Mechanical passivity

The standard electrode trotential, Ep, 2+ Pb = —Q.126V . shows that lead is thermodynamically unstable in acid solutions but stable in neutral. solutions. The exchange current for the hydrogen evolution reaction on lead is very small (-10 - 10"" Acm ), but control of corrosion is usually due to mechanical passivation of the local anodes of the corrosion cells as the majority of lead salts are insoluble and frequently form protective films or coatings. 

A thorough discussion of the mechanisms of absorption is provided in Chapter 4. Water-soluble vitamins (B2, B12, and C) and other nutrients (e.g., monosaccharides, amino acids) are absorbed by specialized mechanisms. With the exception of a number of antimetabolites used in cancer chemotherapy, L-dopa, and certain antibiotics (e.g., aminopenicillins, aminoceph-alosporins), virtually all drugs are absorbed in humans by a passive diffusion mechanism. Passive diffusion indicates that the transfer of a compound from an aqueous phase through a membrane may be described by physicochemical laws and by the properties of the membrane. The membrane itself is passive in that it does not partake in the transfer process but acts as a simple barrier to diffusion. The driving force for diffusion across the membrane is the concentration gradient (more correctly, the activity gradient) of the compound across that membrane. This mechanism of... 

Drug permeation proceeds by several mechanisms. Passive diffusion in an aqueous or lipid medium is common, but active processes play a role in the movement of many drugs, especially those whose molecules are too large to diffuse readily (Figure 1-5). 

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. 

Mechanical Passivity.—In certain instances the dissolution of an anode is prevented by a visible film, e.g., lead dioxide on a lead anode in dilute sulfuric acid this phenomenon has been called mechanical passivity, but it is probably not fundamentally different from the forms of passivity already discussed. The film is usually not completely impervious, but merely has the effect of decreasing the exposed surface of the electrode to a considerable extent the effective c.d. is thus increased until another process in which the metal is involved can occur. At a lead anode in sulfuric acid, for example, the lead first dissolves to form plumbous ions which unite with the sulfate ions in the solution to form a porous layer of insoluble lead sulfate. The effective c.d. is increased so much that the potential rises until another process, viz., the formation of plumbic ions, occurs. If the acid is sufficiently concentrated these ions pass into solution, but in more dilute acid media lead dioxide is precipitated and tends partially to close up the pores the layer of dioxide is somewhat porous and so it increases in thickness until it becomes visible. Such an oxide is not completely protective and attack of the anode continues to some extent it is, however, a good conductor and so hydroxyl ions are discharged at its outer surface, and oxygen is evolved, in spite of its thickness. 

The so-called mechanical passivity, accompanied by the formation of a visible film and oxygen evolution, has been observed with iron, cobalt, nickel, manganese, lead and other anodes in alkali hydroxide solutions, and also with thallium, antimony and bismuth in aqueous sulfuric acid. 

In contrast to active transport, passive transport as a whole does not involve energy consumption and, therefore, only can work down a concentration gradient (or other types of gradients, such as electrochemical potential, thermal, or pressure gradients). In other words, passive transport of molecules equalizes their chemical potential on both sides of the membrane. The process of passive transport can be subdivided into two different mechanisms passive diffusion and facilitated transport. Passive diffusion is a physico-chemical process, whereas in facilitated transport, molecules pass through the membrane via special channels or are translocated via carrier proteins. Both passive diffusion and facilitated transport, in contrast to active transport, follow a gradient, where facilitation merely lowers the activation energy for the transport process. 

Chemicals can cross membranes by one or more of the following mechanisms passive diffusion,... 

Relevant Immune Effector Mechanisms Passive Immunization... 

One requirement that is always added (or should be added) to the functional constraints such as conductivity, color, elasticity, and toughness is chemical and mechanical passivity. The material must not corrode, sinter, wear, or react irreversibly during use. High durability is a nontrivial design constraint but it is not always mentioned explicitly. 

Albery et al. [39, 49] prepared poly(3-thiopheneacetic acid) and its copolymer with thiophene by electrochemical polymerization. Bartlett et al. [50] electrochemically synthesized conducting poly(3-thiophene-acetic acid) films in dry acetonitrile containing tetraethyl ammonium tetrafluoroborate. These films are redox active in acetonitrile, however, stability was reportedly poor in comparison with poly(3-methylthio-phene) and poly(methyl 3-thiopheneacetate) due to traces of water. In dry acetonitrile, the polymer can be electrochemically oxidized and reduced. Upon oxidation in water and methanol, poly(3-thiopheneacetic acid) film converted into a passive film. Based on the electrochemistry and an FT-IR study, Bartlett et al. postulate the mechanism for the electrochemical passivation shown in the Figure 4.33. In the mechanism, passivation of the polymer involves the formation of an intermediate cyclic lactone and subsequent breakdown by reaction with solvent. This process does not destroy the conductivity of the polymer so the process can continue until all the monomer units within the film are converted to a lactone form (Figure 4.33, IV). The electrochemical passivation is not observed... 

Development of theoretical principles of goal-directed influence on a complex chemical process, similar to the described cases of optimal control, is based on the detailed mechanism of the process. Such an approach is the most fundamental with a relatively wide prognostic range. Hence, the principles of goal-directed influence do not interpret the reaction mechanism passively, but allow also correcting the ideas on the mechanisms, making them more realistic. Here a quite simple principle is applicable if the kinetic model is accurate and reliable, then it has heuristic capabilities, that is, it describes correctly the behavior of a reaction system when varying the conditions of the reaction. 

Fig. 9.7. Descriptions of muscle mechanics passive stretch curve, energy loss and elastic efhciency...

Note that writing eq. (17) we have tacitly assumed that the ambient air at the upper level is some kind of a large reservoir mechanically passive (air has a dynamic shear viscosity two orders of magnitude lower than that of standard liquids) and that the interface liquid-air although deformable is mechanically ideal. Thus we have... 

In alkaline media, Ni-based supports were also explored in conjunetion with PtRu and PtRuMo electrocatalysts [200-202]. Pt Ru compositions between 1.1 1 and 2.1 1 atomic ratio supported on Ni were found to yield the lowest faradaic resistances for the oxidation of 1 M ethanol in 1 M NaOH, determined by eleetrochemical impedance spectroscopy [200]. It was speculated that the role of Ni support extends beyond purely mechanical passive interaction with the catalyst, and Ni could contribute to the electrocatalytic activity by its surface and electronic properties as an oxophilic element. Further studies are required in this area. 

Dmgs pass across barriers by a number of mechanisms. Passive mechanisms utilize the forces of concentration differences or pressure differences to move substances from one site to another. Active transport of a drug is typically via a specific transporter, requires energy, and moves solute against its electrochemical gradient. Figure 4 illustrates these definitions. 

E. L. Uttauer and K. C. Tsai, Anodic Behavior of Lithium in Aqueous Electrolytes, ii. Mechanical Passivation, / Electrochem. Soc. 123 964 (1976) Corrosion of Lithium in Aqueous Electrolytes, ibid. 124 850 (1977) Anodic Behavior of Lithium in Aqueous Electrolytes, iii. Influence of Flow Velocity, Contact Pressure and Concentration, ibid 125 845 (1978). 

Accident mitigation (Passive heat removal mechanisms) Target large radioactivity release frequency of less than lO Vreactor-year to be achieved by reliable containment design and passive heat removal mechanisms Passive mechanism is, in general, preferable for mitigating an accident and could be more reliable. 

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