Microelectrochemical Measurements Research Articles

Microelectrochemical Studies of Pit Initiation on High Purity and Ultra High Purity Aluminum

AbstractThe onset of pitting on high purity and ultra high purity aluminum has been studied in the micrometer range by using a microelectrochemical cell in order to evaluate influences that induce localized corrosion. Microelectrochemical tests on polished samples of Al 99.999 and Al 99.9999 in 1 M NaCl showed that pitting at large areas occurred at several hundreds mV more negative than at small areas, suggesting that even very pure Al contains weak points. On rough surfaces weak points are more “activated” than on smooth surfaces. Locally measured pitting potentials of polished samples were shifted to more positive values than those measured on ground samples. Larger areas contain always small scratches, even on polished samples. Hence, the surface roughness showed a minor influence on the onset of pitting on larger areas. Microelectrochemical measurements performed on pure Al 99.99 with small amounts of copper showed that a copper content below 30 ppm is too small to have a beneficial or detrimental effect, whereas a copper content above 300 ppm probably leads to the formation of small inclusions or precipitations that acted as preferential corrosion initiation sites. A copper content around 100 ppm showed a beneficial effect on very small areas but not on larger ones.

Microelectrochemical Techniques for Probing Kinetics at Liquid/Liquid Interfaces

We provide an overview of recent advances in microelectrochemical approaches to investigate the kinetics of various physicochemical processes that occur at the interface between two immiscible electrolyte solutions (ITIES). To place the advances in context, background material on the structure of the ITIES, derived from both experimental studies and computer simulation, is also provided. The main focus of the article is micro-ITIES techniques, single droplet measurements, microelectrochemical measurements at expanding droplets (MEMED) and scanning electrochemical microscopy (SECM). Recent developments in a combined SECM-Langmuir trough technique for probing diffusion processes across Langmuir monolayers at the water/air (W/A) interface are also highlighted, by considering an organic monolayer at a water surface as a special case of a liquid/liquid interface.

Investigation of the Kinetics and Mechanism of Acid Chloride Hydrolysis in an Oil/Water System Using Microelectrochemical Measurements at Expanding Droplets (MEMED)

Acid chloride (ROCl) hydrolysis in a 1,2-dichloroethane (DCE)/water system has been investigated over a wide range of conditions using microelectrochemical measurements at expanding droplets (MEMED). Hydrolysis occurred spontaneously when DCE droplets containing either butyryl chloride, valeryl chloride, hexanoyl chloride, or decanoyl chloride were expanded into an aqueous buffer solution (pH 4−11). The reaction was monitored using a potentiometric Ag/AgCl ultramicroelectrode to measure local changes in Cl- concentration in the aqueous phase near the surface of the droplets during the reaction. For butyryl chloride and decanoyl chloride, the hydrolysis rate increased gradually with increasing pH. For valeryl chloride and hexanoyl chloride, the hydrolysis rate increased with pH in the range of 4−9 but decreased at higher pH values, due to the blocking of the liquid/liquid interface by products. A mechanism for the ROCl hydrolysis reaction, involving rapid (Langmuirian) adsorption of ROCl at the interface f...

Investigation of cupric ion extraction kinetics in a two-phase organic/water system using microelectrochemical measurements at expanding droplets (MEMED)

The extraction kinetics of metal ions from aqueous solution to an organic phase containing an extractant ligand are investigated using microelectrochemical measurements at expanding droplets (MEMED). Specifically, the extraction of cupric ion from an aqueous phase by the oxime ligand, Acorga P50 (HL) in 1,2-dichloroethane (DCE) droplets, is considered. The MEMED approach involves the periodic formation of DCE droplets containing HL, by flowing this organic phase at a slow rate through a fine capillary tip submerged in an aqueous receptor phase. The depletion of cupric ion in the aqueous phase, adjacent to the advancing droplet, is probed amperometrically at a fixed ultramicroelectrode. Time-dependent spatially resolved concentration measurements made in this way could be related to the kinetics of the extraction process, by solving a well-posed mass transport problem. The extraction process is immeasurably slow at pH 1 in the aqueous bulk phase, but becomes faster with increasing pH, ultimately attaining a transport-controlled rate on the MEMED timescale at pH 4. A first-order dependence of the extraction rate on the bulk concentration of Cu 2+ (1.0–6.0 mM) was found, while there was an inverse first-order correlation with proton concentration (pH 2.0–3.0). The dependence of the extraction rate constant on [HL] in the DCE phase was studied in detail. The extraction rate was first-order in [HL] in the range 10–200 mM. A mechanistic model for Cu 2+ extraction by HL with the formation of CuL + at the water ∣ DCE interface as the rate-limiting step, is proposed to explain these observations.

