Rotating electrodes

Eig. 5. Concentration profiles at a rotating electrode or in a stirred solution at appHed potentials Hq, and where the dotted line represents the Nernst... 

Releasing agents 793 Replicate determinations number of, 142 reliability of, 137 Residual current 595 Resistance 504 Resistivity 519 Resonance line sources 790 Results, comparison of 139 Reverse osmosis 90 Reversible back e.m.f. 505 Rf values 234 Rotated electrode 511 R.U. powder 766, 772... 

Eredictions. A rotating cyhnder within a cyhnder electrode test system as been developed that operates under a defined hydrodynamics relationship (Figs. 25-15 and 25-16). The assumption is that if the rotating electrode operates at a shear stress comparable to that in plant geometry, the mechanism in the plant geometty may be modeled in the laboratory. Once the mechanism is defined, the appropriate relationship between fluid flow rate and corrosion rate in the plant equipment as defined by the mechanism can be used to predict the expected corrosion... 

In order to asses the analytical aspects of the rotating electrodes we must consider the convective-diffusion processes at their bottom surface, and in view of this complex matter we shall confine ourselves to the following conditions (1) as a model of electrode process we take the completely reversible equilibrium reaction ... 

The aforementioned experiments at rotating electrodes concerned merely steady-state conditions so-called transients123 at these electrodes, e.g., with potential or current steps, as well as with hydrodynamic modulation, i.e., variation of co with time, are, as a consequence of their non-steady-state conditions, less important in analysis and therefore will not be treated here. 

As titrations require rapid and complete chemical reactions, it is mandatory to work under hydrodynamic conditions hence, with stationary electrodes, which are often used in voltammetric titrations, the solution is stirred by a mechanical or magnetic stirrer with rotating electrodes an additional stirring may sometimes be superfluous. 

This value represents the upper limit of a first order reaction rate constant, k, which may be determined by the RHSE. This limit is approximately one order of magnitude smaller that of a rotating electrode. One way to extend the upper limit is to combine the RHSE with an AC electrochemical technique, such as the AC impedance and faradaic rectification metods. Since the AC current distribution is uniform on a RHSE, accurate kinetic data may be obtained for the fast electrochemical reactions with a RHSE. 

The dependence of the limiting current density on the rate of stirring was first established in 1904 by Nernst (N2) and Brunner (Blla). They interpreted this dependence using the stagnant layer concept first proposed by Noyes and Whitney. The thickness of this layer ( Nernst diffusion layer thickness ) was correlated simply with the speed of the stirring impeller or rotated electrode tip. 

Assuming laminar flow to the rotating electrode surface, i.e. the rotation rate is not sufficiently fast to cause turbulence, Karman and Cochran solved the pattern of flow and showed that the streamlines are as in Figure 2.89. The rotating disc sucks solution up towards it from the bulk and flings it out centrifugally. The layer of liquid immediately adjacent to the surface is... 

A correlation between the spacing of striae and convection downstream of protrusions does not fully describe the process. The initial protrusions arise far from transport control and cannot be attributed to a diffusive instability of the type described in the previous section. Jorne and Lee proposed that striations formed on rotating electrodes by deposition of zinc, copper and silver are generated by an instability that arises only in systems in which the current density at constant overpotential decreases with increasing concentration of metal ion at the interface [59]. 

Mart et al. [793] and Valenta et al. [794] have described two differential pulse ASV methods for the determination of cadmium, lead, and copper in arctic seawater. After a previous plating of the trace metals into a mercury film on a rotating electrode with a highly polished glassy carbon as substrate, they were stripped in the differential pulse mode. The plating was done in situ. 

Rotating electrode cell Wilson Process Systems Rotating cylindrical foil (usually stainless steel) or static cylindrical foil with rotating anode (larger cells) Discontinuous by manual scraping or flexing No V V ... 

Despite the importance of the ORR and long history of study, very little is known about the reaction mechanism.126,130,131 Mechanistic information has been derived almost exclusively from rotating disk electrode (RDE)131,132 and rotating ring disk electrode (RRDE)133-136,62,128 studies. The rotating electrode minimizes mass transfer effects and allows a kinetic current density to be extracted. In the RRDE setup, the ring surrounding the disk electrode detects species weakly adsorbed to the electrode that are ejected due to electrode rotation. The ORR reaction (eqn 4) is... 

Numerous atomization techniques have evolved for the production of metal/alloy powders or as a step in spray forming processes. Atomization of melts may be achieved by a variety of means such as aerodynamic, hydrodynamic, mechanical, ultrasonic, electrostatic, electromagnetic, or pressure effect, or a combination of some of these effects. Some of the atomization techniques have been extensively developed and applied to commercial productions, including (a) two-fluid atomization using gas, water, or oil (i.e., gas atomization, water atomization, oil atomization), (b) vacuum atomization, and (c) rotating electrode atomization. Two-fluid atomization... 

Rotating Electrode Atomization (REP, PREP) -20 Standard deviation 1.3-1.5 Armco Fe, Cu, Al, Zn, Co-Cr, Ti, Zr, Ni alloys. Low carbon steels SlO2 1-10 -0.04 Spherical, very smooth, ultraclcan particles, Relatively high EE High cost, Low capacity and volume, D Relatively Coarse particles I... 

Rotating electrode atomization process has been described in detail by many researchers.[183][184] As illustrated in Fig. 2.20, in the... 

Rotating electrode atomization may be applied to almost all metals and alloys since it does not require a crucible for melting and/ or pouring. In particular, high melting-temperature metals and alloys, such as Ti and Zr, are well suited for the process. However, the production cost is still a drawback associated with the process, since electrode production is generally more expensive than a metal melt. In addition, production rates are relatively low compared to other atomization processes such as gas atomization and water atomization. 

In the VEP, currents used are between 600 and 1200 A at potentials between 30 and 60 V. The vibration frequency of the wire electrode is up to 500 Hz. The materials atomized via VEP include mild steel, Cr-Ni steel, Cu-Ni alloy and tungsten. The VEP is carried out in an inert atmosphere (typically argon) for most alloys, but the arc is struck under water for tungsten wire. Wire diameter is 1-4 mm, and its feed rate is 1.7-4.3 m/min. The feed rate and current density must be determined properly according to the relationship between these two variables. At lower current densities, the wire electrode tends to stick to the rotating electrode. At higher current densities, the wire electrode becomes overheated, causing it to bend or even rupture. 

Droplet Formation in Centrifugal Atomization. The mechanisms of centrifugal atomization of liquid metals are quite similar to those for normal liquids. Three atomization modes have been identified in rotating electrode atomization process, i.e., (I) Direct Droplet Formation, (2) Ligament Disintegration, and (3) Film/Sheet Disintegration.1[189][32°] are aiso applicable to the centrifugal atomiza-... 

Table 4.20. Correlations for Droplet Sizes of Liquid Metals in Rotating Electrode Atomization (REP)...

---