribozymes can be regulated in a classical allosteric way. In this scenario, a binding event in one part of the RNA structure induces conformational changes in a separated part, which constitutes the catalytic centre. As a result activity is switched on (positive regulation) or off (negative regulation). We have developed a hairpin aptazyme responding to flavine mononucleotide (FMN). Ribozyme activity is dependent on binding of FMN and thus is switched on in the presence of FMN in its oxidized form. Under reducing conditions, however, FMN changes its molecular geometry, which is associated with loss of binding and consequently down‐regulation of ribozyme activity. While in previous experiments sodium dithionite was used for reduction of FMN, we now present an assay for electrochemically induced activity switching. We have developed an electrochemical microcell that allows for iterative cycles of reduction/oxidation of FMN in an oxygen free atmosphere and thus for reversible switching of ribozyme activity. The reaction proceeds in droplets of 3 to 10 μL at micro‐ to nanomolar concentrations of the reaction components.

Heat exchangers corrosion protection by using organic coatings

Corrosion phenomena in aluminium heat exchangers represent a problem in terms of durability and efficiency of thermal exchange. This work evaluates the barrier properties of two coatings that represent the state of the art for the protection in the heat, ventilating, air conditioning/refrigerating field (HVAC/R): electrophoretic coating (E-coating) using epoxy resin and spraying of polyurethane coating with addition of metallic pigments. The ability of the coatings to cover the surface of heat exchanger has been evaluated by means of optical microscopy in order to highlight critical zones of the system for the application of the coatings. The electrochemical behaviour of coated heat exchangers has been studied by means of electrochemical impedance spectroscopy in 3.5% NaCl solution. The local electrochemical behaviour of coating defects has been investigated using the electrochemical micro-cell, which enables to perform potentiodynamic polarization measurements on single defects. E-coatings evidenced difficulties to provide uniform thickness of the coating at the extremities of the fins. Spraying of the polyurethane coating containing pigments require particular care in order to fully cover zones of the heat exchanger with difficult accessibility. The electrochemical behaviour of coated heat exchangers is affected by the existence of defects in the coatings.

Numerical modelling of the electrochemical behaviour of 316L stainless steel based upon static and dynamic experimental microcapillary-based techniques

Microcapillary-based techniques allow the selection and interrogation of single metallurgical sites and are therefore becoming increasingly popular to investigate the electrochemical behaviour of metallic phases and non-metallic heterogeneities in alloys. This study has been carried out to assess the differences between current measurements made using a ‘closed’ microcapillary system (the electrochemical microcell technique) with current measurements derived from a flowing ‘open’ microcapillary droplet cell (the scanning droplet cell). The experimental results were compared with calculations derived from a model system adopting a finite element approach. The corrosion system consists of four parallel electrochemical reactions: three cathodic reactions (the oxygen reduction reaction, the hydrogen evolution reaction and water dissociation) and one anodic reaction (metal dissolution reaction). Comparative experimental results have shown there is a large discrepancy between the magnitudes of the cathodic current obtained by the two techniques. The results of this study allow an assessment of the parameters responsible for the mass transport and distribution of species in the closed and open systems. Critical parameters (for example, microcell crevice geometry, specimen surface-capillary gap distance, etc.) which lead to significant modifications of the curves were then identified using the numerical simulation.

LBW of Similar and Dissimilar Skin-Stringer Joints Part II: Electrochemical Characterization