Microelectrochemical measurements of electron transfer rates at the interface between two immiscible electrolyte solutions: Potential dependence of the ferro/ferricyanide-7,7,8,8-tetracyanoquinodimethane (TCNQ)/TCNQ˙– system

The reduction of 7,7,8,8-tetracyanoquinodimethane (TCNQ) in both 1,2-dichloroethane (DCE) and nitrobenzene (NB), by aqueous ferrocyanide, and the back reaction have been studied by scanning electrochemical microscopy (SECM) and microelectrochemical measurements at expanding droplets (MEMED). The main focus has been to elucidate the effect of galvanic potential at the interface between two immiscible electrolyte solutions (ITIES) on electron transfer (ET) rates, with different electrolyte concentrations in the organic phase. SECM studies show that the ET rate constants for both the forward and back reaction depend strongly on the interfacial potential drop, with an apparent ET coefficient close to 0.5. MEMED demonstrates that TCNQ˙− is confined to DCE, but transfers from NB to water under certain experimental conditions, which could complicate kinetic analysis. The ET kinetics for the water/DCE system have been analysed further using Marcus theory. Close to zero driving force, the rate constants for the forward and back reaction are found to be similar and in good agreement with predictions from Marcus theory with a sharp liquid/liquid interface. The results suggest that Butler–Volmer kinetics describe ET at the ITIES when the driving force is low and the ionic strength in both the aqueous and organic phases is relatively high.

Oxidation of 4-Methylanisole by Aqueous Cerium(IV) in a Two−Phase Immiscible Liquid/Liquid System: Interfacial versus Homogeneous Control

The kinetics of the oxidation of 4-methylanisole (MA) by Ce(IV) have been measured in both two-phase (MA/aqueous sulfuric acid) and single-phase (aqueous sulfuric acid) arrangements by using microelectrochemical measurements at expanding droplets (MEMED) and UV−visible spectrophotometry, respectively. In the two-phase arrangement, the electron transfer (ET) reaction appeared to occur interfacially, rather than by dissolution of MA followed by reaction in solution as previously considered. The effective first-order rate constant for the consumption of Ce(IV) at the interface is 2.0 × 10-4 cm s-1. The homogeneous reaction is first-order in both Ce(IV) and MA, with a rate constant for the loss of Ce(IV) of 1.0 mol-1 dm3 s-1. The lifetime of the 4-methylanisole radical cation in aqueous sulfuric acid, formed in the first step of the oxidation process, has been measured independently as ∼10 ms using steady-state ultramicroelectrode voltammetry. The factors leading to the different energetics of the one phase a...

Effects of Temperature and Chloride Concentration on Pit Initiation and Early Pit Growth of Stainless Steel

The influences of temperature and chloride concentration on pitting corrosion of stainless steel AISI 304 were investigated. Conventional large-scale measurements (sample diameter, were conducted to study the effect of temperature on pit initiation, metastable and stable pitting in 0.1 M and in 0.1 M NaCl solutions at 0, 30, 60, and 90°C. Microelectrochemical measurements μm) were performed to investigate pit initiation events at room temperature, 60, and 90°C in in NaCl, and in LiCl solutions of various concentrations (0.01-10 M It was found that the MnS inclusions dissolved at lower potentials when the temperature was increased. In the presence of chlorides, an increase in temperature influenced pit growth and decreased the pitting potential. The temperature was found to aid in stabilizing pit growth and hindering repassivation. Pitting corrosion at single MnS inclusions was initiated by conducting potential sweeps with a microelectrochemical cell until the pitting potential was reached. In the presence of chlorides, pits initiated at the interface of the MnS inclusions and the metal matrix. The extent and morphology of the attack at the interface between the inclusion and the metal matrix varied with chloride concentration and temperature. At high temperatures and high chloride concentrations, numerous additional initiation processes on the passive surface around the larger inclusions are taking place, leading to an etching-type corrosion of this area. © 2001 The Electrochemical Society. All rights reserved.

