Cathodes, aluminum sacrificial

Galvanic corrosion is the enhanced corrosion of one metal by contact with a more noble metal. The two metals require only being in electrical contact with each other and exposing to the same electrolyte environment. By virtue of the potential difference that exists between the two metals, a current flows between them, as in the case of copper and zinc in a Daniell cell. This current dissolves the more reactive metal (zinc in this case), simultaneously reducing the corrosion rate of the less reactive metal. This principle is exploited in the cathodic protection (Section 53.7.2) of steel structures by the sacrificial loss of aluminum or zinc anodes. 

In the future, further studies should be addressed to improve the chemose-lectivity and diastereoselectivity of the reductive coupling process, especially searching for novel reagents and milder experimental conditions. As a matter of fact, a few novel reductive couphng procedures which showed improved efficiency and/or stereoselectivity have not been further apphed to optically active imines. For example, a new electrochemical procedure which makes use of the spatially addressable electrolysis platform with a stainless steel cathode and a sacrificial aluminum anode has been developed for imines derived from aromatic aldehydes, and the use of the N-benzhydryl substituent allowed 1,2-diamines to be obtained with good yields and dl-to-meso ratios... 

P, y-Unsaturated esters (184) have been synthesized by a one-step electrochemical procedure from a-chloroesters (183) and aryl or vinyl halides (Scheme 73b) [294, 295]. This novel electroreductive cross-coupling method is based on the use of a Ni(II)(bpy) catalyst and a sacrificial aluminum anode in a one-compartment cell (Scheme 73). The whole cathodic process progresses at —1.2 V (SCE) (Scheme 73c),... 

Magnesium, zinc, or aluminum blocks are anached to ships hulls, oil and gas pipelines, underground iron pipes, and gasoline storage tanks. These reactive metals provide cathodic protection by acting as a sacrificial anode. 

Impingement attack at the inlet ends can be minimized by providing a suitable amount of cathodic current. While this current does not enter the tube ends to any great depth, it protects the first few inches. The current may be provided by sacrificial anodes made from iron, zinc, aluminum, or magnesium. 

Bordeau and coworkers also reported that cathodic reduction of Me2SiCl2 without solvent using a sacrificial aluminum anode and a stainless steel cathode in an undivided cell produces polydimethylsilane with a very high current efficiency84. 

Cathodic reduction of (chloromethyl)dimethylchlorosilane using an aluminum anode provided polycarbosilanes besides a large amount of di- and trisilacyclic compounds (equation 67)97. On the other hand, the electrolysis in the presence of Me2SiCl2 gave bis(dimethylchlorosilyl)methane, a useful polycarbosilane precursor (equation 68)97. Similarly, polycarbosilanes were prepared from dichlorocarbosilanes using sacrificial aluminum electrodes in DME (equation 69)91. 

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. Another way of keeping metal in a cathodic state is to connect it to an external direct current source. For smaller structures such as boats and domestic water water heaters the current is supplied by a sacrificial anode made of aluminum or zinc. For larger extended structures such as piers and buried pipelines, an external line-operated or photovoltaic power supply is commonly used. (All interstate oil piplines in the U.S. are required by law to employ cathodic protections.)... 

Nanostructured aluminum [74—78], iron [74] and aluminum-manganese alloys [74] have been prepared from a Lewis acid A1Q3/[BMIM]Q mixture (65 mol% AICI3, 35 mol% [BMIMJC1) whereas palladium alloys have been deposited from a Lewis basic system (45 mol% Aid , 55 mol% [BMIMJC1). The electrochemical cell and all parts which are in contact with the electrolyte have to be built from inert materials. As cathode material glassy carbon can be used. A constant ion concentration in the electrolyte can be realized by the use of a sacrificial anode consisting of the... 

A 3-allyloxy-2-bromo ester 264 (Y=0, R2=C02Et) cyclized similarly using 20 mol% of (salen)Co complex 267 as a catalyst under reductive electrochemical conditions at a carbon fiber cathode and a sacrificial aluminum anode under constant current conditions. Tetrahydrofurancarboxylates 266 were isolated in 70% yield and a good 6 1 CK//r[Pg.266]

In a bipolar arrangement, the sacrificial electrodes are placed between the two parallel electrodes without any electrical connection. The two monopolar electrodes are connected to the electric power source with no interconnections between the sacrificial electrodes. This cell arrangement provides a simple setup, which facilitates easy maintenance. When an electric current is passed through the two electrodes, the neutral sides of the conductive plate will be transformed to charged sides, which have opposite charge compared with the parallel side beside it. The sacrificial electrodes are known as bipolar electrodes. It has been reported that EC cell with monopolar electrodes in series connection was more effective where aluminum electrodes were used as sacrificial and iron was used as anode and cathode. And, electrocoagulation with Fe/Al (anode/cathode) was more effective for the treatment process than Fe/Fe electrode pair (Modirshahla et al. 2007). 

Cathodic protection of a hot water tanks using magnesium as the sacrificial anode is shown in Figure 1.71. Some characteristic properties of zinc, magnesium and aluminum anodes are listed in Table 1.30. 