The durability of metal structures depends most upon the corrosion behaviour of its materials as well as the on the electrochemical behaviour of joints and the technology employed to construct the joints itself. This work describes the effect of Laser Beam Welding (LBW) technology on the electrochemical and corrosion behaviour of parent materials and bead of several aluminium alloy joints. Investigation was carried out by using dc electrochemical techniques (open circuit potential monitoring, OCP, and anodic polarization) on selected micro areas of parent materials and bead by means of a suitable electrochemical microcell. With regards to the corrosion behaviour of heat affected zone (HAZ), no data has been presented due to the small area of HAZ when compared to the microcapillary area adopted in this study. Alloys investigated in this work are the AA 2139 and AA 6156 as skin in the form of rolled sheets, and AA 2139 and PA 765 as stringer, in form of extruded parts. In addition, the AA 4047 has been used as filler material in form of wire. Results obtained in this investigation indicated that no major corrosion concerns arise from the LBW of both laminated AA 2139 skin with extruded AA 2139 stringer and laminated AA 6156 skin with extruded AA 2139 stringer. In the former case, the bead exhibits corrosion potential nobler than that exhibited by the skin and stringer of circa 100 mV for the AA 6156 skin with extruded AA 2139 joint. This result is attributed to the presence of Si in the bead due to the process parameters adopted for such a weld. Quite similar corrosion behaviour was observed for the skin, stringer and bead in the latter case. Attention must be paid when laminated AA 2139 skin with extruded PA 765 stringer, and laminated AA 6156 skin with extruded PA 765 stringer are joined by LBW. In this case low potential and high surface activity were observed on the stringer and bead and attributed to the excess of Zn in the PA 765 alloy.

Numerical modelling of the electrochemical behaviour of 316 stainless steel based upon static and dynamic experimental microcapillary‐based techniques: effect of electrolyte flow and capillary size

AbstractThe objective of this work was to determine the parameters that affect the mass transport and the distribution of species in microcapillaries close to the specimen surface. Local experiments were carried out under static and flow conditions on type 316L stainless steel in 1.7 M NaCl, pH = 3, by means of the electrochemical microcell and the scanning droplet cell technique. The polarisation behaviour of pure iron (used as a model system) in an aqueous environment was calculated adopting a finite element approach and was compared to the experimental results. The corrosion system consists of three parallel electrochemical reactions: the oxygen reduction reaction (ORR), the hydrogen evolution reaction (HER) and iron dissolution. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Local Electrochemical Studies and Surface Analysis on Worn Surfaces

Specimens of duplex stainless steel UNS S32304 immersed in distilled water were worn under sliding friction conditions. The electrochemical microcell technique and local Auger spectroscopy were then applied to these heterogeneous surfaces to determine the influence of surface chemical composition and surface defects on the electrochemical behavior of each phase of the stainless steel in and out of the wear track. Experiments were performed both in NaCl, pH 3 (where the passive film is stable), and HCl (where an active peak is observed). In addition, the scanning vibrating electrode technique was combined with a modified microcell to study microgalvanic processes between the surrounding passive surface and the track at the OCP value.

A novel disposable electrochemical microcell: construction and characterization

An alternative technique for the fabrication of disposable electrochemical microcells containing working, reference and auxiliary electrodes on a single device is reported. The procedure is based on thermal-transfer of toner masks onto CD-R (recordable compact discs) gold surfaces to define the layout of the electrodes (contour). In a subsequent step, the layout is manually painted with a permanent marker pen. The unprotected gold surface is conveniently etched (chemical corrosion) and the ink is then easily removed with ethanol, generating gold surfaces without contamination. The final and reproducible area of the electrodes is defined by heat transference of a second toner mask. Silver epoxy is deposited on one of the gold bands which is the satisfactorily used as reference electrode. These microcells were electrochemically characterized by cyclic, linear, and square wave voltammetry, and several electroactive species were used as model systems. The area reproducibility of the electrodes for different microcells was studied and a relative standard deviation better than 1,0% (n = 10) was obtained. Disposable electrochemical microcells were successfully used in analysis of liquid samples with volumes lower than 200 µL and good stability and reproducibility (RSD less than 2.0%) were achieved. These microcells were also evaluated for quantification of paracetamol and dipyrone in pharmaceutical formulations.

Effects of Sliding Wear at Various Loading Conditions on the Local Mechanical-Electrochemical Behaviour of Duplex Stainless Steels

The present paper aims at demonstrating the potential of the electrochemical microcell technique, the scanning vibrating electrode technique and local Auger for determining the influence of the surface chemical composition and surface defects on the electrochemical behaviour of each phase of duplex stainless steels in and out of the wear track.