Properties of electroless and electroplated Ni–P and its application in microgalvanics

Composition, microstructure, surface morphology, mechanical properties and electrochemical behaviour of electroless (el) and electroplated (ep) Ni–P deposits are studied using XPS, SEM–EDX, AFM, nanoindentation measurements, cyclic voltammetry and capacitance measurements. Ni–P layers were compared with ep Ni films and bulk Ni. Ni–P layers prepared by both techniques contain 12–14 wt% phosphorus, present in oxidation states of P 0 and P 3−. El and ep Ni–P deposits are amorphous and are characterised by a relatively low average surface roughness (2 and 4 nm, respectively). The ep layers possess a rhythmic-lamellar microstructure indicating a periodic change of electrodeposition conditions. The el Ni–P layers do not show such laminated structure but exhibit small surface pores, which are absent in the ep layers. Comparable values for the hardness and the reduced elasticity modulus of el and ep coatings are determined from the nanoindentation data. The observed small differences indicate that the mechanical properties of Ni–P deposits depend not only on the phosphorus content but also on the deposit microstructure. Microelectrochemical measurements with the so-called droplet cell show that the electrochemical behaviour of both el and ep Ni–P coatings is practically identical and does not depend on the location on the sample surface. Evolution of O 2 and H 2 on Ni–P are similar to pure Ni (ep and bulk), but the corrosion resistance in acid solution is much better. The very similar properties and electrochemical behaviour of el and ep Ni–P deposits suggest that both materials are suitable for various applications in microsystem technology. For different substrates and microstructures of different size and geometry, deposition conditions have still to be optimised.

Microelectrochemical Measurements of the Dissolution of Single MnS Inclusions, and the Prediction of the Critical Conditions for Pit Initiation on Stainless Steel

A combination of mathematical modeling and experiments on single MnS inclusions was used to investigate the role of MnS inclusions on the initiation of pitting corrosion of 304 stainless steel. Isolation of single MnS inclusions in chloride-containing solutions with use of microcapillaries as electrochemical cells showed that the orientation of the MnS inclusion played a significant role in pit initiation. Large, shallow MnS inclusions failed to initiate pitting corrosion, while the same inclusions, oriented narrow and deep, consistently exhibited the onset of localized corrosion. The formation of a microcrevice was observed between the dissolving MnS inclusion and stainless steel. The microcrevice resulted in a locally occluded region where aggressive ions can accumulate. A mathematical model was developed to explore the local chemistry within a one-dimensional microcrevice of which one wall was MnS and the other was stainless steel. The results of the simulations supported the view of a critical solution chemistry of sulfur species, in a chloride environment, as a possible trigger mechanism for localized corrosion. A critical microcrevice geometry of approximately 1 μm was predicted to be sufficiently large to generate stable pitting in the system under study. © 2001 The Electrochemical Society. All rights reserved.

Orientation in complex chemical landscapes: Spatial arrangement of chemical sources influences crayfish food‐finding efficiency in artificial stream

Fluid dynamics has been shown to alter ecologically important behaviors of aquatic organisms orienting to distant chemical sources. Because the fluid dynamics and chemical plumes change across hydraulic environments, it is unclear which of these factors influence orientation behavior more. This study examined how alterations in chemical signal structure, through changes in source spatial arrangement, affect chemically mediated search behavior. Microelectrochemical measurements of tracer molecules revealed that source arrangement significantly alters the down‐stream fine‐scale structure of chemical plumes. Flume hydrodynamic characterizations (as measured with laser Doppler velocimetry) also differed among source arrangements; however, differences were minor and existed only at select upstream regions of the flume. Crayfish (Orconectes virilis) found the source faster and spent less time in refuges when sources were separated, compared with sources together. Similar numbers of crayfish found the source regardless of source arrangement. Crayfish searched more efficiently with increased spatial complexity at the source. These results supported the hypothesis that spatial and temporal dynamics of chemicals within plumes contain important information that organisms use during olfactory‐mediated orientation in streams.

