Corrosion current densities at a disk-shaped inclusion

Laplace's equation is solved for the geometry of a polarized disk, of radius R, inlaid in a coplanar conductor and immersed in an aqueous electrolyte solution of conductivity κ. This geometry corresponds to a corroding inclusion on the surface of a metal sheet when the rate of corrosion is controlled by ohmic polarization. When the corrosion potential is Φ, the corrosion current density is found to have a minimum anodic value, equal to κΦ/R, in the centre of the disk. Current densities at a variety of sites on the disk-shaped anode and on the annular cathode have been determined numerically. Limiting behaviours have also been delineated. Equipotential surfaces are portrayed in a cross-sectional diagram, which also shows the routes taken by the corrosion current. The behaviour of the current in special regions – near the axis, near the three-phase junction and on remote regions of the cathode – can be described by simple formulas. Electronic Publication     

Simulations of film growth and diffusion during the corrosion process

A simple model is proposed to describe some general aspects concerning the formation of films at metal ionic solution interfaces. Working on a mesoscopic scale, three main processes are retained: the metal corrosion, the reduction of one species present in the ionic solution and the diffusion of one species from the corrosion front to the growth front where the reduction takes place. This model differs from those devoted to the growth on a surface by the existence of two fronts and their coupling. Moreover, a coupling between the diffusive species is taken into account. To emphasize the role of the diffusion, the predictions of the model are compared with those obtained from a simple extension of the Eden model. Due to diffusion a dissymetry is created between the two fronts which obeys different scaling laws. The internal structure of the film during its formation is investigated via the concentration of diffusive species.     

Composite polarizability and the construction of an invariant function of refraction and mass density for solutions

Re-examination of dynamical ionic polarizabilities in water solutions leads to the formulation of a solution function r(c), which combines the indices of refraction and mass densities of solutions. We show that this function should be independent of ionic concentration if the composite polarizabilities of hydrated solute clusters are constant. Using existing experimental data for a number of aqueous salt and organic solutions, we find that the r(c) function is either constant or varies linearly with concentration, in most cases with negligible slope. We use this function to compare ionic polarizabilities of crystals and aqueous solutions and to highlight how solute polarizabilities at infinite dilution scale with the electronic valence shell of cations and anions. The proposed r(c) function can be used generally to verify the consistency of experimental measurements and of simulation results, and it provides a test of assumptions in current theories of ionic polarizabilities.     

Apparent Molar Volume and Apparent Molar Expansibility of Rubidium, Cesium, and Ammonium Cyclohexylsulfamate in Aqueous Solution

Summary. The apparent molar volume of rubidium, caesium, and ammonium cyclohexylsulfamate was determined from the density data of their aqueous solutions at 293.15, 298.15, 303.15, 313.15, 323.15, and 333.15 K. From the apparent molar volume, determined at various temperatures, the apparent molar expansibility was calculated. The limiting apparent molar volume and apparent molar expansibility were evaluated and apportioned into their ionic components. The limiting partial molar ionic volumes and expansibilities are discussed in terms of the various effects of the ion in solution on the structure of water. It was shown that the limiting partial molar ionic expansibilities of the alkali-metal cations increase with their ionic radii. The coefficients of thermal expansion of the investigated solutions at 298.15 K were calculated and are presented graphically together with some alkali-metal cyclohexylsulfamates and tetramethylammonium cyclohexylsulfamate. The densities of the investigated solutions can be adequately represented by an equation derived by Redlich.     

Electrokinetics in nanochannels: part I. Electric double layer overlap and channel-to-well equilibrium

In this paper a new model is described for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. Electrolyte concentration in the nanochannel is predicted self-consistently via equilibrium between ionic solution in the wells and within the nanochannel. Differently than published models that require detailed iterative numerical solutions of coupled differential equations, the framework presented here is self-consistent and predictions are obtained solving a simple one-dimensional integral. The derivation clearly shows that the electric potential field depends on three new parameters: the ratio of ion density in the channel to ion density in the wells; the ratio of free-charge density to bulk ion density within the channel; and a modified Debye-Hückel thickness, which is the relevant scale for shielding of surface net charge. For completeness, three wall-surface boundary conditions are analyzed: specified zeta-potential; specified surface net charge density; and charge regulation. Predictions of experimentally observable quantities based on the model proposed here, such as depth-averaged electroosmotic flow and net ionic current, are significantly different than results from previous overlapped electric double layer models. In this first paper of a series of two, predictions are presented where channel depth is varied at constant well concentration. Results show that under conditions of electric double layer overlap, electroosmosis contributes only a small fraction of the net ionic current, and that most of the measurable current is due to ionic conduction in conditions of increased counterion density in the nanochannel. In the second of this two-paper series, predictions are presented where well-concentration is varied and the channel depth is held constant, and the model described here is employed to study the dependence of ion mobility on ionic strength, and compare predictions to measurements of ionic current as a function of channel depth and ion density.     

