Electrochemical Dissolution Of Metals Research Articles

Nanoelectrochemistry of Metal Dissolution and Deposition Reactions

Electrochemical metal dissolution and depositions reactions are reversible processes involving phase transition, which are fundamentally important in battery and corrosion. The electrode kinetics of these reactions are highly sensitive to the surface structures. In this talk, I will present the study on the initiation of metal dissolution reactions on Ag and Ni, representing model systems for oxide-free and oxide-covered metals, respectively. The local dissolution kinetics is voltammetrically mapped via scanning electrochemical cell microscopy (SECCM). Co-localized characterization of crystal orientation reveals slower dissolution on {111} close-packed planes. Direct measurement of local dissolution kinetics on grain boundaries shows a faster dissolution rate on some but not all grain boundaries. We demonstrate that correlative electrochemical and structural imaging are powerful tools for studying heterogeneity at complex electrochemical interfaces, which can also be used to study metal deposition reactions at the nanoscale.

A Review on Recent Achievements and Challenges in Electrochemical Machining of Tungsten Carbide

Electrochemical machining (ECM) is a modern, unconventional manufacturing technique that uses the electrochemical dissolution of metal to produce pieces with a predetermined shape, dimensions, and surface polish. Tungsten carbide (WC) is used in many fields and applications due to its high wear resistance and hardness. WC is used in cutting tools, mining and drilling equipment, wear parts, industrial machinery, aerospace and defence components, and medical equipment. It gives a workpiece a particular shape and number of boreholes, removes a damaged outer coat, and eliminates surface roughness. Though the ECM of WC is fraught with difficulties and obstacles, altering the electrolytes and refining the technological equipment and process of the ECM of WC can help improve machining accuracy and stability. In the current study, we examined the ECM of WC and compiled a reliable database from reliable sources using the keywords "electrochemical machining" and "tungsten carbide". Research papers constitute the majority of these publications. It was discussed how WC behaves anodically as it applies to ECM, which experiments were conducted with various electrolytes, and how they performed. Several ECM modes have been considered, and each mode has been evaluated in detail for challenges and gaps.

Interfacial Structure and Energy Determine the Heterogeneity in the Electrochemical Metal Dissolution Activity at Grain Boundary

Interfacial Structure and Energy Determine the Heterogeneity in the Electrochemical Metal Dissolution Activity at Grain Boundary

Isotropic Tuning of Electrochemical Etching for the Nanometric Finishing of Metals

Isotropic etching polishing (IEP) based on the merging of isotropic etch pits has been proposed as a generic metal finishing approach. In this work, the tuning of the etching isotropy of various metals, which is the key to realizing the finishing effect of IEP, is studied by theoretical analysis and etching experiments. The isotropic etching of various metals can be realized through mass transfer polarization by adjusting the electrochemical parameters. The addition of sulfuric acid in the electrolyte is the most effective for tuning the isotropy of electrochemical etching. It can decrease the diffusion coefficient of metal ions, thereby increasing the resistance of mass transfer and transforming the electrochemical dissolution of metal into mass transfer polarization. In this study, the atomic and close-to-atomic scale surface finishing of various metals and alloys has been successfully achieved through isotropic etching. After etching at a current of 1.5 A for 3 min, the surface Sa roughness of TA2 is drastically reduced from 242 to 3.98 nm. After etching for 1 min at a current of 3 A, the surface Sa roughness of pure tungsten, NiTi, and CoCrNi decreases from 9.33, 76.4, and 37.6 nm, respectively, to 1.16, 2.01, and 2.51 nm, respectively.

Nanoscale Colocalized Electrochemical and Structural Mapping of Metal Dissolution Reaction.

Understanding the structure-activity relationship in electrochemical metal dissolution reactions is fundamentally important, from designing higher density batteries to mitigating corrosions. The kinetics of metal dissolution reaction is highly dependent on surface structures, including grain boundaries and local defects. However, directly probing the electrochemical activity at these sites is difficult because the conventional bulk electrochemistry measures an averaged kinetics, obscuring the structure-activity correlation. Herein, we report the colocalized mapping of an electrochemical metal dissolution reaction using Ag as a model system. The local dissolution kinetics is voltammetrically mapped via scanning electrochemical cell microscopy (SECCM), which is correlated with local structures obtained via colocalized electron backscattering diffraction (EBSD). Individual pits of ∼200 nm are formed, and their geometries suggest dissolution is fastest in the direction parallel to the {111} planes. Enhanced dissolution kinetics is observed at the high-angle grain boundaries but not at twin boundaries, which are attributed to the different binding energy of Ag atoms. Furthermore, the faster local dissolution correlates with the geometrically necessary dislocation density. The work demonstrates the importance of nanoscale local electrochemical mapping and colocalized microscopic measurement in obtaining the structure-activity relationship for electrochemical reactions at complex interfaces.

