Cathode Current Distribution Research Articles

Steel Pipe Internal Corrosion Protection Using Magnesium Anode: Impact of Protective Potential and Phase Layer Formation on Corrosion Rate

The influence of magnesium anode dissolution in pipe filled with potable water on the steel corrosion rate was investigated. Steel corrosion rate was measured using the weight loss technique, and linear polarization resistance measurements were used to estimate the formation kinetics and protective abilities of the phase layer on the metal surface. The cathodic current distribution along steel pipe was measured using a microammeter and compared with the computer model. The results clearly demonstrate that the reduction of metal corrosion rate is at least 2.8 times in the region where cathodic protection distributes inside the pipe. However, for the remaining part of the metal pipe, the reduction of corrosion rate of 1.5 times was obtained due to the formation of the phase layer on the metal surface that leads to the reduction of oxygen supply. The results obtained in the study can be further applied for designing and maintenance of water supply systems.

DIGITAL APPROACH TO THERMIONIC EMISSION CURRENT TO VOLTAGE CONVERSION FOR HIGH-VOLTAGE SOURCES OF ELECTRONS

The thermionic emission current is used in many vacuum devices such as evaporators, rare gas excimers, or electron beam objects for high-energy physics. The stability of the thermionic emission current is a very important requirement for the accuracy of those devices. Hence, there is a number of control systems that use a feedback signal directly proportional to the emission current in order to stabilize the thermionic emission current. Most of them use feedback from a high-voltage anode circuit to a low-voltage cathode circuit. However, there is a novel solution that uses linear cathode current distribution and processing of two cathode circuit voltage signals for converting the emission current to voltage. However, it is based on old-fashioned analog technology. This paper shows the thermionic emission current to voltage conversion method with the use of a digital control system. A digital realization of a multiplicative-additive algorithm is presented and proper work in closed-loop mode is confirmed.

Evaluation of Applied Stress on Atmospheric Corrosion and Pitting Characteristics in 304L Stainless Steel

The effects of applied stress, ranging from tensile to compressive, on the atmospheric pitting corrosion behavior of 304L stainless steel (SS304L) were analyzed through accelerated atmospheric laboratory exposures and microelectrochemical cell analysis. After exposing the lateral surface of a SS304L four-point bend specimen to artificial seawater at 50°C and 35% relative humidity for 50 d, pitting characteristics were determined using optical profilometry and scanning electron microscopy. The SS304L microstructure was analyzed using electron backscatter diffraction. Additionally, localized electrochemical measurements were performed on a similar, unexposed, SS304L four-point bend bar to determine the effects of applied stress on corrosion susceptibility. Under the applied loads and the environment tested, the observed pitting characteristics showed no correlation with the applied stress (from 250 MPa to −250 MPa). Pitting depth, surface area, roundness, and distribution were found to be independent of location on the sample or applied stress. The lack of correlation between pitting statistics and applied stress was more likely due to the aggressive exposure environment, with a sea salt loading of 4 g/m2 chloride. The pitting characteristics observed were instead governed by the available cathode current and salt distribution, which are a function of sea salt loading, as well as pre-existing underlying microstructure. In microelectrochemical cell experiments performed in Cl− environments comparable to the atmospheric exposure and in environments containing orders of magnitude lower Cl− concentrations, effects of the applied stress on corrosion susceptibility were only apparent in open-circuit potential in low Cl− concentration solutions. Cl− concentration governed the current density and transpassive dissolution potential.

Conversion Method of Thermionic Emission Current to Voltage for High-Voltage Sources of Electrons

The stability of the electron thermionic emission current is one of the most important requirements for electron sources used, inter alia, in evaporators, production of rare gas excimers, and electron beam objects for high energy physics. In emission current control systems, a negative feedback signal, directly proportional to the emission current is transferred from the high-voltage anode circuit to the low-voltage cathode circuit. This technique, especially for high-voltage sources of electrons, requires the use of galvanic isolation. Alternatively, a method of converting the emission current to voltage in the cathode power supply circuit was proposed. It uses a linear cathode current intensity distribution and multiplicative-additive processing of two voltage signals, directly proportional to the values of cathode current intensity. The simulation results show that a relatively high conversion accuracy can be obtained for low values of the electron work function of the cathode material. The results of experimental tests of the dynamic parameters of the electron source and the steady-state Ie-V characteristic of the converter are presented. The implementation of the proposed Ie-V conversion method facilitates the design of the emission current controller, especially for high-voltage sources of electrons, because a negative feedback loop between the anode and cathode circuits is not required, all controller sub-components are at a common electrostatic potential.

