Hydroxide film formation

In general, metals or alloys that are used are covered with oxide or hydroxide films. Formation of cracks and fissures can destroy the passivation. The depth of crevices increases rapidly because it is only there that the metal is not covered with a protective layer of oxide/hydroxide (see Fig. 16.8). The result is an increase in surface roughness and possible problems due to reduction in mechanical strength. 

Impedance analysis is a powerful thermoelectrochemical instrument. It proved useful to study the capacity of cobalt hydroxide film formation in double-layer electrolyte capacitors [56]. Also, experiments with the capacity of electrodes in solid state electrolytes have been published, e.g. with silver hahde/graphite... 

ALKALI AND Cm ORINE PRODUCTS - Cm ORINE AND SODIUM HYDROXIDE] (Vol 1) -for C VD [IHIN FILMS - FILM FORMATION TECHNIQUES] (Vol 23)... 

The compositions of the conversion baths are proprietary and vary greatly. They may contain either hexavalent or trivalent chromium (179,180), but baths containing both Cr(III) and Cr(VI) are rare. The mechanism of film formation for hexavalent baths has been studied (181,182), and it appears that the strength of the acid and its identity, as well as time and temperature, influences the film s thickness and its final properties, eg, color. The newly prepared film is a very soft, easily damaged gel, but when allowed to age, the film slowly hardens, assumes a hydrophobic character and becomes resistant to abrasion. The film s stmcture can be described as a cross-linked Cr(III) polymer, that uses anion species to link chromium centers. These anions may be hydroxide, chromate, fluoride, and/or others, depending on the composition of the bath (183). 

The aqueous decomposition of thiourea to sulfide and cyanamide has been found to be catalyzed by metal hydroxide species and colloidal metal hydroxide precipitates. Kitaev suggested that Cd(OH)2 is actually required for CdS film formation to occur by adsorption of thiourea on the metal hydroxide particles, followed by decomposition of the Cd(OH)2-thiourea complex to CdS. Kaur et al. [241] found... 

The basic features of the ion-by-ion and hydroxide cluster film-forming mechanisms are shown schematically in Figures 2.1 and 2.2, respectively. Film formation involving complex decomposition will proceed in a similar manner (Fig. 2.3 shows this for a molecule-by-molecule deposition). 

Finally, even if these criteria are satisfied, there remains the question of whether the product will adhere to form a film or just precipitate homogeneously in the solution. This is the most difficult criterion to answer a priori. The hydroxide and/or oxy groups present on many substrates in aqueous solutions are likely to be quite different in a nonaqueous solvent (depending on whether hydroxide groups are present or not). Another factor that could conceivably explain the general lack of film formation in many organic solvents is the lower Hamaker constant of water compared with many other liquids this means that the interaction between a particle in the solvent and a solid surface will be somewhat more in water than in most other liquids (see Chapter 1, van der Waals forces). From the author s own experience, although slow precipitation can be readily accomplished from nonaqueous solutions, film formation appears to be the exception rather than the rule. The few examples described in the literature are confined to carboxylic acid solvents (see later). 

All these results show that Cd(OH)2 colloids do adsorb on a substrate (either under conditions where Cd(OH)2 is present in solution or, according to the studies of Rieke and Bentjen and Ortega-Borges and Lincot [48], even when it is not present in solution but under solution conditions close to solid hydroxide formation). The induction period when no deposition is seen in the hydroxide-cluster deposition therefore is understood to mean that a fast and nongrowing Cd(OH)2 adsorption has occurred, which is too fast and/or too httle to measure by the experimental methods used to make the kinetic curves, and that only when the hydroxide starts to convert into the chalcogenide, by reaction of the slowly formed chalcogenide ion with the hydroxide, does real film formation proceed. 

Film Formation. Starch-g-PAN samples used for this study were prepared as described above, except that the small amounts of PAN homopolymer were not removed by DMF extraction. For the graft polymerization with gelatinized starch, the starch-water slurry was heated for 30 min at 85°C before reaction at room temperature with acrylonitrile and ceric ammonium nitrate. Saponifications in these experiments were carried out by stirring. 55 g of graft copolymer with 450 ml of 0.711 sodium hydroxide solution in a sigma mixer for 2 hr at 90-100°C. 

Passivity — An active metal is one that undergoes oxidation (-> corrosion) when exposed to electrolyte containing an oxidant such as O2 or H+, common examples being iron, aluminum, and their alloys. The metal becomes passive (i.e., exhibits passivity) if it resists corrosion under conditions in which the bare metal should react significantly. This behavior is due to the formation of an oxide or hydroxide film of limited ionic conductivity (a passive film) that separates the metal from the corrosive environment. Such films often form spontaneously from the metal itself and from components of the environment (e.g., oxygen or water) or may be formed by an anodization process in which the anodic current is supplied by a power supply (see -> passivation). For example, A1 forms a passive oxide film by the reaction... 

Transpassivity — Certain metals exhibit the property of -> passivity, whereby the metal resists - corrosion under conditions in which it should react significantly, usually due to the formation of an oxide or hydroxide film of limited ionic conductivity (a passive film) that separates the metal from the corrosive environment. An-... 

Due to their polar M-0 bond, nonstabilized alkoxides are very sensitive toward hydrolysis, leading to the formation of metal hydroxides (M-OH), with the concurrent release of an alcohol molecule. Hydrolysis reactions are common in the use of these compounds and they have been studied in detail for the silica system, with the effects of catalysis type (acid or base), extent of hydrolysis, and resulting oligomeric species being well documented. The goal in using these precursors for film formation is to control the extent of hydrolysis and subsequent condensation to yield short-chain (a few to a few dozen metal centers) polymeric species, referred to as oligomers. 

Work on the corrosion of aluminum has suggested that its resistance is largely dependent on the structure of the film thus, complete passivity is considered to be associated with the formation of an oxide, while a hydrated oxide or hydroxide film only confers partial protection. It is postulated that the important factor in inhibition by oxygen-containing anions is their ability to cause the true oxide to be produced rather than the hydroxide which would otherwise form. It is thought that the anion acts catalytically in donating its own O " ion to the film in competition with the hydroxyl ion from the solution. 

This deionized colloidal silica is not a strong binder. The removal of the sodium hydroxide, which appears to act as a binder catalyst, and/or the presence of the alumina on the surface, weakens the overall bond strength developed. However, this acidic version of colloidal silica does form stable mixtures with several water soluble polymers, the most important of which is polyvinyl alcohol (PVA). PVA is itself a good film former and imparts this property of much improved film formation to colloidal silica even when present as only about 3-5% of the total solids. The mixture of PVA and acid stabilized colloidal... 

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