Sacrificial anode advantages

The use of an impressed-current system or sacrificial anodes will both provide satisfactory cathodic protection, but each has advantages and disadvantages with respect to the other (Table 10.24). 

Cobaloxime(I), electrochemically regenerated from chloro(pyridine)-cobaloxime (III) (232), has been employed as a mediator in the reductive cleavage of the C—Br bond of 2-bromoalkyl 2-alkynyl ethers (253), giving (254) through radical trapping ofthe internal olefin (Scheme 95) [390]. An interesting feature of the radical cyclization (253) (254) is the reaction in methanol, unlike the trialkyltin hydride-promoted radical reactions that need an aprotic nonpolar solvent. An improved procedure for the electroreductive radical cyclization of (253) has been attained by the combined use of cobaloxime(III) (232) and a zinc plate as a sacrificial anode in an undivided cell [391]. The procedure is advantageous in terms of the turnover of the catalyst and the convenience of the operation. 

The advantage of using a sacrificial anode has been clearly pointed out. Magnesium was found to be the most convenient, the oxidation of which produces Mg ions which can enter the catalytic cycle to cleave the nickela-cycle intermediate and liberate Ni for further catalytic cycles (Scheme 7). Such a mechanism has been substantiated on the basis of the formation of the nickelacycle and its characterization by cyclic voltammetry. In the absence of Mg (reactions conducted in a divided cell in the presence of ammonium ions) the nickelacycle does not transform and the reaction stops when all the starting nickel compound has been reacted. Upon addition of MgBr2 to an electrochemically prepared solution of the nickelacycle, Ni(II) is recovered [114]. 

The separator is often the weakest component in any electrochemical cell. There are also difficulties in employing ion-exchange diaphragms in aprotic media. Particularly with large industrial cells, it is advantageous to devise reaction conditions that allow the use of an undivided cell. One solution to these problems for an electrochemical reduction process employs a sacrificial anode of magnesium, alumin-... 

In general, electrocarboxylation reactions are carried out in aprotic solvents such as acetonitrile (ACN), N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP) in a one-compartment cell by the use of sacrificial anodes (Al or Mg), as the use of these systems generally provide important advantages [7, 10-12] that include ... 

This method uses a more active metal than that in the structure to be protected, to supply the current needed to stop corrosion. Metals commonly used to protect iron as sacrificial anodes are magnesium, zinc, aluminum, and their alloys. No current has to be impressed to the system, since this acts as a galvanic pair that generates a current. The protected metal becomes the cathode, and hence it is free of corrosion. Two dissimilar metals in the same environment can lead to accelerated corrosion of the more active metal and protection of the less active one. Galvanic protection is often used in preference to impressed-current technique when the current requirements are low and the electrolyte has relatively low resistivity. It offers an advantage when there is no source of electrical power and when a completely underground system is desired. Probably, it is the most economical method for short life protection. 

One example of the application of polarization curves in a predictive manner involves their use in galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are in electrical and ionic contact as is schematically shown in Fig. 29. Galvanic corrosion is used to advantage in sacrificial anodes of zinc in seawater and magnesium in home water heaters. It slows corrosion of millions of tons of structural materials. The darker side of galvanic corrosion is that it also causes major failures by the accelerated dissolution of materials that are accidentally linked electrically to more noble materials. 

The advantages of the electrochemical pathway are that contamination with byproducts resulting from chemical reduction agents are avoided and that the products are easily isolated from the precipitate. The electrochemical preparation also provides a size-selective particle formation. Reetz et al. have conducted several experiments using a commercially available Pd sheet as the sacrificial anode and the surfactant as the electrolyte and stabilizer. Analysis of the (C8Hi7)4N Br-stabilized Pd ° particles produced have indicated that the particle size depends on such... 

In the standard chemical preparation methods, the properties, especially the size and size distribution of the nanoparticles, are defined by the choice of the reaction conditions, reactant concentrations, etc. The use of electrochemical techniques to generate nuclei has the advantage that the supersaturation is determined by the applied potential or current density. Thus, the size of the particles can be controlled by electrochemical instrumentation rather than by changing the experimental conditions. Reetz and Helbig [115] demonstrated how electrochemical methods can be used to produce metal colloids of nanometer size and more importantly how particle size can be controlled in a simple manner by adjusting the current density [159]. First, a sacrificial anode was used as the source of the metal ions, which were then reduced at the cathode. Later, a more general approach was introduced, where metal salts were used as the starting material [160]. The particles were stabilized by alkylammonium or betaine salts. With a suitable choice of surfactants, the electrochemical method can be applied in the preparation of different shapes of particles, e.g., nanorods [161]. 

The advantages of using sacrificial anode CP technique include... 

Sacrificial-anode-type cathodic protection systems have a number of advantages ... 

Aluminium and magne.sium and their alloys are also used in sacrificial anode cathodic protection systems. One advantage of these alloys is... 

The advantage of this technique over the impressed current is that it does not require a power supply since the structure and the anode are coupled through a wiring system or by mounting the anode on the structure forming a galvanic cell. Table 8.2 lists relevant data for common sacrificial anode materials used... 

Examples of impressed urrent, cathodic protection are illustrated in Fig. 10.29. Each method of cathodic protection has its own strengths and weaknesses. Sacrificial anodes have the following advantages ... 

Zinc used as a sacrificial material should be characterized by high purity (99.99% Zn, less than 0.003% Fe). The presence of impurities such as iron, copper, and lead very negatively affects the work of a sacrificial anode. They cause passivation of the surface of zinc as a result of which the polarization current decreases in the protection system and the current output is decreased. In order to improve the sacrificial properties of zinc, small amounts of alloy additives are introduced. The following have an advantageous effect aluminum (0.1-0.5% Al) and cadmium (0.02-0.15% Cd), and aluminum (0.5% Al) and silicon (0.1% Si). 

Sacrificial anodes can be installed as single anodes or in groups. In practice, sacrificial anodes are placed relatively close to the cathode (protected structure) to decrease the resistance of the electric circuit. In water, low potential sacrificial anodes can be mounted directly (through an insulation washer) on the protected surface, while it is better to place high potential sacrificial anodes on appropriate supports at some distance (e.g., 0.6 m) from the cathode, which has an advantageous effect on the potential distribution. In soil, the method of sacrificial anode installation depends on many local factors, e.g.,... 

Sacrificial anodes are relatively inexpensive, easy to install, and in contrast to impressed current systems, can be used where there is no power supply. The method has the added advantage that there is no expensive electrical equipment to buy and current cannot be supplied in the wrong direction. Sacrificial anodes are very suitable in small-scale applications (Fig. 13.4), though they are also used extensively and with equal effect on large-scale structures (Fig. 13.5). 

This sacrificial anode system does not require the use of an external power source as in the impressed current system. A schematic description is shown in Fig. 12.16. Use is made of zinc and magnesium anodes which corrode and supply electrons to steel bars embedded in concrete. The current flow circuitry is same as in the impressed current system. The anode life is, however, shorter than the life of inert anode. As the current generated by the corroding anode is a function of environment, such as temperature and moisture, it is difficult to adjust and control the current. However, a major advantage is that the risk of over-protection which is inherent in impressed ourrent system is minimized. i.e. operator dependence is removed, advantageously. The galvanic... 

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