Search

We studied a potential petroleum source rock deposited in a clastic-dominated ramp environment: the Argiles de Châtillon Formation (Kimmeridgian-Tithonian, Boulonnais area, northern France). The formation was deposited along a proximal-distal gradient on this ramp affected by synsedimentary fault movements. A sedimentological, mineralogical and geochemical study was conducted to decipher the distribution patterns of sedimentary parameters along such a depth increase over the ramp. It comes out that smectite distribution unexpectedly mimics the lateral depth evolution despite the good floatability of the mineral. It is also observed that the Argiles de Châtillon could accumulate noticeable amounts of organic matter in spite of paleoenvironmental conditions that were not specifically prone to organic matter preservation and burial (sedimentation rate, mineral particle grain size, productivity, oxygenation level…). Conversely, reactive iron, when being present in limited abundance, must have allowed sulfide ions to react with organic molecules instead of forming iron sulfides, which fostered organic matter preservation and accumulation. This protecting role of organic sulfide incorporation (additional to other favorable factors) cannot exist if reactive iron is relatively abundant. Lastly, our results (still fragmentary) suggest that smectite minerals could carry reactive iron, which would have occasionally hampered organic-matter preservation.

Abstract: The temperature distribution and shape evolution during electrochemical machining (ECM) are the result of a large number of intertwined physical processes. Electrolyte flow, electrical conduction, ion transport, electrochemical reactions, heat generation and heat transfer strongly influence one another, making modeling and numerical simulation of ECM a very challenging procedure. In part I , a temperature dependent multi-ion transport and reaction model (MITReM) is put forward which considers mass transfer as a consequence of diffusion, convection and migration, combined with the electroneutrality condition and linearized temperature dependent polarization relations at the electrode–electrolyte interface. The flow field is calculated using the incompressible laminar Navier–Stokes equations for viscous flow. The local temperature is obtained by solving internal energy balance, enabling the use of temperature dependent expressions for several physical properties such as the ion diffusion coefficients and electrolyte viscosity. In this second part, the temperature dependent MITReM is used to simulate ECM of stainless steel in aqueous NaNO3 electrolyte solution. The effects of temperature, electrode thermal conduction, reaction heat generation, electrolyte flow and water depletion are investigated. A comparison is made between the temperature dependent potential model and MITReM. [Copyright &y& Elsevier]

Abstract: A new temperature dependent multi-domain model is presented for numerical simulation of the electrochemical machining process with a moving cathode tool. The method includes mass transfer as a consequence of diffusion, convection and migration, combined with the electroneutrality condition and linearized temperature dependent polarization relations at the electrolyte–electrode interface. Concentration and temperature dependent expressions are used for the diffusion coefficients and electrolyte viscosity. The electrolyte flow field is calculated using the laminar Navier–Stokes equations for viscous incompressible flow. Heat is generated in the bulk solution and in the electrical double layer. The electrodes are cooled by natural convection. The level set method is used for tracking the anode interface. The model is applied to the electrochemical machining of steel in a NaNO3 supporting electrolyte. Hydrogen is formed at the cathode, and metal dissolution and oxygen evolution reactions are considered at the anode. The effect of water depletion at the electrodes is modeled by limiting the oxygen and hydrogen evolution reaction rates depending on the local surface water concentration. The heat conduction through electrodes and the heat production by the electrode reactions are found to play an important role. [Copyright &y& Elsevier]

Laminar and turbulent mass transfer in a parallel plate reactor at high Schmidt number obtained from numerical simulation is compared with literature data. In a first step, the fluid flow is determined numerically in the reactor by solving the Navier–Stokes equations. For turbulent flow, a low Reynolds number k—ω model is used to calculate the turbulent viscosity. Using the obtained flow field and turbulent viscosity, the current density distribution is calculated for different flow velocities by solving the equations describing the transport of multiple ions due to diffusion, convection and migration. For the laminar case, a very good agreement with literature data is obtained. For turbulent flow, different numerical models for turbulent mass transfer are proposed in the literature. A detailed study of the behaviour close to the wall of these different turbulence models is presented, together with a comparison of the calculated results with literature correlations. This allows identification of the benefits and disadvantages of each of the turbulence models for the numerical calculation of mass transfer at high Schmidt numbers in a parallel plate reactor. [ABSTRACT FROM AUTHOR]

In this paper, we consider the problem of detecting clustered microcalcification in digitized mammograms using new wavelets with a high Sobolev regularity index. We experimentally assess the superiority of the new wavelets when compared with the classical ones. [ABSTRACT FROM AUTHOR]

Abstract The diffusion and trapping of hydrogen in iron alloys is commonly studied using the Devanathan-Stachurski cell. In this cell, the hydrogen flux through a metal membrane is studied. At the exit side of the membrane, an oxide layer is commonly developed and stabilized as a consequence of the anodic polarization imposed. The process of hydrogen diffusion through the oxide layer and its consecutive oxidation is not fully understood yet. In this paper, we propose a modelling approach that provides additional insights in this process. In particular, we highlight the impact that the presence of the oxide layer can have on the overall hydrogen transport throughout the metal membrane. Moreover, we highlight the complexity of this mechanism and discuss the possible reactions taking place in and at this interface. As a conclusion a possible alternative process to explain the findings is proposed. [ABSTRACT FROM AUTHOR]