Apparent Molar Volume and Apparent Molar Expansibility of Rubidium, Cesium, and Ammonium Cyclohexylsulfamate in Aqueous Solution

The apparent molar volume of rubidium, caesium, and ammonium cyclohexylsulfamate was determined from the density data of their aqueous solutions at 293.15, 298.15, 303.15, 313.15, 323.15, and 333.15 K. From the apparent molar volume, determined at various temperatures, the apparent molar expansibility was calculated. The limiting apparent molar volume and apparent molar expansibility were evaluated and apportioned into their ionic components. The limiting partial molar ionic volumes and expansibilities are discussed in terms of the various effects of the ion in solution on the structure of water. It was shown that the limiting partial molar ionic expansibilities of the alkali-metal cations increase with their ionic radii. The coefficients of thermal expansion of the investigated solutions at 298.15 K were calculated and are presented graphically together with some alkali-metal cyclohexylsulfamates and tetramethylammonium cyclohexylsulfamate. The densities of the investigated solutions can be adequately represented by an equation derived by Redlich.     

Membrane Potential of Composite Bipolar Membrane in Ethanol-Water Solutions: The Role of the Membrane Interface

The membrane potential across a composite bipolar membrane (CBM) composed of a cation-exchange membrane with an anion-exchange membrane is theoretically and experimentally analyzed for LiCl ethanol-water solutions. The theoretical approach is based on an extension of the Donnan equilibrium and the Nernst-Planck equation of monopolar charged membranes for the case of two ion-exchange layers by considering the effect of electrolyte ion pairing in the external solution. The experimental results show that the effective membrane charge densities of the two ion-exchange layers will become smaller than those which are separately estimated for each layer. We have introduced a contact factor, zeta, into the theoretical approach to clarify this phenomenon in this study, and the theoretical predictions were in good agreement with the experimental data. The membrane potential measurements show that CBM has the characteristics of a bipolar membrane and can significantly contribute to a better electrochemical characterization of the CBMs. Copyright 1999 Academic Press.     

Determination of metal corrosion rate using the pH-metry by the method of compensating additives

A possibility for determining the metal corrosion current in the acidic solutions in the absence of external polarization is considered. The calculations are based on the analysis of time dependence of the amount of electrolyte with a lower pH value (as compared with the pH value of the working solution), which was introduced into the measuring cell in order to compensate an increase of the pH value in the course of corrosion.     

Molecular thermodynamics for micellar branching in solutions of ionic surfactants

We develop an analytical molecular-thermodynamic model for the aggregation free energy of branching portions of wormlike ionic micelles in 1:1 salt solution. The junction of three cylindrical aggregates is represented by a combination of pieces of the torus and bilayer. A geometry-dependent analytical solution is obtained for the linearized Poisson-Boltzmann equation. This analytical solution is applicable to saddle-like structures and reduces to the solutions known previously for planar, cylindrical, and spherical aggregates. For micellar junctions, our new analytical solution is in excellent agreement with numerical results over the range of parameters typical of ionic surfactant systems with branching micelles. Our model correctly predicts the sequence of stable aggregate morphologies, including a narrow bicontinuous zone, in dependence of hydrocarbon tail length, head size, and solution salinity. For predicting properties of a spatial network of wormlike micelles, our aggregation free energy is used in the Zilman-Safran theory. Our predictions are compared with experimental data for branching micelles of ionic surfactants.     

A new method of evaluating the average electro-osmotic velocity in microchannels

A new and simple method to evaluate the average electro-osmotic flow velocity in microchannels is presented in this paper. In this method, the average electro-osmotic flow velocity is determined by using the slope of the measured current-time relationship during the electro-osmotic flow of one solution replacing another similar solution. The two solutions have the same electrolyte and a small difference in ionic concentration. Careful experiments were conducted to measure the electrical current change with time during such a displacing process under a constant applied electrical field. KCl and LaCl3 electrolyte solutions and 10-cm-long polyamide-coated silica capillary tubes of 100 and 200 microm in internal diameter were used in this study. The average velocities were determined by using the slope method. A numerical model was also developed to predict the average velocity of such an electro-osmotic flow. An excellent agreement in the average velocities between the slope method and the model predictions was found.     