Some measures involving cathodic protection of aluminum using zinc as the sacrificial anode,47 and protection of aluminum ship hulls are found in the literature.37 Overprotection may lead to alkali (cathodic corrosion) attack. Alclad alloy systems are also some form of self-contained cathodic protection systems. 

Aluminum can be attacked by way of cathodic corrosion in strong alkaline media generated at the cathode when magnesium corrodes sacrificially in static NaCI solutions. This mode of attack destroys compatibility in alloys containing significant iron contamination. 

Sacrificial anode — is a piece of metal used as an anode in electrochemical processes where it is intended to be dissolved during the process. In -+ corrosion protection it is a piece of a non-noble metal or metal alloy (e.g., magnesium, aluminum, zinc) attached to the metal to be protected. Because of their relative -+ electrode potentials the latter is established as the -+ cathode und thus immune to corrosion. In -+ electroplating the metal used as anode may serve as a source for replenishing the electrolyte which is consumed by cathodic deposition. The sodium-lead alloy anode used in the electrochemical production of tetraethyl lead may also be considered as a sacrificial anode. 

The efficient preparation of trimethyl(trinuoroinethyl)silane via the eleetroredaction of bromo-trifluoroniethane in dimethylformamide in the presence of chlorotrimethylsilane using a sacrificial aluminum anode and nickel foam cathode (area 20 cm ) containing tetrabutylam-... 

Cathodic protection can be viewed as a form of galvanic corrosion, put to good use. In this case an active metal (most often zinc, but under special circumstances magnesium or aluminum) is employed as a sacrificial anode. It is attached to the steel structure being protected in one or several locations and does not constitute part of the... 

Early workers [103] detected benzilic acid formed during the reduction of benzophenone in dimethylformamide in the presence of carbon dioxide. The carbon dioxide radical anion system is known to have E" = —2.2V (vs. SCE) [104] and will thus not be formed in preference to the ketone radical anion. Reaction occurs through trapping of aromatic carbonyl radical anions by carbon dioxide, and this has been developed into a convenient synthesis of aryllactic acids. The modern technological process uses constant current conditions. On a small scale, a divided cell with mercury cathode has been used to obtain benzilic acids from substituted benzophenones and carbon dioxide in 70-90% yields [105] and to convert 4-isopropylacetophenone to the corresponding phenyllactic acid in 85% yield [106]. On a technical scale, these reactions are best carried out in an undivided cell using a lead cathode and a sacrificial aluminum anode with dimethylformamide as solvent... 

Aluminum and aluminum-zinc alloy anodes have become the preferred sacrificial anodes for the cathodic protection of offshore platforms. This preference is because aluminum anodes demonstrate reliable long-term performance when compared with magnesium, which might be consumed before the platform has served its useful hfe. Aluminum also has better current/weight characteristics than zinc. Weight can be a major consideration for large offshore platforms. The major disadvantage of aluminum for some applications, for example, the protection of painted ship hulls, is that aluminum is too corrosion resistant in many environments. Aluminum alloys will not corrode reliably onshore or in freshwater [37]. In marine... 

Cathodic protection has been traditionally used for a long time to protect structures in seawater. Zinc bracelet anodes are installed on pipelines immersed in seawater to provide sacrificial CP to the outer surfaces of the steel pipe. Offshore structures for gas and oil exploration, drilKng, and production are commonly protected by the use of aluminum anodes on the steel structures. Cathodic protection is employed by several power plants located near coastal sites for protecting steel structures submerged in seawater. 

Jetties are individual or multiple piles interconnected together to form a structure in the seabed and support a deck. The piles of a jetty usually have half of their length in the seabed and the rest in the high tide and splash zones up to the jetty deck. They are often concrete structures reinforced with steel. Cathodic protection using sacrificial zinc or aluminum anodes is installed after the completion of the jetty. With a deepwater jetty the suspension of more than a single anode or placing of alternate anodes at different levels is necessary. A few and larger anodes are necessary while impressed current method is employed. An... 

Ship hulls Painting cannot always protect hostile marine conditions, in ships and, areas above keel blocks. Stem and mdder areas suffer erosion and corrosion due to the high turbulence caused by the propeller coupled with the galvanic effects of the noble bronze propeller. Effective cathodic protection of ship hulls and similar marine structures in seawater against corrosion can be apphed using either aluminum or zinc alloy sacrificial anodes. Twenty percent of the anodes required for full hull protection are required for stern protection only. 

The sacrificial anode system consists of burial of anodes in the electrical proximity of the tank. The anodes are made of magnesium or aluminum, which are less noble than the steel tank. This enables the flow of current from the sacrificial anode (A1 or Mg) to the cathodic steel tank. Over a period of time, the anodes are consumed and hence replaced with new anodes in order for continued corrosion protection of the tank. 

The two types of cathodic protection are (i) sacrificial and (ii) impressed current systems. The sacrificial anode system typically uses magnesium, zinc, or aluminum and their alloys (Fig. 5.25). These metals or alloys act as anodes when coupled with steel and its alloys. These metals or alloys act as anodes when coupled with steel and preferentially corrode. Magnesium is often used in fresh water media while zinc and aluminum are used in seawater and brackish water media. 

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