Microelectrochemical Measurements at Expanding Droplets: Effect of Surfactant Adsorption on Electron Transfer Kinetics at Liquid/Liquid Interfaces

Adsorption of the nonionic surfactant Triton X-100 at the interface between two immiscible electrolyte solutions (ITIES) and its effect on the electron transfer (ET) reaction between tetracyanoquinodimethane in 1,2-dichloethane and aqueous Fe(CN)64- was studied using microelectrochemical measurements at expanding droplets (MEMED). A numerical model was developed for this process by assuming that adsorption of the surfactant at the ITIES was Langmuirian and that the ET reaction only occurred at the uncovered portion of the ITIES. Theoretical results show that, for typical MEMED conditions, the surfactant adsorption process attains the diffusion-controlled limit if the rate constant is greater than 1 cm s-1. The inhibitory effect of surfactant adsorption on the ET process produces changes in the reactant concentration profile adjacent to the droplet, which depend on the bulk surfactant concentration, equilibrium constant (K), the maximum surface coverage (Γmax), and the ET kinetics. Methods for determining these parameters are suggested. The effect of Triton X-100 on the ET reaction was measured over a wide range of conditions, with bulk aqueous Triton X-100 concentrations in the range 2.5 × 10-5 to 2.5 × 10-4 M, over various time scales. Experimental results were found to be in excellent agreement with theoretical predictions and yielded an ET rate constant of 0.0020 ± 0.0001 cm s-1 for the clean interface, with the potential across the ITIES established with 0.1 M ClO4- in each phase. The diminution in the ET rate with surfactant present was consistent with diffusion-controlled surfactant adsorption, characterized by K = 2.7 × 104 M-1 and a value of Γmax = 3 × 10-10 mol cm-2.

Microelectrochemical studies of charge transfer at the interface between two immiscible electrolyte solutions: electron transfer from decamethyl ferrocene to aqueous oxidants

Experimental studies of electron transfer (ET) reactions at the interface between two immiscible electrolyte solutions (ITIES) were carried out with either Fe(CN)63−, Ru(CN)63− or IrCl62− as oxidants in water and decamethyl ferrocene (DMFc) in 1,2-dichloroethane (DCE), using both scanning electrochemical microscopy (SECM) and microelectrochemical measurements at expanding droplets (MEMED). Either tetrabutylammonium cation (TBA+) or ClO4− were employed in each phase to control the interfacial potential drop. SECM double potential step chronoamperometry was used to show that DMFc+, generated in the ET process, does not cross the interface in the potential range of interest. The ET rate constants were found to depend strongly on the interfacial potential drop, with an apparent measured ET coefficient of 0.38 when TBA+ was used and the aqueous ionic strength was ca. 0.1 mol dm−3. However, the potential dependence of the ET rate was complicated when ClO4− was used to change the interfacial potential drop. Although the rate constant increased when the driving force was increased by changing the aqueous oxidant, the rate constant decreased for a particular oxidant when the potential of the organic phase was made more negative relative to the aqueous phase, by increasing the concentration of ClO4− in the aqueous phase. A similar effect was observed with Fe(CN)63− as the aqueous oxidant and DMFc as the electron donor in a nitrobenzene phase. In contrast, the rate constant for ET was found to be apparently insensitive to the ClO4− concentration in the aqueous phase (and hence potential drop) when the aqueous electrolyte concentration was increased to the salting out levels employed in earlier studies with externally-polarised ITIES. Possible reasons for the behaviour observed and the implications for further studies are discussed.

