Protective removal, electrochemical

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

Electrochemical corrosion protection has the objective of reducing corrosion damage or removing it altogether. Three different processes are discussed in this work ... 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

We do not claim that the Co(III) method is superior to modern methods of chemical synthesis. However it does provide an alternative. The Co(III)-active ester, once made, can be stored for long periods of time, it provides both N-terminal protection and carbonyl-O activation in the one system, it is orange in color (e480 —100 M l cm"1), generally quite water soluble (it is a salt ), and the Co(III) metal plus ancillary ligands can easily be removed by chemical or electrochemical (—1.0 V vs SCE) reduction methods. 

On occasion such reductive deprotection processes can be quite selective. Electrochemical reduction of A,A -di-p-toluenesulfonyl-A-t-butoxycarbonyl derivatives of aliphatic and aromatic diamines selectively removed the p-toluenesulfonyl group attached to a primary amine site63. Yields on the mono-protected products are fair to high selective deprotection of the corresponding A,A -dibenzoyl derivatives occurred in yields >92%. 

The enormous cost of corrosion of iron to society has prompted many efforts to devise ways of reducing or preventing it. Several electrochemical or chemical methods are available. One method is removal of the cathodic species (usually oxygen). Most methods are based on the principle of providing a barrier between the reacting species. The barrier may be physical, i. e. a metal or paint coating or a protective oxide film, or electronic, i. e. making the iron thermodynamically immune. Here, the em-... 

Treatment of 4-methoxy-2-oxazolidinone 86 with indolylmagnesium bromide 87, followed by A-protection with a ferf-butoxycarbonyl (Boc) group affords NJd -di-Boc-4-(3-indolyl)-2-oxazolidinone 88. Subsequent treatment with A-bromosuc-cinimide (NBS) in the presence of azobisisobutyronitrile (AIBN) followed by electrochemical reduction yields the protected 4-(3-indolyl)-2(3//)-oxazolone 90 (Fig. 5.23). The Boc groups are easily removed by pyrolysis."" "" ... 

The Diels-Alder adduct of fulvene and di(2,2,2-trichloroethyl)azodiearboxylate after selective monohydrogenation of the endocyclic pi bond can lead to the bicyclic biscarbamate 107. The electrochemical removal of the TV-protecting carbamoyl groups in a DMF—LiC104—(Hg) system is followed by the oxidation with potassium ferricyanide to give the azo compound 108 which on thermal decomposition forms the linearly fused tricydopentanoid 109 in over 50 % yield (from 107, Scheme 3-41)88a). 

The disadvantage of physically protective barriers is the rapid and localized corrosion that occurs when the protective layer is scratched or removed locally (Figs. 16.10 and 16.11). Thus, in many cases, the utilization of methods involving continuous electrochemical protection is necessary. 

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