Theoretical analysis of mass transfer during anodic dissolution of tungsten rotating disk electrode in alkaline solutions

The mass transfer during the electrochemical dissolution of metals with the participation of electrolyte anions and the formation of stable anionic complex is analyzed by the example of tungsten anodic dissolution in the alkaline solutions. The distributions of ionic concentrations, partial current densities of ions and potential over the solution layer near the anode are determined as a result of numerical solution of the problem. The results of numerical solution are compared to the results of analytical solution and experimental data available from the literature. It is shown that the numerical solution of the problem with regard for migration, convection and concentration dependences of transport properties of solution agrees well with the experimental data. A complex role of ion migration in this process is revealed.

An Insight in Electrocoagulation Process through Current Density Distribution (CDD)

Electrocoagulation (EC) is a flourishing technique in the field of water treatment implementations. Numerous investigations have been performed to assess the performance of EC to eliminate different pollutants; however, the more basic electrochemical features of the technique are usually ignored. Scientists such as McBeath et al. [1] provided an understanding of the essential link of water flow, electrochemical metal dissolution, and current density distribution (CDD) via computational fluid dynamic (CFD) models, mathematical models, and in situ CDD determination tests. They established, in theory, that current distributed over the electrode was inversely proportional to the water flow rate. By means of the CFD models and current distribution determining technique, they noted that current density was distributed unequally and pursued the tendency anticipated via theory. Eliminating natural organic matter was decreased as much as 79% when the inter-electrode distance was diminished from 10 to 1 mm. As a perspective, more efforts are required to better understand the CDD at the anodes surface and electric charges transfer from electrodes to the bulk of the solution. Mechanisms related to interactions of anodic metallic cations and various pollutants should be more investigated.

In-situ determination of current density distribution and fluid modeling of an electrocoagulation process and its effects on natural organic matter removal for drinking water treatment

Electrocoagulation is a burgeoning technology now being considered for niche water treatment applications. Although much research has been conducted to determine the efficacy of electrocoagulation to remove various contaminants, the more fundamental electrochemical aspects of the technology are often overlooked. This research provides insight into the fundamental relationship of water flow, electrochemical metal dissolution and current density distribution through computational fluid dynamic (CFD) models, mathematical models and in-situ current density distribution identification experiments. Theoretically, it was determined that current distributed along the electrode was inversely proportional to the water flowrate. The turbulent flow through the EC reactor was simulated with varying inter-electrode gaps and flowrates, while the average velocity segments across the electrode surface was calculated, corresponding to the same segments used to experimentally determine the current distribution. Through the CFD models and current distribution determining technique, it was observed that current density was distributed unevenly and followed the trend predicted by theory. Areas of lower current density were generally accompanied by higher velocity flow. More uniform current was yielded with larger inter-electrode gaps, due to the greater flow uniformity. While operating with a 1 mm gap, the current and water velocity varied across the electrode by Δ27.6 mA/cm2 and Δ0.220 m/s, and was minimized to Δ3.6 mA/cm2 and Δ0.062 m/s at a 10 mm gap. Although current uniformity was increased, the overall current density decreased significantly due to the greater ohmic resistance associated with the larger gap. The removal of natural organic matter was reduced as much as 79% when the inter-electrode gap was reduced from 10 to 1 mm.

Kinetics of electrochemical dissolution of metals in porous media.

Metals embedded in porous media interact electrochemically with the liquid phase contained in the pores. A widespread form of this, adversely affecting the integrity of engineered structures, is corrosion of steel in porous media or in natural environments. While it is well documented that the rate of this electrochemical dissolution process can vary over several orders of magnitude, understanding the underlying mechanisms remains a critical challenge hampering the development of reliable predictive models. Here we study the electrochemical dissolution kinetics of steel in meso-to-macro-porous media, using cement-based materials, wood and artificial soil as model systems. Our results reveal the dual role of the pore structure (that is, the influence on the electrochemical behaviour through transport limitations and an area effect, which is ultimately due to microscopic inhomogeneity of the metal/porous material interface). We rationalize the observations with the theory of capillary condensation and propose a material-independent model to predict the corrosion rate.

Exploring chemical speciation at electrified interfaces using detailed continuum models.

