Rotating hemisphere

Theory and Experimental Aspects of the Rotating Hemispherical Electrode... 

The rotating hemispherical electrode (RHSE) was originally proposed by the author in 1971 as an analytical tool for studying high-rate corrosion and dissolution reactions [13]. Since then, much work has been published in the literature. The RHSE has a uniform primary current distribution, and its surface geometry is not easily deformed by metal deposition and dissolution reactions. These features have made the RHSE a complementary tool to the rotating disk electrode (RDE). 

The above problems may be alleviated by the use of a rotating hemispherical electrode (RHSE). In this geometry, the flat circular disk on the RDE is replaced with a metal hemisphere as shown in Fig. 1(a). The theory, experimental setup and methods of application to electrochemical studies are similar to those of the RDE. The advantages of the RHSE are ... 

This article presents a brief account of theory and practical aspects of rotating hemispherical electrodes. The fluid flow around the RHSE, mass transfer correlations, potential profile, and electrochemical application to the investigations of diffusivity, reaction rate constants, intermediate reaction products, passivity, and AC techniques are reviewed in the following sections. 

Fig. 1. Flow near a rotating hemisphere electrode. (a) Dye movement at Re = 1300. (b, c) Spiral flow patterns etched on a copper hemisphere.

Fig. 3. Numerical values of A and a for the solution of turbulent flow boundary layer on a rotating hemisphere. The value of meridional angle, 9, is given in degrees. From [22].

To consider the convective mass transfer problem of a rotating hemisphere electrode, we assume that sufficient inert salts are present in the electrolyte that the migrational... 

Eqs. (40)-(45) describe the behavior of a mass transport process occurring at very large Schmidt numbers. For finite values of Sc, Chin [16] extended the 0-series expansion analysis and obtained the following asymptotic series for Sfiav on a rotating hemisphere ... 

For the flow induced by the rotating hemisphere, the leading edge of the flow boundary layer occurs at the pole of rotation. The surface distance, x, from the leading edge is equal to ad. One may also take aQsinff as the characteristic velocity for every local point on the spherical surface. Thus, the quantities, Shx, Rex, and fx can be expressed as... 

For turbulent flow, the local wall shear stress, xr, is given by Eq. (25). Substituting Eqs. (48H50) into Eq. (47) and making use of Eq. (25), one arrives at an expression for the Sherwood number based upon the radius of the rotating hemisphere ... 

Fig. 7. Current distribution on a rotating hemispherical electrode at high rotational speed. From [47].

The experimental setup of a rotating hemispherical electrode (RHSE) is similar to that of a rotating disk electrode [50]. The basic system consists of a removable hemispherical electrode, and a variable speed rotator equipped with a provision, such as the slip-ring contact, to make electric connection to the hemispherical electrode during the experiments. 

Fig. 8(a) shows the design of a rotating hemisphere electrode used in the author s laboratory [14], It is composed of a hemisphere electrode, an arcylic support rod, and a tappered brass holder to be attached to a high speed rotator [Fig. 8(b) ]. The electrode is machined into the form of a metal screw with a hemispherical head, and is threaded into the inert acrylic support rod of a larger radius. The design has the advantages that... 

Fig. 8. Construction of a rotating hemisphere electrode (a) and cell setup (b). From [14].

Fig. 9. Types of rotating spherical electrodes reported in the literature, (a) Rotating micro-sphere electrode, (b, c) rotating hemisphere electodes (d) rotating ring-hemisphere electrodes (e) rotating dropping mercury electrode.

The rotating hemisphere electrode has been used to investigate the effect of AC on the electrodissolution and deposition reactions of zinc in zinc chloride [25] and copper in acid copper sulfate solutions [55], AC was found to increase the rate of nucleation and produce more uniform deposit on the zinc electrode. The corrosion of an iron rotating hemisphere in dilute sulfuric acid was investigated by Haili [31] using the AC impedance measurement. 

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. 

In this section, we consider mass transport-controlled currents to disc and concentric ring electrodes on a planar spinning disc surface. For other less common rotating electrodes, e.g. rotating hemisphere, see Table 3. 

Besides the RDE and DME, other uniformly accessible electrodes under laminar flow have been described. They include the rotating hemispherical and rotating cone electrodes, which were developed to obviate the problem of trapped gas bubbles at the centre of an RDE their use is not widespread. 

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