Corrosion of steels in lead bismuth eutectic (LBE) cooled reactors can be mitigated by forming a protective oxide layer on the steel surfaces. The amount of oxygen necessary to ensure continuous oxide layer formation on fuel cladding depends on the characteristics of the steel and on the local temperature, local oxygen concentration and velocity of the LBE in contact with the steel. The most critical areas from a corrosion point of view are high temperature and low oxygen concentration regions. Wire-wrapped fuel assemblies (FAs) which are foreseen to be used in LBE cooled reactors, are characterized by hot spots and quasi-stagnant areas where oxygen could be depleted. Experimental measurements to verify whether the oxygen concentration in those critical areas is sufficiently elevated for oxide layer formation, are practically impossible. This information can be however obtained by numerical modeling. This paper focuses on the development of a numerical model of oxygen mass transfer in a 19-pin scaled fuel assembly (FA) representative of the MYRRHA reactor core. Oxidation of steels and oxygen transport from the bulk of the LBE to the surface of steels were simulated simultaneously. The simulations provide a local oxygen concentration mapping at steel/LBE interface enabling to identify the regions of the core which could be prone to corrosion due to oxygen depleted LBE. Operation recommendations for the MYRRHA reactor were given based on the simulation results. [ABSTRACT FROM AUTHOR]

The durability and service life of concrete structures strongly depends on the transport of fluids through the cementitious material. These fluids can contain aggressive agents which causes degradation of the concrete matrix and corrosion of the steel reinforcement. In most cases the dominating mechanism for water ingress in cementitious materials is capillary absorption, especially when cracks are present. To be able to predict the durability of a concrete structure containing cracks, it is necessary to know the distribution of water and other aggressive agents inside the cementitious material. Capillary water entrance in uncracked and cracked mortar was simulated in this research by solving the 3D Richard’s equation using Finite Element Method. Realistic boundary conditions, by considering the water evaporation process, were imposed in the model. The developed numerical model was validated by performing gravimetrical water absorption experiments and X-ray radiography on mortar specimens with one or multiple artificial cracks. This paper reports the validation of the numerical model through an experimental program. The numerical results agreed well with the experimental results in both the transient global water content and the transient water distribution for both uncracked and cracked mortar. Therefore, the imposed boundary conditions correctly describe the water uptake phenomenon. To be able to completely describe the durability of cementitious materials, the numerical model still has to be extended regarding the ingress of other substances like CO 2 and chlorides. [ABSTRACT FROM AUTHOR]

Campanian sediments from two French sedimentary basins were studied, using clay mineralogy and stable isotope (δ 13 C and δ 18 O) geochemistry, in order to investigate the Late Campanian Event. The clay fraction of the Campanian sediments from the Tercis-les-Bains section (Aquitaine Basin) and from the Poigny borehole (Paris Basin) is mainly composed of smectite. This background sedimentation was, however, interrupted during the Upper Campanian in the two basins by a substantial increase in detrital inputs, including illite, kaolinite, and chlorite at Tercis-les-Bains, and illite at Poigny. This detrital event, resulting from the enhanced erosion of nearby continental areas triggered by increasing runoff, has also been recognized in the Tethys and South Atlantic oceans. It coincided with a global negative carbon isotope excursion, the Late Campanian Event (LCE). Carbon isotope stratigraphy was used to correlate the two basins with previously studied sections from distant areas. Spectral analysis of the bulk δ 13 C from Tercis-les-Bains suggests a duration of ca. 400 kyr for a pre-LCE negative excursion and ca. 800–900 kyr for the LCE sensu stricto . The detrital event, as characterized by clay mineralogy, spans the interval that comprises the pre-LCE and the LCE, with a duration of 1.3 Myr. Intensification of continental erosion during the LCE may have resulted either from the Late Campanian polyplocum regression and/or from a regional tectonic pulse that triggered the emersion of previous submerged shelf areas and the increase of silicate erosion. As the LCE seems to be recorded at a large geographic scale, it is proposed here that enhanced chemical weathering and an associated decrease in atmospheric p CO 2 levels could have contributed to the long-term Late Cretaceous cooling trend. [ABSTRACT FROM AUTHOR]

Introduction of microcapillary techniques opened a new range of methods to study the behavior of microsystems. Although these measurements are widely employed and results obtained are generally recognized, certain aspects require careful consideration. We present the results of a parametric study performed by means of multi-ion modeling in order to clarify the influence of the microcapillary size and shape together with the permeability of the sealant for oxygen on the polarization curves obtained on metals with different corrosion activity. The obtained results show that when a corroding metal is (almost) non-active ( e.g. aluminum), the oxygen limiting current increases with increase of the ratio between the radius of the capillary main part and the end radius γ = R / r but the corrosion potential remains the same. For more active metals and alloys ( e.g. steel), a change of the aspect ratio γ leads not only to the proportional change of the limiting current, but also to a substantial shift of E corr . Analytical solutions derived for limiting current density in a microcapillary confirmed the simulated results. Permeability of the sealant for oxygen plays a significant role mostly for narrow capillaries with low ratio γ and more active metallic substrates. It is shown that the problem of comparison of polarization data obtained with capillaries of different size can be partially avoided by use of capillaries with the same aspect ratio γ but only if the quality of the sealant (low permeability for oxygen) is assured. [ABSTRACT FROM AUTHOR]

