Oxidation localization

Corrosion resistance of stainless steel is reduced in deaerated solutions. This behavior is opposite to the behavior of iron, low-alloy steel, and most nonferrous metals in oxygenated waters. Stainless steels exhibit very low corrosion rates in oxidizing media until the solution oxidizing power becomes great enough to breach the protective oxide locally. The solution pH alone does not control attack (see Chap. 4, Underdeposit Corrosion ). The presence of chloride and other strong depassivating chemicals deteriorates corrosion resistance. 

The site http //www.oyston.com/history has afascinating history of the topic, mentioning such early anaesthetics as ether, chloroform and nitrous oxide. Local anaesthetics are often injected in the form of liquids or solutions, see the article pharmacology of local anaesthetic agents by a British anaesthetist, Dr J. M. Tuckley, may be found... 

Sato Mooney (1960) pointed out that the oxidation of the conductor itself cannot be responsible for the SP phenomenon because it would oxidize locally in the upper part of the ore-body where oxidizing agents are in direct contact with the ore and there would be no separation of charge and therefore no SP phenomenon. The redox reactions that produce SP therefore occur between reactants in the groundwater environment at the top and bottom of the ore body. This cathodically protects the upper part of the ore body from direct oxidation. Oxidation of the ore body can still occur (hence gossans) but results from local detached oxidation which cannot produce SP. 

Relaxation of blood vessels appears to be at least partially under the control of endothelial cells and their secreted products, especially endothelium-derived relaxation factor (EDRF). Oxidized LDL directly inhibits the endothelial cell-associated vessel relaxation. The generation of increased reactive oxygen species in association with elevated levels of blood cholesterol has also been reported. One of these reactive oxygen species, superoxide (O2), may interact with vasoactive EDRF (nitric oxide) locally in the artery wall, preventing endothelial cell-dependent vasodilation. In addition, a product of the reaction of nitric oxide and superoxide, the reactive peroxynitrite, may act to stimulate lipoprotein oxidation, which, as noted above, is regarded as an early step in atherosclerotic plaque generation. 

Lin RF, Lin TS, Tilton RG et al. Nitric oxide localized to spinal cords of mice with experimental allergic encephalomyelitis An electron paramagnetic resonance study. J Exp Med 1993 178 643-8. 

To explain the various effects of temperature, time of re-oxidation, air flow and substrate thickness additional microscopic and SEM investigations have been carried out on re-oxidized samples. Microscopic pictures of areas of fracture of such cells revealed that a big gradient in the local degree of oxidation appears at higher temperatures (T > 700 C). At lower temperatures no such gradient can be observed, the substrate is re-oxidized homogenous (see Fig. 9). Half of the substrate of the sample re-oxidlzed at 800°C is completely re-oxidized (local DoO 100%), the other half is still completely reduced (local DoO 0%). A le-oxidation front appears in the substrate that separates the reoxidized part from the reduced part. The sample in Fig. 9 (b) re-oxidised at 600°C has a local DoO of about 50% everywhere in the substrate, no re-o. idation front can be observed, no gradient >pears in the local DoO. 

Raman microscopy has been used to probe photo-oxidation of imstabilized PP (90). It was concluded that the catalyst residues, for the type of PP studied, tend to stabilize the polymer in the immediate vicinity, but also form reactive species that diffuse away from the catalyst to initiate oxidation. Localized oxidation was also confirmed by examining the distribution of polystyrene grafl ed onto the hydroperoxide sites in photo-oxidized PP (91). 

On the basis of these results a theoretical model of a preliminary oxidation localization in interphase zones of a polymer sample has been proposed [239]. Apparently, the reaction ability of the compositions depends on the chemical structure of the interphase zone. That the increase of PP concentration in the PP/PP-co-PE composition from 38 to 62% leads to a lowering of reaction ability of samples has been demonstrated. The process of autooxidation begins from the most activeingredient of a polymer composition, PP or PP-co-PE. The interphase zone... 

Amperometric detection of neurotransmitter release from single PC12 cells has been carried out with these carbon-fiber MEAs to measure exocytosis. To measure exocytosis, a carbon-fiber MEA is typically placed close to the cell membrane to detect released transmitter. Released transmitters are oxidized locally and free diffusion is minimized by the small electrode/cell separation. 

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is described in section Bl.25,2.1. The most connnonly employed x-rays are the Mg Ka (1253.6 eV) and the A1 Ka (1486.6 eV) lines, which are produced from a standard x-ray tube. Peaks are seen in XPS spectra that correspond to the bound core-level electrons in the material. The intensity of each peak is proportional to the abundance of the emitting atoms in the near-surface region, while the precise binding energy of each peak depends on the chemical oxidation state and local enviromnent of the emitting atoms. The Perkin-Elmer XPS handbook contains sample spectra of each element and bindmg energies for certain compounds [58]. 

This localization phenomenon has also been shown to be important in a case of catalysis by premicellar aggregates. In such a case [ ] premicellar aggregates of cetylpyridinium chloride (CPC) were shown to enhance tire rate of tire Fe(III) catalysed oxidation of sulphanilic acid by potassium periodate in tire presence of 1,10-phenantliroline as activator. This chemistry provides a lowering of tire detection limit for Fe(III) by seven orders of magnitude. It must also be appreciated, however, tliat such premicellar aggregates of CPC actually constitute mixed micelles of CPC and 1,10-phenantliroline tliat are smaller tlian conventional CPC micelles. 

The passive state of a metal can, under certain circumstances, be prone to localized instabilities. Most investigated is the case of localized dissolution events on oxide-passivated surfaces [51, 106, 107, 108, 109, 110, ill, 112, 113, 114, 115, 116, 117 and 118]. The essence of localized corrosion is that distinct anodic sites on the surface can be identified where the metal oxidation reaction (e.g. Fe —> Fe + 2e ) dominates, surrounded by a cathodic zone where the reduction reaction takes place (e.g. 2Fi + 2e —> Fi2). The result is the fonnation of an active pit in the metal, an example of which is illustrated in figure C2.8.6(a) and (b). 

The most important property of phosphorus(V) oxide is its great tendency to react with water, either free or combined. It reacts with ordinary water with great vigour, and much heat is evolved trioxo-phosphoric(V) acid is formed, but the local heating may convert some of this to tetraoxophosphoric(V) acid ... 

Bronze disease necessitates immediate action to halt the process and remove the cause. For a long time, stabilization was sought by removal of the cuprous chloride by immersing the object in a solution of sodium sesquicarbonate. This process was, however, extremely time-consuming, frequentiy unsuccesshil, and often the cause of unpleasant discolorations of the patina. Objects affected by bronze disease are mostiy treated by immersion in, or surface appHcation of, 1 H-henzotriazole [95-14-7] C H N, a corrosion inhibitor for copper. A localized treatment is the excavation of cuprous chloride from the affected area until bare metal is obtained, followed by appHcation of moist, freshly precipitated silver oxide which serves to stabilize the chloride by formation of silver chloride. Subsequent storage in very dry conditions is generally recommended to prevent recurrence. 

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

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