We present a local free-energy functional-based generic continuum model for material interfaces with a specific emphasis on electrified solid/liquid interfaces. The model enables a description of multicomponent phases at interfaces and includes the effects of specific non-electrostatic interactions (specific adsorption), ion size disparity, and the explicit presence of neutral species. In addition to the optimization of electrostatic, non-electrostatic, and steric forces, the model can be easily modified to explore the effects of other channels for equilibration, including local chemical transformations driven by equilibrium constants and electrochemical reactions driven by the electrode potential. In this way, we show that, upon accounting for these effects, local speciation in the vicinity of the interface can be drastically different from what is expected from restricted models and minor species (from the bulk perspective) may become dominant due to the effects of local pH. We evaluate the ionic contribution to the surface tension at the interface and show how this could impact the structure of air/liquid interfaces. On the same footing, an attempt to describe electrochemical metal dissolution is made. The model allows estimates of the mutual population of newly produced ions depending on their charge and size and could be useful for interpretation of electrochemical and spectroscopic measurements if the dissolution involves different metal ions (species). With these advances, the proposed model may be used as an ingredient within a hybrid ab initio-continuum methodology to model biased interfaces.

Alkaline Plating and Striping of Metal-Amine Complexes for Amine Regeneration in Gas Scrubbing Processes

Traditional thermal amine gas scrubbing technology can be energy and capital intensive owing to the enthalpy of formation of amine-CO2 complexes and the complexity in the process integration of a stripper column. An electrochemically mediated amine regeneration cycle in traditional gas scrubbing process has been proposed for post combustion CO2 capture.1 2 We take the advantage of competitive binding of amine with electroactive species, e.g. copper, to facilitate the desorption of CO2 and regeneration of amine in a modular electrochemical reactor.3 We investigated the energy efficiency of this electrochemical process via bulk electrolysis of the electrolyte. The charges applied in electrochemical metal dissolution are compared quantitatively to gas desorption in the electrolyte. Our controlled volume analysis validates the efficacy of the proposed cycle. We benchmarked the polarization of metal electrodes with different metal-amine complexes using a rotating disk electrode. The Tafel expression of the kinetic data assists in gauging the rate of metal dissolution/plating and provides insight into effects of various anions. We examined the quality of metal deposits at different operating conditions using XPS. The surface composition of the electrodes account for long-term stability of the electrodes. These thermo and kinetic data afford qualitative comparison of cell-level performance in the amine regeneration cycle.

Nanoscale control of plasmon-active metal nanodimer structures via electrochemical metal dissolution reaction

Herein, we report the control of the optical properties of metal nanodimer structures using electrochemical metal dissolution reactions. The reaction rate could be precisely tuned by changing the electrochemical potential and, as a consequence, fine tuning of the size and gap distance of metal nanodimers was achieved as the functions of applied potential and polarization time. The observed linear correlation between the scattering intensity and charge resulting from nanostructure dissolutions suggested that the surface dissolution rate was 0.30 nm min−1, corresponding to the surface dissolution of a single atomic layer per min. The present method can control the change in the volume of the structures, leading to the change in the gap distance of nanodimers at an atomic-scale level.

Study on Precision Microholes Using Pulsed Laser with Jet Electrochemical Machining

The precision machining basically in the micron domain is a great challenge for advanced manufacturing industries. The assistance of laser beam with jet electrochemical machining process will localize the machining zone and helps to electrochemical dissolution of metal from the specific area which improves the machining quality and precision. Keeping in view, a hybrid laser-assisted jet electrochemical machine (LA-JECM) has been indigenously developed and utilized for microhole drilling experiments. The assistance of laser beam increases localization effect by increasing and maintaining the temperature in a predominant mode. A low-power Nd:YAG diode-pumped solid-state laser beam of wavelength 532 nm is used concentrically with electrolytic jet and carefully monitored its application; otherwise, electrolyte will start to boil due to supply of high laser beam energy and hamper the precision. The percentage of reduction in taper, overcut and average spark-affected zone are 32.3; 27.8; and 50.2%, respectively, when machining operations were carried out on LA-JECM over jet electrochemical machine. Taguchi method-based design of experiments is employed for experimental investigation, and acquired results are utilized for optimization of LA-JECM parameters for further improvement and precision.

Application of Ammoniacal Solutions for Leaching and Electrochemical Dissolution of Metals from Alloys Produced from Low-Grade E-Scrap

AbstractPaper reports results of the research on the application of ammoniacal solutions (sulfate, chloride, carbonate) for hydrometallurgical treatment of smelted low-grade e-waste. Two Cu-Zn-Sn-Ag-Pb alloys were analyzed in details and discussed. Leaching of copper from the alloys was slow in sulfate solution, but the dissolution rate decreased in chloride and carbonate baths. Anodic alloys dissolution was uniform in sulfate and carbonate electrolytes, but high degradation of the material was observed in chloride bath. Chemical and phase composition of slimes and cathodic deposits produced during electrolysis were characterized. In all cases, separation of copper from other metals was found, but efficiencies of the processes were low.