A Dynamic Electrolyte Film Corrosion (DEFC) model is presented that integrates a model for uniform film condensation/evaporation in presence of salts with a simple model of iron corrosion. The focus is on the role of the film thickness. A limited number of measurements confirms that it is unavoidable to consider the hygroscopic effects. The model is used to simulate the role of sample thickness, heat transfer and changing rates of temperature and relative humidity in well defined climate chamber conditions. Obtained film thicknesses and corrosion rates are simulated using identical input data in absence of salt and for two salts (NaCl and a seawater-like mixture of 90 %NaCl-10 %MgCl 2 ). The large indirect influence of salts on the corrosion is discussed. Also one-week simulations are performed based on recorded and averaged meteorological data. They show that averaged meteorologic data will lead to incorrect results. [ABSTRACT FROM AUTHOR]

Measurements of oxygen reduction current are performed on a platinum electrode submerged under NaCl electrolyte films of different thickness. The chloride concentration is kept constant or increases due to evaporation. Measurements are supported by the numerical Multi-Ion Transport and Reaction Model (MITReM). In case of constant salt concentration, oxygen reduction current is proportional to the reciprocal of the electrolyte film thicknesses down to 50–75 μm; for lower thicknesses deviation from the Fick's law takes place. For evaporating films, oxygen current is the result of two counteracting phenomena: reducing film thickness and increasing salt concentration leading to decrease of oxygen solubility. [ABSTRACT FROM AUTHOR]

Lithium-Ion Batteries lack performance and are costly for applications such as electric vehicles and electric energy storage systems. Multiphysics based battery models are one of the engineering tools to enhance their performance. In this regard, simulations have not only to provide qualitative but also quantitative valuable information. Our work focuses on the characterization and modelling of Li x Ni 1/3 Mn 1/3 Co 1/3 O 2 based cells which has not been fully addressed yet. First, we present a modified multiphysics model compared to the conventional porous electrode theory for simulating these type of cells. The correspondence with experimental results is satisfactory but the model fails to accurately predict the voltage drop at high current rates. [ABSTRACT FROM AUTHOR]

The local concentration of dissolved oxygen over a metal-coated steel sample of cut-edge geometry was recorded by using either an amperometric probe or an optode (fibre-optic oxygen micro-sensor). Hydroxide ions, being the product of the current determining reaction on the amperometric micro-probe, were found to change the chemical composition of corrosion products. Application of an O 2 selective micro-optode for local measurements allows avoiding the indicated problem and is reported here for the first time when applied to localized corrosion measurements. The concentration of dissolved oxygen was mapped 100 μm above the cut-edge steel coated with Al–Zn–Mg alloy. This was performed quasi-simultaneously with rastering local current density and pH over the same sample area. [ABSTRACT FROM AUTHOR]

Scanning electrochemical microscopy (SECM) with its high spatial resolution and chemical selectivity is a powerful technique for studying a wide range of corrosion processes. A common procedure to detect qualitatively a corrosion activity on a metal substrate consists of measuring the local oxygen concentration. The aim of this work is to clarify the local impact of the O 2 measurement and the undesired effect on the corrosion process. By means of numerical simulation the variations of the anodic and cathodic currents on the aluminum substrate, as well as concentration profiles, are obtained. The multi-ion transport and reaction model (MITReM) is used considering the homogeneous reactions taking place in the solution, transport of species dominating the corrosion process and the electrochemical reactions. The model case study in this work is corrosion of pure aluminum in chloride solution. We concluded that amperometric O 2 SECM measurements lead to a local increase of the solution pH and decrease of partial current densities for O 2 reduction on the metal. This influence becomes significant when the distance to the substrate and when the size of the active surface is comparable with the size of the SECM probe. [ABSTRACT FROM AUTHOR]

Abstract: A Computational Fluid Dynamic (CFD) model of a lead oxide mass exchanger (PbO MX) was developed. The mass exchanger consisted of a packed bed of PbO spheres. The geometry was created using Discrete Elements Method (DEM) software while the meshing, the solving and the post-processing were done by the commercial CFD package CFX. The dissolution process was modeled by implementing in the code oxygen mass transfer through the boundary layer. The dissolution rate was then predicted for different temperatures. Experiments were also performed at the LBE material test loop known as the DELTA loop. Oxygen concentration at the outlet of the PbO MX was measured for different conditions using a potentiometric oxygen sensor and the dissolution rate was determined for five different temperatures. The experimental data were compared with the numerical model. The temperature dependence of the dissolution rate was then determined in terms of Sherwood number by fitting the simulation results while keeping constant Reynolds number. The results showed that the Sherwood number for PbO MX in flowing LBE varies with Sc0.323. [Copyright &y& Elsevier]