Microelectrochemical measurements at expanding droplets (MEMED): investigation of cupric ion stripping kinetics in a two-phase oil/water system

Microelectrochemical measurements at expanding droplets (MEMED) are used as a new approach for investigating the stripping kinetics of metals ions between an organic phase containing an extractant ligand, which complexes the metal ion of interest, and an aqueous acid solution. The approach is illustrated with measurements of the stripping of cupric ion from 1,2-dichloroethane (DCE) droplets containing CuL2 (where L is the oxime ligand, Acorga P50) that are formed periodically through a capillary tip submerged in an aqueous receptor phase containing HCl. First-order dependences of the stripping rate constants on the bulk concentration of CuL2 in the DCE phase (5–20 mmol dm−3) and HCl in the aqueous phase (0.25–2.0 mol dm−3) were found, with an effective bimolecular rate constant of (6.2±1.0)×10−5 cm s−1 M−1. The results obtained demonstrate that the cupric ion stripping process is likely to be heterogeneous, occurring at the immiscible liquid/liquid interface, with the first attack of protons on CuL2 as the rate-limiting step. The prospects for using MEMED more generally to investigate metal extraction/stripping kinetics are discussed.

Potential Dependence of Electron-Transfer Rates at the Interface between Two Immiscible Electrolyte Solutions: Reduction of 7,7,8,8-Tetracyanoquinodimethane in 1,2-Dichloroethane by Aqueous Ferrocyanide Studied with Microelectrochemical Techniques

Experimental studies of electron-transfer (ET) reactions at the interface between two immiscible electrolyte solutions (ITIES) were carried out with Fe(CN)64- as a reductant in water and 7,7,8,8-tetracyanoquinodimethane as an electron acceptor in 1,2-dichloroethane (DCE). The kinetics of this process have been determined using both scanning electrochemical microscopy and microelectrochemical measurements at expanding droplets. ClO4- was employed in each phase to control the interfacial potential drop. The ET rate constants were found to depend on the interfacial potential drop, with an apparent ET coefficient in the range 0.30−0.41 with a moderate aqueous ionic strength 0.1−0.3 M and an electrolyte concentration in the DCE phase of 0.1 M. When the aqueous ionic strength was increased significantly to the levels employed in studies of externally polarized ITIES, the ET rate constant decreased appreciably and was less dependent on the interfacial potential drop. These latter studies yielded results comparab...

Microelectrochemical Measurements at Expanding Droplets (MEMED): Mass-Transport Characterization and Assessment of Amperometric and Potentiometric Electrodes as Concentration Boundary Layer Probes of Liquid/Liquid Interfaces

Microelectrochemical measurements at expanding droplets (MEMED) is a new technique for studying the kinetics of reactions that occur spontaneously at the interface between two immiscible liquids. T...

Microelectrochemical Investigations of Stainless Steels at Elevated Temperatures

Recently developed microelectrochemical methods are powerful techniques to study localized processes in the micrometer range. The investigations confirm the key role of inclusions, predominantly manganese sulfides, in being initiation sites for localized corrosion. However, localized corrosion is also affected by the temperature. Many investigations use the temperature as a pitting criterion, i.e. in critical pitting temperature tests. This work presents the investigation of two commercial DIN 1.4301 stainless steels with different sulfur levels in Na 2 SO 4 and NaCl solutions at various temperatures. In addition to conventional electrochemical techniques and critical pitting temperature tests, microelectrochemical measurements are presented which allow to study localized corrosion in the micrometer scale at temperatures up to 90°C. Results from locally resolved measurements show that higher current densities in large scale experiments originate from many single activation and repassivation processes. The effect of the temperature on these processes is compared with the results from large scale experiments in the temperature range of 0°C to 90°C.

THE MARLOW MEDAL LECTURE Dynamic electrochemistry as a quantitative probe of interfacial physicochemical processes

The application of electrochemical techniques to investigate interfacial processes outside the traditional boundaries of electrochemistry is discussed. By drawing on the well-established merits and principles of electrochemical methods, it is shown how a wide range of processes at electrically insulating interfaces can be investigated quantitatively under conditions where: (i) mass transport and surface kinetic effects can be separated; (ii) interfacial reactive fluxes can be measured and interpreted in terms of interfacial speciation; (iii) the influence of surface structure can be explored. The relative attributes of the channel flow method with electrochemical detection, the channel stopped flow method, microelectrochemical measurements at expanding droplets, scanning electrochemical microscopy and integrated electrochemical-atomic force microscopy are assessed as probes of physicochemical phenomena at liquid/solid, liquid/liquid and liquid/gas interfaces.