Modeling Nickel Electrowinning with Electrode Diaphragms Based on Nernst-Plank Equation and a Volume Force Form of Darcy's Law

The Ni electrowinning process with electrode diaphragms was simulated with an Electrochemical-Computational Fluid Dynamic model by considering a two-phase flow, mass transport, and electrochemical reactions. The effects of electrode diaphragms on the fluid flow, concentration, voltage drop, and current distribution were analyzed based on the simulation results. Results show that the electrode diaphragms have significant effects on the fluid flow and voltage drop in the electrolyte. The electrode diaphragms greatly reduced the strong convective flow caused by rising bubbles in the cell, and thus reduce the transport of H+ (generated at the anode) to the cathode. Based on this model, the effects of different operating parameters on the electrolyte potential and cathode current distribution have been analyzed. The results show that the cell temperature of 60°C, Ni concentration of 80 g/L, and a relatively low current density are the most favorable compared with other settings in the simulation for decreasing the electrolyte voltage drop and reducing the roughness of the deposition.

The Effect of the Shape of the Anode Head on the Nitrogen Soft X-Ray Emission from a Dense Plasma Focus Device: a Numerical Investigation Using the Modified Lee Model Code Accompanied by Experiments

An alternative way for calculating the magnetic field in the Lee Model Code has been used. The effects of the various anode-tip shapes on the DPF behavior have been studied. The current and impedance calculations have been employed for these investigations. For the conical anode tip, the best pinch current and soft x-ray radiation have been seen in simulations which was in good agreement with the empirical results. The investigations were carried out for the voltages of 11, 12, and 13 kV and the pressure range of 1–6 Torrs. The asymmetry in the discharging current has been investigated through three arbitrarily defined cathode current distribution patterns. The simulations showed that asymmetry in the axial discharge current increased the plasma Impedance and consequently reduced the soft x-ray emission.

Cathode-constriction and column-constriction in high current vacuum arcs subjected to an axial magnetic field

The influence of the applied axial magnetic field on the current density distribution in the arc column and electrodes is intensively studied. However, the previous results only provide a qualitative explanation, which cannot quantitatively explain a recent experimental data on anode current density. The objective of this paper is to quantitatively determine the current constriction subjected to an axial magnetic field in high-current vacuum arcs according to the recent experimental data. A magnetohydrodynamic model is adopted to describe the high current vacuum arcs. The vacuum arc is in a diffuse arc mode with an arc current ranged from 6 kArms to 14 kArms and an axial magnetic field ranged from 20 mT to 110 mT. By a comparison of the recent experimental work of current density distribution on the anode, the modelling results show that there are two types of current constriction. On one hand, the current on the cathode shows a constriction, and this constriction is termed as the cathode-constriction. On the other hand, the current constricts in the arc column region, and this constriction is termed as the column-constriction. The cathode boundary is of vital importance in a quantitative model. An improved cathode constriction boundary is proposed. Under the improved boundary, the simulation results are in good agreement with the recent experimental data on the anode current density distribution. It is demonstrated that the current density distribution at the anode is sensitive to that at the cathode, so that measurements of the anode current density can be used, in combination with the vacuum arc model, to infer the cathode current density distribution.

Computational and Experimental Analyses of Galvanic Corrosion of AA7075-T6 Induced By Dissimilar Metal Fasteners and Mitigated By Sol-Gel Coatings