Abstract: A departure from the theoretical current density distribution close to a point current source is observed in both Scanning Reference Electrode Technique (SRET) and Scanning Vibrating Electrode Technique (SVET) measurements. Here we report the use of a general multi-ion transport and reaction model, which accounts for micro-convection to point out the origin of the deviations that are experimentally observed. The validity of the model is proven by comparison against both analytical and measured (SRET and SVET) distributions of the current density in a typical calibration setup. Solutions with different conductivities and different impressed currents are considered. [Copyright &y& Elsevier]

Highlights: [•] A CFD model of oxygen mass transfer in LBE is developed. [•] The model is used to design a venturi type PbO mass exchanger. [•] The effect of re-circulation flow on the oxygen concentration is simulated. [•] Recirculation of 6% the total flow rate can prevent poisoning of the PbO spheres. [Copyright &y& Elsevier]

Abstract: Pulsed electrochemical micromachining is a metal dissolution process where the capacitive behaviour of the double layer enhances the confinement of the machining profile, when very short voltage pulses are applied. During the pulses, the dissolution is confined to electrode regions where the tool–workpiece gap is the smallest. The model used combines the potential distribution in the electrolyte with the load–unload behaviour of the double layer. The model is solved using the Finite Element Method. The pulse and double layer charging are the main focus and therefore no shape change is included in the model at this point. The influence of the double layer, pulse signal parameters and inter-electrode gap size on the dissolution current density (material removal depth), as a function of time, is investigated. An estimation of the double layer loading time is presented, as well as a quantification method for the metal removal confinement, by comparing the calculated error against an ideal removal profile. The influence of pulse characteristics on the dissolution process has also been studied. It was found that a strongly non-linear polarization in combination with nano-second pulses and a small gap size increases the confinement. All the simulation results were obtained on an axisymmetric case by considering several geometrical set-ups. [Copyright &y& Elsevier]

Abstract: Thin passive nickel oxides are investigated in neutral and weakly alkaline pH solutions under steady-state conditions. The chemical species considered in the oxide film are nickel interstitials and vacancies, as well as oxygen vacancies. The set of differential equations used in this study is solved using the finite element method (FEM) and is able to reproduce the experimental data present in the literature. Steady-state oxide thickness variation with the applied electrode potential presents a linear behavior with an average slope of 2nm/V. The role of dominant species in these thin films is investigated in terms of current density produced by the reactions at the interfaces, the reactions involving production and consumption of Ni2+ vacancies playing a major role in the steady-state properties of the oxide. We show that the mass transport of species in the oxide is influenced more by the migration component of the flux than the diffusion component. Our results also show that the flux of Ni2+ vacancies is approximately two orders of magnitude higher than the flux of oxygen vacancies and Ni2+ interstitials, making them the dominant defects in the oxide (thus the p-type electronic character is present). Also, the Ni2+ vacancies were found to have density levels of 1020–1021 cm−3 close to the metal–film interface. Variations of the steady-state thickness and logarithm of the current density with the electrolyte pH, show a linear increase and decrease respectively. Some of these results are compared with data from experiments and simulations done on the iron oxide, showing that Ni forms steady-state passive films that are thinner than the ones formed on Fe under the same environment conditions (pH, temperature, and applied potential). [Copyright &y& Elsevier]

Abstract: A transient multi-ions transport and reaction model for galvanized steel corrosion in a 10molm−3 NaCl is proposed. The pH and the current density evolutions are simulated as a function of the immersion time and the results are compared with measurements. The simulations are two-dimensional and for a time up to 20000s. When considering no flow in the 10mm electrolyte layer above the electrodes, it was impossible to find agreement with the measured pH profile that showed a diffusion layer. The addition of natural convection, which is mimicked by a 0.1mm/s fluid flow towards the electrodes, leads to a much better qualitative agreement of the measured and simulated pH profile. Even in the presence of natural convection, the model predicts a low pH area only above the zinc and the zinc/steel interface. The insertion of two insulating pieces, representing the passivation due to zinc hydroxide deposit extends the low pH area up to 4mm beyond the edges of the anode and shows that the passivating layer can still grow further. The thus obtained plateau of low pH around the anode matches closely the measurements, while the predicted high pH matches the measurements also far away from the anode. [Copyright &y& Elsevier]

Abstract: Numerous studies on the formation of passive oxide films on metals have been made, all aiming to understand and quantify how the thickness of the oxide film depends on the electrolyte solution''s pH and on the applied potential. A one dimensional electrochemical model is presented considering the passive film on a rotating disk electrode (RDE) in contact with electrolyte solutions with the pH in the interval [8.4–11.2]. The mobile species in the oxide are oxygen vacancies, iron interstitials Fe2+, Fe3+ and iron vacancies. Reactions at the interfaces involving the defects in the oxide are combined with equations providing species and potential distributions, both in the oxide film and in the electrolyte solution. This approach offers new ways to link the film–solution interface with locally changing electrolyte composition. The passive film is described in terms of mass balance equations together with Poisson''s equation. The results of our numerical simulations based on the finite element method show that Fe3+ interstitials and oxygen vacancies are the species responsible for the transport of mass in the oxide. These results are discussed and compared with experimental data and other numerical results. [Copyright &y& Elsevier]

