Sacrificial consumption

Effectively, the ruthenium complex undergoes a catalytic cycle, while the Co(III) complex and water are consumed. The overall process of O2 generation therefore involves the sacrificial consumption of the cobalt complex (Eq. 11-7) ... 

Mechanistic elucidations, and analysis of in-poI)aner tests, has revealed [2,4] the importance of the structure of the moiety bound to position 4 of the phenolic nucleus. Three main structural types have been used. The first consists of compounds bearing a methyl or a mono- or a disubsti-tuted methyl group, as in IV to VII. After their sacrificial consumption, these phenols transform into relatively stable quinone methides, generally VIII (A, B = residues of the molecule). 

Pure magnesium should have a driving potential of 850 mV to protect steel but in practice the metal corroded very rapidly with a very low efficiency. The metal suffers low polarization in the presence of chloride or sulfate ions and produce highly soluble chloride and sulfate salts. These ions are usually artificially introduced into the electrolyte as a backfill when a deficiency is expected, the hydroxide which is preferentially formed because of its low solubility becomes enriched with the backfill anions and itself functions as a backfill. Uniform general corrosion can then be obtained and well-designed inserts help to keep most or all of the anode metal available for sacrificial consumption. In freshwater or electrolytes which contain none of these ions, the hydroxide and carbonate may form, but these do not seriously polarize the anode (Morgan, 1993). 

If possible, the cell should be undivided to minimize the construction cost and also the energy consumption (see goal 1). The application of a controlled reaction at the auxiliary electrode taking place at low potential allows for the use of undivided cells in many cases. For oxidations, the cathodic process at the auxiliary electrode may be a proton reduction under formation of hydrogen. For reductions, the anodic process may be the oxidation of formate or oxalate under production of carbon dioxide [68] or the dissolution of sacrificial anodes [69] (see also Sec. V.B). 

Cathodic Protection. Steel can be protected by cathodic current, supplied either from sacrificial anodes or an external direct current source. The method is effective for completely immersed steel—i.e., for surfaces on structures below the low-tide level. Current consumption can be greatly reduced by applying a suitable paint system to the steel before it is immersed in sea water. Such a paint system should be alkali resistant. 

To our knowledge, at present there are no bilayer membrane systems which simultaneously satisfy all the requirements listed. Nevertheless, notable achievements have been made on the way towards their developments. Introducing appropriate catalysts in the vesicles it was possible to accomplish separately both water reduction to dihydrogen and its oxidation to dioxygen at the expense of irreversible consumption of sacrificial electron donors and acceptors, respectively. 

Figure 6.10 Perforation in the coating gives rise to localized attack of steel (left) or consumption of the sacrificial Zn (right)16...

When the promoter is consumed at a faster rate, which is still, however, smaller than that of the catalytic reactant, then the promoter is termed sacrificial promoter [13,14]. This is the case, as we will see, in electrochemical promotion utilizing conducting solid electrolytes. The promoting species is introduced via a Faradaic process on the catalyst surface at a rate of I/2F, where / is the applied current and F is Faraday s constant. At steady state, I/2F also equals the rate of consumption of the sacrificial promoter species on the catalyst surface. 

Low tension waterflooding is a method intermediate between alkaline and micellar/polymer technology. The LTWF employs a dilute surfactant to reduce IFT and mobilize residual oil. A few field trials (26-29) of this process have been tried with mixed success. None of these trials however employed sodium silicates in any part of the flood design. Instead, other alkalis such as sodium carbonate and sodium tripoly- phosphate were used. Some of the reasons proposed for the limited success in these trials were 1) high consumption of the sacrificial agents, leaving the surfactant unprotected, 2) poor sweep of the pay zone, 3) limited mobility control and lower than expected displacement efficiency. Recent work published and obtained in our laboratories has shown that sodium silicates may help to overcome some of these problems better than other alkalis. 

The criteria for cathodic protection are not free from criticism. It is beheved that all the listed criteria are deficient to some extent and therefore qualitative in practical appKcation. However, one should be optimistic that any level of cathodic polarization is beneficial, and a broad range of ca-thodically applied potentials will yield adequate protection. As a result, the use of any criterion listed in Table 4 [24] will produce adequate cathodic protection if applied judiciously. The amount of cathodic protection should be sufficient to reduce the corrosion rate to an acceptable range. Caution should be exercised to avoid overprotection. Overprotection results in the premature consumption of sacrificial anodes or excessive amounts of impressed current demands. Moreover, the application of too much cathodic protection can result in damage to the structure to be protected as a result of hydrogen embrittlement. 