In modern aircraft, there is a need to protect aluminum structures from galvanic corrosion due to dissimilar metals used in complex aerospace structures. Galvanic corrosion between corrosion resistant fasteners (e.g., stainless steel or titanium alloys) and aluminum alloy structures represent a recurring problem. One means of mitigating such attack is via the application of surface treatments to the fasteners designed to limit their cathodic current density. Detailed measurements and modeling of these effects are important to optimization of the surface treatment and estimates of the level of improvement likely to be achieved. The present work combines experimental measurements of galvanic corrosion damage and computational modeling of galvanic current distributions. The goal is quantitatively connect observations of decreased galvanic corrosion damage on AA7075-T6 coupons due to the presence of stainless steel fasteners under thin electrolyte films as would be present in aerospace service. In particular, sol-gel surface treatments were applied to stainless steel and titanium alloy fasteners which were then placed in AA7075-T6 panels before exposure to salt spray testing. Experimental analyses of corrosion damage were performed using white light interferometry generated depth profiles. These data were used to create data for total volume mass loss (TVML). Computational modeling of galvanic current distributions was performed using a commercial FEM package for thin electrolyte film conditions with experimentally-derived electrochemical kinetics as boundary conditions. The polarization curves used and the current distributions for a 0.1 mm electrolyte layer are shown in Figure 1. Integration of the anodic current densities on the AA7075 over the surface produces a corrosion damage metric, the total anodic current (TAC). The effects of sample geometry, including spacing between fasteners and their location relative to the edges of the sample, as well as water layer thickness and solution concentration on the TAC will be reported. The TVML and TAC can be correlated to one another and to total measured mass loss via Faraday’s Law. Comparisons between the TVML and the TAC will be discussed in detail. Of particular importance will be analyses of the effective throwing power of the fasteners as a function of fastener material and the presence or absence of sol gel surface treatments. Figure 1: (a) Polarization curves in 0.6 M NaCl of the materials of interest. (b) False color plot of anodic (positive) and cathodic (negative) current distribution (A/m2) for a 0.1 mm electrolyte layer of 0.6 M NaCl Acknowledgements: The financial support of the Office of Naval Research through Contract # N68335-16-C-0121 (William Nickerson, PM) via Luna Innovations (Adam Goff) is gratefully acknowledged. Figure 1

Mechanism Analysis of Carbon Contamination and the Inhibition by an Anode Structure during Soluble K2CrO4 Electrolysis in CaCl2-KCl Molten Salt

High-efficiency electrolysis of soluble K2CrO4 was realized in the CaCl2-KCl molten salt by adopting a novel anode structure. The carbon contamination due to the accumulation of carbon particles and CO32− reduction was very detrimental to the electrolysis process when a graphite anode was used, especially for the new process that soluble K2CrO4 was directly electrolyzed to metal Cr in the CaCl2-KCl molten salt. In this paper, the carbon migration and CO32− reduction during the K2CrO4 electrolysis were analyzed and discussed. The cathodic current distribution and the competing reduction reactions between CrO42− and CO32− were analyzed. The results indicated that the current efficiency for Cr production was only 45.7% for 12 h electrolysis. Dense carbon precipitations were found on the upper surface of molten salt, which led to the short circuit between anode and cathode. Therefore, the current efficiency is very low for Cr production. Meanwhile, competing reactions between CO32− and CrO42− took place, leading to high carbon contents in the cathode products. By adopting a novel anode structure, i.e. a graphite anode with a porous MgO shroud, the carbon precipitates were eliminated and the short circuit was effectively avoided. Resultantly, the current efficiency for Cr production increased greatly from 45.7% to 88.4% and the carbon content in the cathode products decreased from 2.47 wt% to 0.57 wt%.

Research progress on the cathodic protection current and potential distribution of the tank bottom plate

AbstractCathodic protection is an important method to prevent the corrosion of the tank bottom plate. In this article, the research status in recent decades of the current density and potential distribution of the tank bottom plate in cathodic protection is summarized. The research status and problems of the analytical and numerical methods for tank bottom plate are described and discussed. A detailed summary of boundary condition modeling is carried out. It is proposed that the analytical method will be gradually replaced by numerical methods and the mathematical model and boundary conditions need to be further studied and improved. In addition, the combined use of a variety of numerical methods and a numerical simulation study on the vapor phase corrosion related to tank bottom will be a trend in the development of numerical simulation in the future.

Measurement of Local Electrode Potentials in an Operating PEMFC Exposed to Contaminants

The use of a three-electrode cell, in which the potential of a working electrode can be accurately measured vs. a reference electrode (RE), is a standard practice in liquid-phase electrochemical experiments. However, the geometric constraints of a working PEMFC make use of a RE challenging. Incorporation of a RE into an operating PEMFC typically requires experimental compromises that prevent precise and accurate measurements from being obtained, and do not allow accurate spatial resolution of potential variation along the electrodes. As a result, PEMFC experiments are often performed using the PEMFC anode, which remains within a few mV of 0 V vs RHE during typical operation, as a pseudo RE. Recently, the National Physical Laboratory (NPL) developed an effective technique for accurate in situ monitoring of local electrode potentials in a PEMFC. By inserting REs from the back side of the MEA, the NPL configuration eliminates the edge effects, lack of anode vs. cathode specificity, Ohmic losses, and current density distribution disruptions that have compromised previous fuel cell RE experiments. We report here on the use of this RE technique to monitor local electrode potentials in an operating PEMFC during exposure to CO gas (Figure 1). CO is a common impurity in H2 gas and is known to poison Pt-based anodes. Levels of CO present in H2 fuel are expected to be in the ppb range for cells operating on pure H2 and the ppm range or higher for cells operating on reformate. Use of an RE can reveal the distribution of CO losses, and can also enable losses on the anode to be distinguished from those on the cathode. The results of this work demonstrate a new diagnostic capability that enables improved understanding of local conditions during PEMFC operation during transient or steady state operation. Figure 1. Cell voltage (orange) and anode potential (blue) of a MEA operating at 1 A/cm2 with anode exposed to 20 ppm CO. CO was removed from the anode feed at t = 47 min. Gore MEA, with anode/cathode loading of 0.4/0.1 mg/cm2, 80°C, 100% RH, 100 kPaabs. Figure 1

