Layers lead oxides

The electrode potential of aluminium would lead us to expect attack by water. The inertness to water is due to the formation of an unreactive layer of oxide on the metal surface. In the presence of mercury, aluminium readily forms an amalgam (destroying the original surface) which is. therefore, rapidly attacked by water. Since mercury can be readily displaced from its soluble salts by aluminium, contact with such salts must be avoided if rapid corrosion and weakening of aluminium structures is to be prevented. 

Evidence for the solvated electron e (aq) can be obtained reaction of sodium vapour with ice in the complete absence of air at 273 K gives a blue colour (cf. the reaction of sodium with liquid ammonia, p. 126). Magnesium, zinc and iron react with steam at elevated temperatures to yield hydrogen, and a few metals, in the presence of air, form a surface layer of oxide or hydroxide, for example iron, lead and aluminium. These reactions are more fully considered under the respective metals. Water is not easily oxidised but fluorine and chlorine are both capable of liberating oxygen ... 

Some elements, such as the rare eartlrs and the refractory metals, have a high afflnity for oxygen, so vaporization of tlrese elements in a irormaT vacuum of about 10 " Pa, would lead to the formation of at least a surface layer of oxide on a deposited flhrr. The evaporation of these elements therefore requires the use of ultra-high vacuum techniques, which can produce a pressure of 10 Pa. 

Sodium and potassium are restricted because they react with sulfur at elevated temperatures to corrode metals by hot corrosion or sulfurization. The hot-corrision mechanism is not fully understood however, it can be discussed in general terms. It is believed that the deposition of alkali sulfates (Na2S04) on the blade reduces the protective oxide layer. Corrosion results from the continual forming and removing of the oxide layer. Also, oxidation of the blades occurs when liquid vanadium is deposited on the blade. Fortunately, lead is not encountered very often. Its presence is primarily from contamination by leaded fuel or as a result of some refinery practice. Presently, there is no fuel treatment to counteract the presence of lead. 

Lead oxide (PbO) (also called litharge) is formed when the lead surface is exposed to oxygen. Furthermore, it is important as a primary product in the manufacturing process of the active material for the positive and negative electrodes. It is not stable in acidic solution but it is formed as an intermediate layer between lead and lead dioxide at the surface of the corroding grid in the positive electrode. It is also observed underneath lead sulfate layers at the surface of the positive active material. 

One technical process involves blowing air above the surface of molten lead. (cf. The Barton process in Sec. 4.2.1), but also, at room temperature, reaction (1) soon covers any piece of lead exposed to air with a dull gray layer of lead oxide (cf. The milling process in Sec. 4.2.1). 

The drying of negative plates is not possible without precautions, because of the tendency to spontaneous oxidation. This oxidation reaction is much ac-celerated by water, and the active material of a moist negative electrode is spon-taneously converted into lead oxide when exposed to air. When, on the other hand, the charged plate is dry, a thin layer of oxide covers the surface of the active material, and prevents... 

When specimens of pure lead and a 5% antimony alloy were periodically oxidized and reduced, lead oxide layers were observed with different structures ... 

The hrst working lead cell, manufactured in 1859 by a French scientist, Gaston Plante, consisted of two lead plates separated by a strip of cloth, coiled, and inserted into a jar with sulfuric acid. A surface layer of lead dioxide was produced by electrochemical reactions in the first charge cycle. Later developments led to electrodes made by pasting a mass of lead oxides and sulfuric oxide into grids of lead-antimony alloy. 

At Pt electrodes, adsorption of oxygen species is supposed to be controlled by the Temkin isotherm in the low overpotential region [Damjanovic and Bockris, 1966], whereas in the higher overpotential region, the absence of an oxide layer leads to... 

