Rotating electrode process

GMAW gas metal arc welding PREP plasma rotating electrode process... 

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 ... 

Electrode processes are often studied under steady-state conditions, for example at a rotating disk electrode or at a ultramicroelectrode. Polarog-raphy with dropping electrode where average currents during the droptime are often measured shows similar features as steady-state methods. The distribution of the concentrations of the oxidized and reduced forms at the surface of the electrode under steady-state conditions is shown in Fig. 5.12. For the current density we have (cf. Eq. (2.7.13))... 

With its axisymmetric transport and current distribution, the rotating hemispherical electrode complements the rotating disk as a tool for studying electrode processes. Der-Tau Chin provides a valuable overview and summary of the fundamental theory and applications of this interesting device. 

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]. 

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 ... 

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 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. 

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-... 

Fig. 1.6 Two different immersion cell designs optimized for special applications, (a) Set-up for fast removal and rinsing of a strip-shaped electrode by fast rotation of the shaft (solid arrow). This set-up is useful for measurements of transient electrode processes like...

None of the set-ups discussed so far provides stirring of the electrolyte for bubble removal or for enhancement of the reaction rates. A standard set-up developed to study kinetic electrode processes is the rotating disc electrode [11]. The electrode is a small flat disc set in a vertical axle. The hydrodynamic flow pattern at the disc depends on rotation speed and can be calculated. An additional ring electrode set at a different potential provides information about reaction products such as, for example, hydrogen. However, because this set-up is designed to study kinetic processes and is usually equipped with a platinum disc, it becomes inconvenient if silicon samples of different geometries have to be mounted. 

Since products of the electrode process are quickly transported out of the vicinity of the electrode disk, use of the rotating disk electrode complements the more complex rotating ring disk electrode (RRDE) [32]. Here, redox active products can be detected at the ring electrode, which is held at a separately controlled potential. 

The electrode processes on the voltammetric and the preparative electrolysis time scales may be quite different. The oxidation of enaminone 1 with the hydroxy group in the ortho position under the controlled potential electrolysis gave bichromone 2 in 68% yield (Scheme 4.) with the consumption of 2.4 F/mol [21], The RDE voltammogram of the solution of 1 in CH3CN-O.I mol/1 Et4C104 showed one wave whose current function, ii/co C, was constant with rotation rates in the range from 1(X) to 2700 rpm and showed one-electron behavior by comparison to the values of the current function with that obtained for ferrocene. The LSV analysis was undertaken in order to explain the mechanism of the reaction which involves several steps (e-c-dimerization-p-deamina-tion). The variation of Ep/2 with log v was 30.1 1.8 mV and variation of Ep/2 with logC was zero. Thus, our kinetic data obtained from LSV compare favorably with the theoretical value, 29.6 mV at 298 K, for a first order rate low [15]. This observation ruled out the dimerization of radical cation, for... 

Rotating-Disk Electrode, Study of the kinetics and mechanism of electrode processes under well-defined mass transport conditions is possible through use of methods of the rotating-disk electrode (RDE). The RDE consists of a disk of metal embedded in a cylindrical insulator (e.g., Teflon) holder (see Fig. 6.24). It is rotated about its center. Only the bottom end of the metal disk is exposed to the solution. 

The mass transport rate coefficient, kd, for a RDE at the maximum practical rotation speed of 10000 per min"1 is approximately 2 x 10-2 cms-1 [28], which sets a limit of about 10 3 cms 1 for the electrode reaction kinetics. For the study of very fast electrode processes, such as some outer sphere redox reactions on noble metal electrodes under stationary conditions, higher mass transport rates in the solution adjacent to the electrode must be employed. 

Hydrodynamic modulation has been performed almost exclusively at the rotating disc electrode. It has found use for analytical purposes at rotating and tubular electrodes owing to the fact that non-convectively dependent electrode processes are unaffected by the modulation [236]. 

Reversible, quasi-reversible and irreversible electrode processes have been studied at the RDE [266] as have coupled homogeneous reactions without [267] and with the effect of electrode kinetics [268], The theoretical results are very similar to those of a.c. polarography, being very phase-angle sensitive to coupled chemical reactions in the rotation speed range where convection can be neglected, the polarographic results may be directly applied [269]. 

Study of the charge transfer processes (step 3 above), free from the effects of mass transport, is possible by the use of transient techniques. In the transient techniques the interface at equilibrium is changed from an equilibrium state to a steady state characterized by a new potential difference A. The analysis of the time dependence of this transition is a basis of transient electrochemical techniques. We will discuss galvanostatic and potentiostatic transient techniques. For other techniques [e.g., alternating current (ac) and rotating electrodes], the reader is referred to references in the Further Reading list. 

For the investigator who wants to study electrode processes at depth, a number of more physically oriented methods are available, such as double layer capacitance measurements19 rotating disc and ring disc techniques 25 and radio-. active tracer methods 40a Spectroscopical methods in conjunction with optically transparent electrodes can be used for the study of intermediates 40b), as can also total reflectance spectroscopy 40c). 

The method of resolution of (8.1) was indicated in Sections 5.7-5.9, showing as an example the calculation of the limiting current at the rotating disc electrode. In this chapter we discuss this and other hydrodynamic electrodes used in the study of electrode processes. The rotating disc electrode has probably been the hydrodynamic electrode... 

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