Passivity chemical

Uses. The use of titanium alloys for cast partial dentures offers light weight, low cost, good ductihty, adequate stiffness, chemical passivity toward foods and oral fluids, and biocompatibiHty with the oral tissues. 

Kolotyrkin, Y. M., Electrochemical Behaviour of Metals During Anodic and Chemical Passivation in Electrolytic Solutions , 1st Intern. Congr. Metallic Corrosion, Butterworths, London, 10 (1962)... 

Fig. 9. Model of chemical passivity for CoP20 in NaCl [124]. (Reprinted by permission of The Electrochemical Society).

In conclusion it should be emphasized that the passivation of the pore walls is electronic in nature. It therefore is not specific to silicon, but applies to all semiconducting electrodes. This is in contrast to chemical passivation, which is usually specific to a certain electrode material and electrolyte chemistry. 

Due to chemical passivation in old waste materials, subsequent leaching of the leach residue with sulfuric acid, with perhaps some oxidation, is necessary to achieve very high yields. If the pH is kept about 3 in this second treatment, leaching of iron is negligible. Also, to reduce iron in the leach solution, spent potassium hydroxide electrolyte from discarded batteries can be used for iron hydroxide precipitation. 

Here we describe some of the results. In each of these studies, the compound semiconductor was first etched in either acid or base to remove the oxide. The specific surface groups following the etch are not well understood. However, Pluchery et al. have followed the acid etching of InP by in situ infrared spectroscopy [175] and observed the removal of the oxide. Unlike Si, for which an acid (HF) etch leaves the surface hydrogen-terminated and temporarily passivated, acid etching of InP does not produce a chemically passivated surface. Presumably, the surface is left unprotected, and quickly oxidizes if not passivated by another process. Similar results showing reduction or removal of the oxide are seen for GaAs [174,176,177]. 

Loscutoff, P. W. and Bent, S. F. Reactivity of the germanium surface Chemical passivation and functionalization. Annual Review of Physical Chemistry 57, 467 (2006). 

Bodlaki, D., Yamamoto, H., Waldeck, D. H. and Borguet, E. Ambient stability of chemically passivated germanium interfaces. Surface Science 543, 63-74 (2003). 

Chemical passivation should be started as soon as possible after the cleaning of metal surfaces. Accumulation of new corrosion products can occur if it is not initiated soon after cleaning. It may be achieved by treating equipment... 

White rust and similar visible surface corrosion problems, resulting from a failure to provide a proper chemical passivation program. 

Sometimes the dissolution of the anode aim st ceases Imt, unlike the previous case,a permanent and visible layer of precipitate adheres to the anode this phenomenon is called mechanical passivity the causes of which are in principle the same as those of chemical passivity. 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

Apart from physical roughness and chemical passivation effects, phase transformation or structural changes can also play a major role in the friction and wear performance of diamond coatings.The extent of such changes may be dominated by environmental species or by ambient temperature. 

Charge transfer. 4, 55 Chemical passivation, 513 Chlorine evolution, 131, 134 Oironopoteniiometry, 386, 396, 398 Co-generation, 484 Colc-cole plot, 213 Collection efficiency, 95... 

Chemical passivation was discovered about 200 years ago. A piece of iron placed in concentrated nitric acid was found to be passive, while the metal dissolved readily in dilute HNO, with copious evolution of hydrogen. This type of behavior can be demon.straled in a very simple, yet quite spectacular, experiment. Nitric acid of various concentrations, from 1 mM to 70%, is introduced into a series of test tubes, and an aluminum wire is placed in each solution. No reaction is observed in the most dilute solutions. As the concentration is increased, however, hydrogen evolution becomes visible. At even higher concentrations, reduction of the acid takes place, in addition to hydrogen evolution. This is evidenced by the liberation of a brown gas, NO, which is one of the reduction products. When the concentration has reached 35%, the reaction suddenly stops. There is no gas evolution and the surface of he metal is not attacked. Accurate measurements show no weight loss when aluminum is kept in these solutions for months. Aluminum is passivated in concentrated HNO. A thin oxide film is formed on the surface and further attack is prevented. 

Electrochemical passivation is in many ways similar to chemical passivation. As the potential of an iron sample is increased in the anodic direction, the rate of dissolution increases, reaches a maximum, and then decreases almost to zero. Further increase of the potential has little effect on the current in the passive region until passivity... 

The molecular size of a chemical Passive diffusion is normally limited to molecules whose molecular weight does not exceed 500 Da. However, a small molecule will cross membranes more rapidly than a larger one of equal lipophilicity. 

Figure 9.11 Depiction of an idealised chemical passivation of Shockley surface states. As the bonding strength between the surface and chemical group increases, the energies of the antibonding and bonding orbitals are shifted outside the bandgap.

Heller A., Leamy H. J., Miller B. and Johnston W. D. (1983), Chemical passivation of carrier recombination at acid interfaces and grain boundaries of p-lnP , J. Phys. Chem. 87, 3239-3244. 

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