Experimental investigation of cathode current density distribution in vacuum arc based on ring-shaped electrode

In this paper, based on ring-shaped cathode and cup-shaped axial magnetic field (AMF) anode configuration system, the current density distribution on the cathode surface was measured. Vacuum arc appearance also was observed by a high-speed camera. The experimental results showed that the current density on the cathode was of the order of 10 6 –10 7 A/m 2 . For lower current (3 kA), the current density distribution was relatively uniform. For higher current (6 kA), due to the weaker AMF of cathode side, the arc was more seriously constricted and the current density distribution was more nonuniform, the current density of central area was much larger than that of other peripheral regions in the cathode surface. Compared with two cup-shaped AMF electrode systems, the current constriction was more significant for the same current and gap distance.

Modeling Particulates Effects on the Cathode Current Capacity in Crevice Corrosion

The presence of particulates within thin electrolyte films covering the cathode in corroding systems affects the cathodic current distribution and the total current capacity. These effects are due to increased solution resistance (“volume effect”) and decreased available electrode area (“surface effect”) associated with the presence of the particulates. This work simulates and characterizes the effect of uniform particulate layers embedded within thin electrolyte films on the cathodic current capacity under steady-state conditions. Particulate configurations consisting of varying particle sizes, shapes, arrangements, volume fraction, and electrode coverage were numerically modeled. It is concluded that the effects associated with the particles can be fully accounted for in terms of two corrections: the decrease in the electrolyte conductivity due to volume blockage by the particles is correlated using Bruggeman’s equation, while the electrode surface coverage by the particles is modeled in terms of an area correction to the electrode kinetics. For the range of parameters analyzed, applying these two corrections enables the modeling of particle-containing thin electrolyte films covering the cathode in terms of equivalent homogenous electrolytes with modified properties. The latter can then be analyzed using simpler approaches. Particulate effects on cathode capacity at saturation and saturation length are also quantified.

A 3D model for the free-breathing direct methanol fuel cell: Methanol crossover aspects and validations with current distribution measurements

A three-dimensional model has been developed for the free-breathing direct methanol fuel cell (DMFC) assuming steady-state isothermal and single-phase conditions. Especially the MeOH crossover phenomenon is investigated and the model validations are done using previous cathodic current distribution measurements. A free convection of air is modelled in the cathode channels and diffusion and convection of liquid (anode) and gaseous species (cathode) in the porous transport layers. The MeOH flow in the membrane is described with diffusion and protonic drag. The parameter ψ in the model describes the MeOH oxidation rate at the cathode and it is fitted according to the measured current distributions. The model describes the behaviour of the free-breathing DMFC, when different operating parameters such as cell temperature, MeOH concentration and flow rate are varied in a wide range. The model also predicts the existence of the experimentally observed electrolytic domains, i.e. local regions of negative current densities. Altogether, the developed model is in reasonable agreement with both the measured current distributions and polarization curves. The spatial information gained of mass transfer phenomena inside the DMFC is valuable for the optimization of the DMFC operating parameters.

SUS304鋼に対する純水のミクロな濡れ性：塩水浸漬の影響

The surface composition and micro-wettability of SUS304 steel by pure water before and after immersion in 3.5 mass% NaCl aqueous solution were investigated at room temperature. The surface composition was analyzed using X-ray photoelectron spectroscopy(XPS). Anode and cathode current distribution measurements on specimen surfaces in NaCl aqueous solution were performed with scanning vibration electrode equipment(SVE). The wetting morphologies of micro-pure water on specimen surfaces were observed and wettability was evaluated with an atomic force microscope(AFM) in AC non-contact mode. The relationship between micro-wetting behavior of pure water and the surface composition of SUS304 steel are discussed.The results obtained from XPS shows that considerable organic contamination is present on each surface, and oxides and hydroxides of iron, chromium and nickel formed on the specimen surfaces after wet polishing. The amount of oxide and hydroxide of chromium in the oxide (hydroxide) layer increased, while those of iron and nickel decreased after 1.8×106 s immersion in NaCl aqueous solution. The specimen surface became rougher after immersion, due to corrosion pitting. SVE measurements revealed that the ratio of anodic to cathodic area changed with immersion time, which indicates that oxides and hydroxides were resolved or formed, and local corrosion occurred. The AFM observation and evaluation showed that the wettability of SUS304 steel was reduced after immersion in NaCl solution.