Abstract: A multi ion transport and reaction model has been developed with the intent to improve via simulation the understanding of the first steps of localized corrosion (pitting) of aluminum alloys at the microscale. However, the onset of pitting could not be predicted, but the simulation of all parts of a micropolarization curve, excluding the pitting regime, was possible. Unlike previous models, the model does not use the assumption that the system is at all times in a state of chemical equilibrium. In order to model localized corrosion initiation on Al alloys, one has to consider their complex microstructure which normally has a high number of structural inhomogeneities. We have considered localized corrosion initiation on the aluminum alloy AA2024 in sodium chloride solution. The intermetallic particles found in this widely used alloy can be divided into three major types based upon their composition: AlCuFeMnSi (2nd phase) intermetallics and Al2Cu (θ phase) precipitates, which are more noble than the Al matrix (areas of the alloy without constituent microscale particles), and Al2CuMg (S phase) precipitates, which are the preferential initiation sites for localized corrosion. It was expected that the results from this microscopic model would provide input data for a macroscopic corrosion model. Such a macroscopic model could then be used to simulate the behavior of sensitive assemblies, such as two overlapping aluminum alloy sheets, that can generate an occluded electrochemical cell. It turned out that this modeling approach has value, but requires very careful consideration of the input data. [Copyright &y& Elsevier]

Abstract: The ECM process with a moving cathode tool is simulated with the aim to study the influence of the temperature on the uniformity or copying quality of the removal rate. Two temperature dependent electrode reaction models are presented and used for solving the charge conservation and internal energy balance equations, in combination with the Navier–Stokes equations for viscous incompressible flow. The level set method is used for tracking the anode interface. The heat conduction through electrodes and the heat production by the electrode reactions are found to play an important role. [Copyright &y& Elsevier]

Abstract: The article discusses components and performance of an algebraic multigrid (AMG) preconditioner for the fully coupled multi-ion transport and reaction model (MITReM) with nonlinear boundary conditions, important for electrochemical modeling. The governing partial differential equations (PDEs) are discretized in space by a combined finite element and residual distribution method. Solution of the discrete system is obtained by means of a Newton-based nonlinear solver, and an AMG-preconditioned BICGSTAB Krylov linear solver. The presented AMG preconditioner is based on so-called point-based classical AMG. The linear solver is compared to a standard direct and several one-level iterative solvers for a range of geometries and chemical systems with scientific and industrial relevance. The results indicate that point-based AMG methods, carefully designed, are an attractive alternative to more commonly employed numerical methods for the simulation of complex electrochemical processes. [ABSTRACT FROM AUTHOR]

Abstract: Electrodeposition of zinc with simultaneous hydrogen evolution in an aqueous acidic sulphate solution is investigated in an Inverted Rotating Disk Electrode (IRDE) reactor. A simulation tool, based on the MITReM (Multi-Ion Transport and Reaction Model) model is used to describe the electrochemical behavior. This model can account for mass transport by convection, diffusion and migration and for the presence of homogeneous reactions. The electron transfer at the electrode is described by a Butler–Volmer relation. A reaction mechanism is proposed from surface analysis of the deposited zinc layer. Field Emission Auger Electron Spectroscopy (FE-AES) in combination with Factor Analysis (FA) is used to determine the compositional structure of the formed zinc layer. The model parameters such as diffusion coefficients, rate constants and transfer coefficients are determined by comparison of experimental and simulated polarization curves and current efficiency profiles for different rotation speeds of the IRDE and concentrations of supporting electrolyte. The importance of the homogeneous reactions is shown. The influence of gas bubble evolution (induced micro-stirring effects, reduced electrolyte conductivity and surface blockage) on the electrodeposition process is elucidated. [Copyright &y& Elsevier]

Simulation of the Pulse Electrochemical Machining (PECM) process can provide information on system design and guidelines for practical use. The pulses that are applied to the PECM system have to be described on a time scale that can be orders of magnitude smaller than the physical time scales in the system. If the full detail of the applied pulses has to be taken into account, the time accurate calculation of the variable distribution evolutions in PECM can become a computationally very expensive procedure. In previous work of the authors, approximate techniques were introduced: the hybrid calculation and the Quasi Steady State Shortcut (QSSSC). In other previous work of the authors a model for PECM of steel in NaNO3 was introduced. This model contains a changing polarization behaviour of the double layer as a function of the metal ion surface concentration, which brings a strong non-linearity in the system. In this paper a technique is introduced to integrate the non-linear model into the approximate methods. To achieve this, the strategy of the approximate methods is extended. For the QSSSC, the non-linearity is handled using an extra convergence level. For the hybrid calculation, live averaging is used to take care of the non-linear effects. Performing this, the timesteps used during the high level calculations are no longer dictated by the pulse characteristics. Using this approach, computationally very cheap, yet satisfying results can be obtained. The technique is very general and very powerful and can be used in any multi-timescale system. [ABSTRACT FROM AUTHOR]