Re-defining the kinetics of redox reactions on passive metal surfaces

It is known that the kinetics of redox reactions occurring on the surfaces of passive metals depend upon the properties of the passive film, ostensibly due to quantum mechanical tunnelling (QMT) of electrons and holes between the metal and the redox couple at the barrier layer/solution (bl/s) interface. In this paper, the tunnelling probability is used to inter-convert the exchange current densities for the redox reactions occurring at the bl/s interface and on the hypothetical bare metal surface. We review our previous work on combining QMT theory with the point defect model (PDM), which provides an analytical expression for the bl thickness as a function of voltage. By combining QMT theory and the PDM, we derive a modified form of the generalized Butler-Volmer equation that requires as input only the kinetic parameters for the redox reaction on the hypothetical bare surface and parameters contained in the PDM. The application of the theory is illustrated with reference to the corrosion of carbon steel in concrete pore solution, to calculating the corrosion potential of, and crack growth rate in, sensitized type 304 SS in boiling water reactor (BWR) coolant circuits, and the use of hydrogen oxidation on platinum to determine the thickness of the bl as a function of voltage and temperature. This illustrates a new, powerful technique for probing the formation of passive films on metal surfaces.

     

Theoretical transient currents from two-component bipolar space-charge swarms in media

Using an arbitrary initial space-charge distribution consisting of two bipolar species of charge carriers of different but constant mobilities in a medium, relations for the electric fields and charge-carrier densities, are derived as functions of position and time. The highly nonlinear, one-dimensional equations, which are derived for swarms of charge carriers between parallel plane electrodes, include the effects of the space-charge fields. A general method is outlined which, in principle, can be used to generate second-order differential equations whose solutions predict the time-dependent current caused by the drifting bipolar space-charge swarm. A specific differential equation is derived for initial charge distributions in dielectric media which could be produced by a pulse of ionizing radiation of uniform intensity.     

Chromatographic studies of corrosion sites in metallic materials

Many types of corrosion phenomena are controlled by the ionic composition of a small volume of solution at the surface. Localized corrosion and atmospheric corrosion are two examples in which < 1 μl of solution can cause dramatic damage. Ion chromatographic (IC) techniques have been used to analyze these solutions in order to gain a better understanding of the mechanisms which govern them. Two examples are presented. The presence of minor alloying elements at localized corrosion sites in two aluminum alloys has been demonstrated, indicating non-stoichiometric dissolution of the alloy during localized corrosion. In addition, IC analysis allowed the determination of the species responsible for the atmospheric corrosion failure of electrical connectors, including their likely origin.     

The connection between Young's rule for apparent molar volumes and a Young's rule for density

It is shown that the Young's rule approximation for apparent molar volumes is equivalent to a Young's rule approximation for density, provided that volumetric concentrations are used. If the densities of the constituent binary solutions are evaluated at the molar ionic strength S of the ternary solution, for example, then $$d(S) = y_1 d_1 (S) + y_2 d_2 (S)$$ where the y _i are ionic strength fractions. However, if the binary solution densities d _i are evaluated at the same total equivalent concentration E of the ternary solution, then the coefficients of d_i become equivalent fractions, etc. A small correction term can be included to improve agreement with experiment, and the results are easily extended to multicomponent systems. This form of the density rule does not follow from Young's rule if the various quantities are in molality or other mass units. Relations among various types of apparent volumes are clarified. Three binary evaluation strategies are considered for both volume and mass units, based on evaluating binary solution densities at constant molarity, constant equivalents, and constant ionic strength. Some binary approximation examples are given for aqueous solutions.     

电化学腐蚀法可控合成氧化锌纳米材料

以金属锌板为电极材料,采用去离子水为液相介质,在电化学腐蚀过程中得到了均一组成的ZnO纳米片.通过调节电流密度、反应温度、反应时间等条件,在阳极板上得到了由均一的ZnO纳米粒子构成的不同花样的粒子膜.利用X-射线粉末衍射(XRD)、扫描电子显微镜(SEM)对产物的组成和形貌进行了表征.     

