Metal direct attack

Underdeposit corrosion is not so much a single corrosion mechanism as it is a generic description of wastage beneath deposits. Attack may appear much the same beneath silt, precipitates, metal oxides, and debris. Differential oxygen concentration cell corrosion may appear much the same beneath all kinds of deposits. However, when deposits tend to directly interact with metal surfaces, attack is easier to recognize. 

Second, oxygen attacks the metal directly by precipitating iron oxide at the anode, thus, preventing anodic polarization by Fe ions. 

The form of Figure 1.43 is common among many metals in solutions of acidic to neutral pH of non-complexing anions. Some metals such as aluminium and zinc, whose oxides are amphoteric, lose their passivity in alkaline solutions, a feature reflected in the potential/pH diagram. This is likely to arise from the rapid rate at which the oxide is attacked by the solution, rather than from direct attack on the metal, although at low potential, active dissolution is predicted thermodynamically The reader is referred to the classical work of Pourbaix for a full treatment of potential/pH diagrams of pure metals in equilibrium with water. 

This is the one example in which metal is not the substrate. Corrosion takes on a new meaning the coating here is required to protect the substrate from direct attack by corrosive substances, from water to more powerful household or industrial chemicals, such as grease, alcohols and bleach. We are concerned with the industrial application of thin protective layers to paper (e.g. labels), card (e.g. playing cards) and many wooden articles, including industrially finished doors, window frames and, particularly, furniture. 

A direct attack of nucleophiles on the sulfur atom of the sulfone or sulfoxide group in acyclic or large-ring sulfones and sulfoxides is rather rare, or unknown, excluding metal hydride reductions and/or reductive deoxygenations. The situation is completely different in the three-membered ring systems. 

On the basis of the results obtained from additional experiments with other metals it was concluded that in the presence of active metal such as Sb° or Bi° or Zn° direct attack on the DBDPO would readily occur. However, all attempts to directly measure the extent of Sb° formation from Sb203 during polymer degradation were unsuccessful. 

It would be reasonable to assume that, in a solvent of high dielectric constant, such as nitromethane, the nitrile (84) is formed by direct attack of cyanide ion on an ion-pair (86) in which the bromide ion has the a-D orientation. Departure, assisted by metal ions, of the halide ion from 83 or 86, with possible assistance by the lone pair of the ring-oxygen atom, would lead to an oxonium ion (87) that could... 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

The reactivity of the electron rich dimeric Au derivative (99) towards electrophiles (metallic or not) seems to begin by direct attack of the electrophile on the... 

In another investigation,425 the exchange between [Ce(edta)aq] and hydrated Pb2+, Ni2+ or Co2+ ions again show reaction by dissociation of protonated [Ce(Hedta)aq] as well as by the direct attack of metal ions on [Ce(edta)aq] or [Ce(Hedta)aq]. The kinetic parameters for the Ni2+ or Co2+ ions could be related to the relatively slow (k - 2.6 x 106s 1 for Co2+ and 3.4 x 104 s-1 for Ni2+) water exchange reactions of these ions. The direct attack was interpreted in terms of an intermediate in which one of the carboxylate groups was coordinated to the incoming ion rather than to Ce3+. These reactions were followed by spectrophotometry at 280 nm, where the absorbance of Ce3+aq is much lower than the edta complex. 

The key intermediate in the reduction of metal ions by carbon monoxide and water is the hydroxycarbonyl (18). Initially (18) was proposed to form by a migratory insertion of CO into a M—OH bond, but more recent studies have favored a direct attack of water or hydroxide on a coordinated carbonyl (4,62). This latter view is in accord with the expected reactivity of coordinated CO toward nucleophiles. Intermediate (18) may then decarboxylate to give C02 and either a reduced metal ion or a metal hydride, as in (29) and (30), respectively. 

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

Lewis acids, such as BF3 and AlMe3, can also bind to metals directly (equation 29)53 although they often attack ligands such as CO and N2 (equation 30.)54 Attack at the metal raises v(CO) of attached carbonyls by reducing the available metal electron density, but attack at CO or N2 lowers v(CO) or v(N2) because the lone pair donated to the Lewis add has C—O and N—N bonding character. 

Terminal monoalkenes were alkylated by stabilized carbanions (p a 10-18) in the presence of 1 equiv. of palladium chloride and 2 equiv. of triethylamine, at low temperatures (Scheme l).1 The resulting unstable hydride eliminate to give the alkene (path b), or treated with carbon monoxide and methanol to produce the ester (path c).2 As was the case with heteroatom nucleophiles, attack at the more substituted alkene position predominated, and internal alkenes underwent alkylation in much lower (=30%) yield. In the absence of triethylamine, the yields were very low (1-2%) and reduction of the metal by the carbanion became the major process. Presumably, the tertiary amine ligand prevented attack of the carbanion at the metal, directing it instead to the coordinated alkene. The regiochemistry (predominant attack at the more sub-... 

Reaction with even harder nucleophiles such as organolithiums and Grignard reagents is substantially limited by virtue of the fact that these carbon nucleophiles add by direct attack at the metal center, as opposed to the softer carbon nucleophiles which add by attack on the allyl ligand. Direct metal addition can lead to the opening of alternative reaction pathways, e.g. 3-H elimination, in competition with reductive elimination which accomplishes nucleophile allylation (equation 40). 

The author suggests that reduction of C02 may proceed via direct attack of C02 on a metal hydride intermediate without prior coordination of C02 to the metal center (e.g., analogous to species 6). 

Pitting corrosion is a general term that can be considered a visible sign of the results of concentration cell corrosion and of further induced-corrosion processes such as when chloride attack occurs. Although pits can also occur with acid corrosion, etc., under-deposit corrosion, of course, can also involve direct metal surface attack, from, say, biologically induced corrosion (but that is discussed separately). 

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