Investigation of effect of electrode polarity on electrochemistry and magnetohydrodynamics using tertiary current distribution in electroslag remelting process

Transport phenomena including the electromagnetic, concentration of ions, flow, and thermal fields in the electroslag remelting (ESR) process made of slag, electrode, air, mold, and melt pool are computed considering tertiary current distribution. Nernst–Planck equations are solved in the bulk of slag, and faradaic reactions are regarded at the metal–slag interface. Aiming at exploring electrochemical effects on the behavior of the ESR process, the calculated field structures are compared with those obtained using the classical ohmic approach, namely, primary current distribution whereby variations in concentrations of ions and faradaic reactions are ignored. Also, the influence of the earth magnetic field on magnetohydrodynamics in the melt pool and slag is considered. The impact of the polarity of electrode, whether positive, also known as direct current reverse polarity (DCRP), or negative, as known as direct current straight polarity (DCSP), on the transport of oxygen to the ingot of ESR is investigated. The obtained modeling results enabled us to explain the experimental observation of higher oxygen content in DCSP than that of DCRP operated ESR process.

(ECS 236th) Multiphysics Modeling of the Tertiary Current Distribution in Pulse/Pulse-Reverse Electrochemical Processing of Microstructured Workpieces

The physical phenomena governing the current distribution on an electrode of arbitrary shape are typically categorized as falling into primary, secondary, and/or tertiary effects. Primary current distribution effects are defined by the geometry of the system and the electrical properties of the relevant materials, whereas secondary and tertiary effects incorporate additional position-dependent polarizations that respectively arise from electrochemical-kinetic and mass-transfer/concentration physics. In industrial electrochemical processes, the uniformity of the current distribution across a workpiece is of vital concern. In electrodeposition processes, for example, it is usually desirable for the deposited metal to be as uniformly distributed as possible, regardless of the form of the workpiece. Conversely, in electropolishing processes, it is critical to focus the current density onto the tops of asperities on the workpiece surface, in a highly non-uniform fashion, in order to minimize material removal irrelevant to the goal of decreased surface roughness. In general, the primary current distribution leads to the most non-uniform current distribution possible for a given geometry, from which the secondary and tertiary effects tend to have varying degrees of a “leveling” effect, leading to a comparative increase in processing uniformity.In electrodissolution processes, saturation of the dissolved metal at the workpiece surface is an important mechanism by which the tertiary current distribution effects influence practical electrochemical processes. This saturation phenomenon leads both to an increase in the local overpotential, via concentration polarization, and also has the potential to occlude locally a fraction of the workpiece exposed area due to the formation of insoluble precipitates. As noted, both of these effects tend to increase the uniformity of the resulting overall current distribution, and thus it is important to be able to predict, even if approximately, when a given process will be operating in this regime and to what extent the uniformity of the current distribution might be affected.This talk will summarize results from multiphysics simulations developed to represent this occluded-surface aspect of the tertiary current distribution, in addition to primary and secondary current distribution effects. These simulations incorporate pulse/pulse-reverse waveforms applied to workpieces with structured surfaces, in an attempt to approximate a surface finishing application of industrial relevance. In particular, focus was placed on simulating a “microprofile,” the scenario where surface structures have characteristic dimensions much smaller than the hydrodynamic boundary layer for mass transfer—this choice simplifies the modeling by obviating consideration of the macroscopic fluid dynamics of the system. The effect of pulse waveform parameters on the uniformity of the overall current distribution will be discussed, and simulation results will be shown illustrating the tendency of suitably-chosen waveform parameters to “collapse” toward the workpiece surface the subdomain of the boundary layer in which the local concentration of dissolved material oscillates significantly in response to the applied electric field.

Molten Salt Electrometallurgy

The experimental results are for electro-deoxidation in molten salt and the numerical results are for molten salt processes in electrometallurgy.<div>The internal cathode microstructure is analysed using SEM, Energy dispersive X-ray spectroscopy, and computerised X-ray tomography.<br></div><div>Numerical simulations using COMSOL multiphysics report results for molten salt processes using theory from rotating disk electrodes and multiphase flow. Scale-up of electro-deoxidation is discussed using primary current distribution simulations in various electro-deoxidation cell designs.<br></div><div>Experimental details of electro-deoxidation cell construction is referenced to and outlined in my earlier work: <i>Electrochim. Acta</i> <b>164</b>, 48 (2015)<br></div>

Molten Salt Electrometallurgy

The experimental results are for electro-deoxidation in molten salt and the numerical results are for molten salt processes in electrometallurgy.<div>The internal cathode microstructure is analysed using SEM, Energy dispersive X-ray spectroscopy, and computerised X-ray tomography.<br></div><div>Numerical simulations using COMSOL multiphysics report results for molten salt processes using theory from rotating disk electrodes and multiphase flow. Scale-up of electro-deoxidation is discussed using primary current distribution simulations in various electro-deoxidation cell designs.<br></div><div>Experimental details of electro-deoxidation cell construction is referenced to and outlined in my earlier work: <i>Electrochim. Acta</i> <b>164</b>, 48 (2015)<br></div>

Theoretical and Experimental Study of Copper Electrodeposition in a Modified Hull Cell

The primary current distribution and the resistance of a modified Hull cell are calculated by using conformal mapping technique coupled with numerical evaluation of the resulting integral equations. An approximate analytical expression for the primary current distribution of a modified Hull cell is presented. The primary current distribution along the cathode surface is noticed varying in controlled manner as a function of position on the substrate. The current distributions (primary, secondary, and tertiary) in the cell have also been calculated at different applied average current densities (2, 4.1, and 8.2 mA cm−2) through numerical simulation by using finite element based software. The numerical simulation result of the primary current distribution is then compared with the analytical solution and a good match is found. Experimentally, single Cu metal electrodeposition is carried out at different applied average current densities (2, 4.1, and 8.2 mA cm−2) in a modified Hull. The current distribution (primary, secondary, and tertiary) results obtained from the numerical simulation are compared with the experimental results and a satisfactory match is found. Surface morphology of the Cu deposits is examined using scanning electron microscopy (SEM).

On the Mathematical Modelling of Diffusion-Migration Mass Transport of Protons

A one-dimensional-isothermal model and an analytical solution for the mass transport (migration and diffusion) of protons in a single PEM fuel cell have been developed. Under a linear approximation between current density and electric potential (pseudo-primary current distribution) we have obtained the proton concentration profile along the cathodic and anodic gas diffusers as a function of the position. The graphical shape of the function allows us to discuss the effect of the physicochemical parameters in our model especially on the anodic side of the diffuser. From the dependence of the electric potential and current density with the position we have obtained a theoretical polarization curve. Under the pseudo-primary current distribution, the fitting of the first points yields a high Tafel slope for the cell operation performance. In spite of being a limited model for the cell performance, this is one of the least analytical and exact solved solutions obtained from mass balances in the course of the hydrogen electrooxidation and oxygen electroreduction. We consider that this situation would probably put some more knowledge in the understanding of the PEMFC functional description.