Sacrificial cathodes

Another study on the electrosynthesis of (alkyl) M compounds (M = Ge, Pb, Sn n = 2, 4) provides illustrative examples37. Sacrificial cathodes of Cd, Zn and Mg were used to produce the corresponding metal alkyls which are subsequently oxidized on sacrificial anodes of Ge, Sn and Pb. The cells are of very simple construction, with the proper metal electrodes. Diethylcadmium is utilized in this way for the manufacture of tetraethyllead from lead acetate and triethylaluminum in the following reaction sequence ... 

One of the first series of reports on ultrasonically-enhanced electrosynthesis was by Gautheron, Tainturier and Degrand [69] who used the technique to explore routes to organoselenium and tellurium derivatives. Instead of employing a sacrificial cathode of elemental selenium, their procedure allowed the direct use of selenium powder with carbon cloth as cathode to produce Se and Se. A further benefit was that this method also allowed production of the corresponding tellurium anions. These species could be employed in situ in aprotic solvents such as DMF, THF and MeCN for the synthesis of selenides and tellurides by nucleophilic displacement from haloalkanes. 

The sulphur, selenium and tellurium nucleophiles required for these SrnI reactions can be generated in a preliminary step by reduction of a sacrificial cathode of graphite mixed with elemental sulphur, selenium or tellurium [160,162]. 

Electrochemical reactions with use of a sacrificial cathode [121], which have not yet been reported in relation to phthalocyanine, could have theoretical success in the synthesis of PcM. The cathode dissolution takes place due to the action of free radicals formed in solution near the cathode surface [121] in the case of the synthesis of Pc (which is formed on the cathode surface), UV irradiation of the reaction mixture could serve as a source of free radicals. 

Most macroscale experiments run with C/S sacrificial cathode were conducted at a potential of —0.9 V (vs. SCE). Obviously, transients species coming from the fragmentation of sulfur (radical ions or anions) can act as reducing species and nucleophiles. Very often, it appeared difficult to discriminate between different chemical (and electrochemical) pathways. Preliminary experiments based on the nucleophilicity of reduced products of dissolved sulfur led with poor selectivity to dipropionitrile sulfide (Tomilov [285]) and to alkyl polysulfides (Paris [286]). 

In the late 1990s, sacrificial cathodic protection (SCP) of concrete reinforcement has been introduced. In SCP, dissolution of a less noble metal such as zinc or aluminium connected to the reinforcement provides the current instead of an external power source. 

M. Raupach, M. Bruns, Effectiveness of a zinc-hydrogel anode for sacrificial cathodic protection or reinforced-concrete structures , ICC 15 Int. Corrosion Congress, Granada, 22-27 September 2002 (CD-ROM). 

Design Aspects of Sacrificial Cathodic Protection System... 

A. A. Sagues, S. C. Krane, F. J. Presuel-Moreno, Advanced computational model for sacrificial cathodic protection of partially submerged reinforced concrete marine... 

There is considerable need for additional studies on innovative constmction materials such as corrosion-resistant alloy/clad rebars (both metallic and nonmetallic) and more durable concretes with inherent corrosion-resistant properties. Further research and development in rehabilitation technologies that can mitigate corrosion with minimal maintenance requirements such as sacrificial cathodic protection (CP) systems is desirable. 

Anode Resistance The anode resistance has a significant role in determining the amount of the anode material to be used [84]. The anodes use in the sacrificial cathodic protection system is of three types vertical anodes, horizontal anodes, and grouped anodes. A typical graphite anode vertical and horizontal installation is shown in Figs. 15.12 and 15.13. The resistance of the vertical anode to earth is calculated using the Dwight formula [87]. 

NACE International Publication 01105 Sacrificial Cathodic Protection of Reinforced Concrete Elements A State-of-the-Art Report. 2005 Item No. 24224. 

Sacrificial Cathodic Protection makes al i the steel negative by dissointron of anode metai to generate electrons Resistance must be low tor enough current to pass... 

Traditionally, metallic coatings serve only one or two functions. For example, zinc has excellent corrosion resistance and functions as a sacrificial anode (Tsura, 2005). Zinc galvanizing provides sacrificial cathodic protection and acts as a barrier (Jones, 1996) but does not nsnally supply inhibitor ions. The release of zinc ions during the sacrificial protection of galvanized steels (Tsuru, 2005 Pourbaix, 1974) only provides a small additional benefit compared with galvanic protection provided by the potential driving force. Metallic coatings used to protect Al alloys (Reddy et al., 2000 Walton et al, 1953) consist of a thin layer of nearly pure Al mechanically bonded to standard precipitation age... 

Coatings that act as barriers to corrosion are commonly used and date back to the use of noble metal decorative and corrosion resistant coatings (Cramer and Covino, 2005). However, as in the case of sacrificial cathodic protection, barrier properties are often fixed by rather inflexible alloy compositions and stractures. Porosity in coatings is often minimized by mediating coating thickness. 

Presuel-Moreno, F. J., Goldman, M. E., KeUy, R. G. and Scully, J. R. 2005a. Electrochemical sacrificial cathodic prevention provided by an Al-Co-Ce metal coating coupled to AA2024-T3. Journal of the Electrochemical Society, 152, B302-B310. 

Not all considerations are, however, given to the adverse effects of the proximity of materials. There are several situations where, by judicious choice of dissimilarity between materials, beneficial results can be obtained (e.g. sacrificial cathodic protection, cleaning of metals). 

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