Whereas auxiliary anodes need not be consumed in order to fulfill their purpose, sacrificial anodes are consumed not less than is required by Faraday s law in order to supply an equivalent electric current. In general, the observed rate of consumption is greater than the theoretical. For zinc the difference is not large, but for magnesium it is appreciable, with the cause being ascribed to local-action currents on the metal surface, to formation of colloidal metal particles [13, 14] or, perhaps more important, to initial formation of univalent magnesium ions [15], The latter ions are unstable and react in part with water in accord with... 

The corrosion resistance of zinc is so important that about half the world s annual consumption of zinc is used to protect steel from rust. A vast amount of information now exists on the resistance of zinc (including its alloys and their use as a coating) in a wide variety of corrosive conditions and is summarized herein. The present volume draws upon these earlier works, but also contains much valuable data from previously unpublished and untranslated works. Zinc Its Corrosion Resistance was originally compiled in 1970 by the Battelle Memorial Institute, Columbus, Ohio, which had been commissioned by the International Lead Zinc Research Organization, Inc. (ILZRO) to collate the available information on the corrosion resistance of zinc and zinc-coated steel. The 1983 edition included information that had become available since 1970 plus one new chapter dealing with the performance of zinc as a sacrificial anode. 

Sacrificial Oxidation Electrode Capacity Consumption Notes... 

Sacrificial anode material Composition Typical anodic current density (A.m") Consumption rate (g-A-. yr ) Cost per unit surface area (US /m ) 1-mm-thick anode Notes... 

The molecular origin of electrochemical promotion is currently understood on the basis of the sacrificial promoter mechanism [1]. NEMCA results from the Faradaic (i.e., at a rate I/nF) introduction of promoting species (0 in the case of conductors, H" in the case of H" conductors) on the catalyst surface. This electrochemically introduced 0 species acts as a promoter for the catalytic reaction (by changing the catalyst work function and affecting the chemisorptive bond strengths of co-adsorbed reactants and intermediates) and is eventually consumed at a rate equal, at steady state, to its rate of supply (I/2F), which is A times smaller than the rate of consumption of the catalytic reactant, e.g., atomic O originating from the gas phase [1]. 

Fig. 27 Top SEM images of CaCOs particles grown on a glass slip in the early reaction stage, PEO-6-PMAA, [CaCb] = 10 mM, lgL , 5h. a CaCOs particles with either spherical or hollow structures, b Zoom showing the calcite rhombohedral subunits grown on the surface of the hollow structure and the inner part consisting of tiny primary nanocrystals with a grain size of about 320 nm as indicated by the arrow (sacrificial vaterite template). Bottom Proposed formation mechanism of the calcite hollow spheres a polymer-stabilized amorphous nanoparticles b Formation of spherical vaterite precursors c aggregation of the vaterite nanoparticles d vaterite-calcite transformation starting on the outer sphere of the particles e formation of calcite hollow spheres under consumption of the sacrificial vaterite precursors. Reproduced in part from [61] with permission of the American Chemical Society...

In addition, anodes are classified as sacrificial anodes and impressed-current anodes. The former must be anodic to the stmcture and must dissolve at a low rate, providing electrons to the cathode. On the other hand, the latter must have low consumption rates in cathodic protection designs. Specifically, sacrificial magnesium Mg) anodes are widely used in buried pipelines and domestic or industrial water heater applications. For instance, a Mg anode may protect as much as 8 Km of a coated pipeline buried in the seal [3]. 

In the light of the previous comments one can see that the popular option of the sacrificial layer conld imply several sustainability issnes when it implies periodic production of residnes and consumption of fnrther material with the impact associated with its prodnction (e.g. CO2 emissions in the case of lime). 

In terms of sustainability, there are several direct and indirect impacts to be considered and it is advisable to use the tools classically available in environmental impact assessment. In this regard were highlighted the concern in relation to the possible leaching of products used in biocide treatment and the problems associated with one of the popular choices in terms of interventions which is the use of a sacrificial layer that would imply the impacts associated with the preparation of the replacement mortars (such as periodic consumption of resources and CO2 emissions). The sustainability analyses needs to consider the balance between periodicity of applications and the effects of more permanent solutions. In general there is scarce reflection on the sustainability implications of these procedures for conservation of materials. 

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