Current efficiency in the Hall–He´roult process for aluminium electrolysis: experimental and modelling studies

Results are presented from a laboratory study of the influence of electrolyte composition, temperature, cathodic current density and interpolar distance on the current efficiency with respect to aluminium (CE). The current efficiency was determined from the weight gain of metal, in a laboratory cell designed to attain good and reproducible convective conditions, and with a flat cathode surface which ensures uniform cathodic current distribution. The cell is believed to more closely represent conditions in industrial cells than traditional small-scale cells, and is a good basis for an experimental study of the influence of isolated variable parameters on the current efficiency with respect to aluminium. The results show a nonlinear decrease of CE with increasing electrolyte temperature, a close to linear decrease of CE with increasing NaF/AlF3 ratio in the electrolyte, a slight increase of CE with increasing electrolyte CaF2 concentration, and no influence of electrolyte Al2O3 concentration on CE. A current efficiency model, based on previous work and theory of electrochemistry and mass transport, shows good agreement with the obtained results.

Modelling Secondary Current Distribution in Electrodeposition Cells with the Probabilisitic Method. Part 2—Variation of Secondary Current Distribution on Scaling up Laboratory Size Model Cells

SummaryGeneral problems involved in scaling up laboratory size model cells to prototype or commercial size units are discussed and the specific effect of current distribution variation is considered. The probabilistic method is used as a tool to obtain example simulations which directly indicate the effect that altering the scale of a cell has on the cathodic secondary current distribution.

The axial dependence of the cathode current density in the longitudinal hollow cathode discharge

In the present paper the axial dependence of the cathode current distribution in the longitudinal hollow cathode discharge is investigated. On the basis of experimental results the model of the longitudinal hollow cathode discharge is developed and axial distribution of the cathode current is discussed.

Evaluating the Effect of Auxiliary Anodes on Coating Thickness Distribution in Eelectrodeposition Cells using a Probabilistic Method

SummaryThe probabilistic method of determining coating thickness distribution in electrodeposition cells is theoretically extended to deal with the introduction of auxiliary anodes. Equations are derived that enable cathode current distribution to be determined when the auxiliary anode potential is the same as that of the main anode and also when its potential is set to a different value. Computer simulations are carried out to examine the effect of auxiliary anode potential and position when electrodepositing into recesses and onto peaks, and to determine optimum configurations. The computed results obtained are corrected for current efficiency variation with current density and compared to experimentally obtained thickness distributions o f chromium deposits.

Current Distribution in a Parallel‐Plate Reactor: A Comparison of Steady and Sinusoidal Cell‐Voltage Control

A time‐varying electrodeposition in a parallel‐plate reactor is modeled to study the effect of sinusoidal cell‐voltage control on the uniformity of the cathodic current distribution. An analytic expression for the electrolyte potential was coupled through Butler‐Volmer kinetics, for the same metal reaction at the anode and cathode, to a finite‐element solution of the transient convective‐diffusion equation. The latter included both streamwise and transverse diffusion terms, with well‐developed laminar flow assumed to be occurring in the reactor. At low Peclet number, streamwise diffusion significantly increases the current in comparison to that in its absence and effects a more symmetric current distribution. With increasing Peclet number at a constant cell voltage, the current may pass through a minimum, and the deposit nonuniformity through a maximum, with both effects resulting from the decoupling of cathode and anode diffusion boundary layers. Deposit nonuniformity increases with the ratio of the interelectrode distance to electrode length for both periodic and steady control. The uniformity of the deposit increases with frequency at a given bias and amplitude of a sinusoidal cell‐voltage oscillation. At cycle‐averaged currents which are a large fraction of the mass‐transfer limiting current, sinusoidal cell‐voltage control can result in a more uniform deposit than steady control, for select values of the control waveform. However, at a low value of the dimensionless exchange current density and a small fraction of the limiting current, sinusoidal voltage control causes a more nonuniform deposit than steady control.