Freshly cast lead has a bright, silvery appearance. On exposure to the atmosphere, however, lead in the surface layer combines with atmospheric oxygen and carbon dioxide to form a dark, stable gray coating of mixed lead oxide and basic lead carbonate. This layer usually protects the metal from further oxidation and corrosion (see Fig. 38). Protected by a weathered surface layer, solid lead is stable to further corrosion. Lead is also very ductile and soft, being the softest metal known in antiquity. It is mainly because of these properties that lead was widely used for building, to make pipes and roofs, and in naval construction, for example. Solid lead flows, albeit very... 

Melroy and co-workers88 recently reported on the EXAFS spectrum of Pb underpotentially deposited on silver (111). In this case, no Pb/Ag scattering was observed and this was ascribed to the large Debye-Waller factor for the lead as well as to the presence of an incommensurate layer. However, data analysis as well as comparison of the edge region of spectra for the underpotentially deposited lead, lead foil, lead acetate, and lead oxide indicated the presence of oxygen from either water or acetate (from electrolyte) as a backscatterer. 

Tani, T. Lakeman, C. D. E. Li, J.-F. Xu, Z. Payne, D. A. 1994. Crystallization behavior and improved properties for sol-gel derived PZT and PLZT thin layers processed with a lead oxide cover coating. Ceram. Trans. 43 89-106. 

Buckley, A. N., 1994. A survey of the application of X-ray photoelectron spectroscopy to flotation research. Colloids Surf, 93 159 - 172 Buckley, A. N. and Woods, R., 1995. Identifying chemisorption in the interaction of thiol collectors with sulphide minerals by XPS adsorption of xanthate on silver and silver sulphide. Colloids and Surfaces A Physicochemical and Engineering Aspects, 104,2 - 3 Buckley, A. N. and Woods, R., 1996. Relaxation of the lead-deficient sulphide surface layer on oxidized galena. Journal of Applied Electrochemistry, 26(9) 899 - 907 Buckley, A. N. and Woods, R., 1997. Chemisorption—the thermodynamically favored process in the interaction of thiol collectors with sulphide minerals. Inert. J. Miner. Process, 51 15-26... 

Other commonly employed redox electrodes are metals such as copper, cobalt, silver, zinc, nickel, and other transition metals. Some p-block metals such as tin, lead and indium can also function as redox electrodes. However, s-block metals such as magnesium do not make good redox electrodes since the elemental metal is reactive and forms a layer of oxide coating, which leads to poor reproducibility, poor electronic conductivity and electrode potentials that are difficult to interpret, (see Section 3.3.1). 

Thallium has much the same look (silvery) and feel as lead and is just as malleable. Unlike lead, which does not oxidize readily, thallium will oxidize in a short time, first appearing as a dull gray, then turning brown, and in just a few years or less turning into blackish corroded chunks of thallium hydroxide. This oxide coating does not protect the surface of thallium because it merely flakes off exposing the next layer to oxidation. 

The red lead oxide (the tetragonal alpha modification) is obtained by slow cooling of the lead monoxide melt. The sohdified mass may contain the red alpha form of the oxide resulting from slow cooling of the melt, under an outer layer of yellow beta form that may result from the rapid cooling of the outer portion. 

The oxidation of 2-phenyl-3-arylaminoindoles has been studied in CH3CN, DMF, and propylene carbonate at a platinum electrode with periodic renewal of the diffusion layer. The oxidation proceeds in two one-electron steps, the first leading to the formation of a radical-cation, which in the second step is oxidized at a more positive potential.424 The main concentration of charge and unpaired spin in the radical-cation are at the amino group. In the presence of base, 2-phenyl-3-arylaminoindoles undergo a two-electron oxidation to the corresponding imines. 

Schmidt et al. (139) postulated that in the presence of excess oxygen, platinum was transported as volatile oxides through the gas phase and boundary layer. This mechanism could not adequately explain the reconstruction observed far into the excess ammonia regime. It was suggested that under these and other conditions, other volatile platinum species formed. Moreover, these species might decompose by reaction in the boundary layer, leading eventually to the platinum replating itself. 

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