Microcapsules for controlled release via donnan dialysis

The preparation of ion-exchange microcapsules that exhibit a diameter of 10 40μm, a membrane shell of 0.1-1μm in thickness and with charge densities in the range of 0.8-1.2 meq/g is described. The microcapsules were prepared by interfacial cross-linking of bromomethylated poly(phenylene oxide) (PPOBr) and poly(ethyleneimine) (PEI). These polymers were dissolved in toluene and aqueous solutions, respectively. Upon dispersing the PEI solution in the organic phase, ion-exchange microcapsules were instantaneously formed. These microcapsules contain aqueous solutions of PEI and inorganic salt and are self-sealing. They can be dried and stored in a “dormant”, inactive state. Upon exposure to ambient humidity or aqueous solutions, water diffuses rapidly into the microcapsules, establishing the conditions for Donnan exchange of ions between the external phase and the microcapsule interior. Some preliminary transport measurements of the exchange of encapsulated nitrate ions (a corrosion inhibitor) with chloride ions (a corrosion inducer) in the external phase via a Donnan exchange mechanism are reported. Ion transport and leakage rates are compared with those obtained from flat sheet ion-exchange membranes made from identical materials.     

Electron-transfer reactions at the plasma-liquid interface

Electrochemical reactions are normally initiated in solution by metal electrodes such as Pt, which are expensive and limited in supply. In this Communication, we demonstrate that an atmospheric-pressure microplasma can act as a gaseous, metal-free electrode to mediate electron-transfer reactions in aqueous solutions. Ferricyanide is reduced to ferrocyanide by plasma electrons, and the reduction rate is found to depend on discharge current. The ability to initiate and control electrochemical reactions at the plasma-liquid interface opens a new direction for electrochemistry based on interactions between gas-phase electrons and ionic solutions.     

利用充电曲线数据计算钝态金属的腐蚀速度

对 钝态金 属腐蚀体 系推导了 不含近 似处 理的 充电 曲线 方程 式．提 出 了使 用数 值 微分和线性 回归求 解体系电 化学参数 的计算 机方法．     

Application of Restrictive Primitive SAFT Coupled with Different Hard-Sphere Equations to Model Mean Ionic Activity Coefficients of Electrolyte Solutions

In this work, the primitive SAFT equation of state along with three different hard-sphere equations was used to correlate and predict mean ionic activity coefficients of aqueous electrolyte solutions. The mean ionic activity coefficient of aqueous electrolyte solutions was considered as the contribution of hard-sphere and dispersion effects. The Mansoori (M), Wang-Khoshkbarchi-Vera (WKV) and Ghotbi-Vera (GV) hard-sphere equations were applied in correlating the mean ionic activity coefficient of electrolyte solutions. The comparison among above indicated equations was shown. First, vapor pressure and densities of water in the temperature range of 373.15 to 423.15 K was regressed by SAFT equation of state. In the restrictive primitive mean spherical model, ions were hard spheres without any chain structure. Neither association effects were considered in this study. Clearly, in common used five SAFT parameters were decreased to three, which were calculated by using the experimental mean ionic activity coefficients of electrolyte solutions. The comparison among three hard-sphere equations of state approved that Ghotbi-Vera hard-sphere model (GV) correlated the experimental data accurately than the others; two hard-sphere models. The mean ionic activity coefficients of some electrolyte solutions were being predicted by taking the advantage of the regressed values surely, in a wide range of molality.     

Electroviscous effects on pressure-driven flow of dilute electrolyte solutions in small microchannels

A fundamental understanding of the flow characteristics of electrolyte solutions in microchannels is critical to the design and control of microfluidic devices. Experimental studies have shown that the electroviscous effect is appreciable for a dilute solution in a small microchannel. However, the experimentally observed electroviscous effects cannot be predicted by the traditional theoretical model, which involves the use of the Boltzmann distribution for the ionic concentration field. It has been found that the Boltzmann distribution is not applicable to systems with dilute electrolyte solutions in small microchannels because it violates the ion number conservation condition. A new theoretical model is developed in this paper using the Nernst equation and the ion number conservation, instead of the Boltzmann distribution, to obtain the ionic concentration field. The ionic concentration field, electrical potential field, and flow field in small microchannels are studied using the model developed here. In order to verify this model, the model-predicted dP/dx (applied pressure gradient) Re (Reynolds number) relationship is compared with the experimentally determined dP/dx approximately Re relationship. Strong agreement between the model predictions and the experimental results supports this model.