Macroscopic and Local Electrochemical Studies of Austempered Ductile Iron in Various Aqueous Solutions

The present paper aims at demonstrating the potentialities of the electrochemical microcell technique and surface analysis methods for studying the electrochemical behaviour of austempered ductile iron at the microscale. The objective is to identify precursor sites and to understand basic mechanisms leading to localized dissolution. These Experiments, which were conducted both in 1M NaClO4, pH=10 and 1M NaClO4, pH=3, are compared to the results obtained at the macroscale.

Microelectrochemical Measurements of Dissolution of MnS Inclusions and Morphological Observation of Metastable and Stable Pitting on Stainless Steel

The electrochemical dissolution of MnS inclusions and pit initiation processes on type 303 stainless steel were investigated by an electrochemical microcell technique. In , the electrochemical dissolution of MnS inclusions started at vs , which was followed by the initiation of a hemispherical and smooth wall pit at MnS/matrix boundary. It was suggested that the dissolution products of MnS inclusions induced the formation of a salt film, which caused localized electropolishing. In NaCl, the onset potential of electrochemical dissolution of MnS inclusions was and the stable pit growth occurred at a potential of around . Field-emission scanning electron microscopy observations revealed that many metastable pits with a diameter of approximately were formed at MnS/matrix boundary on the specimen, of which the polarization measurement was stopped immediately after stable pit initiation. From a morphological point of view, the metastable pits were found to be very similar to flat-walled etch pits. The dissolution products of MnS inclusions and chloride ions would attack the metal surface newly exposed by dissolution of the inclusions. It is proposed that the chemistry of dissolved species from MnS inclusions have a substantial influence on pit initiation at MnS inclusions.

Impact des conditions d'usinage sur la zone du matériau affectée par le procédé

Machining processes can introduce residual stresses at the surface of machined workpieces and can modify the microstructure and texture in a volume close to the surface. Such changes are important when controlling the behaviour of these workpieces during the workpiece lifetime. It is thus necessary to quantify residual stresses and texture components and to determine the machining-induced microstructure to understand and predict the behaviour of modified surfaces. In this study, particular attention was paid to pure copper considered as a modelling system. The influence of the nose radius, the cutting speed and the lubricant on the surface stress field and the surface texture was quantified by means of XRD techniques. The microstructural modifications at the specimen surface and in depth were evaluated using scanning electron microscopy. The electrochemical behaviour of modified surfaces were studied from local measurements using the electrochemical microcell technique.

The use of local electrochemical probes and surface analysis methods to study the electrochemical behaviour and pitting corrosion of stainless steels

In this paper, the electrochemical microcell technique is combined with ex situ (SIMS and XPS) and in situ (SVET) techniques to study the electrochemical behaviour and pitting corrosion of inclusion-contaning stainless steels. Dissolution of metallurgical heterogeneities, depassivation and pitting mechanisms are first discussed in 1 M NaClO 4, pH 3, and 1 M NaCl, pH 3, by considering the chemical composition and morphology of inclusions. The influence of the chemical composition of passive films on the electrochemical reactions and the microgalvanic coupling processes occurring at the corrosion potential is also investigated on the same specimen microstructure.

Effects of Cr 2N on the pitting corrosion of high nitrogen stainless steels

Effects of nitrogen (N) content on the precipitation behavior of Cr 2N with its relevant influences on the resistance to localized corrosion of high nitrogen stainless steels (HNSs) were examined using an electrochemical microcell. HNS of Fe–18Cr–20Mn–3Mo–1.07N was highly resistant to pitting corrosion. With aging the alloy at 900 °C, Cr 2N precipitated initially along grain boundaries, then within grain, and eventually formed a lamellar structure, accompanied with a drastic decrease in the resistance to pitting corrosion. It was found from the electrochemical microcell tests that the lamellar Cr 2N was the most susceptible site for pitting corrosion when compared with any other heterogeneity such as a (Mn, Cr)-oxide or a Cr 2N precipitated along the grain boundaries.