Simulation of the species concentrations during the pulse electrochemical machining (PECM) process can provide information on system design and guidelines for practical use. In detailed numerical calculations, the concentrations will be calculated simultaneously with the temperature due to mutual dependencies. The pulses that are applied to the PECM system have to be described on a timescale that can be orders of magnitude smaller than the physical timescales in the system. If the full detail of the applied pulses has to be taken into account, the time accurate calculation of the variable distributions' evolutions in PECM can become a computationally very expensive procedure. A different approach is used by time averaging the pulses applied to the system. Performing this, the timesteps used during the calculations are no longer dictated by the pulse characteristics. Using this approach is computationally very cheap, yet satisfying results can be obtained. In the previous study of the authors (Smets et al., J Appl Electrochem 37(11):1345–1355, 2007 []), the hybrid calculation and the quasi-steady-state shortcut (QSSSC) were introduced. These methods introduce errors, however, which were quantified using analytical solutions and found to be acceptable. The results applied only to rectangular pulses. In this study, the more general case of arbitrary pulse forms is considered using a spectral approach. The concentration and the temperature calculation have different requirements for optimal approximated calculations, and a compromise has to be found between them. An analysis is performed on a simplified model, which provides useful guidelines during simulations. [ABSTRACT FROM AUTHOR]

A so-called blockage geometry consisting of a rod with a fin positioned concentrically within a pipe is used to asses the capabilities of numerical turbulent flow and mass transfer models to predict the turbulent mass transfer coefficients. Measurements of the mass transfer coefficient have been performed for a range of fin diameters and flow rates. The limiting diffusion current measurements were performed using the ferri-ferrocyanide system and nickel electrodes. Different mass transfer turbulence models are used for the calculations and the results are compared with the measurements. The influence of flow rate and fin diameter on the mass transfer rate is examined. [ABSTRACT FROM AUTHOR]

Abstract: The influence of heat transfer on anodic oxide growth during anodizing of high purity Al is studied on a laboratory scale in a wall-jet electrode reactor under different flow conditions. Local electrode temperatures are monitored by temperature measurements on the backside of the anode, whereas the effect of local temperature on film growth is evaluated by FE-SEM analyses. Quantitative information on the conditions of heat transfer is provided by numerical calculations of the flow field, and is applied during simulations of the anodizing process. The impact of the flow field, calculated by two different turbulence models, on the simulations is verified. [Copyright &y& Elsevier]

Simulation of the temperature distribution during the Pulse Electrochemical Machining (PECM) process provides information on system design and guidelines for practical use. The pulses that are applied to the PECM system have to be described on a time scale that can be orders of magnitude smaller than the time scale on which the thermal effects evolve. If the full detail of the applied pulses has to be taken into account, the time accurate calculation of the temperature distribution in PECM can become a computationally very expensive procedure. A different approach is used by time averaging the heat sources of the system. Performing this, the time steps used during the calculations are no longer dictated by the pulse characteristics. Using this approach, computationally very cheap, yet satisfying results can be obtained. In previous work of the authors, the hybrid calculation and the Quasi Steady State ShortCut (QSSSC) were introduced. This method allows to perform simplified calculations while getting satisfactory results. The method introduces errors however, which were quantified using analytical solutions and found to be acceptable. The results applied only to rectangular pulses. In this work, the more general case of arbitrary pulse forms is considered using a spectral approach. [ABSTRACT FROM AUTHOR]

Abstract: The chemical and electrochemical conditions within a crevice of Type 316 stainless steel in boric acid–lithium hydroxide solutions under PWR-relevant conditions were modelled with a computational electrochemistry code. The influence of various variables: dissolved hydrogen, boric acid, lithium hydroxide concentration, crevice length, and radiation dose rate was studied. It was found with the model that 25ccH2/kg (STP) was sufficient to remain below an electrode potential of -230mVshe, commonly accepted sufficient to prevent stress corrosion cracking under BWR conditions. In a PWR plant various operational B–Li cycles are possible but it was found that the choice of the cycle did not significantly influence the model results. It was also found that a hydrogen level of 50ccH2/kg (STP) would be needed to avoid substantial lowering of the pH inside a crevice. [Copyright &y& Elsevier]

Abstract: The crack propagation rate of Type 316 stainless steel in boric acid–lithium hydroxide solutions under PWR-relevant conditions was modelled. A film rupture/dissolution/repassivation mechanism is assumed and extended to cold worked materials by including a stress-dependent bare metal dissolution current density. The chemical and electrochemical conditions within the crack are calculated by finite element calculations, an analytical expression is used for the crack-tip strain rate and the crack-tip stress is assumed equal to 2.5 times the yield stress (plane-strain). First the model was calibrated against a literature published data set. Afterwards, the influence of various variables – dissolved hydrogen, boric acid and lithium hydroxide content, stress intensity, crack length, temperature, flow rate – was studied. Finally, other published crack growth rate tests were modelled and the calculated crack growth rates were found to be in reasonable agreement with the reported ones. [Copyright &y& Elsevier]