Electrochemical synthesis and optical properties of organically capped silver nanoparticles

A top to bottom approach has been adopted to prepare silver nanoparticles by electrochemical dissolution of metal in suitable organic solvents. The method is being simple and economical, also permits in situ capping of nanoparticles with organic molecules. Thioglycolic acid is used as capping/stabilizing agent. Optical absorption, transmission electron microscopy, fourier transform infrared spectroscopy and X-ray diffraction measurements were carried out to study the effect of capping molecules on the size and shape of nanoparticles. It is found that thioglycolic acid is an effective capping agent and hence the resultant sol, even with high density of nanoparticles, is kinetically more stable. The present method can also be extended to synthesize other metal nanoparticles capped with various organic molecules.

An analysis of drifts and nonlinearities in electrochemical impedance spectra

The response of an electrochemical reacting system to potential perturbations during electrochemical impedance spectrum measurement is investigated using numerical simulation. Electrochemical metal dissolution via an adsorbed intermediate species is analyzed and it is shown that applying the potential perturbation causes the average surface coverage to drift. For high frequency perturbations, the final value of the average surface coverage depends mainly on the kinetic parameters and the amplitude of the applied perturbation. Acquiring the data during the first few cycles of perturbations leads to an incorrect calculation of the impedance, particularly for large amplitude perturbations. Repeating the experiments will not identify this drift, while Kramers–Kronig Transform (KKT) can successfully detect this problem. The correct experimental methodology to overcome this effect and obtain the impedance spectra is also described. Another reaction with two adsorbed intermediates is also investigated and it is shown that in certain cases, the violations of linearity criteria can also be detected by KKT. The results illustrate the importance of validating the impedance data with KKT before further analysis.

Oxygen Electroreduction on Core-Shell Nanocatalysts Prepared by Selective Electrochemical Metal Dissolution of PtCu3 and PtCo3 Alloy

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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.

Anodic oxidation of Pd alloys with Pt and Rh

Anodic oxidation of Pd alloys with Pt and Rh has been studied in acidic solution (0.5 M H 2SO 4) using cyclic voltammetry coupled with the electrochemical quartz crystal microbalance (EQCM). The influence of electrode potential and alloy composition on the processes of surface oxide formation/reduction and electrochemical dissolution of metals was investigated. For Pd–Rh alloys containing up to ca. 85–90% Pd on the surface the oxidation of electrode material starts at potentials 200–300 mV lower than on pure Pd or Pd–Pt alloys and 50–100 mV higher than on pure Rh. The formation of a monolayer of surface oxide is completed at ca. 1.36 V for Rh-rich Pd–Rh alloys, and ca. 1.46 V for Pd–Pt and Pd-rich Pd–Rh alloys. The EQCM results indicate that surface oxide formed on Pd alloys has MO stoichiometry. Both Pd and Rh dissolve during alloy polarization to the potential region of surface oxidation. The amount of dissolved metal is inversely proportional to scan rate and increases with the increase in electrode potential and Pd surface content in alloys. Electrochemical dissolution of metals from the alloys proceeds differently than from pure metal electrodes.

Electrochemical behavior of Pd–Rh alloys

Pd–Rh alloys were prepared by electrochemical codeposition. Bulk compositions of the alloys were determined by the energy dispersive X-ray analysis method, while surface compositions were determined from the potential of the surface oxide reduction peak. Cyclic voltammograms, recorded in 0.5 M H2SO4 for Pd–Rh alloys of different bulk and surface compositions, are intermediate between curves characteristic of Pd and Rh. The influence of potential cycling on electrochemical properties and surface morphologies of the alloys was studied. Due to electrochemical dissolution of metals, both alloy surface and bulk become enriched with Pd. Carbon oxides were adsorbed at a constant potential from the range of hydrogen adsorption. The presence of adsorbed CO2 causes remarkable diminution of hydrogen adsorption but it does not significantly influence hydrogen insertion into the alloy bulk. On the other hand, in the presence of adsorbed CO, both hydrogen absorption and adsorption are strongly suppressed. Oxidative removal of the adsorbates results in a characteristic voltammetric peak, whose potential increases with the decrease in Rh surface content. Electron per site (eps) values calculated for the oxidation of the adsorbates change with alloy surface composition, more for CO2 than CO adsorption, indicating the variation of the structure and composition of CO2 and CO adsorption products. The course of the dependence of eps values on surface composition suggests that the products of CO2 and CO adsorption on Pd–Rh alloys are similar but not totally identical.