Sacrificial anode technique

Using the impressed-current technique the driving voltage for the protec-ive current comes from a d.c. power source. The sacrificial anode technique... 

The sacrificial anode technique appears to be competitive with regard to the other electrochemical methods [19] to create the silicon-silicon bond and is very promising for the replacement of alkali metals in many syntheses, especially when high selectivity is required. 

The current and potential fields are uniform around the buried pipeline. Thus, the potential and current field gradients are quantitatively determined in either impiessed-cunent or sacrificial anode techniques. This assumption is not realistic since the complexity of real engineering stmc-tures and heterogeneity of the soil or seawater electrolytes preclude exact theoretical calculations. The margin of error between the fteoretical and experimental results depends on the assumptions considered to develop a particular mathematical model... 

Electrochemical synthesis of tetra and pentasilanes. Larger polysilane oligomers could also be obtained using the aluminum sacrificial anode technique. Thus, the electrolysis of 1-chloro-l-phenyltetramethyldisilane, electrochemically prepared as described above, led to the corresponding tetrasilane in 85 % yield (Equation 14). 

It is interesting that the first large-scale application of cathodic protection by Davy was directed at protecting copper rather than steel. It is also a measure of Davy s grasp of the topic that he was able to consider the use of two techniques of cathodic protection, viz. sacrificial anodes and impressed current, and two types of sacrificial anode, viz. zinc and cast iron. 

There are two principal methods of applying cathodic protection, viz. the impressed current technique and the use of sacrificial anodes. The former includes the structure as part of a driven electrochemical cell and the latter includes the structure as part of a spontaneous galvanic cell. 

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. 

Cathodic protection is an electrochemical technique of providing protection from corrosion [38]. The object to be protected is made the cathode of an electrochemical cell and its potential driven negatively to a point where the metal is immune to corrosion. The metal is then completely protected. The reaction at the surface of the object will be oxygen reduction and/or hydrogen evolution. Cathodic protection may be divided into two types, that produced using sacrificial anodes and the second by impressed current from a d.c. generator [39]. 

The formation of silicon-silicon bonds constitutes the key step in these syntheses. To avoid the use of sodium, a simple, inexpensive, and practical electrochemical technique using an undivided cell, a sacrificial anode, and a constant current density has been developed allowing a facile synthesis of di-, tri-, or polysilanes including polydimethylsilane. 

Reductive silylation of mono- and polyhalothiophens has also been achieved using an Al sacrificial anode [Eq. (12)] [59-61]. This reaction provides a convenient method for the preparation of 2,5-bis(trimethylsilyl)thiophene, which serves as a good precursor of poly thiophene. The sacrificial Mg and Al anode technique was also successfully applied to the reductive silylation of bromopyrroles [62]. 

The steel hulls of ships are constantly in contact with saltwater, so the prevention of corrosion is vital. Although the hull may be painted, another method is used to minimize corrosion. Blocks of metals, such as magnesium, aluminum, or titanium, that oxidize more easily than iron are placed in contact with the steel hull. These blocks rather than the iron in the hull become the anode of the corrosion cell. As a result, these blocks, called sacrificial anodes, are corroded while the iron in the hull is spared. Of course, the sacrificial anodes must be replaced before they corrode away completely, leaving the ship s hull unprotected. A similar technique is used to protect iron pipes that are run underground. Magnesium bars are attached to the pipe by wires, and these bars corrode instead of the pipe, as shown in Figure 21-15b. 

Cathodic protection is an anticorrosion technique widely used in ships and in buried or submerged pipe work. This method seeks to reduce the rate of corrosion of the structure to be protected by joining it to sacrificial anodes. In other words, the structure is joined to another metal (an anode) that corrodes more readily, effectively diverting the tendency to corrode away from the structure. 

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]. 

In some instances, galvanic corrosion can be helpful in the plant. For example, if pieces of zinc are attached to the bottom of a steel water tank, the zinc will become the anode, and it will corrode. The steel in the tank becomes the cathode, and it will not be effected by the corrosion. This technique is known as cathodic protection. The metal to be protected is forced to become a cathode, and it will corrode at a much slower rate than the other metal, which is used as a sacrificial anode. 

Recently this technique has been proposed in conjunction with conventional patch repair of chloride-contaminated structures in order to avoid the initiation of incipient pitting around the repaired zones, by utihsing sacrificial anodes embedded near the periphery of the repair patches [20,21]. Cathodic prevention is now being used in several countries. ... 

Sacrificial Anodes Incontrastto the impressed current technique, the use of sacrificial anodes does not depend on the creation of driven electrochemical cell. Rather, a galvanic cell is formed between the structure and the sacrificial anode in which electrons pass spontaneously from the latter to the former (Fig. 9). Thus, the source of the electrons (the sacrificial anode) must have a more negative electrode potential than the structure. It was for this reason that Humphrey Davy chose zinc or iron to protect copper, and it also explains why magnesium, aluminum and zinc alloys are used to protect steel today. 

The sacrificial CP technique involves the use of a more active metal than that used in the structure to be protected to supply the current needed to control corrosion. The more active metal is called a sacrificial anode [35]. Coupling two dissimilar metals... 

Comparison of Sacrificial Anodes and Impressed Current Techniques It is... 

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

The most severe limitation of the sacrificial anode CP technique is the small driving force, which restricts its use to conductive environments, short current throws and, marine use apart from, wellpower supply, the danger of overprotection, the difficulty of achieving a satisfactory potential profile over a complex shape and the possibility of improper connection causing corrosion of the structure intended to be protected are the major disadvantages of the impressed current CP technique. 

Sometimes the need to be environmentally acceptable may lead to new problems. For instance, ozone was suggested to replace biocides with no data available on the performance in the chlorination of water (60). Corrosion control techniques can have both favorable as well as ill effects and hence one has to exert balanced judgment before embarking on a corrosion prevention method. Organotin antifouling coatings on ships were effective, but they polluted the seawater and hence were banned from further use. The use of cadmium as a sacrificial anode is restricted because of its toxicity. Large amounts of zinc are used to protect steel platforms in the sheltered and shallow waters of the sea, and the effects of zinc on the contamination of waters are not known. 

The application of cathodic protection requires the delivery of electrons to the structure to be protected. This may be achieved by two separate means (1) using impressed-current techniques and (2) by a spontaneous galvanic effect using sacrificial anodes. The two techniques are discussed briefly below. 

Another electrochemical method that has the potential to be an effective alternative to the various traditional techniques employed for the distillery and/or brewery effluent treatment is electrocoagulation. Electrocoagulation is based on the in situ formation of the coagulant as the sacrificial anode dissolves due to the applied current, while the simultaneous evolution of gases at the electrodes allows for organic pollutant ronoval by flotation (Khandegar Saroha, 2012). 

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