Simulation of the species concentrations during the Pulse Electrochemical Machining (PECM) process can provide information on system design and guidelines for practical use. In detailed numerical calculations the concentrations will be calculated simultaneously with the temperature due to mutual dependencies. The pulses that are applied to the PECM system have to be described on a time scale that can be orders of magnitude smaller than the physical time scales in the system. If the full detail of the applied pulses has to be taken into account, the time accurate calculation of the variables distributions evolutions in PECM can become a computationally very expensive procedure. In previous work (Smets et al. J Appl Electrochem 37(11):1345, 2007), a time averaging approach was introduced. Performing this, the timesteps used during the calculations are no longer dictated by the pulse characteristics. Using this approach, computationally very cheap, yet satisfying results can be obtained. This work focuses on the behaviour of the concentration evolution. The concentration and the temperature calculation have different requirements for optimal approximated calculations, and a compromise has to be found between them. An analysis is performed on a simplified model, which provides useful guidelines during simulations. [ABSTRACT FROM AUTHOR]

Simulation of the temperature distribution and evolution during pulse electrochemical machining can be a computationally very expensive procedure. In a previous part of the work [Smets et al. J Appl Electrochem 37(11):1345, 2007] a new approach to calculate the temperature evolution was introduced: the hybrid method, which combines averaged and pulsed calculations. The averaged calculations are performed by time averaging the boundary conditions and the bulk heat sources of the system. The timesteps used during the averaged calculations are then no longer dictated by the pulse characteristics. Using this approach, computationally very cheap, yet satisfactory results can be obtained. The analysis in the previous part of the work was obtained from analytical solutions on simplified models. In this part, the more general case is solved numerically. Multiple geometries are simulated and analyzed and methods are compared. Very satisfactory, yet cheap results are obtained. [ABSTRACT FROM AUTHOR]

Simulation of the temperature distribution during the Pulse Electrochemical Machining (PECM) process provides information on system design and guidelines for practical use. The pulses that are applied to the PECM system have to be described on a time scale that can be orders of magnitude smaller than the time scale on which the thermal effects evolve. If the full detail of the applied pulses has to be taken into account, the time accurate calculation of the temperature distribution in PECM can become a computationally very expensive procedure. A new approach is introduced by time averaging the heat sources of the system. Performing this, the time steps used during the calculations are no longer dictated by the pulse characteristics. Using this approach, computationally very cheap, yet satisfactory results can be obtained. In this part of the work, the hybrid calculation and the Quasi Steady State ShortCut (QSSSC) are introduced. The hybrid calculation is a method, by which averaged and pulsed heat sources are combined in one calculation. The QSSSC is a method for quickly calculating the Quasi Steady State (QSS) in numerical calculations with time stepping. Analytical solutions of simplified cases are studied to provide useful insights into the more general case. It is shown that the averaging technique adopted in this work does not always deliver perfect results. However, using a technique of shifting the pulses in time, the results can become very satisfactory yet still extremely cheap. The more general case, which will be solved numerically, can be found in part II [Smets et al. J Appl Electrochem (Submitted)] of this work. [ABSTRACT FROM AUTHOR]

Pulse Electrochemical Machining (PECM) is a manufacturing process which provides an economical and effective method for machining hard materials into complex shapes. One important drawback of ECM is the lack of quantitative simulation software to predict the tool shape and machining parameters necessary to produce a given work-piece profile. Calculating temperature distributions in the system allows more accurate simulations, as well as the determination of the thermal limits of the system. In this paper temperature transients over multiple pulses are calculated. It is found that the way the system is modeled has a great impact on the temperature evolution in the thermal boundary layer. The presence of massive electrodes introduces extra time scales which may not be negligible. It is advantageous to identify the thermal time scales in the system, to see whether the heat produced during separate pulses will accumulate or not during the process. The occurring thermal time scales in the system are discussed in detail. [ABSTRACT FROM AUTHOR]

Abstract: The crack propagation rate of Type 304 stainless steel in diluted sulphuric acid solutions was modelled based on finite element calculations of the chemical and electrochemical conditions within the crack and an analytical expression for the crack-tip strain rate. The slip/dissolution/repassivation mechanism was assumed. The influence of various variables – dissolved oxygen, sulphuric acid content, stress intensity, crack length, temperature, flow rate, sensitization and yield strength – was studied and found to be in agreement with experimental observations. The model was calibrated against one data point. The effect of temperature on thermal conductivity, electrode reaction kinetics, equilibrium constants, yield strength and crack geometry was considered and the thermal activation energy for the crack propagation rate could almost fully be accounted for. [Copyright &y& Elsevier]

A new two-dimensional model is presented that allows describing the high speed electrochemical machining of steel in NaNO3 solutions. Unlike existing models, local ion concentrations are calculated and used to evaluate local diffusion coefficients and electrolyte conductivity. Secondly, the presence of a super-saturated, honey-like layer on the anode surface is accounted for by introducing a water depletion factor. This factor describes the suppression of the oxygen evolution as the vast increase in ion concentrations reduces the amount of free water molecules at the anode. It is demonstrated that this approach enables to reproduce experimental average efficiency curves over a broad range of electrolyte concentrations with just a limited set of adjustable parameters. [ABSTRACT FROM AUTHOR]

Deep eutectic solvents (DESs) are a new class of solvents with wider potential window than that of water and high electrochemical stability, making them potential candidates for a wide range of electrochemical systems. However, due to the hygroscopic nature of DESs, the presence of latent water is unavoidable. Therefore, understanding the interfacial structure and the electrosorption and distribution of residual water at the electrified interface is of great importance for the use of these solvents in electrochemical systems. Using atomistic molecular dynamics, we explore the electrosorption and distribution of different amounts of water in 1 : 2 choline chloride-urea DES (Reline) at the electrified graphene interface. We found that both the water distribution and the interfacial structure are sensitive to the electrification of the graphene electrode. As a result, it is found that for moderately charged electrodes, water shows a preferential asymmetric adsorption in the vicinity of the positively charged electrode, partly due to strong intermolecular interactions with anions through hydrogen bonds. In contrast, for highly charged electrodes, water adsorbs at both electrodes due to a strongly enhanced external electrostatic interaction between the electrodes and the water dipoles., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Green, stable, and wide electrochemical window deep eutectic solvents (DESs) are ideal candidates for electrochemical systems. However, despite several studies of their bulk properties, their structure and properties under electrified confinement have barely been investigated, which has hindered widespread use of these solvents in electrochemical applications. In this Letter, we explore the electrical double layer structure of 1:2 choline chloride-urea (Reline), with a particular focus on the electrosorption of the hydrogen bond donor on a graphene electrode using atomistic molecular dynamics simulations. We discovered that the interface is composed of a mixed layer of urea and counterions followed by a mixed charged clustered structure of all of the Reline components. This interfacial structuring is strongly dependent on the balance between intermolecular interactions and surface polarization. These results provide new insights into the electrical double layer structure of a new generation of electrolytes whose interfacial structure can be tuned at the molecular level.

Fundamental understanding of the early stages of electrodeposition at the nanoscale is key to address the challenges in a wide range of applications. Despite having been studied for decades, a comprehensive understanding of the whole process is still out of reach. In this work, we introduce a novel modelling approach that couples a finite element method (FEM) with a random walk algorithm, to study the early stages of nanocluster formation, aggregation and growth, during electrochemical deposition. This approach takes into account not only electrochemical kinetics and transport of active species, but also the surface diffusion and aggregation of adatoms and small nanoclusters. The simulation results reveal that the relative surface mobility of the nanoclusters compared to that of the adatoms plays a crucial role in the early growth stages. The number of clusters, their size and their size dispersion are influenced more significantly by nanocluster mobility than by the applied overpotential itself. Increasing the overpotential results in shorter induction times and leads to aggregation prevalence at shorter times. A higher mobility results in longer induction times, a delayed transition from nucleation to aggregation prevalence, and as a consequence, a larger surface coverage of smaller clusters with a smaller size dispersion. As a consequence, it is shown that a classical first-order nucleation kinetics equation cannot describe the evolution of the number of clusters with time, N(t), in potentiostatic electrodeposition. Instead, a more accurate representation of N(t) is provided. We show that an evaluation of N(t), which neglects the effect of nanocluster mobility and aggregation, can induce errors of several orders of magnitude in the determination of nucleation rate constants. These findings are extremely important towards evaluating the elementary electrodeposition processes, considering not only adatoms, but also nanoclusters as building blocks.

The present study investigates the validity of one of the key assumptions underlying the general plate height model of chromatography, i.e., the presumed independency of the individual band broadening contributions. More precisely, it is investigated under which conditions the mass transfer inside the stationary zone (e.g., porous pillars) is independent from the axial transport of species outside this zone, and how strongly any such dependency would affect the validity of the general plate height model of chromatography. For this purpose, detailed calculations of the species concentration distribution inside and outside the porous pillars of a computer-mimic of a porous pillar array column have been made. These simulations revealed a clear interplay between the mass transfer inside and outside the pillars, manifesting itself as an asymmetry of the species concentration distribution inside the pillars. The latter is in disagreement with the basic assumption used to calculate the value of the C(s)-term of the general plate height model. The asymmetry-effect is largest at low reduced velocities, high retention factors and high intra-pillar diffusion coefficients. Fortunately, these are conditions where the C(s)-term is relatively small, which might explain why the general plate height model of chromatography (and based on the symmetry assumption) can represent the band broadening in a porous pillar array within an accuracy on the order of some 1-2%., (Copyright 2010 Elsevier B.V. All rights reserved.)

The band broadening that can be expected in perfectly ordered cylindrical pillar arrays has been calculated for a wide range of intra-particle diffusion coefficients (D(sz)) and zone retention factors (0

The Fuoss-Onsager continuity equations are solved by using the equilibrium pair distribution functions of the mean spherical approximation in the case of equal diameters. An analytical expression is obtained for the relaxation effect on the Onsager coefficients of mixed strong electrolytes. This work also extends the existing expressions for the conductivity of binary and ternary electrolytes to any number of ions.